Comparing climatic suitability and niche distances to explain populations responses to extreme climatic events

Habitat suitability calculated from Species Distribution Models (SDMs) has
been used to assess population performance, but empirical studies have
provided weak or inconclusive support to this approach. Novel approaches
measuring population distances to niche centroid and margin in
environmental space have been recently proposed to explain population
performance, particularly when populations experience exceptional
environmental conditions that may place them outside of the species niche.
Here, we use data of co-occurring species’ decay, gathered after an
extreme drought event occurring in the SE of the Iberian Peninsula which
highly affected rich semiarid shrubland communities, to compare the
relationship between population decay (mortality and remaining green
canopy) and (1) distances between populations’ location and species niche
margin and centroid in the environmental space, and (2) climatic
suitability estimated from frequently used SDMs (here MaxEnt) considering
both the extreme climatic episode and the average reference climatic
period before this. We found that both SDMs-derived suitability and
distances to species niche properly predict populations performance when
considering the reference climatic period; but climatic suitability failed
to predict performance considering the extreme climate period. In
addition, while distance to niche margins accurately predict both
mortality and remaining green canopy responses, centroid distances failed
to explain mortality, suggesting that indexes containing information about
the position to niche margin (inside or outside) are better to predict
binary responses. We conclude that the location of populations in the
environmental space is consistent with performance responses to extreme
drought. Niche distances appear to be a more efficient approach than the
use of climate suitability indices derived from more frequently used SDMs
to explain population performance when dealing with environmental
conditions that are located outside the species environmental niche. The
use of this alternative metrics may be particularly useful when designing
conservation measures to mitigate impacts of shifting environmental
conditions.