Devils and Roadkill

CSSS Santa Fe 2010

Next Meeting: We'll meet in a couple of days over a meal to discuss progress, but feel free to discuss a time if need arises!

Impact of prey predictability on contact network dynamics of individual foragers

We are interested in using cellular automata/agent based modelling approaches to investigate how a predictable increase of prey density (from roadkill) impacts the contact network dynamics of a group of individual foragers (Tasmanian Devils). We are interested in how the frequency at which devils interact (and with whom) scales with the ratio of prey density on roads to background levels. (Gavin Fay)

The context for this foraging work is that determining how supplementing the prey base (from roadkill) might impact the transmission dynamics of Devil Facial Tumour Disease (DFTD) among the devil population. Additional interests of ours are therefore related to modelling dynamics of infection within the network, which could also be connected with population dynamics models, and changes in life history traits (infected populations are now semelparous, with a large drop in age of sexual maturity).

Working group / interest

Background

Tasmanian Devils, the largest marsupial carnivore, are currently experiencing a dramatic reduction in population due to DFTD, a rather nasty infectious cancer which has become prevalent through much of the state. DFTD infection relies on transmission of infected cells from contact, most likely due to biting, which these critters do a lot of during mating and around prey carcasses. A hot conservation topic right now is forestry plans to build roads opening up a wilderness area in the north of the state to ecotourism opportunities. The devil population in this area has until now remained disease free. There are concerns that the road will increase the likelihood that DFTD will spread to the diseasse-free population: Devils are scavengers and frequently feed on roadkill, the creation of a road may then provide an opportunity for increased frequency of contact between infected and disease-free devils. It might be interesting to investigate how introducing a fixed-location source of additional prey items (ie a road) to a devil population would change the contact network for Devils, and then also to what extent the increased contact frequency would have to be to facilitate transmission of DFTD from an infected devil population to a disease-free one.

Methods/Approach/Ideas

ABM approach - Model Devils as random walkers on a grid, that move diffusively toward prey locations. Use Devil densities from mark recapture estimates. Prey map is stochastic, with intensity of spatial process given by a base mortality rate. Parts of grid that are road have higher mortality rate (tunable ratio relative to base mortality rate). Keep track of who meets who, and how often. Build network, with weights of links driven by frequency of interaction. Analyse network structure when conducting simulations with/without a road. Other scenarios include, values for ratio of prey densities, how rapidly devils move toward prey, etc. (Gavin Fay)

Start off with random grid. Possible extensions could include importing GIS of parts of Tasmania for road locations. Approach above assumes that interaction occurs given same location - if we are going further with disease transmission etc, then we could have a probability of fighting/injury given an interaction.

Basic Model

From meeting on 06/14/10 (Megan Olsen):

The Basic model will NOT have disease, mating, natural death, etc. It will only have what is explicitly listed in this section.

The world:

Bounded grid (not a torus)
Overlap (multiple agents on a single grid point) is allowed
Essentially a Cellular Automata (decision rules are based on stochasticity and states in neighborhood)

Agents/Predators (Devils):

Moore neighborhood of interaction (8 neighbors, this includes cardinal points and diagonals)
Random walk on grid
If smell of prey is within neighborhood, will follow it with some (high) probability
Pr(Fight|Overlap) will be considered, but set to 1 (i.e. they will always fight if on same gridpoint)
Will follow strongest gradient of smell when there is a choice; if multiple directions are equivalent will choose randomly

Prey (Roadkill):

Have a defined "lifetime," the strength of their smell will decay with their lifetime until they disappear from end of lifetime or from being eaten
Smell diffuses within some radius; diffuses evenly in all directions

Start State: