Author Name: George Angell; BSc (Hons) Animal Science
Invasive species can cause detrimental effects to both island and mainland ecosystems with many introduced species being invasive predators. Invasive predators have been shown to prey on adults, juveniles, neonates and eggs of native species and can lower the effective population size. Eradication attempts have been made on some species of invasive mustelids with the majority of research coming from New Zealand and Britain looking at stoats/ferrets and mink respectively. While research has shown that invasive mustelids do harm the native environment, in many cases, this research cannot be extrapolated due to the specific environmental components such as both native and other introduced species being present. In some research it has been shown that eradication and control projects can exacerbate the negative effects to native species due to the complex interactions between predators and prey. More research is needed to identify the optimal method to eradicate or control invasive mustelids in environments where their removal could facilitate further negative effects on native species through the rise of other introduced predators. This could be through researching humane, effective multi-species eradication projects as the removal of the secondary invasive predator could alter the invasive mustelid diet and further impact on native species.
All biota have a natural range and if due to anthropogenic activities they should occur elsewhere, they can be defined as alien (Global Invasive Species Database, 2014). An alien species is invasive should its introduction cause, or be likely to cause economic or environmental harm, or harm to human health (IUCN, 2011; Strubbe, Assaf and Francois, 2011; The National Invasive Species Council, 2006). However, not all alien species are invasive as some can have positive effects (Goodenough, 2010; Cassey et al., 2005; Mooney and Cleland, 2001) but many have adverse effects to native flora and fauna (Witczuk, Pagacz and Mills, 2013; Simberloff, 2005; Lim et al., 2003).
It is widely accepted that invasive species cause substantial damage to native environments (Canning, 2011; Aravind et al., 2010; Mwebaze et al., 2010; Mehta et al., 2007; Holzapfel et al, 2006; Gurevitch and Padilla, 2004) with some arguing that invasive species are the leading cause of biodiversity loss, more so than habitat destruction (Didham et al., 2005; Pimentel, Zuniga and Morrison, 2005; Crooks and Soule, 2001). This is due to direct impacts such as predation, competition, interference and hybridisation (Warren et al., 2011; Canning, 2011; Moore, Roy and Helyar, 2010; Collins, Latta and Roemer, 2009; Strubbe and Matthysen, 2009; Prys-Jones, Prys-Jones and Lawley, 1981), and indirect impacts such as the spread of novel diseases and the spread of invasive vegetation through seed preference (LaPoint, Atkinson and Samuel, 2012; Olias et al., 2011; Samuel et al., 2011; Linnebjerg et al., 2010; Kueffer et al., 2009).
Several mustelid species have been introduced across the globe into countries outside of their natural range for a variety of reasons. The American mink (Mustela vison) was released from fur farms in Europe (Moore, Roy and Heylar, 2010; Bonesi and Palazon, 2007), weasels (Mustela nivalis), stoats (Mustela erminea) and ferrets (Mustela putorius furo) were released on the New Zealand mainland to control rabbit (Lepus curpaeums) populations (Edge et al., 2011), and ferrets became feral in Ireland from escaped pets/working animals (Bodey, Bearhop, and McDonald, 2011). Elliot et al. (2010) state that there have been few deliberate or accidental releases of stoats onto islands surrounding New Zealand, however many are populated because stoats are capable, long-distance swimmers (Veale, Clout and Gleeson, 2012; McMurtrie, et al., 2011). Mustelids are obligate carnivores with a highly generalised diet and as such pose a large threat to native species (Malecha and Antczak, 2013; Bonsesi and Palazon, 2007; King and Powell, 2007) as generalist behaviour could be a predetermining factor as to why some predators are well adapted to becoming invasive (Shah and Shaanker, 2014; Prentis et al., 2008; Duncan, Blackburn and Sol, 2003; Sol, Timmermans and Lefebvre, 2002).
It has been hypothesised by Errington (1946) that predators consume the excess production of a prey population and thus predation is compensatory as opposed to additive, as the ‘doomed surplus’ are consumed. Although this hypothesis was developed 68 years ago, it has been cited 433 times in recent studies and proved true in the study conducted by Banks (1999). Despite this, the research did not take into account any sub lethal effects the invasive red fox (Vulpes vulpes) may have had on native bush rat (Rattus fuscipes) populations. Some studies have shown invasive predators can have low predation rates and have greater sub lethal effects (Nogales et al., 2013; Creel and Christianson, 2008; Pangle, Peacor, Johannsson, 2007). Beckerman, Boots and Gaston (2007) investigated the effects of fear of cats (Felis catus) in urban bird populations in the UK and showed that fecundity can be reduced, substantially limiting population numbers. This could mean low-predation rates could be reflective of low population numbers. This can exacerbate the effects of predation (Nogales et al., 2013; Massaro, Stanbury and Briskie, 2012; Salafsky et al., 2008).
