Abstract
Protecting key hotspots of marine biodiversity is essential to maintain ecosystem services at large spatial scales. Protected areas serve not only as sources of propagules colonizing other habitats, but also as receptors, thus acting as protected nurseries. To quantify the geographical extent and the temporal persistence of ecological benefits resulting from protection, we investigate larval connectivity within a remote archipelago, characterized by a strong spatial gradient of human impact from pristine to heavily exploited: the Northern Line Islands (NLIs), including part of the Pacific Remote Islands Marine National Monument (PRI-MNM). Larvae are described as passive Lagrangian particles transported by oceanic currents obtained from a oceanographic reanalysis. We compare different simulation schemes and compute connectivity measures (larval exchange probabilities and minimum/average larval dispersal distances from target islands). To explore the role of PRI-MNM in protecting marine organisms with pelagic larval stages, we drive millions of individual-based simulations for various Pelagic Larval Durations (PLDs), in all release seasons, and over a two-decades time horizon (1991–2010). We find that connectivity in the NLIs is spatially asymmetric and displays significant intra- and inter-annual variations. The islands belonging to PRI-MNM act more as sinks than sources of larvae, and connectivity is higher during the winter-spring period. In multi-annual analyses, yearly averaged southward connectivity significantly and negatively correlates with climatological anomalies (El Niño). This points out a possible system fragility and susceptibility to global warming. Quantitative assessments of large-scale, long-term marine connectivity patterns help understand region-specific, ecologically relevant interactions between islands. This is fundamental for devising scientifically-based protection strategies, which must be space- and time-varying to cope with the challenges posed by the concurrent pressures of human exploitation and global climate change.
Highlights
Multiple anthropogenic pressures are causing dramatic ecological changes to the oceans at the global scale [1]
We focus on the five most important Northern Line Islands (NLIs, Fig 1), which represent an ideal setting to analyze connectivity patterns. Despite their similar oceanographic characteristics, the recent history of the NLIs is characterized by an anthropogenic trajectory from virtually unimpacted (e.g. Kingman Reef and Palmyra Atoll in the north) to heavily impacted (e.g. Christmas Island-Kiritimati), following a gradient of fishing pressure and human impact increasing from north to south [8]
Despite some small fluctuations in recostruction performances induced by the interannual variability of the oceanic currents, the yearly median values of the relative errors produced by scheme S are always around 2%/4% for the longitudinal/latitudinal components of the velocity field, while median relative errors are around 5%/10% for schemes T and L
Summary
Multiple anthropogenic pressures are causing dramatic ecological changes to the oceans at the global scale [1]. Preservation of the marine environment requires the creation and the protection of sanctuaries, as well as the conservation and, if necessary, the restoration of the most critical seascape ecological functions [2]. Connectivity determines the amount of between-patch dispersal, being one of the most important mechanisms ensuring the preservation of marine biodiversity [4]. Ecological connectivity is a key concept for a variety of conservation and management issues in marine environments. These include securing the persistence and long-term viability of species assemblages [5], enhancing ecosystem resilience (i.e. facilitating recovery after disturbance, see [6]), and improving productivity and ecosystem functioning in the sea [7]
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