Community Ecology

Chapter 15

Community ecology

• A community is

• Interspecific interactions

– classified according to effect on population concerned

Interspecific interactions• Competition: -/-

• Mutualism: +/+

• Predation/herbivory: +/-

• Parasite/pathogen: +/-

Each organism has a niche

• Ecological niche: a multidimensional description of a species; its way of living

• The more similar 2 niches are, the more intense the competition for resources

When niches overlap• Competitive exclusion

• OR• Resource partitioning

• Both of these result in a reduction or removal of competition

• Competition seems to be indirect, not head-to-head; both species trying for same resource, one is just a bit better at getting it

When niches overlap

Key properties to a community

• Prevalent vegetation• Species diversity• Response to disturbance

Vegetation & species diversity

• The more diverse the vegetation, the more diverse the population of animal species

• The more diverse the community, the more resistant it is to the negative effect of pathogens

Diversity defined by 2 components

• Species richness

• Relative abundance

Species richness & relative abundance together

determine community diversity

Keystone species impact diversity• A keystone species greatly

influences which other species are present

• If a keystone species is removed from community, the composition changes drastically

Community response to disturbance

• What is a disturbance?

• Not always negative; small scale disturbances can contribute to species diversity

• Succession is the change in species composition over time following a disturbance

Primary succession• Starts in lifeless area without even soil

– 1st life forms are autotrophic bacteria– photosynthetic mosses & lichen– grasses & shrubs– small trees– larger, longer-living species that eventually

become prevalent vegetation• Primary succession can take 100s or 1000s

of years

Primary succession

Primary succession

Secondary succession

• Occurs when disturbance destroyed existing community but left soil intact

• 1st grasses & shrubs followed by larger vegetation. . . – because animals depend on plants, animal

community goes through succession too; ex. diversity of birds increases as trees replace shrubs

• Secondary succession can happen in decades or even a few years

Secondary succession

More about succession• Succession can lead to a climax community

– the specific species in a climax community is dependent on the temperature & rainfall

• The most diverse communities are at some intermittent step of succession

Summary• Community ecology is the study of

interspecies interactions• Interactions can be classified as beneficial or

harmful• Interactions shape the community structure• The presence or absence of some species

play a larger role in the community structure than others (keystone species)

• How communities respond to disturbance help define the community

Predation• An interaction where one species eats

another, including herbivory– the predator benefits; the prey or plant don’t

• One of the most important forces shaping the composition & abundance of species in a community

• Defenses against predation– physical– behavioral

Physical prey defenses• Mechanical defenses

– spines, quills, armor, claws, etc.

• Chemical toxins– make prey poisonous or

unpalatable– used by many plants;

can be harmful to one species and harmless to another

– some animals also use this defense

Physical prey defenses, cont.

• Warning coloration– danger! poisonous!

& mimics

• Camouflage– patterns of

coloration that allow an organism to blend into its environment

Behavioral prey defenses• Hiding or escaping

– requires vigilance to avoid predators– safety in numbers

• Alarm calling– many species, especially birds &

mammals, warn others of danger• Fighting back

– some species mob the predator– spitting, vomiting, eliminating waste,

dropping tails, eviscerating, etc.

Parasitism• Parasitism: a symbiotic relationship where one

species benefits (the parasite) & one is harmed (the host)

• Two types of parasites– ectoparasites live outside the host’s body

• lice, leeches, ticks, fleas– endoparasites live inside the host’s body

• tapeworms, roundworms, Plasmodium

• Different from other forms of predation – the parasite is much smaller then the host – parasites stay in contact with the host for long periods

of time, weakening the host as it uses the host’s resources

Positive interspecies interactions

• Mutualism: everyone benefits– both species gain & neither is

harmed– common in virtually every

community• honeybees & flowers; cleaner fish &

the “dirty” fish

• Commensalism: one species benefits & the other neither benefits or is harmed– cattle egrets & livestock