Character Displacement - Conceptual Development

Conceptual Development

“Character displacement is the situation in which, when two species of animals overlap geographically, the differences between them are accentuated in the zone of sympatry and weakened or lost entirely in the parts of their ranges outside this zone” (Brown and Wilson 1956). While the term “ecological character displacement” first appeared in the scientific literature in 1956, the idea has earlier roots. For example, Joseph Grinnell, in the classic paper that set forth the concept of the ecological niche (1917), stated, “It is, of course, axiomatic that no two species regularly established in a single fauna have precisely the same niche requirements.” The existence of character displacement is evidence that the two species do not completely overlap in their niche requirement.

Following the dissemination of the concept, character displacement was viewed as an important force in structuring ecological communities, and biologists identified numerous examples. During the late 1970s and early 1980s, however, the role of competition and character displacement in structuring communities was questioned and its importance greatly downgraded (Losos 2000). Many found the early examples unconvincing and suggested it to be a rare phenomenon. Criticisms with earlier studies included the lack of rigor in statistical analyses and the use of poorly rationalized characters (Losos 2000, Dayan and Simberloff 2005). Additionally, theory seemed to indicate that the conditions that allowed character displacement to occur were limited (Losos 2000). This scrutiny helped motivate theoretical and methodological advances as well as the development of a more rigorous framework for testing character displacement (Losos 2000).

Six criteria have been developed to establish character displacement as the mechanism for differences between sympatric species (Schluter and McPhail 1992; Taper and Case 1992). These include: (1) differences between sympatric taxa are greater than expected by chance; (2) differences in character states are related to differences in resource use; (3) resources are limiting, and interspecific competition for these resources is a function of character similarity; (4) resource distribution are the same in sympatry and allopatry such that differences in character states are not due to differences in resource availability; (5) differences must have evolved in situ; (6) differences must be genetically based. Rigorously testing these criteria necessitates a synthetic approach, combining areas of research like community ecology, functional morphology, adaptation, quantitative genetics and phylogenetic systematics (Dayan and Simberloff 2005), While satisfying all six criteria in a single study of character displacement is not often feasible, they provide the necessary context for research of character displacement (Losos 2000, Dayan and Simberloff 2005).