Download Vanni et al 2009 - units.miamioh.edu

This article was originally published in the Encyclopedia of Inland Waters
published by Elsevier, and the attached copy is provided by Elsevier for the
author's benefit and for the benefit of the author's institution, for noncommercial research and educational use including without limitation use in
instruction at your institution, sending it to specific colleagues who you know,
and providing a copy to your institution’s administrator.
All other uses, reproduction and distribution, including without limitation
commercial reprints, selling or licensing copies or access, or posting on open
internet sites, your personal or institution’s website or repository, are
prohibited. For exceptions, permission may be sought for such use through
Elsevier's permissions site at:
http://www.elsevier.com/locate/permissionusematerial
Vanni M J, Duncan J M, González M J and Horgan M J. (2009) Competition
Among Aquatic Organisms. In: Gene E. Likens, (Editor) Encyclopedia of Inland
Waters. volume 1, pp. 395-404 Oxford: Elsevier.
Author's personal copy
Competition Among Aquatic Organisms
M J Vanni, J M Duncan, M J González, and M J Horgan, Miami University, Oxford, OH, USA
ã 2009 Elsevier Inc. All rights reserved.
Introduction
Intraspecific Competition
Competition can be defined as a negative interaction between organisms resulting from a shared
requirement for a resource that is in limited supply.
When competing, individuals use limiting resources
that would otherwise be available for other individuals. Consequently, competing individuals obtain
resources at lower rates, and are likely to grow
more slowly, have fewer offspring, and have a lower
chance of surviving than they would in absence of
competition.
Ecologists classify competition based on the identity of interacting individuals. Intraspecific competition occurs between individuals of the same species,
while interspecific competition occurs between individuals of two or more species. Competition is also
categorized according to the mechanism by which it
occurs. Interference competition occurs when an individual directly prevents another from obtaining
resources. For example, a fish that aggressively chases
away other fish to monopolize a nesting site is an
interference competitor. In contrast, individuals can
also compete via exploitative competition, which
occurs when individuals deplete resources that
would otherwise be available to others; in this case
there is no direct agonistic interaction. For instance,
the depletion of nutrients by an individual algal cell
reduces nutrient availability to other algal cells.
Both intraspecific and interspecific competition can
occur simultaneously and both can occur via exploitative or interference mechanisms. Most groups of
aquatic organisms (bacteria, protists, algae, plants,
and animals) compete both intraspecifically and
interspecifically, and via both mechanisms, although
competition does not always occur. Because competition occurs within a network of species interactions
(Figure 1), the outcome of competition can depend on
the presence of other species, especially predators of
potential competitors.
The study of competition among freshwater organisms has a long history, parallel with that of competition studies in terrestrial environments. Aquatic
ecologists were also among the first to study the
complexities of competitive interactions, including
competition in size-structured populations and variable environments and the interactions between competition and predation.
Intraspecific competition is a common and important
interaction for many aquatic species. A classic laboratory study by L. B. Slobodkin showed reduced growth,
survival, and reproduction of Daphnia when population size was high, as a result of exploitative competition, and served as the basis of subsequent studies on
competition in zooplankton. One outcome of intraspecific competition is logistic population growth
(called sigmoidal or S-shaped growth); population
growth is nearly exponential when numbers are low,
but then growth rate is reduced progressively as the
population expands, and eventually the population
approaches its carrying capacity. Logistic growth of
aquatic populations has been demonstrated repeatedly in laboratory studies of aquatic algae, bacteria,
protozoans, and metazoans. It has been demonstrated
less often in the field, probably because it is difficult to
observe colonization events that usually precede logistic growth.
Interference competition can also be an important
mechanism of intraspecific competition. Many zooplankton taxa make autotoxins, which are chemicals
that inhibit feeding or increase mortality in conspecifics. For example, individuals of the rotifer Synchaeta
pectinata produce an autotoxin that reduces growth
rate and increases mortality of other individuals of
the same species. Autotoxin effects have also been
demonstrated in a marine phytoplankton species. It
is likely that autotoxic effects are common among
freshwater organisms, but little research has been
directed toward this phenomenon.
One consequence of intraspecific competition is
stunted growth of fish in dense populations. Fisheries
managers observed long ago that fish in a crowded
population (or with low food availability) often show
low (stunted) growth rates and thus are much smaller
than individuals growing in a population with few
individuals (or with abundant resources). Stunted
growth has many implications. Small and large fish
often rely on different food resources, so a stunted
population may have food web effects different from
those of a population with larger individuals. In addition, smaller individuals may be more vulnerable to
predators, especially other fish that are gape-limited.
Stunted populations also may be less desirable for
recreational and commercial harvest.
