Charles Schutte
11/18/2005
The Search for a Mechanism of Coexistence in Ecological Literature
At the time when MacArthur’s 1958 paper, “Population Ecology of some
Warblers of Northeastern Coniferous Forests”, was published in the journal
Ecology, it was an “established principle” (Grinnell, 1922) that for a bird to exist in
an area, proper food, breeding and nesting grounds, and shelter, must be
available in that area, and that these requirements depend on the structural
features of that bird, that is, their physical characteristics.
Grinnell (1922)
defined these variables as the “ecological niche”, and stated that each species of
bird occupies a specific niche. Niches fill quickly, and intraspecific competition
for the resources of a given niche limits population size. If more than one
species of bird were to share the same niche, it was expected that the species
best able to compete for the limited resources of the niche would quickly drive
the other species to local extinction (Grinnell, 1922). Under these assumptions,
for species to coexist, they must differ in habitat or range, or be limited by
different factors (MacArthur, 1958).
MacArthur studied five species of warbler, the Cape May warbler
(Dendroica tigrina), myrtle warbler (D. coronata), black-throated green warbler
(D. virens), blackburnian warbler (D. fusca), and the bay-breasted warbler (D.
castanea), precisely because they appeared to violate this principle. These
warblers are considered congenerics, meaning that there is little difference
between their requirements for food and habitat. Physically, these five species
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are structured in much the same way, having very little variation in beak
dimensions (MacArthur, 1958), for example. MacArthur’s goal was to determine
what factors control the abundances of these species, and what keeps
competition from driving all but one of them to extinction. “…differences in food
and space requirements are neither always necessary nor always sufficient to
prevent competition and permit coexistence.” (MacArthur, 1958). He hoped to
find a mechanism of density-dependent control through which a given species,
when sufficiently abundant, would limit its own increase more than it would inhibit
the increase of other species, resulting in stable coexistence (MacArthur, 1958).
MacArthur found through his observations that the different species of
warblers preferred different feeding zones within coniferous trees. They also
behaved differently in that they hunted in different ways to different extents,
moved in different manners and directions, and varied in their peak times of
activity. This meant that the different species of warbler were each exposed to
different types of food resources, or the same kinds of food resources at different
places and times. MacArthur concluded that these five warblers partitioned the
resources of their habitat such that each species was limited by a different factor.
Coexistence of these different species was the result of sufficiently large
difference in habitats existing between them such that each limited its own
population more strongly than it inhibited the others.
This is fundamentally different from the earlier conceptualization of the
niche. The earlier paradigm saw the niche as a fundamental set of
environmental variables defining the habitat of a specific species based on the
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physical attributes of that species, (Grinnell, 1922). Competition results in only
one species being able to occupy any given niche, as the species best equipped
to exploit the resources of that niche will drive out all other species. MacArthur
(1958) saw that multiple species could occupy the same niche by partitioning the
resources of that niche through habitat selection. A simple behavioral
modification of foraging in different locations exposed closely related species to
different subsets of food resources within a classic niche, and allowed them to
coexist.
This paradigm shift sparked several new lines of research, the most
obvious of which was to test this new idea in other types of animals. An example
of this was a study of three closely related orb-weaving spiders (Brown, 1981).
These three spiders were capable of preying on the same types of insects, and
all fed preferentially on large insects (Brown, 1981). Spider and prey densities
were significantly correlated (Brown, 1981), suggesting that spider density is
limited by food, and creating potential for competition between the spiders. It
was determined that the three species of spiders displaced their webs vertically
through the vegetation, resulting in exposure to different types of prey (Brown,
1981). A greater divergence in web height was noted in drier areas with lower
prey availability (Brown, 1981). This study presented strong inferential evidence
of niche partitioning mitigating the affects of interspecific competition within the
three species of spider studied (Brown, 1981), supporting MacArthur’s 1958 work
by attaining similar results in a completely different type of animal. Although this
research did not take place soon after MacArthur’s, it is an interesting example of
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its application, and shows that investigators were still thinking about his ideas
years later.
Perhaps more important than how other researchers explored MacArthur’s
idea “horizontally”, is how it was carried forward in new ways. MacArthur and his
colleagues started this trend themselves in 1961 when they found that the
diversity of birds in an area depends its foliage profile. The foliage profile was
defined as the foliage density plotted versus foliage height (MacArthur et. al.,
1961). A patch was defined as a certain foliage profile required by a given
species for its habitat (MacArthur et. al., 1961). This is a formalization of the idea
that different birds forage at different locations within the vegetation of a given
area (MacArthur, 1958). This definition suggests that the diversity of bird species
depends on the diversity of patches in an area (MacArthur et. al., 1962).
MacArthur and his colleagues tested this hypothesis in 1962 by attempting to
predict a bird census by measuring the habitat. It was found that a fairly accurate
census could be made by measuring the amount of foliage in three horizontal
layers at different heights above the ground (MacArthur et. al., 1962).
Another study performed across many different study areas with high
variation in vegetation complexity supported MacArthur’s (1958, 1961, 1962)
work. It found that bird species diversity was correlated with foliage height
diversity and percent vegetation cover (Willson, 1974). Essentially, as the
complexity of vegetation structure increased across a gradient, so did the
diversity of birds (Willson, 1974). Willson (1974) speculated that this may have
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been the result of an increase in structural heterogeneity leading to increased
opportunities for partitioning of foraging sites, which minimizes competition.
