Where consumers control plant reproduction in coastal wetlands: The environmental stress model in plants' versus consumers' perspectives

Abstract

Understanding where consumers control plant reproduction in a time of changing plant–consumer interactions with environmental change can inform conservation. A seminal hypothesis—the environmental stress model (the ESM)—posits that consumer control of plants varies predictably with environmental stress. The ESM has been tested often along gradients of an abiotic stressor, yielding highly mixed support, although plants and consumers are often differently affected by a stressor. Here, to provide a conciliation, we introduced and tested a new ESM—the ESM in plants' versus consumers' perspectives. In field exclusion experiments conducted at three marsh sites that varied broadly in plant productivity, crab abundance and salinity within a coastal landscape, we tested the new ESM in predicting the impact of crab herbivores on the sexual reproduction of Phragmites australis. We analysed how their impact varied with plant productivity and crab abundance—proxies for environmental stresses in plants' and consumers' perspectives, respectively. We found that environmental stresses in plants' and consumers' perspectives were uncoupled across this coastal landscape. Neither crab abundance nor crab grazing intensity was correlated with plant productivity. Plant productivity and crab abundance were inconsistently related to key abiotic stressors including soil salinity. Depending on site, crab herbivores substantially suppressed Phragmites sexual reproductive effort (by >90%, by regulating a series of population‐ and individual‐level reproductive processes) or had little effect. Where crab herbivores had stronger impacts was not predicted by plant productivity, but largely by crab abundance via a threshold model. Above an abundance threshold, crabs increasingly suppressed Phragmites sexual reproductive effort. Synthesis. Our study revealed that consumer control of plant reproduction may be highly variable, even within a coastal landscape and for the same species, and that consumer abundance, rather than plant productivity, was a key predictor of this variation. As plants and consumers become increasingly uncoupled in many ecosystems under environmental change, the newly introduced ESM in plants' versus consumers' perspectives may be more versatile than previously recognized to inform conservation actions.