395
Encyclopedia of Inland Waters (2009), vol. 1, pp. 395-404
Author's personal copy
396
Biological Integration _ Competition Among Aquatic Organisms
Intraspecific
competition
Interference
competition
Exploitative
competition
Apparent
competition
or mutualism
Intraguild
predation
Keystone
predation
Predators
Potential
competitors
Resources
Figure 1 Diagram representing different kinds of competitive interactions. Potential competing species are shown in the middle
row, their resources in the bottom row, and their predators in the top row. Arrows indicate direction of consumptive flows. The curved
arrow for intraspecific competition denotes competition among individuals of the same species, and the two-headed arrow for
interference competition indicates direct agonistic competition. Modified from Blaustein L and Chase JM (2007) Interactions between
mosquito larvae and species that share the same trophic level. Annual Review of Entomology 52: 489–507.
Intraspecific competition can also lead to increased
variability in body size. Competition is often highly
asymmetric, meaning that it affects some individuals
much more than others. This could be because some
individuals are inherently better competitors, or
because some individuals arrive at a site (or are
born) earlier than others and thus preempt resources.
Superior or early-arriving individuals may reach a
relatively large size while inferior competitors or
late arrivers suffer reduced body size. Often there is
a gradient in competitive ability or arrival times, and
a population growing under intraspecific competition
displays a wide distribution of sizes among individuals of equal age. Such asymmetries have been
demonstrated in fish, amphibians, and insects. Differences in size initiated by intraspecific competition can
become magnified over time by size-dependent competitive superiority. An individual that gains an initial
advantage (e.g., by arriving early or by having a
slightly larger initial size) will grow more rapidly
than the average individual. This individual may use
a wider range of resources (e.g., larger fish can consume a wider range of prey items), leading to a further
gain in size relative to other individuals. This difference in size may become more pronounced over time.
Size differences can also set up hierarchies in which
large individuals are superior via interference competition because larger individuals may be better at
guarding territories, gaining access to mates, or surviving aggressive interactions with conspecifics.
(1910–1986), who studied competition among aquatic
protozoans (Paramecium and others), showed that
for some pairs of species, a superior competitor
always reduced the inferior species to extinction. In
other cases, two species coexisted but nevertheless
showed reduced stable population sizes when grown
in each other’s presence, compared with when each
was grown alone. These studies had a large effect on
the growing field of ecology and stimulated much
experimental work on competition in aquatic and
terrestrial communities. In the 1950s, P. W. Frank’s
laboratory studies of Daphnia populations were
among the first experimental studies of interspecific
competition in animals. They revealed competitive
asymmetry between two similar species, causing one
species consistently to outcompete the other.
Early laboratory studies such as Frank’s laid the
groundwork for more sophisticated studies of interspecific competition within food webs of dozens or
hundreds of potentially interacting species. Modern
studies include many factors that can affect competition, such as the size structure of populations, abiotic
conditions, and the abundance, ratio, and temporal
variation of limiting resources. Much progress has
been made in understanding these and other aspects
of competition in freshwater ecosystems. Studies of
competition among aquatic organisms have contributed greatly to our understanding of aquatic communities and have enriched the broader field of ecology.
Body Size and Competition
Interspecific Competition
The study of interspecific competition among aquatic species has an accomplished history. G. F. Gause
The species with the larger body size is often competitively dominant over smaller species. In a highly
influential study published in 1965, J. L. Brooks and
Encyclopedia of Inland Waters (2009), vol. 1, pp. 395-404
Author's personal copy
Biological Integration _ Competition Among Aquatic Organisms
S. I. Dodson proposed the ‘size-efficiency hypothesis’
for freshwater zooplankton, which states that larger
species outcompete smaller species because the former are more efficient grazers of phytoplankton.
Thus, when fish, which preferentially consume larger
zooplankton species, are scarce, large species should
dominate because they competitively exclude smaller
species. Field studies have repeatedly shown that
large zooplankton species usually dominate when
the intensity of fish predation is low, while small
species are common only when fish predation is
intense and large zooplankton are scarce. Experimental studies also show that large cladoceran species
show positive population growth at lower food concentrations than do smaller cladocerans and rotifers;
specifically, they are more tolerant of low food concentrations that can be produced during competitive
situations.
While much evidence supports the size-efficiency
hypothesis, other research shows that larger species
do not always prevail in competitive situations. For
example, small-bodied cladocerans and rotifers often
coexist, even though they differ greatly in size. Other
research shows that multiple kinds of environmental
conditions determine whether the large or small species is competitively dominant. For example, the
dominant species among Daphnia species of different
size depends on food abundance and quality. Also,
high concentrations of inorganic particles (e.g., silt
and clay) or filamentous cyanobacteria can change
the outcome of competition; in general, these particles favor small species because they reduce the feeding efficiency of large species. Several studies also
show that the dominance of large zooplankton species, and the associated scarcity of small species, in
lakes with low fish predation may be caused by predation of larger zooplankton on smaller zooplankton
rather than competitive exclusion based on shared
foods. Thus, while large zooplankton species are
often better competitors than small species, this is
not always true, and it is not clear to what extent
interspecific competition can explain the dominance
of large zooplankton in lakes where planktivorous
fish are scarce.
