Competition occurs when two consumers require a resource that is in insufficient abundance to meet the needs of both. Members of the same species can compete (intraspecific competition) as can members of different species (interspecific competition). Although intraspecific competition can affect an individual’s ability to acquire resources, interspecific competition has received more attention because of the insight it provides into the coexistence of different species in an assemblage, which addresses the more general question of how biodiversity is created and maintained. In general, individuals can compete for food, feeding and resting sites, and refuges from predators and the elements (competition for mates and breeding sites is viewed as part of the reproductive biology of a species rather than as traditional competition).
To avoid or reduce competition, organisms may change the way they exploit a resource. Competition may lead to differences in resource use (resource partitioning), such as when two sympatric (“living together”) species feed on different sizes of a prey type or eat similar prey but in different microhabitats. Competition is more strongly implicated if these same predators feed on identical prey when they are allopatric (“living separately”). Also, competitive interactions can be suspected if potential competitors shift their resource use when resources become seasonally limiting, or if population reductions of one species occur when a suspected competitor is introduced into an area. However, ecological differences among species can also be caused by differences in nutritional requirements, foraging or locomotory capabilities, predator vulnerability, and phylogeny. Also, introduced species can alter predator–prey relationships or serve as vectors for parasites and diseases, which would also affect population densities of previous residents. Consequently it is generally necessary to perform experimental manipulations of resource abundance or distribution, or of population densities of suspected competitors, to prove that competition is in fact the cause of the dissimilarities. In such experiments, competition can be invoked if an inferior competitor or less aggressive species that occupies suboptimal regions in sympatry expands its habitat or feeding habits when the superior competitor is eliminated. Reciprocal removal of the inferior competitor should have little effect on the habits of the superior species. Such experiments may be conducted fortuitously or deliberately.
A fortuitous manipulation of resource partitioning, mediated by both competition and predation, is conducted annually in Lake Tjeukemeer, the Netherlands (Lammens et al. 1985). Bream (Abramis brama, Cyprinidae) and European Eels (Anguilla anguilla, Anguillidae) occur year round in fairly stable numbers in the lake, where their chief foods are waterfleas and juvenile midges, respectively. Smelt (Osmerus eperlanus, Osmeridae), a zooplanktivore, enter the lake each spring as juveniles when water is pumped from a nearby lake as part of a water stabilization program. Smelt do not persist in the lake because the adults are almost all consumed by predatory Pikeperch (Sander lucioperca, Percidae). When large numbers of juvenile smelt are recruited into the lake, they depress zooplankton populations, having their strongest effects on the size classes of zooplankton most used by Bream. Bream respond to reductions in zooplankton resources by switching to benthic invertebrates such as midge larvae, thereby depressing that resource. Eels then respond to depletion of their primary food by switching to piscivory. When Smelt are abundant, both Bream and Eels suffer reductions in condition (weight/ length) and Bream show poor gonad development. In years when Smelt recruitment is low, Bream and Eels switch back to their waterflea/midge diets and their growth and reproduction improve.
A well-studied example that includes the experiments necessary to establish the causes of shifts in resource use involves sunfishes in North America. As many as eight species of centrarchid sunfishes and basses may co-occur in a single lake. Many of these species are very similar morphologically. How do they coexist without competing? When stocked separately in ponds as year-old fish, three species, Bluegill Sunfish (Lepomis macrochirus), Green Sunfish (L. cyanellus), and Pumpkinseed Sunfish (L. gibbosus), use similar habitats and feed on similar food types. All three concentrate their time and effort on vegetation associated invertebrates. When stocked together, Bluegill and Pumpkinseed shift their habitats and diet in apparent avoidance of the competitively superior Green Sunfish. Bluegill shift to feeding on zooplankton in open water, and
Pumpkinseed include more benthic prey in their diet. Green Sunfish maintain a diet of vegetation-associated insects. All three species show reduced growth rates, indicating competitive reduction of resources for each species, but Bluegill show the greatest declines. When Bluegill and Green Sunfish are stocked together in ponds with little open water habitat (i.e., no alternative habitat for the Bluegill), Green Sunfish show better growth, fuller stomachs, and a higher survival rate than Bluegill. Competition among these species has an ontogenetic component that is felt most strongly by young fish. As the fish grow older, they begin to specialize more on different habitats. Bluegill become more adept at maneuvering in open water and suction feeding on zooplankton, and Pumpkinseed develop pharyngeal dentition with which they can crush mollusks that live in sediments. Hence the potential for competition is reduced in older fish under natural conditions (Werner & Hall 1979; Werner 1984; Mittelbach 1988; Osenberg et al. 1988; Wootton 1999).
