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10 Ecosystems: CHAPTER ● Earth ● Biosphere ● Biome ● Ecosystem ● Community ● Population ● Organism ● Systems ● Organs ● Tissuses ● Cells ● Organelles ● Molecules ● Atoms relationships and populations Chapter 10 Relationships and populations Key knowledge Components of ecosystems: communities of living organisms, ecological groupings; ecological niche ● Relationships between organisms: feeding including parasite/host, predator/prey; of mutual benefit including mutualism and symbiosis ● Population dynamics: carrying capacity of ecosystems; factors affecting distribution and abundance of ● organisms, including birth and death rates, migration ● Techniques for monitoring and maintaining ecosystems A satellite image of the Earth shows an orb with swirls of white covering misty continents. Shining through is the intense blue of the oceans. This is our Earth, our place in the universe. Organisms find their place to live where water and ice (hydrosphere), land (lithosphere) and air (atmosphere) interact. This dynamic, ever-changing layer around the Earth is about 20 km thick, from the bottom of the oceans to the highest peaks of the mountains. It is called the biosphere. Nowhere is there conclusive evidence that living things exist elsewhere in the universe. Enhanced views of Earth from space present patterns of browns and blues that identify large terrestrial and aquatic biomes. Each biome has features that distinguish it from another. Terrestrial biomes roughly correspond to major variations in vegetation types, climate, topography (surface features, altitude) and soil type. Tropical rainforests, temperate deciduous forests, deserts, grasslands and tundra are examples of major terrestrial biomes. Figure 10.1 The Earth from space. 301 80°N 60°N 40°N Tropic of Cancer Equator Tropic of Capricorn 40°S Tundra Temperate forest Chaparral and/or evergreen hardwood (Mediterranean) Desert Temperate grassland Tropical rainforest Taiga (coniferous forest) STU Figure 10.2 Distribution of the world’s major terrestrial biomes. DE NT C D Terrestrial biomes bioTERMS biodiversity the variety of different living things: the different plants, animals and microorganisms, their genes and the ecosystems of which they are a part 302 Unit 2 Savanna grassland Other biomes (ice, mountains, semi-arid) Just as the biosphere is composed of biomes that are different from each other, patterns of ecosystems in the whole of Australia can be distinguished. Biogeographic areas, or bioregions, are distinguished by ecological characteristics of the landscape or seascape. They can also be related to the patterns of use of land and sea. Lakes, oceans and rocky shores are examples of aquatic biomes. How permanent the body of water is, its salinity, depth and availability of nutrients all influence the kinds of organisms that inhabit them and the relationships between them. The national terrestrial bioregions have been broken down into 21 Victorian terrestrial bioregions as shown in Figure 10.3. Another five bioregions occur in Victorian waters. Bioregional networks are being established to facilitate partnerships between the wide range of land managers who share responsibilities for biodiversity conservation. The land managers are rather like the managers of a chain of supermarkets. Chapter 10 Relationships and populations LM Lowan Mallee GO Goldfields OR Otway Ranges MM Murray Mallee GVU Central Victorian Uplands WP Warrnambool Plain WI Wimmera NIS Northern Inland Slopes OP Otway Plain VP Victorian Volcanic Plain VR Victorian Riverina GIP Gippsland Plain GP Glenelg Plain VA Victorian Alps WPR Wilsons Promontory DT Dundas Tablelands HS Highlands – Southern Fall EGU East Gippsland Uplands GG Greater Grampians HN Highlands – Northern Fall EGL East Gippsland Lowlands OTW Otway (Marine) CV Central Victoria (Marine) VE Victorian Embayments (Marine) FLI Finders (Marine) TWO Twofold Shelf (Marine) Figure 10.3 Bioregions of Victoria. Ecologists study the relationships between living things and their surroundings. This is called ecology (from the Greek oikos for home and logos for study), a term first used by Ernst Heinrich Heinkel in 1869. In the early days, ecology was largely descriptive, providing qualitative data based on observational studies. It has become a more exact science since measurements providing quantitative data have contributed to the development of models and theories that help us understand relationships better. STU Ecology DE NT C D Bioregions 303 Figure 10.4 Ernst Heinrich Heinkel (1888–1958). For nearly two centuries of European settlement people’s ideas of relationships between themselves, their surroundings and other living things tended to be restricted to a European view. The rich knowledge that indigenous peoples had of interactions between the environment and the living things that inhabit their world was largely ignored. Fortunately this is changing. There is increased recognition of the need to integrate both perspectives and areas of knowledge to understand our unique environment better. Today, with increased urbanisation, many people have little opportunity to develop first-hand understanding of relationships ‘in the wild’. Our relationships tend to be with our immediate surroundings, each other and, if we have them, our pets. Most of these relationships, direct and indirect, are to do with our wellbeing. Many people obtain what they need from a local store or supermarket. They learn where various items are; they may be annoyed if they cannot find something but at least an assistant can help. Some of you may work at packing shelves or at a checkout. Everyone expects facilities to support them; they don’t expect the refrigerators to fail, the doors to get stuck or suppliers to fail to deliver the goods. A supermarket is an example of a system in which there are inputs, processing and outputs. Complex interactions occur between the people who are part of the system. They make sure that the supermarket runs smoothly, that supplies meet demand and people obtain what they need. Ecosystem bioTERMS ecosystem a self-sustaining unit made up of living things (a community) interacting with and within a particular habitat interactions interplay or association between organisms of the same or different species community the sum of all the living organisms living in a habitat at a particular time It is much the same with an ecosystem. An ecosystem is not really a place, although we tend to use the word in that way. The concept of an ecosystem is useful; it provides a framework for studying the interactions between living things and their non-living surroundings, usually referred to as their environment. For example, a forest ecosystem includes not only all the organisms that make up the community living there, but also aspects of the physical environment, such as rain, the inorganic minerals of the soil, sunlight and atmospheric oxygen and carbon dioxide. All these factors interact with each other in a system we call an ecosystem. habitat + community = ecosystem The English ecologist Sir Arthur Tansley first used the term ‘ecosystem’ in 1935. He realised that organisms and their immediate environment need to be considered together as a functional unit. For example, it is easier to understand why a platypus lives the way it does if it is studied in relation to the conditions in the stream that it lives in, the organisms that it eats and its relationships with the plants and other animals in its immediate surroundings. REVIEW 1 Distinguish between the following terms: a biosphere and biome b ecology and ecosystem c habitat and community d biotic and abiotic factors. 304 Unit 2 2 Draw a concept map that shows the relationship between the following: biosphere, habitat, ecosystem, community, biome, environmental factors. 3 Distinguish between qualitative and quantitative data. Chapter 10 Relationships and populations Describing and naming ecosystems An ecosystem is usually named after the most dominant species in the community, together with its overall appearance or ‘layout’. Examples of individual ecosystems include river red gum woodland, mangrove swamp, alpine herb field, wetlands, spinifex grassland, coral reefs, heath land, rock pools and caves. All ecosystems of the same type have essentially the same features – rather like the recognisable stores of a particular chain of supermarkets. A coral reef, for example, will have more features in common with every other coral reef than it does with a woodland. As with biomes, the distribution of ecosystems depends on factors such as soil type and climate, particularly temperature and rainfall. Can you see why the ecosystem types roughly correspond to the vegetation types shown in Figure 10.6. Vegetation is classified according to: • the percentage of ground shaded or covered by the tallest layer of vegetation • the form (tree, shrub, grass) of the tallest layer Open forests have between 30 and 70% of the ground shaded by trees between 10 m and 30 m tall. If the trees are taller, the forest is described as a tall open forest. If the percentage cover is between 10 and 30%, it is a woodland. Figure 10.5 Some Australian ecosystems. bioTERMS dominant species the most common or most obvious species in a particular community 305 Rainforest Heath Wet sclerophyll forest Shrubland Dry sclerophyll forest Grassland Woodland STU Figure 10.6 Distribution of vegetation types in Australia. DE NT C D Vegetation types 306 Unit 2 Ecosystems are relatively self-contained. An ecosystem tends to support itself by cycling or exchanging materials within that ecosystem. In a forest, for instance, leaves fall and decompose and their nutrients are returned to the soil. In turn, the plants remove these nutrients from the soil and use them in growth. But this is a fairly simple explanation of how an ecosystem operates. If you travel from one ecosystem to the next it may not be easy to notice where one ecosystem stops and another begins. In ecosystems next to each other, physical conditions, such as soil type or temperature, gradually change or merge; the ecosystems overlap. Ecosystems are seldom closed. Some of the animals, such as birds, will be a part of two or more different communities as they move from one area to another; the ecosystems are open. Despite this difficulty, the concept of the ecosystem has proved very useful to ecologists. Chapter 10 Relationships and populations Table 10.1 The