Available at: http://digitalcommons.calpoly.edu/theses/1491
Date of Award
MS in Biological Sciences
Francis X Villablanca
This study focuses on a secondary contact zone between two sister species of woodrat, Neotoma fuscipes (dusky-footed woodrat) and N. macrotis (big-eared woodrat). Along the Nacimiento River, on the border of southern Monterey and northern San Luis Obispo counties, the ranges of these sister species of woodrats meet and overlap forming a secondary contact zone. The zone of secondary contact is estimated to include a 500-meter (~1,650 linear feet) portion of the Nacimiento River riparian corridor.
This research examines quantifiable morphological change that is likely associated with heightened inter-specific competition within the contact zone. When in sympatry the sister species may compete for resources indirectly through exploitative competition, or directly through contest competition, or through a combination of these two processes. The prediction that heightened competition has resulted in distinctive morphological character shifts between allopatric and sympatric populations was tested my examining size and shape of adult woodrats along a 20-kilometer transect. It was confirmed that adults woodrats of the two sister taxa are morphologically distinct (N = 602) and that the phallus morphology was indeed a reliable means to identify adult male woodrats as to species (p < 0.0001, N = 331). A two model approach was used to examine convergence and divergence in size and shape of woodrats across the transect. Neotoma fuscipes exhibited a statistically significant divergerence from N. macrotis with regard to breadth of rostrum (p < 0.0001, N = 414) in a region of sympatry along the Nacimiento River. Based on the results on one statistical model, N. macrotis exhibited a statistically significant convergence with regard to body-size (p = 0.0240, N = 587) and length of hind foot (p < 0.0001, N = 563) towards those of N. fuscipes between zones of sympatry and allopatry. Alternatively, based on the results of a second statistical model that accounted for environmental variation within the system both species exhibited a statistically significant divergence with regard to body-size (p = 0.0054, N = 587) and towards that of N. fuscipes between zones of sympatry and allopatry. Also, N. macrotis exhibited a statistically significant convergence with regard to length of ear (p = 0.0022, N = 563) towards that of N. fuscipes. Based on the results of both models, detectable re-patterning of size-independent traits was observed to varying degrees.
The morphological character shifts between sympatric populations and allopatric populations of woodrats suggest that ecological interactions between the species are occuring. Specifically, across the contact zone, patterns of variation in body-size and other morphological character traits are consistent with expectations of a combination of contest and exploitative competition.