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Trophic ecology of the Eurasian otter (Lutra lutra)

Drake, Lorna 2020. Trophic ecology of the Eurasian otter (Lutra lutra). PhD Thesis, Cardiff University.
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Abstract

Generalist apex predators have broad diets, consuming species from a range of trophic levels and connecting multiple energetic pathways. Investigating the diet of generalist apex predators thus provides a unique opportunity to describe trophic interactions over space and time, and can be used to gain an insight into ecosystem level changes. Broad spatio-temporal and demographic variation in the diet of the Eurasian otter (Lutra lutra), an apex predator of freshwater ecosystems, were investigated using a biobank of tissue samples and data collected from dead otters in England and Wales over 23 years. Molecular methods were utilised to gain a greater insight into otter diet, with stable isotope analysis revealing broad-scale variation in nutrient assimilation, and DNA metabarcoding provided a more precise description of the species consumed by otters. Isotopic signatures suggested nutrient assimilation by otters was driven by changes in basal nutrient levels and prey availability. Variation in nitrogen isotopes may reflect landscape scale differences in anthropogenic inputs from fertilisers, which were suggested to enrich basal nitrogen signatures within the environment and thus enrich signals throughout the trophic network, with some additional changes driven by differences in the consumption of high trophic level prey by otters. Variation in carbon isotopes of otters reflected changes in the availability of marine-derived nutrients, with otters sampling such nutrients primarily via consumption of marine prey at the coast and anadromous fish inland. Analysis of DNA metabarcoding data revealed otters consumed a broad range of prey from a variety of habitats. Stocked fish, invasive species and protected species occurred in the diet of otters infrequently, with the exception of the critically endangered European eel (Anguilla anguilla), which was one of the most frequently consumed prey items. Dietary variation primarily reflected seasonal changes in prey availability and differences in prey distributions across the landscape pertaining to longitude and coastal proximity. These findings displayed the opportunistic foraging behaviour of otters and their ability to switch predation to more abundant prey. Prior to ecological analyses of DNA metabarcoding data, a range of minimum sequence copy thresholds were tested to remove erroneous data and provide a more accurate description of the diet of otters. Of these thresholds, combining the removal of 2 a percentage of reads per sample with removal of the maximum read count in a blank control per taxon performed the best. Overall the findings of this study demonstrate the high levels of dietary plasticity exhibited by otters. Such adaptability in foraging is suggested to have aided population recovery and distribution expansion by British otter populations and provide resilience to future environmental stressors. The study also highlights how an understanding of the dietary variation exhibited by generalist apex predators can be utilised to investigate trophic interactions and nutrient flows across freshwater ecosystems.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Biosciences
Subjects: Q Science > Q Science (General)
Date of First Compliant Deposit: 13 November 2020
Last Modified: 13 Nov 2020 17:22
URI: http://orca.cf.ac.uk/id/eprint/136365

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