Competition
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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