Species distribution models typically use four common proxies for water in order to predict how water availability will impact future species distribution. However, the affects of changes in vegetation structure and functioning on the water balance of terrestrial ecosystems are not accounted for. Hickler sought to assess whether the water availability measure commonly used in species distribution models properly account for hydrological effects of changes in vegetation structure and functioning (2009). In his study, ecosystem models LPJ- GUESS and LPF- DGVM which took into account soil water content along with variables based on climate. The projections for future water availability from four common proxies for water and the soil water content predicted by ecosystem models differ substantially. Winnie Wong
Hickler, T., Stepan F., Miguel B. A., Oliver S., Wilfried T., and Martin T. S., 2009. An ecosystem model-based estimate of changes in water availability differs from water proxies that are commonly used in species distribution models. Global Ecology and Biogeography 18, 304-313.
The paper focused on water availability in Europe. The LPJ-GUESS model includes a number of factors that influence water availability not account for in common proxies for water. LPJ- DGVM is a simplified ecosystem model that was also assessed. The four commonly used proxies for water availability used were total annual precipitation, total precipitation during water scarce periods (June, July and August), annual water deficit, sum of actual evapotranspiration divided by equilibrium evapotranspiration. Water stress experienced by plants are a function of water availability and both soil after and atmospheric variables are biologically meaningful. For tree species, the water content of soil is a more direct measure of water availability than variables purely based on climate.
The changes projected by the four proxies correlated with each other white these variable showed no or very weak correlations with the two ecosystem models. Projected changes in the four proxies for water availability indicate a net effect of increasing temperate and annual rainfall in many areas will be drying over most of Europe. In contrast the ecosystem models the sign of change (positive or negative) differs in 32% of the total study area. These significant differences in results are important because results from species distribution models has been used for guiding policies for adaptation to climate change.