Diving through the thermal window: implications for a warming world

Diving in hot water: a meta-analytic review of how diving vertebrate ectotherms will fare in a warmer world

Diving ectothermic vertebrates are an important component of many aquatic ecosystems, but the threat of climate warming is particularly salient to this group. Dive durations typically decrease as water temperatures rise; yet, we lack an understanding of whether this trend is apparent in all diving ectotherms and how this group will fare under climate warming. We compiled data from 27 studies on 20 ectothermic vertebrate species to quantify the effect of temperature on dive durations. Using meta-analytic approaches, we show that, on average, dive durations decreased by 11% with every 1°C increase in water temperature. Larger increases in temperature (e.g. +3°C versus +8-9°C) exerted stronger effects on dive durations. Although species that respire bimodally are projected to be more resilient to the effects of temperature on dive durations than purely aerial breathers, we found no significant difference between these groups. Body mass had a weak impact on mean dive durations, with smaller divers being impacted by temperature more strongly. Few studies have examined thermal phenotypic plasticity (N=4) in diving ectotherms, and all report limited plasticity. Average water temperatures in marine and freshwater habitats are projected to increase between 1.5 and 4°C in the next century, and our data suggest that this magnitude of warming could translate to substantial decreases in dive durations, by approximately 16-44%. Together, these data shed light on an overlooked threat to diving ectothermic vertebrates and suggest that time available for underwater activities, such as predator avoidance and foraging, may be shortened under future warming.

Water loss and temperature interact to compound amphibian vulnerability to climate change

Ectotherm thermal physiology is frequently used to predict species responses to changing climates, but for amphibians, water loss may be of equal or greater importance. Using physical models, we estimated the frequency of exceeding the thermal optimum (T_opt ) or critical evaporative water loss (EWL_crit ) limits, with and without shade- or water-seeking behaviours. Under current climatic conditions (2002-2012), we predict that harmful thermal (>T_opt ) and hydric (>EWL_crit ) conditions limit the activity of amphibians during ~70% of snow-free days in sunny habitats. By the 2080s, we estimate that sunny and dry habitats will exceed one or both of these physiological limits during 95% of snow-free days. Counterintuitively, we find that while wet environments eliminate the risk of critical EWL, they do not reduce the risk of exceeding T_opt (+2% higher). Similarly, while shaded dry environments lower the risk of exceeding T_opt , critical EWL limits are still exceeded during 63% of snow-free days. Thus, no single environment that we evaluated can simultaneously reduce both physiological risks. When we forecast both temperature and EWL into the 2080s, both physiological thresholds are exceeded in all habitats during 48% of snow-free days, suggesting that there may be limited opportunity for behaviour to ameliorate climate change. We conclude that temperature and water loss act synergistically, compounding the ecophysiological risk posed by climate change, as the combined effects are more severe than those predicted individually. Our results suggest that predictions of physiological risk posed by climate change that do not account for water loss in amphibians may be severely underestimated and that there may be limited scope for facultative behaviours to mediate rapidly changing environments.

Diving beyond Aerobic Limits: Effect of Temperature on Anaerobic Support of Simulated Predator Avoidance Dives in an Air-Breathing Ectotherm

Diving optimality models predict air breathers to routinely dive within aerobic limits, but predator avoidance dives may be an exception. Lengthening submergence times during a predation threat may enhance survival probability, and we therefore hypothesized that predator avoidance dives in juvenile estuarine crocodiles (Crocodylus porosus) would be partially anaerobically fueled. We also predicted that reliance on anaerobic metabolism would increase at elevated temperatures to offset the faster depletion of body oxygen stores. Crocodiles were maintained at 28° and 34°C for 60 d and subsequently underwent simulated predator avoidance dive trials at two test temperatures (28° and 34°C). Blood was sampled immediately on surfacing to measure plasma lactate concentrations relative to nondiving (control) values. Aerobic dive limits (cADL; min) were also calculated using known body mass and oxygen storage relationships and rates of diving oxygen consumption and compared with observed dive durations. Postdive plasma lactate levels were elevated beyond resting levels at both test temperatures, indicating that aerobic thresholds were surpassed during simulated predator avoidance dives. Similarly, ≥90% of dive durations exceeded cADLs at both test temperatures. Postdive plasma lactate concentrations were independent of water temperature and thermal acclimation treatment. Together, these findings suggest that reliance on anaerobiosis during simulated predator avoidance dives is important regardless of temperature.

