competitive exclusion

To: Birding-aus <>
Subject: competitive exclusion
From: "John Leonard" <>
Date: Fri, 20 Oct 2006 11:41:40 +1000
At the moment in Canberra we're having a bit of a crake and rail influx. On
our main 'pond' Kelly's Swamp, aka Jerrabomberra Wetlands, Baillon's,
Australian and Spotless Crakes, and Lewin's Rail have all been seen

This leads me to think how it is that all these crakes and rails can live in
the same area without competitively excluding one another. I suppose that
Lewin's Rails are bigger and so can do something that the smaller ones
can't, but how can the smaller crakes all co-exist (how many different ways
are there to pick over the mud and reed roots?)

My understanding of competitive exclusion is that no two species inhabiting
a habitat can share a particular food resource to any extent for long
without one competitively excluding the other. Exceptions would be where
food resources are super abundant for a time, or, which I suppose might be
the case with the crakes, where the habitats are only inhabited sporadically
and not to full capacity.

However what about stable habitats with large numbers of resident spp? The
example that springs to mind is the forests of Costa Rica, where there
dozens of spp of woodcreepers and allies, and woodpeckers and piculets,
which all, seemingly, run up and down trees and pick insects off them, or
from cracks in the bark, or from under loose bark, seem to co-exist. There
are size differences, and differences in beak length and shape, and in
different forests you get different suites of birds depending on altitude,
whether Pacific or Caribbean slope &c &c, but in any one place you can
expect to find at least six spp with these feeding strategies that seem to
co-exist in these places. In Australia I guess something like coastal wet
schlerophyll might be a similar habitat that is stable and where certain
niches seem to have several spp co-existing in them (eg Yellow-throated and
White-browed Scrubwrens and Pilotbirds as medium to small leaf-litter

Any suggestions on how these species can all pack into these niches?

John Leonard

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