Plants can have positive effects on each other. For example, the accumulation of nutrients, provision of shade, amelioration of disturbance, or protection from herbivores by some species can enhance the performance of neighbouring species. Thus the notion that the distributions and abundances of plant species are independent of other species may be inadequate as a theoretical underpinning for understanding species coexistence and diversity. But there have been no large-scale experiments designed to examine the generality of positive interactions in plant communities and their importance relative to competition. Here we show that the biomass, growth and reproduction of alpine plant species are higher when other plants are nearby. In an experiment conducted in subalpine and alpine plant communities with 115 species in 11 different mountain ranges, we find that competition generally, but not exclusively, dominates interactions at lower elevations where conditions are less physically stressful. In contrast, at high elevations where abiotic stress is high the interactions among plants are predominantly positive. Furthermore, across all high and low sites positive interactions are more important at sites with low temperatures in the early summer, but competition prevails at warmer sites.

Invading exotic plants are thought to succeed primarily because they have escaped their natural enemies, not because of novel interactions with their new neighbors. However, we find that Centaurea diffusa, a noxious weed in North America, has much stronger negative effects on grass species from North America than on closely related grass species from communities to which Centaurea is native. Centaurea's advantage against North American species appears to be due to differences in the effects of its root exudates and how these root exudates affect competition for resources. Our results may help to explain why some exotic species so successfully invade natural plant communities.

Interactions among species determine local-scale diversity, but local interactions are thought to have minor effects at larger scales. However, quantitative comparisons of the importance of biotic interactions relative to other drivers are rarely made at larger scales. Using a data set spanning 78 sites and five continents, we assessed the relative importance of biotic interactions and climate in determining plant diversity in alpine ecosystems dominated by nurse-plant cushion species. Climate variables related with water balance showed the highest correlation with richness at the global scale. Strikingly, although the effect of cushion species on diversity was lower than that of climate, its contribution was still substantial. In particular, cushion species enhanced species richness more in systems with inherently impoverished local diversity. Nurse species appear to act as a 'safety net' sustaining diversity under harsh conditions, demonstrating that climate and species interactions should be integrated when predicting future biodiversity effects of climate change.

28Human activities have caused a near-ubiquitous and evolutionarily-unprecedented increase in 29 environmental sound levels and artificial night lighting. These stimuli reorganize communities 30 by interfering with species-specific perception of time cues, habitat features, and auditory and 31 visual signals. Rapid evolutionary changes could occur in response to light and noise, given their 32 magnitude, geographical extent, and degree to which they represent unprecedented 33 environmental conditions. We present a framework for investigating anthropogenic light and 34 noise as agents of selection, and as drivers of other evolutionary processes, to influence a range 35 of behavioural and physiological traits, such as phenological characters and sensory and 36 signalling systems. In this context, opportunities abound for understanding contemporary and 37 rapid evolution in response to human-caused environmental change. The overcast night sky radiance in urban areas has been found to be as much as four orders of 55 magnitude larger than in natural settings (Figure 1) [5]. Similarly, increased noise levels affect a 56 sizable proportion of the human population. In Europe for instance, 65% of the population is 57 exposed to ambient sound levels exceeding 55 dB(A) [6], roughly equivalent to constant rainfall. 58Of the land in the contiguous U.S., 88% is estimated to experience elevated sound levels from 59 anthropogenic noise (Figure 1) [7]. These effects are not limited to terrestrial environments; 60 ocean noise levels are estimated to have increased by 12 decibels (an ~16-fold increase in sound 61 intensity) in the past few decades from commercial shipping alone [8], while an estimated 22% 62 of the global coastline is exposed to artificial light [3] and many offshore coral reefs are 63 chronically exposed to artificial lighting from cities, fishing boats, and hydrocarbon extraction 64 [9]. 65The changes in light at night and noise levels are occurring on a global scale similar to 66 well-recognized ecological and evolutionary forces such as land cover and climate change. In 67 4 parallel with research involving climate change [10], much of our understanding of organismal 68 response to noise and light is restricted to short-term behavioural reactions. Organismal 69 responses might be associated with tolerance to these stimuli in terms of habitat use [11,12] Status of research on anthropogenic light and sound in ecology 98Night lighting and noise are highly correlated in many landscapes (e.g., [21]). It is critical to 99 understand whether the selective pressures these stimuli exert are additive, synergistic (Figure 2), 100 or if they mitigate one another. Few studies have examined the influence of each simultaneously 101 (e.g., [21]). In one study, flashing lights combined with boat motor noise suppressed antipredator 102 behaviour in hermit crabs (Coenobita clypeatus) more so than noise alone [22]. Future research 103 should quantify both light and sound simultaneously in the same population. Existing r...