Abstract
Species distribution models (SDMs) are routinely applied to assess current as well as future species distributions, for example to assess impacts of future environmental change on biodiversity or to underpin conservation planning. It has been repeatedly emphasized that SDMs should be evaluated based not only on their goodness of fit to the data, but also on the realism of the modeled ecological responses. However, possibilities for the latter are hampered by limited knowledge on the true responses as well as a lack of quantitative evaluation methods. Here we compared modeled niche optima obtained from European‐scale SDMs of 1476 terrestrial vascular plant species with empirical ecological indicator values indicating the preferences of plant species for key environmental conditions. For each plant species we first fitted an ensemble SDM including three modeling techniques (GLM, GAM and BRT) and extracted niche optima for climate, soil, land use and nitrogen deposition variables with a large explanatory power for the occurrence of that species. We then compared these SDM‐derived niche optima with the ecological indicator values by means of bivariate correlation analysis. We found weak to moderate correlations in the expected direction between the SDM‐derived niche optima and ecological indicator values. The strongest correlation occurred between the modeled optima for growing degree days and the ecological indicator values for temperature. Correlations were weaker for SDM‐derived niche optima with a more distal relationship to ecological indicator values (notably precipitation and soil moisture). Further, correlations were consistently highest for BRT, followed by GLM and GAM. Our method gives insight into the ecological realism of modeled niche optima and projected core habitats and can be used to improve SDMs by making a more informed selection of environmental variables and modeling techniques.
Highlights
Global biodiversity is currently declining at an unusually high rate (Butchart et al 2010, Barnosky et al 2011, Tittensor et al 2014)
We evaluate to what extent SDMs of 1476 vascular plant species in Europe adequately capture niche optima based on a comparison with independent ecological indicator values
We found the strongest correlation between the modeled niche indicator values of each species (MIVs) of annual growing degree days and the ecological indicator values (EIVs) of temperature (ρ = 0.53, p < 0.0001, n = 1380; Fig. 2a) and the weakest correlation between the MIV of total annual precipitation and the EIV of soil moisture (ρ = 0.10, p = 0.0086, n = 656; Fig. 2c)
Summary
Global biodiversity is currently declining at an unusually high rate (Butchart et al 2010, Barnosky et al 2011, Tittensor et al 2014). Species distribution models (SDMs) are quantitative relationships between the occurrence of a species and a set of environmental factors (Elith and Leathwick 2009). These models help to reveal and understand the habitat preferences of a species and can be used to project its distribution as a function of changing environmental conditions, which in turn provides vital information in order to underpin biodiversity policy (Araújo et al 2011, Visconti et al 2016, Morán-Ordóñez et al 2017). Possibilities are limited by a lack of knowledge on the true responses and a lack of quantitative approaches to evaluate the modeled responses (Austin et al 2006, Austin 2007)
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