Download Part C: The Biosphere - Environmental Intermediate

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Part C: The Biosphere
Unit 3: Properties of Biological Communities
1. Definition of Community
The following definitions need to be known:
- Ecosystem: a group of inter-dependent organisms and the environment in which they
live (e.g. bees, flowers, butterflies and grass in a garden.)
- Habitat: The place where an organism lives (e.g. a river, rocky shore)
- Community: All the organisms living in a particular habitat (e.g. in soil (the habitat), the
community consists of earthworms, fungi, plants, etc.). A community is thus a group of
populations of different species living in the same place at the same time.
2. Interactions between Organisms
In the natural world, no organism is an autonomous entity isolated from its surroundings. It is
part of its environment, rich in living and non living elements all of which interact with each other
in some fashion. Biological interactions occur since organisms in an ecosystem interact with
each other.
Biological interactions are the relationships between two species in an ecosystem. There are
many types of relationships which are classified either on the effects or on the mechanism of the
interaction.
The interactions between two individuals will vary greatly in these aspects as well as in duration
and strength.
The interactions between two species need not be through direct contact. Due to the connected
nature of ecosystems, species may affect each other through intermediaries such as shared
resources or common enemies.
2.1 Competition
Competition is a relationship between two individuals which would like to obtain a similar limited
resource. This occurs since individuals in a species need limited environmental factor for growth
such as light, food, water, mates. Therefore, since the resource is limited, these must compete
with each other in order to obtain it for survival.
Competition is a very important factor in evolution. As a result, several species less suited to
compete for the resources may either adapt or die out. According to evolutionary theory, this
competition for resources plays a critical role in natural selection.
Two types of competition exist: Competition between members of the same species is known as
intraspecific competition. Competition is also present between individuals of different species
and is known as interspecific competition. Intraspecific competition occurs for mates, territory
and food.
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2.1.2 Three Classic Studies of Interspecific Competition
Scientists have classically studied three interactions
2.1.2.1 Paramecium: a laboratory "bottle" experiment
Paramecium is a single-celled protozoan that feeds on bacteria. It can be
raised easily in a test-tube, in a soup of bacteria, which is its food supply.
What will happen if two species are raised separately, and then together? As
the figure below shows, both species thrive when raised alone, reaching the
carrying capacity set by the volume of the test-tube and the supply of
bacteria. However, when raised together, the growth rate is slowed for both.
Eventually, one species declines and the other takes over and grows to the
carrying capacity of the test-tube. Evidently these two protozoans are
competing for limiting resources (bacteria and space), and share the same niche. Competition
has caused only one species to persist and the other to die out.
Results of Paramecium bottle experiment
2.1.2.2 Warblers in a spruce forest
Warblers are migratory, spending the winter in the tropics, and flying to
northern forests in early spring to set up territories, mate, and rear their
young.
Cape May Warbler
An important study was made of five species of warblers. These warblers are similar in body
size, beak size, and all feed on insects. A study of the feeding behavior of these five warblers
found many differences in the way they made use of the habitat. These species differed in what
part of a spruce tree they frequented, whether they captured insects whilst flying, from needles,
or under bark, and so on. The conclusion was that each species occupied a distinct niche. This
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study therefore showed that even superficially similar species really are different, upon close
inspection.
2.1.2.3 Barnacles in the inter-tidal zone: a field experimental study
Barnacles occur on near-vertical rock walls in the inter-tidal
zone, which means that they are submerged at some times
and exposed to the air at others.
A study in Scotland examined the vertical distribution of two
species: Balanus, which occurs lower in this zone, and
Chthamalus, which occurs higher. Barnacles filter-feed small
plankton from the water, so can only feed when submerged.
Observations of the two species could suggest that competition may be occurring. Through
experiments it has been shown that larvae of each species settle throughout the zone of the
other, but adults are not found throughout (figure below). It was established that Chthamalus
could thrive in the lower zone if protected from Balanus. Otherwise, Balanus, which is a larger
and more robust species, crushed neighboring Chthamalus as it grew in size. Chthamalus was
excluded from the lower inter-tidal. At the upper limit, however, both species were limited by
their ability to withstand desiccation. Chthamalus, the inferior competitor, was better adapted to
desiccation and so occupied the higher portion of the inter-tidal zone.