ground cover features of the major Australian ecosystems Ecosystem Ground cover Desert In patches, widely spaced Grasslands In arid regions typically hummocks and tussocks with a very low cover of <30% Scrublands Foliage cover 30–70% Woodlands Widely spaced canopy cover 10–30%; well-developed shrubs and grasses Alpine Low, fairly continuous cover Open (sclerophyll) forest Fairly open canopy cover 30–70%; good understorey and ground cover Closed (rainforest) forest Dense canopy cover 70–100%; distinct layers or storeys within forest Reefs and marshes Dense growth 307 How does an ecosystem work? In an earlier chapter we investigated the factors in the environment that organisms have to deal with if they are to survive. In this section we are looking more closely at how relationships between organisms can affect survival. The community Analysing an ecosystem involves studying how different organisms interact in a community. Finding out the number of different kinds of organisms in an ecosystem gives an indication of its biological diversity or biodiversity. Organisms can be classified in a number of ways, depending on the purpose of the classification: according to their structure, how they obtain their nutrients, the role they have in the ecosystem, and how they can be used to give information about an ecosystem. Table 10.2 Species types in ecosystems. Species Description Endemic Those that are native to a particular area Exotic Those that have come from or been introduced from elsewhere Indicator A particular species that give clues about the health of an ecosystem Keystone A species that is critical to the survival of an ecosystem because so many other species depend on it BIOBOX 10.1 FROGS IN DECLINE ure Fig 10.7 aw b T he b is endangered. aw frog No one seems to know why frogs and other amphibians around the world are dying – not even the scientists of 60 nations who contributed to the Global Amphibian Assessment. Amphibians are believed to be the best indicators of the health of ecosystems. They are regarded as ‘canaries in the coalmine’. Canaries used to be taken down coalmines to detect the quality of the air. If they died, it indicated that the air was not breathable for the miners. The sensitive skin of the amphibians detects slight changes in water and air quality. If numbers decline, then it is a signal that all is not well. Australia and New Zealand species seem to have been hit particularly hard. Australia has 214 species of frogs: 15 are classified as endangered, 12 as vulnerable and four as extinct. Worldwide nearly 1900 species – almost one-third of global species – are threatened with extinction. 308 Unit 2 Chapter 10 Relationships and populations REVIEW 4 Distinguish between an open and a closed ecosystem. 5 How are ecosystems named? 6 Explain the significance of a keystone species. Relationships and interactions between living things A necklace of parasites for a carpet python? A karri forest that depends on a fungus? A fish living in the mouth of another fish? The living world is full of interesting and even bizarre examples of relationships. Every living thing is profoundly affected by the presence or even absence of other living things. All the living factors that affect an organism are referred to collectively as its biotic environment and this, in turn, can shape or be shaped by the abiotic environment. Organisms of a soil community, for example, are affected by the texture, mineral and water content of the medium in which they live. But the properties of the soil itself are affected by the activities of burrowing worms and decomposers. The burrows of the worms allow air and water to reach into the soil and the decomposers increase the fertility of the soils by recycling organic material. Human exploitation, disease and climate change have been put forward to account for the decline in species. Even when a suitable habitat remains, numbers often still decline, so the cause is still a mystery. In Australia the fungal disease of amphibians, chytridiomycosis, a disease linked to drought and climate change, has contributed to the decline. So how do we find out what kinds of organisms are in an ecosystem? Keys and field guides can help you identify the organism, once you have found it. Keys help you to put the organism into a classification group and give it its individual or species name. But to do this you need to be able to identify parts of organisms and compare similarities and differences. a b bell-like flower blue bell narrow leaf plants trumpet-like flower bell-like flower wild daffodil dead nettle broad leaf top petal does not overhang lower petal 1. narrow leaf broad leaf 2. bell-like leaf trumpet-like flower go to 2 go to 3 blue bell wild daffodil heart-shaped leaf lesser celandine club-shaped leaf primrose 3. top petal overhangs lower petal top petal does not overhang lower petal 4. heart-shaped leaf club-shaped leaf dead nettle go to 4 lesser celandine primrose Figure 10.8 (a) Keys and (b) field guides. 309 Figure 10.9 Sea anemones compete for food sources. A wallaby doesn’t exist on its own. It interacts not only with other wallabies but also with the vegetation it eats, the ticks, mites, flies and other parasites that pester it, and the wedge-tailed eagle or dingo that may attack its young. Understanding relationships between organisms and their interactions with each other can help us understand how an ecosystem works. Some interactions can be straightforward but others are extremely complex. It is only by experimentation and painstaking study of behaviour (ethology) that we can come near to unravelling them. Competitor or collaborator? Communities are complex interactions of different populations and the individuals of which they are composed. Many are in competition with each other; they require the same resources to fulfil their needs for survival. Competition within and between species is a common feature of all communities. Seemingly harmless sea anemones, for example, compete for the same food source. They can detect slight genetic variations in intruders of the same species. Both rivals discharge a battery of stinging cells, normally used to paralyse and catch prey. In the end, one will admit defeat, close up and creep away. Members of some species solve the problem of catching their prey by collaborating with each other as wolves do in hunting for their prey or dolphins when herding schools of fish (see Chapter 9). These are examples of intraspecific interactions – relationships between members of the same species. On the other hand, the association the wolves and dolphins have with their prey is interspecific – a relationship between members of different species. Predator and prey An obvious feeding relationship is a predator–prey relationship in which one organism kills another or consumes part of it for its food. Predators can be classified by the way they obtain prey. Searchers, such as wrens, bream and mice, spend much of their time foraging for small invertebrate prey. Pursuers spend time and energy stalking large vertebrate prey before the chase. They can afford to wait for a long time between meals. Some predators just lie in wait for their meal to pass by. Animals employ a huge range of strategies that enable them to locate, capture and consume their prey. Although there is usually a preferred prey species, it is unusual for a predator to depend on only one species. It is an advantage for a predator to be a member of a network of food chains. If one prey species becomes in short supply, the predator can turn to others. A dynamic relationship exists between predator and prey that is usually balanced but sometimes conditions can change and upset this balance. 310 Unit 2 Under favourable conditions, with increasing availability of prey, the number of predators can increase, although it usually remains less than that of the prey. During a period of adverse conditions, the prey population can decrease. When this occurs, there is increased intraspecific competition. Predators turn to alternative prey species and the effect on them can be so severe that some may become endangered or extinct. The tale of the fox and others In the 1950s in New South Wales when poison was used to kill their preferred food (rabbits), foxes turned to the more vulnerable bettongs. It was a similar story in other parts of Australia where rabbits were prolific; predators such as wedge-tailed eagles, foxes and dingoes resorted to other prey. Foxes are responsible for devastating many smallto medium-sized Australian mammal populations including the rock wallaby and the tammar wallaby, but particularly the bettong. What remains of these populations survives in remote places or those inaccessible to foxes. Before the release of the calicivirus in 1995, rabbits were found in the stomachs of 63% of foxes in an area surveyed in the Flinders Ranges of South Australia. After the release the percentage fell to less than 16%. It was not quite the same for feral cats – they tended not to turn to native animals for food. Many died of starvation or failed to breed. So what did the foxes eat? They turned to carrion (dead animals) and small mammals and invertebrates such as locusts, grasshoppers and centipedes. Though the impact was initially quite severe, there is some, but not conclusive, evidence of recovery. At least the drop in rabbit numbers allowed native vegetation to recover. Size of population Chapter 10 Relationships and populations prey predator Too much of a good thing The predator and prey relationship can go wrong. Foxes, for example, go on the rampage and kill more prey than they need for survival. This kind of surplus killing is uncommon in predator and prey species that have coevolved over thousands or millions of years, yet it does happen. In these cases localised surplus killing is triggered by some unusual event. One example of such a killing is when over 80 Thomson’s gazelles were killed by predators in Africa on a very dark night with unusually heavy rain. Dingoes have been known to surplus kill. They are pursuers rather than pouncers but they attack domestic stock or kangaroos around waterholes. It is less exhausting. Time Figure 10.10 The predator–prey relationship. Figure 10.11 The bettong became the foxes’ prey. bioTERMS coevolve the evolving and adapting together of species 311 ACTICA PR .1 10 CTIVITY LA bioTERMS coexistence living together Coexistence – peaceful and otherwise An interesting aspect of predation is that in some cases it can affect the coexistence of a number of other species. The starfish is a natural predator of mussels in the intertidal zones of shores. Removal of the starfish in experiments resulted in the expansion of the population of mussels. They displaced the other sessile (fixed) organisms, such as barnacles, as they spread. When the starfish were returned, the mussels were again preyed upon and the barnacles were able to re-occupy the space. The predator – the starfish – allowed the coexistence of two species with the same requirement for space. REVIEW 7 Compare, using examples, interspecific and intraspecific relationsh ips. 10 What is meant by a dynamic relationship between predator and prey? 8 What is the distinctive feature of a predator–prey relationship? Give three examples of predator–prey relationships. 