Metabolic requirements:

Our model runs on user-specified requirements for metabolizable energy and protein. It also requires a user-specified daily rate of dry-matter intake of food (oven-dry weight of total food intake in a day). These values are empirically determined. We have provided a recommended set of metabolic requirements, based on the scientific literature, which the user can use for reference. However, the user can choose different values, however they wish. Metabolic requirements must be specified by the user for each run of the model. “Carrying capacity” values will vary dramatically, depending on user-specified metabolic requirements. This is a very important consideration in any evaluation of habitat, as summer range is very different than winter range, and habitat suitable for only “maintenance” levels in summer may have much lower carrying capacities at “reproductive” levels.

Metabolic requirements of animals vary greatly seasonally and as a function of age, sex, reproductive status and body condition. For both deer and moose, summer is a time of growth, body reserve accumulation, and reproduction (lactation). Winter is a time of weight loss and deterioration of body condition (reserves). We usually work with adult females as our unit for analysis (see definition of carrying capacity). Summer metabolic requirements for adult females depend on their body condition going into summer, their reproductive status (maintenance only, or maintenance plus lactation for one or more fawns), and their need to accumulate body reserves (fat and protein) before going into winter. Their winter metabolic requirements depend on their body condition (reserves) going into the winter and their activity levels (or winter weather) influencing the rate at which they draw on their energy reserves – with the difference between the two (reserves and draw-down rate) needing to be made up for by food resources.

Our recommended set of metabolic requirements for summer includes lactation requirements for both single and twin fawns. However, the values are for peak lactation requirements, not “average” requirements across the entire summer. Peak requirements are relatively high, and peak requirements for twin fawns are especially high. Peak requirements can be met by the adult female through a combination of using her own body reserves and relying on food intake. Thus, peak requirements, especially for twin fawns, might be an unreasonably high standard for evaluating carrying capacity of a habitat. Therefore, we usually use the recommended requirements for “single fawn” and refer to that as simply “reproduction” (unspecified as to singletons or twins). Of course, the user can specify whatever they wish for these requirements.

Our recommended set of metabolic requirements for winter includes two alternatives, depending upon the user-assumed body condition of the animals going into winter: one set of requirements for “fat” animals coming off high-quality summer range; the other set for “lean” animals coming off poor-quality summer range. “Fat” animals have greater body reserves than do “lean” animals, and therefore have lower requirements from their food resources in order to survive the winter. Our “requirements” are the requirements from the food resources. We assume similar activity budgets, winter weather conditions, and appetites (dry-matter intake rates) for both alternatives.

Metabolic requirements can be specified in terms of digestible energy (in kilocalories or kilojoules), dry-matter digestibility of the diet (percent dry matter), digestible protein (percent dry matter), or crude protein (percent dry matter). The choices must be specified by the user. Use of either digestible energy or digestible dry matter (both relate to metabolizable energy requirement) includes an assumed constant metabolic energy coefficient; use of dry-matter digestibility of the diet includes a further assumed constant gross energy content of all forages. Metabolic requirements are dealt with by digestible protein or crude protein. Digestible protein includes effects of plant tannins on reducing protein digestibility. Crude protein does not include effects of tannins and should be viewed as highly suspect – although, often times, tannin effects are unknown, and only data for crude protein are available.

Details of sources and assumptions are provided in the recommended set of metabolic requirements link.