Chapter 8: Community Ecology

By: Chandana, Eric, Beoung & Chaitanya

Section 8.1: Community Structure and Species Diversity


  • Species Equilibrium Model or Theory of Island Biogeography: The number of species found on an island is determined by a balance between two factors: the immigration rate (of species new to the island) from other inhabited areas and the extinction rate (of species established on the island). The model predicts that at some point the rates of immigration and extinction will reach an equilibrium point that determines the island's average number of different species (species diversity).

Main Ideas

  • Ecologists use 3 characteristics to describe a biological community.

    • Physical appearance: relative sizes, stratification and distribution of its populations and species. Large terrestrial community are patches of vegetation. Transition varies around the edges of community structure where one community makes a transition to a different type of community. Increased edge area from habitat fragmentation makes many species more vulnerable to stresses.

    • Species diversity: combination of a community’s number of different species and the abundance of individuals within each of its species. Latitude, pollution, habitat diversity, NPP, habitat disturbance and time can affect the species diversity in a community

    • Niche structure: number of ecological niches, how they resemble/differ from each other and how the species interact with one another.

  • Large islands tend to have more species of a certain category. To explain the differences in species richness with island size, species equilibrium model or the theory of island biogeography is used.

    • The model projects that at some point, the rates of immigration and extinction should reach equilibrium that determines the island’s average number of different species. Island’s size and distance from the mainland affect its immigration and extinction rates, thus its species diversity.

Section 8.2: Types of Species


  • Native species: Species that normally live and thrive in a particular ecosystem.

  • Non-native species: Species that migrate into an ecosystem or are deliberately or accidentally introduced into an ecosystem by humans.

  • Indicator species: Species that serve as early warnings that a community or ecosystem is being degraded.

  • Keystone species: Species that play roles affecting many other organisms in an ecosystem.

Main Ideas

  • Native plants are plants whose original home is this ecosystem, whereas non-native plants are invasive species that came from different ecosystems and migrated to this ecosystem

  • Indicator species alerts people if deleterious changes are occurring the ecosystem. An example of indicator species are birds because they are ubiquitous and are easily affected by an environmental change. Amphibians are also considered as indicator species. The disappearance of amphibians usually prognosticates that the ecosystem is not in a good shape.

  • Keystone species are species that help the environment run fluently; they also determine the number and the type of species in an ecosystem. Keystone species help with pollinating, and etc.

  • Foundation species are significant because they create and shape the environment. An example of foundation species are elephants; elephants break and uproot trees, thus they help create forest openings in the woodlands. Also, foundation species help increase the rate of nutrient cycling.

Section 8.3: Species Interactions-Competition and Predation


  • Interspecific Competition: Attempts by members of two or more species to use the same limited resources in an ecosystem.

  • Resource Partitioning: Process of dividing up resources in an ecosystem so that species with similar needs use the same scarce resources at different times, in different ways, or in different places.

  • Predation: Situation in which an organism of one species captures and feeds on parts or all of an organism of another species.

Main Ideas

  • The five basic species interactions are predation, parasitism, mutualism, competition, and commensalism.

  • Competition between species for food, sunlight, water, soil and etc. is known as interspecific competition.

  • In competitive situations, some species evolve and adapt to the environment to thrive. Over a long period of time, some species gain more specialized traits to permit them to use shared resources different times of the day.

  • Predator-prey relationships is when a species feeds on other species. This is beneficial because it gets rid of the weak, sick, and old species. Predators use various techniques such as camouflage to catch the prey, whereas the prey uses techniques such as fast movement and protective shells to survive.

Section 8.4: Species Interactions-Parasitism, Mutualism, and Commensalism


  • Parasitism: the relation between two different kinds of organisms in which one receives benefits from the other by causing damage to it.

  • Mutualism: the relation between two different species of organisms that are interdependent.

  • Commensalism: the relation between two different kinds of organisms when one receives benefits from the other without damaging it.