An adaptive transgenerational effect of warming but not of pesticide exposure determines how a pesticide and warming interact for antipredator behaviour

The impact of pesticides on organisms may strongly depend on temperature. While many species will be exposed to pesticides and warming both in the parental and offspring generations, transgenerational effects of pesticides under warming are still poorly studied, particularly for behaviour. We therefore studied the single and combined effects of exposure to the pesticide chlorpyrifos (CPF) and warming both within and across generations on antipredator behaviour of larvae of the vector mosquito Culex pipiens. Within each generation pesticide exposure and warming reduced the escape diving time, making the larvae more susceptible to predation. Pesticide exposure of the parents did not affect offspring antipredator behaviour. Yet, parental exposure to warming determined how warming and the pesticide interacted in the offspring generation. When parents were reared at 24 °C, warming no longer reduced offspring diving times in the solvent control, suggesting an adaptive transgenerational effect to prepare the offspring to better deal with a higher predation risk under warming. Related to this, the CPF-induced reduction in diving time was stronger at 20 °C than at 24 °C, except in the offspring whose parents had been exposed to 24 °C. This dependency of the widespread interaction between warming and pesticide exposure on an adaptive transgenerational effect of warming is an important finding at the interface of global change ecology and ecotoxicology.

Physiological mechanisms constraining ectotherm fright-dive performance at elevated temperatures

Survival of air-breathing, diving ectotherms is dependent on their capacity to optimise the time available for obligate underwater activities, such as predator avoidance. Submergence times are thermally sensitive, with dive durations significantly reduced by increases in water temperature, deeming these animals particularly vulnerable to the effects of climate change. The physiological mechanisms underlying this compromised performance are unclear but are hypothesised to be linked to increased oxygen demand and a reduced capacity for metabolic depression at elevated temperatures. Here, we investigated how water temperature (both acute and chronic exposures) affected the physiology of juvenile estuarine crocodiles (Crocodylus porosus) performing predator avoidance dives (i.e. fright-dives). Diving oxygen consumption, ‘fright’ bradycardia, haematocrit and haemoglobin (indicators of blood oxygen carrying capacity) were assessed at two test temperatures, reflective of different climate change scenarios (i.e. current summer water temperatures, 28°C, and ‘high’ climate warming, 34°C). Diving oxygen consumption rate increased threefold between 28 and 34°C (Q10=7.4). The capacity to depress oxygen demand was reduced at elevated temperatures, with animals lowering oxygen demand from surface levels by 52.9±27.8% and 27.8±16.5% (means±s.e.m.) at 28°C and 34°C, respectively. Resting and post-fright-dive haematocrit and haemoglobin were thermally insensitive. Together these findings suggest decrements in fright-dive performance at elevated temperatures stem from increased oxygen demand coupled with a reduced capacity for metabolic depression.

Coming up for air: thermal-dependence of dive behaviours and metabolism in sea snakes

Cutaneous gas exchange allows some air-breathing diving ectotherms to supplement their pulmonary oxygen uptake, which may allow prolongation of dives and an increased capacity to withstand anthropogenic and natural threatening processes that increase submergence times. However, little is known of the interplay between metabolism, bimodal oxygen uptake and activity levels across thermal environments in diving ectotherms. Here, we show in two species of sea snake (spine-bellied sea snake; Hydrophis curtus and elegant sea snake; H. elegans) that increasing temperature elevates surfacing rates, increases total oxygen consumption, and decreases dive durations. The majority of dives observed in both species remained within estimated maximal aerobic limits. While cutaneous gas exchange accounted for a substantial proportion of total oxygen consumption (up to 23%), unexpectedly it was independent of water temperature and activity levels, suggesting a diffusion-limited mechanism. Our findings demonstrate that rising water temperature and a limited capability to up-regulate cutaneous oxygen uptake may compromise the proficiency with which sea snakes perform prolonged dives. This may hinder their capacity to withstand ongoing anthropogenic activities like trawl fishing, and increase their susceptibility to surface predation as their natural environments continue to warm.

