Seed predation behaviors

Seed predation occurs after seed dispersal has occurred and serves to reduce the number of seeds by simulating seed consumption by predators. Only one predator population can be modeled.

Seed predation parameters

Functional response seed predation

This behavior simulates functional response seed predation, where the number of predators is a function of the amount of food that has been consumed. Since seed predator life cycles are often very short, this behavior runs as a mini-model within the context of the larger simulation. It simulates weekly timesteps of seed fall, seed consumption, and predator response for as much of the year as the user desires. The mini-model simulates a single year, which starts at the beginning of seed fall. All species are assumed to drop seeds at the same time, at a constant rate. You set the number of weeks that seedfall should occur. Once seedfall is over, there are no new additions to the predator food pool. There is then an optional period of germination in which the food pool of seeds is further reduced by some proportion of those remaining seeds germinating into seedlings (seedlings cannot be eaten). Once the model has finished running, the leftover seeds that were not consumed by predators, or those seeds that escaped through germination, are available as input to the Establishment behaviors.

How it works

The behavior's mini-model begins with the number of seeds of each species to which it is applied that are available in each cell of the Dispersed Seeds grid. Each cell gets its own run of the mini-model. The pool of seeds in one cell for all species to which this behavior applies is treated as a single food pool for one year's time (even if the model timestep length is greater than one year). For all the species to which this behavior is applied, the seeds are eaten with equal enthusiasm; the proportions of each type are captured before predation, and any leftover seeds are redistributed according to those proportions. The initial number of predators is calculated from the "Predator initial density - num/sq m" parameter or, if the mini-model has run before and the "Preserve predator densities between SORTIE timesteps" parameter is set to true, from the final density of the last mini-model run.

The behavior mini-model run begins at the part of the year in which seedfall occurs. The number of seeds in the seed rain is the total seed pool, which is evenly divided over the user-defined seed rain length. The predator population has as a food source the number of seeds added during the current week's rain (if the rain is going on) plus any leftover seeds from previous weeks which have not been consumed. Beginning at a certain week in the spring, the number of seeds available to the mice is further reduced by a certain percentage each week to simulate germination. Once germination begins, it continues until the predator model finishes running. In order to correctly calculate mouse consumption and ensure that the seeds which germinate are actually available later, the behavior keeps track of the seeds actually consumed; it is this number which is subtracted from total seeds at the end.

Seed offtake for each week is calculated as
O = IRs * N

where Per capita seed offtake is
IRs = cs(1 - e-(S*D))

where The number of predators in each cell's population is calculated as
Nt = Nt-1 * ert-1

where The instantaneous rate of change, r, is calculated as
r = a + d(IRs) + g(N)

where

How to apply it

This behavior may be applied to seeds of any species. Any species to which it is applied must also have a Disperse behavior applied as well.


15-Aug-2004 10:11 PM