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V. Food Chains and Food Webs A. food chain: series of organisms that transfer food between the trophic levels of an ecosystem 1. all food chains begin with producers 2. no ecosystem is simple enough to show with just one food chain B. food web: network of food chains representing the feeding relationships among the organisms of an ecosystem 1. includes all the food chains of an ecosystem 2. changes in one population of a food web send ripples through the food web so that the entire web can be affected VI. Diversity and Stability A. The number of links in a food web varies for each ecosystem. 1. older, more mature ecosystems often have more species than young ecosystems (and therefore, more links) B. Some ecologists think that webs with more links are more stable because they can resist disturbance better. VII. Biological Magnification A. definition: increasing concentration of a pollutant in an organism at higher trophic levels in a food web *** The effects of pollution caused by humans can be magnified in a food web. (ex. DDT and eagles, p. 120) VIII. Energy in an Ecosystem A. Energy and Food 1. biomass: total amount of organic matter present in a trophic level a. the biomass in each trophic level is the amount of energy (in the form of food) available to the next trophic level 2. Most of the energy that enters through organisms in a trophic level DOES NOT become biomass a. Energy is lost through activities i. Mammals keeping a high body temp. b. Some of the biomass is lost in organic matter that is difficult to digest. i. Bones, hooves, shells, fibers 3. When all of the energy losses are added together, only about 10% of energy entering one trophic level forms biomass in the next trophic level called “the 10% law” a. The 10% law is the main reason why food chains have 5 links or less. i. Owls are not preyed upon by a high level of carnivores. There is not enough biomass or owls to support another level of consumers. B. Ecological Pyramids 1. ecological pyramid: diagram that shows the relative amounts of energy in different trophic levels in an ecosystem a. the pyramid is divided into sections, each representing one trophic level b. can show energy, biomass, or number of organisms in a food web i. figure 9, page 123 ii. generally follows the 10% law C. Matter moves between trophic levels 1. carbon, hydrogen, oxygen, and nitrogen are the most common elements in food 2. with any of these elements missing, food cannot be produced 3. for this reason, the growth of producers in most ecosystems is limited by the lack of one or more of these elements, not by the amount of energy from the sun 4. in the next section, we’ll examine how these elements are cycled through the environment and reused Ecological Succession Change is always occurring in ecosystems. Organisms affect their environments and so do other forces – fire, flood, and other natural events. As the environment changes, the organisms living in that environment change as well. Many times, different communities follow one another in a definite pattern, and this is called ecological succession. I. Primary Succession A. The sequence of communities forming in an originally lifeless habitat is called primary succession. 1. occurs in new habitats without life such as cooled lava fields, sand dunes, exposed rock from a retreating glacier B. The first step in primary succession is the formation of soil from exposed rocks. 1. Rocks are first colonized by organisms called lichens. 2. lichen: a fungus and an alga living in a symbiotic relationship a. Lichens secrete acids that break down the rock and form organic material by photosynthesis. b. The actions of lichens, combined with weathering, form soil. It may take several hundred to several thousand years to form soil. 3. Scientists call the lichen community a pioneer community because it is the first community to colonize a new habitat. (Plants that first colonize disrupted areas can also be considered pioneer species.) C. Once soil has formed, grasses and other small plants such as mosses begin to grow. 1. Seeds come from wind or animals. 2. Plants eventually block sunlight to the lichens and the lichens die. D. The grass and moss community makes the soil deeper and more fertile; taller weeds and shrubs with deeper roots begin to grow. 1. As the shrub community grows, it further deepens the soil and makes it richer in nutrients. E. Once the soil is deep enough, pines and other shallow rooted trees move in. 1. As the trees block the sunlight, the shrubs are replaced by the trees and the plants of the forest floor. F. Pine forest is replaced by hardwood forest with trees such as oak, hickory, birch, white spruce. Hardwood forest is often the last step in primary succession. 1. If the ecosystem is left undisturbed, no more drastic changes in the habitat will occur. 2. climax community: a community that does not undergo further succession II. Secondary Succession A. secondary succession: occurs where a community has been cleared by a disturbance that does not destroy the soil, such as fires, storms, human activity 1. Secondary succession resembles the later stages of primary succession. a. Fast growing grasses, mosses b. Shrubs and tall weeds c. Pines d. Slow growing hardwood trees 2. Succession is complete with a climax community. B. Old Field Succession – occurs when farmland is abandoned. 1. Once a field is no longer cultivated, grasses and weeds start to take over the land. Taller plants follow, (killing the grasses and weeds with shade and a lack of water), and then trees move in, continuing the pattern (pines to slower growing trees). 2. Old field succession happens much faster than other types of succession; after about 100 years the land can be returned to the climax community that existed before the land was cleared for farming. C. Some habitats never develop climax communities. 1. Some frequently disturbed communities simply do not last long enough – it takes several hundred years. 2. Common example – grasslands that have frequent fires. Ironically, disruption is the key to keeping these communities balanced and stable. III. Aquatic Succession A. Aquatic succession is very similar to other forms of succession. 