9 Explain, by using an example, why a prey population is usually larger than the predator population. 11 Explain the advantage to a predator of having more than one prey species. 12 Distinguish between coexistence and coevolution of species. Give examples to demonstrate the difference. 13 Account for surplus killing by some species. Countering attack Figure 10.12 This butterfly has fake eyes and fake antennae. 312 Unit 2 Why is it that prey species are not wiped out by their predators? Prey species have evolved counteradaptations – structural, physiological and behavioural features – which enable them to avoid their predators. We saw some examples of these in an earlier chapter. Bats are very successful survivors and echolocation may be the key. It allows them to detect their prey at night when many other predators cannot forage or hunt. Certain insects have evolved counteradaptations. The listening membranes of some species of moth detect the sonar pulses of the bat. This advanced warning gives the moths the opportunity to escape. Other moths even produce their own sonar, confusing the bats. Large fake eyes on the wings of butterflies and fins of fish seem to distract the predator from the important end of the animal’s body. This gives the potential prey a chance to get away, largely undamaged. Other animals, such as tortoises, have tough armour, which proves to be a barrier to all but the most determined predator. Some animals deter predators by chemical defence – some squids, skunks and insects squirt nasty fluids. Chapter 10 Relationships and populations BIOBOX 10.2 MIXED MESSAGES Figure 10.13 Colours and patterns as warning signals. Keep off! The monarch butterfly is coloured and patterned in a way characteristic of many animals that are highly poisonous to others. This visual signal is somehow understood by animals of all kinds as a warning. A strong pattern of black and yellow or orange is one of the most common and is displayed, for example, by arrow-poison frogs, monarch butterflies, bees, wasps and hornets. This sharing of a common pattern by different poisonous animals is known as Mullerian mimicry and is advantageous to them all by deterring predators. Fooling others – Batesian mimicry Some animals, such as hoverflies, clear-winged moths and some beetles, display a danger patterning too, but they are frauds – they have no stings nor are they poisonous. This kind of mimicry – Batesian mimicry – benefits the mimic because its potential predator avoids it. The model – the animal being copied – could be affected because a few successful meals of a mimic may lead the predator to try the real thing. Batesian mimicry is known in the plant world, too. Different species have evolved to resemble each other to their mutual advantage. Find me if you can – camouflage and disguises Animals can avoid being preyed upon by disguising or camouflaging themselves. This is known as crypsis. Many insects can hardly be distinguished from thorns or leaves and a Costa Rican caterpillar looks like a viper. Some predators use disguise. Floating ‘seaweed’, in reality a seahorse, can snap up a passing shrimp. A praying mantis is indistinguishable until it moves and catches its unsuspecting victim. Figure 10.14 A viper or an elephant hawkmoth caterpillar? 313 Preying on plants The term ‘predator–prey’ is usually associated with predators and animal prey but it is often used in relation to animals that feed voraciously on plants. In one year the leaf-chewing insects of the snow gums, Eucalyptus pauciflora, in the Snowy Mountains, removed 38% of the leaf area and 76% of shoots. Up to 50% of the eucalypt leaf area is regularly chewed by stick insects, cup moths and other kinds of insects and their larvae. Christmas beetles can strip a tree in a very short time. Although insects and other herbivores can stunt the growth of trees and alter the species composition of forests, there is one advantage to this activity – the continuous downpour of frass, the faeces of the insects and their larvae. Frass provides a rich source of nutrients for the decomposers in the forest litter. This process enables nutrients to be recycled more rapidly than would happen otherwise. Some insects, particularly ants, remove up to 70% of seeds after they fall. These animals are referred to as seed predators and they affect the rate of germination and consequently the renewal of forests. aly Euc .15 0 1 re Figu pt g ed ma a d es leav s by a y la wfl Figure 10.16 Venus flytrap. e. rva A chemical interaction In many species of trees, chemical defence can be switched on by the attack of leaf-cutting insects – an induced defence. The damage to the tissues causes the release of a chemical that is thought to diffuse from the leaf to neighbouring leaves. These respond by increasing their levels of toxic or repellent compounds, with the effect of deterring further feeding and damage to the plant. bioBYTE The mystery of the suicidal Norwegian lemmings can possibly be explained by a kind of plant chemical lemming interaction. When lemmings start to feed on sedges and cotton grass in the Arctic tundra, the plants respond by producing a toxin that neutralises the lemmings’ digestive juices. Plants stop doing this if grazing is light but, if heavy, as when populations of lemmings explode, the toxins accumulate, so the lemmings cannot digest their food and the more they eat the hungrier they get. Having stripped the land of food resources, on reaching a lake or sea edge, it could be that the lemmings attempt to swim to other areas in a desperate search for more food. Plants preying on animals Animal-eating plants? This sounds rather odd but there are many examples of plants relying on animals to supplement their diet. In nutrient-poor environments pitcher plants, often called carnivorous plants, trap the insects and detritus that fall into their specially adapted pitchers. The action of enzymes produced by the pitcher plant and the activity of microorganisms break down the organic matter, thereby making it available to the plant. Pitcher plants may have lodgers, too – small frogs who wait for unwary insects to fall in. Pitcher plants capture their prey passively but sundews and Venus flytraps are more active in how they go about it. REVIEW 14 What are counteradaptations? Give an example. 15 Do you think herbivores should be referred to as predators? Justify your answer. 314 Unit 2 16 Describe a chemical interaction between a plant and an animal that feeds on it. Explain any advantages and disadvantages in such a relationship. Chapter 10 Relationships and populations Symbiotic relationships Symbiosis is the general term used to describe the relationship in which individuals of two or more different species live and in which at least one of the species benefits. There are three main types: 1 parasitism: one organism benefits at the expense of the other 2 mutualism: both species in the relationship benefit and neither is harmed 3 commensalism: one organism benefits and the other neither benefits nor is harmed. Parasitism! Parasites can be fascinating – for some. Most species, including humans, harbour parasites in every conceivable part of their body. a The parasites are extraordinarily well adapted in life cycle, structure and physiology to find their host and survive the hazards of being dependent. The hosts themselves have coevolved strategies for surviving the effects of their unwelcome invaders. In a predator–prey relationship, one animal benefits at the entire expense of the other, but in other exploitative relationships, it is different. In parasitism, the parasite benefits and the host is harmed. But it is not in the interest of the parasite to damage the host to the extent of killing it. An ectoparasite is an organism b that lives on the surface of another and obtains its food from the host, for example a tick on a lizard or an aphid on a leaf. An endoparasite lives inside the tissues of its host, for example a tapeworm in a sheep, malaria parasites in blood or a leaf miner that burrows in a leaf. Parasitic flatworms live in the gut and absorb nutrients. Blood flukes feed on blood in the tissues, whereas leeches prefer to suck it from the outside. Worms of many kinds inhabit the insides of their hosts. Nematodes worm their way into plant roots, while some inhabit animals. Mites are ectoparasites. They have been called ‘terrestrial plankton’ because they are so numerous and exist everywhere – even on us. About 2 million dust mites keep us company in bed, living off the skin we shed! Colonies of mites inhabit a single listening membrane of a moth, using different parts to lay their eggs, discard their wastes and feed on the moth’s blood. The relatives of the mites, the ticks, are also bloodthirsty creatures living on a variety of hosts, particularly warm-blooded vertebrates. Ticks are common on cattle, numbats, bandicoots, kangaroos and birds. About 70 species live in Australia but only about 13 bite humans. A few can cause serious harm to their host. The Australian paralysis tick, Ixodes holocyclus, has the unenviable reputation of being the most toxic tick in the world. Figure 10.17 A parasitic way of life. (a) The top left red blood cell is infected with a malarial parasite, Plasmodium, an endoparasite. (b) This grass skink has red mites, which are ectoparasites, in its armpit. 315 Parasites of plants Plants suffer from parasites, too. Different fungi infect nearly every kind of plant either through the roots or through the leaves. Dodder plants have thin stems that wind around the host plant. Fine suckers grow out, penetrate the plant stems and absorb the nutrients. The cherry ballart (native cherry) is not obviously a parasite, being a tree, but it obtains most of its nutrients from the roots of nearby host trees such as eucalypts. REVIEW 17 Why are predation and parasitism regarded as exploitative relationships? 