It is commonly known that terrestrial vertebrate ectotherms and endotherms differ in thermal physiology. One often notices the lizard on a rock, basking in the sun because the lizard’s greater sensitivity to its environment compels the organism’s need to retain a moderate body temperature on its own. Then there is the bird with a less variable body temperature, flying around freely. How do those physiological differences affect organism distribution/diversity and fundamentally, climate change responses? In turn, how do those effects influence the world’s ecological patterns on a broad scale? These questions were investigated in a recent study by biology professors Lauren B. Buckley and Allen H. Hurlbert from the University of North Carolina and Walter Jetz from Yale University. New literary data and models were gathered, analyzed, and compared to better understand how biodiversity will respond to environmental change.
Macrophysiology, the study of large-scale variation in physiological traits, has long been studied and become increasingly significant recently. Eminent climate change issues have created the necessity to combat those issues by first knowing how species will react to climate change. Broad-scale analyses have focused on endotherms, but the three professors largely included ectotherms in their study to emphasize the contrasts between ectotherms and endotherms in their abundance, diversity, and distribution due to physiological, energetic, and ecological limits.
Buckley and her colleagues first review the basic energetic performances of ectotherms and endotherms as a base to their study. Although ectotherms closely follow environmental temperatures, they still thermoregulate effectively. Endothermy, with higher metabolism and body temperature, is more costly in energy but has its performance advantages, like greater endurance.
The researchers then continued with analyses on how endotherms and ectotherms differ in their responses to temperature, energy availability, and water availability in their environment. Endotherms rely more on and thermoregulate more precisely with energy (food) and water availability, while ectotherms have buffering periods of low resource availability. Instead, ectotherms depend more on energy assimilation; ectotherms’ performance curve depicts how the rate of energy assimilation is a function of temperature (hump-shaped relationship).
These varying responses of ectotherms and endotherms result in various geographic patterns of energy use, thermal limits, and seasonal activity. On energy use, calculations and graphs were done to examine how temperature, field metabolic rate (FMR), and net primary productivity (NPP) correlate with each other. For endotherms, FMR is relatively stable (may decrease slightly) with temperature, while for ectotherms, FMR usually increases. On thermal limits, overall, tropical organisms exhibit narrower thermal tolerance. Endotherms are less affected by thermal tolerance, with mostly only thermoneutral, rather than lethal, limits. On seasonal patterns of energy use and activity time, the analysis was based on the basal and resting metabolic rate (MR, W) of an idealized bird and lizard. The distributions of ectotherms are more greatly constrained by potential activity time, especially in high latitudes. Endotherm richness is more related to primary productivity, while ectotherm richness is better predicted by temperature and not much associated with abundance.
With global change so prominent, there will be ecological consequences for the distribution, abundance, and diversity of terrestrial vertebrate groups. For example, low temperatures reduce activity times and distributions of ectotherms but increase energy expenditures of endotherms. Also, there has been a proposal that the drastic climate changes of mountains will cause the evolution of greater thermal specialization of tropical organisms due to the greater constancy of the tropics. Overall, endotherm abundance is limited by temperature and energy availability, while ectotherms are dominated by thermal constraints. About species richness, it is difficult to link differences to particular causes (geography, temperature, or energy availability) because biological differences exist for each species.
With all this research on ecophysiology, it is clear that increased temperatures on Earth will dramatically affect activity times and energy needs of ectotherms and endotherms. Overheating and shorter activity times (heat avoidance) for ectotherms and die-offs, limited foraging activity, and water loss for endotherms are some of the more dire consequences. There is still much to learn about ectotherm and endotherm diversification that is limited by physiology in an evolutionary and biogeographic perspective. Nevertheless, the study clearly indicates ultimate macrophysiological constraints on ecological structure.
- Lauren B. Buckley, Allen H. Hurlbert, Walter Jetz. Broad-scale ecological implications of ectothermy and endothermy in changing environments. Global Ecology and Biogeography, 2012; DOI: 10.1111/j.1466-8238.2011.00737.x http://onlinelibrary.wiley.com/doi/10.1111/j.1466-8238.2011.00737.x/abstract