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2.1.2 The Result of Competition
So what happens when two species compete with each other? Can two species ever coexist?
Does the same species always win out? These results have been answered through many
"bottle" studies with flour beetles, Drosophila, yeast, various plants.
Interspecific competition among ecologically similar species is the major factor that determines
the structure of animal and plant communities. In general, when two species competing for a
resource occur together and compete, these either coexist or else are subject to competitive
exclusion. The main question is however, can competing species coexist or not, and what are
the major factors that affect coexistence.
In general, it has been shown that one species wins, and one species is eliminated. This is the
competitive exclusion principle which stats that:
No two species of similar requirements can long occupy the same niche (coexist).
When two species competing for the same resource occur, these are normally subject to
competitive exclusion. This is since one of the two competitors will always have an ever so slight
advantage over the other. Therefore, the other species that leads to extinction of the second
competitor in the long run or to an evolution of the inferior competitor towards a different
ecological niche. As a consequence, competing related species often evolve distinguishing
characteristics in areas where they both coexist, as in the case of the warblers.
Competition is a powerful force of natural selection. There are many examples of closely related
species that differ "just enough" to reduce niche overlap and permit coexistence.
Competition likely affects species diversity. In the short run, competition should cause a
reduction in the number of species living within an area, preventing very similar species from cooccurring. In the long run, however, competition is likely to increase species diversity, by acting
as a force for specialization and divergence.
Does the same species always win? In the case of paramecium, the same species always
won. Studies with flour beetles, in contrast, found that changes in temperature and moisture
could shift the outcome in favor of one or the other species.
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2.2 Predation
Predation is defined as the removal of organism from a population by another organism. The
animal that hunts and kills other organisms is called the predator, while the prganism which is
killed is called the prey. This is a +/- relationship, in which case the predator benefits (food) and
the prey is negatively affected (it is killed)
Predators are either carnivores or omnivores. Carnivores consume meat, whilst omnivores, such
as humans, eat both meat and plants. Herbivores can also be considered as predators since
they eat plants. However, plants are eaten only in part and are left alive.
Different hierarchies of predators exist for example though small birds prey on insects, they may
in turn be prey for snakes, which may in turn be prey for hawks. A predator at the top of its food
chain (that is, one that is preyed upon by no organism) is called an apex/ top predator;
examples include the Great White Shark, tiger and crocodile and even omnivorous Humans.
Many predators are specialist, that is they specialize in hunting only one species of prey. Others
are more opportunistic and will kill and eat almost anything.
The difference between a predator and a parasite is that for a predator kills the prey, whilst the
parasite feeds on the prey whilst it is alive.
Examples: Several examples are given below
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2.3 Parasitism
Parasitism occurs when an organism (parasite) feeds on another (host) without killing it, yet
causing some harm. Numerous examples of parasitic organisms occur in all kingdoms (Bacteria,
fungi, protozoans, plants and animals) which parasitise a wide range of hosts.
In contrast, a parasitoid is an organism that spends a significant portion of its life history
attached to or within a single host organism which it ultimately kills (and often consumes) in the
process. Thus they are similar to typical parasites except in the fate of the host. Examples are
numerous species of paraitoid wasps.
Various types of parasites exist. In general we distinguish two types:
- Parasites that live inside the body of the host are called endoparasites (e.g.,
-
hookworms that live in the host gut); and
Parasites that live on the outside are called ectoparasites (e.g., some mites).
An epiparasite is a parasite that feeds on another parasite.
Many endoparasites acquire hosts by passive mechanisms, such as the nematode Ascaris
lumbricoides, an endoparasite of the human intestine. A. lumbricoides produces large numbers
of eggs which are passed from the host's digestive tract into the external environment, relying on
other humans to inadvertently ingest them in places without good sanitation. Ectoparasites, on
the other hand, often have elaborate mechanisms and strategies for finding hosts. Some aquatic
leeches, for example, locate hosts by sensing movement and then confirm their identity through
skin temperature and chemical cues before attaching theselves.
