Predation Reduce Competition
When resources are limiting, a superior competitor can eliminate other species from a community. However, predators can prevent or greatly reduce competitive exclusion by reducing the numbers of individuals of competiting species. a given predator may often feed on two, three, or more kinds of plants or animals in a given community. the predator's choice depends partly on the relative abundance of the prey options. in other words, a predator may feed on species A when it is abundant and then switch to species B when A is rare. Similarly, a given prey species may be a primary sources of food for increasing numbers of species aas it becomes more abundant. in this way, superior competitors may be prevented from outcompeting other species.
Such patterns are often characteristic of biological communities in marine intertidal habitats, for example, in preying selectively on bivalves, sea stars prevent bivalves from monopolizing such habitas, opening up space for many other organism (See Picture 1). When sea stars are removed from a habitat, species diversity falls precipitously, the seafloor community coming to be dominated by a few species of bivalves. because predation tends to reduce competition in natural communities, it is usually a mistake to attempt to eliminate a major predator such as wolves or mountain lions from a community. the result is to decrease rather tahn increase the biological diversity of the community, the opposite of what is intended.
Parasitism May Counter Competition
Parasites may effect sympatric species differently and thus influences the outcome of interspecific interactions. in a classic experiment, Thomas Park of the University of Chicago investigated interactions between two flour beetles, Tribolium castaneum and T. confusum with a parasites Adelina. In the absence of the parasite, T. castaneum is dominant and T. confusum normally goes extinct. when the parasites is present, however, the outcome is reversed and T. castaneum perishes. similar effect of parasites in natural systems have been observed in many species. for example, in the Anolis lizard of St. Maarten mentioned previously, the competitively inferior species is resistant to malaria, whreas the other species is highly susceptible. Only in areas in which the malaria parasite occurs are the two species capable of coexisting
Indirect Effects
In some cases, species may not directly interact, yet the presence of one species may effect a second species by way of interactions with a third species. such effects are termed indirect effects. for example, in the Chihuahuan Desert, rodents and ants both eat seeds. Thus, one might expect tem to compete with each other. however, when all rodents were completely removed from large enclosure, ant populations first increase, but then declined. the initial increase was the expected result of removing a competitor; why did it reverse? the answer reveals the intricacies of natural ecosystems. rodents prefer large seeds, whereas ants prefers smaller seeds. further, in this system plants with large seeds are competitively superior to plants with small seed. Thus, the removal of rodents leads to an increase in the numberof small seeds available to ants, wich thus leads to a decline in ant populations. thus, the effects of rodents on ants is complicated; a direct negative effect of resource competition and an indirect, posituve effect mediated by plant competition.
Species that have particularly strong effects on the composistion of communities are termed keystone species. predators, such as the starfish, can often serve as keystone species by preventing one species from outcompeting others, thus maintaining high levels of species richness in a community.
There a re, however, a wide variety of other types of keystone species. some species manipulate the environment in ways that create new habitats for others species. Beavers, for example, change running streams into small impoundments, changing the flow of water and flooding areas
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