Zak Mather-Gratton

Back in 2018 we discussed some of the impacts of climate change on amphibians worldwide (Froglife 2018). Since then, climate change has continued to accelerate, with global average temperatures in 2020 more than 1oC warmer relative to pre-industrial levels. The 2015 Paris Agreement has not, so far, caused nations to reduce their emissions anywhere near fast enough (Global Carbon Project 2020). The extent of future warming is projected to reach somewhere between 2 – 4^oC or above, dependent upon a range of scenarios (IPCC 2014). The magnitude and speed of this change is unprecedented, and the impacts upon wildlife are already being seen.

We know that wildlife and their habitats – collectively termed the ‘biosphere’ – are in a fine, dynamic and interconnected balance. Unpicking the exact drivers of population changes is not straightforward, because they themselves are interacting in complex ways. What’s clear is that climate change is an additional stress factor for wildlife worldwide, on top of (and often accelerating) habitat loss and fragmentation, pollution and the impact of invasive species. In the UK climate change is predicted to cause hotter, drier summers and warmer, wetter winters. There will be greater extremes of weather, as well as sea level rise (Met Office 2019).

So how might the UK’s terrestrial reptiles be affected by climate change?
To begin to answer this, we can consider the direct effects of climate change on reptile physiology and behaviour, and indirect impacts from changing trophic interactions (populations of predators and prey), and from changes to reptile habitats (Le Galliard et al. 2012).

UK reptiles are more or less at the northernmost edge of the range for their species, although the adder, common lizard and slow worm have Scandinavian populations at higher latitudes. These three species are viviparous, meaning embryos develop within the body of the adult and born live. The smooth snake is ovoviviparous, which can be described as a more primitive form of viviparity, where the eggs develop within the mothers body. Sand lizards and grass snake are oviparous, meaning they lay their eggs externally, and embryos are therefore more reliant on environmental factors for their development. 

All UK reptiles however, have cousins residing in the warmer southerly climes of Iberia and Central Europe and the Balkans. So, we could assume that warmer climate here will improve survival in existing reptile populations, while enabling a northward shift of their distributions. Unfortunately, the reality is not that simple. Dunford and Berry (2012) modelled UK reptile population changes under low and high emissions scenarios for 2050 and 2080. Grass snakes appeared to be the only species consistently predicted to shift northward while maintaining the majority of their present distribution (see Cathrine, 2014 for tantalising evidence of Scottish grass snake populations), while slow worms showed a mixed northward shift with a contraction of southern populations. For all other species modelling predicted widespread population decline, greater under a higher emissions scenario (Dunford and Berry 2012).

Being ectothermic, reptiles are highly sensitive to their environment, with a tight bell curve describing their optimal climatic conditions (Le Galliard et al. 2012). Temperature and rainfall strongly influence reptile behaviour, and a changing climate has great potential to create mismatches – for example if temperature and rainfall is suboptimal either for reptiles or their prey species, especially at key points in the year such as hibernation emergence. Climate change is already affecting reptile prey populations and will continue to do so. For example Ewald et al. (2015) identified numerous invertebrate groups demonstrating sensitivity to extreme climate events over the last four decades. These trends must be considered alongside the ongoing impact of other pressures, particularly pesticide use (van Klink et al. 2020), all of which creates a complex picture. Climate impacts will vary between species groups and depend upon other biogeographical factors, meaning that impacts will not be the same everywhere for a given species.

In the UK, warming should increase reptile growth and maturation rates, due to longer periods of activity. Milder winters will likely reduce hibernation lengths for our reptiles. Earlier spring emergence, and activity extending further into the autumn and winter months would likely bring reproduction forward in the year, as gestation and incubation length is generally shorter in warmer climates. Larger body sizes in warmer climates may result in greater reproductive success (Le Galliard et al. 2012). Warm temperatures also enable a greater number of number of broods per year, observed in slow worms (Smith 1990), and common lizards (Bestion et al. 2015). However, life cycle changes may have additional impacts on population demographics and survival. Bestion et al. (2015) created experimental common lizard populations in climate-controlled chambers, with populations either subject to current average temperatures or those matching IPCC predictions for the coming century. Warming increased growth rates, brought reproduction forward, and resulted in a higher number of broods per season. However, this life cycle acceleration was coupled with a reduction in adult survival rates. Of several possible causes, the authors proposed the most likely to be greater metabolic requirements for larger individuals, with warmer temperatures increasing energetic requirements which could not be met fully by increased foraging, particularly when warm weather restricted their activity.

