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广西快乐十分开奖结:Maternal microbes complicate coexistence for tropical trees
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How can hundreds of tree species coexist in a single hectare of tropical forest when the environmental conditions, as well as the species’ basic requirements, appear so similar? A leading explanation, particularly in tropical forests, is called the Janzen–Connell hypothesis after the two ecologists who proposed it in the early 1970s (1, 2). In this mechanism, specialized predators and pathogens concentrated near adult plants differentially reduce survival and growth of conspecific offspring relative to seedlings of other species. These natural enemies limit regeneration of common species and confer a relative advantage to rare species because they have more enemy-free regeneration sites. This has driven decades of empirical work to understand whether enemies are specialized enough in nature to cause this pattern and allow coexistence by this mechanism, and a recent metaanalysis supports the prediction that seed or seedling survival is greater away from conspecific adults (3). In PNAS, Eck et al. (4) show that natural enemies can be even more specialized, specializing on individual genotypes within a wild population. Through simulation models exploring the theoretical implications of this field result, Eck et al. (4) show that this greater specialization may weaken the stabilizing effects of natural enemies on species coexistence but also may select for greater dispersal over evolutionary time.
The body of theory developed since the original formulation of the Janzen–Connell hypothesis has primarily asked (i) How specialized must the natural enemies be? and (ii) At what scale must these patterns operate? With regard to natural enemies, a long-standing conclusion is that species specificity of natural enemies allows coexistence. If a rare plant species shares a generalist enemy with an abundant plant species, then this abundant generalist enemy could extirpate the rare species from the community. With regard to patterns, the spatial scale matters—both in terms of enemy …
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