06 December 2009

Examples of Competitive Exclusion



 Pied-billed Grebe

1) HABITAT exclusion usually takes place between congenerics with no morphological specialization. For example, take Hermit Thrush and Swainson's Thrush in Maine. You find the Hermit in open undergrowth and the Swainson's in dense undergrowth. Swainson's is in both habitats on islands lacking Hermit! Or take Grebes in North Dakota. The Horned Grebe is found in small, deep lakes; the Eared Grebe is in large shallow lakes (both without vegetation); and the Pied-billed Grebe is found in all sizes with vegetation.


2. FEEDING SITE / MICROHABITAT. In the same general habitat Common Murre and Thick-billed Murre feed near same islands. Common Murres dive to middle of ocean while the Thick-billed goes to bottom. Perhaps the most famous study, and one that is in almost every Biology textbook, is MacArthur's warblers: five Dendroica warbkers feed in spruce trees but you find Cape May Warbler at very top; Blackburnian Warbler on the outside top; Black-throated Green Warbler on the mid-outside; Bay-breasted Warbler in the center; and the Myrtle Warbler feeds at the bottom and on the ground. Try finding this study with Google.

3. FOOD SIZE. Food size usually is determined by size rather than species. Willow Pitarmigan and Rock Ptarmigan winter together in river bottoms. The Rock eats small birch buds, while the Willow eats willow buds.

4. PERHAPS NEST SITE. North American Thrushes (?)

5. Competiton itself may affect competitive exclusion. An old study of Eastern Rock Nuthatch and Western Rock Nuthatch suggested that, in the zone of overlap, these species have different sized bills, whereas the bill size is similar where they do not overlap. Another study in the Galapagos Islands suggests that Small-billed Ground Finch have larger bills on islands that lack Medium-billed Ground Finch.

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