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Untangling the influences of air quality and climate change on biodiversity in space and time

G. Phoenix,Abdulssalam H Khafsha

Published 2023 in Global Change Biology

ABSTRACT

One of the greatest challenges in understanding the impact of global change on biodiversity is disentangling the influence of the multitude of potential causes of biodiversity change. Species face an array of pressures including many facets of climate, pollution, land use and more, so it is not easy to know which factor is most responsible for changes in biodiversity. Other pertinent questions arise, such as to what extent is our current biodiversity driven by conditions now, or by historic conditions forming the biodiversity landscape we see today. These are questions directly addressed in this issue by Hutsemékers et al. (2023) in their work on epiphytic bryophytes and the influence of air pollutants and climate over the past four decades. Epiphytic bryophytes make a sensible choice for this work. They can be sensitive to atmospheric pollutants due to having leaves often only one cell thick, and absorbing water and nutrients directly through the leaf surface (Nash & Wirth, 1988; Pescott et al., 2015). Uptake of water in this way, and their temperature sensitivity, also means they can be susceptible to changes in climate. Focussing on these plants, Hutsemékers and colleagues used data from 1980 to now in southern Belgium on species distributions. They then combined this with data on the air pollutants nitrogen dioxide (NO2), sulphur dioxide (SO2), ozone (O3) and fine particles (PM10), along with climate data on temperature and precipitation (including various means, maxima and minima). From these they determined the relationship between the air quality and climate parameters, and both how species composition changed over time and its current spatial variation. The work found that improving air quality (declining pollutant loads) was the main cause of changes in species composition over time, but also that pollutant loads have declined enough so that current spatial climate variability is now the main driver of current species distributions. Over the four decades, species frequencies increased, resulting in greater species richness. Over time, that diversity consisted less of acidloving species with increasing proportions of acidsensitive species as well as nitrogenloving species. It was also notable that the change in species composition over time was much greater than the current spatial change in composition across southern Belgium. The main environmental trends were that temperatures warmed, and SO2 and NO2 decreased. But, to what extent did these environmental changes influence the epiphytic bryophyte compositional change in space and time? Hutsemékers and colleagues' analyses show that improving air quality over time (mainly declining NO2 and SO2) was responsible for more than onethird of the temporal change in species composition over the four decades, with temporal changes in climatic conditions of little importance. In contrast, current climate spatial variability across the study region was found to be the main driver of current spatial compositional variation. In other words, improving air quality has allowed a substantial recovery of the epiphytic bryophyte flora, and air quality has improved so much that it now allows climate's influence on current species distributions to emerge. The authors point out that the increase in nitrogenloving species is puzzling, given declining NO2. Their suggestion is that past very high pollution loads may have been too much (i.e. toxic) even for nitrogenloving species, but that SO2 and NO2 have declined enough so that NO2 levels are now ‘favourable’ and so allow nitrogenloving species to spread. In addition to the important work disentangling the temporal and spatial drivers of biodiversity change in this flora, a number of additional insights arise from this work. First, the considerable change over time points to this flora not suffering from dispersal limitation, and indicates a flora that is well capable of recovery as environmental conditions become more suitable. Also, it is good to see that the implementation of legislation that has improved air quality has been successful enough so that air quality no longer has a signal in the current spatial variability of this flora. These points are important in the global context. N deposition, for instance, is classically considered to

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