Microhabitats can be recruitment refuges and sinks

© 2019 Inter-Research. Recruitment is an important demographic bottleneck in the life history of many plant and animal species. Microhabitats provided by surfaces or plants or animals can reduce mortality during this critical life-history period. We investigated how microhabitats influence postsettlement processes on a range of ascidians, bryozoans and barnacles. We compared postsettlement mortality and growth on surfaces that were flat or were covered by artificial barnacle mimics or live adult barnacle microhabitats. We also tested whether the effects of surface type changed under different levels of environmental stress by manipulating the orientation of treatments. Orientation had the strongest influence on survival, growth and recruitment, with individuals on downwards-facing surfaces performing the best and those on upwards-facing surfaces with significantly higher mortality. The bryozoan Watersipora subatra had higher survival in micro - habitats, but was not influenced by surface orientation. In contrast, barnacles showed increased mortality in microhabitats on upwards-facing panels, but were not affected by other orientations. The presence of adult barnacles caused decreased growth of barnacle recruits, but had no effect on the bryozoan. In comparison, the ascidians (Botryllus sclosseri and Diplosoma listerianum) were mostly not influenced by microhabitats. The high rates of sedimentation on upwards-facing surfaces provide the most plausible answer for the strong influences of surface orientation on survival and growth. While microhabitats can act as refuges for some species, the same microhabitat type can lead to higher mortality in other species. The response of each species to microhabitats ultimately depends on the source of mortality at each site.