Species roles and link roles : a richer perspective on network ecology

Type of content
Theses / Dissertations
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Thesis discipline
Biological Sciences
Degree name
Doctor of Philosophy
University of Canterbury
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Cirtwill, Alyssa R.

Food webs and other ecological networks can be seen as maps of species and their interactions (e.g., predation, pollination, and parasitism). Such mappings frame the complex intricacies of biological communities in a way that is analytically tractable, but also obscure species-level information. This can lead to a gap between studies of networks and the deep literature surrounding species’ idiosyncratic ecologies. Species roles— descriptions of the way each species is embedded into its community —offer one way to bridge this gap. As roles provide a species-level perspective on network structure, patterns in species roles can often be related to species traits in a way that the overall structure of a network usually cannot. Thus, role-based approaches give network ecologists a way to use species’ natural histories to understand patterns in network structure while also making network analyses more approachable for ecologists with different specialities. This thesis uses a variety of definitions of species roles to explore a variety of ecological networks, demonstrating the broad range of questions to which species roles may be applied. The first chapter provides an overview of several different role concepts used in network ecology, and the second through fifth chapters each use one or more role concept to investigate specific ecological questions. Chapter two uses species roles to incorporate a predator-prey network into the Theory of Island Biogeography. Chapter three uses species roles to compare the overlap of plants’ interaction partners in plant-pollinator and plant-herbivore networks, while chapter four explores the changes to plants’ and insects’ roles in a single plantpollinator network over 15 years of climate change. Chapters five and six are focused on aquatic food webs that include parasites. Chapter five compares the roles of parasites and free-living species, as well as different types of interactions between them (i.e., predation among free-living species, parasitism, antagonism among parasites, and concomitant predation on parasites inside their hosts). Chapter six uses the roles of feeding links between free-living species to better understand the trophic transmission of parasites. Finally, in an appendix we show how individual variation in fishes diets affect their parasite loads. The key findings of this thesis are i) that using species roles to incorporate information from food webs improves the predictions of the Theory of Island Biogeography, ii) that more closely related plants had more similar sets of interaction partners despite a great deal of variation across networks and between plant families, iii) that the roles of plants and pollinators have shown different changes after 15 years of warming, suggesting that phenological uncoupling may be a risk for this system, iv) that parasites and free-living species have different roles in food webs, but only when concomitant predation was considered, and v) that many properties of feeding links between free-living species affect the outcomes of these links for parasites. As well as providing answers to the driving questions behind each chapter, this thesis demonstrates the breadth of potential applications for species roles. We conclude species roles provide a framework that speaks to the heart of one of the fundamental unsolved questions in ecology— how species’ traits relate to the structure of ecological networks.

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