Larger fish often are better competitors than their
smaller counterparts in the absence of predation.
However, because body size distributions and feeding
habits overlap to a great extent among species, the
outcome of competition can depend on the size distributions of competing species (Figure 2). Newly
hatched fish are small, and even species that differ
greatly in maximum size have offspring that are of
similar size. As fish grow they feed on larger prey, so
species that differ greatly in maximum size can overlap extensively in diet, especially as juveniles. Even
a species that is prey to another fish species can
397
compete with its predator (a piscivore) under some
conditions because juveniles of the piscivore species
may share the same food resources as adults of the
prey species. For example, juvenile bass and adult
sunfish both consume invertebrates. Under these conditions, the prey species (sunfish) may actually be the
better competitor and thereby limit the survival of
young predator (bass) individuals (Figure 2). This can
create a bottleneck that limits the survival of juvenile
piscivores, preventing the piscivore from establishing a
viable population. In contrast, if the piscivore population is dominated by large individuals predation may
limit the number of prey individuals surviving, thereby
reducing competition between adults of the prey species and juveniles of the piscivore. Such interactions are
likely to be important among most species, but have
been studied primarily for fish and amphibians.
Among aquatic primary producers, the importance
of size in competition may depend on the limiting
resource. For example, aquatic plants (including
macrophytes and algae) can intercept light, thus
shading out smaller species. Even so, sometimes
being large is disadvantageous for benthic (bottom
dwelling) algae because of increased susceptibility of
large algal mats to grazing or physical scouring. Also,
for phytoplankton and bacteria, smaller species may
be competitively superior when nutrients are limiting
because small size provides a higher ratio of surface
area to volume, which facilitates uptake of limiting
nutrients.
Priority Effects
A priority effect occurs when a competitively dominant species is not predetermined according to species
identity, but rather by the order of birth or arrival.
This has been shown most convincingly in amphibians
of temporary ponds, where body size plays a key role.
The first species arriving at a pond to breed has the
competitive advantage because its tadpoles are larger
relative to tadpoles of competitors. Therefore, the
competitive dominant depends on body size, which
is strongly influenced by arrival time. In amphibians
of temporary ponds, body size may be more important
than species identity in determining competitive dominance. Priority effects have also been observed in
phytoplankton, zooplankton, and insects.
Competition and Niche
A species’ ecological niche can be defined as the range
of resources and conditions allowing the species to
maintain a viable population. Theoretically, if two
species have the same niche, one species will exclude
the other. The corollary is that the niches of coexisting
species must differ. Niches of coexisting species can
Encyclopedia of Inland Waters (2009), vol. 1, pp. 395-404
Author's personal copy
398
Biological Integration _ Competition Among Aquatic Organisms
Adult
bass
Juvenile
bluegill
Adult
bluegill
Juvenile
bass
Invertebrates
Adult
bluegill
Zooplankton
Juvenile
bluegill
Juvenile
pumpkinseed
Littoral
invertebrates
Adult
pumpkinseed
Snails
Figure 2 Competition in size-structured fish populations. Consumptive flows are indicated by the solid arrows and competitive
interactions by the dashed, two-headed arrows. The top diagram shows size-specific interactions between bluegill (Lepomis
macrochirus) and largemouth bass (Micropterus salmoides). The bottom diagram shows size-specific competition between bluegill and
pumpkinseed (Lepomis gibbosus). On the right are photographs of largemouth bass (top), bluegill (middle), and pumpkinseed (bottom).
Diagrams based on information from Olson MH, Mittelbach GG, and Osenberg CW (1995) Competition between predator and prey:
resource-based mechanisms and implications for stage-structured interactions. Ecology 76: 1758–1771; Osenberg CW, Mittelbach,
GG, and Wainwright PC (1992) Two-stage life histories in fish: The interaction between juvenile competition and adult performance.
