The highest level of certainty an ecological modeler knows is that there are some apparently unavoidable pitfalls. One is mortality (any biological modeler reading this will nod in spite of himself). Sooner or later, in a meeting like the one I am this week, you'll hear something spirited like "...but your mortality function is not based on any mechanism, so what the ... are we (non-modelers) supposed to do with your results..."
Mortality rates of the small plankton are notoriously difficult to measure in the field, and thus, this term is one of the most difficult to constrain. Most single species copepod models have developed empirical relationships with temperature and/or food (for seasonality purpose) and many include some form of density dependence (for numerical purpose). Those choices arise from the trade-offs between the availability of data and the necessity to move forward and do actual modeling.
The case of temperature-dependent functions illustrates this situation: the Gulf of Maine time series suggest that herring predation may limit Calanus finmarchicus abundance. Predation by herring is the highest in the summer and the seasonal changes could then be approximated as a function of temperature. If spring conditions were warmer, we might expect that herring would begin feeding earlier, and thus, the temperature dependent mortality would adequately reflect interannual changes in a mechanistic way. However, it seems unlikely that herring predation would respond to a temperature anomaly of a few days, and it is unclear whether a warming throughout the year would correspond to higher mortality.
A novel approach of mortality in copepod models requires a mortality function that reflects some aspects of the dynamical response of predator populations to copepod abundance. This requirement becomes essential to enable realistic projections under climate variability and change. Our knowledge of copepod predators remains limited, and attempting to model the populations of all of the major predators of the life stages of our copepod would just be unfeasible. Following the "middle-out" framework, in future iteration of our models we want to use a compromise mortality function. This new function will make use of several populations of predators, each representing predation by progressively larger animals preying on progressively larger copepods. We will use classical size-dependent feeding behavior for the predators, namely a type II ingestion function (rapid increase at low food concentrations) for small predators and type III function (depressed feeding at low concentrations) for large predators. The result will be that on one hand, the predation rate on smaller copepods (early life stages) will increase largely through changes in the abundance of the predators, while on the other hand predation on larger copepods (later life stages) will respond to changes in their own abundance through the variable ingestion rate of the larger predators. That, is a bold move!
This picture has nothing to do with what I just talked about... I just feel that the pictures on the blog are discriminatory toward the earthly mammals ! And raccoons are cute (at 3pm, not 3am, though...).