Emily May

Topic

Improving our understanding of vertebrates in nutrient recycling and ecological stoichiometry

Department of Biology

Date & location

• Tuesday, July 23, 2024
• 8:30 A.M.
• Clearihue Building, Room B021

Examining Committee

Supervisory Committee

• Dr. Rana El-Sabaawi, Department of Biology, University of Victoria (Supervisor)
• Dr. Diana Varela, Department of Biology, UVic (Member)
• Dr. Steve Perlman, Department of Biology, UVic (Member)
• Dr. Brian Starzomski, School of Earth and Ocean Sciences, UVic (Outside Member)

External Examiner

• Dr. Halvor Halvorson, Department of Biology, University of Central Arkansas

Chair of Oral Examination

• Dr. Ian Putnam, Department of Mathematics and Statistics, UVic

Abstract

Nutrient-rich, large, long-lived, and mobile, vertebrates may uniquely impact nutrient recycling and thus ecosystem function by storing copious nutrients within their bodies, by producing large quantities of waste, and by transporting nutrients long distances within and across ecosystems. Currently, vertebrates face myriad threats, leaving many ecosystems to face a future with reduced vertebrate biomass and diversity; understanding how vertebrates affect nutrient cycles thus provides valuable insight into how vertebrate extinctions will impact ecosystems. Broadly, I characterize variation in how vertebrates store phosphorus (P) within their bodies and recycle P from their diets in their excreted and egested wastes. To accomplish this, I link bone investment, bodily elemental content, and waste production. Bone, a P-rich tissue (mean of ~12%) unique to vertebrates, can alter vertebrate-driven nutrient recycling both by increasing P storage within vertebrate bodies and by changing P demand and thus P release in waste. Given this, my dissertation has three goals: (1) quantify how vertebrates vary in bone content and whole-body %P, (2) determine how this variation affects dietary intake and waste production, and (3) evaluate how bone may affect P demand differently from other P-rich bodily components. To fully characterize this, I used ecological stoichiometry (ES), a framework that uses mass-balance to conceptualize organismal and ecological processes, including nutrient uptake and processing by animals. I therefore begin my dissertation by developing methods to measure all factors effectively and simultaneously, which is rarely achieved in current literature. Using these methods, I then performed a field study to examine how intraspecific bone variation in the threespine stickleback (an evolutionary model fish species with well-characterized bone variation) affected whole-body %P, dietary %P, and P release in excreta and egesta. ES predicts that bone increases whole-body %P and should therefore increase P demand and decrease P release. Counter to these predictions, I found that although stickleback with more bone indeed had higher whole-body %P, bone did not clearly impact P demand. Stickleback with more bone showed similar diets, nutrient absorption, and egestion to those with less bone. Additionally, stickleback with more bone excreted more P, perhaps implying that bone mitigates P demand rather than causing it. More broadly, I found considerable variation in bone, whole-body %P, and waste production within and across sites, most of which was unexplained by size, sex, or environmental characteristics. Bone also causes interspecific and ontogenetic differences. In a systematic review of vertebrate elemental content, I found whole-body %P ranging from 0.39 – 6.91% P, suggesting strong bone variation across taxa (interspecific) and life stages (intraspecific). Given my previous findings, this variation may impact P demand and release in unexpected ways. Thus, I finish by integrating osteological and stoichiometric literature to examine how bone’s unique physiology affects its relationship with P demand and release. While ES implicitly assumes all tissues impact demand equally, bone’s unique characteristics change how it affects elemental demand. Primarily, bone’s ability to self-destroy and provide mineral nutrients to the rest of the body allows bone to directly mitigate a vertebrate’s increased P or calcium demands. This may make vertebrates with more bone more resilient to dietary P limitation, rather than the opposite. Ideally, future research will build on these findings to improve our overall understanding of vertebrate-driven nutrient recycling.