1. Lakes often begin like barren rock; water is low in nutrients and supports only a few organisms. 2. More and more water plants colonize the shore and the surface of the lake. 3. Sediments and organic matter begin to collect and with additional plants, a marsh is formed. 4. Land plants colonize the marsh and the lake becomes a meadow (which may later undergo land succession and become a forest). IV. Island Succession A. Occurs much the same way as succession on land. 1. Volcanic eruptions often create new islands and they are colonized very quickly. 2. Any organism on an island must have ancestors that were carried there by wind, water and other organisms. 3. Islands often have large bird populations because birds can reach islands much more easily than land animals. B. Unfulfilled Roles 1. Because many organisms from the mainland have no way of getting to an island, many roles are unfilled in island ecosystems. 2. The “lucky” organism that finds a mate on the island can produce offspring that (over many generations) can evolve to fill several different roles because there is no competition. 3. The result: it’s possible to get many different species from only a few ancestors. Interactions in the Ecosystem: Habitats and Niches I. Niches A. niche – the role of an organism in the ecosystem 1. includes both biotic and abiotic factors a. biotic: food sources, predators b. abiotic: temperature, sunlight, water 2. In addition to an organism’s habitat, a niche includes what the organism does with its habitat. B. All the members of a species are adapted to the same niche. 1. A species’ niche is unique to that species. a. Two species can occupy niches that are very similar, but no 2 species can occupy the exact same niche. b. If two species try to share the same niche, they will compete for resources. i. Whichever species is best adapted and gets the resources will force the second species to move to another area or die out (in that area). c. Competitive exclusion: the extinction of a population due to direct competition with another species for a resource 2. Sometimes one species’ activity helps to define the niche of another species. a. Example: Two species of barnacles living on the coast of Scotland have very similar niches. Both species live on rocks in the intertidal zone of the ocean shore – Chthamalus stellatus (species A) and Balanus balanoides (species B). Species A occurs on higher rocks that are usually exposed to air and Species B lives on lower rocks usually covered by water except during low tide when they are exposed. Species B survives in lower zones because it is vulnerable to drying out while Species A is more resistant to drying out so it can survive on the higher rocks. Scientist J.H. Connell performed this experiment: He removed all of Species B from a small area of shore. He found that Species A began to grow on the lower rocks. The hypothesis – Species A could live on all parts of the rocky shore, but Species B drove out Species A in the places where Species B could survive. The result: Species A was limited to the higher rocks by Species B, even though it could live on all of the rocks if Species B were absent. 3. fundamental niche: the theoretical niche of a species; where/conditions the organism could live 4. realized niche: the niche that the organism actually uses 5. Note – the fundamental niche is always equal to or larger than the realized niche. II. Niche Diversity A. Niche diversity is often determined by the abiotic factors in the habitat. 1. example – Marsh: lots of organisms, but overall diversity of low because the physical conditions of the marsh are generally the same from place to place. In a desert, there are fewer organisms but the diversity is large due to vast differences between moisture and temperature. 2. One biotic factor that has a huge influence on niche diversity is predation. B. Predator: an organism that actively hunts or feeds on other organisms 1. Predators increase niche diversity in an ecosystem. 2. By decreasing the population of the prey species, predators make resources available for other species. The actions of a predator can create other niches. 3. keystone predator: a predator that causes a large increase in the diversity of its habitat a. example: sea stars in tidepools – Robert Paine, Washington state b. When sea stars were removed from tidepools, the number of mussels increased dramatically. The mussels outcompeted many of the other species in the tidepools for resources. The number of species in the tidepools dropped from 15 with the sea stars present to 8 without the sea stars. By keeping the population of mussels in check, the sea stars enabled other species to survive. III. Evolving to the Niche A. Populations evolve by adapting to niches in the environment. 1. In the same idea, populations can also evolve to avoid competition with another species. 2. example: warblers (type of bird) a. five different species, all feed on insects in the branches of spruce trees b. Although it seems that they compete with each other for food, each species looks for food in a slightly different part of the tree – outer edges of high branches, inside high branches close to the trunk, middle of the tree close to the trunk, etc. c. The niches overlap a little but are sufficiently different to allow the five species to coexist. Each niche is very narrow. B. Specialized vs. Generalized Species 1. specialized species – organism with a very small or narrow niche a. examples – giant panda (one main food source – bamboo), warblers b. Specialized species are very vulnerable to extinction because they can’t respond well to changes in the environment. If something happens to the bamboo, the giant panda will not survive. 2. generalized species – organism with a very wide niche a. examples – mice, roaches b. Generalized species often have several alternative food sources. Sometimes generalized species are described as being opportunistic. c. These species are able to adapt to environmental changes with relative ease. C. Convergent Evolution 1. convergent evolution: the development of similar adaptations in two species with similar niches 2. Similar ecosystems would have similar niches, which in turn place similar demands on the organisms fulfilling those niches. 