Mammal invaders on islands: impact, control and control impact

Abstract

The invasion of ecosystems by exotic species is currently viewed as one of the most important sources of biodiversity loss. The largest part of this loss occurs on islands, where indigenous species have often evolved in the absence of strong competition, herbivory, parasitism or predation. As a result, introduced species thrive in those optimal insular ecosystems affecting their plant food, competitors or animal prey. As islands are characterised by a high rate of endemism, the impacted populations often correspond to local subspecies or even unique species. One of the most important taxa concerning biological invasions on islands is mammals. A small number of mammal species is responsible for most of the damage to invaded insular ecosystems : rats, cats, goats, rabbits, pigs and a few others. The effect of alien invasive species may be simple or very complex, especially since a large array of invasive species, mammals and others, can be present simultaneously and interact among themselves as well as with the indigenous species. In most cases, introduced species generally have a strong impact and they often are responsible for the impoverishment of the local flora and fauna. The best response to these effects is almost always to control the alien population, either by regularly reducing their numbers, or better still, by eradicating the population as a whole from the island. Several types of methods are currently used : physical (trapping, shooting), chemical ( poisoning ) and biological (e.g. directed use of diseases ). Each has its own set of advantages and disadvantages, depending on the mammal species targeted. The best strategy is almost always to combine several methods. Whatever the strategy used, its long-term success is critically dependent on solid support from several different areas, including financial support, staff commitment, and public support, to name only a few. In many cases, the elimination of the alien invasive species is followed by a rapid and often spectacular recovery of the impacted local populations. However, in other cases, the removal of the alien is not sufficient for the damaged ecosystem to revert to its former state, and complementary actions, such as species re-introduction, are required. A third situation may be widespread : the sudden removal of the alien species may generate a further disequilibrium, resulting in further or greater damage to the ecosystem. Given the numerous and complex population interactions among island species, it is difficult to predict the outcome of the removal of key species, such as a top predator. This justifies careful pre-control study and preparation prior to initiating the eradication of an alien species, in order to avoid an ecological catastrophe. In addition, long-term monitoring of the post-eradication ecosystem is crucial to assess success and prevent reinvasion.

Biological invasions in the Antarctic: extent, impacts and implications

“…Some persistent or invasive mammals have been removed in conservation actions, though these actions themselves have further impacts (Micol & Jouventin, 1995 ;Myers et al, 2000;Bester et al, 2002;Courchamp et al, 2003). Further eradication programmes are under consideration (the removal of reindeer from South Georgia) or trial (rat eradication on some offshore islets of South Georgia).…”

“…Biological invasions are amongst the most significant threats to biodiversity (McKinney & Lockwood, 1999 ;Sala et al, 2000, Courchamp, Chapuis & Pascal, 2003, posing both a significant threat to individual species (Case, 1996;Williamson, 1996), and being responsible for major changes to ecosystem structure and functioning (Heywood, 1989;D'Antonio & Dudley, 1995 ;Mack et al, 2000). Their extent and significance are likely to increase with global environmental change (Dukes & Mooney, 1999 ;Hughes, 2000;Smith et al, 2000 ;McKinney, 2001 ;Prinzig et al, 2002;Walther et al, 2002).…”

Distributional patterns of endemic, native and alien species along a roadside elevation gradient in Tenerife, Canary Islands

“…In contrast, less attention has been paid in understanding such processes in more pristine and high-elevation environments, where most of the world's protected areas are located (Pauchard et al 2009). Mountain and island ecosystems are known to be particularly prone to invasions by alien species (e.g., Barni et al 2012): the reason for island ecosystems to be highly susceptible and vulnerable to the intentional and unintentional introduction of invasive species include their comparatively low Distributional patterns of endemic, native and alien species along a roadside elevation gradient in Tenerife, Canary Islands habitat diversity, their simplified trophic webs and high rates of endemism (Courchamp et al 2003). Island communities have undergone long lasting processes of mutual evolution due to their isolation from other ecosystems (Steinbauer et al 2013).…”

Rats dying for mice: Modelling the competitor release effect

“…In their race against biodiversity erosion, conservation managers are often faced with dilemmas and the resulting choices often translate into a need to give priority to fighting simple direct causes of species loss, leaving little space for interacting processes. In the context of biological invasions, a major cause of population extinctions, this often amounts to controlling invading predators to protect local animal communities (Diamond & Veitch 1981;Towns & Ballantine 1993;Courchamp et al 2003). Yet, processes that are less obvious than direct predation can operate even in very simple systems such as insular ecosystems (Glen & Dickman 2005), which makes risky the necessary removal of exotic predators.…”