Interacting species have a tremendous influence on the size of each other's populations. The various mechanisms for these biotic influences are quite different from the way in which abiotic factors effect the size of populations. Biotic factors also regulate the size of populations more intensely. Finally, the influence of biotic interactions can occur at two different levels. Interspecific effects are direct interactions between species, and the intraspecific effects represent interactions of individuals within a single species.
Neutralism is the most common type of interspecific interaction. Neither population directly affects the other. What interactions occur are slight and indirect. The simple presence of the two species should not directly affect the population level of either. An example of neutralism would be the interaction between rainbow trout and dandelions living in a mountain valley.
When two or more organisms in the same community seek the same resource (e.g., food, water, nesting space, ground space), which is in limiting supply to the individuals seeking it, they compete with one another. If the competition is among members of the same species, it is calledintraspecific. Competition among individuals of different species it is referred to as interspecific competition. Individuals in populations experience both types of competition to a greater or lesser degree.
Competition may be the result of two different processes: exploitation or interference. Competition by exploitation occurs between individuals when the indirect effects of two or more species or individuals reduce the supply of the limiting resource or resources needed for survival. The exclusion of one organism by another can only occur when the dominant organism requires less of the limiting resource to survive. Further, the dominant species must be able to reduce the quantity of the resource to some critical level with respect to the other organism. Resource exploitation, however, does not always cause the exclusion of a species from a community. It may just cause the species involved in this interaction to experience a reduction in their potential growth.
Competition by interference occurs when an individual directly prevents the physical establishment of another individual in a portion of a habitat. Established plants can preempt the invasion and colonization of other individuals by way of dense root mats, peat and litter accumulation, and mechanical abrasion.
Amensalism is an interaction where one species suffers and the other interacting species experiences no effect. One particular form of amensalism is allelopathy which occurs with plants. Allelopathy involves the production and release of chemical substances by one species that inhibit the growth of another. Allelopathic substances range from acids to bases to simple organic compounds. All of these substances are known under the general term: secondary substances.
Secondary substances are chemicals produced by plants that seem to have no direct use in metabolism. A good example of a secondary substance is the antibiotic juglone which is secreted by Black Walnut (Juglans nigra) trees. This substance is known to inhibit the growth of trees, shrubs, grasses, and herbs found growing near Black Walnut trees. In the chaparral vegetation of California, certain species of shrubs, notably Salvia leucophylla (mint) and Artemisia californica (sagebrush) are known to produce allelopathic substances. Often these chemicals accumulate in the soil during the dry season reducing the germination and growth of grasses and herbs in an area up to 1 to 2 meters from the secreting plants.
Mutualism is the name given to associations between pairs of species that bring mutual benefit. The individuals in the populations of each mutualist species grow and/or survive and/or reproduce at a higher rate when in the presence of individuals of the other species. In most ecology or biogeography textbooks mutualisms are generally underemphasized or ignored. Yet this type of interaction is an extremely widespread phenomena. For example, most rooting plants have mutualistic associations with fungal mycorrhizae. Mycorrhizae increase the capability of plant roots to absorb nutrients like nitrogen and phosphorus. In return, the roots of the host provide support and a constant supply of carbohydrates for consumption.
Mutualistic interactions between species can be of two types: symbiotic or nonsymbiotic. In a symbiotic mutualism, individuals interact physically and their relationship is biologically essential for survival. At least one member of the pair cannot live without close contact with the other. For example, the fungal-algal symbiosis that occurs in lichens. The morphological structure of a lichen is a mass of fungal hyphae that forms around a small colony of algae cells. In this mutualism, the alga produces carbohydrates and other food by products through photosynthesisand metabolism, while the fungus absorbs the required minerals and water to allow for these processes to occur.
More common in nature is the nonsymbiotic mutualism. In this interaction, the mutualists live independent lives yet cannot survive without each other. The most obvious example of an interaction of this type is the relationship between flowering plants and their insect pollinators (Figure 9f-1).
Figure 9f-1: Bees and many species of flowering plants interact with each other in a mutualistic fashion. In this interaction, the flower becomes pollinated by the insect, while the bee receives food in the form of pollen and nectar.
Predation, Parasitism, and Pathogens
Pathogens, parasites, and predators obtain food at the expense of their hosts and prey. These processes are basic to the entire grazing food chain above the autotroph level. Predators tend to be larger than their prey and consume them from the outside (Figure 9f-2). A parasite or pathogen is smaller than its host and consumes it either from the inside or from the outside of the organism.
Figure 9f-2: The tiger (Panthera tigris) hunts at night preying on a variety of animals, including deer, wild hog, and wild cattle. Tigers are ambush predators that try to approach their prey as closely as possible. They often attack their prey from behind, biting its neck or throat in the capture process. (Source: Wikipedia)
It is easy to believe that the predator-prey interaction is somehow detrimental to the prey population. This idea has led to extensive efforts to control predator populations in the name of wildlife conservation. However, functional relationships between predator-prey between species, within natural ecosystems, have coevolved over long periods time creating a dynamic balance between their interacting populations. Thus, the population sizes of predator and prey species are interregulated by delicate feedback mechanisms that control the densities of both species.
A classic example of the balance between predator and prey involves the prickly pear cactus, Opuntia spp. In the 19th century, prickly pear cactus was introduced into Australia from South America. Because no Australian predator species existed to control the population size of this cactus, it quickly expanded throughout millions of acres of grazing land. The presence of the prickly pear cactus excluded cattle and sheep from grazing vegetation and caused a substantial economic hardship to farmers. A method of control of the prickly pear cactus was initiated with the introduction of Cactoblastis cactorum, a cactus eating moth from Argentina, in 1925. By 1930, densities of the prickly pear cactus were significantly reduced.
Some times predator species can drive their prey into localized extinction. In complex communities, this does no particular harm to the predator if several other species exist as alternative prey.
Pidwirny, M. (2006). "Biotic Interactions and the Distribution of Species". Fundamentals of Physical Geography, 2nd Edition. 11/1/2012. http://www.physicalgeography.net/fundamentals/9f.html
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