Our fragmented UK landscape creates major barriers for all wildlife. Reptile species are generally philopatric (tending to return to or remain near the same particular area), and so are especially sensitive to habitat loss, with populations often centred around relatively small habitat pockets or managed reserves. So even if a warmer climate could result in populations expanding northward, whether this will occur in reality depends on whether there are routes and habitats for them to do so. Araujo et al. (2006) simulated climate change driven UK reptile population dynamics under assumptions of unlimited and zero dispersal ability. Unlimited dispersal resulted in population expansion, with some local extinctions in southerly locations. Zero dispersal resulted in population contractions for all UK species, indicating climate change driven population collapse. This demonstrates the importance of considering all factors influencing population viability and dispersal in combination with climate change pressures.

Adders are usually reliant on specific habitat patches and are highly sensitive to habitat destruction and hibernation area disturbance due to their low recolonization abilities, even relative to other reptile species. On a local level, given typically small population sizes and low mobility, climate change driven habitat quality declines could result in local extinctions particularly as they may not have suitable adjacent habitat, nor the inclination to move (Gleed-Owen and Langham 2012). Although McInerny (2018) indicates that implementing appropriate mitigation can enable adder populations to persist alongside human modifications to local habitats, the adders ongoing decline nationwide is concerning (Baker et al. 2002, Gardner et al. 2019), which even a low emissions climate change scenario is likely to exacerbate (Dunford and Berry 2012).

The dual pressures of habitat loss and climate change are even more apparent for our two most limited reptiles, the sand lizard and the smooth snake. Sand lizards can be found on lowland heath as well as coastal sand dunes. Smooth snakes are only known to be present on lowland heath in the south of England. Populations are generally isolated with little capacity for dispersal, creating a barrier to adaptation to changing conditions. Climate change could threaten habitat quality, for example increased frequency of heavy rainfall events could destroy sand lizard nests and reduce juvenile survival rates (Edgar and Bird 2006), while increased fire prevalence and sea level rise could destroy important habitat features. Dunford and Berry (2012) paint a particularly dire picture for the sand lizard, as its reliance upon highly specific landscape features such as south facing slopes and complex habitat mosaics makes the species highly sensitive to habitat loss. A small population of sand lizards has persisted on the Scottish Isle of Coll, since their introduction for research in the 1970s. Comparing the climate change resilience of this northern and relatively isolated population with that of other UK populations will be an interesting topic for study over the next few decades.

Heathland habitats are one of our most heavily impacted by urbanisation in the UK. Although direct loss through land-use change is now controlled by planning and environmental legislation, development in surrounding areas continues to increase pressure on remaining isolated heathland patches, through increased fire risk, predation by domestic animals and disturbance (Hayhow et al. 2019). Fagundez (2013) describes further pressures facing heathlands under climate change, including fires, and shifts in vegetation composition. We are used to a static conservation model for our heaths. As climate change progresses, the ideal climatic conditions for heath will likely expand in Britain (Thomas et al. 1999), and Loidi et al. (2010) point out that extensive lowland heaths throughout southern Europe support a greater diversity of reptiles than we see here. However, whether habitats will be able to expand is an entirely human choice (Coll et al. 2016, Hayhow et al. 2019).

All of the pressures we have presented here are anthropogenic in origin, born from human choices, and our expectations of how we can and should manage the land and its resources. The enormity of shifting these practices cannot be underestimated, but at least it is within our abilities to do it!

Written by Zak Mather-Gratton 

References
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Bestion, E., Teyssier, A., Richard, M., Clobert, J. and Cote, J., 2015. Live fast, die young: experimental evidence of population extinction risk due to climate change. PLoS Biol, 13(10), p.e1002281.

Cathrine, C., 2014. Grass Snakes (Natrix natrix) in Scotland. Glasgow Naturalist, 26(Part 1), pp.36-40.

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