Modeling of processes of a mistletoe population in the Southwestern United States

Background/Question/Methods

 The need to understand and predict geographic distributions of species is of major importance in biodiversity science. Recent advances in species and ecological niche modeling have seen the advantage of mechanistic and process-based modeling as a better way to understand what determines the presence of a species in its geographic range. In this work several equations were derived to describe the processes of colonization and extinction for the desert mistletoe (Phoradendron californicum) under a spatially explicit metapopulation framework.  The equations represent the processes of the propagation of the seeds by a bird species, the infection of patches with different number of host trees, and the effect of climate on mortality rates.  The equations were used in a computer model which was implemented on geographic grids of six scales (1, 5, 10, 20, 50, and 100 km). The data used in the model include data collected in the field, image classification and other online databases. We later applied kriging to estimate probability values in areas with no observed records.

Results/Conclusions

The output of the model allow to estimate the probability of finding the desert mistletoe in a cell of the grid, but also allow to evaluate specific parts of the model in relation to the colonization and extinction rates for all grid cells. Unsolved questions include the validity of the model output, as well as the best way to find initial parameter values for each simulation.  The output (probability of infection, colonization, and extinction) were in some cases sensitive to these initial estimates. Rules should be developed for testing multiple simulations, with different choices of initial parameter estimates, and selecting one with valid output.

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