Parasites can also be one of the following:
- Obligate parasite - the parasite cannot complete its life cycle without a host;
- Facultative parasite - the parasite can complete its life cycle independent of a host.
- Stem parasite - the parasite attaches to the host stem.
- Root parasite - the parasite attaches to the host root.
- Holoparasite - plants that are completely parasitic on other plants and have virtually no
-
chlorophyll.
Hemiparasite - plant that is at least partly photosynthetic but also usually parasitic.
Hemiparasites may just obtain water and mineral nutrients from the host plant. Many
obtain at least part of their organic nutrients from the host as well.
Below are examples of parasitic organisms:
2.3.1. Parasitic plants
- Orobanche (Broomrape): The broomrape is a holoparasitic root parasite attacking a variety of
plants, including broadbeans;
- Cuscuta (Dodder): a emiparasitic stem parasite attacking a large variety of plants including
thyme.
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2.3.2 Intestinal parasites:
The are parasites that populate the gastro-intestinal tract (These are not endoparasites!). In
humans, they are often spread by poor hygiene related to feces, contact with animals, or poorly
cooked food containing parasites.
-Taenia (Tape worm)
Taenia is a genus of tapeworm that includes some important
parasites of livestock and humans. All members of this genus
have an "armed" scolex, meaning that there are hooks on the
"head" region, used to grab onto the walls of the intestine.
The lifecycle of of the Tapeworm is given below.
Head (scolex) of Taenia
2.3.3 Endoparasites
Various examples are present, most of which give rise to serious diseases. A few examples are:
- Giardia lamblia (the most common intestinal protozoan in the United States): causes
Giardiases;
- Trypanosoma: protist causing sleeping sickness;
- Leishmania: protist causing Leishmaniasis;
- Plasmodium: protist causing malaria. This has two hosts that is mosquitos and humans.
Therefore it has a very complex life cycle which is described in the diagram below:
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Liver flukes: very complex lifecycle
involving two hosts. Two examples are
given below:
The Chinese liver fluke
The Sheep Liver FLuke
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2.3.4. Ectoparasites
Examples include the following
- Acarina (Ticks, some mites);
- Tsetse Flies; and
- some leeches.
2.4 Symbiosis
Symbiosis means living together. It therefore describes obligatory relationships which may be
beneficial or harmful top one or both organisms. Symbiosis can be divided into commensalism,
amensalism and mutualism.
2.4.1 Commensalism
A symbiotic interaction between two living organisms, where one organism benefits and the
other is neither harmed nor helped. The term commensalism derives from the Latin com mensa,
meaning sharing a table. Examples are:
- Phoresy: Using a second organism for transportation. Examples are the remora on a shark, or
mites on dung bugs. Both temporary and permanent phoresy exist; and
- Inquilinism: Using a second organism for housing. Examples are epiphytic plants (such as
many orchids) which grow on trees, or birds that live in holes in trees.
2.4.2 Amensalism
A symbiotic interaction between two species in which one impedes or restricts the success of
the other without being affected positively or negatively, by the presence of the other. Usually
this occurs when one organism produces a chemical compound that is detrimental to another
organism. For example, the bread mold Penicillium secretes penicillin, which is a chemical that
kills bacteria. A second example is the black walnut tree and eucalyptus. Their roots secrete a
chemical that harms or kills some species of neighboring plants.
2.4.3 Mutualism
An interaction between two or more species where both species derive benefit. Mutualisms can
be lifelong interactions involving close physical contact They can also be briefer such as those
between flowering plants and pollinators.
Several examples are given below:
2.4.3.1 Lichens
Lichens are in fact a symbiotic lifeform between a fungus and a alga. This symbiosis is so
intense that seemingly a new lifeform generates: the lichen. This has a very specific structure
shown in the diagram below. In brief a network (mycelium) of fungal hyphae host blue green
algae within the network.
The advantage is such that these symbionts are capable of living in conditions much harsher
than the components making up the lichen would. In fact, they tolerate numerous dry habitats
and may colonize bare stone. Lichens often are the first to settle in places lacking soil,
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constituting the sole vegetation in some extreme environments such as those found at high
mountain elevations and at high latitudes. Some survive in the tough conditions of deserts, and
others on frozen soil of the arctic regions.
