CHAPTER 54: COMMUNITY ECOLOGY

What is community ecology?

Community ecology is the scientific study of interactions between populations of different species living close together and how these interactions affect the structure of the community.

General Review

Interspecific interactions — Key relationships between species and oraganisms; including predation, competition, herbivory, and symbiosis.

Interspecific competition — Occurs when individuals of different species compete for important resources. This has a negative impact on both sides.

Competitive exclusion — The elimination of an inferior competitor when two species compete for the same limited resource and one species has even a slight advantage.

Ecological niche — An organisms ecological role, how it survives, and the sum of its use of biotic and abiotic resources. A fundamental niche, is the potential and initial niche of an organism while a realized niche is the portion of its fundamental niche that an organism actually occupies.

Resource partitioning — The differentiation of niches (usually due to past competition) that enables similar species to coexist in an environment.

Character displacement — The tendency for characteristics to diverge more in sympatric (geographically overlapping) populations than in allopatric (geographically seperate) populations.

Predation — An interaction where the predator kills and eats the prey. Animals display defensive adaptations:

Cryptic coloration (camouflage)
Aposematic coloration (bright colors to warn predators of toxic chemicals in the prey)
Batesian mimicry (a harmless species mimicking a harmful or unpalatable one)
Müllerian mimicry (2 or more unpalatable species which resemble one another)

Herbivory — Interactions where organisms eat parts of plants or algae. Plants may use chemical toxins or thorns as defense.

Species diversity — Variety of different kinds of organisms which make up the community. Species richness describes the number of different species present, while relative abundance refers to the porportion of the total each species makes up. One widely used index to calculate diversity is the Shannon diversity.

Community organization models — A change at on end of the trophic pyramid will cause a change at the other end:

The bottom-up model describes a unidirectional influence over the higher trophic levels that the lower trophic levels have.
The top-down model describes a community where the predators limit the lower trophic levels.

Both communities A and B have equal species richness, but A has greater diversity.

Symbiosis

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

Parasitism — An interaction where the parasite derives its nourishment from and therefore harms its host. Endoparasites live within the body of their hosts wheras ectoparasties feed on the external surface of hosts.

Mutualism — An interation which benefits both species involved. When these species still have the ability to survive alone, it is called facultative mutualism. If at least one of the species can no longer survive without its partner, the relationship is called obligate mutualism.

Commensalism — An interaction where one species benefits whle the other is neither harmed nor benefited. This is hard to document in nature since close association between species nearly always results in at least a slight effect on both.

Notable Experiment

Ecologist Joseph Conell studied two barnacle species and found that interspecific competition makes the realized niche of the barnacles much smaller than their fundamental niches.

Dominant and Keystone Species

Dominant species — The species in a community that is most abundant or has the greatest biomass. A species might become dominant because it is competitively superior or is more successful at avoiding predators and disease (such as the case of invasive species).

Keystone species — Keystone species exert strong control over community structure not through abundance but instead through their important niches. Removal experiments can help identify keystone species.

Foundation species — Organisms which alter the environment physically with their behaviors or collective biomass. if these species have a positive impact on the biotic community, then they are known as facilitators.

Trophic structure

Trophic structure refers to the feeding relationships and classes between organisms. A food chain shows the general transfer of energy up the trophic levels. A food web shows specific feeding relationships (consisting of many food chains).

Food chains are usually fairly short, consisting of only a few links in length. There are two main hypotheses explaining why:

Energetic hypothesis — Suggests that the inefficient energy transfer of 10% from each trophic level is the limiting factor. This hypothesis is supported by most available data.

Dynamic stability hypothesis — Proposes that longer food chains are less stable. Environmental factors leading to population fluctuations at lower trophic levels would have an exceedingly great impact on top predators if the food chain is too long.

Notable Experiment

Robert Paine found that without the sea star Pisaster ochraceus, mussel species monopolized the area an eliminated most other life. Thus, Pisaster is a keystone species.

ECOLOGICAL SUCCESSION

Disturbances

A disturbance is a destructive event, such as a storm, fire, drought, or human activity which changes a community by removing organims or altering resoure availability. The nonequilibrium model describes how most communities are constantly changing due to disturbances.

Ecological succession — The process by which a disturbed area is colonized by species which are eventually replaced by other species. When succession initiates in a virtually lifeless are with no soil, it is called primary succession. On the other hand, secondary succession occurs where an existing community has been cleared but soil remains.

Early arrivals can either facilitate or inhibit the establishment of later species. If an early species neither helps nor hinders a later species, then the later species tolerates the previous conditions.

BIODIVERSITY AFFECTING FACTORS

Latitudinal Gradients

Plant and animal life is generally moar abundant and diverse in the tropics, gradually decreasing as latitude increases.

Evapotranspiration — The evaporation of water from soil in addition to the transporation of water from plants. It encompasses the factors of solar energy and water availability, which are correlated with biodiversity.

Pathogens

Pathogens are disease causing microorganisms. Recent studies have highlighted the role of pathogens in limiting both terrestirail and marine communities.

Zoonotic pathogens — Pathogens transfered to humans by other animals, directly or indirectly.

Vector — Animals which facilitate the spread of zoonotic diseases. Vectors are often parasites such as mosquitos.

Notable Experiment

Ecologists Robert MacArthur and E. O. Wilson studied the number of plant species on each of the Galápagos Islands, which greatly vary in size. They found that the a steady increase in species richness with island size.

Area Effects

The species-area curve states that when all other factors are equal, a larger geographic area results in a community with a greater number of species.

Island equilibrium model — A model where equilibrium is eventually reached on an island when the species immigration rate equals the extinction rate. Two physical features of an island affect immigration and extinction rates, as well as the final number of species:

Size: potential colonizers are more likely to reach larger islands, which also have more space and resources to sustain populations
Distance from shore: closer islands usually have higher immigration rates