Community Diversity

dynamics of community species composition

Community Diversity

• At its simplest, community diversity can be thought of as the raw number of species occupying a small area (sometimes called alpha diversity)

Community Diversity

• Generally, the species abundance in a community will follow a lognormal distribution

Community Diversity

• Metrics that aim to capture community diversity should include two aspects of diversity:• absolute number of species (species

richness)• relative abundance of each species (species

evenness)

Community diversity – species richness and evenness

Community Diversity Metrics

• Two measures used to quantify community diversity that incorporate both species richness and evenness:• Shannon-Wiener Index

H’ = -Σpilogepi (H’ increases with increasing species richness and evenness)

• Simpson’s Indexγ = Σpi

2

(γ= the probability that any two individuals drawn from the sample will be the same species)

s

i=1

Community Diversity – Rank Abundance Curves

• Can also visualize relative abundance and species diversity in a community by plotting relative diversity of species against their rank in abundance• In these plots, greater evenness in species

abundances is indicated by lower slope

Community Diversity – Rank Abundance Curves

Community Diversity – Phylogenetic Diversity

• evolutionary trees are increasingly being used as a tool to assess community diversity

• A phylogenetic tree representing the relationships of all the species in a community can be constructed and used to determine the evolutionary scope of the community

• based on summing ‘branch lengths’ for trees

Flora of a biodiversity hotspot, the cape of South Africa

Forest et al. 2007

Community Diversity

• Factors that influence community diversity include:• environmental complexity• species interactions• disturbance• history

Community Diversity – Environmental Complexity

• Increased environmental heterogeneity / complexity translates into increased niche “space” for species to occupy, hence, increased species diversity• complexity can be influenced by different

factors for different types of organisms…

Community Diversity – Environmental Complexity

• Animals may rely primarily on spatial habitat complexity, e.g., volume of tree space available for occupation by birds (MacArthur)

• Plants may rely more on chemical / nutrient composition of substrate to divide up niche space among different co-occurring species, reduce effects of competition

• microscopic organisms (e.g., plankton) also avoid competitive exclusion, increase community diversity through chemical heterogeneity of their environment

Community Diversity – Species Interactions

• predation can act to increase community diversity if the predator acts to suppress a dominant competitor or consumer species, therefore allowing resources (niche space) to be divided among a larger number of species

Community Diversity – Species Interactions

• competition can act to increase community diversity if the competitive interaction causes the species to partition resources (niche space) more finely

Example using Darwin’s finches: Competition drives character displacement in beak size

Studies of niche breadth in Anolis spp. fit the competition hypothesis.In this case, prey length was used as an indicator of “niche breadth.”

Narrow niche breadthin island with 5 species

Broader niche breadthin island with one species.

Community Diversity – Disturbance

• Disturbance: a discrete event in time that disrupts population, community or ecosystem structure and changes resources or the physical environment

• Joseph Connell: Intermediate Disturbance Hypothesis• species diversity will be highest in

communities that experience intermediate levels of disturbance

Community Diversity – Disturbance

• High disturbance: favors dominance of a few ruderal/weedy species with fast life cycles

• Low disturbance: favors dominance of a few competitively dominant species

• Intermediate disturbance: enough time for a wide variety of organisms to colonize; not enough time for competitive exclusion

Sousa 1979

Measuring Diversity in Two Redwood Forest Communities

Field Guide

# individuals pi lnpi pi(lnpi)

2 0.05556 -2.890291761 -0.16058461

4 0.11111 -2.197234577 -0.244134734

5 0.13889 -1.974073026 -0.274179003

6 0.16667 -1.791739469 -0.298629217

7 0.19444 -1.637631647 -0.318421097

8 0.22222 -1.504087397 -0.334238301

9 0.25 -1.386294361 -0.34657359

Community Dynamics - Succession

Communities are not static, but constantly change in response to disturbance,

environmental change, and their own internal dynamics

 

Succession

• The process of succession occurs as the composition and diversity of organisms in a community change following exposure of a new area for colonization

Succession

• Primary succession is a process that occurs on newly exposed geological substrate (little to no organic activity in substrate)

Succession

• Secondary succession is a process that occurs when disturbance removes a community without destroying the soil

Succession

• Pioneer community – the first species to colonize an open area after disturbance

Succession

• Climax community – the final assemblage of species achieved in succession: the populations of the climax community remain stable until perturbed by disturbance

Succession

• Time required to get from pioneer to climax community is highly context dependent:• Intertidal boulder – 1.5 years• Taiga – 1500 years• Temperate forest – 150 years

Succession

• Generalities regarding the process of succession:• species composition changes • species diversity increases

Succession

• Several mechanisms have been proposed to explain how succession occurs:

• Facilitation (Clements 1900s)• Tolerance (MacArthur and Horn 1970s)• Inhibition (Connell and Slatyr 1980s)

Succession

• Facilitation - pioneer species are the only ones capable of colonizing during early stages of succession, pioneer species modify their environment in such a way that it becomes less suitable for themselves and more conducive to the growth of other (later successional) species.

Succession

• Tolerance – all species tolerant of the conditions in a particular environment (even climax species) can be present in pioneer community. Less emphasis on earlier species modifying environment for later species; final species composition of climax community simply reflects the environment’s selection of all species that can tolerate environmental conditions.

Succession

• Inhibition – proposes that earlier occupants of an area modify the environment in a way that makes it less suitable for both early and late successional species. Late successional species can only invade an area if space is opened up by the death of earlier successional species. The climax community in this model comprises species that are long-lived and resistant to physical damage and biological factors (competition, etc.). Earlier successional species more vulnerable to physical and biological stresses.

Inhibition Model: algal succession on intertidal boulders (Sousa 19979)

control

treatment(Ulva sporelingsremoved by hand)

Observed process of succession is often a combination of different models

Community dynamics

• Climax community stability – some form of disturbance likely to occur fairly frequently in at least some parts of a community / ecosystem

• Community stability defined as the ability of a community to tolerate disturbance or the ability of a community to reassemble itself after a disturbance (resilience)