18 Use examples, including both animals and plants, to distinguish between ectoparasites and endoparasites. 19 Explain why parasites seldom kill their host. Figure 10.18 Parasite of plants: dodder plant. bioTERMS mutualism a relationship that exists between organisms of different species and that benefits both and harms neither association a relationship or interaction between species Mutualism Not all relationships cause harm. There are all possible grades of mutualism, from a rather loose association in which the partners seem to gain little from each other, to ones that are so intimate that the two partners can be regarded as a single organism. Pollinators Pollination is essential for plants to reproduce. Some plants are pollinated by the wind but many rely on insects, birds, small mammals and even reptiles to transfer pollen from one flower to another. Usually they are rewarded for their efforts: pollen and nectar are rich in nutrients. Figure 10.19 These pygmy possums, while collecting nectar from eucalyptus blossom, are also acting as pollinators. Generalist pollinators pollinate a range of different species, whereas specialists will pollinate only the kind of plant with which they have coevolved. In these partnerships, both members depend so much on each other that if one disappears the other would remain sterile or starve. 316 Unit 2 Chapter 10 Relationships and populations Figure 10.20 The rare spider orchid, Caladenia robinsonii. bioBYTE The rare spider orchid is on the brink of extinction because of destruction of its habitat. It is thought that only 20 plants remain in the wild. The orchid relies on a wasp to pollinate it. The orchid has coevolved to resemble the wasp, fooling it into attempting to mate with it. In the process, the orchid is pollinated. BIOBOX 10.3 THE PLIGHT OF POLLINATORS The decline of some plant species has been traced to the decline of their pollinators. An epidemic of parasitic varroa mites in North America and Europe is thought to be responsible for the massive drop in honeybee colonies. The bees are the major pollinators for over a hundred commercial crops and, without them, production costs of crops, particularly of California almonds, have risen to over $6 billion per year. Other pollinators – bats, birds, butterflies and other insects – are in worldwide decline, largely due to the overuse of pesticides. Declining bat populations affect the survival of eucalyptus and neem trees in tropical areas. In Central America, the decline in hummingbirds has threatened the survival of many plants. In Britain, a quarter of the 250 native bee species are endangered and these are responsible for most of the pollination of wild plants and fruit crops, such as strawberries, apples and pears. Figure 10.21 Pollinators: reward for effort. In some instances, the loss of a single pollinator species – a keystone species – can cause the collapse of entire ecosystems. The loss of flying foxes in some Pacific islands could have repercussions along food chains because mammals depend on the fruit of trees pollinated by the flying foxes. To halt the decline, scientists have suggested constructing ‘nectar corridors’ along which pollinators can migrate from one habitat to another. 317 Seed dispersers – sometimes a sticky business An interesting relationship involves parasitism and a seed disperser. The mistletoe bird relies almost exclusively on mistletoe berries for its food. Even having passed through the bird’s digestive system, the berry retains its stickiness and remains attached to the bird’s rear end. The mistletoe bird rubs on, say, the eucalypt branch to remove the annoying berry. In the process, the berry becomes attached to the branch. It grows out small roots that penetrate the eucalypt. Eventually they reach the transport vessels and absorb the sap. Figure 10.22 Making the most of mistletoe. Lichens – do one and one make one? Lichens were a mystery for a long time. They result from the close physical and metabolic interaction between an alga or cyanobacterium and a fungus. The partnership has been successful as lichens can survive in extreme conditions of altitude, low and high temperatures and desiccation (drying out). Lichens are used as indicator species because of their sensitivity to atmospheric pollutants. Many kinds have become extinct and more are endangered because of changes in the atmosphere. bioTERMS soredia (unit of reproduction) seed disperser an animal that forages and disperses seeds facultative describes an association that may be obligatory for one species, but not another Figure 10.24 (below) Lichens show many different forms, such as blisters, streamers, colourful nodules and these crusty leaf-like structures. algal layer fungal layers fungal hyphae (threads) ‘rootlets’ for attachment to substrate Figure 10.23 Section through a lichen, showing fungal hyphae (absorbing threads) among the algal cells. The fungus is nearly always the dominant organism in a lichen but it is unable to survive without its alga or cyanobacterium partner. The fungus relies on the products of photosynthesis taking place in the alga. This is an obligatory association for the fungus. The algae or cyanobacteria, on the other hand, are protected from desiccation though they can usually survive on their own. As the host is not essential for them, the association for the alga or cyanobacterium is described as facultative. 318 Unit 2 Chapter 10 Relationships and populations It is difficult to classify lichens because of the complex fungus–algae relationship. Are they one organism or two? Are they prokaryotes, protists or fungi? The present taxonomic solution is to put the 20 000 or so different lichens into a group of their own – the too hard basket. Symbiotic digesters Complex communities of microorganisms live in the gut of many animals. Without them, cows, sheep, koalas, termites and many others would not be able to digest their food. The microorganisms secrete enzymes that break down cellulose. In return, the host provides a relatively safe environment for them – unless they get moved along and are themselves digested! Termites are voracious feeders. They can attack a eucalypt tree, feed on the central heartwood, which is dead, and hollow it out rapidly. In doing so, they make shelter available for other animals. The tree is not usually killed, as the living part of the trunk is not attacked. What kinds of relationships exist between the bacteria, the termites, the tree and the animals that live in the hollows? Relationships of the enclosing kind The ultimate intimate association is achieved when one of the partners lives inside the cells of the other. Many coral polyps, jellyfish, clams and sea slugs have algae living in their tissues. The algae need nitrates and phosphates for their metabolism. These are made available in the waste material of their animal partner. The partner is careful to ensure that its algae are always adequately exposed to light as they benefit from the organic compounds produced in photosynthesis. Some sea slugs from the Great Barrier Reef actually stimulate the colonies of algae to reproduce. The sea slugs grow out tentacles along their sides and the algae become squatters. The sea slugs come to rely entirely on the food produced by the colonies. But how did they get there in the first place? The algae were present in the coral the slugs fed on; they passed from the slug’s gut into the tentacles. Figure 10.25 Ultimate intimacy can be seen in the relationship between the Hydra and the green algae in its cells. Figure 10.26 (a) The remora and shark (b) Sea anemones and fish. b a Commensalism Commensalism is a one-sided affair. Only one of the two organisms involved, the commensal, benefits from the association. Some relationships are easy to identify as commensal, such as the relationship between the remora and the shark. The remora gets a free ride – and possibly a free meal of leftovers – by attaching the suction pad it has on the back of its head to the shark. Many fish gain protection by swimming between the tentacles of sea anemones. The tentacles have stinging cells that paralyse and capture prey but do not affect the fish. 319 BIOBOX 10.4 MORE MUTUAL MATTERS There are countless examples of other interesting relationships. One such is between a crab and its carefully placed anemones. The anemones scavenge on the food particles that escape from the crab. Perhaps they also benefit by being taken into areas they could not otherwise go. The anemone can stand on rocks and shells but not on soft sand or mud. The anemones themselves perhaps give some kind of protection to the crabs. Why do we think so? The claws of crabs with anemones on them tend to be smaller than those of species that do not carry anemones. Interestingly, the claws of hermit crabs that have the anemone perched on the back of their borrowed shell are large. ure Fig 7Ac 10.2 h its anemones – a beneficial relationship rab wit . Travelling companions and a cleaning service Some animals act as carriers for their smaller partners. A little mouse in the forests of Costa Rica carries beetles clinging to its fur round its eyes and neck. The beetles do not feed on the mice but on the fleas that occupy the mice’s nests. Because the beetles keep down the number of fleas, the mice benefit. Ox-pecker or tick birds are seen perched on the back of buffalos, zebras, rhinoceroses and other large herbivores. Coevolution has ensured that the birds are able to probe for the ticks with flattened beaks and hang on to the beast when it is moving with long claws. In addition, they warn their carriers of approaching predators. Sometimes, the oxpeckers take more than the ticks – they like to sample the blood in an open wound. Galapagos finches have a similar relationship with giant tortoises, which signal their need to be cleaned by rising up on their legs and remaining immobile. The finches Figure 10.28 Us eful travelling companions. gain access to the softer parts of the tortoise, which are irritated by parasites. Large fish and whales have a similar cleaning service provided by smaller fish or shrimps. Dead bits of tissue, fish lice and fungi are all removed, sometimes at recognised ‘cleaning stations’. 