Community Ecology

Species richness

The number of species in a community

Species evenness

the relative abundance of the different species in a community

Species diversity

Species richness and evenness

Shannon-Wiener Diversity Index

Index to quantify species diversity

• If similar in richness then the more even is more diverse

Why measure species diversity

• It offers insight into a community’s health and stability

• A higher H indicates productivity and stability

• A lower H can signal a community is under stress

Ecosystem Analyses

1. Energy and nutrient flow through communities

2. How communities resist environmental stresses

3. The overall function and resilience of communities

Trophic Level

• Trophic levels are linked by the feeding relationships within the community

Grazing food web

Primary Producers (Plants)

Primary Consumers (Herbivores)

Secondary Consumers (Carnivores)

Tertiary Consumers (Secondary carnivores)

Apex Predators (Vultures, eagles, lions, tigers)

Detrital food web

Detritivores and decomposers

Direct Interactions effects

When the actions of one species directly impacts another species

• Cheetahs kill gazelles

  • Bee pollinates flowers

Indirect Interactions effects

When the actions of one species on another is mediated by a third species

• Birds indirectly benefiting a plant by reducing the number of caterpillars

• Trophic cascade (effects go down the trophic levels)

Trophic cascade

Introducing or removing a top keystone predator

• It effects trick down throughout trophic levels

Bob Paine

• Trophic cascade in intertidal communities

• Sea stars were a top predator and created diversity within the intertidal community by regulating the population of dominant mussels.

Keystone predator

• A predator that has a large but indirect effect on its native habitats

• Sea stars (Bob Paine)

Ecological succession

The change in species composition of a community over time

Observing succession directly

• Document changes in community composition over time

  • The island of Krakatau in Indonesia

  • Researchers have documented the colonization of new ecosystems after volcano erupted.

• Difficult because succession happens over decades

Observing succession Indirectly

• Examine communities that began succession at different times

  • The size of the lakes in Michigan have shrunk

  • Ecologists assumed that plants farthest from the lake were the oldest and reflected more advance stages of succession

• Difficult to know whether the initial conditions were the same in different communities

Primary Succession

• Begins from scratch

• Takes a long time (hundreds to thousands of years)

• first lichens develop on bare rock

Secondary succession

• Occur in areas of devastation where above ground vegetation is eliminated

  • from fires, logging, tornadoes…

• Takes less time (decades to hundreds of years)

Pioneer species

Less competitive stage so…

• Small

• Reproduce early

• Little parental investments

• High reproductive rate

• long seed viability

• low shade tolerance

Climax species

More competitive stage so…

• Large

• reproduce late

• Exhibit more parental investment

• low reproductive rate

• short seed viability

• high shade tolerance

Island Biogeography

• Understanding what determines species richness and diversity in communities

• Islands are consider natural laboratories

  • Examining the impact of the island size and distance from the mainland on species richness

Island Biogeorgraphy Takeaways

• Size matters

  • Large: Greater species abunadnace

  • Small: Greater species extinction

• Distance to mainland matters

  • Closer: Higher diversity

  • Farther: Lower richness

• Rates of immigration vs extinction

• Equilibrium theory of species diversity

Large, close islands

• High species richness

• High Immigration

• Low extinction

• High competition

Small, distant islands

• Low species richness

• Low immigration

• High extinction