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Satellite tracks predator-prey interactions

Australia: By linking ecological theory with freely-available Google Earth satellite imagery, landscape-scale footprints of behavioural interactions between predators and prey can be observed remotely, according to a paper published in an issue of Nature Publishing Group’s open access journal, Scientific Reports. The report, Landscape of fear visible from space, is authored by Elizabeth M. P. Madin, Joshua S. Madin and David J. Booth.

A Google Earth image survey of the lagoon habitat at Heron Island within Australia’s Great Barrier Reef revealed distinct halo patterns within algal beds surrounding patch reefs. Ground truth surveys confirmed that, as predicted, algal canopy height increases with distance from reef edges. A grazing assay subsequently demonstrated that herbivore grazing was responsible for this pattern. In conjunction with recent behavioural ecology studies, these findings demonstrate that herbivores’ collective anti-predator behavioural patterns can shape vegetation distributions on a scale clearly visible from space. By using sequential Google Earth images of specific locations over time, this technique could potentially allow rapid, inexpensive remote monitoring of cascading, indirect effects of predator removals (e.g., fishing; hunting) and/or recovery and reintroductions (e.g., marine or terrestrial reserves) nearly anywhere on earth.

Remote surveys and ground verification: Google Earth was used to remotely explore the landscape-scale features of the shallow (< 5m depth) lagoon benthos. Images were visually scanned for evidence of grazing halos surrounding isolated patch reefs. Once halos were identified, patch reef coordinates were entered into hand-held GPS units, which were subsequently used to navigate by foot and/or boat to individual patch reefs within halos identified in the image survey. Once located, ground verification consisted of determining canopy height of algae surrounding the apparent halos. For 3 replicate transects per halo for three halos, canopy height was measured as the algal canopy height every 3m radiating from reef edges outwards to 30m. Height was measured at its maximum point at each distance within 1m on either side of transects. Grazing assay: With the exception of the species used, assay followed the protocol described in Hay 10. Clumps of Hincksia sp., the thin, filamentous alga whose beds provide the basis for the observed grazing halos, were clipped to 5.5cm lengths then attached to clips on benthic assay units. Assay units were placed every 3m in straight transects radiating outwards from the reef’s edge to 12m. Three transects with two replicate clumps per unit were used. After being left in-situ for 72h, units were retrieved and clumps re-measured to the nearest 0.5cm. This assay measured the percent change in algal clump lengths over the three-day period. Assuming that all clumps would have experienced the same growth rate over this period, this provides a metric of relative grazing intensity at varying distances from the reef.

Source: The Nature