Detecting biodiversity refugia using remotely sensed data

Context: Habitats characterized by improved soil moisture availability can function as microrefugia (hereafter referred to as “refugia”) for the persistence of rare plant species in dry environments. Such areas are dominated by Mediterranean woody vegetation (shrubland and woodland). An analysis of these refugia elucidates their spatial distribution at the landscape scale. Objectives: Explore whether potential refugia, detected using the upper quantile of the normalized difference vegetation index (NDVI), are related, in space and time, with the survivability of rare species in dry environments. Methods: We used upper NDVI quantile (25%) values to predict potential refugia in nine selected areas in northern parts of Israel from 1992 to 2011. Next, we developed an index based on the ratio of density (number of observations per area) of rare species in non-refugia versus refugia patches, per site (density of rare species index, DRSI). Finally, we examined the temporal stability of the DRSI using ANOVA and Augmented Dickey–Fuller (ADF) tests. Results: Refugia classifications and DRSI values for all areas were stable over time (1992–2011). The DRSI values were significantly lower than 1; that is, the density of rare species in the predicted refugia areas was higher than in non-refugia areas. Conclusions: We assumed that patches of dense woody vegetation, determined by the upper 25% quantile of the NDVI, could be used to identify potential biodiversity refugia in dry environments. This assumption was validated by the DRSI results; it confirms that the local conditions in refugia support rare species.