Modelling the Abundance of Rangifer: The Frame Size Model

Rangifer herds across the globe experience cycles of abundance and scarcity over a multi-decadal time scale and a number of theories have been presented to help explain their cycles. Focussing at the individual energetic level, the FRAME SIZE model was developed to examine the role of Rangifer frame size under resource availability scenarios. The energy requirements of animals scale with metabolic body size (kg 0.75) and failure to meet energy and protein demands through diet leads to costs to the individual affecting growth, survival and reproduction. On the other hand when resources are not limited large frame size has an advantage in terms of survival and age of first reproduction. We thus examine the energetic and demographic consequences of having a large frame size (high metabolic demand) when either resources are limited or demand is high. In the model we track the number of animals in various frame size classes and project populations over the length of a normal population cycle under a number of resource availability scenarios (cyclic, uniform, stochastic) and resource demand scenarios (e.g. to mount an immune response to parasite loads). We present data from existing databases to help validate output. We use the model to develop a monitoring program to allow management groups to better understand population cycles in their caribou herds, to explore a number of theories on population regulation, and, under current population declines, to provide a basis for discussion on the role of global change to the vulnerability of herds.