When landscape modification is advantageous for protected species. The case of a synanthropic tarantula, Brachypelma vagans

Research by Salima Machkour-M’Rabet, Yann Hénaut, Sophie Calmé, and Luc Legal

Post by Charlotte Kanzler

            Habitat fragmentation and its effects on wild species is a problem which ecologists must confront in an increasingly urban world. Fragmentation occurs when development or another disturbance splits a piece of habitat, decreasing the amount of suitable habitat and increasing the distance (isolation) between those useable patches. This can make it more difficult for a species to disperse from one patch to another, and, as consequence, decreases the genetic diversity of the population, which may suffer from inbreeding. In spite of many theoretical reasons why habitat fragmentation is harmful to a population, empirical evidence reveals a more complicated picture, with some research bearing out these negative effects, while other studies show a neutral or positive effect on biodiversity. The inconsistency of these results make it valuable to study the effects of habitat fragmentation under a variety of conditions and in a variety of species.

            Though fragmentation has some relevance to many species, the researchers in the article in question focused on tarantulas: specifically, certain species in the South American tarantula genus Brachypelma. This genus is at risk because of their members’ limited distribution, high juvenile mortality rates, and long maturation time (7-8 years in males, 9-10 years in females), as well as their high value in the pet trade. These factors combined to make Brachypelma an endangered genus, potentially more vulnerable to fragmentation than other tarantula species.

In the article under question, researchers wanted to figure out if habitat fragmentation was jeopardizing populations of Brachypelma vagans. To do so, Machkour-M’Rabet and contemporaries captured thirty different tarantulas each from the open central plazas of six different villages in the Mexican Caribbean. Five of the populations were continental; the one isolated island population, nestled on Cozumel island, was traced back to a release of tarantulas used in a movie production after filming in 1971.

The region in the Yucatan Peninsula from which Brachypelma vagans were drawn for this study. Note the population isolated on Cozumel Island. Image from Machkour-M’Rabet et al., 2011.

 The plazas on which the tarantulas were collected were oases of suitable habitat surrounded by unsuitable forest, agricultural field, and human development. A piece of the limb was taken from each tarantula (not ideal, but they grow back) in order to perform genetic analysis. Researchers were seeking the presence or absence of loci which could be used to determine the amount of gene flow and the genetic differentiation between sampled populations. Contrary to expectations, the genetic diversity within each population was high, indicating that adults were still able to travel between populations. This movement offset processes such as genetic drift which, reduce genetic variability. Though the island population had less diversity than the others, it was still within acceptable ranges, evidencing its recent isolation.

The results of the Machkour-M’Rabet study indicated that the tarantulas existed in a metapopulation, a population comprised of populations connected by dispersal events - dispersal that, instead of being limited by human development, may have been aided by it (for example, tarantulas can move more easily along gently-used dirt roads cutting through a forest). They assumed that dispersal events were fueled mainly by the males of the species, which range out to find solitary females shortly after they reach maturity.

A typical tarantula burrow, of the species Aphonopelma iodius. Image from http://wildaboututah.org/images/pitts.2004.05.18zc_51804b_DSCN4563.jpg.

           

            Though they assume that dispersal was caused by the males of the species, the authors of the study never explicitly linked male dispersal, habitat fragmentation, and genetic diversity - i.e., they never actually tracked the males during breeding season. This leaves open the question of how important the dispersing males are to the persistence of each population, genetic or otherwise. Though the population may not suffer from genetic homogeneity, if too many males die while migrating, the populations may still be at risk. My grant proposal revolves around some of the questions left unanswered in this report, as well as in the realm of tarantula research at large. Specifically, I’d like to take a closer look at the effect of habitat fragmentation on male mating dispersal in a common desert tarantula, Aphonopelma chalcodes. However, I will assess my results through the lens of population ecology: how differential mortality between sexes could impact population persistence. The effect of sex-based dispersal on metapopulations is not only an understudied aspect of tarantula ecology, but of metapopulation dynamics as a whole: another piece in the puzzle of population dynamics in the face of human intervention.

           

Reference:

Machkour-M’Rabet, S., Hénaut, Y., Calmé, S., Legal, L.2011. When landscape modification is advantageous for protected species. The case of a synanthropic tarantula, Brachypelma vagans. Journal of Insect Conservation 16:479-488.