320 Unit 2 Chapter 10 Relationships and populations Epiphytes Climbing plants, such as lianas, use trees to support them in their reach for light. Seeds germinate on the forest floor and the rapidly growing shoots spread out. If they reach a vertical surface they take hold of it. Other seeds, such as those of orchids, are wafted high up on the branches and take hold there. In these instances, the tree offers support without apparently gaining anything in return. BIOBOX 10.5 LIFE SUPPORT Figure 10.29 Bromeliad ponds. Some of the commonest epiphytes in forests are the bromeliads. They germinate high up in the canopy and anchor themselves tightly by wrapping their roots around a branch. The leaves grow in such a way that they channel rain water and form a miniature pond, which becomes home to an assortment of small animals. Dragonflies and mosquitoes lay their eggs here, tiny frogs pass through their life cycles here and slugs, worms, beetles and small reptiles also form part of the community. Even birds come to visit and drink from the ‘pond’, leaving their droppings for the microorganisms. The bromeliad benefits from this arrangement by being able to extract nutrients from the decaying organic matter in the ‘pond’, which otherwise would not be available. This relationship is therefore mutualistic, but what of the relationship between the bromeliad and the tree? As the name suggests, the strangler fig does not have such a kindly relationship with its supporting tree. The relationship starts well enough but ends disastrously. As the lattice of roots grows downwards, the roots are able to use the nutrients from the forest floor, leaving less for the supporting tree. This, together with the fig’s upwardly growing bushy branches that compete successfully for light, eventually results in the death of the supporting tree. Strange, hollow lattices are left standing once the trunk rots away. Commensalism begins the relationship, but at what point does it end? REVIEW 20 Describe three relationships or interactions between plants and their pollinators. 21 Identify the components of the ‘mistletoe’ relationships described in the text. 22 What kind of relationship is demonstrated by lichens? Explain your answer. 23 Identify the relationships between termites and the microorganisms of their gut, termites and the tree that they hollow out, and termites and the possums that shelter in the hollowed-out tree. 24 Draw up a table that compares the characteristics of parasitism, mutualism and commensalism. Include examples of each kind of relationship. What general description is used for these relationships? Explain. 321 Population studies Relationships between organisms can change. We have seen how the balance between interacting species can be upset. Studying populations can help us predict such changes and their consequences; it can give us an idea of biodiversity within ecosystems and, on a much larger scale, the biosphere. Ecologists describe the total number of a particular species in a particular place at a particular time as the population. For example, the number of manna gums (Eucalyptus viminalis) in a heath-land community or the number of straw-necked ibis (Threskiornis spinicollis) in a wetland in a particular month; or even the number of foxes in an urban area. Populations in an ecosystem are dynamic. Maybe you have noticed changes in the number and kind of plants or insects from one season to another or from year to year. Even the ratio of males to females can change. After mating, for example, male Antechinus die from stress and disappear from the population. Some female spiders eat their partners! Figure 10.30 Reproducing dangerously – Antechinus. bioTERMS open ecosystems ecosystems in which there is immigration and emigration closed ecosystems ecosystems in which there is little or no immigration and emigration Growth of populations In open ecosystems migration between populations of animals and plants (dispersal) can occur. This will affect overall numbers in the ecosystems involved, their distribution and, in the longer term, birth and death rates. In closed ecosystems the growth rate depends only on birth and death rates. A population is increasing if the birth rate (br) and immigration rate (ir) exceed the death rate (dr) and emigration rate (er). Rate refers to the number of individuals per hundred, thousand or whatever unit is appropriate. growth rate = (br + ir) – (dr + er) For example: growth rate = (100 per thousand + 65 per thousand) – (37 per thousand + 25 per thousand) = 103 bioBYTE Therefore, the population has grown by 103 individuals per 1000. Growth rates can also be expressed as a percentage: +10.3% if there is an increase or –10.3% if there is a decrease. CALM’s Foxglove team (Conservation And Land Management) in Western Australia monitored changes in population of native species. This followed control of foxes in designated areas. Woylies (brush-tailed bettongs) were trapped, fitted with radio transmitters and released. The information the team obtained helped to develop strategies for controlling foxes on a much larger scale elsewhere. 322 Unit 2 Quantifying risk Information that is provided by population studies is useful for monitoring ecosystems. Knowing the age structure of a population is significant in predicting future growth trends. Figure 10.31 shows population pyramids for two different human populations. Population A has a greater proportion of children than does population B. What is the significance of this? Scientific data can help in the management of ecosystems. It can be used to plan how to ensure sufficient numbers of young live to reach reproductive age, how to develop strategies for protecting endangered species and maintaining ecosystems, and how to restore degraded ones. Chapter 10 Relationships and populations 90 80 Age (years) 70 60 50 40 30 20 Males Females Males Females 10 50 40 30 20 10 0 10 20 30 40 50 12 10 8 6 4 2 0 2 4 6 8 10 12 Population A (millions) Population B (millions) Figure 10.31 Population pyramids. Distribution Understanding how an ecosystem operates takes more than knowing what kinds of species are part of the system or how they relate to each other. It is important to know their distribution – exactly where in the physical space members of the different species are found. Members of a species are seldom spread evenly throughout the entire ecosystem. There are patterns in the way populations are distributed. • random distribution: organisms are spaced irregularly – the location of an organism does not affect the location of another (more common for plants than for animals) • uniform or continuous distribution: organisms are evenly spaced – the presence of one determines how close or distant another will be. It is common in relatively high-density populations of some animals that set up breeding territories • clumped or grouped distribution: a number of individuals is grouped together and the groups make up the population as a whole. This is sometimes to do with social behaviour as in schools of fish, or clumping of vegetation in favourable mini-habitats. clumped uniform Figure 10.32 Distribution patterns: clumped, uniform and random. random 323 Knowing the distribution and abundance (how many) of a species can help keep track of populations of significance. Knowledge of particular plant species can give clues about the distribution and abundance of animals that depend on them. The forestry industry needs to know about the distribution and abundance of valuable tree species and the fishing industry about fish stocks. Keeping track of pest and plague species, such as mice and locusts, gives forewarning of potential outbreaks that would require management. Measuring distribution Figure 10.33 Differences in physical conditions result in vertical stratification in a tropical rainforest. The line transect (trans across, sect section) method is best to use if environmental factors such as land surface change along the distance to be sampled. Gradual changes like this are referred to as environmental gradients. Vertical transects can show vertical distribution of species. Different conditions at different levels result in stratification, for example in forests. Light: 70% Wind: 15 km/h Humidity: 67% Light: 50% Wind: 12 km/h Humidity: 75% Light: 12% Wind: 9 km/h Humidity: 80% Light: 6% Wind: 5 km/h Humidity: 85% Light: 1% Wind: 3 km/h Humidity: 90% Light: 0% Wind: 0 km/h Humidity: 98% Source: Biozone Learning Media, Year 11 Biology Student Resource and Activity Manual Environmental factors change across a tidal mudflat or a marine rock platform. In these cases a profile can be drawn. 324 Unit 2 Chapter 10 Relationships and populations BIOBOX 10.6 LOCUSTS Figure 10.34 Australian plague locust. Australia has about 700 endemic species of grasshoppers and locusts but the one that usually hits the headlines and the news screens is the Australian plague locust, Chortoicetes terminifera. Locust populations increase enormously in favourable climatic conditions – usually following periods of rainfall. Many minor plagues occur regularly but there have been four major plagues in the past 50 years. Although plagues of this locust have been experienced in inland areas of New South Wales, South Australia and even Victoria, most have their origin in the Channel Country of south Queensland. Can you imagine eating about one-third to half of your body weight each day of your life? Plague locusts do. No wonder farmers and others want advance warning of their approach. After hatching, the hoppers – the first of several stages in the locust’s life cycle – move away in dense marching bands that can cover several kilometres. Densities of 1000/m2 have been recorded. It takes little imagination to realise that these eating machines can strip crops, grazing land and native vegetation in a matter of hours. It is not surprising that the young locusts, called instars, have to split their ‘skins’ several times before adulthood to make room for expansion! The adult locusts form swarms of varying density and can contain up to several million individuals. The swarms are capable of being carried up to 50 m high and of flying 10–20 km per day. Long-distance migration takes place at night, as the locusts travel with the help of the night breezes. Plagues such as these have economic and environmental implications. It is understandable why chemicals are used to control the locusts. However, it is reported that these are moderately toxic for birds and fish and highly toxic for aquatic and terrestrial arthropods. The use of natural bioinsecticides is increasingly common. An oil-based formulation containing Metarhizium, a fungus, has been developed by the CSIRO to control plague locusts. The spores are easy and cheap to produce, and can be used in a spray or distributed as granules. They can be stored readily and are extremely effective. This method of control depends on the interaction between the locust and the fungus. persistent intermittent Figure 10.35 Distribution of Australian plague locusts in Australia. 