E-Thesis 72 views
Top-down effects of Eurasian otter (Lutra lutra) diet on the trophic functionality of freshwater systems in the UK and Ireland / JOSE GALLARDO
Swansea University Author: JOSE GALLARDO
Abstract
The Eurasian otter (Lutra lutra) has been described as both a generalist and specialist predator, but there is little literature on the trophic effects of otter diet on a freshwater system. We analysed otter dietary data from across the UK and Ireland to identify any potential patterns in the trophi...
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Swansea, Wales, UK
2023
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| Institution: | Swansea University |
| Degree level: | Master of Research |
| Degree name: | MRes |
| Supervisor: | Forman, Dan. |
| URI: | https://cronfa.swan.ac.uk/Record/cronfa65370 |
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| Abstract: |
The Eurasian otter (Lutra lutra) has been described as both a generalist and specialist predator, but there is little literature on the trophic effects of otter diet on a freshwater system. We analysed otter dietary data from across the UK and Ireland to identify any potential patterns in the trophic functionality and characteristics of predated freshwater fish species across sites, in order to deduce any possible trophic effects of top-down control. These were inferred by comparing our results with previous research on otter feeding ecology, prey characteristics and the cascading trophic effects of more studied carnivore species. We also tested whether prey trophic level patterns were consistent between sites. Five functional/morphological categories were selected for cross-comparison analysis between prey types (trophic level, resilience, mean adult length, body form, and aquatic zone). Each prey type identified was assigned a value/group within each category and the mean RFO% of each species across sites was used as a proxy occurrence value, with overlapping values added together. The results showed that freshwater fish prey followed a consistent trophic pattern across sites. Otters primarily predated on species between trophic levels 3.2-3.6, with the latter representing 30.3% of all freshwater fish predated upon. Trophic levels of prey types also remained consistent between sites indicating a trophic niche that otters repeatedly prioritised. We determined that otter diet mostly consisted of small-medium (10-25cm) fish, primarily demersal and benthopelagic species with resilience levels ranging from medium-low. We also theorised a potential range of otter prey selection based on locomotion specialisation and net energy gain. Upon review of the diet and trophic effects of prey species, we believe that otters may increase primary production at the demersal zone through top-down control of secondary consumers. We presented arguments for potential increases to biodiversity; population increases of small invertebrates; potential reduction in helminth populations; and possible control of invasive species. We also identified possible areas of disturbance risk to otter’s primary foraging grounds, which could alter the theorised trophic effects imparted by their diet. We argued that there is sufficient evidence to show that otter feeding ecology is more specialist than generalist in the UK and Ireland and discussed some inherent issues with these classifications. Lastly, we identified some key issues with the methodologies and reporting of data in some of the reviewed studies on otter diet. We argued that standardisation of methods and data transparency could prevent limitations on the speed at which we advance our understanding within this field. |
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| Keywords: |
Otter, Lutra lutra, top-down control, trophic cascades, freshwater systems, ecology |
| College: |
Faculty of Science and Engineering |