Ecologists continue to debate how different species manage to coexist. If two species use identical resources, such as food, invariably one will be more efficient and out-compete the other. The classical explanation is that each species has evolved morphological or physiological traits that allow it to exploit some resources more efficiently than all other species. Such partitioning of resources essentially provides each species with exclusive access to resources necessary for its survival. Although coexistence is often attributed to interspecific differences in morphology, direct evidence is relatively rare. Dabbling ducks, which include the ubiquitous mallard, are a good example. Dabbling ducks are primarily filter-feeders. They use lamellae, which are comb-like projections on the bill, to sieve food particles from pond water. Many ecologists, including Darwin, suggested that ducks coexist because interspecific differences in the spacing of bill lamellae allow each species to consume food particles of different sizes. Research published in the March issue of the American Naturalist by Brent Gurd of Simon Fraser University has demonstrated that interspecific differences in lamellar length, not spacing, allow ducks to partition food by size.
"Lamellar spacing alone does not lead to resource partitioning," said Gurd, "ducks with small spacing are more efficient than species with wide spacing because they retain a wider range of particle sizes than species with wide spacing. In order for variation in bill morphology to lead to resource partitioning, each trait must impose both costs and benefits on foraging ducks. It is the trade-off between these costs and benefits that allows resources to be partitioned."
"Foraging ducks are faced with the cost of separating food particles from detritus particles like sand and silt" said Gurd. "To do this, they alter the position of the upper and lower bill while they are feeding. This alters the size of the gap be
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