2.0 Literature Review
2.1 Eradication of invasive Mustelids
There are arguments that invasive predators should be controlled or eradicated (Buxton et al., 2014; Young et al., 2013; Reardon et al., 2012; Elliott et al., 2010; Baxter et al., 2008; Salo et al., 2007; Moore, Roy and Helyar, 2003). Due to the generalist diet of invasive mustelids (Malecha and Antczak, 2013; Melero et al., 2012; Chapple, Simmonds and Wong, 2012), many native species are at risk, be it from becoming the mainstay of the diet (Nogales et al, 2013; Valenzuela et al., 2013) or from low level predation rates keeping the population levels at risk to stochastic effects (Bodey et al., 2011; Salo et al., 2007).
Bodey et al. (2011) found there was no significant immediate effect of feral ferret removal on lapwing (Vanellus vanellus) populations on Rathlin Island, UK. This could be because it was a two year study looking at one native species on one island. Should other native species have been monitored over numerous islands on a longer term study, a more significant effect of predator removal may have been found (Bodey et al., 2011). A study carried out by Bodey, Bearhop and McDonald (2011) has shown invasive feral ferret numbers increased by 50% following control efforts, thus potentially exacerbating the negative effects to native species. Although both papers are written by the same authors, very little other research looks at the ferret population on Rathlin Island.
Salo et al. (2008) show that invasive American mink population and home ranges in South-West Finland, can be limited by the re-introduction of the native apex predator the white-tailed sea eagle (Haliaeetus albicilla). Although the sample size of 18 mink was small, this study highlights a key point that can be transferred to the eradication of mustelids. That is, in certain ecosystems, the effects of a predator can be beneficial. Tompkins and Veltman (2006) simulated the control of stoats in New Zealand and found that invasive rat (Rattus spp.) and mice (Mus spp.) numbers would increase, possibly having a greater negative effect on native species. This is known as mesopredator release (Prugh et al., 2009) and is supported by the findings of Rayner et al. (2007) who showed that in high altitudes, the removal of cats from Little Barrier Island, New Zealand, lowered the breeding success of Cooks petrels (Pterodroma cookii) as pacific rat (Rattus exulans) populations increased. On the other hand, a simulation by Fukasawa (2013) found that after the removal of the Indian mongoose (Herpestes auropunctatus) from Amami Island, the two endemic rat (Tokudaia osimensis, Diplothrix legata) populations benefitted more than the invasive black rat (Rattus rattus). Ruscoe et al. (2011) found that mesopredator population levels were limited by competition rather than predation. As many studies use simulations to present the theory of mesopredator release, it can be argued that the real extent of mesopredator release is unknown, but with research such as Rayner et al. (2007) and further advances in eradication techniques on island ecosystems (Howald et al., 2007) further research on impacts of predator eradication can be carried out and due to the complex interactions between predators and prey, multi species eradications are called for (Hervias et al., 2013; Orchan et al., 2013; Young et al., 2013; Innes and Saunders, 2011).
2.2 Common invasive Mustelids
A report by Clout and Russel (2004) lists successful eradications of stoats on islands around New Zealand. Keitt et al. (2011) suggest a failure rate of 13% for the eradication of mustelids, but state that many attempts are listed as unknown so this may be inaccurate. McMurtie et al. (2011) is one such study that failed to eradicate stoats in the initial time frame. However as the methodology is being extrapolated and developed from those used on smaller islands (Elliot et al., 2010), and the rate of re-invasion is much higher than expected, the project is still ongoing.
