Fine-scale environmental heterogeneity and conservation management: beach-cast wrack creates microhabitats for thermoregulation in shorebirds.
Conservation management to protect coastal ecosystems sometimes overlooks site-specific fine-scale heterogeneity. For example, while habitat loss is a known key driver of population declines in many shorebirds, these birds are also dependent on high-quality habitats to maximize energy stores. Here we describe the microhabitats provided by beach-cast wrack (washed up macroalgae and seagrasses), a resource threatened by harvesting and beach cleaning, and how shorebirds utilize these. We measured the temperature and absolute humidity at 10 cm above three substrates (fresh wrack, aged wrack and sand) and then related bird behaviour (roosting vs. foraging) to climatic and environmental data. Freshly beach-cast wrack mostly provided cooler and less humid habitats, but warmer temperatures than aged wrack or sand in the early mornings. Microtopography created by shelter from prevailing winds and wrack depth modified these general trends. Generally, temperature predicted where shorebirds overall and the two most common species, the double-banded plover Charadrius bicinctus and red-necked stint Calidris ruficollis, were observed. During most of the day, foraging and roosting were more likely to occur on the warmer aged wrack. In the early morning, when fresh wrack provided the warmer temperatures, birds tended to roost and forage on fresh wrack. Synthesis and Applications. Beach-cast wrack creates a complex mosaic of unique microclimates varying in space and time, which allows shorebirds to minimize energy expenditure by selecting the thermally most favourable habitats for roosting and foraging. Removal of beach-cast wrack for commercial and aesthetic reasons thus reduces habitat quality and increases energy expenditure in shorebirds. Associated declines in energy stores may be contributing to declines in shorebird populations. Management of coastal ecosystems and shorebirds therefore needs to consider and maintain fine-scale environmental heterogeneity at local scales.