From wing to wing: the persistence of long ecological interaction chains in less-disturbed ecosystems

Abstract

Human impact on biodiversity usually is measured by reduction in species abundance or richness. Just as important, but much more difficult to discern, is the anthropogenic elimination of ecological interactions. Here we report on the persistence of a long ecological interaction chain linking diverse food webs and habitats in the near-pristine portions of a remote Pacific atoll. Using biogeochemical assays, animal tracking, and field surveys we show that seabirds roosting on native trees fertilize soils, increasing coastal nutrients and the abundance of plankton, thus attracting manta rays to native forest coastlines. Partnered observations conducted in regions of this atoll where native trees have been replaced by human propagated palms reveal that this complex interaction chain linking trees to mantas readily breaks down. Taken together these findings provide a compelling example of how anthropogenic disturbance may be contributing to widespread reductions in ecological interaction chain length, thereby isolating and simplifying ecosystems.

Figure 1. Description of a long interaction chain linking forests to manta rays.

This chain is retained in less disturbed native forest but its integrity is compromised in human-altered palm forest. Bar graphs comparing processes in native (N) and palm (P) forests (mean ± SE) indicate that reductions in native tree abundance (A) reduce seabird abundance (B), which diminish the contribution of seabird derived nutrient subsidies to terrestrial ecosystems (C,D), which severely impair the movement of nutrients to the marine environment (E), reducing zooplankton abundance (F), and ultimately eliminating manta ray (Manta birostris) utilization of native forest coastlines (G). Delta values depict the difference between mean δ¹⁵N of native forest and palm forest material (Δδ¹⁵N = δ¹⁵N_N − δ¹⁵N_P). Positive delta values measured at multiple points along this lengthy interaction chain reveal that taxa in native forest zones are causally linked to one another via dependency upon isotopically elevated seabird derived nutrients.