Ecology 73: 255–267; and Persson L (1988) Asymmetries in competitive and predatory interactions in fish populations. In: Ebenman B
and Persson L (eds.) Size-Structured Populations: Ecology and Evolution, pp. 203–218. Berlin: Springer.
be similar, but not identical. Such ‘niche partitioning’
has been shown in many aquatic organisms. Niche
partitioning of fish species provides examples. For
instance, sunfish (family Centrarchidae) preferentially feed in a pond habitat (e.g., littoral vegetation
or open water) where they gain the most energy per
unit effort. Habitat preference (hence niche occupancy) depends, however, on the presence of other
competitors; a species may have a wide niche in
absence of competitors but a narrow niche in the
presence of a competitor. Furthermore, the niche of
a species often varies predictably with age (size). Such
‘ontogenetic niche shifts’ often are driven by changes
in food, as discussed earlier. Within a species, small
individuals may have a different feeding niche than
larger individuals, and feeding often is associated
with habitat preference. For example, small fish may
feed on zooplankton in open waters while larger fish
may feed on benthic invertebrates near shore. As
mentioned earlier, the strength and outcome of interspecific competition may depend on the size (age)
structure of the populations. Thus, the niche dimensions of a species may depend on the presence or
absence of a competing species and the size distributions of competing species. Furthermore, habitat
selection also is strongly influenced by predation. In
the absence of predators, small individuals may forage where the rate of food intake is optimal. In the
presence of a predator, these individuals may feed
where risk of predation is lower.
Resource-Ratio Competition
D. Tilman developed a model of competition among
phytoplankton that has influenced ecological research
to a great extent over the past 25 years. The model
Encyclopedia of Inland Waters (2009), vol. 1, pp. 395-404
Author's personal copy
Biological Integration _ Competition Among Aquatic Organisms
uses the minimum resource requirements (R*) of species and the ratio at which resources are supplied (the
resource ratio) to predict competitive outcomes. An
important prediction of the model is that the resource
ratio determines the outcome of competition, i.e.,
which species will eliminate competitors or whether
species will coexist.
As an example, two species of algae may be competing for nitrogen and phosphorus (Figure 3(a)). If
one species has a lower minimum requirement (a
lower R*) for both N and P, that species will always
drive the other to extinction via exploitative competition. When the two species are better competitors for
a different resource, the resource ratio determines the
outcome (Figure 3(b)). In this case, one species (species A) is the better competitor for P and competitively excludes the other species (species B) when
P is in short supply, i.e., when the N:P supply ratio
is high. When N is limiting (i.e., the N:P supply ratio
is low), species B will exclude species A. When the
resource ratio is intermediate, the two species coexist
because each species is limited by a different resource
(Figure 3(b)). Thus, when two species (A and B)
compete for two resources and the two are superior
competitors for different resources, three outcomes
S2
NA*
A
NB*
PB*
Resource 2 (e.g., P)
B
PA*
(b)
E D+E
PB*
Resource 2 (e.g., P)
Resource 1 (e.g., N)
A
Resource 1 (e.g., N)
Resource 1 (e.g., N)
B
PA*
(a)
S3
S2
NA*
are possible: species A wins, species B wins, or A and
B coexist. The outcome depends on the resource ratio;
the outcome does not depend on the absolute concentrations or supply rates, because the organisms reduce
resources to equilibrium concentrations defined by
R*, regardless of supply rates.
The Tilman resource-ratio model has been applied
extensively to phytoplankton, and it is now generally
accepted that resource ratios are important in determining phytoplankton community composition. Laboratory competition experiments generally confirm
the veracity of the model under equilibrium conditions, although surprisingly few studies have explicitly tested the model’s predictions. For example, when
diatom species compete for silica (Si) and phosphorus
(P), the outcome of competition depends on the
Si:P ratio. Field surveys also offer support for the
resource-ratio model. Different diatom species dominate at different Si:P ratios, as predicted by laboratory
competition experiments. Similarly, low N:P supply
ratios can favor N-fixing cyanobacteria (taxa for
which N* is essentially zero because they can utilize
atmospheric N2) whereas higher N:P ratios often
favor taxa other than cyanobacteria. In addition, at
least one study has successfully used the resource
S1
S1
NB*
399
(c)
D C+D
C
B+C
B
A+B
A
E
D
C
B
A
Resource 2 (e.g., P)
Figure 3 Resource-ratio competition. (a) Species A is the better competitor for both resources because it can survive at lower
minimum concentrations of N and P. When competition is for P (Supply Point 1, or S1), under equilibrium conditions the species grow
towards their carrying capacities and reduce concentrations of N and P (indicated by the arrow emanating from S1). Eventually the
P concentration is driven below Species B’s minimum P concentration (PB*) but remains above PA*. When this happens, Species A still
exhibits positive population growth, but Species B declines, and Species A eventually excludes Species B via competition for
P. Because Species A has a lower minimum concentration for N, it will also outcompete Species B when N is limiting (Supply Point S2).
