For all the remaining nests (18%), the Google Earth high resolution satellite image revealed in their vicinity the presence of small canopy gaps (undetected by the NDVI). The analysis on the NDVI was effective for 82% of cases (i.e., nests occurred within 30 m from favorable habitats). Nests remained clustered because of the heterogeneous habitat conditions, but spatial distribution has changed due to canopy alteration along time. Both old and new information sources showed that ants prefer intermediate canopy cover conditions in their vicinity.

A threshold distance of 30 m was used to assess location of ant nests compared to favorable habitats. For past conditions (no spectral information available), presence of edges and more open canopies was assessed on a Corona spy-satellite image and based on information from old forest management plans. The NDVI product was obtained using Google Earth Engine and Sentinel 2 repository. Habitat suitability at present was determined using a Normalized Difference Vegetation Index (NDVI) image as a proxy for stand canopy cover.

Both historical and recent sources of information were used.

We compared the distribution of ant mounds in the 1960s with recent inventories (2018), looking at changes in canopy cover over time, in a managed forest. Dynamics of habitat conditions drive important changes in distribution and abundance of animal species making monitoring an important but also a challenging task when data from the past are scarce.

Google earth satellite image