Diving in a warming world: the thermal sensitivity and plasticity of diving performance in juvenile estuarine crocodiles (Crocodylus porosus)

Air-breathing, diving ectotherms are a crucial component of the biodiversity and functioning of aquatic ecosystems, but these organisms may be particularly vulnerable to the effects of climate change on submergence times. Ectothermic dive capacity is thermally sensitive, with dive durations significantly reduced by acute increases in water temperature; it is unclear whether diving performance can acclimate/acclimatize in response to long-term exposure to elevated water temperatures. We assessed the thermal sensitivity and plasticity of 'fright-dive' capacity in juvenile estuarine crocodiles (Crocodylus porosus; n = 11). Crocodiles were exposed to one of three long-term thermal treatments, designed to emulate water temperatures under differing climate change scenarios (i.e. current summer, 28°C; 'moderate' climate warming, 31.5°C; 'high' climate warming, 35°C). Dive trials were conducted in a temperature-controlled tank across a range of water temperatures. Dive durations were independent of thermal acclimation treatment, indicating a lack of thermal acclimation response. Acute increases in water temperature resulted in significantly shorter dive durations, with mean submergence times effectively halving with every 3.5°C increase in water temperature (Q 10 0.17, P < 0.001). Maximal dive performances, however, were found to be thermally insensitive across the temperature range of 28-35°C. These results suggest that C. porosus have a limited or non-existent capacity to thermally acclimate sustained 'fright-dive' performance. If the findings here are applicable to other air-breathing, diving ectotherms, the functional capacity of these organisms will probably be compromised under climate warming.

Predicting organismal vulnerability to climate warming: roles of behaviour, physiology and adaptation

A recently developed integrative framework proposes that the vulnerability of a species to environmental change depends on the species' exposure and sensitivity to environmental change, its resilience to perturbations and its potential to adapt to change. These vulnerability criteria require behavioural, physiological and genetic data. With this information in hand, biologists can predict organisms most at risk from environmental change. Biologists and managers can then target organisms and habitats most at risk. Unfortunately, the required data (e.g. optimal physiological temperatures) are rarely available. Here, we evaluate the reliability of potential proxies (e.g. critical temperatures) that are often available for some groups. Several proxies for ectotherms are promising, but analogous ones for endotherms are lacking. We also develop a simple graphical model of how behavioural thermoregulation, acclimation and adaptation may interact to influence vulnerability over time. After considering this model together with the proxies available for physiological sensitivity to climate change, we conclude that ectotherms sharing vulnerability traits seem concentrated in lowland tropical forests. Their vulnerability may be exacerbated by negative biotic interactions. Whether tropical forest (or other) species can adapt to warming environments is unclear, as genetic and selective data are scant. Nevertheless, the prospects for tropical forest ectotherms appear grim.

Determining environmental causes of biological effects: the need for a mechanistic physiological dimension in conservation biology

The emerging field of Conservation Physiology links environmental change and ecological success by the application of physiological theory, approaches and tools to elucidate and address conservation problems. Human activity has changed the natural environment to a point where the viability of many ecosystems is now under threat. There are already many descriptions of how changes in biological patterns are correlated with environmental changes. The next important step is to determine the causative relationship between environmental variability and biological systems. Physiology provides the mechanistic link between environmental change and ecological patterns. Physiological research, therefore, should be integrated into conservation to predict the biological consequences of human activity, and to identify those species or populations that are most vulnerable.