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.
In this document:
Seed predation parameters
Functional response seed predation behavior
Neighborhood seed predation behavior
Seed predation parameters
- Density-Dependent Coefficient The coefficient describing the effect that density-dependent factors have on the predator population instantaneous rate of change. See equation below. Used by the Functional response seed predation behavior.
- Foraging Efficiency The predator population foraging efficiency. See equation below. Used by the Functional response seed predation behavior.
- Max Decline Rate in Absence of Food, predators/week The maximum rate of decline in the predator population in the absence of any food, in predators per week. Used by the Functional response seed predation behavior.
- Max Intake Rate - seeds per predator per day The maximum number of seeds that can be eaten by one predator in one day. Used by the Functional response seed predation behavior.
- Neighborhood Predation - Masting "A" The "A" term in the seed offtake equation under masting conditions. Used by the Neighborhood seed predation behavior.
- Neighborhood Predation - Masting "B" The "B" term in the seed offtake equation under masting conditions. Used by the Neighborhood seed predation behavior.
- Neighborhood Predation - Masting "C" The "C" term in the seed offtake equation under masting conditions. Used by the Neighborhood seed predation behavior.
- Neighborhood Predation - Masting Seed Density, #/m2/yr The density of edible seed that indicates masting has occurred. Used by the Neighborhood seed predation behavior.
- Neighborhood Predation - Masting Load Factor The loading factor for each species for calculating the PCA value of the neighborhood under masting conditions. Used by the Neighborhood seed predation behavior.
- Neighborhood Predation - Minimum Neighbor DBH (cm) The minimum DBH of trees to be included when calculating the PCA value of the neighborhood. Used by the Neighborhood seed predation behavior.
- Neighborhood Predation - Neighbor Search Radius (m) The radius to search for trees when calculating the PCA value of the neighborhood. Used by the Neighborhood seed predation behavior.
- Neighborhood Predation - Non-Masting "A" The "A" term in the seed offtake equation under non-masting conditions. Used by the Neighborhood seed predation behavior.
- Neighborhood Predation - Non-Masting "B" The "B" term in the seed offtake equation under non-masting conditions. Used by the Neighborhood seed predation behavior.
- Neighborhood Predation - Non-Masting "C" The "C" term in the seed offtake equation under non-masting conditions. Used by the Neighborhood seed predation behavior.
- Neighborhood Predation - Non-Masting Load Factor The loading factor for each species for calculating the PCA value of the neighborhood under non-masting conditions. Used by the Neighborhood seed predation behavior.
- Number of Weeks in Which Seedfall Occurs The number of weeks at the beginning of the behavior mini-model run that seedfall occurs. The number of seeds that fall is evenly distributed over that number of weeks. Used by the Functional response seed predation behavior.
- Predator Initial Density - num/sq m The initial density of the predator population, in numbers per m2. This will be used the first time this behavior is run. Whether this density is used for subsequent behavior mini-model runs depends on the value of the "Preserve predator densities between SORTIE timesteps" parameter. Used by the Functional response seed predation behavior.
- Predator Population Demographic Efficiency The seed predator population demographic efficiency. See equation below. Used by the Functional response seed predation behavior.
- Preserve Predator Densities Between SORTIE Timesteps If true, this means that the final predator density at the end of the behavior mini-model run is the initial density for the next mini-model run. If false, every time the behavior mini-model runs, it is re-initialized with the value of the "Predator initial density - num/sq m" parameter. Used by the Functional response seed predation behavior.
- Proportion of Seeds Removed Each Week During Germination Period During the period that germination occurs, this is the fraction of the seed pool that is removed due to those seeds having germinated. Expressed as a value between 0 and 1. Used by the Functional response seed predation behavior.
- Seed Predation Output Filename, If Desired The filename where the mini-model will store its intermediate results for later analysis, if desired. This value is not required. Used by the Functional response seed predation behavior.
- Week During Predation Model Run Germination Begins The week (timestep) during the behavior mini-model run that germination begins to occur. If you do not want germination to occur, set this value greater than or equal to the "Weeks to run seed predation model (1 - 52)" parameter. This value must be between 0 and 52. Used by the Functional response seed predation behavior.
- Weeks to Run Seed Predation Model (1 - 52) The number of weeks (timesteps) to run the behavior's mini-model. This number must be between 1 and 52. Used by the Functional response seed predation behavior.
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
- 0 is offtake (total number of seeds consumed)
- IRs is per capita seed offtake
- N is the number of predators
Per capita seed offtake is
IRs = cs(1 - e-(S*D))
where
- cs is the Max Intake Rate - seeds per predator per day parameter
- S is the number of seeds per predator per day
- D is the Foraging Efficiency parameter
The number of predators in each cell's population is calculated as
Nt = Nt-1 * ert-1
where
- Nt is number of predators for the current timestep
- Nt-1 is number of predators in the previous timestep
- rt-1 is instantaneous rate of change in predator abundance for the previous timestep
The instantaneous rate of change, r, is calculated as
r = a + d(IRs) + g(N)
where
- a is the Max Decline Rate in Absence of Food, predators/week parameter
- d is the Predator Population Demographic Efficiency parameter
- g is the Density-Dependent Coefficient parameter
- N is number of predators
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.
Neighborhood seed predation
This simulates seed predation as a function of tree neighborhood and masting events. The same equations are used to calculate the amount of seed eaten, but there are different parameters for masting and non-masting timesteps.
How it works
Masting timesteps are those with a heavy density of edible seeds. You set the threshold for this density in the Neighborhood Predation - Masting Seed Density, #/m2/yr parameter. The seed density is set as an annual average so the density will be calculated the same way for different-length timesteps. If the total plotwide average annual seed density is greater than this value, the timestep is treated as a masting timestep. Only those seeds of species to which this behavior applies are considered in the density.
The seeds in each grid cell of the Dispersed Seeds grid are treated separately for predation, according to their local neighborhood. The neighborhood gets a PCA (principle components analysis) value by considering the relative basal area of each species, of the total basal area of individuals within Neighborhood Predation - Neighbor Search Radius (m) meters of the grid cell center that have a DBH greater than Neighborhood Predation - Minimum Neighbor DBH (cm). The value is calculated as follows:
where:
- X is the PCA value summed over N species
- pn is either the Neighborhood Predation - Masting Load Factor or the Neighborhood Predation - Non-Masting Load Factor of species n
- RBAn is the relative basal are of species n
The amount of seed eaten for each species is calculated as:
where:
- Y is the proportion of that species' seed that is eaten
- A is either the Neighborhood Predation - Masting "A" or the Neighborhood Predation - Non-Masting "A" parameter for that species
- B is either the Neighborhood Predation - Masting "B" or the Neighborhood Predation - Non-Masting "B" parameter for that species
- C is either the Neighborhood Predation - Masting "C" or the Neighborhood Predation - Non-Masting "C" parameter for that species
- X is the PCA value for the grid cell's neighborhood, above
The eaten seeds are removed from the Dispersed Seeds grid.
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.
22-May-2007 01:37 PM