Download LECTURE 17 COMMUNITY STRUCTURE

BIO 300
ECOLOGY
LECTURE 17
COMMUNITY STRUCTURE
I. WHAT ARE COMMUNITIES?
A. Some definitions
1. Species- A group of populations that have the potential to interbreed in nature.
2a. Populations- a group of organisms of the same species that occupy a particular space at the
same time.
2b. Deme- a small local population.
3. Community-a naturally occurring assemblage of species populations which occur together in
space and time.
a. A simple perception of communities - an oak community or pond community - spatially
defined.
b. But by definition, communities are more complex. Consider the complexity of interactions
among populations.
4. A community has collective properties, such as species diversity, community biomass and
productivity.
5. Community is more than just the sum of its parts. Emergent properties appear when the
community is the focus.
A more ecological definition of a community is -an association of interacting populations,
usually defined by the nature of their interaction or the place in which they live.
B. Niche Differentiation- Relationship of species with environment.
1. We can distinguish 3 aspects of the relationship of a species to environment.
a. The area of a species is its geographic range. Its distribution in space.
b. The habitat of the species refers to the kind of environment the species occurs in.
c. The species position in a community in relation to the other species is its niche.
d. Niche is a term for the way a species population is specialized within a community.
2. Ways in which a niche can be differentiated.
a. Differential resource utilization- (resource partitioning).
b. Spatial and temporal separation of resources.
c. Niche differentiation based on conditions. Two species may utilize precisely the same
resources, but if their ability to do so is influenced by environmental conditions.
II.
Equilibrium versus nonequilibrium views of community structure.
(Closed and open communities)
1. Equilibrium view - Over a period of time, the number and type of species comprising a
community stabilize.
2. Nonequilibrium view – Change is the norm.
A. F.E. Clement's view of communities (The Organismic or Holistic View)
1. The community is like an organism.
a. There are functional relationships among the associations of species in a community.
2. Clement perceived communities as discrete units with sharp boundaries.
a. A lodgepole pine forest of the southeastern United States, which occurs in a drier area, is
distinctly different than a fir forest which grows in cooler, moister habitats.
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b. Differences between southwest and northeast-facing slopes.
3. Concept of community as an organismic or holistic unit is based on the belief that species
belonging to a community are closely associated with each other, and the ecological limits of
the species are coincident with the distribution of the community.
a. E.g. a cattail-water lily community would be restricted to bodies of water.
b. These species assemblage are closely linked species locked into the association by
mandatory biotic interactions that cause the community to function as a unit.
4. Closed communities are natural ecological units, with distinct boundaries. The edges of such
communities, called ecotones, are regions of rapid replacement of species along the gradient,
that is, from one habitat type to another.
a. Allows for accidentals and for species that have ranges that are broad enough to overlap.
5. Implicit in this viewpoint is that these associations among the species in the community have
reached equilibrium.
B. The Continuum View
1. H.A. Gleason believed that the community was merely a fortuitous association of organisms
whose adaptations enabled them to live together under the particular limitations of the
environment. A plant association, he said, was 'not an organism, scarcely even a vegetational
unit, but merely a coincidence.'
2. Gleason believed that each species was distributed independently of others that co-occurred in
a particular association; an organization referred to as an open community.
3. Such open communities have no natural boundaries. Their limits are arbitrary with respect to
geographical and ecological distributions of their component species.
a. Although some animals are biotically dependent on aspects of the community (e.g. Fish and
frogs use an aquatic habitat for breeding.)
b. Other animals are not as closely linked. (e.g. Dragonflies may use the pond for reproduction,
but adults may fly a mile or two away to catch prey.)
6. In an open community the distribution of species across an environmental gradient occurs at
random with respect to the other species. Species are not clumped into groups.
7. Ecologists use Gleason's view of communities as being random assemblages of species as
support for the concept that communities are not in equilibrium.
C. Both concepts have validity in nature. Sharp boundaries do exist between associations.
1. The physical environment changes abruptly- i.e. a transition between aquatic and terrestrial
systems, such as between southwest and northeast-facing slopes.
2. Classic example of serpentine rock communities.
a. Serpentine rock has high amounts of nickel, chromium, iron and magnesium and low
amounts of copper and calcium.
b. Many plant species cannot grow on these sites. However, these sites have many other
species that can tolerate these conditions. They could grow on non-serpentine soil, but
they cannot compete.
D. And continuums do exist.
1. The eastern deciduous forest presents a physiognomically uniform appearance across a broad
geographic region. However, ecologists are aware that over that broad area, different species
of trees and other plants occur in different areas.
