Topic 1: What determines the number of species in a community?

1. Understand the different types of diversity (alpha, beta, gamma), and how diversity varies across latitude

2. Explain the differences between evolutionary and ecological hypotheses for latitudinal gradients in species richness, and why it is so difficult to determine which of these actually generate the gradient

3. Describe the difference between the museum and cradle hypotheses, and explain what evidence could be used to support each

4. Describe (verbally & mathematically) a species-area relationship5. Explain which processes are important for a within-region (nested) versus between-

region species-area relationships6. Use Island Biogeography Theory to explain how species richness varies with island size

and isolation7. Explain a dynamic equilibrium8. Distinguish between processes that are neutral with respect to species identity, and

those that are based on species niches9. Contrast dispersal limitation versus biotic limitation of local species richness10. Explain the difference between a species’ fundamental and realized niche11. Understand the potential influence of competition, facilitation, and predation on local

species richness 12. Design and interpret experiments to test for species interactions & dispersal limitation.

Topic 1: Learning outcomes

Diversity: it’s a matter of scale!

The different types of diversity:

Welcome to the ecological fraternity, “Alpha Beta Gamma”

Alpha diversity: The number of species at a particular site

Species ASpecies BSpecies CSpecies DSpecies ESpecies FSpecies G

XX

ForestWhat’s the alpha diversity?

A.0 B.1C.2D.7E.XX (Dos Equis)

Beta diversity: “turnover” in species composition from one site to the next

Species ASpecies BSpecies CSpecies DSpecies ESpecies FSpecies G

XXX X

XX X

XX

ForestSavanna

Grassland

What’s the gamma diversity in regions 1 and 2?

A.0, 0B.3, 3C.0, 3D.7, 7E.7, 5

Gamma diversity: Total diversity within a region –the combination of alpha and beta diversity

Species ASpecies BSpecies CSpecies DSpecies ESpecies FSpecies G

XXX X

XX X

XX

ForestSavanna

Grassland

Species ASpecies BSpecies CSpecies DSpecies ESpecies FSpecies G

XXX

XXX

XX

X

ForestSavanna

GrasslandRegion 1 Region 2

What’s the beta diversity in regions 1 and 2?

A.3, 2B.3, 3C.4, 3D.4, 2E.7, 5

Beta diversity: can be measured as gamma diversity minus average alpha diversity

Species ASpecies BSpecies CSpecies DSpecies ESpecies FSpecies G

XXX X

XX X

XX

ForestSavanna

Grassland

Species ASpecies BSpecies CSpecies DSpecies ESpecies FSpecies G

XXX

XXX

XX

X

ForestSavanna

GrasslandRegion 1 Region 2

< 100

100 - 200

200 - 500

500 – 1,000

1,000 – 1,500

1,500 – 2,000

2,000 – 3,000

3,000 – 4,000

4,000 – 5,000

> 5,000

The simplest metric of diversity is Species Richness: the number of species in an area

(Here the number of plant species per 100km x 100km pixel)

Species richness

(S)

Equator Poles

Latitude

The overall global pattern in species richness:highest in the tropics, lowest near the poles

Observable pattern: There are VASTLY more species in the tropics (low latitude) than at the

poles (high latitude)

Birds, amphibians, & mammals combined

A single 50 ha plot in Borneo (that’s a 1 km by ½ km rectangle) has more tree species (~1100) than the entire temperate zone (North America, Europe, northern Asia) combined!!

WHY are there so many more species in the tropics than at the poles?

What mechanisms might account for these patterns? How can they be tested?

(Stay tuned fornext class!)

13

First of all: what are the “tropics”?

-More direct sunlight heats up the tropical atmosphere

-The hot air rises and, as it hits the upper atmosphere, it cools

-Cold air can hold less water, so the water is dumped as rain-The now dry air descends at mid-latitudes, warming up again, and creating deserts-That’s why the tropics tend to be both hot and wet

Equator

Southern tropic line(tropic of Capricorn)

Northern tropic line(tropic of Cancer)

Equator

Southern tropic line

Northern tropic line

First of all: what are the “tropics”?

The tropics

Neotropics

Afrotropics

SE Asia

My personal latitudinal gradientInterior Alaska

Latitude: 65°

BorneoWorld’s 3rd largest islandLatitude: 2°

This is what interior Alaska looks like....

This is what Borneo looks like....

Borneo rainforest … alpha and beta diversity

Alpha diversity: Really high

Beta diversity: Really high

Gamma diversity: Through the roof

Alaskan tundra … alpha and beta diversity

Alpha diversity: Medium

Beta diversity: Low

Gamma diversity: Pretty low

25

Many more species in Borneo!

Borneo 288 species ~600 species 145 species 150 species~5,000 species

Alaska 76 species~200 species 0 species 2 species ~20 species

MammalsBirdsSnakesFrogsTrees

1. Understand the different types of diversity (alpha, beta, gamma), and how diversity varies across latitude

2. Explain the differences between evolutionary and ecological hypotheses for latitudinal gradients in species richness, and why it is so difficult to determine which of these actually generate the gradient

3. Describe the difference between the museum and cradle hypotheses, and explain what evidence could be used to support each

4. Describe (verbally & mathematically) a species-area relationship5. Explain which processes are important for a within-region (nested) versus between-

region species-area relationships6. Use Island Biogeography Theory to explain how species richness varies with island size

and isolation7. Explain a dynamic equilibrium8. Distinguish between processes that are neutral with respect to species identity, and

those that are based on species niches9. Contrast dispersal limitation versus biotic limitation of local species richness10. Explain the difference between a species’ fundamental and realized niche11. Understand the potential influence of competition, facilitation, and predation on local

species richness 12. Design and interpret experiments to test for species interactions & dispersal limitation.

Topic 1: Learning outcomes