Thus, Species A is always the superior competitor, regardless of the ratio at which resources are supplied. (b). Each species is the better
competitor for one of the resources, and the outcome of competition depends on the ratio at which N and P are supplied. Species A is
the better competitor for P (PA* < PB*), and thus will outcompete Species B when P is in short supply relative to N, i.e., when the N:P
supply ratio is high (Supply Point S1). When N is limiting (i.e., the N:P supply ratio is low, as indicated by Supply Point S2), Species B will
win because it is the better competitor for N, i.e., NB* < NA*. The two species will coexist if the resource ratio is intermediate, i.e., if the N:
P supply ratio falls within the triangle indicated by the dashed lines, and illustrated by Supply Point S3). (c). Nonequilibrium conditions
promote species coexistence. Here, five species (A–E) compete for two resources. When the supply ratio is constant, only one or two
species can coexist and all others go extinct. The identity of the coexisting species (i.e., A, A þ B, B, etc.) depends on the N:P supply
ratio. However, when the N:P supply ratio varies temporally, several species can coexist because the identity of the competitive
dominant changes as the supply ratio varies. Temporal variation in the N:P supply ratio is indicated by the oval, which encompasses the
coexistence regions of all five species. Thus, all five species can persist.
Encyclopedia of Inland Waters (2009), vol. 1, pp. 395-404
Author's personal copy
Biological Integration _ Competition Among Aquatic Organisms
ratio model to predict the competitive outcome
between two rotifer species feeding on two phytoplankton species.
Competition under Variable Conditions
Many competition models assume equilibrium conditions and many experimental studies employ such
conditions. Yet in nature, equilibrium conditions
rarely occur, so it is important to understand how
competition plays out under variable conditions.
Ecologists have begun this quest with both theory
and experiments. The general consensus is that spatial or temporal variability tends to increase the
number of species that can coexist. Furthermore, in
nature many species often coexist. G. E. Hutchinson
(1903–1991) coined the famous term ‘paradox of the
plankton,’ which asks why well-mixed pelagic environments of lakes or oceans maintain a large number
of phytoplankton species (tens or hundreds), even
though the component species largely require the
same resources. Therefore, niche diversification
among species in mixed water should be minimal.
Hutchinson suggested that under variable conditions,
the identity of the competitive dominant changes,
usually too quickly for competitive exclusion to
occur.
Disturbances, i.e., discrete events that impose mortality (such as storm flushing a stream or wind mixing
a lake), can also reduce the severity of competition by
reducing populations to densities below those at
which competition occurs. The role of disturbance
may be particularly important in streams, where
high flow events can impose considerable mortality
on organisms, effectively ‘re-setting’ competitive
interactions.
The resource-ratio model can be extended to variable conditions (Figure 3(c)). When the resource supply ratio varies temporally, several species can coexist
because the identity of the competitive dominant
changes when the supply ratio varies. If the N:P
ratio changes more rapidly than the time needed for
competitive exclusion to occur, more species can persist than under equilibrium conditions. Experimental
work supports this hypothesis. For example, pulsed
nutrient supply (Si and P) increased the number of
coexisting phytoplankton species when compared
with supply at steady state.
Exposure to light is also variable in mixed water;
phytoplankton experience high light intensity when
they circulate near the surface and low light
when they are in deeper water. Coexistence depends
on the species’ reactions to nutrients and light. For
instance, when air was bubbled into the bottom of a
lake to increase turbulence (mixing), dominance
shifted from the buoyant, toxin-producing cyanobacterium Microcystis to preferable diatom and green
algal species because diatoms and green algae
respond better than cyanobacteria to fluctuating
light (Figure 4). In addition to demonstrating the
importance of temporal variation in mediating competition, this example shows how knowledge of
competition can be used to enhance water quality.
Interference Competition
Many aquatic species compete with each other via
agonistic (aggressive) interactions. For example, many
species, including many fish and insects, actively
exclude other individuals (conspecifics as well as
other species) from territories. In streams, some benthic
insects (e.g., caddisflies and blackflies) defend feeding
territories where they capture suspended particles.
Their densities can be quite high, leading to simultaneous exploitative and interference competition.
Indeed, many benthic species (plants or animals) probably simultaneously compete via both interference
competition for space and exploitation for food. For
example, zebra and quagga mussels (Dreissena polymorpha and D. bugensis) can grow densely on shells of
larger bivalve species, leading to reduced growth or
Critical depth
100
Turbulent diffusion (cm2/s −1)
400
10
No blooms
Diatoms
and
greens
win
Critical
turbulence
1
0.1
Microcystis wins
0.01
0.1
1
10
100
1000
Water-column depth (m)
Figure 4 Effects of a variable light climate on phytoplankton
competition. The diagram shows model predictions, defined by
the zones labeled ‘diatoms and greens win,’ ‘Microcystis wins,’
and ‘No blooms.’ The shaded region indicates conditions that are
predicted to lead to coexistence of the two algal groups. Diatoms
and green algae are predicted to competitively exclude
Microcystis when light intensity is variable (i.e., when turbulent
diffusion, or lake mixing, is high), whereas Microcystis is
predicted to win when conditions are calm and variability in light
intensity is low. The points and error bars show the outcome,
under normal conditions (open circle) and when the lake was
artificially mixed to increase turbulence (solid circle). From
Huisman J, Sharples J, Stroom JM, et al. (2004) Changes in
turbulent mixing shift competition for light between
phytoplankton species. Ecology 85: 2960–2970.
