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