Ch. 16 - Species abundance & diversity

Community

Different species that inhabit a defined area

Guild

Group of organisms that make their living in a similar way (ex. birds that live and feed in a specific part of a tree canopy)

Life-form

Classification of structure and growth dynamic of plants

Relative abundance

proportion of a particular species relative to other species in a community.

Preston’s Distribution of Commonness and Rarity

An ecological model that uses a lognormal distribution to show most species are moderately abundant with even fewer species being extremely rare or extremely common.

Species diversity is based on two factors:

Species richness & Species evenness 

Species richness

the number of different species in a given area.

Species evenness

relative abundance of species in a community.

Shannon Weiner Diversity Index

Method for quantifying species diversity that considers both species richness and species evenness.

Shannon Weiner Value

H’

• is always positive 

• Higher H’ value = higher diversity

Rank Abundance Curve

A plot of relative species abundance against their rank in abundance.

Based on rank abundance curve how can you tell which community has greater species evenness?

a more even slope will have greater species evenness.

According to this rank abundance curve which community is more diverse?

The mountain stream community

Why is it important to quantify diversity within a community?

• it allows scientist to measure the health, stability, and track the impacts that any environmental changes have on a community

  • provides a baseline for comparison

  • provide a numerical representation of a community's structure and how this may be impacted by disease, climate change, pollution, etc.

  • Help us better understand ecological roles that impact communities: such as pollination, nutrient cycling, and decomposition.

Environmental complexity creates more

Species diversity

Examples of environmental complexity supporting greater diversity

• Robert and John MacArthur found that greater foliage height supported more diverse bird communities

• David Tilman found that greater variation of nutrients in lakes supports more diverse phytoplankton communities

Equilibrium

State of community/ecosystem when environmental conditions remain reasonably stable.

• opposing forces cancel each other out 

Disturbance

Any event that removes all or part of a community.

Potential sources of disturbance:

• abiotic (fire, hurricanes, floods)

• biotic (disease, predation, competition, parasitism, invasives)

• human-caused (pollution, climate change, agriculture)

What factors must be considered when looking at the impacts of disturbance on communities

• impacts vary with organisms and environments

• spatial and temporal scales

• frequency and intensity 

Intermediate Disturbance Hypothesis 

Predicts that intermediate levels of disturbance promote higher levels of species diversity. 

Examples of intermediate disturbance hypothesis.

• in the intertidal zone boulders of medium size that experience intermediate disturbance have greater diversity of algae and marine organisms compared to the smaller and larger ones.

• Prairie dogs disturb the vegetation around the areas they create burrows however this disturbance has a positive effect on plant diversity by creating open patches for new plants to colonize. 

Urban Ecology

Study of ecology in urban settings

• landscapes exposed to some degree of human land use can have a positive impact on species diversity.