Vertebrates under ultraviolet radiation

Chlorofluorocarbons are widely used as refrigerants and propellants, but can reach the stratosphere where they catalyse the destruction of ozone molecules, leading to a depletion of stratospheric ozone (Molina & Rowland, 1974). The consequence of this loss of ozone is a corresponding increase in ultraviolet radiation (UVR, 100–400 nm) reaching the surface of the Earth. The Montreal Protocol on Substances that Deplete the Ozone Layer, signed in 1987, has largely halted further damage to the stratospheric ozone layer, and stratospheric ozone is expected to recover over the coming decades (e.g. the Antarctic ozone layer is emerging since 2000, Solomon et al., 2016). Although the Montreal Protocol and its amendments have prevented global-scale increases in UVR at the Earth's surface over the past four decades, future changes in UVR reaching the Earth's surface have been suggested to be mostly dominated by changes in the climate (e.g. aerosols and cloud cover) rather than by changes in stratospheric ozone levels (Bernhard et al., 2023). It has been projected that in the tropics, UV radiation (primarily UV-B, 280–315 nm) will increase by 3%–8% by the end of this century, as a result of decreases in clouds and ozone levels (Bais et al., 2011). Assessing the effects of UVR (especially of increased UVR levels in the context of climate change) on biological systems across all levels of biological organisation is of great importance for scientists studying global change biology. In their meta-analysis paper in this issue of Global Change Biology, Downie et al. (2023) conducted a comprehensive and in-depth analysis using 895 observations from 47 different vertebrate species (fish, amphibians, reptiles and birds) and 51 physiological metrics (i.e. cellular, tissue and whole-animal metrics), across 73 independent studies, to examine the response patterns of UVR effects on vertebrate physiology. Vertebrates, which have evolved in the presence of UVR over millennia, have developed various behavioural and physiological strategies to cope with UVR exposure. Because of cutaneous coverings such as fur, feathers and scales that reflect UVR or act as a sufficient barrier against UVR in reptiles, birds and mammals, these vertebrates are expected to be more resistant than other vertebrate groups, such as fish and amphibians, that lack such physical protection. By carefully analysing data obtained over the last four decades, Downie et al. provided solid evidence in support of this hypothesis. Furthermore, they demonstrated that both fish and amphibians are significantly more negatively affected by UVR exposure relative to reptiles and birds. This information is critical to identify taxa that are particularly vulnerable to UVR, and to improve the predictions of future diversity changes in the context of global change. Another interesting result from this study is the latitude-dependent vertebrate vulnerability to UVR. The authors have verified that vertebrates from tropical habitats are more negatively impacted by UVR than those from higher latitudes. Although vertebrates from low latitudes might be expected to have a higher tolerance to elevated UVR levels, Downie et al. found that even slight increases in UVR exposure levels are detrimental for various vertebrate species. Thus, many vertebrate species in tropical regions may already be at the limit of their capacity to manage and contend with UVR-associated molecular damage; they may have little scope to respond to further increases in UVR. This information is of great importance for a better understanding of the impacts of predicted increases in UVR levels associated with climate change in tropical regions (Bais et al., 2011). Downie et al. (2023) also show that the vulnerability of vertebrates to UVR is life-stage dependent. They demonstrate that adults and larvae are the most susceptible life stages, while the effect of UVR on embryos or juveniles was found to be not significant. The reasons for this result may be twofold: First, the melanin in many fish and amphibian embryos or the vitelline jelly in frog eggs can absorb UVR (up to 90% absorption of UV-B), thus protecting more sensitive biomolecules from damage (Licht, 2003); second, some fish eggs have sophisticated UVR damage-repair mechanisms (Dethlefsen et al., 2001). Hence, larval fish and tadpoles appear to be more sensitive to UVR exposure because of the loss of the protective defences provided by their eggs; moreover, their larger surface area to volume ratios make them vulnerable to external stressors and they may be physiologically under-developed to cope with high UVR stress (Alves & Agustí, 2021). However, as emphasised by Downie et al., the effects of UVR exposure that occur at one life stage may carry over to subsequent life stages. Therefore, it will be interesting to further explore how these carry-over effects may buffer or amplify the UVR sensitivity of subsequent life stages, and influence future population viability. In parallel with increased UVR exposure, organisms are facing multiple environmental changes because of climate change and intensified anthropogenic activities, such as global warming. On the one hand, warming within thermal limits can compensate for the negative effects of UVR in a variety of terrestrial and aquatic organisms (Jin et al., 2019). On the other hand, the increasing temperature in the tropics, where UVR is predicted to increase by 3%–8%, is driving the relocation of vertebrates from tropical to temperate regions or shifting their habitat ranges to higher altitudes with higher ambient UVR levels to seek thermal refuge (Barnes et al., 2023). Therefore, the study by Downie et al., which provides a structured assessment of the impacts of UVR on a wide range of physiological, behavioural and morphological traits in vertebrates, should inspire further research on how elevated UVR exposure interacts with other environmental changes, such as warming. The assessment they provide is crucial to better understand the resilience of vertebrates to multiple environmental changes and to develop solutions to either manage or ameliorate the effects of these changes on UVR sensitive species. The author have no conflicts of interest to declare. No data were used for this commentary.