The Interplay between Environmental Filtering and Spatial Processes in Structuring Communities: The Case of Neotropical Snake Communities

Both habitat filters and spatial processes can influence community structure. Space alone affects species immigration from the regional species pool, whereas habitat filters affect species distribution and inter-specific interactions. This study aimed to understand how the interplay between environmental and geographical processes influenced the structure of Neotropical snake communities in different habitat types. We selected six studies that sampled snakes in forests, four conducted in savannas and two in grasslands (the latter two are grouped in a non-forest category). We used the net relatedness and nearest taxon indices to assess phylogenetic structure within forest and non-forest areas. We also used the phylogenetic fuzzy-weighting algorithm to characterize phylogenetic structure across communities and the relation of phylogenetic composition patterns to habitat type, structure, and latitude. Finally, we tested for morphological trait convergence and phylogenetic niche conservatism using four forest and four non-forest areas for which morphological data were available. Community phylogenetic composition changed across forest and non-forest areas suggesting that environmental filtering influences community structure. Species traits were affected by habitat type, indicating convergence at the metacommunity level. Tail length, robustness, and number of ventral scales maximized community convergence among forest and non-forest areas. The observed patterns suggested environmental filtering, indicating that less vertically structured habitats represent a strong filter. Despite the fact that phylogenetic structure was not detected individually for each community, we observed a trend towards communities composed by more closely related species in higher latitudes and more overdispersed compositions in lower latitudes. Such pattern suggests that the limited distribution of major snake lineages constrained species distributions. Structure indices for each community were also related to habitat type, showing that communities from non-forest areas tend to be more clustered. Our study showed that both environmental filtering and spatial gradients play important roles in shaping the composition of Neotropical snake communities.

生境过滤（habitat filters）与空间过程（spatial processes）均可对群落结构产生影响。仅空间因素即可影响物种从区域物种库（regional species pool）中的迁入过程，而生境过滤则会调控物种分布与种间相互作用（inter-specific interactions）。本研究旨在探究环境与地理过程的交互作用，如何塑造不同生境类型下的新热带区蛇类群落（Neotropical snake communities）结构。我们共遴选了6项以森林为采样生境的蛇类研究、4项以稀树草原为采样生境的研究，以及2项以草原为采样生境的研究（后两项草原研究被归入非森林类别）。我们采用净亲缘关系指数（net relatedness index）与最近分类单元指数（nearest taxon index），评估森林与非森林生境内部的系统发育结构（phylogenetic structure）。我们还运用系统发育模糊加权算法（phylogenetic fuzzy-weighting algorithm），刻画群落间的系统发育结构特征，并分析系统发育组成模式与生境类型、结构及纬度的关联。最后，我们针对4处森林生境与4处非森林生境（均配有可用的形态学数据）开展分析，检验形态性状趋同（morphological trait convergence）与系统发育生态位保守性（phylogenetic niche conservatism）两大现象。森林与非森林生境间的群落系统发育组成存在显著差异，这表明环境过滤对群落结构存在调控作用。物种性状受生境类型影响，这提示元群落（metacommunity）水平上存在性状趋同现象。尾长、躯体粗壮度以及腹鳞（ventral scales）数量，是森林与非森林生境间群落性状趋同程度最高的指标。本次观测到的模式印证了环境过滤的作用，表明垂直结构较弱的生境是较强的过滤因子。尽管单从每个群落本身无法检测到显著的系统发育结构，但我们观测到一种趋势：高纬度地区的群落由亲缘关系更近的物种构成，而低纬度地区的群落系统发育组成则更为发散。该模式表明，蛇类主要演化支系的分布范围受限，对物种分布形成了约束。各群落的结构指数同样与生境类型相关，结果显示非森林生境中的群落往往具有更强的系统发育聚类特征。本研究证实，环境过滤与空间梯度（spatial gradients）均在新热带区蛇类群落的组成塑造过程中发挥了关键作用。