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Physiological and Biophysical Ecology

Homeostasis is a cornerstone of DEB theory, especially in the context of the organism's biochemistry. Organisms must also maintain homeostasis in terms of their body temperature and water balance under changing external environments, and these issues fall under the topic of physiological ecology. Organisms have evolved a wide range of morphological, behavioural and physiological responses to cope with environmental variation on different time scales, including changes to the thermal response curve, acclimation, behavioural responses and torpor.

Moreover, just as energy and mass balances can be used in DEB theory to understand how organisms take up and use substrates from their environment, they can also be used to understand the exchange of heat and water between the organism and its environment. The discipline of biophysical ecology provides the theory and equations for making such inferences, and these equations interface with DEB theory through metabolic heat and water production. For ectotherms, biophysical models of heat transfer allow the estimation of the body temperature, given the convective, conductive, evaporative and radiative environment and the metabolic heat production. For endotherms, the same equations are used but body temperature is treated as given and the necessary metabolic heat production (in cold environments) or evaporative heat loss (in hot environments) is determined.

Of fundamental importance in physiological and biophysical ecology is that the environment is characterised as it is actually experienced by the organism, i.e. the `microclimate' of the organism must be determined. For terrestrial environments, sophisticated models exist for determining how terrain and atmospheric conditions interact to affect spatial and temporal patterns of wind speed, air temperature, longwave and shortwave radiation, and vapour pressure near the ground, as well as temperature and moisture conditions in the soil. For aquatic environments, models exist that can account for stratification of temperature profiles with depth, the pH, dissolved oxygen and salinity, and even the complex combinations of conditions that occur where land and water meet, such as the intertidal zone or the edge of a pond.

Mechanistic niche models

The niche of an organism, as defined by G. Evelyn Hutchinson, can be seen as the combinations of environmental conditions that allow an organism to survive and reproduce. It is necessarily a multidimensional environmental space, and it also has a temporal aspect to it because the environmental requirements change through time. A useful distinction is between the fundamental and realised niche - the latter representing the influence of predators, competitors and parasites while the former considering just the bare necessities for life (i.e. temperature, food and water).

The field of mechanistic niche modelling aims to understand how the functional traits of an organism interact with its environment to determine the limits to its survival and reproduction across space and time, i.e. to define the fundamental niche in environmental space and map it to physical space and time. The goal is to make this connection as much as possible through the explicit application of physically-based models. This can be achieved through the integration of biophysical models of heat and water exchange, with DEB models of the trajectory of growth, development, reproduction and senescence, and physical models of microclimatic conditions. Such models can be seen as determining the ever-present thermodynamic constraints on life, the `thermodynamic niche'. If we think of the problem of distribution and abundance as a jigsaw puzzle, these models give us the `edge pieces' that bound the possibilities for persistence.

Mechanistic niche models make the strongest inferences where it can be shown that certain places, at certain times, are physically impossible for a species to persist. In other words, where it can be shown that homeostasis breaks down such that death occurs before successful reproduction. This could be due to a hard limit, e.g. the organism dies from starvation, heat/cold stress, or desiccation at some point in the life cycle. It could also be a softer limit, e.g. the organism cannot reproduce fast enough to balance mortality imposed by the physical or biotic environment. Such softer limits can potentially be inferred directly, or the outputs of such models can be used as inputs for population dynamics models (vital rates) or take into account population processes as described

From AmP entries to mechanistic niche models

The NicheMapR software package for the R programming environment integrates microclimate and biophysical models with DEB models ...