Studies of ecological demography and the impact of climate change across latitudes

Abstract

Temperatures are increasing throughout the world, but these increases are much less pronounced in tropical than in arctic regions and it has often been assumed that the ecological effects of climate change will therefore be smaller in the tropics. In ectotherms, however, there is evidence that temperatures in the cooler regions may still be too cold and that physiological performance might actually increase if the climate there warms - temperatures in the tropics are already too hot so rising temperatures there may actually have a more negative effect. In this thesis, I test whether these principles may apply more generally to endotherms and not just in the case of ectotherms. I focus not on physiological indicators of performance but on the relationship between temperature and fitness itself. By drawing together published studies from across the world, I show that even in endothermic birds, conditions are often too cold in the mid- and higher latitudes where much of the world’s research effort has so far been focused, with significantly more studies finding an increase rather than a decrease in vital rates as temperatures there rise. In order to compare the impact of arctic and tropical climate change in a single population, I examine migratory species which are exposed to both latitudes in the course of their annual cycles. I develop the analytical methods needed to reconstruct the relationships between fitness and temperature at different latitudes. These methods involve modelling any effect which temperature might also have on our chances of detecting the birds, so I can distinguish ‘detection probability effects’ from genuine effects on population growth. Using the signal of arctic weather in time-series of population growth rates, my methods allow me to estimate the summer breeding locations of populations counted in the Tropics in winter. By applying these methods to the Grey Plover Pluvialis squatarola, which winters in Hong Kong and breeds in summer in the western Russian Arctic, I show that temperatures on their arctic breeding grounds are still too cold, while temperatures on their tropical wintering grounds are already too hot. The temperature increases may be greater in the Arctic, but the ecological impact appears to be more negative in the Tropics. And I construct a seasonal matrix model to examine whether climate change might exert more leverage on a population’s growth rate during the summer months when both survival and reproduction could be affected. My model suggests that the effects of climate change on survival and reproduction are only separate and cumulative if they affect reproduction via the number of eggs laid – not if they only affect reproduction via the survival of young. Clutch size in some species may be determined by temperatures on the breeding grounds, but in others it may depend on body condition which in turn may be influenced by temperatures on the wintering grounds and on migration. Overall, my thesis demonstrates that even in endotherms, the most negative ecological impacts need not occur simply in the regions with the greatest climate change.