Population structure of Poecilia mexicana (native) and Poeciliopsis gracilis (non-native) in a subtropical river

Despite the increase of exotic species in bodies of water, studies on the population structure of coexisting native and exotic fish species are rare. We evaluated the population structure of the native Poecilia mexicana and non-native Poeciliopsis gracilis species in an environmental gradient and its relation to the habitat characteristics in the Jalpan River, located in the state of Queretaro, Mexico. We calculated size structure, sex ratio, gonad stage, and condition factor for both species. Visual-based habitat quality, riparian quality, and physical and chemical characteristics of water were also evaluated. A total of 322 individuals of P. gracilis and 762 of P. mexicana were captured with different fishing nets due to the habitat heterogeneity. Study sites were classified into three categories of habitat conditions: poor, marginal, and sub-optimal. P. mexicana females showed a complete structure of sizes in the poor habitat category. P. gracilis did not show a complete structure in any habitat category. The sub-optimal category of habitat conditions had the lowest number of specimens for both species. Poor category showed a sex ratio of 1:1 (females: males) for P. mexicana while in P. gracilis, females were more abundant than males (sex ratio 5:1). Fulton's factor was higher for P. mexicana than for P. gracilis. Our results showed that habitat quality of the river affected size structure, stages of development, and sex ratio of the two species, but did not show conclusive evidence of favored species or reproductive competition.

Effects of human-made resource hotspots on seasonal spatial strategies by a desert pitviper

Habitat heterogeneity and local resource distribution play key roles in animal search patterns. Optimal strategies are often considered for foraging organisms, but many of the same predictions are applicable to mate searching. We quantified movement and space use by a pitviper to test whether Native Habitats (NH) and human-made Resource Hotspots (RH) facilitate alternative seasonal spatial strategies as a result of critical resources, including potential mating partners, being widely dispersed in NH and clustered in RH. Independent of habitat category, seasonal patterns resembled an intermediate mating system with elements of prolonged male mate-searching and female-defense. However, individuals using primarily NH or RH exhibited alternative strategies. NH rattlesnakes displayed greater movement and larger home ranges than RH rattlesnakes across behavioral seasons. NH males increased movement distances and home ranges during the mating season, while RH males displayed minimal or no seasonal shifts. NH females also elevated movement distances during the mating season, while RH females showed no significant seasonal differences. Despite contrasting spatial patterns, mating success and female-defense effort were not significantly affected by habitat category. This unique study system highlights the potential for interactions among sexual selection, habitat heterogeneity, and behavioral plasticity to facilitate divergent search tactics within populations.