Ecological character displacement inPlethodon: Biomechanical differences found from a geometric morphometric study

Abstract

Ecological character displacement describes a pattern where morphological differences between sympatric species are enhanced through interspecific competition. Although widely considered a pervasive force in evolutionary ecology, few clear-cut examples have been documented. Here we report a case of ecological character displacement between two salamander species, Plethodon cinereus and Plethodon hoffmani. Morphology was quantified by using linear measurements and landmark-based geometric morphometric methods for specimens from allopatric and sympatric populations from two geographic transects in south-central Pennsylvania, and stomach contents were assayed to quantify food resource use. Morphological variation was also assessed in 13 additional allopatric populations. In both transects, we found significant morphological differentiation between sympatric populations that was associated with a reduction in prey consumption in sympatry and a segregation of prey according to prey size. No trophic morphological or resource use differences were found between allopatric populations, and comparisons of sympatric populations with randomly paired allopatric populations revealed that the observed sympatric morphological differentiation was greater than expected by chance. The major trophic anatomical differences between sympatric populations relates to functional and biomechanical differences in jaw closure: sympatric P. hoffmani have a faster closing jaw, whereas sympatric P. cinereus have a slower, stronger jaw. Because salamanders immobilize prey of different sizes in different ways, and because the observed sympatric biomechanical differences in jaw closure are associated with the differences in prey consumption, the observed character displacement has a functional ecological correlate, and we can link changes in form with changes in function in this apparent example of character displacement.

Analysis of Two-State Multivariate Phenotypic Change in Ecological Studies

“…To discern if the level of phenotypic change of one population is greater than another, the test statistic jD E1 À D E2 j is calculated (e.g., Adams and Rohlf 2000). To discern if two populations differ in the direction of their phenotypic change vectors, their vector correlation is calculated as the inner product of the two vectors scaled to unit length: Cheverud and Leamy 1985, Schluter 1996, Klingenberg and Leamy 2001, Be´gin and Roff 2003.…”

Character Shifts in the Defensive Armor of Sympatric Sticklebacks

“…Numerous studies have demonstrated evidence consistent with morphological divergence as a result of interspecific competition (e.g., Fjeldså 1983;Dayan and Simberloff 1994;Chiba 1999;Adams and Rohlf 2000). For example, Adams and Rohlf (2000) reported a case of character displacement between two salamander species. Sympatric Plethodon hoffmani populations have evolved a faster closing, albeit weaker, jaw than allopatric populations, whereas sympatric P. cinereus populations have evolved a slower, stronger jaw.…”

“…This pattern is most frequently attributed to ecological character displacement, evolutionary shifts of morphological traits in response to interspecific competition for resources between similar species (Brown and Wilson 1956;Grant 1972;Schluter 2000b). Numerous studies have demonstrated evidence consistent with morphological divergence as a result of interspecific competition (e.g., Fjeldså 1983;Dayan and Simberloff 1994;Chiba 1999;Adams and Rohlf 2000). For example, Adams and Rohlf (2000) reported a case of character displacement between two salamander species.…”

Multivariate and geometric morphometrics in the analysis of sexual dimorphism variation in Podarcis lizards

“…In lizard studies, total size is usually represented by snout-vent length (SVL) (Hews, 1996;Butler and Losos, 2002;Kratochvil et al, 2003), and shape variation is analyzed via statistical size correction of the rest of the body-parts. However, in recent years the development of geometric morphometrics has allowed the direct study of shape variation (Bookstein, 1984;Rohlf, 1990;Rohlf and Slice, 1990;Corti, 1993;Adams and Rohlf, 2000;Adams et al, 2004). Although the application of geometric morphometrics in SD studies still needs to be explored, some authors report interesting SD patterns revealed by the application of such techniques in various animal taxa (Hood, 2000;Rosas and Bastir, 2002;Loy et al, 2004;Rufino et al, 2004;Valenzuela et al, 2004;Bruner et al, 2005).…”