No Pigeon Is an Island

1. Nine species of large and small pigeons live in the rain forests of New Guinea, each with its own role in the forest.



2. But the trees, insects, decomposers--every living thing--are also interacting directly or indirectly with their neighbors.

47.1   Which Factors Shape Community Structure?

1. A habitat is a place where an organism lives; it is characterized by distinctive physical features, vegetation, and the array of species living in it.
   
   
   1. A community is an association of interacting populations of different species living in a particular habitat.
   
   
   
   2. Five factors shape the structure of the community:
      
      
      1. Interactions between climate and topography dictate rainfall, temperature, soil composition, and so on.
      
      
      
      2. Availability of food and resources affects inhabitants.
      
      
      
      3. Adaptive traits enable individuals to exploit specific resources.
      
      
      
      4. Interactions of various kinds occur among the inhabitants; these include competition, predation, and mutualism.
      
      
      
      5. Physical disturbances, immigration, and episodes of extinction affect the habitat.
   
   
   3. Several community properties are the result of the factors above.
      
      
      1. Varying numbers of species are found in feeding levels from producers to consumers.
      
      
      
      2. Diversity tends to increase in tropical climates, creating species richness.
      
      
      
      3. Relative abundance refers to the number of individuals of each species; dispersion describes how the individuals are dispersed through the habitat.


2. The Niche
   
   
   1. The niche of each species is defined by the sum of activities and relationships in which it engages to secure and use the resources necessary for its survival and reproduction.
   
   
   
   2. The potential niche is the one that could prevail in the absence of competition; the realized niche results from shifts in large and small ways over time as individuals of the species respond to a mosaic of changes.


3. 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:
      
      
      1. Neutral relationship: neither species directly affects the other (example: eagles and grass).
      
      
      
      2. Commensalism: one species benefits and the other is not affected (example: bird's nest in tree).
      
      
      
      3. Mutualism: there is a symbiotic relationship where both species benefit.
      
      
      
      4. Interspecific competition: both species are harmed by the interaction.
      
      
      
      5. Predation and parasitism: one species (predator or parasite) benefits while the other (prey or host) is harmed.

47.2   Mutualism

1. The yucca moth feeds only on the yucca plant, which is completely dependent on the moth for pollination--classic example of mutualism.



2. 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.

47.3   Competitive Interactions

1. There are two major 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:
      
      
      1. In exploitation competition, all individuals have equal access to a resource but differ in their ability (speed or efficiency) to exploit that resource.
      
      
      
      2. In interference competition, some individuals limit others' access to the resource.


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


3. Resource Partitioning
   
   
   1. Similar species share the same resource in different ways.
   
   
   
   2. Resource partitioning arises in two ways:
      
      
      1. Ecological differences between established and competing populations may increase through natural selection.
      
      
      
      2. Only species that are dissimilar from established ones can succeed in joining an existing community.

47.4   Predator-Prey Interactions

1. Coevolution of Predators and Prey
   
   
   1. Predators get their food from prey, but they do not take up residence on or in the prey.
   
   
   
   2. Many of the adaptations of predators and their victims arose through coevolution.


2. Models for Predator-Prey Interactions
   
   
   1. By the type I model, each individual predator will consume a constant number of prey individuals over time, regardless of prey abundance.
   
   
   
   2. In the type II model, the consumption of prey by each predator incereases, but not as fast as increases in prey density.
   
   
   
   3. By the type III model, a predator response is lowest when prey density is at its lowest level and predation pressure lessens.


3. The Canadian Lynx and Snowshoe Hare
   
   
   1. Stable coexistence results when predators prevent prey from overshooting the carrying capacity.
   
   
   
   2. 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.

47.5   An Evolutionary Arms Race

1. Camouflage is any adaptation in form, color, patterning, or behavior that allows a prey or predator to blend with its surroundings.



2. Warning coloration in toxic prey offer bright colors or bold patterns that serve as a warning to predators.



3. In mimicry, prey not equipped with defenses may escape predators by resembling toxic prey.



4. Moment-of-truth defenses allow prey animals defend themselves by startling or intimidating the predator with display behavior.



5. Predator responses to prey are adaptations used by predators to counter prey defenses.

47.6   Parasite-Host Interactions

1. Evolution of Parasitism
   
   
   1. Natural selection tends to favor parasite and host adaptations that promote some level of mutual tolerance and less-than-lethal effects.
   
