How is establishment probability affected by the size and composition of the release group?

In document A tactics-based approach for improving the outcomes of eastern bettong (Bettongia gaimardi) reintroductions (Page 80-83)

One of the factors commonly associated with reintroduction failure is the small size of founder groups, because small populations are vulnerable to extinction (Pimm 1991). Therefore, increasing the size of a release group (release cohort or founder group) represents an intuitive tactic for improving the probability of success (Box 2.1). The effectiveness of this tactic is suggested by the positive relationship between the number of individuals released and reintroduction success (Griffith et al. 1989; Fischer and Lindenmayer 2000). However, the number of individuals available is often restricted by a range of factors, including the need to minimise the detrimental effects to a source population (Dimond and Armstrong 2007), and the substantial cost and logistical difficulty associated with acquiring many individuals from small population (Van Houtan et al. 2009).

Given that the size of a founder group is finite, establishment probabilities can be improved through careful design of the release group (Armstrong and Seddon 2008). When designing the optimal composition of a release group(s), the traits considered often encompass genetic,

demographic and social characteristics (IUCN 2013). Although we acknowledge the importance of genetics in reintroductions, we do not include them here because they have been reviewed

59 Generally, there are two opposing tactics adopted when designing the composition of release groups: the first approach is to mimic the composition of a reference population; the alternative approach is to establish unnatural biases (IUCN 2013).

Translocating only a sub-set of a source population or mixing previously unfamiliar individuals often causes social disorganisation that acts as a stressor and influences post-release performances (Letty et al. 2007). One tactic that can be adopted to minimise the detrimental effects of social disruption is to reintroduce groups of familiar individuals. This could have a range of potential benefits including reducing post-release aggression, encouraging mating or facilitating anti-

predator behaviour. Releasing established social groups has been shown to improve the probability of establishment in black-tailed prairie dogs (Cynomys ludovicianus) in the USA (Shier 2006). However, experiments undertaken with New Zealand forest birds and Australian tammar wallabies (Macropus eugenii) have not indicated any beneficial effects of familiarity, either because

relationships were not maintained post-release, or because the hypothesised effects of familiarity did not occur (Armstrong 1995; Armstrong and Craig 1995; Armstrong et al. 1995b). Although these experiments did not show any benefits of familiarity, releasing intact colonies of black-eared miners (Manorina melanoti) appeared to facilitate post-release social cohesion in this socially complex species (Clarke et al. 2002), leading to a successful reintroduction (colonies still present in 2013; R.L. Boulton pers. comm.). A similar tactic appeared to facilitate post-release settlement of brown treecreepers (Climacteris picumnus) (Bennett et al. 2012), although this reintroduction was not successful in the longer term.

An alternative method to gain similar benefits is to house previously unacquainted individuals together before release to allow for relationships to be established. This approach did not influence post-release survival of translocated hihi (Notiomystis cincta) on Kapiti Island, New Zealand (Castro et al. 1995), but has proved beneficial for other species including African wild dogs

60 (Lycaon pictus) (Gusset et al. 2006). A variation on the theme of familiarity is ensuring the release of animals with similar vocal dialects to avoid reproductive discrimination. This has been

identified as a potential issue in translocations of North Island kokako (Callaeas wilsoni) in New Zealand (Rowe and Bell 2007) and noisy scrub birds (Atrichornis clamosus) in Australia (Kemp et al. 2015).

The demographic structure of a release group in regard to age, sex and reproductive status can influence reintroduction outcomes (Letty et al. 2007; IUCN 2013). The optimal sex ratio for a release group will often be dictated by the mating system of the species. For example, for the polygynous bridled nailtail wallaby (Onychogalea fraenata), creating a female bias can increase the potential population growth rates without affecting genetic viability (Sigg et al. 2005). Conversely, an equal sex ratio will usually be appropriate for monogamous species such as the New Zealand robin because population growth is limited by the availability of both males and females (Jamieson 2011). Sex-biased dispersal can also shape the optimal composition of a founder group, as observed during a translocation of bridled nailtail wallabies where the release of male-only groups increased dispersal due to mate-finding behaviour (Hayward et al. 2012). The age structure of release groups also needs to be considered for species with age-dependent

behaviours. This has been observed in South Island saddleback (Philesturnus carunculatus) where settlement and survivorship is greater in birds released as sub-adults compared to adults due to differences in territorial statuses when released (Masuda and Jamieson 2012). An alternative approach is to preferentially select adults with dependent young in order to reduce dispersal capabilities, as observed in translocated black stilts (Himantopus novaezelandiae) (van Heezik et al. 2009).

When planning a reintroduction project, the design of the founder group is paramount because its size and composition can influence reintroduction outcomes (IUCN 2013). Although a founder

61 group will ideally be designed to maximise the probability of success, other factors must also be taken into consideration, including the effect harvesting will have on the source population (Richardson et al., 2015). Currently, the conservation community has a good theoretical

understanding of how the structure of a founder group can influence future population dynamics. However, more empirical studies are needed to distinguish between perceived and real effects (Kemp et al. 2015). Through the accumulation of empirical evidence, it may also be possible to improve population models which could be used to guide the design of future reintroductions (Chauvenet et al., 2015).

In document A tactics-based approach for improving the outcomes of eastern bettong (Bettongia gaimardi) reintroductions (Page 80-83)