The development of all the research discussed thus far is based on the
underlying assumption that availability of food resources limits populations,
leading to competition, which structures communities. This assumption was first
challenged in 1960 by Hairston who stated that, “the world is green”. The fact
that the world is green suggests that an abundance of uneaten plants exist, so it
makes little sense for food to be a limiting resource for herbivores (Hairston,
1960). Hairston (1960) proposed that natural enemies must keep herbivore
populations below their carrying capacities, and predation, not competition for a
limiting resource, must drive community structuring.
Holt (1977) carried this idea forward with his definition of apparent
competition. Apparent competition occurs when a predator simultaneously
causes a decrease in the population density of one prey species and an increase
in the population density of another prey species (Holt, 1977). In other words,
two species interact with one another indirectly through the mediation of a shared
natural enemy. In cases where food resources are not limiting, apparent
competition may be more important than competition in defining community
structure.
In 1993, Martin further challenged the competition paradigm by presenting
a process that provides an alternative mechanism for the coexistence of species.
This process was nest predation. The potential-prey-site hypothesis states that
increases in the density of a patch type used by a particular species decreases
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the probability of predation by forcing would-be predators to search more
potential prey sites before finding an occupied site (Martin, 1993). Martin found
support for this hypothesis through his own research, and by reanalyzing the
data from Willson’s 1974 study on avian habitat structure. This reanalysis
showed that 20% more of the variation in species number was explained by
preferences in nesting site than by foraging preference (Martin, 1993).
Furthermore, it was found that predation rates increased for species that placed
a high proportion of their nests in nest sites similar to those used by other
coexisting species (Martin, 1993). Martin (1993) concluded that the importance
of nest site partitioning was greater than that of foraging site partitioning between
coexisting species, so nest predation must be a stronger force than competition
for food in driving this pattern.
Recently, the concept of apparent competition has again become popular.
Morris (2005) argues that apparent competition plays an important role in
structuring herbivorous insect communities that do not appear to compete for
food resources. She constructed a leaf-miner—parasitoid quantitative food web
to summarize the interactions between each insect species being studied. She
then removed all of the host plants for a particular specialist leaf-miner from
sample plots in order to remove that particular leaf-miner from the community.
She found that the removal of this leaf-miner decreased the density of a shared
parasitoid, which increased the density of other species that were hosts for that
particular parasitoid, in a clear case of apparent competition (Morris, 2005).
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Morris (2005) speculates that in tropical communities, competition kernels
may be relatively small for insect herbivores like the leaf-miners that are
frequently specialists in a highly diverse and heterogeneous landscape.
Competition kernels are a measure of the strength of competition experienced by
an individual due to the number of neighbors it has, and how close these
neighbors are (Morris, 2005). Apparent competition can increase the size of the
kernel, because it takes into account the predator-mediated influence of other
species that would otherwise not have been included because they feed on
different host plants (Morris, 2005). Apparent competition can be very important
in structuring these herbivorous insect communities, but its affects are dependent
on the range over which the natural enemies can move (Morris, 2005). Thus,
certain subpopulations of insects within a metapopulation may be affected by
apparent competition, and others not, depending on the mobility of the shared
host. This could result in spatial heterogeneity in species composition at scales
greater than that of the range of a given predator or parasitoid. Measuring this
scale over which indirect affects are important will likely be a primary target of
future research.
References:
Brown, K.M. 1981. Foraging Ecology and Niche Partitioning in Orb-Weaving
Spiders. Oecologia 50: 380 – 385.
Grinnell, J. 1922. The trend of avian populations in California.
56: 671 - 676.
Science
Hairston, N.G. 1960. Community Structure, Population Control, and
Competition. American Naturalist 94: 421.
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Holt, R.D. 1977. Predation, Apparent Competition, and Structure of Prey
Communities. Theoretical Population Biology 12: 197.
MacArthur, R.H. 1958. Population ecology of some warblers of northeastern
coniferous forests.
Ecology 39: 599 - 619.
MacArthur, R.H., and J. MacArthur. 1961. On bird species diversity.
594-598.
Ecology.
MacArthur, R.H., J. MacArthur, and J. Preer. 1962. On Bird Species Diversity.
II. Prediction of Bird Census from Habitat Measurements. The American
Naturalist 96: 167-174.
Martin, T. 1993. Nest predation and nest sites: new perspectives on old
patterns. BioScience. 43: 523.
Morris, R.J., O.T. Lewis, and H.C.J. Godfray. 2005. Apparent competition and
insect community structure: towards a spatial perspective. Annales Zoologici
Fennici 42: 449 – 462.
Willson, M.F. 1974. Avian Community Organization and Habitat Structure.
Ecology 55: 1017 – 1029.
Overall, well-written. Traces niche concept both backwards to Grinnellian niches
and forwards. Future directions?
Well written, and a great start with addressing major directions onward from
MacArthur, but then lost some momentum with just a string of anecdotes about
papers that built on MacArthur without discussing overall direction at present and
likely direction into the future.
20 B+
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