Uncovering patterns of freshwater positive interactions using meta-analysis: Identifying the roles of common participants, invasive species and environmental context.

Positive interactions are sensitive to human activities, necessitating synthetic approaches to elucidate broad patterns and predict future changes if these interactions are altered or lost. General understanding of freshwater positive interactions has been far outpaced by knowledge of these important relationships in terrestrial and marine ecosystems. We conducted a global meta-analysis to evaluate the magnitude of positive interactions across freshwater habitats. In 340 studies, we found substantial positive effects, with facilitators increasing beneficiaries by, on average, 81% across all taxa and response variables. Mollusks in particular were commonly studied as both facilitators and beneficiaries. Amphibians were one group benefiting the most from positive interactions, yet few studies investigated amphibians. Invasive facilitators had stronger positive effects on beneficiaries than non-invasive facilitators. We compared positive effects between high- and low-stress conditions and found no difference in the magnitude of benefit in the subset of studies that manipulated stressors. Future areas of research include understudied facilitators and beneficiaries, the stress gradient hypothesis, patterns across space or time and the influence of declining taxa whose elimination would jeopardise fragile positive interaction networks. Freshwater positive interactions occur among a wide range of taxa, influence populations, communities and ecosystem processes and deserve further exploration.

Islands within islands: Diversification of tailless whip spiders (Amblypygi, Phrynus) in Caribbean caves.

Islands have played a key role in understanding species formation ever since Darwin's work on the Galapagos and Wallace's work in the Malay Archipelago. Like oceanic islands, habitat 'islands', such as mountaintops and caves similarly may drive diversification. Here we examine patterns of diversification in the tailless whip spider genus Phrynus Larmarck, 1809 (Amblypygida: Phrynidae) a system that shows evidence of diversification under the influence of 'islands within islands'. We estimate phylogeographic history and measure genetic diversity among representatives of three nominal Phrynus species from epigean and cave systems of Puerto Rico and nearby islands. Data from five loci (mitochondrial 12S, 16S, Cox1; nuclear H3, 28S) were used to generate phylogenetic hypotheses and to assess species monophyly and phylogeographic relationships. Genetic divergences and population limits were estimated and assessed using the Geneious barcoding plugin and the genealogical sorting index. We find that mtDNA sequence divergences within each of the three Phrynus species range between 15% and 20%. Genetic divergence is structured at three spatial scales: among islands in a manner consistent with the GAARlandia hypothesis, among bedrock formations within Puerto Rico, and among caves within these formations. Every isolated cave system contains a unique mtDNA genetic lineage of Phrynus, with divergence among cave systems far exceeding that within. In some localities epigean specimens nest among cave taxa, in others caves are monophyletic. Remarkably, clades that show up to 20% mtDNA sequence divergence show little or no variation in the nuclear markers. We interpret this pattern as resulting from extreme conservation of our nuclear markers rather than male sex-biased dispersal, based on high conservation of 28S and H3 between our individuals and other amblypygid genera that are restricted to Africa. While this study includes but a tiny fraction of Caribbean caves, our findings suggest Phrynus may be much more diverse than hitherto thought, at least in terms of mtDNA diversity, and that the arthropod fauna of caves may represent a dimension of biodiversity that is yet to be discovered in the Caribbean biodiversity hotspot.