Community Ecology

Chapter 53

Community Ecology• A community is an assemblage of

species (populations) living close enough together for potential interaction in a habitat

Community Ecology• The boundaries of a community can be

set by the experimental question (the decomposers on a rotting log, the benthic community of Lake Erie, the trees and shrubs in Yellowstone National Park)

• Communities vary in their species richness and relative abundance

Interspecific Interactions• Interspecific competition – competition

for resources between different species when resources are in short supply

Interspecific Competition• Competitive exclusion principle – The

concept that when populations of two similar species compete for the same limited resources, one population will use the resources more efficiently and have a reproductive advantage that will eventually lead to the elimination of the other population

The Ecological Niche• An organism’s ecological niche is

described as its place in an ecosystem – its habitat and use of biotic and abiotic resources

• The competitive exclusion principle holds that two species with identical niches cannot coexist in a community

Resource Partitioning• Resource partitioning – slight variations

in niche that allow ecologically similar species to coexist; competition is a selection factor in evolution

Predation• Predation – a predator eating prey• Includes herbivory and parasitism• Eating and being eaten are prerequisite

to reproductive success• Adaptations to increase success in

predation may include acute senses, speed and agility, camouflage coloration, and physical structure such as claws, fangs, teeth, and stingers

Predation• Plants may defend themselves with

mechanical devices, such as thorns, or chemical compounds

• Animals can defend against predation by hiding, fleeing, or defending

• Cryptic coloration allows animals to blend in to their environment

• Mechanical and chemical defenses discourage predation

• Aposematic coloration warns predators not to eat animals with chemical defenses

Predation

Predation• Mimicry may be used by prey to exploit

the warning coloration of other species• Batesian mimicry – a palatable or

harmless species mimics an unpalatable or harmful model

• Müllerian mimicry – two or more unpalatable species resemble each other

Cuckoo BeeCuckoo Bee Yellow JacketYellow Jacket

Parasites• A parasite derives its nourishment from

another organism, the host, which is harmed in the process

• Parasites that live within the host are endoparasites ; those that feed on the surface of a host are ectoparasites

• In parasitoidism, insects lay eggs on hosts, on which their larvae then feed.

• Parasites do not usually cause lethal harm to the host on which they feed

Mutualism• In mutualism, interactions between

species benefit both participants• Mutualistic interactions require the

coevolution of adaptations in both species

• Many cases of mutual symbiosis may have evolved when organisms became able to derive some benefits from their predator or parasite Mutualism between Mutualism between

acacia trees and antsacacia trees and ants

Commensalism• In commensalism, only one member

appears to benefit from the interaction• Examples include “hitchhiking” species

and species that feed on food incidentally exposed by anotherThe Patella gets it

food from the plant, the Euklonia, which is not harmed or damaged in the process.

Example of Example of MutualismMutualism

Trophic Structure• The trophic structure

of a community is its feeding relations

• A food chain shows the transfer of food energy from one trophic level to the next

Food Webs• A food web

diagrams the complex trophic relationships within a community

What Limits the Length of a Food Chain?

• Within a food web, each food chain consists of only a few links (usually five or fewer)

• Two hypotheses– Energetic hypothesis – food chains are limited

by inefficiency of energy transfer (only about 10%) from one trophic level to the next

– Dynamic stability hypothesis – suggests that short food chains are more stable than long ones because an environmental disruption that reduces production at lower levels will be magnified at higher trophic levels as food supply is reduced all the way up the chain

Dominant Species• Species in a community that have the

highest abundance or largest biomass are a major influence on the distribution and abundance of other species

• A species may become a dominant species due to its competitive use of resources or success at avoiding predation

• What happens if the dominant specie is removed from a habitat such as the American chestnut trees?

Keystone Species• A keystone species has a large impact

on community structure as a result of its ecological role

• Experiments with a predatory sea star (Pisaster) demonstrates the importance of keystone species

Sea Otters as Keystone Predators in the North Pacific

Structures of Communities• The structure of a community may be

controlled bottom-up by nutrients or top-down by predators

• According to the bottom-up model of community organization, N → V → H → P, an increase in nutrients yields an increase in biomass at each succeeding trophic level

• The top-down model, N ← V ← H ← P, assumes that predation controls community organization; also called the trophic cascade model

Disturbance and Community Structure• The nonequilibrium model emphasizes

the nonstable, changing structure and composition of communities as a result of disturbances

• Disturbances such as fire, drought, storms, overgrazing, or human activity change resource availability, reduce or eliminate some populations, and may create opportunities for new species

• Small scale disturbances may enhance environmental patchiness and help maintain species diversity

Disturbance and Community Structure

Disturbance and Community Structure

Disturbance and Community Structure

Ecological Succession• Ecological succession is the transition in

species composition in a community, usually following some disturbance

• Primary succession occurs in areas previously devoid of life (new volcanic island or the moraine left by a retreating glacier)– Autotrophic bacteria → lichens → mosses →

grasses → shrubs → trees• Secondary succession occurs in existing

communities that have been disturbed by fire, logging, or farming, but the soil remains intact– Herbaceous species → woody shrubs → trees

Biodiversity• A communities biodiversity correlates

with its size and geographic location• Biodiversity is greatest in the tropics and

on large versus smaller islands• Biodiversity is measured by both species

richness and relative abundance• Ecologists measure biodiversity as

heterogeneity, which considers both diversity factors: richness and relative abundance

Which Forest is More Diverse?

Biodiversity• Tropical habitats

support much larger numbers of species of plants, animals, and other organisms than do temperate and polar regions

Biodiversity• A species–area curve illustrates the

correlation between the size of a community and the number of species found there

Biodiversity• Island biogeography states that the size

of the island and its closeness to the mainland (or source of dispersing species) are important variables directly correlated with species diversity