1

12 February 2021

Species Diversity Today�s topics: •  Food webs

•  Keystone species and indirect effects •  Species diversity

•  Richness and Evenness •  Global patterns of diversity

•  Islands and fragmentation •  Equilibrium of colonization and

extinction •  Species-area effects

Fig. 54-11

Carnivore

Carnivore

Carnivore

Herbivore

Plant

A terrestrial food chain

Quaternary consumers

Tertiary consumers

Secondary consumers

Primary consumers

Primary producers

A marine food chain

Phytoplankton

Zooplankton

Carnivore

Carnivore

Carnivore

A Food Web in the Grasslands of Yellowstone National Park

Removing Wolves Initiated a Trophic Cascade

With sea otters Without sea otters Another Trophic Cascade Another Trophic Cascade

Largemouth Bass removed

Source: Estes et al. 2011

Bass Planktivorous fish Zooplankton Phytoplankton

2

Species interactions can evolve

A. ricordii

A. insolitus usually perches on shady branches.

A. distichus perches on fence posts and other sunny surfaces.

A. aliniger A. distichus

A. insolitus

A. christophei

A. cybotes A. etheridgei

Niche partitioning

Competition among lizards causes species to forage in different microhabitats.

Niche evolution in lizards

Green Anole Brown Anole

Green anoles perch higher in trees and evolved larger toepads on islands where Brown anoles were introduced.

Los Hermanos

G. fuliginosa G. fortis

Beak depth

Daphne

G. fuliginosa, allopatric

G. fortis, allopatric

Sympatric populations

Santa María, San Cristóbal

Beak depth (mm)

Perc

enta

ges

of in

divi

dual

s in

eac

h si

ze c

lass

60 40 20 0

60 40 20 0

60 40 20 0

8 10 12 14 16

Sizes of Galapagos finch beaks are more different on islands where two species compete.

Defense Mechanisms and Evolutionary Arms Races

Some places have high diversity

3

Boreal forest

Desert

Other habitats have low species diversity

“Diversity” depends on species richness and evenness

A B C D

Community 1 A: 25% B: 25% C: 25% D: 25%

Community 2 A: 80% B: 5% C: 5% D: 10%

Which of these communities is more diverse?

Tropical habitats support much larger numbers of species than do temperate and polar regions.

Mammals and Plants show the same pattern

Plant Species Richness

Mammals

Imagine a newly formed island some distance from the mainland…

What will determine the rate of arrival of new species?

Species richness will be a balance between immigration and extinction

Number of species on island

Rat

e of

imm

igra

tion

or e

xtin

ctio

n

Extin

ctio

n

Imm

igration

How would these curves change for a larger island? How will that affect the equilibrium number of species?

4

Area and Isolation Influence Species Richness on Islands

Number of plant species on the Galápagos Islands in relation to island area.

Endemic Galapagos prickly pear cactus tree

Species increase with area even at continental scales.

Area (hectares)

Num

ber o

f spe

cies

1,000

100

10

1 0.1 1 10 100 103 104 105 106 107 108 109 1010

The Theory of Island Biogeography Can Be Tested

Experimentally remove insects from small mangrove islands. Will the species number come back to equilibrium?

The Theory of Island Biogeography Can Be Tested (Part 2) How well does that experiment fit predictions of island biogeography theory?