In the relationship, the fungus gives the algae protection whilst the algae gives nutrients to the
fungus, which nutrients it harvests from photosynthesis.
2.4.3.2 Nitrogen Fixation
Legumes such as clover, beans, alfalfa and peanuts, contain symbiotic bacteria called rhizobia
within nodules in their root systems. These produce nitrogen compounds from atmospheric
nitrogen and help the plant to grow and compete with other plants.
2.4.3.3 Mycorrhiza
A mycorrhiza (typically seen in the plural forms mycorrhizae or
mycorrhizas, Greek for fungus roots) is the result of a symbiotic
association between a fungus and a plant. This symbiosis takes
place at the root level, where individual hyphae extending from the
mycelium of a fungus colonize the roots of a host plant.
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2.4.3.4 Ant-aphid mutualism
The aphids are protected against predators by the ants who cultivate the aphids for their
secretions of honeydew, a food source.
2.4.3.5 Evolutionary Importance of Mutualism
Mutualism is important in evolution from a co-adaptation and a co-evolution point of view.
Coevolution is a change in the genetic composition of one species (or group) in response to a
genetic change in another, whilst co-adaptation is a joint adaptation to an environmental
stimulus.
Mutualists have co-evolved together and have a survival advantage over organisms occupying
the same habitat, or else the individual organism in the association. Therefore, they are selected
for, This is due to co-adaptation to harsh environments, allowing for a better survival.
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2.5 Summary of Interactions
The following is a summary of the interactions described and more:
Effect on X
Effect on Y
Type of interaction
0
0
Neutralism
-
0
Amensalism
+
0
Commensalism
-
-
Synnecrosis
+
+
Mutualism
+
-
Predation or Parasitism
Some types of relationships listed by the effect they have on
each partner. '0' is no effect, '-' is detrimental, and '+' is
beneficial.
 Neutralism is a lack of interaction. Since all species sharing an environment interact in some
way, a complete lack of interaction is rarely seen in nature. However, the term can also signify a
relationship in which each species derives neither benefit nor detriment to any measurable
degree; and
 Synnecrosis is detrimental to both species. It is a rare and necessarily short-lived condition as
evolution selects against it.
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3. The Ecological Niche
3.1 Definition and explanation
A niche is a term describing the relational position of a species or population in an ecosystem. It
is defined as the role amd relationships of an organism within its community.
The niche for each species describes how a population responds to the abundance of its
resources and enemies (e. g., by growing when resources are abundant, and predators,
parasites and pathogens are scarce) and how it affects those same factors (e. g., by reducing
the abundance of resources through consumption and contributing to the population growth of
enemies by falling prey to them). The abiotic or physical environment is also part of the niche
because it influences how populations affect, and are affected by, resources and enemies.
The description of a niche also includes descriptions of the organism's life history, habitat, and
place in the food chain.
A niche can be depicted as a graph of productivity vs environmental characteristics as given
below. This can be in one or two dimensions. On the, a niche is often shown as a hypervolume
graph, that is a graph with several facets, as given below:
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A generalist species would have a wide niche breadth. However a specialist species would
have a niche which is less wide. A specialist species may have resulted from restriction of the
niche of a generalist species.
3.2 Fundamental and realized niche
The full range of environmental conditions (biological and physical) under which an organism
can exist describes its fundamental niche. However, due to pressure from, and interactions
with, other organisms (e.g. superior competitors) species are usually forced to occupy a niche
that is smaller narrower than the fundamental niche. Organisms will occupy the part of the niche
to which they are mostly highly adapted. This is termed the realized niche.
Different species can hold similar niches in different locations and the same species may occupy
different niches in different locations. The Australian grasslands species, though different from
those of the Great Plains grasslands, occupy the same niche. Once a niche is left vacant, other
organisms can fill into that position. For example, the niche that was left vacant by the extinction
of the tarpan has been filled by other animals (in particular a small horse breed, the konik).
When plants and animals are introduced into a new environment, they can occupy the new
niches or niches of native organisms, outcompete the indigenous species, and become a
serious pest.
