Stable isotopes reveal evidence of predation by ship rats on seabirds on the Shiant Islands, Scotland

Abstract

Summary1. Introduced predators are a major threat to native island populations, yet direct evidence of predation is often lacking, especially when it is difficult to detect by traditional dietary methods. 2. Historical declines of nesting seabirds on the Shiant Islands, Outer Hebrides, roughly coincided with the accidental introduction of ship rats Rattus rattus in c . 1900. Rats have been implicated in declines of seabirds, but the Shiant population is one of two remaining naturalized R. rattus populations in Britain, prompting calls for their protection. 3. Live-trapping studies with stable isotopes and gut content analysis were used to investigate whether ship rats prey on Shiant Islands seabirds. Another aim of this study was to determine whether marine-derived foods subsidize rat populations, permitting higher densities, greater productivity and larger body size than expected from terrestrial resources alone. 4. Comparisons of stable carbon and nitrogen isotopic signatures of rat tissues with those of seabirds, marine invertebrates, marine algae and land-based foods revealed that seabirds and other marine prey were the primary source of protein for rats living in colonies or near the shore. These results were corroborated by gut content analysis, and suggest a greater role for active predation of seabirds by rats than has previously been apparent at this locality. 5. Seabird colonies and especially coastal areas supported higher numbers of rats than more inland habitats. Coastal and colony-dwelling rats were more active reproductively and were larger than those living inland. 6. Although rats are capable of surviving solely on terrestrial foods, their ability to use marine prey may buffer populations during lean times, i.e. outside the seabird nesting season, and may in part explain their success and status as pests on islands world-wide. Overall, this work reveals the value of stable isotopes in identifying predation by exotic species, but also underscores potential uncertainties inherent in all diet-based methods in distinguishing predation from scavenging.

Dietary niche differentiation among three species of invasive rodents (Rattus rattus, R. exulans, Mus musculus)

“…The difference in d 15 N among rodent species averaged \1.5 %, which does not typically justify assigning distinct trophic levels to different species (Peterson and Fry 1987;Lajtha and Michener 1994;Post 2002). Many isotope studies have examined the degree to which introduced rodents ate seabirds and the proportion of diet attributable to marine and terrestrial sources (Stapp 2002;Caut et al 2008a;Quillfeldt et al 2008;Ruffino et al 2011). Marine inputs to rodent diets are unlikely at Kahanahaiki because the site is [3 km from the ocean, there are no seabirds, and home-ranges are typically \4 ha for each rodent (Shiels 2010).…”

“…The only isotopic study available with wild house mice was by Quillfeldt et al (2008) in the Falkland Islands where d 13 C were similar to those in our study (-21 and -24 %); yet the d 15 N for mice ranged from 12 to 31 % and were indistinguishable from potential food items, including terrestrial plants (8-35 %), terrestrial invertebrates (14 %), and upland birds (16-19 %). Therefore, using isotopes to estimate trophic levels and determine the types of species which consumers feed upon may be challenging without supplemental dietary analysis such as stomach contents (Stapp 2002;Caut et al 2008a;Quillfeldt et al 2008; this study).…”

“…Identifying species consumed by introduced animals provides insight into native species' vulnerability and can inform strategies for managing native and non-native species (Stapp 2002;Caut et al 2008a;Bonnaud et al 2011;St Clair 2011). Rodents (Rattus rattus, black or ship rat; R. norvegicus, Norway rat; R. exulans, Pacific rat; Mus musculus, house mouse) have been introduced to many ecosystems and are among the most widespread and problematic invasive animals affecting islands Angel et al 2009;Drake and Hunt 2009).…”

“…Rodents (Rattus rattus, black or ship rat; R. norvegicus, Norway rat; R. exulans, Pacific rat; Mus musculus, house mouse) have been introduced to many ecosystems and are among the most widespread and problematic invasive animals affecting islands Angel et al 2009;Drake and Hunt 2009). Introduced rodents may consume a wide variety of food items, including plants (e.g., fruits, seeds, vegetative material) and animals (e.g., arthropods, mollusks, birds ;Sugihara 1997;Stapp 2002;Drake et al 2011), and their diets can shift depending upon a number of factors, including food availability, the chemical and nutritional quality of food items, and the rodents' competitive ability relative to other animals that coexist in the environment (Clark 1981(Clark , 1982Caut et al 2008a;Ruffino et al 2011).…”

Maritime mammals: terrestrial mammals as consumers in marine intertidal communities

“…In the arid deserts adjacent to the Gulf of California, coyotes Canis latrans feed on a variety of marine organisms, including sea turtles, marine mammals, fish, birds, invertebrates, and algae (Rose & Polis 1998 (Fay & Stephenson 1989). On islands off Baja California, the deer mouse Peromyscus maniculatus is a granivore in wet years, but feeds extensively on intertidal organisms in dry years, as revealed by stable isotope analysis (Stapp et al 1999, Stapp 2002. On a British Columbia island, Keen's mice Peromyscus keeni appear to expand their diet to marine invertebrates before seabird eggs become available, which is work also based on stable isotope analysis (Drever et al 2000).…”