Community Ecology

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5

In contrast to dominant species, they are not necessarily abundant in a community.

Field studies of sea stars exhibit their role as a keystone species in intertidal communities.

Sea otter populations and their predation shows how otters affect ocean communities. Sea otters are keystone predators in the North Pacific.

Slide 28

Seastar are keystone predators. They are key in preserving species diversity in their ecosystem.

With Pisaster (control)

Without Pisaster (experimental)

Number of species

present

Year

20

15

10

5

0

1963

’64

’65

’66

’67

’68

’69

’70

’71

’72

’73

RESULTS

EXPERIMENT

Slide 29

Sea otters are keystone predators in the North Pacific

(a) Sea otter abundance

Otter number

(% max. count)

100

80

60

40

20

0

400

300

200

100

0

(b) Sea urchin biomass

Grams per

0.25 m2

10

8

6

4

2

0

1972

Number per

0.25 m2

1985

1997

Year

(c) Total kelp density

Food chain

1989

1993

Slide 30

Foundation Species (Ecosystem “Engineers”)

Foundation species (ecosystem “engineers”) cause physical changes in the environment that affect community structure.

For example, beaver dams can transform landscapes on a very large scale.

Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community.

Slide 31

Beavers are a Foundation Species = ecosystem“engineers”

Slide 32

Bottom-Up and Top-Down Controls

The bottom-up model of community organization proposes a unidirectional influence from lower to higher trophic levels.

In this case, presence or absence of mineral nutrients determines community structure, including abundance of primary producers.

The top-down model, also called the trophic cascade model, proposes that control comes from the trophic level above.

In this case, predators control herbivores, which in turn control primary producers.

Slide 33

Disturbance influences species diversity and composition

Pollution can affect community dynamics.

Biomanipulation can help restore polluted communities. Bio remediation is an effective strategy to restore polluted and damaged areas.

Decades ago, most ecologists favored the view that communities are in a state of equilibrium.