1

Geographic Ecology

Chapter 22

2

3

4

Outline

• Introduction• Island Area, Isolation, and Species Richness

Terrestrial Aquatic

• Equilibrium Model of Island Biogeography• Latitudinal Gradients in Species Richness• Historical and Regional Influences

5

Introduction

• MacArthur defined geographic ecology as the search for patterns of plant and animal life that can be put on a map. Above level of landscape ecology. Vast breadth

Chapter only focuses on a few aspects.

6

Island Area, Isolation, and Species Richness

• Preston found fewest bird species live on smallest islands and most species on largest islands.

• Nilsson et al. found island area was best single predictor of species richness among woody plants, carabid beetles, and land snails.

7

Island Area, Isolation, and Species Richness

8

Habitat Patches on Continents: Mountain Islands

• As Pleistocene ended and climate warmed, forest and alpine habitats contracted to the tops of high mountains across American Southwest. Woodlands, grasslands, and desert scrub,

invaded lower elevations. Once continuous forest converted to

series of island-like fragments associated with mountains: Montane.

9

10

Lakes as Islands

• Lakes can be considered as habitat islands. Differ widely by degree of isolation.

Tonn and Magnuson found the number of species increases with the area of an insular environment.

11

Marine Islands

• MacArthur and Wilson found isolation reduces bird diversity on Pacific Islands.

• island area and species richness in Azore Islands: Birds show clear influence of isolation on

diversity, pteridophytes do not.

12

Marine Islands

13

Isolation and Habitat Islands on Continents

• Lomolino et al. found a strong negative relationship between isolation and the number of montane mammal species living on mountaintops across the American Southwest.

14

15

Equilibrium Model of Island Biogeography

• MacArthur and Wilson: Model explaining species diversity on islands = immigration and extinction rates.

Rates of immigration highest on new island with no organisms.

As species accumulate, rate of immigration declines since fewer arrivals arenew species.

16

Equilibrium Model of Island Biogeography

17

Equilibrium Model of Island Biogeography

• Rate of extinction rises with increasing number of species on an island for three reasons: More species creates larger pool of

potential extinctions. As number of species increases,

population size of each must diminish. As number of species increases,

competitive interactions between species will increase.

18

Equilibrium Model of Island Biogeography

• Point where two lines cross predicts the number of species on island.

• Proposed rates of extinction determined mainly by island size. LG near islands support highest S. SM far islands support lowest S. SM near and LG support intermediate S.

19

Equilibrium Model of Island Biogeography

20

Species Turnover on Islands

• Equilibrium model predicts species composition on islands is dynamic. Change referred to as species turnover.

• Diamond found birds in nine CA Channel Islands in a stable equilibrium Species turnover result of equal levels of

immigration and extinction.

21

Species Turnover on Islands

22

Experimental Island Biogeography

• Simberloff and Wilson studied insect recolonization in Florida Keys. Chose 2 stands of mangroves as control

islands, and 6 others as experimental islands.

Defaunated islands Followed recolonization for 1 yr.

– Species number constant, but composition changed considerably.

23

Spray insecticide on 2 islands, 6 no spray

24

Experimental Island Biogeography

25

Manipulating Island Area

• Simberloff tested effect of island area on species richness = cut mangroves out. area reduced, S decreased.

control island S increased slightly. reduced area < species with reduction in

area. Area has positive influence on S.

26

Manipulating Island Area

27

Latitudinal Gradients in Species Richness

• Most groups of organisms are more species-rich in the tropics.

• Brown grouped hypotheses into six categories:

28

29

•

1. Time Since Perturbation More species in the tropics because

tropics are older and disturbed less frequently.

More time for speciation, less frequent disturbance reduces extinction rate.

30

2. Productivity High productivity = high species

richness. More energy to divide among

population.

3. Environmental Heterogeneity More heterogeneity, more potential

habitat areas and niches.

31

4. Favorableness Tropics more favorable environments.

No extremes to limit diversity.

5. Niche Breadth and 6. Interspecific Interactions

Various themes Brown: biological processes play

secondary role.– Ultimate causes are physical

differences.

32

Area and Latitudinal Gradientsin Species Richness

• Rosenzweig: immigration can be largely discounted at broad scales, thus speciation is primary source of new species.

Species removal via extinction. Tropics richness is greater due to higher

rates of speciation and / or lower rates of extinction.

33

Continental Area and Species Richness

• Rosenzweig - strong positive relationship between area and species diversity.

34

Historical and Regional Influences

• Latham and Ricklefs: contrast in diversity of temperate zone trees that cannot be explained by area effect. Temperate forest biome in Europe,

Eastern Asia, and Eastern North America all have roughly equitable area, but support vastly different levels of biological diversity.

Eastern Asia: 3x NA and 6x Europe.

35

36

Diversity of Temperate Trees

• Latham and Ricklefs: Must examine conditions trees in these regions faced during the last glacial period. Mountains in Europe form east-west

oriented barriers. During last ice age, temperate trees had

southward retreat largely cut-off. Lower species richness as

consequence of higher extinction rate.

37

Historical and Regional Influences

Appalachian Mountains in N.A. run north-south; temperate trees had retreat path as temperatures cooled.

Also no mountain barriers in Asia.

• Most temperate tree taxa originated in Eastern Asia and dispersed to Europe and N.A. After dispersal lines were cut, speciation

continued in Asia.

38Molles: Ecology 2nd Ed.

Historical and Regional Influences

39Molles: Ecology 2nd Ed.

Historical and Regional Influences

40

Review

• Introduction• Island Area, Isolation, and Species Richness

Terrestrial Aquatic

• Equilibrium Model of Island Biogeography• Latitudinal Gradients in Species Richness• Historical and Regional Influences

41