Lecture 17 - Community Structure and Diversity

Communities

groups of interacting species that occur together at the same place and time

Alpha diversity metrics

Richness, Evenness, Diversity

Species richness

number of species

Species evenness

compares relative abundances (commonness and rarity) of species

Species diversity

combines richness and evenness

– Shannon diversity

– Inverse Simpson diversity

Shannon diversity

– Average degree of uncertainty in predicting the species of an individual if that individual were picked at random from the community

– Sensitive to rare species

Inverse Simpson diversity

– Number of randomly selected pairs of individuals that must be drawn from your community to get a pair where both individuals are the same species

– Weights common species more

pi

proportion of individuals in the ith species

S

number of species in the community (richness)

N

total number of individuals in community

n

number of individuals in each species

S

number of species in the community (richness)

H

shannon diversity

EH

Shannon evenness

• Comparing richness and Shannon diversity plots, what do they tell you about evenness in control and warmed treatments?

• Comparing the Shannon and Simpson diversity plots, what do they tell you about what is driving the unevenness?

The warmed treatment is less even than the control.,

The unevenness is being driven primarily by changes in the dominant species, not by rare species.

Species accumulation curves

species richness plotted as a function of total number of individuals counted

Species area curve

species richness vs. area sampled

Direct interactions

Between two species

Indirect interactions

The relationship between two species is mediated by a third (or more) species

Trophic cascade

occurs when the rate of consumption at one trophic level results in a change in species abundance or composition at lower trophic levels

Apparent competition

a negative effect of one species on another species, mediated through the action of shared natural enemies

Interaction strength

effect of one species on the abundance of another

Foundation species

have large effects on other species by virtue of their considerable abundance or biomass

Keystone species

effects are large in proportion to their biomass or abundance