Abstract
Naturally dark nighttime environments are being widely eroded by the introduction of artificial light at night (ALAN). The biological impacts vary with the intensity and spectrum of ALAN, but have been documented from molecules to ecosystems. How globally severe these impacts are likely to be depends in large part on the relationship between the spatio-temporal distribution of ALAN and that of the geographic ranges of species. Here, we determine this relationship for the Cactaceae family. Using maps of the geographic ranges of cacti and nighttime stable light composite images for the period 1992 to 2012, we found that a high percentage of cactus species were experiencing ALAN within their ranges in 1992, and that this percentage had increased by 2012. For almost all cactus species (89.7%) the percentage of their geographic range that was lit increased from 1992–1996 to 2008–2012, often markedly. There was a significant negative relationship between the species richness of an area, and that of threatened species, and the level of ALAN. Cacti could be particularly sensitive to this widespread and ongoing intrusion of ALAN into their geographic ranges, especially when considering the potential for additive and synergistic interactions with the impacts of other anthropogenic pressures.
Highlights
Concern is being widely expressed as to the negative environmental implications of the introduction of artificial light at night (ALAN), through the use of electric lighting
Of the 1,435 species analysed, a high percentage (80.7%) had some areas of detectable ALAN within the bounds of their geographic ranges in 1992. This increased to 89.7% of species in 2012 (Table 1). In both years we found species with their geographic ranges lit throughout, i.e. every pixel within their range had digital number (DN) values ≥5.5 (0.6% of species in 1992 and 1.6% of species in 2012, Table 1)
The overall trend of increasing erosion of natural darkness across the geographic ranges of cacti was apparent when comparing the percentages of their geographic ranges that were lit in different periods (Fig. 2a)
Summary
Concern is being widely expressed as to the negative environmental implications of the introduction of artificial light at night (ALAN), through the use of electric lighting (including, but not limited to, street lighting[1,2,3,4,5]). Studies of many other taxa are plainly required before any general conclusions can be drawn Amongst plants this is challenging given the paucity of taxonomic groups for which global geographic ranges have been mapped for all or most of the species. Naturally distributed almost entirely on the American continent (with the exception of Rhipsalis baccifera which is the only species naturally distributed in Africa and Sri Lanka) and occurring across a wide range of climatic and ecological conditions[26], it is somewhat emblematic of, and predominantly distributed in, arid lands (Fig. 1a) These ecosystems have been shown to be disproportionately influenced by ALAN27. Indirect effects of particular concern are those of ALAN on pollinators and dispersers (e.g. bats and insects[32])
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