Encyclopedia of Inland Waters (2009), vol. 1, pp. 395-404
Author's personal copy
Biological Integration _ Competition Among Aquatic Organisms
mortality of the larger bivalve species via both interference and exploitative competition.
Some zooplankton species mechanically interfere
with each other in a size-dependent manner. Daphnia
can capture rotifers while feeding and in the process
rotifers may be damaged or even ingested (Figure 5).
Generally, such mechanical interference is important
only between very large Daphnia and small rotifers.
When the size difference is less or when the rotifer has
spines or armor, interference competition rarely
occurs. Thus, both interference and exploitative
mechanisms can contribute to the competitive superiority of large zooplankton.
Allelopathy can be important among aquatic organisms, particularly in algae, plants, and bacteria. All
major groups of aquatic primary producers generate
allelochemicals that act against competitors, especially interfering with enzyme action or photosynthesis.
Emergent wetland plants use allelochemicals against
each other. Floating macrophytes such as Eichhornia
are allelopathic as they need to compete with other
primary producers for nutrients dissolved in the water.
Rooted macrophytes often get nutrients from sediments, but they can be shaded by epiphytic algae that
grow on them or phytoplankton that grow above
them. These macrophytes often use allelopathy to compete with primary producers that interfere with their
ability to gather light. Such interactions are involved in
abrupt shifts between phytoplankton dominance and
401
macrophyte dominance in shallow lakes. Allelopathy
also occurs among benthic and pelagic microalgae
and cyanobacteria. Allelopathy appears to be strongest
in still waters where released chemicals can affect neighboring competitors. In streams and rivers, allelochemicals would be washed away without providing benefit
to the organism that produced them. The exception may
be localized interactions among stream benthic algae.
Competitive Effects of Exotic Species
Exotic species are those introduced by human activities to areas where they are not native (Figure 6).
Some exotics have significant effects on native species, including those mediated by competition. Nonnative zebra and quagga mussels (Dreissena spp.)
can competitively exclude or reduce populations of
mollusks native to North America, as mentioned
earlier. Via their filtering, dreissenids have greatly
reduced phytoplankton in some environments (e.g.,
the Hudson River, New York, USA), and this has probably reduced the abundance of other species relying
on phytoplankton. In addition to dreissenids, examples of competitive effects of exotics on native species
include fish (e.g., effects of brown trout on galaxiids
in New Zealand streams), crustaceans (e.g., effects of
exotic crayfishes on native crayfishes around the
world), and macrophytes (e.g., effects of water hyacinth Eichornia on native plants).
3.3
A. p.
A. e.
Relative susceptibility
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Synchaeta
oblonga (juveniles)
Synchaeta oblonga
Ascomorpha ecaudis
Keratella
cochlearis f. typica
Keratella testudo
Synchaeta pectinata
C. u.
(3) (2) (2) (3) (2) (2) (1)
Polyarthra remata
200 µm
K. t.
K. b.
(2) (2) (1) (2)
Keratella crassa
D. p.
0.0
Kellicottia
bostoniensis
Asplanchna
priodonta
Conochilus
unicornis
S. o.
K. c.
P. r.
S. p.
K. cr.
Rotifer
Figure 5 Interference competition between Daphnia and rotifers (drawn to the same scale at left). At right, the relative susceptibilities
of the rotifer taxa to interference competition with Daphnia are shown. From Gilbert JJ (1988) Susceptibilities of ten rotifer species
to interference from Daphnia pulex. Ecology 69: 1826–1838.
Encyclopedia of Inland Waters (2009), vol. 1, pp. 395-404
Author's personal copy
402
Biological Integration _ Competition Among Aquatic Organisms
(a)
(c)
(b)
(d)
Figure 6 Representative invasive exotic species that negatively affect native species via competition. (a) The macrophyte Eichornia
crassipes (water hyacinth), which can outcompete native plants for light and nutrients; (b) the zebra mussel (Dreissena polymorpha)
growing on top of a native bivalve; (c) brown trout (Salmo trutta), which often reduces the abundance of native fishes; and (d) the rusty
crayfish (Orconectes rusticus), which has numerous effects on native crayfish and other benthic invertebrates.
Evolutionary Consequences of Competition
Over many generations competition can cause subpopulations to diverge morphologically, ecologically,
and genetically as each group becomes specialized in
ways that reduce competition. Genetic differences
can proceed to speciation, meaning that the two subpopulations become two reproductively isolated species. A familiar example is the spectacular adaptive
radiation of cichlids in ancient tropical lakes. Over
millions of years, dozens to hundreds of species have
emerged, each with slightly different niches, presumably as a consequence of historical competitive
interactions. Speciation events have also occurred
in sticklebacks in small temperate lakes. Within a
lake, two different species of sticklebacks can exist
because one is a pelagic specialist feeding on zooplankton and the other is a benthic specialist. In
lakes with just one species, that species exhibits characteristics intermediate of the two specialist species.
