Gustavo Arnaud

Ecological aspects of the land crab Gecarcinus planatus (Decapoda: gecarcinidae) in Socorro Island, Mexico

Abstract The results of a study are presented on the Socorro Island crab Gecarcinus planatus , a species inhabiting this oceanic island in a wide range of communities and altitudes. In February 1990, two transects in the forest community at Socorro Island were sampled and all crabs captured were recorded. Through mark-recapture methods the population density was estimated, and the individual home ranges were determined by means of Component Bivariate Model. The structure and phenology of the vegetation was recorded in the same two transects. It seems that crab density is related to cover of trees, particularl to Ficus cotinifolia, Guettarda insularis , and Bumelia socorrensis . In addition, the phenology of G. insularis also seems to be important to crab distribution. Home range of G. planatus is inversely associated to crab population density. Vegetation communities in Socorro Island are endangered by sheep overgrazing, so densities and home ranges of the crab in this forest may be affected in the future. To determine the real impact of exotic species on the structure and function of the communities present in Socorro Island (such as feral sheep) it will be necessary to study the relationships between vegetation and the different animal populations in the areas not strongly disturbed.

Feeding Ecology of the Endemic Rattleless Rattlesnake, Crotalus catalinensis, of Santa Catalina Island, Gulf of California, Mexico

Abstract Crotalus catalinensis is a rattleless rattlesnake endemic to Santa Catalina Island, in the Gulf of California, Mexico. It has been hypothesized that the lack of a rattle in this species is a stealth adaptation for hunting birds in vegetation. We provide detailed data on the diet of C. catalinensis from samples obtained during nine trips to the island in 2002–2004. Over two-thirds (70%) of the diet of C. catalinensis was composed of the Santa Catalina Deer Mouse (Peromyscus slevini). The remaining prey were lizards (Dipsosaurus catalinensis, Uta squamata, and Sceloporus lineatulus). There was an ontogenetic shift in diet and higher feeding activity during the dry season. The diet of this species is only a small subset of the diet of its supposed closest relative, C. ruber, probably as a result of limited diversity of prey on the island. The lack of birds in the diet of C. catalinensis argues against the supposed importance of birds as an essential feature for the hypothesis relating the lack of a rattle with a stealth hunting technique for birds in vegetation. However, since P. slevini is partially arboreal, there remains the possibility that the lack of a rattle is an adaptation for stealth hunting for mice in vegetation.

Exploring Hypotheses about the Loss of the Rattle in Rattlesnakes: How Arboreal Is the Isla Santa Catalina Rattleless Rattlesnake, Crotalus catalinensis?

Abstract Rattle loss in the Isla Santa Catalina (Baja California Sur, México) rattlesnake, Crotalus catalinensis, was hypothesized to be a stealth adaptation for hunting for birds within the vegetation. Recently, however, it was shown that birds are rarely captured by C. catalinensis, while a semi-arboreal mouse is very important in its diet. It was thus suggested that the loss of the rattle could be a stealth adaptation for hunting for mice within the vegetation. But both these hypotheses imply a high degree of arboreality in C. catalinensis, and there is no detailed study on microhabitat use in this species. Here we use field data recently obtained (2005 to 2008) to test the hypothesis of a high degree of arboreality in C. catalinensis. During nine visits to Isla Santa Catalina we searched for snakes in suitable habitats. Of 92 observations in which the snakes were exposed, in seven occasions (7.6%) they were on the vegetation. There was no difference between sexes or between young and adult snakes in the frequency of use of vegetation. Thus, C. catalinensis is a primarily terrestrial rattlesnake which uses the vegetation only occasionally, similar to its sister species Crotalus ruber. Results of a previous phase of our studies at Isla Santa Catalina are similar to those presented herein. These results make the hypothesis of the loss of the rattle as a stealth adaptation for hunting on the vegetation very unlikely, since such a low frequency of use of vegetation may not function as a selective agent that could lead to an adaptive loss of the rattle.

Stable isotope analyses suggest Mus musculus occupies the place of the “possibly extinct” Peromyscus guardia in Angel de la Guarda Island, (Mexico)

According the IUCN Red List criteria, species must be tagged as “possibly extinct” when there is a small chance that they may be extant. This means that the formal acceptance of the extinction is very dependent on search effort. Here, we report two unsuccessful attempts to detect the critically endangered (possibly extinct) Peromyscus guardia on Angel de la Guarda Island (Gulf of California, Mexico). Instead, we found that invader house mice seem to be very abundant just at the same habitat (beaches) where P. guardia were lastly reported. We analyzed the isotopic niche of the house mice, confirming that the species is currently occupying the presumed trophic niche (common use of marine resources: stranded remains and littoral invertebrates) of the last known populations of P. guardia. The apparent replacement of the specialized endemic species by the ubiquitous house mouse is a new evidence of the current trend towards global biodiversity homogenization.

Invasive migration of a mainland rodent to santa catalina island and its effect on the endemic species Peromyscus slevini

Santa Catalina Island has an endemic mouse, Peromyscus slevini, which is the only native rodent species on the island. However, specimens of P. fraterculus have been recorded on the island. P. fraterculus is the most common species of Peromyscus off Santa Catalina Island in the Baja California peninsula. The records show the absence of P. slevini in the 1990s and an increasing number of P. fraterculus during the 2000s. P. slevini has recently been collected in 2007. The current situation in Santa Catalina Island shows a strong expansion of P. fraterculus and the restriction of P. slevini to the canyons. This study confirms for the first time the recent invasion of a peninsular native species in one island on the Gulf of California. Peromyscus fraterculus is a better desert adapted species in contrast to P. slevini. Under those conditions, in the near future, P. slevini could be extinct.