| Abstract |
- A central challenge for ecology is to understand the dynamic nature of species
interactions. A classic approach to community ecology assumes that individuals within a
species are functionally identical and that consumer-resource dynamics can be predicted
solely by using species abundances. However, one species can consist of multiple
functional groups, as diet differences among life stages of a single species may be greater
than the diet differences among species. Because of variation in diets over a lifetime, we
must ask: under what circumstances can individuals be simplified into a collective
species, and when does it matter to focus on intraspecific variation? I examined the role
of stage structure in species interactions and its consequences across levels of biological
organization.
In chapter 2, I conducted a functional response experiment in the laboratory to
examine the role of body size, temperature, ontogenetic stage, and larval stonefly density
on crayfish feeding rates. I found that the effect of temperature was stage-dependent as
juveniles increased feeding with temperature, whereas adult feeding rate was similar
across temperature treatments. Adult crayfish reduced their attack rate with increased
stonefly density, which is either reflective of their biology or an artifact of experimental design. Nonetheless, size- and temperature- dependent functional response models that
included stage structure performed better than models without stage structure,
highlighting the importance of incorporating stage structure into measures of species
interaction strengths.
In chapter 3, I used metacommunity theory as a lens to explore how crayfish
ontogenetic stage and sex alter the effect of diet on gut microbial communities. There
was little overlap in the microbial community between crayfish in the lab and those in the
wild, indicating that gut microbes are transient. The similarities in microbial community
composition between food items and crayfish suggest that their gut microbes are mainly
from their food and not from the surrounding environment. There were stage- and sexdependent
effects on gut microbial communities, likely due to differences in behavior and
physiology between the stages and sexes. Though stage, sex, and diet influenced gut
microbial communities, they had low explanatory power. Addressing connectivity
between hosts or feedbacks between the host and the environment on gut microbial
communities are potential avenues for future work.
Species introductions can alter the relationship between trophic interactions and
ecosystem processes. Often, introduced species reduce the abundance and diversity of
biota in recipient food webs. However, ontogenetic diet shifts in the introduced species
can alter the presence, degree or direction of these impacts on native species, making it
difficult for scientists and managers to predict the ecological consequences of species
introductions. In chapter 4, I conducted a manipulative field experiment to assess the
effects of crayfish species identity and ontogenetic stage on benthic invertebrate composition and abundance as well as leaf litter breakdown by native signal crayfish
(Pacifastacus leniusculus) and introduced ringed crayfish (Faxonius neglectus neglectus).
Treatments with signal crayfish or adult crayfish had higher reductions in leaf litter
relative to treatments with introduced crayfish and juvenile crayfish. Alpha and beta
diversity of benthic invertebrates was similar among treatments, but there were fewer
shredders in treatments with adult crayfish. Thus, I show that ontogenetic stage and
native vs. non-native status both matter for understanding the impact of species
introductions on local ecological communities and ecosystem processes.
In Chapter 5, I presented the challenges of uniting ecological theory and empirical
data. I designed an experiment that investigated the roles of consumer body size and
ontogenetic stage, environmental temperature, and resource quality on consumer-resource
interactions. My goal was to bridge the Metabolic Theory of Ecology and Ecological
Stoichiometry to describe how the balance leaf litter nutrient availability and crayfish
elemental demand govern their feeding rates and nutrient cycling over ontogeny, but my
models did not fit the data well. This developed into a philosophical dilemma: as
scientists, do we choose to work in systems that will produce data that we know will fit
our model, or do we test a model in various systems to see how generalizable our
predictions can be? I explored the culture of science, focusing on the existence of
paradigms in ecology that discourage “negative results” and the obstacles one might face
in conducting functional response experiments. Specifically, I reviewed experimental
design and statistical methods used in functional response literature and how they can be
biased to maintain paradigms in ecology.
My dissertation contributes to our understanding on the role of intraspecific
variation in ecology. I examined consumer-resource interactions (Ch. 2), community
structure (Ch. 3), and ecosystem processes (Ch. 4 and 5) to assess the effect of stage
structure on species interactions and their consequences across multiple scales of
organization. I illustrated the challenges of bridging ecological theory and empirical data
and the approaches that hinder or advance the field. Overall, my work has demonstrated
the importance of incorporating stage structure in ecological studies and that this
information advances both ecological theory and applied efforts.
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