Following a presentation of my work to fellow biologist/ecologists, it seemed that what people wanted to see what a motivation for why individuals move around, rather than simply hard-coding them to move around x% of the time. This meant a shift away from the least-cost model to something more intuitive - food.
At first glance, this seems a relatively straight-forward shift to make. Now, instead of individuals choosing a location based on some negative cost value (and opting for the least-cost move) now they would move around according to their food requirements. In principle, the landscape-effect is similar. Forest is still good whilst agricultural areas/urban areas etc are not so good. However, the shift to 'emergent' behaviour, i.e. individuals moving around to meet some food requirement involves quite a lot of behind the scenes work within the code itself. To put in brief, the two flowcharts outline the decisions made by individuals within each given scenario, 1. moving to find enough food and 2. moving according to least-cost principles. Circles in red are decisions, those in thick red are standard default decisions.
Pros and Cons
1. Food
Now it becomes clear, whilst moving for food may be more intuitive and more 'realistic', it is a lot more complicated, involving a large number of additional parameters, is more likely to cause some un-conceieved bias in the results, and hence is far more sensitive to initial conditions and far less robust that a simpler movement process. There is also no explicit recognition of mortality risk, protection, mating opportunities, physiological costs etc that are encompassed with the single cost value used in the least-cost model. In short, the only factor affecting where a jaguar moves is the availability of food.
In addition, mortality risk is modelled separate to movement choice. Individuals are not 'aware' of the mortality risk of any particular location and are unable to choose a location according to this risk. Of course, this could be added as another factor, but this would increase the complexness of the model, add further parameters and make it even more sensitive.
Some additional parameters include:
1. Amount of food required by an individual (per step or as an average per day still to be decided)
2. Reduction in food per time step - should this be different depending on if the individual physically moves compared to staying in the same cell? should there be a reduction per time step and another depending on how the individual moves? Should this be a set amount or a proportion of the total food resource/food required per step/day?
3. Increase in food availability in cells that have been visited by a jaguar - should this be proportional, an incremental increase or a logistic increase? - i.e. type I, type II or type III response to predation?
4. Should the food be homogenous or clumped/variable in distribution?
5. Should the food change in spatial representation over time?
2. Cost
But then again, cost doesn't necessarily make the best option either. Here, whilst the model decision making is simple and straight-forward there is now no motivation for individuals to move. Movement is not 'emergent' but hard-coded. Males move x% of the time, females y% of the time. However, there is much less risk of the model being sensitive to initial conditions as there are only two parameters to be concerned about: relative differences in cost between habitats and movement rates for males and females.
In addition to this, movement decisions are now based on a range of factors, implicitly outlined by the cost of any particular habitat. Not only does food get represented in the cost, but also mortality risk, cover, mating opportunities, physiological costs of moving through the terrain.
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