Reactive persistence, spatial management, and conservation of metapopulations: An application to seagrass restoration.

Assessing the conditions for persistence of spatially structured populations, especially those that are exploited by humans or threatened by global change, is of critical importance to inform management and conservation efforts. Observations for entire metapopulations are usually incomplete and rarely, if ever, sufficiently long to deduce population persistence from spatial patterns of abundance. Instead, insights based on metapopulation theory are often used for interpreting the demographic trajectories of real populations and for informing management decisions. The classical theoretical tool used to assess conditions for metapopulation persistence is the "invasibility criterion," which characterizes the asymptotic, or long-term, stability of a small colonizing population. Essentially, when the linear operator governing the metapopulation dynamics of an invasion event has a positive eigenvalue, recovery and resistance to extinction (resilience) are implied. The converse, however, is not necessarily the case-an invasion may grow over multiple generations, even when the eigenvalues indicate that extinction will eventually occur, a situation referred to here as "reactive persistence." For the management, restoration, and conservation of real metapopulations subject to continual disturbance, this transient behavior is often more relevant than the asymptotic behavior over long time scales. We develop the theoretical tools for assessing reactive persistence, demonstrating how the conditions for asymptotic and reactive persistence differ in both the patch-occupancy models suited to many terrestrial populations and those where local patch extinctions can be disregarded in the dynamics, often suited to marine species. After presenting the mathematical basis for generalizing the invasibility criterion to include reactive persistence, we illustrate how these concepts and tools can be applied in practice, using as a case study the population ecology and restoration of the seagrass Zostera muelleri (Irmisch ex Ascherson, 1867) in the Port of Gladstone in the Great Barrier Reef World Heritage Area Australia. It is shown how the analysis of the transient dynamics of the Z. muelleri metapopulation can be used to guide restoration efforts. Moreover, it is demonstrated that these reactive persistence concepts provide a more appropriate basis for site prioritization for restoration interventions than traditional stability analysis.

Incorporating the Connectivity Timescale in Metapopulation Partitioning.

The often complex spatial patterns of propagule dispersal across a metapopulation present a challenge for species management, motivating efforts to represent the connectivity in simpler but meaningful ways. The reduction of complexity may be achieved by partitioning the metapopulation into groups of highly connected patches called "subpopulations." To have relevance for management, these subunits must be defined from ecological or evolutionary principles. The probabilities of dispersal-mediated propagule interchange between sites, commonly organized into a connectivity matrix, entail a timescale that is usually ignored in subpopulation analyses, limiting their utility and possibly leading to misinterpretation and wrong management decisions. Recognition of the essentially dynamical role played by metapopulation connectivity naturally leads to the incorporation of the generational timescale into the partitioning analysis. An algorithm is proposed to determine the subpopulations-both their cardinality and their composition-as a function of the generational timescale and of a limiting probability of connection, illustrated with a novel empirical estimate of mesopelagic connectivity. The proposed framework allows the unambiguous determination of the timescales corresponding to dispersal barriers and the identification of effective ecological units across the spectrum of management-relevant time horizons.

Coexistence of competitors in marine metacommunities: environmental variability, edge effects, and the dispersal niche.

Theoretical studies have shown that coexistence between competitors can be favored in a spatially heterogeneous environment by a number of mechanisms, which ultimately allow the expression of persistent or transitory variation in species competitive abilities, colonization, or reproduction. Four distinctive paradigms to model metacommunities have been identified according to assumptions about the biology of the species and essential aspects of the environment. Missing from these are mechanisms of coexistence that can arise from the dispersal process itself without explicit spatial heterogeneity or biological trade-offs. These mechanisms have only recently received attention, but they may be common in marine communities and other systems in which dispersal is obligatory and modulated by the physical environment. We investigate coexistence in spatially homogeneous metacommunities where there is no partitioning of resources, no competition-colonization trade-off, and no possibility of source-sink dynamics. Coexistence is shown to be possible through three distinct mechanisms related to the dispersal process itself. Firstly, in a neutral scenario, inclusion of temporal variability in the connectivity matrix, emulating an intrinsic attribute of ocean character and other turbulent environments, can promote the invasion of an equally matched competitor and, in a hierarchical competition scenario, the persistence of an otherwise unviable, inferior competitor (the dispersal variability mechanism). Secondly, a sufficiently large difference in the shape of the time-independent dispersal kernels of the two species, which may result from differences in larval-release timing, buoyancy, or behavior, can produce stable coexistence in the center of their shared range (the dispersal-shape mechanism). Thirdly, asymmetry in the dispersal process due to biased advection renders the metapopulation model reactive, such that small variations in the upstream abundances can be sufficient for the subordinate species to stably persist (the dispersal-bias mechanism). These results demonstrate that a subordinate species may persist by occupying a dispersal niche that differs sufficiently from that of the dominant species. Further theoretical research is necessary to develop simple empirical tests for these and other dispersal-based coexistence mechanisms.