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Positive plant–plant interactions are thought to drive vegetation patterns in harsh environments, such as semi-arid areas. According to the stress-gradient hypothesis (SGH), the role of positive interactions between species (facilitation) is expected to increase with harshness, predicting associated variation in species composition along environmental gradients. However, the relation between stress and facilitation along environmental gradients is debated. Furthermore, differentiating facilitative interactions from other underlying mechanisms, such as microtopographic heterogeneity, is not trivial. We analysed the spatial co-occurrence relationships of vascular plant species that form patchy vegetation in arid lapilli fields (tephra) from recent volcanic eruptions on La Palma, Canary Islands. Assuming a harshness gradient negatively correlated with elevation because the lower elevations are more arid and water availability is considered the most limiting resource, and that an outcome of facilitation is plants co-occurring in the same patch, from the SGH we expected a greater degree of co-occurrence at lower elevation. We tested this at both the species and the individual plant level. We analysed the species composition of 1277 shrubby vegetation patches at 64 different sampling points, ranging from the coast to around 700 m a.s.l. Patch morphology and microtopographic heterogeneity variables were also measured, to account for their potential effects on the species composition of patches. We used generalized linear models and generalized mixed-effects models to analyse species richness, number of individuals in patches and percentage of patches with positive co-occurrences, and a pairwise co-occurrence analysis combined with a graphical network analysis to reveal positive links between 13 of the species. We found that the percentage of patches with positive co-occurrences increased at higher elevations, in contrast to the predictions of the SGH, but in accordance with a refined stress-gradient hypothesis for arid sites, in which characteristics of the interacting species are incorporated.
Supported by the German Alliance Against Depression, 82 regions in Germany launched their own community-based multi-level intervention programs targeting both depression and suicidal behavior prior to January 2016. Sixteen of these regions have implemented the full 4-level intervention program comprising 1) training of General Practitioners, 2) a public awareness campaign, 3) training of community facilitators and 4) support for depressed patients and their relatives for at least three years. The aim of the study was to examine possible suicide prevention effects in these sixteen 4-level intervention regions (comprising a population of 6,976,309) by 1) comparing the annual suicide rates during the 3-year intervention period to a 10-year baseline and 2) comparing these differences to corresponding trends in Germany after excluding all intervention regions (Germany-IR). Primary outcome was the annual rate of suicides. Analyses included negative binomial regression models. When examining differences between suicide rates during the intervention period compared to the baseline period, only a trend towards a significant reduction was found. This reduction of suicides in the sixteen 4-level intervention regions did not differ from that in Germany-IR as control. The interpretation of these findings has to take into account that the training of General Practitioners, police and other community facilitators might have improved the recognition of suicides, thus increasing detection rates. Furthermore, destigmatizing effects of the public awareness campaigns might have increased the number of suicides by lowering suicide threshold (“normalization”) for those at risk and by decreasing the rate of suicides deliberately hidden by suicide victims or their relatives.