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a. Classifications of plant communities become more and more finely split, reaching absurd
levels of distinction because the boundaries were not highly defined. ( e.g. New England
Beech-Maple, Midwest Oak-Hickory Forests, Appalachian oak-chestnut forest).
2. Therefore, within a broadly defined region (grassland, deciduous forest) lies a continuum of
species, which gradually replace each other along gradients of physical conditions
(temperature, moisture, light.)
3. Maple species in southeastern PA.
III. Equilibrium view of Community Structure -the role of competition.
A. Packing them in - changes in niche dimension.
1. Review of fundamental niche and realized niche.
a. Fundamental niche - the total range of environmental conditions under which a species
may survive.
b. Realized niche - the portion of the fundamental niche occupied by a population in face of
competition from populations of other species; the actual range of environmental
conditions under which a population survives and reproduces.
2. There are 3 possible ways in which a niche (or niche relationship) may be altered to
accommodate more species in a habitat. (see Krohne for examples)
a. Increased niche overlap - species share more of the resources
b. Decreased niche breadth - high diversity communities may result from each species
having a smaller niche breadth
c. Increased range of resources - additional species may be fitted into the community by
extending the range of available resources that are utilized.
B. Packing them in is a matter of perspective.
1. Limited human perspective can sometimes alter our interpretation of the true nature of things.
2. In one-dimension, there may appear to be extensive niche overlap.
a. BUT, examined in two dimensions there is far less niche overlap.
C. Niche overlap and the degree of competition
1. The theoretical model of the niche assumes
a. that competition is intense
b.that only one species can occupy a niche space
c. that competitive exclusion takes place in areas of overlap
2. We generally assume that the amount of niche overlap is proportional to the degree of
competition for that resource. This is not always the case.
3. Considerable niche overlap does not always mean high competitive interactions.
a. Competition occurs only when a resource is in short supply. Extensive niche overlap may
indicate that little competition exists because resources are in abundant supply.
b. Don Strong described up to 13 species of beetle feeding on the leaves of Heliconia.
4. Under conditions of intense competition, the fundamental niche of one species may be
completely within or correspond exactly to another.
There are two possible outcomes to this a. If the niche of species E contains the niche of species F, and species E is superior, then
species F will be eliminated.
b. If species F is competitively superior, it will eliminate species E from the part of the niche
spaces that species F occupies. The two species then coexist within the same fundamental
niche.
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D. Patterns of Species Distributions - observational evidence for competition
1. J.M. Diamond (Chapter 9) used observation to infer that competition is what structures the
distribution of cuckoo species along the Bismarck Archipelago near New Guinea.
2. "Supertramps" - species that are not competitive in communities with other species.
3. Diamond's incidence function - As the number of species on an island increases, the
probability that a given species will be found in the community on that island increases.
4. For Supertramps, as species diversity increases (as it becomes more crowded with other
species), they cannot tolerate the competition, so they become eliminated.
a. Similar to revenge of the wimps - colonize recently disturbed areas with little competition.
b. Diamond was examining true islands, but tree-fall gaps are islands too - islands of light in
a dark forest.
E. Critiquing the Role of Competition - testing the null hypothesis
1. Not all observations of the absence of niche overlap or minimal niche overlap are indicative of
competition. I.e., cannot always infer that competition is the mechanism.
2. Don Strong and Dan Simberloff get back to basicsa. Scientific process has taught us that we cannot prove that any hypothesis is 'right' but,
b. We can only demonstrate that the alternative hypothesis is false.
3. Strong, Szyska and Simberloff try testing the null hypothesis.
a. If null hypothesis is not rejected, it doesn't mean that competition is not occurring, but that
it may not be as important as we think it is.
IV. Equilibrium view of Community Structure -role of predation
A. Classic example of how predation organizes community structure.
1. Work of R. Paine investigated intertidal community of invertebrates and algae in Washington.
Pisaster ochraceous - a starfish is a keystone predator. It is central to the organization of the
community. Without it, the organization of the undisturbed community changes drastically.
V. Non-equilibrium theories
A. Previously we have assumed that populations reach some equilibrium.
1. Why aren't early successional species eliminated from plant communities or animal
communities.
2. Variation in space and time.
a. Once source of variation that may reduce the importance of equilibrium processes
(predation and competition) is disturbance - particularly devastating disturbance.
3. Intermediate Disturbance Hypothesis
a. J.H. Connell emphasized role of disturbance to explain the number of coexisting species
in a community.
b. The absence of disturbance allows the elimination of species through competitive
exclusion.
c. Competition becomes the primary mechanism determining community structure.
d. The Intermediate Disturbance Hypothesis does not negate the equilibrium processes of
competition and predation - it just prevents them from organizing the structure of the
community.
e. Anything that removes species may be considered a disturbance -either biotic or abiotic.