325 Vertical height (m) 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 rocky ledge supralittoral zone intermittent splashing periwinkles algae visible in winter splash zone highest tide level usual tidal range lowest tide range sea anenome, starfish, octopus, brown algae with floats periodic wetting and drying out algae (leafy green) periwinkles, limpets, chitons, rock crabs wave pounding kelps (brown algae) with holdfast, barnacles, sea squirts rock pools sublittoral zone Figure 10.36 Transect profile of a marine rock platform. Measuring abundance It is not always possible to determine the population of a species. Birds and insects fly here and there, and some animals move too fast to count. It may be difficult to work out what actually makes up an individual organism. How do you count individual plants of grass? bioTERMS sampling techniques ways of estimating populations of species quadrat a frame of known dimensions used to estimate populations of species in a given area Figure 10.37 Quadrat sampling. Studies of the small native daisy, Rutidosis leptorrhynchoides, are aiming to establish what population size is needed to avoid in-breeding and sustain the population. Direct observation Making direct observations and recording sightings at particular intervals might be possible but it is time-consuming and, in the case of male fur seals in the breeding season on windswept shores, quite dangerous! Satellite images have been used to determine land cover in relatively inaccessible regions. In aquatic ecosystems, plankton nets are used to ‘sweep’ or sample the organisms and aircraft traverse areas to count kangaroos. Even when it is possible it may not be necessary to count all members of a given population. Various sampling techniques can provide estimates of a population. A sample is a small group of organisms selected from the total population in a given area or volume. This sample represents the whole population. Choosing a particular site because it is easy to get to or is more interesting, or selecting only two sample specimens, reduces the reliability of the data obtained. It does not give a true picture of the whole population. To represent the population as a whole reliably, the samples must be collected in an unbiased way. Quadrat For organisms that are fixed or do not move very much the quadrat method of sampling can be used to estimate distribution and abundance. It can also be used to calculate density of a population. Several quadrats representative of the area are sampled at random. For each quadrat: • the number of individuals of each species is counted and recorded, or 326 Unit 2 Chapter 10 Relationships and populations • the relative numbers of each species is estimated using a scale from abundant (3) to absent (0), or • percentage cover is estimated, and • the totals of the quadrats are averaged. A simple mathematical calculation can give the total number or percentage cover for each species in the whole area. The density can be calculated too. average density of members of species (estimated) total number of individuals counted area of each quadrat number of quadrats Figure 10.38 Playing tag – tagging a turtle. population N=? CTIVITY LA .2 10 The capture–mark–recapture method is commonly used to sample motile species. A random sample of individuals of a species is taken and an overall estimation of the abundance of the species is made. Step 1: capture – animals are caught randomly and in such a way that they are unhurt. Small animals are trapped in cages or pitfalls in the ground, birds are trapped in fine nets and some animals are caught easily when they ‘freeze’ in spotlights. Flying insects are attracted to light traps. Step 2: mark – each captured animal is marked so that the mark is not obvious to predators or harmful to the organism. Insects are usually marked with a blob of paint, whereas birds are tagged on the leg or wing. The animals are returned to their habitat and left to mix with the unmarked individuals. Step 3: recapture – later, a random sample is taken and the number of marked individuals counted. From this information the total population can be estimated. The procedure has to be planned carefully so that the chances of each individual being caught are equal. ACTICA PR Capture–mark–recapture sample 1 26 captured All marked individuals are returned to the population. All captured individuals are marked. M = 26 sample 2 21 captured (n) 3 marked (m) Figure 10.39 Catch me if you can. M = 26 N=? Marked individuals have spread throughout the population. 327 Total population (N ) = no. marked in first sample (M) total number of animals recaptured (n) no. of recaptured animals that are marked (m) Mn N _______ m 20 50 For example, total population ________ 10 1000 _____ 10 100 Population density Studying the density of populations in an ecosystem is useful. Density refers to the number of individuals in a given area, such as the number of kangaroos per hectare. Sometimes it is difficult to distinguish individual plants, as with grass. In that case, density is the amount of biomass (dry) per unit area (for example, 0.6 kg of grass per m 2). In the case of diatoms in a pond it is the number per unit volume, for example 300/mL. bioBYTE A Greening Australia program aims to return small woodland birds to the landscape where agriculture has affected populations of birds. But this depends on a sufficient amount of native vegetation to support them. It has been estimated that to reoccupy an area, birds need a minimum of 10–21 hectares of native vegetation with a dense understorey. To support breeding and sustain the population, 100 hectares is needed. Birds are useful bioindicators and they provide free pest control for woodland trees. Carrying capacity Knowing the density of populations can help assess an ecosystem’s ability to provide sufficient resources to support populations. This is the carrying capacity of the ecosystem and it can change from time to time as environmental conditions change. It has been estimated, for example, that 100 km 2 of moist tall eucalypt forest is the minimum area possible to support 10 000 sugar gliders as a viable population. Information such as this is a guide to the minimum size a park or any closed ecosystem needs to be to conserve animals. REVIEW 25 Ecosystems can be open or closed to particular species. What effect does this have on growth of a population? 26 What characteristics of populations are usually studied when analysing ecosystems? 27 A particular population of kangaroos has 1000 births during the year; 72 individuals also join the population while 108 leave. There are 345 deaths. Work out the growth rate for this population for the year. 28 Describe three basic patterns of distribution of populations of organisms. 328 Unit 2 29 Draw up and complete the table below summarising methods of estimating distribution and abundance of populations of organisms. State what each method is best used for. Method Brief description Used for … 30 Explain the meaning of ‘carrying capacity’ of an ecosystem. Chapter 10 Relationships and populations Dynamic populations Population regulation Number of individuals in the population When a few members of a species colonise a new and favourable habitat, its Population Population Population growing slowly growing growth population increases rapidly. exponentially decelerating This population growth cannot be sustained – resources are used. The population begins to level off. Despite minor fluctuations in populations, there tend to be upper and lower limits. For a given species in a particular habitat, there is a certain equilibrium population that the ecosystem can support. Factors in the environment, During this phase population growth increases as population gets under way. Often starts collectively referred to as slowly because initially there is a shortage of environmental resistance, act on reproducing individuals which may be widely dispersed. a population. If the population rises above the equilibrium or set point, competition for resources such as food and space begins to take effect. The increased ability of diseasecausing organisms and parasites to spread also increases deaths and possibly reduces breeding. This could be to such an extent that the population falls. If it falls below the set point, there is less competition and the population begins to rise again. This kind of negative feedback process, or homeostatic control, keeps the population more or less constant. increase in population equilibrium population (near to the carrying capacity of the environment) fall in population raised environmental resistance, e.g. increased competition for food or increased predation lowered environmental resistance, e.g. less competition for food or less predation negative increase in population Population more or less constant Birth rate and death rate balance each other, resulting in equilibrium. Environmental resistance sets in, increasing the death rate and/or decreasing the birth rate. Time This phase represents the maximum growth rate under optimal environmental conditions – no environmental resistance, birth rate exceeds death rate. bioTERMS equilibrium population the population at set point environmental resistance environmental factors that affect population; density-dependent and density-independent set point the level of equilibrium a population reaches feedback fall in population Figure 10.40 The S-curve: generalised graph of population growth. population returns to norm near to the carrying capacity of the environment homeostatic control a negative feedback process that, overall, maintains equilibrium negative feedback Figure 10.41 Homeostatic mechanism of population control. 