Freestone, Amy; Behm, Jocelyn E.; Sewall, Brent J.; Muniz Dias, Gustavo (Temple University. Libraries, 2021)
      Biogeographic barriers have limit the movement of organisms, leading to speciation and shaping the development of unique regional biotas. Human-aided circumvention of biogeographic barriers, however, has increased the connectivity of isolated biotas, while changes in climate have been altering species geographic ranges, restructuring ecological networks, modifying ecological niche arrangements and increasing the frequency and intensity of natural pulse disturbances. In this context, examination of some of the most fundamental processes underlying the patterns of species diversity and community structure across biogeographical scales is therefore vital, especially as humans increasingly modify these relationships established over the course of geological history. The latitudinal diversity gradient is the most well-established and predominantly studied biogeographic diversity pattern on Earth. Higher tropical biodiversity with a decline towards higher latitudes occur in both hemispheres and has been observed for various systems including terrestrial, freshwater and marine. In my dissertation, I explored latitudinal variation in factors that can inform and modify fundamental ecological principles such as coexistence, community stability, and resistance. I studied communities of marine invertebrates across over 7000 km of continuous coastal habitat spanning 47-degree latitudinal range on the Eastern North and Central Pacific coast, from tropical Panama to subarctic Alaska. Experiments were based on the manipulation of recently recruited or adult communities of sessile marine invertebrates, such as barnacles, marine worms and encrusting colonial organisms. These organisms inhabit coastal ecosystems across continental scales and can colonize artificial substrates such as hull of commercial vessels that travel across oceans serving as vectors for marine biological invasions. Among my three aims I was able to: (1) examine how the complementarity of ecological niches varies from the tropics to subarctic among recruiting taxa, and how the differentiation of these recruitment pulses through time help inform coexistence across large spatial scales; (2) document distinct resilience responses to pulse disturbances between lower and high latitude sites, while exploring the interplay between compositional and functional recovery in biological communities; and finally, (3) implement a novel component to pre-existing models to predict probability of biological invasions based on the distinct pressures of propagule introduction, environmental similarity between donor and recipient regions, and the potential for biotic resistance. My dissertation yields empirical evidence of processes varying with latitude, advancing our knowledge in some of the most fundamental questions in classic and contemporary ecology. Large-scale documentation of the mechanisms and processes that maintain global patterns of biodiversity are uppermost in the design of global conservation strategies, especially in a more connected world.
    • Spatial and Temporal Variability in Marine Invasion and Trophic Dynamics

      Freestone, Amy; Sanders, Robert W.; Cordes, Erik E.; Sewall, Brent J.; Petraitis, Peter Steven (Temple University. Libraries, 2014)
      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.