Is a species distribution model output reliable to test niche limitation at the northern range limit of a Pacific coastal dune plant?

Jacqueline Grubel* and Christopher G. Eckert (Faculty Supporter) It is widely thought that the size, shape and location of a species’ geographical distribution are a spatial expression of its realized niche, and this assumption is central to evolutionary biology, biogeography and conservation. Yet, the hypothesis that geographical range limits are niche limits is not well supported by experimental translocations of species beyond their range limits. Beyond range populations often exhibit fitness high enough for self-replacement. In contrast, environmental niche models based on bioclimatic data often suggest a decline in habitat suitability beyond range limits, thereby supporting niche limitation. However very few studies have evaluated whether species distribution models (SDMs) accurately predict the viability of populations in nature, and scant results to date are not supportive. Long-term transplant with the short-lived, Pacific costal dune endemic plant Camissoniopsis cheiranthifolia (Onagraceae) suggest that populations are viable beyond the northern range limit over multiple generations. We constructed an SDM based on a large range-wide database of species records plus standard bioclimatic variables and substrate type. We also included sea surface temperature, which greatly modifies the climate of dune habitat. Preliminary results suggest that our SDM reliably predicts the fitness of experimental populations. However, both approaches indicate that something other than niche limitation enforces the northern range limit of this species. Results from this well-studied dune plant suggest that range limitation via constraints on dispersal may play an important role in limiting northern range expansion.

Increasing temperature threatens an already endangered coastal dune plant

ABSTRACTClimate change has the potential to reduce the abundance and distribution of species and threaten global biodiversity, but it is typically not listed as a threat in classifying species conservation status. This likely occurs because demonstrating climate change as a threat requires data-intensive demographic information. Moreover, the threat from climate change is often studied in specific biomes, such as polar or arid ones. Other biomes, such as coastal ones, have received little attention, despite being currently exposed to substantial climate change effects. We forecast the effect of climate change on the demography and population size of a federally endangered coastal dune plant (Lupinus tidestromii). We use data from a 14-year demographic study across seven extant populations of this endangered plant. Using model selection, we found that survival and fertility measures responded negatively to temperature anomalies. We then produced forecasts based on stochastic individual based population models that account for uncertainty in demographic outcomes. Despite large uncertainties, we predict that all populations will decline if temperatures increase by 1° Celsius. Considering the total number of individuals across all seven populations, the most likely outcome is a population decline of 90%. Moreover, we predict extinction is certain for one of our seven populations. These results demonstrate that climate change will profoundly decrease the current and future population growth rates of this plant, and its chance of persistence. Thus, our study provides the first evidence that climate change is an extinction threat for a plant species classified as endangered under the USA Endangered Species Act.