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3.3 Competition and niche breadth
According to the competitive exclusion principle, no two species
can occupy the same niche in the same environment for a long
time. If they do occupy the same niche, or else the hypervolume
overlaps, intraspecific competition for a resource occurs. The
result would be competitive exclusion, that is one species is
selected against and dies, as occurred in the example of
Paramecium. Alternatively, as seen further above in the example of the warblers, if two or more
species have an identical niche, these must change in order to coexist. Therefore, an
evolutionary change in the niche occurs, as depicted in the diagram below
Interspecific competition thus narrows the niche when coexistence occurs. Alternatively, if
competitive exclusion occurs and one species dies out, the species that is selected for can also
occupy the niche of the species that died out.
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4. Ecological Succession
4.1 Basic Concepts
Within any community some species may become less abundant over some time interval, or
they may even vanish from the ecosystem altogether. Similarly, over some time interval, other
species within the community may become more abundant, or new species may even invade
into the community from adjacent ecosystems. This observed change over time in what is living
in a particular ecosystem is ecological succession.
This is the predictable and orderly changes in the composition (species occurring) or physical
structure of a community over time. This occurs in all ecosystems and may be initiated either by
formation of new, unoccupied habitat (e.g., a new island through lava flow or a severe landslide)
or by some form of disturbance (e.g. fire, severe windthrow, logging) of an existing community.
Succession occurs from an initial pioneer community (dominated by pioneer species such as
lichens and mosses), through a series of different communities (called seres) until a climax
community is formed where composition changes no longer occur with time. The climax
community is the highest community to form for the particular place for example a woodland
community. It is often seen as the end of succession since it is the most advanced community
which a set of conditions can hold. The concept of a climax community assumes that the plants
colonizing and establishing themselves in a given region can achieve stable equilibrium.
4.2 Forms of succession
Two types of succession occur:
- Primary succession: This succession occurs naturally from a new habitat for example
-
bare rock of a new island, from sand on a beach and from water.
Secondary succession: Occurs after a particular habitat has been disturbed. For
example, grassland created by logging, abandoned fields and other habitats created by
man. Secondary succession is usually much quicker than primary succession since there
is already an existing seed bank of suitable plants in the soil and the soil is rather fertile
and deep.
4.3 Secondary succession
Secondary succession occurs when for example an abandoned field develops into an Oak
Forest. This occurs as given below:
The first stage of succession was characterized by the pioneering colonization of annual plant
species on bare ground and nutrient poor soils of the abandoned field. These annual species
had short lifespans (one growing season), rapid maturity, and produce numerous small easily
dispersed seeds. The annuals were then quickly replaced in dominance in the next year by
biennial plants and grasses. After about 3 to 4 years, the biennial and grass species gave way
to perennial herbs and shrubs. These plants live for many years and have the ability to
reproduce several times over their lifespans.
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After about 5 to 15 years, the sites were then colonized by a number of different softwood tree
species including loblolly pines (Pinus sp.). As the softwoods increased in their numbers and
grew in height, they began forming a forest canopy. This canopy reduces the amount of light
reaching the forest floor. The resulting shaded under storey conditions caused the exclusion of
many light loving perennial herb and shrub species. Low light conditions also inhibited the
germination of pine seedlings. Perennial herb and shrub species that were adapted to low light
conditions now began to take over the ground cover. The canopy also changed the microclimate
of habitat near ground level. It was now more humid, has moderated temperatures, and less
wind. These conditions, plus the development of a soil litter layer, allowed for the germination of
hardwood species, like oak (Quercus spp.) and various species of hickory (Carya spp). The
seedlings of these tree species also tolerate low light levels.
By about 50 to 75 years after the initial colonization of the pioneer species, the hardwoods
started to replace the softwood species in the developing forest. At this stage in the succession,
the pines had maximum heights of about 25 meters, while the oaks and hickories were on
average about 10 meters tall. Because of their shorter lifespans (50 years), many of the
softwood species were beginning to die out and the gap that was created was then filled by a
subdominant hardwood tree. Hardwood species, like oak and hickory, can live for more than 100
years. Sites more than 100 years old were found to be dominated by mature oak forests.
