Species Diversity

Biogeography

Explains geographic patterns of diversity by looking at the distribution of organisms

Cosmopolitan

occurs almost world wide

Endemic

occurs in one locale only

Disjunct

occurs in separate locales

Causes of disjunct distributions

• Dispersal

• Extinction

• Geologic changes to the distribution, such as the break up of a continent

2 Critical Processes to Look at in the Distribution of Organisms

• Dispersal (immigrate elsewhere)

• Extinctions (established there and the species does not go extinct)

Why is biogeography important?

It explains ecosystem formation, helps test equilibrium models (e.g., Island Biogeography), and has conservation applications.

What causes disjunct distributions?

Dispersal, extinction of intermediate populations, and geological changes.

What two critical processes determine species distribution?

Dispersal and extinction.

What is the typical latitudinal pattern of species richness?

Higher species richness in the tropics (latitudes 0–20).

Name one exception to latitudinal diversity patterns.

Seabirds, which may not follow the tropical richness trend.

List theories explaining high tropical biodiversity.

• Higher predation reduces competition.

• Higher productivity in warmer climates.

• Higher metabolic rates → faster mutagenesis (Rohde).
  None are conclusive.

What is the species-area effect?

Larger islands contain more species.

What is the species-isolation effect?

More isolated islands have fewer species.

Who developed the Theory of Island Biogeography?

MacArthur and Wilson (1967).

How does extinction rate change with island size?

Higher on small islands because of smaller populations and more vulnerability.

How does immigration rate change with species number?

Immigration declines as species number increases.

How does distance affect immigration?

Far islands receive fewer immigrants due to lower chance of dispersal.

What is ecological equilibrium on an island?

Immigration and extinction balance, keeping species number constant even if composition changes.

What is turnover?

Continuous change in species composition over time.

What did Diamond (1969) find in the Channel Islands study?

Species richness remained stable, but turnover was high.

In conservation, what type of reserve is better?

Usually one large, connected area rather than many small isolated patches.

What factors control species diversity within communities?

Competition, predation, mutualism, abiotic factors (temperature, pH, salinity), and disturbance.

What is resource partitioning?

Coexistence through using different aspects of the same resource spectrum.

What classic example did MacArthur study?

Warblers partitioning tree feeding zones in New England forests.

What does the Intermediate Disturbance Hypothesis predict?

Greatest diversity occurs at intermediate levels of disturbance.

Why do high disturbance sites have low diversity?

Only good colonizers survive.

Why do low disturbance sites have low diversity?

Competitive exclusion dominates.

What did Tilman’s grassland experiment show?

Good dispersers dominate early; good competitors dominate later—supporting IDH.

What three factors determine species diversity in the Menge & Sutherland model?

Stress, predation, competition.

When is predation most important in this model?

At low stress levels.

What do neutral models assume?

Species are equal competitors; chance governs resource acquisition.

What is the Recruitment Limitation Hypothesis?

Poor dispersal causes incomplete mixing, allowing coexistence of similar competitors.

What did Hubbell’s Panama study show?

Seed dispersal is patchy; poor competitors can survive because they don’t always land near strong competitors.

How does species diversity affect community function?

Higher diversity increases function and stability.

What was Tilman’s finding on drought resistance?

Plant communities with higher species richness were more drought-resistant.

What is ecological resilience?

The speed at which a community returns to equilibrium after disturbance.