King et al. (2009) hypothesise that invasive mustelids are difficult to eradicate because they show intelligence through trap avoidance and traps are moved before they have a chance to find them due to their large home ranges (Gillies, Graham and Clout, 2007; King and Murphy, 2005). Elliot et al. (2010) claimed to have eradicated stoats from Anchor and Chalky island within 4 months with low trapping densities. These densities were as low as one trapping tunnel per 7 hectares with one kill trap every 28 hectares. Gilles, Graham and Clout (2007) discuss the Department of Conservation in New Zealand’s guidelines for trapping invasive stoats, and find that in concurrence with their home range study, one trap every 20 hectares (1000x200m) will achieve 80% chance of being encountered. Although Elliot et al. (2010) did use relatively low trapping densities, the home ranges of stoats on islands could be larger than those proposed by Gilles, Graham and Clout (2007) because of varying habitats and lower prey abundance.
2.2.2 American Mink
As of 2007 when Bonesi and Palazon looked at the status, impacts and control of the invasive American mink in Europe, it had invaded 28 countries. They found much of the research carried out on impacts of American mink have been carried out in Britain and for the majority of countries, there are no published studies of the national distribution of mink. This is supported by Bryce et al. (2011) who stated that there are few international articles dealing with eradication techniques and cost-effectiveness of control efforts. Despite this, some research is now emerging looking at the impacts across other countries within Europe (Iordan et al., 2012; Melero et al., 2012; Fey et al., 2009; Salo et al., 2008) and South America (Pescador, Diaz and Peris, 2012; Valenzuela et al., 2012; Fasola et al., 2009).
Heylar (2005) found that American mink had smaller territories and higher densities on coastlines than inland. This is supported by Bodey et al. (2010) who looked at the behavioural responses of mink to eradication efforts. They found by looking at whisker composition, mink rely more on marine resources and move towards the coast as the eradication programme progresses and also suggested that the prey species available can alter the sex ratio of minks caught. They found that female mink preyed more upon voles (Arvicola amphibius) when they were present, with male mink utilising more marine resources. This could be important for future eradication projects by allowing specific targeting of females to lower the effective population size (Kimura and Crow, 1963). Yet these findings are limited by small sample size and although animal samples were selected at random, only 10% of the total animals captured were utilised. As more samples were available, a greater sample size should have been chosen, a point future research may wish to improve. As Bodey et al. (2010) are the same authors publishing research about feral ferrets on Rathlin Island, it suggests that research opportunities in this field are limited and made available to a select few. While research from these experts could be seen as positive, potential limitations of their methodologies could be addressed.
MacDonald et al. (2002) found that water voles were more abundant in areas without American mink in Belarus and that European mink (Mustela lutreola) scats contained significantly less voles than American mink scats. Fey et al. (2009) support this by showing that invasive mink can cause trophic cascades on small islands in the Baltic Sea, however their findings are limited as they could not observe the same number of islands in their control group. Melero et al. (2012) contradict Fey et al. (2009) stating that the invasive mink may be able to live in co-existence with native species, dependent on the native species present. They suggest that a predator with a specific diet and one with a highly generalised diet could co-exist provided spatial divergence can occur, supported by Sidorovich et al. (2010) and Bonesi, Chanin and Macdonald (2004). Melero et al. (2012) found that the mink only had a significant negative effect on three native fish species. This study was carried out in Catalonia, Northeast Spain containing a population of the invasive American crayfish (Pacifastacus leniusculus) which constituted around 85% of the mink’s diet. Due to the mink’s generalist diet (Banks et al., 2008; Valenzuela et al., 2013) a reduction or removal of crayfish could put more predation pressure on native species (Melero et al., 2012). If this study was carried out in an area without crayfish, significant negative impacts on native species may have been found as Bonesi, Chanin and Macdonald (2004) found the diet of the American mink consisted of mainly terrestrial mammals. Yet this study used a small sample size from only one location within the UK and this was due to an author’s previous experience in the area. Their sampling efforts also differed between study times which limits the validity of their results but a strength is that sampling times were carried out in the winter when competition is believed to be greater (Churchfield, Rychlik and Taylor, 2012).
Invasive species can negatively affect native ecosystems with invasive predators sometimes causing substantial damage. Invasive mustelids can inflict damage on native prey populations by reducing their effective population size. Eradication attempts have been made on invasive mustelids with the majority of the research arising from Britain and New Zealand, however more research is emerging from other countries in Europe and those with invasive populations in South America. Both stoats and mink have been shown to negatively affect native species, more so in areas where there are limited numbers of invasive prey species such as rats, mice or crayfish. Due to the complex interactions between predator and prey species, some eradication or control attempts can exacerbate the negative effects of invasive mustelids and in some cases multi-species control is desired.
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