52.1 Species Interactions

• The fitness of an individual is defined as its ability to produce viable, fertile offspring.

• The four common types of interactions:

  • Commensalism occurs when one species benefits but the other species is unaffected.

  • Competition occurs when individuals use the same resources-resulting in lower fitness for both.

  • Consumption (including herbivory, predation, and parasitism) occurs when one organism eats or absorbs nutrients from another.

    • The interaction increases the consumer’s fitness but decreases the victim’s fitness.

  • Mutualism occurs when two species interact in a way that confers fitness benefits to both.

• The competition that occurs between members of the same species is called intraspecific competition.

• Interspecific competition occurs when individuals from different species use the same resources.

  • Note that species may compete directly for the same resources, such as a lion fighting off a hyena to control access to a zebra carcass.

  • Species may also compete indirectly for the same resources, such as when a finch consumes all of the available seeds on a bush, leaving no seeds for species visiting the bush later.

• Early work on interspecific competition focused on the concept of the niche，the range of resources that the species is able to use, or the range of conditions it can tolerate.

  • A fundamental niche is the total theoretical range of environmental conditions that a species can tolerate.

  • A realized niche is the portion of the fundamental niche that a species actually occupies, given limiting factors such as competition with other species.

• An evolutionαry change in resource use, caused by competition over generations, is called niche differentiation or resource partitioning.

• The evolutionary change that occurs in species traits, and that enables species to exploit different resources, is called character displacement.

  • Character displacement makes niche differentiation possible.

• In agriculture and forestry, the use of predators, herbivores, and parasites as biocontrol agents is a key part of integrated pest management: strategies to maximize crop and forest productivity while using a minimum of insecticides or other types of potentially harmful compounds.

• When predators and prey, herbivores and plants, or parasites and hosts interact over time, a coevolutionary arms race often results-a repeating cycle of reciprocal adaptation.

52.2 Community Structure

• You might also be familiar with linking consumption interactions sequentially into a food chain such as “aspen is eaten by elk, and elk are eaten by wolves.”

• Overlapping food chains can then be combined into a food web, a summary of some or all of the consumption interactions in a community.

• A keystone species has a much greater impact on the distribution and abundance of the surrounding species than its abundance and total biomass would suggest.

52.3 Community Dynamics

• A disturbance is any strong, short-lived disruption to a community that changes the distribution of living and/or nonliving resources.

• The recovery that follows is called succession.

  • Primary succession occurs when a disturbance removes the soil and its organisms as well as organisms that live above the soil surface.

• Secondary succession occurs when a disturbance removes some or all of the organisms from an area but leaves the soil intact, including the seeds and microorganisms within.

  • Fire and logging are examples of disturbances that initiate secondary succession on land.

• Pioneering species tend to have “weedy” life histories.

  • In ecology, a weed is a plant that is adapted for growth in disturbed soils.