An assumption that underlies evolutionary ecology is that there are direct relationships between individual fitness, behaviour, and population processes. This assumption is rarely tested because there are few studies that describe direct interactions between physiological, behavioural, and population processes in the field: the practical difficulties involved in taking contemporaneous measurements are normally prohibitive. However, contemporaneous measurement of biological processes at multiple levels of investigation are important in formulating generalisations in ecology because the interpretation of information combined from unconnected studies is limiting. The importance of one variable would be determined, in part, by the values of all other intercorrelated variables and, hence, there is no single level of importance. Terrestrial amphibians are ideal for such studies because they have relatively simple physiological responses which are sensitive to local environmental change. Their responses to the environment, at all levels of interaction, are essentially constrained by the availability of food, water and temperatures suitable for activity, and both the constraints and the responses can be measured in the field.
Aspects of the physiology and behaviour of Bufo marinus (L.) in a tropical savanna environment were studied contemporaneously with a mark-recapture population study. The object of the study was to examine the interactions between the environment, individual condition, behaviour, and population processes of B. marinus, and to assess the relative effects of each in possibly influencing changes in local abundance over time. At each level of interaction multiple regression analysis was used to identify those environmental variables which had the most important direct effects.
The availability of food and water influenced the condition of individual B. marinus and, as a consequence, their ability to survive and reproduce. The amount of energy that was partitioned towards growth, fat storage, gonads and water balance was influenced by body size and by the prevailing environmental conditions. The amount of time spent at different behaviours was influenced by individual condition, the conditions for reproduction and rehydration at water, and foraging conditions away from water. By being very mobile, either locally or when dispersing, toads were able to make greater use of resources in different habitats and, as a consequence, showed increased individual condition.
The total effects of environmental and physiological constraints on individual condition and behaviour influenced the population dynamics of B. marinus in the study area, and ultimately their local distribution and abundance. The environmental variable that had the most direct influence on ecological processes throughout all levels of study was the availability of food, ants in particular. The number of B. marinus in the study area declined by 93% over 2.4 years, in tandem with a decline in the availability of small ants. In conclusion, there were direct, most likely causal, relationships between the environment and individual condition, behaviour, and population processes which influenced the local distribution and abundance of toads. Although temperature, food and water were obviously limiting factors, the availability of food was clearly the dominant factor that regulated post-metamorphic abundance during the study.