329 The factors mentioned above are density-dependent factors. The greater the density of a population, the more individuals die or fail to reproduce. Densityindependent factors, such as severe weather conditions, volcanic activity or habitat destruction by clearing or fire, are those that affect all individuals in a population regardless of age or stage. Figure 10.42 Populations are regulated by density-dependent and density-independent factors. Density-dependent factors Competition Disease Parasites Predation Food supply Density-independent factors Indirectly affect the food supply Population density influences the effects of these factors These factors are influenced by the kangaroo density Factors affect individuals to the same extent regardless of population density Organisms crowded together: • compete more for resources • are more easily found by predators • spread disease and parasites more readily Physical factors Acidity Rainfall Humidity Temperature Salinity Catastrophic events Volcanic eruptions Tsunami Fire Drought Earthquake Flood Can cause ill health or death Influences the individual’s ability to reproduce Source: Biozone Learning Media, Year 11 Biology Student Resource and Activity Manual Percentage of population surviving Figure 10.43 Survivorship curves. In some cases, populations of species are not able to recover from such extreme environmental resistance and become extinct in that ecosystem. A combination of factors, working together, determines the survival of populations. If we examine survivorship of various species, patterns emerge. Species can be classified according to their probability of survival. This can be represented in a graph. 100% 10% 1% Table 10.3 Probability of survival of species. 0.1% Age Classification Description Examples Early loss Species that produce many offspring. Many, even thousands, of young are produced but the probability of survival is low in early life. Probability increases with age. Fish, frogs and many plants Constant loss Species whose probability of survival of individuals does not change with age. There is a fairly constant loss at all ages. Many bird species Late loss Survival of individuals decreases with age. Humans, elephants bioTERMS survivorship the probability of survival of species 330 Unit 2 Chapter 10 Relationships and populations Population distribution and abundance Density The number of organisms per unit area Distribution The location of individuals within an area Total abundance The total number of organisms Population growth rate The change in the total population per unit over time Population composition Migration, immigration and emigration effect • density and distribution • population composition • dynamic (overall change) in the population Sex ratios The number of organisms of each sex Population fertility The reproductive capacity of the females Age structure The number of organisms of different ages Population Males, females, young = Birth rate The number of organisms born per unit over time – Mortality rate The number of organisms dying per unit over time Figure 10.44 Features of populations. Source: Biozone Learning Media, Year 11 Biology Student Resource and Activity Manual REVIEW 31 Draw a simple annotated diagram that summarises homeostatic control of populations. 32 What does environmental resistance mean? Distinguish between density-dependent and density-independent factors. 33 What do survivorship curves show about the survival of populations? Restoring populations Although the rate of species loss worldwide is alarming, there are many examples of endangered populations of species being restored. Some of the recovery is due to natural cycles of population change but many are the result of careful management and legal protection of species. Whale species, for example, are staging extraordinary recoveries after being hunted to the verge of extinction. By 2050 it is predicted that the population of southern right whales in Australian waters may approach their original population. Populations of humpback whales are expanding in the north Atlantic, north Pacific and off Western Australia. Controlling populations When does a living thing become a pest? Sometimes populations of species increase enormously at the expense of others when environmental conditions change. The development of particular agricultural practices, the introduction of species accidentally or deliberately, and the concentration of people in urban environments have created excellent conditions for certain plants and animals to exploit. Many fungi and insects, without natural predators, have become a problem, wreaking havoc on crops of all kinds. Think of the mouse plagues common in Australian since European settlement or the invasive weed, Paterson’s curse, which lives up to its name. 331 Chemical and biological control Figure 10.45 Biological control agents: (a) cane toad (b) green tree ants. a b bioTERMS culling reducing the size of a population by killing some members of the population 332 Unit 2 The use of chemical pesticides (chemical control) is a quick and effective method of getting rid of pests, but there is a downside – the pesticides can be ecologically damaging as well as costly. Nowadays scientists favour the use of biological control agents, which exploit relationships between organisms, or an integrated approach when rapid response is needed in the early stages to manage a problem. There are four kinds of biological control agents: 1 general predators 2 specialised predators 3 parasites 4 microbial diseases. Almost all ecological disasters resulting from biological control have been due to the first method. For example, cane toads were introduced in north-eastern Queensland to control sugar pests. They are now in Kakadu, wreaking havoc on wildlife, and are expected to reach Sydney in 20 years! Introduced plant weeds, some escaped from aquaria, choke many waterways. A small South American weevil, the natural predator of the water weed Salvinia, was released in 1980 with great success in many areas. Moths and flea beetles, introduced in 1977, are reducing the impact of the alligator weed. Another beetle from South America is controlling water hyacinth. There tends to be a balanced relationship now between the control agents and the plants. Wasps are being used to control the native stem-girdler moths, which can decimate macadamia and pecan crops in Queensland. The wasps lay their eggs in the moth eggs, which are consumed by the wasp larvae. Many other wasps have been harnessed to control a range of pests, such as the Heliothis moth, which feeds on cotton plants. Recent research has shown that green ants are a cost-effective method of controlling most pests that attack mangoes. Growers are being encouraged to introduce the ferocious predators into their mango orchards! Nematodes, spread by their host, successfully combated the Sirex infestations of the South Australian softwood plantations, which had suffered a loss of nearly 5 million trees. ‘Myco-insecticides’ are showing promise as control agents. Many species of fungi in the soil attack grubs (larvae), which gnaw away at the roots of many plants, including pasture grasses, sugar cane, potatoes and other crops. Selected fungi are now being used as biological control agents of scarab beetles and other target insects. Culling The thought of culling populations of particular species that are pushing their ecosystem beyond its carrying capacity can cause strong reactions in people. Populations of some species of kangaroos have increased and expanded enormously since Europeans arrived. Land use was changed in such a way that it affected many endemic species of plants and animals. Six kangaroo species, for example, have become extinct, seven are endangered but the remaining 35 species are viable in their rangelands. The Australian red and grey kangaroos have thrived in those areas where permanent water is made available for cattle. As populations of kangaroos have increased there has been increased competition for resources. Each year over 2 million kangaroos are culled under strict regulations. The kangaroo industry is worth over $200 million each year. A small proportion of the meat is processed for human consumption, most of it for export. Chapter 10 Relationships and populations To cull or not to cull? The population of flying foxes in the Melbourne Botanical Gardens grew to such an extent that the valuable collections of plants were being destroyed. Because of stringent regulations nowadays concerning the introduction of exotic species many of the plants would not be able to be replaced. Rather than cull, attempts were made to move flying foxes to other areas, eventually with success. Will this solve the problem for the Botanic Gardens and what will happen in the flying foxes’ new home? Reintroducing populations Increased understanding has been gained through research and through indigenous knowledge of relationships between organisms. This has led to a deeper recognition of the part biodiversity plays in maintaining ecosystems. There are many examples of efforts to conserve remnant populations of native animals and vegetation and to reintroduce species into their preferred habitats. The ‘worth’ of these species lies not only in the part they play in sustaining ecosystems but also, in many cases, in sustaining agricultural systems. The only green grasses left on many farming properties throughout prolonged drought were remnant populations of native species. Some estimates say Australia has been left with a mere 2% of the grasses it once had. Farmers are recognising the benefit of planting these but seeds are scarce and collection for the establishment of seed banks is costly. Indigenous knowledge of the use of many native plants is no longer as widespread as formerly. bioTERMS remnant populations small, isolated groups of a species surviving after the rest of the population has not survived Figure 10.46 The niche of (a) river red gums and (b) a platypus. Putting it together – the ecological niche a Every ecosystem has a series of niches occupied by species characteristic of that ecosystem. The concept of niche has changed as ecologists have come to understand more about the relationships between an organism, its environment and other organisms. The first simple definition of niche described it as an organism’s role or position in its community. Since then the definition has been b broadened. It includes a species’ requirements, the sets of physical conditions and resources in its environment and its adaptations (including tolerance ranges) to meet those requirements and utilise the resources. Its interactions with other organisms of the same or different species, where, how and when it feeds, and its patterns of activity also contribute to the description of an organism’s niche. 