Latitudinal variation in a saltmarsh foundation species and its herbivores

Background/Question/Methods

A +1.5°C increase in the global climate could occur as soon as the year 2030, and it is becoming increasingly unlikely that we will avoid +2°C of warming. Latitudinal gradients have long been used as natural laboratories to test effects of warming on species, but studies are often limited in two ways: 1) by focusing on abundance or occupancy, and 2) failing to account for multi-trophic interactions. Abundance and occupancy can’t account for species-level changes in form or function that may result from warming – but we have long known temperature has strong effects on functional traits. Furthermore, if we fail to account for interactions, we could be blind to effects that might override or alter direct effects of warming. For example, it is generally understood that warmer temperatures are associated with tougher leaves (higher leaf mass per area, higher tensile strength, and higher carbon-to-nitrogen ratio). However, warmer temperatures are also associated with higher herbivory, and higher herbivory is associated with tougher leaves. Should we expect species-level changes in functional traits along latitudinal gradients where there are no changes in abundance or occupancy? Should we expect species interactions or temperature to have a greater impact on functional traits?

Results/Conclusions

Saltmarshes are the ecosystems in which the most progress has been made in addressing these questions, but previous studies lack data resolution along latitude and have not integrated species interactions into a trait-based approach. We conducted a broad-scale biogeographic sampling effort across a broad range of temperature regimes, from northern Maine to central Florida, in order to determine how a saltmarsh foundation species (smooth cordgrass; Spartina alterniflora) and its herbivores vary in form and function. We measured cordgrass functional traits, herbivory, and abundance of two snail species that differ in the feeding habits -- the marsh periwinkle (Littoraria irrorata) and the common marsh snail (Melampus bidentatus). Smooth cordgrass exhibits increasing toughness with increasing temperatures (proximity to the equator), but this pattern is not matched by snail abundance or herbivory. This is an indication that in smooth cordgrass, traits may be driven mostly by abiotic conditions. Further research is needed to understand how this intraspecific variation in functional traits will affect ecosystem services – a question that is increasingly important for the highly productive, rapidly declining marshes of the Eastern US.