Eeb 100 - Lecture 7

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Community

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A group of species that inhabit an area and interact directly/indirectly. Communities may rely on each other for food or the may compete with each other. They can also not interact at all.

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Specific characteristics of a community

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1. Relative abundance of a species (biological structure)

2. Physical Structure

3. Interactions among species

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Relative Abundance

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Relative abundance is expressed as the proportion each species contributes to the total number of individuals of all species in a community.

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Which group is more diverse?

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Group 1

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Rank Abundance Diagram.

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• A rank abundance diagram plots rank abundance (x-axis) against corresponding relative abundance (y-axis). Used to examine patterns of relative abundance within communities. The most abundant species is plotted closest to the left (smallest number).

This plot demonstrates species richness and species evenness. (Longer the line the more species richness community has.)

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Rank Abundance has two features of diversity...

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species richness (S) – the number of species in the community

species evenness – how equally individuals are distributed among the species (max species evenness occurs when each species in a community is equally abundant.)

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What would a rank abundance curve look like if each species was relatively abundant?

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It would be a straight line parallel to the X axis.

If the community had one species, the y-intercept would equal 1.0. If the community had ten species, the y-intercept would equal 0.1

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What is the simplest quantitative measure of community structure?

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Species richness, however, it does not account for differences for relative abundance in a community

( Two communities may both have only 2 species in them, but one community can have one species be more abundant than the other species, while the other community has both species evenly distributed). We wouldn't be able to tell this if we just looked at species richness. In order to bypass this, we look at species diversity

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Species diversity is defined by...

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Species richness and evenness. Species diversity considers both number and relative abundance of species within the community. Simpsons index looks at species diversity.

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Simpsons Index (D)

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Probability that two randomly selected individuals from the community will belong to the same species

D=1 with no diversity.

D approaches 0 with high diversity.

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Simpsons Index of Diversity

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1-D

This value increases with species diversity. Value represents probability that two individuals randomly selected will belong to different species.

0 = no diversity

Values approach 1 with higher diversity.

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Simpsons Reciprocal Index

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1/D

This value increases with species diversity

1 = no diversity (lowest possible value)

Maximum value is species richness S (number of species in community)

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Simpsons Index - Evenness

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Complete evenness = all species equally abundant

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Community 2 because 1/2 is closer to 1

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Dominance

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Although some communities differ in the number of species richness, they can share a common quality in that one species is particularly dominant over the rest. Competition within communities can determine dominance.

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Dominance can be described by 3 criteria...

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1. Number of individuals

2. Relative Abundance

3. Relative Biomass

Can't just describe based on the abundance and number of individuals because one less abundant species can account for more biomass than all others combined, and then this species would be considered more dominant. Because of this, we must take into account all 3 criteria.

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Keystone Species

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A keystone species has a disproportionate impact on the community relative to its abundance (meaning it's not abundant but it's very important to the community).

Keystone species function in a unique and significant way within a community

-- create or modify habitats

-- influence interactions among other species

The removal of a keystone species can lead to changes in community structure loss of biodiversity

Ex: Otter decline leads to increase in sea urchins, which leads to decreased kelp and a decrease in organisms who rely on kelp for shelter/food.

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Food Chain vs Food Web

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Food Chain where species get their food. Has arrows representing the flow of food energy from prey to predator.

A Food Web are just multiple food chains.

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Trophic Level

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all organisms at the same number offeeding levels away from the energy source (sun)

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Food Web Facts

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Restoration Ecology

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Goal of returning a community or ecosystem to a stateapproaching its condition before disturbance

Requires an understanding of the processes influencingthe structure and dynamics of communities

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Least Intensive Effort of Restoration of Existing Communities

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Eliminating invasive species

replanting native species

reintroducing natural disturbances

short-term periodic fires ingrasslands

low-intensity ground fires in pineforests

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More Intensive Effort

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Recreating community from scratch

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How do you rebuild an ecological community?

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Community structure changes through timein a series of gradual, successional stages. (A recently burned field will bequickly filled with weedy plants, followed by small bushes and shrubs, thenovertaken by pine trees and other larger trees.

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Early Successional Species

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Often have

high growth rates

smaller size

high degree of dispersal and colonization

high rates of per capita population growth

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Late Successional Species

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Often have:

longer lifespans

larger size

lower rates of dispersal and colonization

lower rates of per capita population growth

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Primary and Secondary Succession

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Disturbance for secondary succession doesn't necessarily remove all individuals.

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Where do colonists come from?

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...

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Autogenic: example, the verticalprofile of light in a forest is a direct result of the interceptionand reflection of solar radiation by the trees

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Diversity changes through successional time. Diversity increases as newspecies colonize the site. Peak in diversity in the mid stages corresponds to the transition fromearly to late successor species.

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Succession of plant species affectsheterotrophic organisms. Some organisms, like birds, can only livein certain environmental conditions that are met by specific plants. A changein plant composition in the environment will displace the bird species, whichwill then be replaced by another bird species that can survive these newconditions. xR4\Sw%

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