4.4 Mechanism of Primary Ecological succession
Two examples are given below:
- Succession from an aquatic to a terrestrial habitat: This is a hydrosere that is a succession
which starts in water. A wetland, which is a transitional area between open freshwater and dry
land, provides a good example of this and is an excellent place to see several stages of a
hydrosere at the same time.
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In time, an area of open freshwater such as a lake, will naturally dry out, ultimately becoming
woodland. During this process, a range of different habitats such as swamp and marsh will
succeed each other. This succession from open water to climax woodland is likely to take at
least two hundred years (probably much longer). Some intermediate stages occur.
1st sere: Open water: Deep freshwater will not support rooted, submerged plants because there
is not enough light for photosynthesis in the depths. There will be micro-organisms and plankton
floating in the water.
2nd sere: Sumerged plants: Over time, sediments will be transported into the lake (or pond) by
streams or rainwater draining into it from the land. Large amounts of sediment can be deposited
in this way.
The water depth will gradually decrease, allowing rooted, submerged plants, such as starwort
and pondweed to grow. Waterlilies, which are rooted, but with floating leaves may also become
established.
3rd sere: Marshland: By this stage, the water may be too shallow to support fully submerged
plants. Instead, emergent plants,such as Yellow Iris, reed and reedmace, grow partly in and
partly out of the water. The form a type of community called the marshland.
4th sere: Willow and Alder forest: The soil is still wet, but no longer completely waterlogged.
Thereforem willows start growing and dominatee the ground. Willows will support a diverse
range of invertebrates (more than 450 different species). This means that there is plenty of food
for insectivorous birds.
Climax Oak Forest: The soil is deep and tree species present include Oak, Ash and Beech.
They are slow-growing, but because they are also tall and long-lived, in time, they will come to
dominate an area.
Succession from bare rock to a hardwood forest:
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Rocks are colonized by lichens and mosses at first. These form the pioneer community which is
the first sere. During this phase soil is deep and lacks nutrients. However, lichens create soil by
dissolving rocks and mosses trap sediments, thus initiating soil formation. Eventually when the
soil gets deep enough, the pioneer community is replaced by the second sere that of the
grassland. This is made up of herbaceous species such as grasses and legumes which are
ruderal (r) species. Ruderals grow rapidly and a high turnover occurs. The soil is still shallow
and lacks nutrients. However, legumes have a symbiotic relationship with bacteria that perform
nitrogen fixation. Therefore, soil nitrogen is increased. In addition, when these plants die they
form part of soil humus. Therefore soil richness and depth increases.
The third sere eventually replaces the grassland. This would be made up of small bushes which
have deeper roots and contribute more to soil fertility and depth. The fourth and fifth sere are
eventually formed both of which are forests. However, the fifth is made up of larger trees. This is
the climax community which is made up of equilibrium (K) species. The climax community is
stable and will remain in place as long as the present conditions are present.
4.5 Causes of Succession
Succesion can be of two types. Autogenic succession occurs through a natural process of
change from one community to another until the climax for a particular habitat and conditions is
reached. The stimulus for change is an internal one. For example gradual soil improvement
could allow a new species to develop.
Conversely, an autogenic succession occurs due to an external change affecting the
community. For example a sudden flood of an area could bring about a change in species, and
from one succesional series, the community jumps onto another.
The ultimater aim would be to reach the climax community permitted under such conditions.
4.6 Disturbance of succession
Succession is a relatively delicate procedure which needs stable habitat conditions in order to
reach a climax. Disturbance, which is defined as a temporary change in environmental
conditions is known to affect succession. These may be natural or anthropogenic for example
fire, grazing, flood, landslide, drought
If conditions change temporarily due to disturbance a pronounced change in ecosystem
structure that lasts longer than the change in the environment will occur. Ecosystem changes
include a reversion of succession to a previous sere due to altered populations or behaviour of
difference species as they respond to the stressful conditions imposed by the disturbance.
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4.7 Succession in the Maltese Islands
Succession in the Maltese Islands is quite interesting, and is typical of a Mediterranean habitat.
In the Maltese Islands the following succession occurred and is still occurring:
Bare rock
Steppe
Karst landscape
Garrigue
Maquis
Woodland
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