The Island Syndrome in Rodent Populations

Abstract

Populations of rodents isolated on islands often show systematic differences in demography, reproduction, behavior, and morphology when compared to mainland populations. These differences, termed the island syndrome, include higher and more stable densities, better survival, increased body mass, and reduced aggressiveness, reproductive output, and dispersal. We synthesize information in the literature on island rodent populations and construct a conceptual model to explain the island syndrome. Population density and other manifestations of the island syndrome are predicted to increase with island isolation and to decrease with island area. The effect of isolation is direct by limiting dispersal, while the effect of area is less direct. As area increases, predators, competitors, and habitat structure increase in diversity. We suggest that the intensity or absence of density-depressing factors (primarily predation in communities comprised of only a few morphologically and trophically divergent species) is primarily responsible for the area effect and serves as a principal factor differentiating island from mainland populations of rodents. Other characteristics associated with the island syndrome may be the result of both short-term and long-term processes or responses. Short-term responses include reproductive, body size, and behavioral changes that may be phenotypic reaction norms in response to higher island densities. Thus, higher densities lead to reduced reproductive output, which then leads to greater body size. Initial behavioral differences may be owing to better survival and reduced dispersal, which result in less population turnover, greater neighbor familiarity, and less aggression. Long-term changes may be due to directional selection for reduced dispersal, increased body size in response to increased intraspecific competition, reduced reproductive output (smaller litters and delayed maturation) in response to reduced mortality schedules, and reduced aggressiveness. While the model is specific to rodent populations, the conclusions may be applicable to other systems if characteristics such as body size, vagility, and community composition are considered.

Home range and population density of black rats (Rattus rattus) on a seabird island: a case for a marine subsidised effect?

“…Ship rats on islands were heavier and larger (Yom-tov et al 1999) than on mainland New Zealand which supports the micro-evolutionary portion of the theory, probably because a large suite of competitors and predators were absent (Adler & Levins 1994;Yom-tov et al 1999;Ventura & Lopez Fuster 2000). Rapid morphological responses in rodents and other small mammals to changing ecological conditions are now well recorded (Pergams & Ashley 2001;Pergams & Lawyer 2009;Cucchi et al 2014).…”

“…As the population ecology of island rodents may differ from that of continental populations (Gliwicz 1980;Adler & Levins 1994;Polis et al 1997) understanding the responses of rats to island ecosystems will assist with their management. There is evidence for increased population densities, larger body sizes and reduced reproductive rates in rodents on islands (Key et al 1998;Innes 2005), which have been attributed to the 'island syndrome' (Adler & Levins 1994;Russell et al 2011b) as a response to reduced dispersal, competition and predation.…”

“…There is evidence for increased population densities, larger body sizes and reduced reproductive rates in rodents on islands (Key et al 1998;Innes 2005), which have been attributed to the 'island syndrome' (Adler & Levins 1994;Russell et al 2011b) as a response to reduced dispersal, competition and predation. Although this effect is recognised, marine subsidies from dense aggregations of seabirds are also known to lead to increased population densities and other changes in population parameters in various animals including rodents (Stapp & Polis 2003;Mulder et al 2011;Ruffino et al 2013), which may be a confounding factor.…”

Phylogeography of the weasel (Mustela nivalis) in the western-Palaearctic region: combined effects of glacial events and human movements

“…nivalis is one of the most common mustelids occurring in the islands of the Palaearctic region, inhabiting all Mediterranean islands larger than 240 km 2 with the exception of Ibiza and Cyprus (Masseti, 1995;De Marinis and Masseti, 2003). As is often reported for small mammals (Blondel and Vigne, 1993;Alder and Levins, 1994;Michaux et al, 2002), insular and mainland populations of M. nivalis differ morphologically. These variations include larger body size and darker coat colouration in insular populations (Beaucournu and Grulich, 1968;Alcover and Jaume, 1983) and led to the description of potentially one subspecies per island (Beaucournu and Grulich, 1968;De Marinis, 1996).…”

Annual Variation in Habitat Use by White-footed Mice, <em>Peromyscus leucopus</em>: The Effects of Forest Patch Size, Edge and Surrounding Vegetation Type

“…We predicted that fewer mice would move from the forest edge to the agricultural fields surrounding smaller than larger forest patches, but our results did not support the inhibited dispersal hypothesis. Diaz et al (1999) found no difference in movements of Apodemus sylvaticus to the exterior in relation to patch size, whereas Krohne and Hoch (1999) found that P. leucopus populations in Indiana had lower dispersal rates from smaller patches, a factor that may have resulted in higher densities in those patches (Adler and Levins 1994). Nupp and Swihart (1996) concluded that both reduced dispersal and greater abundances of food contributed to the high densities of P. leucopus they observed in small forest woodlots.…”