Population Interactions

No natural habitat exists that contains only a single species of plant or animal. Every species needs, at a bare minimum, some other species -- most commonly as a source of food. Even a plant species that is fully capable of photosynthesis cannot survive entirely alone, since it still depends on soil microbes to break down organic matter in the soil and release the inorganic nutrients the plant needs to grow.

Interactions between organisms can be classed as intraspecific, occurring between individuals of the same population, or interspecific, occurring between members of two different species (which may involve two plants, two animals, or one plant and one animal). Interspecific interactions can be beneficial, detrimental, or neutral -- neither harming nor benefiting -- to one or both of the species involved; Table 13.6 classifies the major categories of interspecific interaction. Neutralism describes an interaction with no significant effect on either species. Negative interactions include direct-interference competition, resource-use competition and amensalism, each harmful to at least one of the two species. Positive interactions include mutualism (symbiosis), commensalism and protocooperation, each beneficial to at least one species while harming neither. Finally, some interactions are simultaneously positive for one species and negative for the other …