Field tests of a general ectotherm niche model show how water can limit lizard activity and distribution

Abstract

Mechanistic forecasts of how species will respond to climate change are highly desired but difficult to achieve. Because processes at different scales are explicit in such models, careful assessments of their predictive abilities can provide valuable insights that will be relevant to functionally similar species. However, there are surprisingly few comprehensive field tests of mechanistic niche models in the literature. We applied a general, thermodynamically grounded modeling framework to determine the fundamental niche of an extremely well‐studied herbivorous ectotherm, the sleepy lizard Tiliqua rugosa. We then compared the model predictions with detailed long‐term field observations that included sub‐hourly data on microclimate, activity levels, home ranges, and body temperatures as well as annual to decadal patterns of body condition and growth. Body temperature predictions inferred from gridded climatic data were within 10% of empirically observed values and explained >70% of observed daytime activity patterns across all lizards. However, some periods of activity restriction were explained by predicted desiccation level rather than by temperature, and metabolically driven activity requirements were much lower than potential activity time. Decadal trajectories of field growth and body condition could also be explained to within 10% of observed values, with the variance in trajectories being attributable to whether individuals had access to permanent water. Continent‐wide applications of the model partly captured the inland distribution limit, but only after accounting for water limitations. Predicted changes in habitat suitability under six climate change scenarios were generally positive within the species’ current range, but varied strongly with predicted rainfall. Temperature is regarded as the major factor that will restrict the distribution and abundance of lizards and other terrestrial ectotherms under climate change. Yet our findings show how water can be more important than temperature in constraining the activity, habitat requirements, and distribution limits of terrestrial ectotherms. Our results demonstrate the feasibility of first‐principles computation of the climatic limits on terrestrial animals from gridded environmental data, providing a coherent picture for how species will respond to climate change at different scales of space and time.

Water restriction induces behavioral fight but impairs thermoregulation in a dry‐skinned ectotherm

“…The earlier activity of water‐restricted females could thus be a strategy to forage for water and perform standard activities in conditions in which desiccation risk is minimized. This result is consistent with those of empirical studies and quantitative models suggesting that changes in water availability can be a dominant driver of seasonal or spatial changes in daily activity patterns in squamate reptiles, and more generally in terrestrial ectotherms (Davis and DeNardo , Wells , Kearney et al ).…”

“…Water is a critical resource and a limiting factor for terrestrial animals living in chronically warm and dry environmental conditions such as xeric or semi‐xeric environments (Davis and DeNardo , Zylstra et al , Long et al , Kearney et al ) as well as during droughts and warm spells in mesic or tropical habitats (Marquis et al , Anderson and Andrade ). Thus, terrestrial animals have evolved plastic behavioral responses to cope with spatio–temporal fluctuations in water availability and maintain a homeostatic hydration state.…”

“…Such behavioral fight responses can improve water balance by limiting water loss and/or by increasing metabolic or dietary water intake. For example, some lizard species reduce their behavioral or locomotor activity during periods of water restriction to reduce evaporative water loss (Lorenzon et al , Davis and DeNardo , Kearney et al , Pirtle et al ) while other species shift their activity towards time periods with moister conditions and a higher availability of free standing water (Davis and DeNardo , Kearney et al ). Plasticity of microhabitat selection is also critical for some organisms to avoid water stress and it may include selection of shadier basking or retreat sites during periods of activity and differential use of moist and cold shelters during periods of inactivity (e.g.…”

When water interacts with temperature: Ecological and evolutionary implications of thermo‐hydroregulation in terrestrial ectotherms

“…There is indeed great scope to improve our understanding of the behavioral responses of ectotherms to variation in water availability and hydric conditions relative to thermal conditions, and to disentangle temperature and water effects on behavior (Kearney et al, 2018;Pintor et al, 2016;Rozen-Rechels et al, 2018). There is indeed great scope to improve our understanding of the behavioral responses of ectotherms to variation in water availability and hydric conditions relative to thermal conditions, and to disentangle temperature and water effects on behavior (Kearney et al, 2018;Pintor et al, 2016;Rozen-Rechels et al, 2018).…”

“…. Mechanistic models implemented to quantify simultaneously water and nutrient dynamics are therefore essential to predict the consequences of energy and water budgets on the life-history traits and population dynamics of ectothermic species(Kearney, Munns, Moore, Malishev, & Bull, 2018;Kearney et al, 2013). These processes altogether determine the mass balance equations for nutrient-energy allocation into life-history traits.…”