Modeling grazer-mediated effects of demographic and material connectivity on giant kelp metapopulation dynamics

Abstract

From dispersal-based metapopulations to meta-ecosystems that arise from flows of non-living materials, spatial connectivity is a major driver of population dynamics. One potentially important process is material transport between populations also linked by individual dispersal. Here, I explored material and demographic connectivity in metapopulations of giant kelp Macrocystis pyrifera, a foundation species that produces both detritus and reproductive spores. Kelp detritus (drift) subsidizes grazers, helping maintain the kelp forest ecosystem state. Drift could potentially be exchanged among kelp patches, but this is less studied than spore dispersal. Therefore, I built an ordinary differential equation (ODE) model to investigate conditions under which drift and/or spore connectivity promotes the kelp forest state. I fit statistical models (generalized linear mixed models, GLMMs) to observational data and used the GLMM’s predictions to validate the ODE model. My results suggest kelp patch dynamics are best explained by connectivity of both drift and spores, and that the impacts of these forms of connectivity depend on local grazer (urchin) abundance. Both models predicted greater kelp persistence in well-connected patches across a range of urchin densities. These effects were largely driven by drift, which reduced grazing in recipient patches and thereby enhanced spore recruitment. While testing these predictions will require greater empirical quantification of interpatch drift transport, my findings indicate drift connectivity may be an important spatial process in kelp forest systems. More broadly, this work highlights the role of meta-ecosystem dynamics within a single ecosystem type, reinforcing the need to expand traditional metapopulation perspectives to consider multiple forms of spatial connectivity.