Species interact in a variety of ways with the other species that co-occur at a site. These interactions produce combinations of positive, neutral, or negative effects for species pairs. However, other species may alter the nature or strength of particular pairwise interactions (e.g., predators can reduce the intensity of competition among prey species by maintaining their populations below levels that induce competition).

Some species compete for a shared resource, with the result that the per capita share of the resource is reduced. This interaction has negative effects on both species. Competition can be by exploitation, when all individuals have equal access to the resource, or interference, when individuals of one species preempt use of, or defend, the resource. In cases of asymmetrical competition, the superior competitor can exclude inferior competitors over a period of time (competitive exclusion), unless the inferior competitor can escape through dispersal or survival in refuges where superior competitors are absent.

Predator-prey interactions involve a predator killing and eating prey and therefore have a positive effect on the predator but a negative effect on the prey. Predators and parasites affect prey populations similarly, but predators generally are opportunistic with respect to prey taxa and kill multiple prey per individual, whereas parasites generally are more specialized for association with particular host species and may or may not kill the host. Predators show preferences for prey size or defensive capability that maximize capture and utilization efficiency.

Symbiosis involves an intimate association between a symbiont and its host species, often co-evolved to maximize the probability of association and to mitigate any host defense against the symbiont. Symbiosis includes parasitism, com-mensalism, and mutualism. Parasitism is beneficial to the parasite but detrimental to its host. Although parasitism usually is considered to involve animal hosts, insect herbivores have a largely parasitic association with their host plants. Para-sitoidism is unique to insects and involves an adult female ovipositing on or in a living host, with her offspring feeding on and eventually killing the host. Most hosts of parasitoids are other arthropods, but at least one sarcophagid fly is a parasitoid of tropical lizards. Commensalism benefits the symbiont but has neutral effects for the host. Usually the symbiont uses the host or its products as habitat or as a means of transport with negligible effects on the host. Mutualism benefits both partners and is exemplified by pollinator-plant, ant-plant, ant-aphid, and detritivore-fungus interactions.

A variety of factors influence the nature and intensity of interaction. Abiotic factors that affect the activity or condition of individuals of a species may alter their competitive, predatory, or defensive ability. Resource availability, particularly the quality and patchiness of resources, may mitigate or exacerbate competition or predation by limiting the likelihood that competitors, or predators and their prey, co-occur in time and space. Other species can influence pairwise interactions indirectly. For example, predators often reduce populations of various prey species below sizes that would induce competition. Induced plant responses can influence predator-herbivore interactions and competition among herbivores in time and space. Species whose presence significantly affects diversity or community structure have been considered keystone species. A number of insect species function as keystone species.

Competition and predation/parasitism have been recognized as important mechanisms of population regulation and have been amenable to mathematical modeling. Mutualism has been viewed largely as a curiosity, rather than an important regulatory interaction, and modeling efforts have been more limited. However, mutualism may promote both populations and reduce their risk of decline to unstable levels. The network of interactions affecting a particular species may maintain population size within a narrower range with less frequent irruptions than occurs when populations are released from their regulatory network. The extent of mutual regulation (stabilization) of populations through this network of interactions has been widely debated but has significant implications for the stability of community structure and ecosystem processes governed by these interactions.

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