Invasion dynamics of an introduced squirrel in Argentina

Abstract

Biological invasions are one of the major threats to both ecosystem and economic functioning. Their management typically involves culling of the pest or removal of its habitat. The Asiatic red-bellied beautiful squirrel Callosciurus erythraeus is the first known introduction of a squirrel into South America. It established from five releases in 1973, using exotic trees to spread through Argentinean Pampas. It now causes substantial economic damage in agricultural and urban areas across 680 km 2 , and its continued spread threatens indigenous species. We developed a spatially explicit model of the invasion for the likely range of life-history parameters, matched against empirical data on patch occupancy in 2004. The two best-fitting models suggest the current population to be on the cusp of an explosive expansion. These models were used to predict future trends under alternative scenarios of strategic culling or habitat removal aimed at slowing the spread. The predictions for 18 yr into the future were that 1) the present lack of systematic management will lead to a 5fold increase in area of occupancy, 2) removal of habitat down to half carrying capacity will thin the population but accelerate its spread, 3) 10 yr of culling above the maximum sustainable yield (MSY) will precipitate declines in abundance and patch occupancy towards extinction, but with immediate recovery upon cessation of the cull. We recommend continuous culling above the MSY in priority patches, aimed at slowing arrival to valuable conservation areas. This study demonstrates the need for prompt action to terminate invasions before they establish. The squirrel invasion is now irreversible after 30 yr of slow spread across fragmented habitat. Although culling requires public awareness campaigns and sustained governmental commitment, it is now the best feasible strategy for managing this invasion.

From a lag in vector activity to a constant increase of translocations: invasion of Callosciurus squirrels in Argentina

“…Apart from a lag in population growth or range expansion of each new invasion focus (Guichón and Doncaster 2008), we now describe a lag-phase in the foundation of new populations that could be explained by increased vector activity over time through translocations (Crooks 2005). Similarly, the introduction events listed by Bertolino and Lurz (2013) for redbellied squirrels in Japan also indicate the occurrence of 17 new introductions after a lag period of approximately 20 years, though we cannot discriminate between multiple introductions and translocations within the country.…”

“…The high relative density of squirrels at Arrecifes release site, which was similar to densities reached in the main invasion focus in Luján , could be explained by the large number of founding individuals, the time elapsed since release, and the highly fragmented habitat surrounding the ranch (M Borgnia and M Hertzriken, personal observation). Habitat fragmentation may not only affect squirrel abundance in a forested patch (Koprowski 2005) but may influence expansion rate (With 2002;Bridgman et al 2012), which could also be modified by habitat carrying capacity (Guichón and Doncaster 2008). Processes involved in the successful squirrel establishment in each invasion focus still need more studies though at present the propagule pressure hypothesis that has broad consensus in invasion ecology (Lockwood et al 2005;Jeschke 2014) does not seem to play an important role while the enemy release hypothesis (Heger and Jeschke 2014) cannot be discarded .…”

“…3), but opposition to control actions is stronger in tourist and urban areas such as La Cumbrecita and Escobar (Fig. 3), even if the latter site has high conservation priority (Guichón and Doncaster 2008;Benitez et al 2013;Borgnia et al 2013). Prevention of new releases, which are illegal, should be a first national action that must prioritise areas of high conservation value and where native squirrels Sciurus aestuans and S. ignitus are present (Cassini and Guichón 2009;Borgnia et al 2013).…”

“…The Pampas region has been largely modified by agricultural activities and urban-industrial development. Suitable arboreal habitat for red-bellied squirrels is highly fragmented and mainly composed of exotic tree species in small patches, wind curtains and tree lines along roads and railways, and in residential areas and urban parks and streets (Guichón and Doncaster 2008). Arboreal vegetation of the study sites was mainly composed of introduced species such as Cupressus spp., Eucaliptus spp., Ligustrum spp., Morus spp., Pinus spp., Populus spp., and Quercus spp.…”

Genetic characterization ofCallosciurus(Rodentia: Sciuridae) Asiatic squirrels introduced in Argentina

“…In 1970, 10 squirrels were introduced close to Luján city, in the province of Buenos Aires (Argentina), and founded the first population of exotic squirrels in South America (Guichón & Doncaster 2008). These arboreal squirrels were morphologically identified as C. erythraeus thai (Aprile & Chicco 1999); however, the subspecies could also correspond to C. erythraeus atrodorsalis (Cassini & Guichón 2009).…”