333 bioTERMS fundamental niche the ideal niche that an organism would occupy if there were no limiting factors realised niche the actual niche an organism occupies subject to limiting factors An American zoologist, G. E. Hutchinson, distinguished between the fundamental niche and the realised niche. The fundamental niche is the ‘ideal’ niche a species would occupy if there were no competitors, predators or parasites. The realised niche is narrower. It results from an organism’s inability to exploit the resources of its habitat because of restrictions. A species may not, therefore, be distributed evenly throughout its potential geographic range. Abiotic factors suitable for the laughing kookaburra extend virtually all the way down the eastern coast of Australia and include Tasmania. But the species is not distributed evenly throughout this geographical range; successful competitors occupy the kookaburra’s niche in certain areas. As conditions and resources differ from place to place within an ecosystem, many niches are open to different species. Animal species that share the same ecosystem generally differ in their food, use of space and even timing of activities. Different forest birds feed at different heights above the ground; different animals feed at night and during the day. Species of waders, with different lengths of leg and different-shaped beaks, can exploit different parts of the mud flats and reduce competition. This kind of use of resources is called resource partitioning. Figure 10.47 Resource partitioning: feeding heights of birds in an eastern Australian eucalypt forest. spine-tailed swift Canopy striated thornbill leaden flycatcher white-throated treecreeper Secondary tree layer brown thornbill rufous fantail Understorey yellow-throated scrubwren Ground 334 Unit 2 ground thrush Chapter 10 Relationships and populations Between them, the 13 species of Darwin’s finches on the Galapagos Islands have evolved to exploit a wide range of ecological niches. The finches can be grouped into six main types, each having a beak specially adapted for dealing with a particular kind of food. Some are ground dwellers, some live in trees; some are herbivores and some insectivores. Survival of individual organisms within an ecosystem depends on a number of factors: their relationship with other members of the same species, and their interactions with other species and with other factors operating in their environment. In Chapter 11 we will see how ecosystems as a whole are maintained. REVIEW 34 a Under what circumstances does an organism become a pest? b Give advantages and disadvantages of the use of chemicals to control pests. 35 Draw up a table that summarises methods of biological control. Include examples of control relationships. 36 Use an annotated diagram to summarise ways to restore the balance of populations of species in an ecosystem. 37 a Describe the ‘ecological niche’ of an organism. A concept map may be useful. b What is the difference between a fundamental niche and a realised niche? 335 Visual summary commensalism chemical mutualism biological parasitism competitor symbiotic relationships collaborator control culling Community predator living things Carrying Capacity Biological Diversity prey Ecosystems counteradaptation Biomes biosphere ecology Naming Habitat surroundings dominant species growth rate Populations of Species quadrat random sampling techniques uniform capture – mark – recapture distribution density observation grouped transect 336 food supply density dependent predation competition disease parasites Unit 2 density independent physical factors catastrophic events Chapter 10 Relationships and populations Key terms association dominant species open ecosystems biodiversity ecology parasitism biological control ecosystem pollinators carrying capacity ectoparasite population chemical control endoparasite quadrat closed ecosystems environmental gradients random distribution clumped or group distribution environmental resistance realised niche coevolution equilibrium population remnant population coevolve facultative association resource partitioning coexistence frass sampling techniques commensalism fundamental niche seed disperser community homeostatic control seed predator competition interaction set point counteradaptations interspecific surplus killing culling intimate association survivorship density intraspecific symbiosis density-dependent mutualism system density-independent obligatory association uniform or continuous distribution Apply understandings Copy and complete the table overleaf, which summarises the main kinds of relationships and interactions between living things. You may need to refer to the information in the Bioboxes for additional information. Give at least two examples of each kind of relationship. Some of the interactions are subsets of others. Identify these. Using the information given in this chapter and an atlas, suggest the names of the ecosystems that are labelled A–D on the map in Figure 10.48. Identify rainfall and temperatures that would be experienced in each labelled area. B C A D Figure 10.48 337 Relationship or interaction Description Example Colour patterns of a non-poisonous animal similar to those of a poisonous one Coevolution Different species living together sharing the same resource Commensalism Rivalry between species for particular resources Crypsis Relationship in which an organism lives in the tissues of its host Ecto-parasitism Mullerian mimicry Mutualism Transfers pollen from anther of stamens to stigma An animal that kills for food Seed dispersal Microorganisms living in the gut of a host a Draw a graph to show what would happen to a population of rabbits if there were no predators in the area where the rabbits lived. b Introduce a predator such as a fox into your population and show on your graph what may be expected to happen to the rabbit population. Refer to Figure 10.49, the surface map (aerial view) of a sample plot in an area where eucalypts grow. a Calculate the total surface area covered by species A. What percentage cover is this? How does it compare with species C? b How would you classify the area represented by the plot? c What limitations are there in using one sample plot to represent a larger area such as a forest? d Draw a transect profile across XY. What particular information is missing that would be helpful in drawing a more accurate profile? e Describe how the information presented would have been obtained. f Suggest reasons for the difference in distribution of the different species. Elephant dung contains a great deal of fibrous matter, including seeds. Some of their plant food species have evolved to produce seeds that have a coating of rind to protect them from the elephants’ digestive juices. Unless the seeds have passed through the elephants’ digestive system first, they are unable to germinate. Analyse and explain the relationship described and use a visual medium for communicating your analysis. A brown moth, so small it is barely visible and with a preference for chardonnay, threatened to become the Victorian wine industry’s greatest scourge. The light brown apple moth can devour up to $2500 worth of grapes per hectare. 338 Unit 2 Chapter 10 Relationships and populations X N A D Scale 1 cm = 2 m B Species Height Eucalypt species A 40 m Eucalypt species B 26 m Eucalypt species C 15 m C A C Species D shrubs Figure 10.49 y Winemakers in the past resorted to chemical spraying. Now thousands of Trichogramma wasps, native to Australia, are released. The wasps lay their own eggs inside the moth eggs and, as they hatch, the wasp larvae eat the moth caterpillars. a What are the advantages and disadvantages of chemical spraying? b What kind of relationship is being made use of to control the moths? Identify the partners. c Draw a simple graph to show what happens to the population of apple moths and the population of Trichogramma wasps over time. Label carefully. d Suggest why the moths favour chardonnay grapes over others. Investigate and inquire Work in a group of about four. Each person is to choose a different category of relationship or interaction between living things (for example mutualism, parasitism) and is to find two Australian examples to investigate. Identify the components of the relationship or interaction and include which organism is harmed/benefited/not affected. Decide how your group will collate and present its findings. Investigate an Australian example of pest management incorporating biological control. Make sure you identify the reason for control, the pest to be controlled and how it is to be managed. Include a bibliography in your report. Different species of ants have an amazing range of relationships and interactions with their own kind and with other organisms. They milk caterpillars, store honeydew and nectar in specialised workers and use their own grubs to weave threads to bind their nests, to give just a few examples. Analyse some ant interactions and prepare a poster to present your findings. Use a range of references – electronic and print – and maintain a bibliography. 339 A student is to estimate the population size and density of beetles in an area shown in the grid. She collected data from 10 randomly placed quadrats each 10 cm 10 cm (Figure 10.50). Figure 10.50 9 8 7 6 5 4 3 2 1 0 0 1 2 3 4 5 6 7 8 9 Table of results Quadrat coordinate Number of beetles Quadrat coordinate Number of beetles [0.0] 10 [5.9] 14 [1.4] 0 [6.7] 3 [2.3] 45 [7.3] 6 [3.8] 32 [8.2] 12 [4.5] 48 [9.6] 16 a Calculate the population size and the population density of the beetles in the total area. b Investigate two methods that could have been used to obtain the count of beetles. Report on a web search for examples of remnant populations of vegetation. Frame at least three focus questions to help you with your investigation. Report on an investigation into examples of reintroduction into habitats of endangered or extinct populations. Frame at least three focus questions to help you with your investigation. There is increasing interest in basing an agricultural system on farming native species rather than on introduced European species. Present arguments for and against this proposal. 340 Unit 2