The
late Robert MacArthur, one of the most distinguished
ecologists of this century, had a lifelong interest in
birds. One of his earliest scientific works was based on his
observation that keen ornithologists seemed to have an
"intuition" about which species of birds would be found in a
given habitat. He reasoned that if a birder could predict
that Northern Parulas, Ovenbirds, or Red-eyed Vireos would
be found in a given woodlot, then he or she must have based
the prediction on a visible feature or combination of
features of the woodlot. MacArthur endeavored to discover
what those features were and to see, for a start, if they
could be used to forecast, not exactly which species of
birds would be found in a woodlot, but how many different
species would live there. Using his own experience as
a bird watcher, he started with the hypothesis that bird
species diversity had something to do with the vertical
structure of the vegetation -- whether most of the foliage
was concentrated near the ground, up high, evenly
distributed, or whatever. It was a reasonable starting point
because, after all, some birds are associated with open
fields and others with mature forests. One does not seek
Worm-eating Warblers in prairies or meadowlarks on heavily
wooded slopes. To test his hypothesis, MacArthur developed a
quantitative index for the distribution of vegetation
density from the ground to the tops of the trees (if any).
He called that index "foliage height diversity." To calculate the index, a
white board is mounted at different heights on a pole, and
the proportion of it obscured by leaves at each height is
recorded. The proportions are combined into a single number
that is high if roughly the same amount of vegetation is
found at each height (grass, shrubs, and trees intermixed),
and low if the foliage is concentrated at a single height --
as in a grassland or in a forest with no
undergrowth. MacArthur counted the bird
species and calculated his index in a series of habitats. He
found that the diversity of bird species was proportional to
the index of foliage height diversity. He had made an
important discovery. From an avian point of view, the
physical structure ("physiognomy") of a plant community (how
the foliage is distributed vertically) is often more
important than the actual species of plants making up the
vegetation. Subsequent work by
ecologists John Wiens and John Rotenberry has shown that, on
a more local scale, floristic composition can hold the key
to bird diversity. Association with particular floras
appears to be largely a result of the differential ability
of plant species to provide food for birds. The basis of the
association is obvious for fruit, seed, and nectar feeding
birds, but less so for insectivores. However, different
kinds of plants can support quite dissimilar faunas of
insect herbivores, and thus be more or less attractive to
warblers, flycatchers, and the like. MacArthur's discovery was
the kind of principle that science seeks: a simple pattern
underlying seemingly great complexity, and one that
stimulates research that leads to further understanding of
how nature works. SEE:
Habitat
Selection;
MacArthur's
Warblers;
Bird
Communities and Competition. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.