   
   
   2. Usually death results only when a parasite attacks a novel host or when the number of parasites overwhelm the host's defenses.


2. Kinds of Parasites
   
   
   1. Parasites may live on the surface of the host (ectoparasites) or within the host's body (endoparasites).
   
   
   
   2. Microparasites include bacteria, viruses, and protistans; macroparasites include flatworms, nematodes, and small arthropods.
   
   
   
   3. Social parasites depend on the social behavior of another to complete the life cycle; for example, cowbirds lay their eggs in the nest of other birds, which unknowingly incubate and hatch the cowbirds' eggs.


3. Regarding the Parasitoids
   
   
   1. Parasitoids are a type of insect larvae that kill other insect larvae by feeding on their tissues.
   
   
   
   2. This provides natural control of insect populations.


4. Parasites as Biological Control Agents
   
   
   1. Parasites and parasitoids have five attributes that make them good control agents:
      
      
      1. They are well adapted to the host species and their habitat.
      
      
      
      2. They are exceptionally good at searching for hosts.
      
      
      
      3. Their growth rate is high relative to that of the host species.
      
      
      
      4. They are mobile enough for adequate dispersal.
      
      
      
      5. The lag time between responses to changes in the numbers of the host population is minimal.
   
   
   2. 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.

47.7   Forces Contributing to Community Stability

1. A Successional Model
   
   
   1. Ecological succession is the predictable developmental sequence of species in a community.
      
      
      1. Pioneer species are the first to colonize an area, followed by more competitive species.
      
      
      
      2. 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.
      
      
      1. Pioneer species help to improve soil fertility; they are usually small, low-growing plants with a short life cycle and an abundance of seeds.
      
      
      
      2. Gradually other, usually larger, species join or replace the pioneer species.
   
   
   3. In secondary succession, a community reestablishes itself to a climax state after a disturbance that allows sunlight to penetrate.


2. The Climax-Pattern Model
   
   
   1. It was once thought that the same general type of community would always develop in a given region because of constraints imposed by climate.
   
   
   
   2. According to the climax-pattern model, a community is adapted to a total pattern of environmental factors--climate, soil, topography, wind, fires, etc.--to create a continuum of climax stages of succession.


3. Cyclic, Nondirectional 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.


4. Restoration Ecology
   
   
   1. Natural restoration of the climax community takes a long time.
   
   
   
   2. Active restoration involves human intervention to speed the re-establishment of a damaged ecosystem.

47.8   Factors Contributing to Community Instability

1. How Keystone Species Tip the Balance
   
   
   1. A keystone species is a dominant species that can dictate community structure.
   
   
   
   2. For example, when sea stars (keystone predator on mussels) were removed from a habitat, mussels increased in number and in turn preyed on enough other species to reduce the community from 15 to 8.


2. How Species Introductions Tip the Balance
   
   
   1. Geographic dispersal of species can occur in three ways:
      
      
      1. A population might expand its home range by slowly moving into outlying regions that prove hospitable.
      
      
      
      2. During the course of a lifetime, individuals may be rapidly transported across great distances (jump dispersal), as in bilge water of large ships.
      
      
      
      3. A population may move out from its home range over geologic time, as by continental drift.
   
   
   2. Some introduced species have proved beneficial: soybeans, rice, wheat, corn and potatoes; others are notoriously bad: water hyacinth, kudzu, rabbits in Australia, gypsy moths, zebra mussels, and Africanized bees.

47.9   Focus on the Environment: Exotic and Endangered Species

47.10   Patterns of Biodiversity

1. What Causes Mainland and Marine Patterns?
   
   
   1. The number of species increases from the Arctic regions to the temperate zone to the tropics.
   
   
   
   2. Diversity is favored in the tropics for three reasons:
      
      
      1. More rainfall and sunlight provides more food reserves.
      
      
      
      2. Species diversity is self-reinforcing from herbivores to predators and parasites.
      
      
      
      3. Traditionally, the rate of speciation has exceeded the rate of extinction.


2. What Causes Island Patterns?
   
   
   1. Islands distant from source areas receive fewer colonizing species (distance effect).
   
   
   
   2. Larger islands tend to support more species (area effect).
   
   
   
   3. Species numbers increase on new islands and reach a stable number that is a balance between immigration rate for species new to the island and the extinction rate for established species.

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