Fast‐growing oysters show reduced capacity to provide a thermal refuge to intertidal biodiversity at high temperatures

Abstract

Ecosystem engineers that modify the thermal environment experienced by associated organisms might assist in the climate change adaptation of species. This depends on the ability of ecosystem engineers to persist and continue to ameliorate thermal stress under changing climatic conditions-traits that may display significant intraspecific variation. In the physically stressful intertidal, the complex three-dimensional structure of oysters provides shading and traps moisture during aerial exposure at low tide. We assessed variation in the capacity of a faster- and slower-growing population of the Sydney Rock Oyster, Saccostrea glomerata, to persist, form three-dimensional structure and provide a cool microhabitat to invertebrates under warmer conditions. The two populations of oysters were exposed to a temperature gradient in the field by attaching them to passively warmed white, grey and black stone pavers and their growth, survivorship and colonisation by invertebrates was monitored over a 12-month period. Oysters displayed a trade-off between fast growth and thermal tolerance. The growth advantage of the fast-growing population diminished with increasing substrate temperature, and at higher temperatures, the faster-growing oysters suffered greater mortality, formed less habitat, and were consequently less effective at ameliorating low-tide air temperature extremes than slower-growing oysters. The greater survivorship of slower-growing oysters, in turn, produced a cooler microclimate which fed back to further bolster oyster survivorship. Invertebrate recruitment increased with habitat cover and was greater among the slower than the faster-growing population. Our results show that the capacity of ecosystem engineers to serve as microhabitat refugia to associated organisms in a warming climate displays marked intraspecific variation. Our study also adds to growing evidence that fast growth may come at the expense of thermal tolerance.

Structural traits dictate abiotic stress amelioration by intertidal oysters

“…Heat stress on rocky shores generally increases with the amount of primary substrata exposed to the sun (Lathlean, ). In a previous field study manipulating the density of horizontally orientated oysters, we found that substrate temperature increased as oyster cover decreased (McAfee et al, ). In the present mesocosm experiment, however, vertically orientated oysters caused cooler tile temperatures than horizontally orientated oysters, despite their occupation of a smaller amount of tile space (an experimental artefact of standardizing oyster biomass).…”

“…Where efforts to restore ecosystem engineers do not achieve the structural traits critical to environmental amelioration, the desired ecological outcomes may not be achieved (Irving & Bertness, ). In the present case, oysters can provide refugia for intertidal biodiversity from environmental extremes (McAfee et al, ), but this is dependent on the structural traits of the oyster beds and, in order for this function to be maximized, vertically structured configurations are most effective. Hence, in order to maximize the benefits of oyster reef restoration projects in providing climate refugia to associated biodiversity, vertical growth forms should be encouraged through restoration methodologies.…”

“…Additionally, infrared images (Testo, i80 camera) were taken from ~1 m above each tile within a minute of being exposed to heating, and every 30 min thereafter to assess small‐scale variation in temperatures across tiles. Body temperatures of oysters were estimated using biomimetic oysters (hereafter “robo‐oysters,” Helmuth et al, ) produced by encasing an iButton (programmed as above) inside an empty oyster shell with 3 M Scotchcast 2,131 (McAfee et al, ). Due to limited numbers of sufficiently large oysters, one robo‐oyster per tile was placed on three of the tiles assigned to the four‐hour duration treatment ( n = 3).…”

“…Consequently, on mid‐intertidal substrata in many parts of the world, these habitat‐forming bivalves are often dominant space occupants, in some instances approaching 100% cover (Bahr & Lanier, ; McAfee, Cole, & Bishop, ). In such environments, they may significantly increase biodiversity by providing associated organisms with an increased surface area for attachment and grazing (Seed, ), microhabitats that ameliorate environmental stressors such as high temperatures, desiccation and waves (McAfee et al, ; Oliveira, Sousa‐Pinto, Weber, & Bertocci, ), and protection from predators (Grabowski, ). Field experiments manipulating temperature and oyster cover indicate that the cool microhabitats provided by oysters are increasingly critical to survival of associated species as temperatures warm, although there is considerable variation in this role which may be related to differences in the structural traits among oyster beds (McAfee et al, ).…”

The value and opportunity of restoring Australia's lost rock oyster reefs

“…subtidal habitats), the complex structure of S. glomerata provides refugia from predators (McAfee & Bishop ). In dampening the impacts of climate and biotic stress, the restoration of S. glomerata reefs can be considered a climate adaption strategy, providing associated organisms more time to adapt to the changing environment (McAfee ).…”