Can We Use Plant Performance As Indicators To Infer Plant-Crab Interactions In Coastal Saltmarshes?

Increasing effort has been devoted to restoring coastal ecosystems to counteract their degradation globally. Restoration success of coastal ecosystems often relies on harnessing biotic interactions that shape the performance of foundation plant species. Crabs acting as essential ecosystem engineers and consumers are commonly present in coastal saltmarshes. Previous work has focused on the grazing effect of crabs on plants. In line with the idea of top-down control, plant performance (in terms of biomass or height) is often used as indicators of plantanimal interactions. However, crabs could also produce facilitative effects through non-consumptive behavior. It remains unclear if and to what extent the net effect of crabs on coastal plants is dependent on environmental context. Therefore, systematic assessments are needed to test if the reliability of plant performance as an indicator of plant-crab interactions is context-dependent. Here, we conducted field survey in two similar intertidal saltmarsh ecosystems along the Yellow Sea coast, eastern China, and examined the effects of crab burrowing, soil properties, and nutrient availability on plant (Suaeda salsa) performance using multiple regression and structural equation models. We found that crab burrow density was significantly correlated with plant height, which signals strong plant-crab interactions. Surprisingly, we observed opposite signs (positive vs. negative) of plant-crab relationships between the two study sites that have highly similar composition and structure of plant communities. A possible explanation is that soil compactness can mediate crab burrowing behavior, resulting in differential facilitative effects on the plants. Thus, the usefulness of plant height serving as an indicator of plant-crab interactions depends on environmental context to a substantial degree. More comprehensive indicators taking into account soil compactness may facilitate robust inference of plant-crab interactions. Our results highlight landscape-scale spatial heterogeneity and context-dependency of plant-animal interactions in coastal saltmarshes, and provide a useful implication to inferring nuanced biotic interactions using compressive biotic and abiotic indicators in coastal ecosystems.