Habitat Connectivity and Quality Alter Population Growth and Community Structure of Marine Crustaceans

Abstract

Communities are linked by dispersers that arrive from neighboring habitats and sustain emerging populations. Differences in recruitment establish dissimilarities in composition and diversity among communities and alter levels of connectivity. In heterogeneous habitats, source populations provide propagules to neighboring sinks, which rely on incoming recruits to sustain populations. However, recruitment is not static, and changing environmental conditions that alter habitats can shift population growth and community structure. In this thesis, I evaluate whether changing environmental conditions, such as habitat loss and future climate change, drive shifts in community assembly and population growth. Using marine peracarids as a model system, I examined the effects of isolation on community assemblage trajectories in seagrass beds and explored the effects of elevated temperature and CO₂ on population growth and quality of amphipods that inhabit temperate reef ecosystems. Here, I show that the identity and abundance of recruits leads to local (α) and between-community (β) diversity differences and that the state of communities is dependent on incoming source recruits. Isolation from sources elicits different responses within and among species, which minimizes differences among communities and shifts species abundances. Furthermore, I show how environmental conditions (i.e. elevated temperature and CO₂), that enhance resource availability in sinks, can boost reproductive output and population growth of an amphipod species (Cymadusa pemptos). My test of the hypothesis that future climate conditions enhances sexual characters, reducing variation in mating success and boosting population growth, suggests that future climate might relax constraints on mechanisms of reproduction. As future climate conditions drive population outbreaks, growing establishment of artificial structures that are increasingly used as habitats provide amphipods with alternative settlement space. Together, these studies highlight the importance of examining population growth and community assembly in spatially structured systems. Sources and sinks that are undergoing shifts in abundance and quality of habitats drive change in the density and distribution of mobile species, and when favorable conditions align, such as greater resource availability and reduced competition and predation, populations can boom. However, the potential for population growth depends on how individuals distribute across spatially structured habitats. Since the identity and abundance of incoming recruits drives sink population growth and community structure, it is essential to understand the effect each species’ dispersing stage (i.e. male, female and juvenile) and resource requirements have on sink development.