Detailed experimental and molecular studies have
verified that recent speciation produced these species,
and that speciation is strongly driven by competition.
When competition is intense, natural selection promotes divergence of competing species to minimize
competition. Given such an evolutionary history,
extant species may show little evidence of competition. Even so, a lack of present-day competition could
also mean that the species never have competed. Ecologists favoring the former explanation for lack of
competition, without necessarily having adequate
evidence, have sometimes been accused of invoking
the ‘ghost of competition past.’ Recently, ecologists
hatched Daphnia from dormant resting eggs in lake
sediments and found that clones isolated from older
sediments (several decades) were strongly influenced
by interspecific competition, while clones hatched
from more recent sediments showed reduced competition and could coexist with other extant Daphnia
species. These results are consistent with the hypothesis that natural selection typically reduces the intensity of interspecific competition.
Encyclopedia of Inland Waters (2009), vol. 1, pp. 395-404
Author's personal copy
Biological Integration _ Competition Among Aquatic Organisms
Competition within Food Webs
Competition occurs within food webs through direct
and indirect species interactions (Figure 1). Thus, the
nature and outcome of competition often depends on
species other than those competing for resources.
Effects of Predation on Competition
Many studies show that predation can reduce the
abundance of potential competitors (Figure 1), thus
increasing resources for the competitors. For example, predators can depress herbivores, leading to an
increase in growth of plants. This process is known
as a trophic cascade. Even if the resources of the
prey do not increase because of predation, each surviving individual of the prey species will have more
resources. Thus, predation can reduce intensity of
competition among prey species.
If predators selectively prey on a competitive dominant, predation can promote coexistence among prey
species. Such ‘keystone predation’ was first described
by R.T. Paine in his classic studies in the marine intertidal zone, but has also been shown in freshwater
communities. For example, zooplankton can promote
species diversity of phytoplankton by grazing most
heavily on the superior competitors. A recent synthesis suggests that predation is most likely to promote coexistence of prey if competitors (1) compete
for space; (2) are efficient consumers of their own
resources; (3) are consumed by several specialist predators; or (4) show a trade-off between competitive
ability versus defense against predation. One or more
of these conditions is often met in aquatic communities. Predation does not always decrease the intensity of competition, but rather can have various effects
on it. For example, effects of predation may interact
with environmental conditions such as productivity,
and may affect superior and inferior competitors
differently.
Pathogens can have effects similar to those
described earlier for predators. For example, fungal
pathogens can alter competition between amphibians
by inducing species-specific effects on tadpole growth
rate and time to metamorphosis. The interaction
between competition and disease can also go both
ways – for example, larval mosquitoes whose growth
is reduced by interspecific competition may show
increased prevalence of viral pathogens.
Predators often induce behavioral changes in prey
that indirectly affect competitive interactions among
prey. When predators are present, prey often must
occupy habitats that are suboptimal for resource acquisition but offer protection from predators. The prey thus
may suffer increased competition, leading to decreased
growth, reproduction, or survival. For example, when
403
predation in intense, herbivorous stream insects spend
more time on the bottom of rocks where the abundance
of algae (their food) is much lower than on the upper
surface. Similarly, small fish sometimes aggregate in
littoral vegetation to avoid piscivores. In these cases,
prey experience increased competition because of their
high densities in a habitat that offers them protection
from predators.
Intraguild Predation
A species may both compete with and prey upon
another species (Figure 1). This phenomenon is
known as ‘intraguild predation’ because predators
often belong to the same ecological guild. Intraguild
predation is common among aquatic organisms and
often is mediated by interactions that vary with size.
For example, juveniles of a piscivorous fish can compete with adults of prey fish (as discussed earlier).
Similarly, juvenile copepods (nauplii) compete with
herbivorous zooplankton, but adults of the same
copepod species prey on herbivorous zooplankton.
Intraguild predation is common in many interactions
between size-structured species, including fish,
insects, crustaceans, and amphibians.
Apparent Competition and Apparent Mutualism
When competitor species share a common predator,
they may interact indirectly through the predator,
positively or negatively (Figure 1). If the presence of
prey species A causes a numerical increase of the
predator, this may negatively affect prey species
B. This is called ‘apparent competition’ because the
negative relationship between the two prey species
may be mistaken for interspecific competition. This
interaction between prey species may, however, be
positive over shorter time scales. If the predator
preys heavily on prey species A, in the short term this
can reduce predation on prey species B. This interaction is referred to as ‘apparent mutualism.’ In temporary ponds, prey species may be apparent mutualists
with each other early in the season, but apparent
competitors later in the season, i.e., after the predator
shows a numerical response that subsequently
depresses the abundance of one of the competitors.
