Gage H. Dayton

Community Ecology of Desert Amphibians

The distribution and abundance of organisms is influenced by historical, abiotic, and biotic factors. My work focuses on determining how abiotic and biotic factors shape the composition and structure of species at multiple spatial scales. My approach relies on extensive field surveys, laboratory and field experiments, and GIS modeling. My research program has been centered on the factors influencing the distribution and abundance of amphibians that inhabit desert environements.

The Role of Microhabitat

I examined microhabitat segregation of the aquatic environment among four desert anurans: Scaphiopus couchii (Couch’s Spadefoot), Bufo debilis (Western Green Toad), Bufo punctatus (Red-spotted Toad), and Gastrophryne olivacea (Narrow-mouthed Toad). I surveyed the occurrence of species at breeding sites, and examined variation in microhabitat characteristics among breeding sites. I also conducted reciprocal transplant experiments of tadpoles in mesh enclosures to test the species performance in different habitats in the absence biotic factors. The four species exhibited non-random patterns of co-occurrence at breeding sites across the Chihuahuan Desert landscape. Canonical axes derived from discriminant analysis revealed species differed in breeding habitat characteristics along an environmental gradient dominated by substrate and bank cover. At one end of the gradient B. punctatus was associated with sites characterized by rocky substrate and low bank cover, at the other end of the axis B. debilis was associated with sites that had clay and silt substrata and high bank cover. Scaphiopus couchii and G. olivacea exhibited intermediate scores but were more similar to B. debilis than to B. punctatus. Reciprocal transplant experiments of larval anurans among sites where they naturally occurred and sites where they were not known to occur, revealed no significant differences in survivorship for any of the species. Taken together, my results suggest that although abiotic components of the breeding site differed among some species, it seems that breeding site characteristics play a minor role in the non-random patterns of occurrence of these species across the landscape.

The Role of Predation

The impacts that predators have on prey behavior, growth, survival, and ultimately the composition of many ecological communities, should depend upon prey defenses and the susceptibility of prey to predators. In short-lived pools in the desert predators are relatively rare and several studies have shown that amphibians that breed highly these habitats have larvae that are very susceptible to predation. One of the studies I conducted hypothesized that prey populations inhabiting short-lived, species-poor, aquatic environments are unconditioned to predators and therefore are highly susceptible to predation. However, I hypothesized that the resultant decrease in prey density and increase in per capita resources (due to high susceptibility to predators) should enhance overall cohort survival because of enhanced growth of surviving prey. To test these ideas, I performed experiments to examine multiple effects of predators on tadpoles of Couch’s Spadefoot Toad (Scaphiopus couchii), that inhabit highly ephemeral pools in desert environments. Chemical and lethal cues of predators elicited no response in behavior, development, or morphology, indicating a lack of defensive mechanisms. Survivorship was significantly reduced in predator treatments. This prey thinning led to accelerated time to metamorphosis, an advantage to survivors who must metamorphose before ephemeral ponds dry. These experiments demonstrated that in short-lived environments rarely colonized by predators, prey may exhibit little or no response to the presence of predators. As a result, direct effects of predators may actually result in an increase in overall cohort survival because of reduced density-dependent effects on prey. Although predators are relatively rare in highly ephemeral aquatic environments they may play an important role in facilitating the long-term persistence of their prey by reducing prey density. The manuscript for this study has been submitted and is under review.

Landscape Level Patterns

A fundamental step in conserving biodiversity is identification of quality habitat needed to sustain populations of target species. I used coarse scale environmental features (soil type, slope, elevation, and proximity to drainage channels) to predict habitat suitability for four species of desert amphibians in Big Bend National Park, USA: Scaphiopus couchii, B. debilis, Bufo punctatus, and Gastrophryne olivacea. Model output was evaluated using data from 7 years of breeding site surveys. Overall our models performed very well for evaluating where species were likely to occur; however, results varied among species. Suitable habitat for B. punctatus and G. olivacea extended over greater proportions of the study area and encompassed a wide variety of habitats compared to suitable habitat for S. couchii and B. debilis. Our models performed better at predicting where S. couchii and B. debilis were likely to occur compared to B. punctatus and G. olivacea. The variation in the predicted suitable habitat among these, as well as the agreement between model output and breeding site use, elucidates the fact that developing single species habitat suitability models is a more appropriate approach then trying to develop to multi-species models. Our study provides the first habitat suitability models for desert amphibians and provides important information for conservation biologists and land managers concerned with preserving amphibian diversity in xeric landscapes.

Predicted habitat suitability for Scaphiopus couchii and Bufo debilis in Big Bend National Prk.

Baseline Surveys

A fundemental first step in all ecological stuides is to determine where the focal organisms occur in the study area. I conducted surveys for amphibians in Big Bend National Park, Texas, Cañon de Santa Elena Protected Area, Mexico, and the Maderas del Carmen Protected Area, Mexico. In Big Bend National Park we found Red-spotted Toads (Bufo punctatus), Western Green Toads (B. debilis), Texas Toads (B. speciosus), Narrow-mouthed Toads (Gastrophryne olivacea), Couch’s Spadefoots (Scaphiopus couchii), Rio Grande Leopard Frogs (Rana berlandieri), Bullfrogs (Rana catesbeiana), Canyon Treefrogs (Hyla arenicolor), and Spotted Chirping Frogs (Syrrhophus guttilatus). In Cañon de Santa Elena Protected Area we found B. debilis, B. punctatus, B. speciosus, G. olivacea, S. couchii, R. berlandieri, R. catesbeiana, S. guttilatus and H. arenicolor. In the Maderas del Carmen Protected Area we found B. punctatus, B. debilis, B. speciosus, G. olivacea, S. couchii, R. berlandieri, and H. arenicolor. Two species, Southwestern Woodhouse’s Toads (Bufo woodhousii) and Mexican Spadefoots (Spea multiplicata), have been collected in Big Bend National Park and are thought to occur in the Mexican Protected Areas, but were not detected in any of our surveys. The results of our surveys provide a baseline inventory of amphibians in protected areas in the Big Bend region of the Chihuahuan Desert at the beginning of the 21st century.

Localities of Hyla arenicolor throughout Big Bend National Park, Canon de Santa Elena Protected Area, and the Maderas del Carmen Protected Area

Photos and Film Clips of Desert Scenes

Videos

Scaphiopus Chorus- Scaphiopus couchii (Couch’s spadefoot) calling immediately after a rain. Gastrophryne olivacea (Plains Narrow-mouthed Toad) can be heard in the background.

Flood- Tornillo Creek is bone dry 95% of the year. During monsoon storms flash floods are common.

Photos

Permanent sites such as this pool in Las Pilas (Cañon de Santa Elena) provide habitat for Rana berlandieri (Rio Grande Leopard Frog) and Hyla arenicolor (Canyon Treefrog). Species that breed in ephemeral pools are excluded from these sites to due heavy predation pressure.

A typical ephemeral pool in the Chihuahuan Desert. Scattered pools such as this one provide breeding habitat for Bufo punctatus (Red-spotted Toad).

The flow of the Rio Grande has been greatly reduced over the past century do to upstream irrigation and water consumption. Large and medium scale floods have all but ceased and the river has become channelized by the encroachment of non-native vegetation such as Arundo donax (Giant Reed) seen above.