Chapter 8: Community Ecology

Eugene Han, Sylvia Titus, Riley Krutsch, and AJ Hauer

Case Study: Flying Foxes: Keystone Species in Tropical Forests

  • Durian- a putrid smelling fruit, delicacy in Southeast Asia

  • Flying foxes- various species of nectar, pollen, and fruit eating bats, pollinate flowers of durian trees

    • Endangered species due to deforestation, hunting for meat, and pest control

    • keystone species- a species that plays an important role in sustaining an environment, many other species rely on in to survive e.g. flying foxes in the tropical environment

  • Mutualism- an interaction between two species in which both benefit, e.g. flying foxes and durian trees

  • Niche- the role of a species in an environment

8-1: Community Structure and Species Diversity

  • 3 characteristics to describe a biological community

    • Physical appearance- the relative sizes, stratification, and distribution of its populations and species

    • Species diversity- combination of species richness(# of diff. species) and species evenness(# of individuals in each species)

    • Niche structure- the number of ecological niches, how they resemble or differ from each other, and how species interact with each other

  • Factors that affect species diversity- latitude: distance from equator, pollution: especially in aquatic ecosystems, habitat diversity, NPP, habitat disturbance, and time

  • Species equilibrium model or theory of island biogeography- claims that a balance between two factors(size and distance from the nearest mainland) determines the number of different species found on an island and the rate at which existing species become extinct on the island

Case Study: Why are amphibians vanishing? Warnings from Frogs

8-2: Types of Species

  • Native species- those that normally live and thrive in a particular community

  • Nonnative species or invasive species or alien species- species that have evolved somewhere else and migrated or are deliberately or accidentally introduced into a community(most don’t survive because they can’t find a niche in the new environment)

    • Beneficial- chickens, cattle, and fish, many domesticated animals and food crops

    • Detrimental- Killer bees, displaced bee population in Brazil, migrated upward throughout Central America and Southwestern United States

  • Indicator species- species that serve as early warnings of damage or danger to a community, e.g. trout need clean water and high DO, reflects water quality; canary in the mines

  • Disappearance of many amphibian species may indicate a global decline in environmental quality; good indicator species because of their eggs(no protective shell) and thing skin through which they take in water and air(susceptible to pollutants)

  • Factors that affect frogs and other amphibians- habitat loss and fragmentation, prolonged drought, dehydration, pollution, increase in UV radiation, increased flatworm parasitism, overhunting, introduction of nonnative predators, competitors, and disease organisms

  • When amphibian populations begin declining;

    • Suggests that environmental quality is declining worldwide

    • They play important ecological roles in biological communities; negative effects for other species in the food chain

    • Amphibians contain compounds that have potential as pharmaceutical products

  • Keystone species- plays a vital role in the ecosystem; if it is removed, the ecosystem will collapse, has a large effect on the species and community around it

  • Ecological roles that are often played by the keystone species: pollination(birds, bats, bees etc.) or top predators(wolf, leopard, lion, alligator)

  • Losing a keystone species can lead to population booms for some species and extinction for other species, a collapse of the ecosystem

  • Foundation species- play a major role in shaping communities by creating and enhancing habitat that benefits other species


8-3 Species Interactions: Competition and Predation

  • Intraspecific competition occurs when members of the same species compete for a limited resource.

  • Territoriality is a common type of intraspecific competition in which an organism claims the rights to an area and defends it against other members of the same species.

  • Interspecific competition occurs when members of different species compete for resources within an ecosystem.

  • Interference competition means that one organism is limiting another’s access to a valuable resource. Territoriality can be a type of interference competition if the territorial species also chases away members of other species that may want to use similar resources.

  • Exploitation competition means that two species are both free to try to acquire a resource, but one just does it a little better than the other.

  • Two species whose niches overlap too much by requiring the same resources cannot coexist indefinitely in the same ecosystem. One of the species will always be at least a little better than the other, and eventually eliminate the inferior species in that area. This process is called the competitive exclusion principle.

  • Only three outcomes are possible for “the lesser” of the competing species in an ecosystem: move, adapt, or die.

  • Predation occurs when members of one species (predator) feeds directly on members of another species (prey).

  • Predators that feed on mobile prey generally choose one of two strategies to capture their target.

    • Pursuit predators use physical adaptations such as the speed of the cheetah, the eyesight of the eagle, or the cooperative tactics of the wolf pack to find and capture prey.

    • Ambush predators use camouflage adaptations such as color and shape to blend in with the background. They can get very close to their prey without being seen before actually attacking them.

  • Prey species have evolved many different strategies to avoid the pursuit and ambush attacks of predators.

    • Poisonous animals often use bright colors such as orange, red, and yellow to advertise themselves. This is called warning coloration - the opposite of camouflage. Other species have evolved to look like these poisonous species even though they’re not. These are mimic species.

