The Trophic Niche of Lizards in María Cleofas Island

I woke up after spending the whole night in motion and listening to the engine noise as a crib song. Some years ago, it had been a crab boat in Alaska and it was now equipped to do biological research in the northwest of Mexico. I am here after a few talks with Armando Escobedo; he told me about an amazing project to describe the diet of the lizards living on Maria Cleofas Island. My experience with lizards and trophic niches was scare, but I was motivated to learn about it. “Not every day I have the opportunity to meet the Marías Islands.”

This island with a biblical name, María Cloefas Island, is home to four lizard species. Anolis nebulosus (Clouded Anole), Aspidoscelis communis (Colima Giant Whiptail), Ctenosaura pectinata (Western Spiny-tailed Iguana), and a recently described endemic leaf-toed gecko, Phyllodactylus cleofasensis. After taking breakfast on the boat deck, Rafael (an undergraduate student like me), Armando and I were taken to the island in a small boat to evaluate the dietary variation of lizard species.

Fieldwork team and lizard species in María Cleofas Island

The main goal of our study was to describe the trophic niche of the lizard community, given that the species differ in foraging strategy. We expected to observe higher prey diversity in the active forager (Aspidoscelis communis) compared to the three sit-and-wait foraging species. Also, due to their different habitat use, we expected that the arboreal species (Anolis nebulosus and Ctenosaura pectinata) would share similar dietary niches, and that the terrestrial species (Aspidoscelis communis) might exhibit a partial dietary niche overlap with them. Finally, we expected that the saxicolous and nocturnal species (Phyllodactylus cleofasensis) would have the most distinct prey diversity.

We visited the island during eight weeks between 2017 and 2018. We performed diurnal and nocturnal surveys in all available habitats to manually capture individuals of the four lizards on the island. We obtained stomach contents from a total of 115 individuals using the stomach flushing technique. From this total of samples, 37 belonged to Anolis nebulosus, 11 to Aspidoscelis communis, 36 to Phyllodactylus cleofasensis, and 31 to Ctenosaura pectinata. Despite the movement of the ship, I could check the stomach contents under the stereoscope, and begin to determine the occurrence of each prey item eaten by each lizard species, for later calculatation of their prey diversity and determination of whether the lizards were generalists or specialists, as well as their degree of inter-individual specialization. Furthermore, we looked for similarities within species; therefore, we calculated their food resource overlap and their similarity index. In addition, we performed some analyses to examine differences in each food niche method and to determine if there was a difference in the prey eaten by each species and between years of surveys.

Insects, skin remains, and vegetal matter found in the stomach contents of the species.

We discovered, surprisingly, a wide variety of arthropods within the stomach contents of the lizards, regardless of their foraging strategy and habitat use! We identified 19 types of prey items such as insects, arachnids, gastropods, and centipedes, with a clear prevalence of beetles, spiders, and vegetation matter in the diets of the lizards. The diet of Anolis nebulosus was the most diverse, composed of 15 items, mostly arthropods, some vegetation matter, and their own skin remains. Aspidoscelis communis consumed 11 prey items, mostly arthropods, while Phyllodactylus cleofasensis consumed 10 prey items, mostly arthropods, some vegetation matter, and their own skin remains. We found nine items for Ctenosaura pectinata; surprisingly, we found a lower amount of vegetation matter, and the rest were arthropods.

Prey items found in the stomach contents of each species.

The Clouded Anole showed the highest richness of prey items in their stomachs; however, it was not the species with the highest prey diversity. Despite this, Anolis nebulosus exhibited greater prey diversity compared to other insular and continental populations. This expansion of its trophic niche could be attributed to the low predation pressure and high intraspecific competition on the island, which also influenced the phenotypic and behavioral plasticity of the species.

The actively foraging species Aspidoscelis communis showed the highest diversity of prey values. Phyllodactylus cleofasensis showed a significant variety of prey, while Ctenosaura pectinata displayed the lowest values across all three measures of food niche diversity. Thus, Anolis nebulosus, Aspidoscelis communis¸ and Phyllodactylus cleofasensis were generalist species with an increase in inter-individual specialization, while Ctenosaura pectinata remained close to the threshold between specialist-generalist feeding habits and little or no inter-individual specialization.

Prey-group diversity index and trophic niche breadth index of each species.

Based on habitat preferences, we expected that arboreal species (Ctenosaura pectinata and Anolis nebulosus) would exhibit similar prey diversity, and also that the actively foraging species, Aspidoscelis communis, would exhibit a greater prey variety compared to Ctenosaura pectinata. Our results through the different trophic niche approaches aligned with these expectations. Surprisingly, Phyllodactylus cleofasensis, despite being a nocturnal forager, demonstrated similar individual specialization as Anolis nebulosus.

Niche overlap and similarity among the lizard species.

The results from the discriminant functional analysis showed distinctive dietary patterns among lizard species. Aspidoscelis communis exhibited a diet divergence from the other lizard species; part of its diet could potentially be confused with Ctenosaura pectinata’s diet, while the diet of Phyllodactylus cleofasensis showed similarity with the diet of Anolis nebulosus (and vice versa). Finally, the diet of Ctenosaura pectinata had a relatively low overlap with Anolis nebulosus.

Diet overlap derived from the number of prey items per stomach among lizard species.

Our research on the lizard community of María Cleofas Island has not only demonstrated the wide dietary diversity among species, but has also expanded our understanding of trophic relationships in island ecosystems. Moreover, with this study, we have challenged conventional assumptions about resource partitioning and dietary niche diversity in insular ecosystems.

Don’t forget to take a look at the original paper; you will find some other amazing observations!