Abstract
Theory suggests that species distributions are expanded by positive species interactions, but the importance of facilitation in expanding species distributions at physiological range limits has not been widely recognized. We investigated the effects of the nurse shrub Tamarix chinensis on the crab Helice tientsinensis on the terrestrial borders of salt marshes, a typical coastal ecotone, where Tamarix and Helice were on their lower and upper elevational distribution edges, respectively. Crab burrows were abundant under Tamarix, but were absent in open areas between Tamarix. Removing Tamarix decreased associated crab burrows with time, while simulating Tamarix in open areas by shading, excluding predators, and adding Tamarix branches as crab food, increased crab burrows. Measurements of soil and microclimate factors showed that removing Tamarix increased abiotic stress, while simulating Tamarix by shading decreased abiotic stress. Survival of tethered crabs was high only when protected from desiccation and predation. Thus, by alleviating abiotic and biotic stresses, as well as by food provision, Tamarix expanded the upper intertidal distribution of Helice. Our study provides clear evidence for the importance of facilitation in expanding species distributions at their range limits, and suggests that facilitation is a crucial biological force maintaining the ecotones between ecosystems.
Highlights
Science and Conservation, Nicholas School of the Environment, Duke University, 135 Duke Marine Lab Road, Beaufort 28516, North Carolina, USA
We investigated the effects of the nurse shrub Tamarix chinensis on the crab Helice tientsinensis on the terrestrial borders of salt marshes, a typical coastal ecotone, where Tamarix and Helice were on their lower and upper elevational distribution edges, respectively
We report two-year field experiments examining the interactions between the shrub Tamarix chinensis and the crab Helice tientsinensis on the terrestrial borders of two northern Chinese salt marshes
Summary
Science and Conservation, Nicholas School of the Environment, Duke University, 135 Duke Marine Lab Road, Beaufort 28516, North Carolina, USA. An ideal habitat to examine this role of facilitation is ecotone, which is the transitional area between different ecosystems Ecotones, such as terrestrial borders of salt marshes in the intertidal[9,10,11] and alpine treelines[12,13], are species distribution edges where species adapted to their native ecosystems face environmental stress of their tolerance limits. We report two-year field experiments examining the interactions between the shrub Tamarix chinensis and the crab Helice tientsinensis on the terrestrial borders of two northern Chinese salt marshes (see Methods) These borders are a typical coastal ecotone[10], which is flooded only during spring and storm tides, and are dry and hypersaline, leading to extreme thermal and desiccation stress for marine animals[9,16,17]. Helice burrows are abundant under Tamarix, but absent in open areas between Tamarix
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