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Chapter 41 Overview: Communities in Motion

A biological community is populations of various species living close enough for interaction

For example, the “carrier crab” carries a sea urchin on its back for protection against predators

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Concept 41.1: Interactions within a Community

Ecologists call relationships between species in a community interspecific interactions

Interspecific interactions can affect the survival and reproduction of each species. Effects can be summarized as positive (), negative (−), or no effect (0)

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Competition Interspecific competition (−/− interaction) occurs

when species compete for a resource that limits their growth or survival

Strong competition can lead to competitive exclusion, local elimination of a competing species. Two species competing for the same limiting resources cannot coexist in the same place

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Ecological Niches and Natural Selection Ecological niche, the specific set of biotic and

abiotic resources used by an organism (it’s ecological role)

Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community

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Figure 41.2

A. ricordii

A. distichus perches onfence posts and othersunny surfaces.

A. insolitus usuallyperches on shadybranches.

A. insolitus

A. alinigerA. distichus

A. cybotes

A. etheridgei

A. christophei

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Predation

Predation (/− interaction) refers to an interaction in which the predator kills and eats the prey

Some feeding adaptations of predators are claws, teeth, stingers, and poison

Behavioral defenses of the prey include hiding, fleeing, forming herds or schools, active self-defense, camouflage, toxins

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Figure 41.5

(a) Crypticcoloration

Canyontree frog

(b) Aposematiccoloration

(c) Batesian mimicry: A harmless species mimicsa harmful one.

(d) Müllerian mimicry: Two unpalatablespecies mimic each other.

Poisondart frog

Nonvenomoushawkmoth larva

Venomous greenparrot snake

Cuckoo bee

Yellow jacket

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Herbivory

Herbivory (/− interaction) refers an herbivore eats parts of a plant or alga

In addition to behavioral adaptations, some herbivores may have chemical sensors or specialized teeth or digestive systems

Plant defenses include chemical toxins and protective structures

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Parasitism

In parasitism (/− interaction) the parasite, derives nourishment its host, which is harmed

Some parasites change the behavior of the host in a way that increases the parasites’ fitness

Mind Control of rodents!!

Symbiosis is a relationship where two or more species live in direct and intimate contact with one another.

3 General Types: Parasitism, Mutualism, Commensalism

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Mutualism

Mutualistic symbiosis, or mutualism (/ interaction), benefits both species

Mutualisms sometimes involve coevolution of related adaptations in both species

(a) Ants (genus Pseudomyrmex) inacacia tree

(b) Area cleared by ants around anacacia tree

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Commensalism In commensalism (/0 interaction), one species

benefits and the other is neither harmed nor helped

Commensal interactions are hard to document in nature because any close association likely affects both species

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Concept 41.2: Diversity and trophic structures

Two fundamental features of community structure are species diversity and feeding relationships

Sometimes a few species in a community exert strong control on that community’s structure

Species diversity depends on species richness and abundance

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

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

DCBA

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Communities with higher diversity are

More productive and more stable in their productivity

Able to produce biomass (the total mass of all individuals in a population) more consistently than single species plots

Better able to withstand and recover from environmental stresses….More resistant to invasive species

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Trophic Structures Food chains link trophic levels from producers to

top carnivores

A food web is a branching food chain with complex trophic interactions

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Species with a Large Impact

Certain species have a very large impact on community structure. Such species are highly abundant or play a pivotal role in community dynamics

Dominant species are those that are most abundant or have the highest biomass

Keystone species exert strong control on a community by their ecological roles, or niches

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

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In western N. America the uncommon sea star preys on mussels, which are the dominant species and strong competitor for space. Removing the sea star caused species richness to decline because the mussels monopolized the rock and eliminated most invertebrates and algae.

Experiment Results

Year

Nu

mb

er o

f sp

ecie

sp

rese

nt

20

15

10

01963’64

Without Pisaster (experimental)

With Pisaster (control)

5

’66’65 ’67 ’69’68 ’70 ’72’71 ’73

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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, the presence or absence of mineral nutrients determines community structure.

The bottom-up model can be represented by the equation

N V H PwhereN mineral nutrientsV plantsH herbivoresP predators

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The top-down model proposes that control comes from the trophic level above

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

Scientist have used this approach to control algae blooms by removing fish so there is more zooplankton to eat the algae

N V H P

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Concept 41.3: Disturbance influences species diversity and composition

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

Recent evidence of change has led to a nonequilibrium model, which describes communities as constantly changing after disturbances

Describe some disturbances.

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The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance

The Yellowstone forest is an example of a nonequilibrium community

(a) Soon after fire (b) One year after fire

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Ecological Succession

Ecological succession is the sequence of community and ecosystem changes after a disturbance

Primary succession occurs where no soil exists when succession begins

Secondary succession begins in an area where soil remains after a disturbance

Early arrivals may facilitate the appearance of later species by making the environment favorable

Early species may inhibit the establishment of later species

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Retreating glaciers provide a valuable field research opportunity for observing succession

Succession in Glacier Bay, Alaska, follows a predictable pattern of change in vegetation and soil characteristics

1. The exposed area is colonized by pioneering plants, including liverworts, mosses, fireweed, Dryas, and willows

2. After about three decades, Dryas dominates the plant community

3. A few decades later, alder invades and forms dense thicket.

4. In the next two centuries, alder are overgrown by Sitka spruce, western hemlock, and mountain hemlock

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Figure 41.19-5

Spruce stage Alder stage

Pioneer stage Dryas stage

Alaska

GlacierBay

Kilometers

19411907

1860

1760

0 5 10 15

1 2

34

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Human Disturbance Humans have the greatest impact on biological

communities worldwide

Human disturbance to communities usually reduces species diversity

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Figure 41.23