Evidence for 'critical slowing down' in seagrass:a stress gradient experiment at the southern limit of its range

The theory of critical slowing down, i.e. the increasing recovery times of complex systems close to tipping points, has been proposed as an early warning signal for collapse. Empirical evidence for the reality of such warning signals is still rare in ecology. We studied this on Zostera noltii intertidal seagrass meadows at their southern range limit, the Banc d'Arguin, Mauritania. We analyse the environmental covariates of recovery rates using structural equation modelling (SEM), based on an experiment in which we assessed whether recovery after disturbances (i.e. seagrass & infauna removal) depends on stress intensity (increasing with elevation) and disturbance patch size (1 m(2) vs. 9 m(2)). The SEM analyses revealed that higher biofilm density and sediment accretion best explained seagrass recovery rates. Experimental disturbances were followed by slow rates of recovery, regrowth occurring mainly in the coolest months of the year. Macrofauna recolonisation lagged behind seagrass recovery. Overall, the recovery rate was six times slower in the high intertidal zone than in the low zone. The large disturbances in the low zone recovered faster than the small ones in the high zone. This provides empirical evidence for critical slowing down with increasing desiccation stress in an intertidal seagrass system

Are hedgerows effective corridors between fragments of woodland habitat? An evidence-based approach

Anthropogenic modification of the countryside has resulted in much of the landscape consisting of fragments of once continuous habitat. Increasing habitat connectivity at the landscape-scale has a vital role to play in the conservation of species restricted to such remnant patches, especially as species may attempt to track zones of habitat that satisfy their niche requirements as the climate changes. Conservation policies and management strategies frequently advocate corridor creation as one approach to restore connectivity and to facilitate species movements through the landscape. Here we examine the utility of hedgerows as corridors between woodland habitat patches using rigorous systematic review methodology. Systematic searching yielded 26 studies which satisfied the review inclusion criteria. The empirical evidence currently available is insufficient to evaluate the effectiveness of hedgerow corridors as a conservation tool to promote the population viability of woodland fauna. However, the studies did provide anecdotal evidence of positive local population effects and indicated that some species use hedgerows as movement conduits. More replicated and controlled field investigations or long term monitoring are required in order to allow practitioners and policy makers to make better informed decisions about hedgerow corridor creation and preservation. The benefits of such corridors in regard to increasing habitat connectivity remain equivocal, and the role of corridors in mitigating the effects of climate change at the landscape-scale is even less well understood

Short-term response and recovery of bluebells (Hyacinthoides non-scripta) after rooting by wild boar (Sus scrofa)

Species reintroduction programmes should include consideration of potential impacts on key species in the recipient community. Wild boar (Sus scrofa) have been reintroduced into Britain after a 700-year absence. There is an urgent need to understand how this ecosystem engineer will affect plant communities in the habitats that it invades. We investigated the impact of rooting by wild boar on bluebells (Hyacinthoides non-scripta), a species that is highly valued for its impressive floral displays and is an important and legally protected component of the UK forest flora. We monitored bluebell performance over three growing seasons in woodland habitats that are routinely rooted by boar in southern England. H. non-scripta cover and reproductive performance were monitored in small-scale experimental exclosures to exclude boar, compared to open control plots, set up on areas that either had or had not been previously rooted. Immediate effects were that rooting significantly reduced the percentage cover and density of H. non-scripta plants, by up to 95 and 60 %, respectively, and also adversely affected the number of flowering stems. However, there was evidence that cessation of rooting brought about by excluding the boar enabled substantial recovery in percentage cover and the density of flowering stems within 2 years. A positive effect of rooting on germination may have assisted this recovery. Thus, the impact of wild boar rooting on bluebell populations is locally severe, but there is potential for rapid recovery if plants are protected. Long-term effects of sustained or frequently repeated rooting still need to be investigated

Dugongs: Seagrass community specialists

Dugongs exploit a relatively wide diet including seagrasses, macro-invertebrates and algae within intertidal and subtidal tropical and subtropical seagrass communities. The importance of seagrass genera to dugongs differs among locations and may change at the same location during times of seagrass loss. Dugongs feed by excavating or cropping, depending on seagrass morphology and the nature of the sediment. An individual dugong can disturb a considerable area of seagrass in a single day, especially in areas with low biomass. The local impact on seagrass biomass can be very significant and cause a loss of over 50% of production. The food quality of the seagrass forage eaten by dugongs is similar to the forage eaten by many wild, large, terrestrial, herbivorous mammals. Dugongs are less effective at masticating fibrous seagrasses than low-fibre seagrasses. This limitation may be most important under lengthy periods of food scarcity, such as at times of seagrass loss. The effect of dugongs feeding on seagrasses is complex and can be measured at several spatial scales including: (1) the individual feeding scar, (2) the area disturbed per day by an individual animal, and (3) the effect of a large group of animals on an individual plant community and using several responses variables: (1) microbial processes, (2) above- and below-ground plant biomass, (3) plant species composition, (4) plant nutrients, (5) invertebrate community composition and detritus, plus (6) the time taken by each of these variables to return to the pre-disturbed condition. Marked temporal fluctuations in dugong mortality and fecundity track major changes in the seagrass communities on which dugongs depend for food

Clonal architecture in an intertidal bed of the dwarf eelgrass Zostera noltii in the Northern Wadden Sea: persistence through extreme physical perturbation and the importance of a seed bank

Genotypic structure and temporal dynamics of the dwarf seagrass, Zostera noltii, were studied in an intertidal meadow that has persisted since prior to 1936 near the Wadden Sea island of Sylt. Samples were collected from two 10 × 10 m plots separated by 250 m from May 2002 to June 2005 and from four 1 × 1 m plots from June 2003 to September 2004. All the samples were genotyped with nine microsatellite loci. No genotypes were shared between the plots separated by 250 m. Genetic diversity was higher in the Wadden Sea than in the other regions of its geographic range. The average clone size (genets) (SD) in the two plots was 1.38 (0.26) and 1.46 (0.4) m², respectively, with a range up to 9 m² and <20% persisted for >4 years. A high genetic and genotypic diversity was maintained by annual recruitment of seedlings despite a dramatic decrease in ramet density that coincided with the severe heat stress event of 2003. Fine-scale (1 m²) analysis suggested that extensive loss of seagrass cover precluded space competition among the genets, while a persistent seed bank prevented local extinction. Long-term persistence of Z. noltii meadows in the intertidal Wadden Sea was achieved by high genet turnover and frequent seedling recruitment from a seed bank, in contrast to the low diversity observed in large and long-living clones of Z. noltii and other seagrasses in subtidal habitats