A
biological community
consists of all of the organisms -- microbes, plants, and
animals -- that live in an area. A community, together with
the physical environment to which it is tied by a series of
processes (such as the production of oxygen by plants and
its use by animals, and the reciprocal production of carbon
dioxide by animals and its use by plants), is called an
ecosystem. How the species composition of communities is
determined, and how those species interact with each other
and with their inanimate surroundings, are major foci of
ecological research. In order to simplify the
investigation of communities, ecologists usually study some
subset of the organisms present, either a functional complex
such as a group of herbivores and the plant species they
feed on, or a taxonomic group, such as the resident insects
or birds. Thus one may read about "mammal communities" or
"bird communities." Unlike mammals (which are often
nocturnal and secretive), birds are relatively easy to
observe; hence, a great deal of research has been done on
avian communities. Much of that research has
been focused on a seemingly simple question: Are the birds
that compose a community merely a chance association of
species that share similar tolerances to a physical
environment, or do biological interactions among the birds
determine which species are included in a community and
which are excluded from it? In particular, interest has
centered on whether competition -- two or more species using
the same limited resource -- is responsible both for
excluding some species from communities and for causing
certain kinds of differences to evolve between the species
that do live together. So far, no simple answers to these
questions about species composition have emerged. Many avian ecologists do
believe that competition can be an important factor
influencing both the composition of bird communities and the
behavioral and morphological characteristics (structure,
color, etc.) of the birds themselves. That belief is based
on diverse lines of evidence, such as many observations of
closely related birds apparently dividing up resources such
as food or suitable space for territories, or one species
excluding another from apparently desirable habitat. For
example, in the northwestern United States males of both
Red-winged and Yellow-headed Blackbirds set up territories
in open marshes. The Redwings arrive earlier in the spring
and occupy the entire marsh. When the Yellowheads fly in,
they take over the best territories (areas of cattails and
other plants in deep water that harbor the richest insect
life) and force the Redwings into the shallower, drier, more
marginal habitats. The Redwings are able to breed
successfully in these areas, however, while the Yellowheads
are unable to exploit the less productive sites
successfully. The Yellowheads are bigger,
perhaps a result of natural selection favoring the ability
to oust Redwings from high-quality areas, perhaps to eat
larger seeds in the winter, or perhaps both -- or perhaps
neither! Many such details remain uncertain, but it is clear
that the Redwings have a broader niche (loosely, "way of
life") than do the Yellowheads. Nevertheless, Yellowheads
are better competitors within their own narrow niche, and
thus are able to exclude the Redwings from it. Territorial
habitat is a scarce resource; the Yellowheads take the
richest and the Redwings get the rest. Dividing up resources
(resource partitioning) often takes the form of similar bird
species either feeding in different parts of the same
habitat (as do some closely related warblers that live
together) or else taking food of different sizes. The former
is evident from differences in foraging behavior; the latter
is often inferred from differences in bill size of the
species. One approach to detecting
the existence of competitive interactions in bird
communities is to determine whether closely related species
are distributed independently over a large sample of similar
habitats, or whether the presence of one or more species has
an influence on the others. For four years, ecologists
Catherine Toft, David Trauger, and Horatio Murdy analyzed
statistically the distribution of five duck species breeding
in 236 ponds in the Northwest Territories of Canada. Three
of the ducks were dabblers: Mallard, American Wigeon, and
Green-winged Teal. Two were divers: Lesser Scaup and
Ring-necked Duck. The species showed significant differences
in both time of hatching and pond-size use. For instance,
the dabblers nested early, running the risks of springtime
inclement weather. The divers nested later, risking the loss
of their broods with the arrival of the fall freeze.
Mallards showed little preference for pond size, teal
strongly preferred small ponds, and wigeon and divers
preferred large ponds (and were almost never found in small
ones). Detailed analysis of the results led to the
conclusion that competition, past and present, was
responsible for patterns of pond use, and partially
responsible for the temporal differences in dabbler and
diver breeding. The most dramatic current competition was
between the two divers; the scaup and Ring-necks would often
occupy the same pond in different years, but not occur there
together. Some of the most interesting
studies of avian communities that consider the distributions
of related species have been done not on North American
birds, but on assemblages of birds found in different parts
of New Guinea and on other islands of the southwest Pacific.
The species compositions of these communities have been
documented by ecologist Jared Diamond, who found many cases
of "checkerboard" distributions, sort of an expanded version
of the scaup and Ring-neck situation, with certain
combinations of closely related species never found. For
instance, Diamond searched for two species of Macropygia
cuckoo doves on 33 islands of the Bismarck Archipelago. He
found one of the species on 14 islands, the second on 6
other islands, and neither on 13 islands. The two kinds of
cuckoo doves never lived together on the same island.
Similarly, two small nectarivores, the Black Sunbird
(Nectarinia sericea) and the Bismarck Black Honeyeater
(Myzomela pammelaena) occupy 41 islands in the archipelago.
The sunbird is found on 18, the honeyeater on 23, but no
island plays host to both. The two birds, although they
belong to different families, are quite similar in color,
size, foraging techniques, and habitat preference. Diamond
concluded from finding such patterns in several groups of
ecologically similar species that certain combinations of
species were "forbidden" by competition. That conclusion has
been challenged, but Diamond's interpretation seems correct
to us. His work provides some of the most persuasive
evidence that competition can play a key role in determining
which species are found in a given bird
community. So competition occurs, but
how common it is, and the degree to which it is responsible
for the composition of bird communities worldwide, remain
unresolved. The arguments are complex and technical. They
involve a wide range of questions, some of which reflect the
difficulties of studying birds. How are the resources fed
upon by birds best measured? How can accurate censuses of
bird populations be obtained? Do spot observations of birds
feeding provide unbiased information about foraging
behavior? Might birds behave differently in relatively open
situations where they are readily observed from the way they
behave when out of sight in thick foliage? How does one
interpret observed differences in bill size or shape among
closely related species? Are these differences a result of
natural selection produced by competition, by the need for
members of the same species to recognize one another, or by
other factors? Other questions involve
difficult statistical issues -- such as how one determines
if an assembly of bird species is a "random" assortment,
that is, a chance subset of the species that theoretically
might be members of a community. How strongly do factors
(such as predation and random disturbances) other than
competition affect community composition? Careful
observations and experiments on many more bird communities
will be required before these questions can be answered with
assurance. SEE: Bird
Guilds;
Bills;
Dabblers
vs. Divers;
MacArthur's
Warblers;
Metabolism. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.