Founder group sizes on the Tiritiri Matangi (19 individuals) and Motuora (20 individuals) were low, as a result of the logistical difficulties of translocating larger numbers of animals. There has been much debate regarding the viability of translocated populations as a function of founder propogule size (Griffith et al., 1989; Craig & Veitch, 1990; Simberloff, 1990; Craig, 1991; Dodd & Seigel, 1991; Armstrong & McLean, 1995). Small founder numbers may have negative implications on the viability of the translocated populations. Griffith et. al. (1989) showed that translocation success increased with increasing founder group size in birds, with an asymptote at approximately 40 animals. However, numerous examples have suggested that the relationship between founder group size and probability of success does not always hold true (Armstrong & McLean, 1995; Thomas & Whitaker, 1995; Towns &
(Petroica spp.) have been established from groups of about five individuals (Flack, 1977). These are R-selected strategists, those with relatively short life spans and high reproductive success, have an increased ability to establish in new environments. Another example is the
Anolis lizards that are known to colonise areas with only five founder individuals (Losos & Spiller, 1999). These strategies are typical of many invasive species (Lodge, 1993; Veltman et al., 1996; Facon et al., 2006). In contrast, K-strategists like New Zealand’s lizards may require extended periods of time to gain population numbers. Towns et. al. (2001) predicted that from 30 translocated Oligosoma suteri, it may take 12 years or more to reach population numbers of 200 individuals. The more extreme K-selected strategists, like H. duvaucelii, may have even lower rates of increase possibly taking centuries to build up to population densities that the islands could support (Towns & Ferreira, 2001).
In addition to reduced population viability, small founder populations are at risk of extinction due to demographic and environmental stochasticities (Goodman, 1987; Armstrong & McLean, 1995). Stochasticity includes irregular chance events, such as a short lived species having a poor breeding season (i.e. demographic stochasticity) or an influx of a highly infectious fatal disease (i.e. environmental stochasticity). Both demographic and environmental stochasticity may act to reduce population numbers (Towns et al., 1990b; Keedwell, 2004). For example, the small founder H. duvaucelii populations may be susceptible to high post-release mortality caused by predation, in which case the population size may become too small, thereby jeopardising the viability of the population (Allee, 1938). The releases of H. duvaucelii were deliberately biased towards sexually mature adults, 50:50 sex ratios and presence of gravid females as this was predicted to increase the potential population expansion. Burke (1991) recommended that deliberate sex ratio manipulation can improve success of translocations however others suggest that release groups should reflect
the demographic structure of the source population (Dodd & Seigel, 1991). The demographic composition of the H. duvaucelii source population was not known and due to the small founder population size, biasing sex ratios would have resulted in a very low number of male founders. Although, a female biased sex ratio may have acted to increase the number of available breeding females, the low number of males would have meant that any mortalities or large-scale dispersals by male H. duvaucelii could have potentially reduced the male population size available to females, to such low levels that breeding success became jeopardised (Allee, 1938). For example, one male H. duvaucelii was located with telemetry over 220 m away from the monitoring area (see Chapter 4). This individual was subsequently re-located back into the monitoring area, since such a large-scale dispersal would have represented a loss to the breeding population. No interactions or copulations were observed during the study however range analysis indicated animals were sufficiently close enough to encounter one another (see Chapter 4).
The release of gravid females aimed to simulate a first breeding season, and amplify the number of geckos in founder population in a short period of time. H. duvaucelii are known to take up to four years to become sexually mature (Thony, 1994). Therefore, the presence of neonates in the first year of release may act to inaugurate the population’s expansion. The capture of these neonates confirmed that at least one female on Tiritiri Matangi and two females on Motuora gave birth during the study. The fact that the neonates were able to survive and gain body condition, provides a good indication that once breeding begins the population is likely to become subject to negative density-dependant compensation and expand. Recently, a translocated population of H. duvaucelii on Mana Island has begun showing signs of establishment, 10 years after 40 individuals were released (T. Whitaker pers. comm., 2008; Jones, 2000).
Concerns regarding compromised long-term genetic viability, as a result of low founder size on both islands, are relevant. Small populations are highly susceptible to inbreeding depression and declines in genetic variation over time (Frankham, 1994; Armstrong & McLean, 1995). It is argued that New Zealand’s fauna have naturally high levels of inbreeding and therefore, may be less susceptible to its effects (Craig et al., 2000; Jamieson et al., 2007). For example, populations of 10-20 H. duvaucelii have been suggested as a sufficient number to establish a population on Motukaha Island (D. Newman & C. Daugherty
pers. comm. in Jansen (1991)). In the present study, the release of gravid females was predicted to increase the founder genetic pool if offspring survived to sexual maturity. The offspring may potentially carry half their genetic material from a different father from the source population. This would theoretically increase the genetic variation in the translocated population. However in the longer-term, further management may be required if these populations are to expand greatly. Population integrity can be maintained in small populations through the introduction of further individuals with new genetic variation and/ or via meta- population management; interchanging individuals between separate populations to maintain or increase genetic diversity (Craig, 1994; Ussher, 1999b). At present, these issues require consideration and the implication of long-term post-release monitoring to reveal the extent of management required in the future.