Simulated megaherbivore grazing as a driver of seagrass flowering

Simulated effects of tidal inundation and light reduction on Zostera muelleri flowering in seagrass nurseries

Zostera muelleri is an abundant seagrass species distributed through intertidal and shallow subtidal waters on the subtropical coasts of Australia. The vertical distribution of Zostera is likely defined by tidal influences, particularly desiccation and light reduction stresses. These stresses were expected to affect the flowering of Z. muelleri; however, it is difficult to quantify the effects of tidal inundation with field studies due to multiple confounding environmental factors affecting flowering (e.g., water temperature, herbivory, nutrients). A laboratory aquarium experiment compared the effects of two levels of tidal height (intertidal and subtidal) and light intensity (shaded and unshaded) on flowering timing, abundance, the ratio between flowering shoots and vegetative shoots, the morphology and duration of flower development. The earliest and greatest flowering intensity was recorded in the subtidal-unshaded group, with no flowers observed in the intertidal-shaded group. Notably, the peak flowering time was the same across shaded and unshaded treatments. Shading prolonged the timing of the first flowering and reduced the density of flowering shoots and spathes, while tidal inundation had a more significant effect on the density of flowering shoots and the density of spathes. Results showed that Z. muelleri could flower under low light conditions or tidal stress but not when exposed to both stresses simultaneously in a laboratory 'nursery setting'. Therefore, applying subtidal-unshaded conditions appears to be beneficial for seagrass nurseries aimed at improved flower abundance despite the plants previously being collected from and adapted to intertidal meadows. Further studies that explore the suitable conditions for triggering and optimising the flowering will be beneficial in designing cost-effective seagrass nurseries.

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.