Spatial and Temporal Variability in Marine Invasion and Trophic Dynamics

Abstract

Species interactions are central to the study of community ecology, but these interactions can change with context. For instance, predator-prey interactions can vary with species introductions, spatial scale and temporal scale, and we are still learning how such factors can influence the strength of these interactions. Studying species interactions via multifaceted approaches and at different scales aids in the understanding of local and large scale processes, and can lead to predictions of how our ecosystems will persist in the face of continued anthropogenic alteration of the globe. The present series of studies sought to explore spatial and temporal variability in marine predator-prey interactions and invasion dynamics. The first objective was to assess biogeographic variability in predator invasions in the field. The second examined spatial variation in niche breadth via field collections, laboratory dissections, and database development, and the third involved a series of laboratory and field experiments as well as population modeling to examine temporal variability in native and non-native behavioral interactions. Specifically for the first objective, I examined the strength of marine invasive species-induced trophic cascades across latitude, hypothesizing that a non-native tertiary consumer could facilitate non-native basal prey establishment through the consumption of a native secondary consumer. I further predicted that the ecological importance of this cascade may be reduced in the subtropics relative to the temperate zone due to stronger predation pressure at lower latitudes. I found evidence of a trophic cascade in both regions, but it was only maintained under ambient predation pressure in the temperate zone. My results also suggest that strong predation pressure on the non-native intermediate predators in the subtropics may explain the weakened cascade under ambient conditions. For the second objective, I tested the hypothesis of increased specialization at lower latitudes using Brachyuran crabs as a model system and diet as my measure for niche breadth, while controlling for range size, body size and evolutionary relatedness. I compiled a dataset on 39 crab species' diets from existing studies and conducted my own diet analyses on species collected in a temperate, subtropical and tropical region, resulting in a global comparison. I found that latitudinal position was correlated with range size for temperate species, but not for tropical species, and found no correlation between the other focal variables and latitude. These results suggest that ecological mechanisms (i.e. competition strength) may be driving patterns of niche breadth in the temperate zone, while evolutionary mechanisms may be more important in predicting niche breadth patterns in tropical systems. For the third objective, I examined the influence of native and non-native prey naïveté on intermediate predator invasion success. I hypothesized that 1) naïveté is greatest in earlier stages of invasion across all trophic levels, decreasing the longer a non-native species is established in a system, 2) Native prey naïveté results in resource effects which increases invasion success, or 3) predator effects on non-native species would outweigh the importance of basal native prey naïveté, preventing an increase in non-native population growth. Through laboratory trials, I found support for naïveté being stronger at earlier stages of invasion, for both native basal prey and non-native intermediate predators. I also found weak predation on the more recently established intermediate predator in the field. However, my population model predicted that growth independent of basal prey naïveté. These results suggest that physiological traits, such as conversion efficiency and growth rates of the invasive crab may be driving its population growth more-so than foraging benefits. My studies surrounding the variability of species interactions are the first to examine the strength of invasive species-induced trophic cascades across latitude, one of very few marine empirical studies to examine diet breadth at a large spatial scale, and the first to examine multi-trophic behavioral effects on invasion success respectively. They highlight the importance of studying multi-trophic interactions, as examining more pieces of the food web is increasingly important in developing a broader understanding of interactions and adaptations within invaded communities. My research also highlights the importance of studying interactions from a macroecological perspective. Tracking both invasions and native species interactions through space and time provides insight into marine community dynamics and may elucidate possible mechanisms of species coexistence.