3. Organisms evolve to meet the demands that the environment (niche) places on them, and organisms that evolve to occupy similar niches may also be alike. 4. Example: The wings of birds and bats are similar in general structure. Yet these are two different animals (bird vs. mammal). Birds and bats evolved flight independently, but the demands of flying are very similar for both organisms (weight, surface area, compactness). Similar demands similar solutions resulting in wings that are very much alike. 5. Example: Ichthyosaur and the dolphin – The dolphin is a mammal and the ichthyosaur is an extinct reptile. Both are adapted to the demands of living in the water – streamlined body, fin shape and placement, eye placement, narrow snout, musculature, etc. D. Coevolution 1. coevolution: process of two species evolving in response to long-term interactions with each other 2. Coevolution often occurs between predator/prey species and species that cooperate with one another. 3. Predator/Prey example: Plants and caterpillars a. Many plants have evolved poisonous chemicals to prevent insects from eating them and some caterpillars have evolved the ability to resist these poisons. Some caterpillars go so far as to feed only on poisonous plants. b. Co-evolved: one changed in response to the other 4. Cooperation example: Acacia tree and stinging ants in Central and South America a. Acacia trees have large hollow thorns that provide a nesting site and protect the ant colony. The tree also provides the ant with food; the ants are totally dependent on the tree, able to nest nowhere else and having no other source of food. b. The ants attack any animal landing on the tree, killing it or driving it away. The ants clear nearby vegetation, eliminating the tree’s competition for sunlight and water. c. Experiments have shown that without the ants, the tree cannot grow properly or successfully. The ants and the tree have coevolved to the point of complete codependence, neither one being able to survive without the other. IV. Relationships in the Ecosystem A. Predator and Prey 1. Sizes of predator and prey populations are closely linked. a. The larger the prey population, the more predators that it can support. b. The smaller the prey population, the fewer predators that it can support. 2. Example: snowshoe hare and the lynx – chart of populations, p. 207 a. Always greater number of prey (hares) than predators (lynx) – recall the 10% law. b. There is a 1-2 year delay in response of the predator population changing after the peaks and dips of the prey population. 3. Population Cycles a. Large herbivores (hare, muskrat) have peaks in populations approximately every 10 years; often their predators have matching cycles. b. Small herbivore (mice) have 4 year cycles and the predators of these animals also have 4 year cycles. B. Symbiosis – general description of a relationship in which 2 species live in close association 1. commensalisms: one species benefits, other species is unaffected (neither helped nor harmed) a. example: barnacles on whales b. the flow of the water over the barnacles as the whale moves benefits the barnacle (filter feeders) and the whale is unaffected 2. mutualism: both species benefit a. example: acacia tree and stinging ants b. example: flowers and pollinating insects 3. parasitism: one species benefits, other is harmed a. parasitism – relationship in which one organism feeds on the tissues or body fluids of another b. host – organism on which the parasite feeds c. Parasites are harmful to the host and may be fatal, but most parasites do not kill their hosts outright. Why??? d. Examples – fleas, ticks, variety of worms e. A true parasite is adapted to living on or in the body of its host. i. Depends on the host for many functions which the parasite cannot perform on its own. ii. Tapeworm: no sensory organs, no means of locomotion iii. It lives in the intestines of animals (doesn’t really need to move). Tapeworms spread from one animal to another through contact with feces that contain segments of the tapeworm that have broken off inside the infected animal. V. Populations A. Population Growth 1. All populations have the ability to grow very quickly in the perfect environment. a. Exponential growth: population growth in which the rate of growth in each generation is a multiple of the previous generation b. In reality, conditions are never perfect – the resources are always limited. c. A population CAN grow exponentially, but not for long. 2. Carrying capacity – the number of individuals that can be supported by an ecosystem a. Populations generally start out small and then increase rapidly (a short period of exponential growth). b. The growth rate slows as carrying capacity is approached. c. Growth stops at carrying capacity. d. The growth curve of a population where the birth and death rates have become equal is called an “S” shaped curve. B. Limiting Factors 1. limiting factor – a force that slows the growth of a population 2. There are 2 types of limiting factors: densitydependent and density-independent. 3. density-dependent limiting factors: dependent on population size a. As a population grows, these factors act more strongly to limit growth. b. Examples – lack of food, predation, disease, parasites (thrive in crowded populations because new hosts are easily found) 4. density-independent limiting factors: affect the same percentage of a population regardless of its size a. examples: natural disasters such as hurricanes, tornadoes, floods, fires; climate; human disturbance 5. Populations with density-dependent limiting factors show an S-shaped growth curve. Populations with density-independent limiting factors show a curve called a boom and bust curve (sharp exponential increases followed by sharp collapse). a. These populations with boom and bust curves are usually adapted to take advantage of regular occurring densityindependent factors, like seasonal warm temperatures. b. Common for insect populations