Climate change and animal performance in hot deserts

For animals living in hot environments day-to-day life requires significant amounts of water, and when it is really hot, birds must evaporate large quantities of water to maintain body temperature below lethal levels. This process is in direct conflict with the need to stay hydrated to maintain cellular function. Understanding how birds thermoregulate in the heat is a prerequisite for understanding their survival and persistence. Our research has shown that even desert species vary greatly in their capacities to deal with heat, the largest order of birds, the passerines dissipate heat by panting and appear to have the most limited capacity for heat dissipation. This makes passerines particularly vulnerable to heat injury during heat waves.  Our observations indicate that these differences may also importantly affect water balance and the long-term prospects for occupancy of hot deserts.

Our most recent work on reproduction in arid zone birds has demonstrated that even ecologically similar species often vary greatly in their response to warming and drought. Over a seven-year period, increasing air temperatures and drought in central New Mexico produced severe declines (>90%) in a Burrowing Owl population while the sympatric Loggerhead Shrike population increased by 30%. Interestingly, reproduction advanced by almost 30 days in shrikes, but was delayed by more than 14 days in owls. In both populations, reproductive success ranged well below the average for each species, but was driven by very different ecological processes among owls and shrikes.  As one might suspect, the drought and heat caused and measured decline in terrestrial arthropod abundance and was accompanied by nest failures and declining body masses in adults and nestlings.  Clutch size, the number of young produced and body condition did not decline in shrikes. Interestingly, higher air temperatures and drought produced higher predation pressure (snakes and coyotes) in these open cup nesters and thus it appears that top-down processes were driving reproductive success in shrikes and bottom-up processes were driving reproduction in owls. Both species are of special management concern and declining range-wide in the west, but as these data show even apparently similar sympatric species can experience very different selective pressures with warming and drought.

Cactus use by consumers in desert ecosystems

art provided by L. Wren Walraven

I am very interested in how resource systems provide nutrients for desert consumers and how climate variability affects resource abundance and use. Deserts are places where water is in short supply and resources are often scarce. Columnar cacti, such as saguaros are prominent features of many arid ecosystems and because of their extensive distribution and abundant production of large succulent fruit they are thought to play an important role in structuring food webs. My research in the Sonoran Desert traces cactus nutrients through the food web by taking advantage of the remarkably distinctive carbon and deuterium signatures in the tissues of cacti. By measuring the carbon isotope ratios of consumer tissues we can use these distinctive cacti values to make quantitative estimates of nutrient transfer between the cactus and individual consumers.

art provided by L. Wren Walraven

Our research has shown, for example, that saguaro fruit comprises more than 40% of the carbon intake by the bird community during the hot summer months. Although the saguaro provides avian consumers with large amounts of water and energy, we have found that its importance varies significantly with foraging ecology and digestive physiology. Insectivorous species such as flycatchers, thrashers and woodpeckers have gentle guts that are designed for digesting soft-bodied insects. As a consequence, when feeding on saguaro fruit, these species obtain water and energy from the fruit pulp, but cannot extract the proteins and lipids from the seeds, which they excrete whole. Granivores, such as finches, sparrows and doves, in contrast, have guts that are designed to grind up seeds and readily extract these other nutrients. Thus, digestive physiology plays a critical role in determining the nutrients obtained by individual species.

Our research on the small mammal community tells a very different story; cactus resources are of very limited importance to most members of the rodent community. The rodent community at large ignores cacti except for a brief period when the saguaro’s seed rain produces a superabundant food source. Our measurements of consumer tissues indicate that only the white-throated wood rat makes extensive use of cacti stems and appears to use cacti as its primary source of water. These results demonstrate that stable isotope approaches can detect both the spatial grain at which animals use the environment and describe even subtle differences in how individuals and populations use a single resource.

Mechanisms and models underlying the use of stable isotopes in ecology

Rigorously interpreting ecological data that rely on stable isotope measurements of animal tissues requires understanding the underlying physiological processes that produce these patterns in nature. Towards these ends, we have conducted experiments in the lab directed at understanding how body size, thermoregulatory strategy, and an animal’s pace of life affects dynamic metabolic processes such as growth, reproduction, and deuterium, nitrogen and carbon turnover in a variety of vertebrates. We have found, for example, that deuterium is poor tracer of water movement in birds unless large differences exist between water sources - a condition that is rarely found in nature.  We have spent considerable time with laboratory experiments that describe carbon dynamics in slow-growing animals such as lizards and tortoises. We have produced the first carbon turnover rate data for tortoises and lizards, which shows that turnover rates in ectotherm tissues are dramatically slower than in those of comparably sized endotherms.