Friend or foe? The controversial status of native predators in New Zealand restoration projects using weka as a case study species – Asher CookPosted: May 4, 2014
Introduced predators have had a catastrophic impact on New Zealand’s unique flora and fauna (King, 1984; O’Donnell, 1996; Holdaway, 1999). Generally, given their non-native status, the control or eradication of introduced predators is a logical procedure that results in clear conservation benefits for native biodiversity (Moorhouse et al. 2003; Whitehead et al. 2008). However, an important but often overlooked issue arises when predation pressure is exerted by native species. In this case, the distinction between predator control and conservation gain is blurred and is exacerbated when the predator is, itself, a threatened species. This paradoxical situation raises an important question – are New Zealand’s native predators friend or foe? This essay will (1) focus on weka (Gallirallus australis) as a real-life example of a native New Zealand predator, (2) present the arguments for and against their inclusion in restoration projects and (3) provide a generalised decision-making framework designed to help conservation practitioners who are considering the reintroduction of a native predator species.
New Zealand’s native predators
New Zealand has a number of native species that are at least part-time predators of native vertebrate fauna – avian examples include swamp harrier, pukeko, morepork, New Zealand Falcon and weka (van Winkel & Ji, 2012; Dey & Jamieson, 2013). They consume a wide range of prey including birds, lizards and bats, and several of these predator species have been controlled to protect rare native species (Seaton & Hyde, 2013). Both weka and New Zealand falcon are threatened with extinction (Miskelly et al. 2008).
Why can native predators pose problems in New Zealand?
Generally in natural predator-prey scenarios the prey species are robust enough to withstand predation from native predators (Salo et al. 2007). Yet, many of New Zealand’s surviving endemic species have small and fragmented populations in which even relatively low rates of predation by native species may cause population decline (especially when coupled with predation by introduced species). This means that in some scenarios native predators pose a significant threat to native biodiversity. Pukeko, for example, are controlled in pateke/brown teal breeding areas due to the threat they pose at high densities to eggs and ducklings (O’Connor et al. 2007; Dey & Jamieson, 2013).
The weka is an iconic large flightless endemic rail (figure 1). Originally found throughout mainland New Zealand, they are now extinct over large tracts of their former range due to a combination of predation (particularly by mustelids), drought susceptibility and habitat loss (Beauchamp, 1997; Miskelly & Beauchamp, 2004). Three of the four subspecies are listed as threatened according to the New Zealand Threat Classification System (Molloy et al. 2002) with the northern and Stewart Island weka listed as ‘nationally vulnerable’ and the western weka as ‘declining’ (Miskelly, 2008). Though invertebrates and fruit constitute most of their diet they also opportunistically prey on lizards, snails, eggs and juvenile ground nesting birds (Harper, 2006).
Weka have the potential for both negative and positive spin-offs for native biodiversity. Therefore they provide an important case study to compare the advantages and disadvantages of including a native predator in a restoration project.
Is it friend…?
There are a number of reasons that support the inclusion of weka within restoration projects. Conservation benefits include increased dispersal of large-fruited plants such as hinau, tawa and taraire (Beauchamp& Butler, 1999). This is especially important because most other species capable of dispersing large seed are now either extinct or absent across large areas of the country (Clout, 1989). In addition, weka also prey upon mammalian pests (namely rats and mice) so can provide a natural form of pest control within mainland sanctuaries – this is particularly significant in unfenced sites where total eradication of introduced mammals is currently infeasible (Saunders & Norton, 2001).
Most of the high-impact weka scenarios have occurred on seabird-dominated offshore islands more than 1 km from the mainland where weka do not naturally occur (Miskelly & Beauchamp, 2004). The grounds for eradication from off-shore islands (where weka often reach high densities) should not necessarily be applied to mainland sanctuaries which are generally within their natural range. In ‘natural’ mainland populations it is likely that other native vertebrate species could cope with weka predation (unless prey populations are particularly small or vulnerable) (Salo et al. 2007). Weka also hold strong cultural significance to some iwi and their availability for sustainable harvest remains an important issue (Beauchamp & Butler,1999).
