ECO - Community Ecology [LESSON]

Community Ecology

Populations are individual species, but all the interacting organisms, of all the different populations that live in a particular area make up the community. Community interactions, such as competition, predation, and various forms of symbiosis, can powerfully affect an ecosystem.

Let's begin with a video Review of Community Ecology.

Competition

Competition occurs when the organisms of the same or different species attempt to use an ecological resource in the same place at the same time. For example, two different species of mountain lions may compete for the same prey, or weeds and grass may compete for the same nutrients available in the soil. Regardless of the species involved, both organisms are negatively impacted. In some cases, one species may have a competitive advantage over the other species, and may over time result in the complete elimination of the species. This is referred to as competitive exclusion.

Another way to respond to competition between species is to separate resources in a niche. An ecological niche is the sum of a species' use of biotic and abiotic resources in its environment. This is different from where the organism lives, that is the habitat. A niche is more so how the organism specifically interacts or fits in within the environment. This specifically includes the range of conditions that the species can tolerate, the methods by which it obtains needed resources, the number of offspring it has, its time of reproduction, and all its other interaction with its environment.

Two species cannot coexist permanently in a community if their niches are identical. If competitive exclusion does not lead to the extinction of the other organism, the niche of one species becomes modified. This leads to the development of the fundamental niche and the realized niche. The fundamental niche is the range of conditions that a species can potentially tolerate and the range of resources it can potentially use. The realized niche of a species is the range of resources it actually uses for that particular environment. Generalists are species with broad niches; they can tolerate a range of conditions and use a variety of resources. Opossums are generalists. Species that have narrow niches, such as the koala, are called specialists.

Predation

Predation is an interaction in which one organism captures and feeds on another organism. The organism doing the capturing is called the predator and the organism being captured is called the prey. This is probably an obvious review, so we will go into a little more depth as to why this is important to the study of ecology.

The predator prey relationship is one of the primary influences of animal adaptations. Most predators have adaptations such as claws, teeth, fangs, stingers, or poison to help them capture and subdue their prey. Additionally, predators have acute sensory adaptations which allow them to locate and identify potential prey. Likewise, prey have adaptations that help them avoid attack and capture. While some defensive adaptations include behavioral patterns, others include morphological and physiological defense mechanisms. Review the interactivity below to review some of these adaptations.

Click through and read about the Review of Community Ecology below. 

Symbiosis

Symbiosis is any relationship where two or more species live in direct and intimate contact. There are three categories of symbiosis, mutualism, commensalism, and parasitism. Mutualism is a symbiotic relationship in which both species benefit. One example we discussed previously was that of lichen. Both the algae and the fungi work together in a mutually beneficial relationship. Commensalism is an interaction between species in which one benefits and the other is not affected. Parasitism is a symbiotic interaction in which one organism benefits at the expense of the other organism. The benefiting organism is called the parasite and the organism harmed is called the host.

Determine the type of symbiotic relationship that exists between organisms described below in the Symbiotic Relationships activity below.

Competitive Exclusion Principle

The competitive exclusion principle tells us that two species can't have exactly the same niche in a habitat and stably coexist. That's because species with identical niches also have identical needs, which means they would compete for precisely the same resources.

The figure above shows two types of Paramecium (Kingdom Protista). When grown individually, both species do very well. However, when they are grown together, both grow more poorly and P. aurelia eventually outcompetes P. caudatum.

Since no two species can occupy the same niche according to the competitive exclusion principle, the options are extinction from that area or resource/habitat partitioning. Competitive exclusion may be avoided if one or both of the competing species evolves to use a different resource, occupy a different area of the habitat, or feed during a different time of day. The result of this kind of evolution is that two similar species use largely non-overlapping resources and thus have different niches. This is called resource partitioning, and it helps the species coexist because there is less direct competition between them.

Predator-Prey Cycles

A predator is an organism that consumes all or part of the body of another—living or recently killed—organism, which is its prey. If a lion eats a zebra, the lion is the predator and the zebra is the prey. However, prey species can also be fungi or plants.  A cow eating the grass is also an example of a predator-prey relationship called herbivory.

Populations of predators and prey in a community are not always constant over time. Instead, in many cases, they vary in cycles that appear to be related. The most frequently cited example of predator-prey dynamics is seen in the cycling of the lynx, a predator, and the snowshoe hare, its prey.  The population cycles of lynx and hare repeat themselves approximately every 10 years, with the lynx population lagging one to two years behind the hare population. The classic explanation is this: As hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well. When the lynx population grows to a threshold level, however, it kills so many hares that the hare population begins to decline. This is followed by a decline in the lynx population due to scarcity of food. When the lynx population is low, the hare population begins to increase, and so on. Recent studies suggest that these patterns are not all attributed to predator-prey relationships, but we can still assume that most of it is.

Try the Community review below to check your knowledge.

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