I. Which Factors Shape Community Structure?
A. A community is an association of interacting populations of different species living in a particular habitat.

1. A habitat is a place where an organism lives; it is characterized by distinctive physical features and vegetation; the following factors affect the habitat:
a. Interactions between climate and topography dictate rainfall, temperature, soil composition, and so on.
b. Availability of food and resources affects inhabitants.
c. Adaptive traits enable individuals to exploit specific resources.
d. Interactions of various kinds occur among the inhabitants; including competition, predation, and mutualism.
e. Physical disturbances, immigration, and episodes of extinction affect the habitat.

2. Several community properties are the result of the factors above.
a. Varying numbers of species are found in feeding levels from producers to consumers.
b. Diversity tends to increase in tropical climates, creating species richness.

B. The Niche
1. The niche of each species is defined by the sum of activities and relationships in which its engages to secure and use the resources necessary for its survival and reproduction.

2. The niche can also be thought of as the "role" each species plays in the habitat.

C. Categories of Species Interactions

1. Interactions can occur between any two species in a community and between entire communities.

2. There are several types of species interactions:
a. Neutral: neither species directly affects the other (example: eagles and grass).
b. Commensalism: one species benefits and the other is not affected (example: bird's nest in tree).
c. Mutualism: a relationship where both species benefit.
d. Interspecific competition: both species are harmed by the interaction.
e. Predation and parasitism: one species (predator or parasite) benefits while the other (prey or host) is harmed.

II. Mutualism

A. The yucca moth feeds only on the yucca plant, which is completely dependent on the moth for pollination—classic example of mutualism that is obligatory.

B. This example is a form of symbiosis which implies an intimate and rather permanent interdependence of the two species on one another for survival and reproduction.

III. Competitive Interactions

A. There are several categories of competition.
1. Competition within a population of the same species (intraspecific) is usually fierce and may result in depletion of a resource.

2. Interspecific competition is less intense because requirements are less similar between the competitors.

3. There are two types of competitive interactions regardless of whether they are inter- or intraspecific:
a. In exploitation competition, all individuals have equal access to a resource but differ in their ability (speed or efficiency) to exploit that resource.
b. In interference competition, some individuals limit others' access to the resource.

B. Competitive Exclusion

1. Competitive exclusion suggests that complete competitors cannot coexist indefinitely.
a. When competitors' niches do not overlap as much, the coexistence is more probable.
b. Differences in adaptive traits will give certain species the competitive edge.

2. A keystone species is a dominant one that dictates community structure; for example: sea stars control the abundance of mussels, limpets, chitons, and barnacles.

C. Resource Partitioning

1. Def: Subdividing of some category of similar resources that lets competing species coexist.
a. Similar species share the same resource in different ways.

2. Resource partitioning arises in two ways:
a. Ecological differences between established and competing populations may increase through natural selection.
b. Only species that are dissimilar from established ones can succeed in joining an existing community.

IV. Predation and Parasitism

A. "Predator" Versus "Parasite"

1. Predators get their food from prey, but they do not take up residence on or in the prey.

2. Parasites get their food from hosts, and they live on or in the host for a good part of their life cycle; they may or may not kill the host (usually not).

B. Dynamics of Predator-Prey Interactions

1. The dynamics, ranging from stable coexistence to recurring cycles, depend on:
a. the carrying capacity of prey population in the absence of predation,
b. the reproductive rates of the prey and predator,
c. the behavioral capacity of the individual predators to respond to prey density.

2. Stable coexistence results when predators prevent prey from overshooting the carrying capacity.

3. Fluctuations in population density tend to occur when predators do not reproduce as fast as their prey, when they can eat only so many prey, and when carrying capacity for prey is high.

C. Dynamics of Parasite-Host Interactions

1. True parasites live in or on a host organism and gain nourishment by tapping into its tissues.
a. Parasites and hosts tend to survive together; usually parasites only kill hosts without coevolved defenses.
b. Ectoparasites live on a host's surface; endoparasites live inside a host's body.
c. Microparasites include bacteria, viruses, and protozoans; macroparasites include flatworms. roundworms, and small arthropods.

2. Social parasites complete their life cycle by drawing on social behaviors of another species; for example the cowbird never builds its own nest but gets other birds to incubate its eggs.

3. Parasites have five attributes that make them good biocontrol agents:
a. They are well adapted to the host species and their habitat.
b. They are exceptionally good at searching for hosts.
c. Their growth rate is high relative to that of the host species.
d. They are mobile enough for adequate dispersal.
e. The lag time between responses to changes in the numbers of the host population is minimal.

4. Care must be taken in releasing more that one kind of control agent in a given area due to the possibility of triggering competition among them and lessening their overall level of effectiveness.

V. Coevolutionary Arms Race

A. Prey species use creative defenses to avoid predation.

1. Camouflage: adaptations in physical form, patterning, color an behavior that blend with the surroundings and avoid detection.

2. Warning coloration: color and patterning of toxic/dangerous organsims that potential predators quickly learn to recognize and avoid.

3. Mimicry: evolution of physical or behavioral similarity to a toxic/dangerous organism by a harmless one.

4. Moment-of-truth defenses: last resort behaviors or chemical defenses used by prey.

B. Predator Responses to Prey

1. Predators counter prey defenses with their own adaptations.
a. Stealth: behavioral adaptation- "sneaking up" or lying motionless to capture prey.
b. Camouflage: same as above, however in the predator's favor.
c. Defense avoidance: adaptations to avoid prey defenses- may be behavioral or anatomical.

VI. Forces Contributing to Community Stability

A. A Successional Model

1. Ecological succession is the predictable development of species in a community.
a. Pioneer species are the first to colonize an area, followed by more competitive species.
b. A climax community is the most persistent array of species that results after some lapse of time.

2. Primary succession happens in an area that was devoid of life.

3. In secondary succession, a community reestablishes itself to a climax state after a disturbance.

B. Cyclic Changes

1. Community stability may require episodes of instability that permit cyclic replacement of equilibrium species, thus maintaining the climax community.

2. A good example are the necessary fires in the forests of California that rid the areas of underbrush.

C. Restoration Ecology

1. Natural restoration during secondary succession is a slow process.

2. In active restoration humans take action to speedup the re-establishment process.

VII. Community Instability

A. Over the short-term, disturbances can hamper the growth of some populations, and long-term changes in climate or other environmental variables may have destabilizing effects.

1. Over several generations, a population may expand its home range by gradually diffusing into hospitable outlying regions.

2. During the course of a lifetime, individuals may be rapidly transported across great distances (jump dispersal), as in bilge water of large ships.
a. Some introduced species have proved beneficial: soybeans, rice, wheat, corn and potatoes.
b. Others are notoriously bad: water hyacinth, kudzu, rabbits in Australia, gypsy moths, zebra mussels, and Africanized bees.