Main Ideas

  • Parasitism occurs when one species feeds on part of another organism. It can be viewed as a special form of predation. Parasites are harmful from the host’s point of view, but they play an important ecological role: they promote biodiversity.

  • Mutualism: two species interact where both benefit.

    • Pollination mutualism: between flowering plants and animals.

    • Nutritional mutualism: lichens grow on trees consist of algae and fungi, where the fungi provide a home for the algae, and their bodies collection moisture and minerals used by both species.

    • Gut inhabitant mutualism: armies of organisms live in an animal’s digestive tract.

  • Commensalism benefits one species while there is no effect on the other. Examples are redwood sorrel and epiphytes.

Section 8.5: Ecological Succession- Communities and Transition


  • Ecological Succession: the gradual and orderly process of change in an ecosystem brought about by the progressive replacement of one community by another until a stable climax is established.

  • Primary Succession: succession that occurs on surfaces where no soil exists.

  • Secondary Succession: succession on a site where an existing community has been disrupted.

  • Pioneer Species: first species to populate an area during primary succession.

  • Early Successional Plant Species: Plant species found in the early stages of succession that grow close to the ground, can establish large populations quickly under harsh conditions, and have short lives.

  • Midsuccessional Plant Species: Grasses and low shrubs that are less hardy than early successional plant species.

  • Late Successional Plant Species: Mostly trees that can tolerate shade and form a fairly stable complex forest community.

  • Disturbance: A discrete event that disrupts an ecosystem or community. Examples of natural disturbances include fires, hurricanes, tornadoes, droughts, and floods. Examples of human-caused disturbances include deforestation, overgrazing, and plowing.

Main Ideas

  • Community structures can change with new environmental conditions.

    • Ecological succession is a gradual change in species compositions

    • Primary ecological succession is the establishment of biotic communities on lifeless ground; it is a long process. Pioneer species tack to bare rock. Early successional plants are tiny annuals that reseed. Midsuccessional plants are low shrubs and tree that need soil and sunlight. Late successional plants are trees that create shade. Primary succession can also take place.

    • Secondary ecological succession is a series of communities with different species in places with soil or bottom sediment.

  • 3 factors affect how and the rate at which succession occurs: facilitation is an area suitable for second species by the first’s actions, inhabitation is that the early species delay establishment of later species, and tolerance is that later species are unaffected by plants at earlier stages.

  • Changes in environmental conditions can set back succession. Disturbances (fire, drought, mining) can set back successions to the beginning stages. Late catastrophic disturbances can devastate a community and create unfilled niches.

    • Intermediate hypothesis disturbance hypothesis says that moderate level can create space for colonizing species, and there is a chance of survival of some mature species.

  • Scientists cannot predict the succession in a community towards a stable community in balance with the environment. The equilibrium model is a balance of nature. Ecologists view the community as a continuous change.

    • Succession reflects a struggle for each species to attain food, light, nutrients and space to gain an advantage by occupying as much of its fundamental niches as possible. Biotic changes describe the changes in a succession. Mature community fits better to describe a stable community with a patch of vegetation.

Section 8.6: Ecological Stability, Complexity, and Sustainibility


  • Persistence: the ability of a living system to resist being disturbed or altered.

  • Constancy: the ability of a living system such as a population to keep its numbers within the limits imposed by available resources.

  • Resilience: the ability of a living system to repair damage after an external disturbance that is not too drastic.

  • Complexity: the number of species in a community at each trophic level and the number of trophic levels in a community.

  • Precautionary Principle: the principle that the introduction of a new product or process whose ultimate effects are disputed or unknown should be resisted.

Main Ideas

  • Living systems maintain some degree of stability or sustainability through constant change in response to changing environmental conditions.

  • There are three aspects of stability or sustainability in living system : inertia (persistence), constancy, and resilience.

  • Having many different species can provide some ecological stability or sustainability for communities, although this might not apply to every community.

  • We have to take precautionary measures to prevent serious harm.