2.3. Community Ecology

Community ecology is the study of co-existing, interdependent populations (of different species - Figure 1). In many cases relatively few species exert a major controlling influence on the entire community. Interactions among populations can be complex and may be sensitive to disturbance or fluctuations. Control of one component of the community can have a positive or negative impact on other organisms in the community. The major interactions within the community are competition, parasitism, predation, mutualism and commensalism.     

2.3.1. Competition

Competition between organisms is for limited supplies of essential resources, such as food in the case of animals, and water, nutrients and light in the case of plants. There are two types of competition: interspecific and intraspecific. The competition between individuals of the same species is intraspecific competition. Intraspecific competition can be of contest type (some individuals survive at the expense of others) or scramble (all individuals obtain insufficient resources) type. On the other hand, the competition between individuals of different species is interspecific competition. In such a case, one species is likely to be superior to the other in a given habitat.  

2.3.2. Predation

Predation is the consumption of one organism by another where the consumed organism (prey) was alive when the predator first attacked it. Predatory behavior is widespread among insects, spiders and mites. More than 40 families of insects, all 60 families of spiders, and 27 families of mites are predators.

2.3.3. Parasitism

Parasitism is a relationship between two species in which the host is harmed, but not killed immediately, and the species feeding on it (parasite) is benefited. A parasite is an organism that obtains its organic nutrients from one or very few host individuals without causing immediate death. A parasitoid is an insect that parasitizes and kills other insects. Parasitoids are parasitic only in their immature stages, killing the host before emerging as a mature larva or adult. However, parasitoids are often referred to as insect parasites. Members of 43 families of the order Hymenoptera and some members of 12 families of the order Diptera are parasitoids of arthropods. Parasitoids that insert their eggs into a host’s body are called endoparasitoids, and those that lay their eggs externally and whose larva develops externally are called ectoparasitoids. Parasitoids of non-parasitoid hosts are primary parasitoids and parasitoids that attack other species of parasitoid are hyperparasitoids.    

2.3.4. Mutualism

Mutualism is the relationship between two species that have developed a positive, reciprocal dependency, and both populations benefit from this association. Through the relationship, both the species strengthen their chances for survival, fitness, or growth. Mutualism is widespread.. The relationship between certain species of ants and mealybugs provides an example of mutualism. Mealybugs produce a waste material, known as honeydew, consisting of highly concentrated plant sugars. Ants harvest this material and in the process protect mealybugs from natural enemies.  Mealybugs are an important pest of pineapple.  Commercial pineapple plantations in many parts of the world find it advantageous to control ants, so that natural enemies are able to keep mealybug populations in check (Jahn and Beardsley 1996, 2000; Rohrbach et al. 1988).    

2.3.5. Commensalism  

Commensalism is the association between two species in which one species benefits from the association, and the other is unaffected. An example is algae growing on a turtle's shell that benefits from the substrate provided but it does not apparently harm or benefit the turtle.