Modelling the thermal environment of incubation informs predictions of habitat suitability for a species with temperature-dependent sex determination
Species distribution models are used increasingly to examine habitat suitability and predict range shifts within the context of modern climate change. However, climate envelopes and other commonly used correlative methods are of limited use if the primary conservation concern is not the availability of suitable or sufficient habitat. In species with temperature-dependent sex determination (TSD), for example, environmental variation with minimal detectable effect on the distribution or fitness of adults may dramatically skew offspring sex ratios, eventually reducing population viability. Likewise, climate warming may be beneficial, in the short term, for adults; however, developing embryos may experience detrimental thermal and hydric conditions in-ovo. An accurate definition of habitat suitability for these species must include the thermal suitability of sites both for successful completion of embryonic development and, over the long term, for maintenance of balanced sex ratios. We used a spatially explicit, mechanistic microclimate model coupled with a biophysical model of embryonic development to locate thermally suitable nesting sites and predict offspring sex ratios for tuatara, a New Zealand-endemic reptile, on two latitudinally and environmentally distinct islands under current and projected future climate scenarios.
If climate change observed over the next 100 years more closely matches the maximum, rather than the minimum, projected magnitude of warming, hatchling sex ratios on both islands are likely to become heavily male-biased. Changes in either nesting phenology or nest depth are unlikely to compensate for the effects of climate change on sex ratios. In addition, the now-tight relationship between egg-laying dates and sex ratios may become de-coupled under maximum warming, creating an ecological trap. We argue that spatially explicit data that quantify the thermal suitability of sites for incubation and sex determination should be considered critical to conservation of tuatara and other species with TSD. In addition, our results highlight the importance of defining habitat suitability at multiple scales. Environmental variation, particularly climate change, may have differential effects at distinct life stages and at individual vs. population levels.