Many other investigations have demonstrated strong competitive interactions among fishes in various habitat types (e.g., tropical streams, Zaret & Rand 1971; temperate streams, Schlosser 1982; temperate marine nearshore, Hixon 1980b, Holbrook & Schmitt 1989; coral reefs, Hixon & Beets 1989, Munday et al. 2001, Holbrook & Schmitt 2002; see reviews in Ross 1986; Ebeling & Hixon 1991; Grant 1997; Hixon 2006). In general, of the kinds of resources for which fishes can compete, competition for food resources, or at least differences in trophic resource use, is more common among fishes than are interspecific differences in habitat use; the reverse is true in terrestrial communities (Ross 1986).
Some traits that reflect apparent adjustments to presentday competition may result from historical interactions between species, the so-called “ghosts of competition past”. The influence of historical competition is frustratingly difficult to determine: are two species different today because of their current impacts on one another or because of past interactions? Experimental manipulations of the resource in question are almost always needed to prove competition, but obviously one cannot manipulate the history of two species and hence we can only speculate on but not demonstrate historical competition (Connell 1980).
Historical factors must also be considered when comparing ecological characteristics of species from unrelated taxonomic groups. The more distantly related two fish species are, the less similar they tend to be ecologically (Ross 1986). For example, generalist predators on coral reefs tend to be active at twilight or at night, have large mouths, and feed on fishes, whereas specialists are diurnal, have small mouths, and feed on sessile or small invertebrates (Hobson 1974, 1975, 2006). Resource partitioning along both trophic and temporal resource dimensions could be invoked here. However, generalist reef species tend to belong to more primitive acanthopterygian (spiny-rayed) groups (squirre lfishes, scorpionfishes, groupers), whereas specialists belong to more advanced groups (butterfl yfishes, wrasses, triggerfishes). Feeding habits, morphology, and activity times in one lineage are likely to have evolved independently of what happened in a later evolving lineage. Differing ecologies may therefore simply reflect differing phylogenetic histories. Interpreting differences in resource use as a result of competition may also be erroneous because of physiological differences among species. Some of the strongest impacts of introduced species on natives involve competitive displacement, suggesting that competition has been historically reduced via evolutionary adjustments (see Competition). In Lake Michigan, a coregonine salmonid, the planktivorous Bloater, Coregonus hoyi (designated Vulnerable by IUCN),
was replaced by introduced planktivorous Alewives, Alosa pseudoharengus (Clupeidae) in the 1960s, apparently as a result of competition for plankton. As Alewife numbers grew, Bloaters declined in abundance, shifted to a diet of benthic invertebrates at an earlier age, and apparently evolved fewer and shorter gill rakers (Crowder 1984). Five other coregonine species were extirpated from Lake Michigan during the same period.
Other indirect evidence of competitive displacement ofnatives by invaders includes habitat displacement of (federally Threatened) Spikedace, Meda fulgida (Cyprinidae), by Red Shiners, Cyprinella lutrensis, a well-known invasive. Red Shiners were introduced into the lower Colorado River. Spikedace disappeared simultaneously and progressively as Red Shiners proliferated, while dams and water withdrawals led to degraded habitat. Both species occupied slow current regions when alone, but where they cooccurred the more aggressive Red Shiners remained in slow current areas while Spikedace were displaced into regions of swifter current (Douglas et al. 1994). Introduced trout are often implicated in competitive displacement of native trout (e.g., Gatz et al. 1987; Fausch 1988). In a Michigan stream, introduced Brown Trout displaced native Brook Trout from the best foraging habitats, forcing brookies into faster water where the energetic costs of maintaining position were higher and where they were more likely to be caught by anglers (Fausch & White 1986; see also Waters 1983). Rainbow Trout displaced two native Japanese salmonids (Dolly Varden, Salvelinus malma, and White-spotted Char, S. leucomaenis) because of timing differences in spawning. Natives spawned in fall but rainbows spawned the next spring, at a time when embryos of the fall-spawning natives were developing in the gravel. The digging and spawning activities of the introduced species disturbed the redds of the natives (Taniguchi et al. 2000).
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