The conservation potential of small habitat fragments for reptiles: insights from monitoring, genomics, and translocations

Landscape modification for agriculture has shaped ecosystems globally, driving widespread population declines and range reductions of native fauna. However, some species persist and even thrive in modified landscapes. Growing evidence challenges the often-assumed low biodiversity value of small fragments of remnant habitat. Thus, modified landscapes present an opportunity to integrate conservation into human landscapes at a global scale. To do so, a deeper understanding of the effects of landscape modification on remnant fauna communities is required. Here, rapidly advancing genomic approaches can assess genomic and population metrics to inform biodiversity management in fragmented landscapes. Translocations are one management tool with the potential to restore ecosystem function, spread extinction risk and increase community engagement in conservation, but which have been rarely tested in fragmented landscapes. In this thesis, I sought to: (1) assess the effect of landscape modification on fauna at the population, community, and genomic level, and (2) trial wild-to-wild translocation as a method of maximising biodiversity value in fragmented landscapes.

Chapter one provides a brief introduction to the thesis. In chapter two, I investigated the influence of landscape modification on herpetofauna and small mammal species, communities and traits. Richness and diversity were similar between habitat fragments and reference ‘pseudo-fragments’, but community composition differed, driven by individual species showing consistently higher abundances in either the fragments or pseudo-fragments. I found evidence for three traits influencing a species’ response to modification; diet; habitat breadth and habit.

In chapter three, I used a genomic approach to assess gene flow, genetic diversity, inbreeding and effective population sizes for three terrestrial lizard species in habitat fragments and pseudo-fragments. For one species, the painted dragon (Ctenophorus pictus) genomic differentiation implicated the agricultural matrix as a barrier to gene flow. The same may also be true for the other two species but with differing demographic and life-history characteristics leading to a time-lag in their genetic response to land modification. I also found evidence of lower effective population size and lower genetic diversity in fragments indicating management interventions may be necessary to ensure the long-term persistence of these populations.

In chapter four, I trialled an experimental translocation of two agamid lizard species as a method of combatting reduced dispersal in fragmented landscapes. Survival, body condition, movement, and microhabitat use were similar between translocated individuals and resident animals. Temporary penning of animals before release (soft-release) had similar results to immediate release. Though longer-term monitoring is required to evaluate translocation success, promising short-term results indicate that ‘mainstreaming’ translocations of common but patchily distributed species might be a viable conservation intervention (Watson and Watson, 2015).

In chapter five, I leveraged data from chapter four to explore the influence of environmental and intrinsic variables on microhabitat use. I found that temperature and time of day strongly influenced microhabitat use. I also identified an ontogenetic change in habitat use where juvenile mallee tree dragons predominantly use low perches in shrubs but move to higher perches and trees as they mature.

My findings show that, despite similar richness and diversity, land modification alters herpetofauna and small mammal community composition, with traits influencing species abundance in response to land modification. I also found evidence of genetic effects of land modification such as reduced gene flow and genetic diversity in fragments, demonstrating the utility of genomic approaches in monitoring fragmented fauna populations. Translocation trials showed promising short-term results, providing support for the potential of this method to combat localised extinctions and genetic issues in fragments identified in previous chapters. I found that microhabitat use varied in response to environmental and individual variables, suggesting that lizards require diverse microhabitats to navigate thermal landscapes, and to cater to different life-stages. Broadly, I demonstrate that, despite some negative impacts of habitat modification on biodiversity, small habitat fragments can sustain diverse fauna communities. Fragments could be used as reservoirs of biodiversity in modified landscapes, which can be built upon through active restoration measures to further enhance conservation potential. Chapter six places my findings in the broader research context and discusses some future directions identified in this thesis.