Ecological Monitoring with Spy Satellite Images—The Case of Red Wood Ants in Romania

Dietrich Klimetzek,Petru Tudor Stăncioiu,Marius Paraschiv,Mihai Daniel Niță

doi:10.3390/rs13030520

Abstract

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. 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. Both historical and recent sources of information were used. Habitat suitability at present was determined using a Normalized Difference Vegetation Index (NDVI) image as a proxy for stand canopy cover. The NDVI product was obtained using Google Earth Engine and Sentinel 2 repository. 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. A threshold distance of 30 m was used to assess location of ant nests compared to favorable habitats. Both old and new information sources showed that ants prefer intermediate canopy cover conditions in their vicinity. Nests remained clustered because of the heterogeneous habitat conditions, but spatial distribution has changed due to canopy alteration along time. The analysis on the NDVI was effective for 82% of cases (i.e., nests occurred within 30 m from favorable habitats). 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). These results show that historical satellite images are very useful for explaining the long-term dynamics of ant colonies. In addition, the use of modern remote sensing techniques provides a reliable and expedite method in determining the presence of favorable small-scale habitat, offering a very useful tool for ecological monitoring across large landscapes and in very different areas, especially in the context of ecosystem dynamics driven and exacerbated by climate change.

Highlights

The study site is located in the hill region of Northeastern part of Romania, approximately 16 km SE of the city of Iasi near the village Poieni (County of Iasi) in a state-owned forest (Figure 1)
The nearest neighbor analysis underlined the strong clustering of RWA, which formed statistically significant hotspots
This could be compensated for by a wealth of additional information from literature, forest management plans and especially old satellite images. This last source provided very important spatial information for rectifying the position of the old map, and the distribution of nests. Even more important, it helped for the identification of edges, reliably identified on Corona 1964 satellite image

Summary

Introduction

Existence of any living organism depends on the environmental conditions available at a certain moment in time and space, referred to as habitat quality [1], containing the range of environments suitable for a particular species [2]. Naturally or under management, environments change and in the same place, the habitat conditions for certain species become more favorable or unfavorable over time. A continuous change in population size of species, and in their spatial distribution, is a normal rule. In forests under active management seeking sustained yield, forestry works are planned both in terms of space and time and natural disturbances more or less controlled

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Discussion

Conclusion