  • The chemicals used by prey to defend against predators are used by people for many purposes, including medicine (aspirin, morphine), recreational drugs (caffeine, nicotine, cocaine), and poisons (insecticides).


8-4: Symbiotic Species Interactions: Parasitism, Mutualism, and Commensalism

  • Symbiosis is a situation where two species have a very close relationship with each other. The end result for each of the species involved may be positive, negative, or neutral.
  • Parasitism is a type of symbiosis where one organism feeds off another by living either on it or in it. The parasite benefits, while the host is harmed.

    • The typical goal of a parasite is to draw nourishment from the host without killing it. The host organism may become weakened over time, but if it died, then the parasite would have to go to the trouble of finding a brand new host.

  • Endoparasites live inside their hosts. Examples include roundworms, tapeworms, and disease-causing bacteria and protists.

  • Ectoparasites live on their hosts. Examples include fleas, ticks, lice, and fungi like athlete’s foot and dermatophytes.

  • Mutualism is a type of symbiosis where both organisms have evolved to exploit the other so that each benefits. Mutualistic species are not “being nice to each other”. It has just worked out so that each uses the other in a positive way.

  • Pollination is a typical form of mutualism. The pollinating species, typically a bird, insect, or bat, gets a small amount of food from the flower in exchange for transporting the plant’s pollen from one individual to the next.

  • Commensalism is a type of symbiosis where one species benefits and the other is neither helped nor hurt. This would be like if you copied correct test answers from somebody and didn’t get caught.

    • For example, barnacles attach themselves to the skin of certain species of whales. They do not feed on the whale; they just use the animal as a ride around the ocean to access plankton.


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8-5: Ecological Succession: Communities in Transition

  • Ecological succession occurs when the species living in a particular area change according to changing environmental conditions. Succession can either be primary or secondary.

    • Primary succession is the establishment of a community starting from scratch on lifeless ground. This includes places like bare rock exposed from a retreating glacier, newly cooled lava from a volcano, or an abandoned parking lot.

      • Pioneer species are the first to grow in the initially tough conditions. These include mosses and lichens – extremely hardy plants that don’t require a lot of soil and moisture to grow, and whose seeds can get blown in by the wind.

      • As time goes by, the rock will start to be broken apart through weathering and erosion and very slowly, soil will begin to build up.

      • Early successional plants will then replace the pioneer species. These are mostly grasses and herbs that grow very close to the ground.

      • When enough soil is created (decades later), the early successional species will be replaced by midsuccessional plant species. These include more herbs, grasses, and shrubs, and eventually sun-loving tree species.

      • When these late successional species become established for a period of time, the process of succession is complete and the area is known as a mature or climax community.

      • The entire process of primary succession typically takes several centuries to complete, something like 500-700 years.

    • Secondary succession occurs when an already existing community is somehow disturbed or removed. The key is that at least some soil is left behind.

      • Secondary succession occurs after forest fires, in areas where the trees have been cut down, and on abandoned farmland.

      • Weeds and grasses grow first, leading to shrubs and bushes, then sun-loving trees and eventually shade-tolerant trees and a mature community.

      • As the plant species change through succession, the animal species do too, as they are dependent on the vegetation for their food and habitat.

  • A disturbance is any change in the environment that causes a disruption in an ecosystem. Disturbances can be natural (flood, hurricane, drought) or man-made (deforestation, construction, forest fire). They can be catastrophic (tornado, earthquake) or gradual (climate change, groundwater depletion).

  • The intermediate disturbance hypothesis states that communities with frequent moderate disturbances tend to have the greatest species diversity.

    • This is because ecosystems with many minor disturbances should have areas at all stages of succession. More stages of succession = higher biodiversity.

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8-6 Ecological Stability, Complexity, and Sustainability

  • Constant change in response to environmental changes helps living systems maintain stability and sustainability

  • Negative and positive feed back loops help alert organisms to what changes need to be made in order to survive in the changing environment

  • 3 aspects of stability and sustainability

    • Inertia or persistence- the ability of an ecosystem to resist disturbances

    • Resilience- the ability of an ecosystem to bounce back after a disturbance

    • Constancy- the tendency of the populations within the ecosystem to maintain steady numbers

  • Complexity-the number of species in a community(species richness) at each trophic level and the number of trophic levels in a community; a measure of community biodiversity

  • Scientists used to believe that the more diverse an ecosystem was, the more stable it was, but recent studies contradict this and are unsure what determines one ecosystem more stable as another

  • Precautionary principle- when there is evidence that a human activity can harm our health or bring about changes in environmental conditions that can affect our economies or quality of life, we should take measures to prevent harm even if some of the cause-and-effect relationships have not been fully established scientifically