Dissertations, Theses, and Capstone Projects

Date of Degree

9-2024

Document Type

Dissertation

Degree Name

Ph.D.

Program

Biology

Advisor

Richard R. Veit

Committee Members

Lisa Manne

Jarrod Santora

David Lahti

Shaibal Mitra

Subject Categories

Behavior and Ethology | Biology | Ecology and Evolutionary Biology | Marine Biology | Ornithology

Keywords

Antarctic, California Current, Interspecific associations, Local enhancement, Seabirds, South Georgia

Abstract

Marine predators today face multifaceted stressors from changing climate to changing prey availability, and these changes are occurring at unprecedented speeds. Any alteration to a species’ environment has the potential to disrupt social networks and thus, how animals obtain and transfer information. Interspecific social interactions in the form of local enhancement and facilitation are a critical, but not yet well-understood, component of seabird foraging; of immediate concern is that anthropogenic changes may be altering the quantity and quality of positive species interactions, with consequences for the species involved.

While we know these interactions are not immutable, less is known about the extent to which these interactions change over time and what drives these changes. This thesis documents at-sea interspecific interactions in two systems that have undergone (and are currently undergoing) great ecological change: South Georgia in the sub-Antarctic and the California Current. I analyze data from these regions over both short (year-to-year) and long (multidecadal) timeframes.

I used Recurrent Group Analysis to identify pairs of species that were positively associated at sea; for every possible species pairing, Recurrent Group Analysis considers the distribution of Species A and the distribution of Species B and evaluates if the number of joint occurrences is greater than would be expected. I propose a new framework for how such associations could impact the fitness of the species in these regions: a density dependent feedback loop where if a population drops below some abundance threshold, feeding success, and ultimately fitness, is reduced due to loss of opportunities for positive interactions.

Chapter 1 is a literature review about how seabirds use, and interact with, their physical and social environments while foraging. This is discussed in the context of their sensory and cognitive capabilities. To my knowledge, this is the first review dedicated to the foraging cognition repertoire of seabirds.

In Chapter 2 I investigate species associations at South Georgia between 1985 and 1993. Species diversity differed between years, concurrent with a shift in the proportion of species found to be associated at sea. To my knowledge, the information presented in this chapter is the first peer-reviewed report of pelagic winter seabird and marine mammal species at South Georgia, thus providing a critical reference point for the conservation of sub-Antarctic marine predators moving forward.

Chapter 2 ends with a revised species list and updated sea-surface temperature trends from a more recent survey conducted 30 years later. Five species were sighted in 2023 that were not sighted on prior comparable surveys (all marine mammals) and seven species were not sighted in 2023 that had been seen previously (all birds). The average sea-surface temperature that we recorded in July of 2023 was 0.7°C warmer than the average of the July temperatures reported between 1985 and 1993.

With only four years of data spread across 37 years, the extent to which the South Georgia dataset can identify the environmental factors influencing the occurrence of at-sea associations is somewhat limited. In Chapter 3, however, I analyze 20 years of data from the California Current, allowing me to investigate the extent to which interspecific associations vary interannually and under what conditions. Concurrent data on seabird fledgling success from Southeast Farallon Island off central California allowed me to test if a species’ propensity to form interspecific associations at sea has fitness consequences at the colony level. While recent studies have linked the use of local enhancement and facilitation to short-term benefits (e.g., fitness proxies such as decreased search time and increased prey intake) no study, to my knowledge, has tested for a relationship between at-sea foraging interactions and any direct fitness benefits.

I found high concordance amongst the tendency of seabirds and marine mammals to associate with one another across all spatial scales studied. More species were found to associate as the scale increased, however. After aggregating the data onto a 0.5°x0.5° grid, ten species were found to form long-term (cross-year) associations and an additional 12 species formed at least one short-term association (i.e., during specific years, but not across years). Most of these pairings were low fidelity, however, with the majority occurring in only one out of 20 survey years. None of the environmental variables tested were found to be significant predictors of the percentage of species pairings that occurred in a given year except for Young of Year Anchovy.

Perhaps surprisingly, none of the species tested exhibited significantly greater fledgling success in years in which they formed associations compared to years in which they did not. However, fledgling success was sometimes significantly affected by their associations with individual species.

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