Long-term consequences of genetic rescue on island populations of South Island robins. (2019)
Type of ContentTheses / Dissertations
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
Population bottlenecks can lead to the loss of genetic diversity, including genetic variants that may be adaptive. In turn, small population size and isolation increases the chances that related individuals mate, and the risk of inbreeding depression and extinction. One method that can help sustain bottlenecked species is the translocation of individuals from genetically diverse populations into genetically depauperate populations to increase genetic diversity and fitness – a strategy known as genetic rescue. However, few studies have looked at the long-term effects of attempted genetic rescue and how often it leads to sustained increases in fitness. The objective of this thesis was to examine the long-term effects of attempted genetic rescue on two populations of South Island robin (Petroica australis), located on Motuara Island and Allports Island, at the northern end of New Zealand’s South Island. Each population was established in 1973 with five birds, but by 1999, both populations were showing signs of inbreeding depression. In an attempt to alleviate the inbreeding, a genetic rescue attempt was made during 2008-2009. This rescue attempt was unusual in that it involved a reciprocal translocation of inbreed donors. I assess both the dynamics of genetic change and fitness-related measures ranging from breeding success, survival, population size, immune system function and cognitive abilities, and when possible, compare these between at the start of the genetic rescue attempt to the present day. This broad approach is necessary as focusing on just a few life stages over a short time frame can underestimate inbreeding depression and the extent to which genetic rescue can lessen its effects.
Population-wide genetic diversity has changed over the 10 years since the translocations. Following the initial increase in genetic diversity in interbred birds in 2008-10, genetic diversity declined by 2016-18. However, this decline has been relatively slow; both expected heterozygosity (He) and allelic richness (Ar) are still higher than pre-translocation levels. Similarly, individual fitness as measured by a number of traits, remains higher in 2016-18 than before the rescue. In both populations, juvenile robins with higher genetic diversity were significantly more likely to survive to one year of age. Interbred F1-F3 individuals arising from the 2008-10 translocation were also significantly more likely to survive to 10 years of age than their pre-translocation counterparts. However, I found no clear evidence for correlations between genetic diversity and individual measures of breeding success within the current populations. The increased survival of both juveniles and F1-F3 interbred adults could increase lifetime reproductive success, and may explain the increase in population size on both islands since 2008-10.
I next assessed immune system function as possible drivers of increased survival. I artificially challenged robin with a phytohaemagglutinin (PHA) test, and measured ecto- and endo-parasite loads, and body morphology. I then assessed correlations between these traits with individual genetic diversity. Birds on both islands continued to show strong reactions to the PHA (as with the 2008-10 interbred individuals), but overall there was no correlation between genetic diversity and immune system response. Mite loads showed a significant inverse correlation with genetic diversity, but overall, there was only weak evidence that genetic diversity was correlated with parasite load and morphology.
Despite the potential impact of genetic diversity on fitness, few studies have investigated the relationship between genetic variation and intelligence. I assessed several cognitive measures in the robin populations, including male song structure and the performance of individuals in two experimental cognitive tasks. I found that genetic diversity affected some aspects of cognitive performance in the tasks, but not in song complexity.
My study confirms that the genetic rescue attempt has been successful in two South Island robin populations via reciprocal translocation of inbred donors. Increases in fitness were still evident 8-10 years later. To ensure the long-term viability of small populations, it is recommended that conservation management include genetic rescue, as well as traditional approaches of habitat restoration and predator control.
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