Competition and Regime Shifts
Competition for light and nutrients also can play a
role in determining alternate ecosystem states, or
regime shifts, in shallow lakes. When phytoplankton
are abundant, they can reduce the amount of light
reaching macrophytes. Such a turbid lake may have
little littoral vegetation. In contrast, when phytoplankton are scarce, macrophytes may have more
Encyclopedia of Inland Waters (2009), vol. 1, pp. 395-404
Author's personal copy
404
Biological Integration _ Competition Among Aquatic Organisms
light and become more abundant. In clear lake conditions, macrophytes sequester nutrients and can also
suppress phytoplankton via allelopathy, as discussed
earlier. In contrast, under turbid lake conditions, phytoplankton sequester nutrients that would otherwise
be locked up in macrophytes and can outcompete
macrophytes for light. It is often extremely hard to
force a lake to change state. Competition is one of
many mechanisms that can be important in initiating
and maintaining alternate states for lakes.
Intraguild predation – A predator–prey interaction in
which the predator and prey also compete with each
other.
Conclusions
Priority effect – A competitive interaction in which the
species arriving (or being born) first outcompetes
the other species.
Competition influences the organization of aquatic
communities. Early laboratory studies showed the
potential importance of interspecific competition
and laid the groundwork for subsequent theory and
field-based studies. Future studies will continue to
elucidate the ways in which competition operates,
and more effectively incorporate competition into
issues of environmental concern such as the competitive effects of exotic species, the interactions between
competition and disease, and the role of competition
in conservation of biodiversity.
Glossary
Allelopathy – The production and release of chemical
substances by an organism that inhibit the growth,
survival, or reproduction of another organism.
Apparent competition – An interaction between
two potentially competing species that share a predator, whereby one potential competitor species
causes the predator to increase in abundance, leading to a decrease in the other potential competitor
species.
Apparent mutualism – An interaction between two
potentially competing species that share a predator,
whereby the predator prefers to feed on one potential competitor species, thereby alleviating predation on, and hence benefiting, the other potential
competitor species.
Exploitative competition – A form of competition
in which competing individuals do not directly
interact, but rather compete by consuming shared
limiting resources.
Interference competition – A form of competition in
which competing individuals directly interact with
each other agonistically.
Intraspecific competition – Competition between
individuals of the same species.
Keystone predation – Predation on the dominant
competitor that alleviates competition among prey
species.
Niche – The range of conditions and resources allowing a species to maintain a viable population.
Further Reading
Blaustein L and Chase JM (2007) Interactions between mosquito
larvae and species that share the same trophic level. Annual
Review of Entomology 52: 489–507.
Chase JM, Abrams PA, Grover JP, et al. (2002) The interaction
between predation and competition: A review and synthesis.
Ecology Letters 5: 302–315.
Ebenman B and Persson L (1988) Size-Structured Populations:
Ecology and Evolution. Berlin: Springer.
Gilbert JJ (1988) Suppression of rotifer populations by Daphnia:
A review of the evidence, the mechanisms, and the effects on
zooplankton community structure. Limnology and Oceanography 33: 1286–1303.
Gross EM (2003) Allelopathy of aquatic autotrophs. Critical
Reviews in Plant Sciences 22: 313–339.
Holt RD and Lawton JH (1994) The ecological consequences of
shared natural enemies. Annual Review of Ecology and Systematics 25: 495–520.
Miller TE, Burns JH, Mungia P, et al. (2005) A critical review of
twenty years’ use of the resource ratio theory. American Naturalist 165: 439–448.
Olson MH, Mittelbach GG, and Osenberg CW (1995) Competition between predator and prey – Resource-based mechanisms
and implications for stage-structured dynamics. Ecology 76:
1758–1771.
Passarge J, Hol S, Escher M, and Huisman J (2006) Competition
for nutrients and light: Stable coexistence, alternative stable
states, or competitive exclusion? Ecological Monographs 76:
57–72.
Polis GA, Myers CA, and Holt RD (1989) The ecology and evolution of intraguild predation – Potential competitors that eat each
other. Annual Review of Ecology and Systematics 20: 297–330.
Schluter D (2001) Ecology and the origin of species. Trends in
Ecology and Evolution 16: 372–380.
Simon KS and Townsend CR (2003) Impacts of freshwater invaders
at different levels of ecological organisation, with emphasis on
salmonids and ecosystem consequences. Freshwater Biology 48:
982–994.
Tilman D (1982) Resource Competition and Community Structure.
Princeton, NJ: Princeton University Press.
Interspecific competition – Competition between
individuals of different species.
Encyclopedia of Inland Waters (2009), vol. 1, pp. 395-404