Weka are prone to rapid population decline and as a result they are now more threatened than many of the species that restoration projects are designed to protect (Miskelly, 2008). North island weka populations in the Gisborne region plummeted during the mid-1980s through to the early 1990s and disappeared from a majority of their former range (Beauchamp et al. 1998). Similar sharp declines were also observed in a Northland population over a similar period when the local population declined from approximately 400 birds to just three individuals in only eight years (Beauchamp, 1997). Many of the large, stable weka populations now occur on offshore islands (Chathams, Kawau, Kapiti) where they have been introduced and act as an important insurance policy to their fragile mainland counterparts.
Their threatened status and susceptibility to sudden population decline suggests that at least limited inclusion in restoration projects is essential to their long-term survival.
Or is it foe…?
Arguments against their inclusion in restoration projects focus mainly on the negative impact they can have on other native vertebrates. Weka have a documented history of predation on a wide range of native species. These include sooty shearwater chicks on the southern Titi Islands (Harper, 2006), petrels on Macquarie Island (Brothers, 1984), lizards on islands free of introduced mammals (Hitchmough, 1998), saddleback (Lovegrove, 1992; Roberts, 1994), North Island kaka, little spotted kiwi (Miskelly & Beauchamp, 2004), snipe (Miskelly, 1987) and Fiordland-crested penguins (St. Clair & St. Clair, 1992). Furthermore, they are thought to have contributed to the extinction of the Macquarie island parakeet (Taylor, 1979). As a result, weka have been removed from at least nine island restoration projects (figure 2) and have been excluded from the reintroduction lists of many mainland projects (Miskelly & Beauchamp, 2004). However, in most examples it remains unclear whether weka predation rates are high enough to induce population decline. Further quantitative research is needed to more accurately determine their impact and would bring more scientific rigour to the ongoing debate.
Their potential for negative impacts on such a wide range of native species suggests that ubiquitous inclusion in restoration projects is neither logical nor feasible.
Posing the important questions
There are a number of important questions that conservation practitioners face when considering their stance on weka – these questions can be applied to other similar situations involving native predator species. The first clear distinction to consider is whether the predator species is already present within the boundaries of the restoration project. Logically, if it is already present then decision making will focus on whether or not to remove the predator. Conversely, if it is absent, focus will be placed on the whether to undertake a reintroduction.
Listed below (figure 3) are some important questions to contemplate when considering the reintroduction of a native predator:
In the case of question (1) if the extinction threat (refer to Miskelly, 2008) of the potential prey species are higher than the extinction threat of the predator, a reintroduction effort should not be considered given that even low rates of additive predation can exert severe population pressure on small populations (O’Donnell, 1996). In this situation other reintroduction sites with less threatened prey species should be considered.
In cases where the extinction threat of the prey species is low but concern remains over the impact of the reintroduced predator, population modelling (incorporating the expected predation rate) can give some idea of the likely scale of impact. In this case, question (2) should be considered because empirical site-specific population data is required for accurate modelling and informative predictions.
Before practitioners proceed with a reintroduction, questions (3) and (4) should also be considered. Evidence of coexistence in similar situations provides strong evidence that a reintroduction could be successful for both predator and prey. In circumstances where there are no examples of coexistence then reintroduction efforts should be considered with extreme caution. Question (4) is also an inquiry of fundamental importance given that predator impacts are usually intensified outside of their natural range (Salo et al. 2007). As mentioned earlier, most high-impact weka scenarios have occurred on offshore islands outside of their natural range (Miskelly & Beauchamp, 2004).
It is well-documented that introduced mammalian predators have had a detrimental impact on New Zealand’s biodiversity (Holdaway, 1999; Croll, 2005). Therefore, the control of predators is generally a logical procedure that leads to clear benefits for native biodiversity. However, when predation pressure is exerted by native species the distinction between predator control and conservation gain is made unclear. In this scenario there is no easy answer.
Sometimes native predators are our friends. Sometimes they are foe.
The example of weka in New Zealand emphasises the need for flexibility in our approach toward native predators in restoration projects.
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 This list of questions should be used in conjunction with those asked in regular reintroductions. See Seddon et al. 2007 (http://onlinelibrary.wiley.com/doi/10.1111/j.1523-1739.2006.00627.x/full) and Armstrong et al. 2008 (http://ac.els-cdn.com/S0169534707003345/1-s2.0-S0169534707003345-main.pdf?_tid=7ee08b5c-bc6c-11e3-a7b0-00000aab0f27&acdnat=1396666172_79fb8b9cdd14260d86ca36bb2f595a7f) for more background information.