The spatial configuration of tropical landscapes is characterized by the existence of archipelagoes constituted by forest remnants of different sizes immersed in a matrix of lands transformed to human-dominated systems. Overall, this extensive perturbation reduces the abundance of forest-dependent species. However, it is assumed small rodents can thrive in forest fragments due to their relative small area requirements, high reproductive rates and as an indirect consequence of the absence of some competitors and predators. We examined if population of small rodents (Heteromys desmarestianus and Peromyscus mexicanus) are able to persist in a hyperfragmented landscape in the tropical rainforest of Los Tuxtlas. We selected 6 fragments: 3 small (between 2-5 hectares) and 3 large (60-200 ha). In each of these fragments we set between 30-50 Sherman traps for periods between 3-6 days during 3 sampling periods. We establishing 10 transects (2x50 m) in each of the fragments in which we recorded and indentified all the trees with a diameter at breast height >1.0 cm; we conducted a literature search to identify how many of the recorded species were potentially consumed by rodents. Finally, we collected blood samples of the captured rodents to make smears to analyze the neutrophil-lynfocite ratio.
Results/Conclusions
In total we recorded a total of only 15 H. desmarestianus individuals and 46 P. mexicanus. Only 3 of the H. desmarestianus were recorded in small fragments. A similar situation occurred in P. mexicanus where only 2 individuals were recorded in small fragments. We found that two of the large fragments differed in their tree composition when compared to the rest of the fragments but only one of them had a greater frequency of tree species consumed by small rodents. Due to the reduced number of rodents recorded, we only contrasted neutrophil /lynfocite ratios between P. mexicanus rodents sampled the largest fragment (231 ha) and other of the large fragments (57 ha). We found significant differences (Wilcox test, W=47, p=0.027) in the neutrophil /lynfocite ratio (greater ratio in the smaller fragment than in the larger fragment). Our results indicate that even for small rodent predominant forest fragment size in Los Tuxtlas (median = 0.89 ha) might be too small to support viable populations. On the other hand, rodents in larger fragments (57 ha) show some evidence that might be subjected to relatively high levels of stress yet, further research in needed to corroborate this finding.