The interacting effect of habitat amount, habitat diversity and fragmentation on insect diversity along elevational gradients.

Aim: Elevational gradients are a useful approach to evaluate how environmental factors affect animal diversity. Decades of studies on the elevation-diversity gradient have revealed that this gradient varies greatly with taxa and geographic regions. One potential explanation for this may be the dependence of the relationship on landscape features. We explore the impact of fragmentation, habitat diversity and habitat amount on insect diversity (alpha and beta) and abundance along elevational gradients. We hypothesize that insect diversity and abundance will relate negatively with elevation, but positively with these landscape features. We also hypothesize that landscape features will interact in a way that the positive effect of a given variable on insect diversity may be offset by the others. Location: Reunion Island (Indian Ocean) Taxon: The insect order thrips (Thysanoptera) Methods: Insects were sampled along replicated elevational gradients, and at each sampling plot landscape features and abiotic variables were estimated within buffers surrounding the site. Insect alpha diversity was estimated using abundance-based rarefaction methods, whereas beta diversity was estimated calculating the "local contributions to beta diversity" metric. The effect of elevation, rainfall, landscape features and their interactions was assessed on insect alpha and beta diversity and abundance during two consecutive seasons using linear mixed effects models. Results: We found that thrips alpha and beta diversity was negatively related with elevation, but the relationship varied between seasons and rainfall regimes. Among the different landscape features considered, we found that habitat amount had the strongest effect on diversity. The effect of habitat amount on diversity, however, was offset in areas of low habitat (or land cover) diversity. Main conclusions: Generalizing the factors that underlie the elevation diversity gradient has become a cornerstone in ecological theory because it can help to understand the impact of human activities on diversity. Here we show that taking landscape information into account may help to fulfil this objective because landscape effects co-vary with elevation with often intricate consequences for diversity.

研究目的：海拔梯度是评估环境因子如何影响动物多样性的有效研究途径。数十年来针对海拔-多样性梯度的相关研究表明，该梯度随类群与地理区域的不同存在显著差异。对此现象的一种潜在解释是，该关系依赖于景观特征。本研究探究了景观破碎化、生境多样性与生境面积沿海拔梯度对昆虫多样性（α多样性（alpha diversity）与β多样性（beta diversity））及多度的影响。我们提出如下假说：昆虫多样性与多度将随海拔升高呈负相关，而与上述景观特征呈正相关；同时景观特征间存在交互作用，即某一变量对昆虫多样性的正向效应可能被其他变量抵消。
研究地点：留尼汪岛（Reunion Island，印度洋）
研究类群：蓟马（缨翅目Thysanoptera）
研究方法：研究者沿重复设置的海拔梯度样带开展昆虫采样，并在每个采样样地内，通过样点周边缓冲区估算景观特征与非生物变量。昆虫α多样性采用基于多度的稀疏法（rarefaction）进行估算，β多样性则通过计算"β多样性局域贡献量（local contributions to beta diversity）"指标进行估算。本研究于两个连续季节中，采用线性混合效应模型（linear mixed effects models）分析了海拔、降雨量、景观特征及其交互作用对昆虫α多样性、β多样性与多度的影响。
研究结果：本研究发现，蓟马的α与β多样性随海拔升高呈负相关，但该关系在不同季节与降雨格局间存在差异。在考量的各类景观特征中，生境面积对多样性的影响最为显著；然而，在生境（或土地覆盖）多样性较低的区域，生境面积对多样性的正向效应会被抵消。
主要结论：阐明海拔多样性梯度背后的驱动因子并实现普适性推演，已成为生态学理论的核心基石之一，因其有助于理解人类活动对生物多样性的影响。本研究表明，纳入景观信息可助力实现这一目标，因为景观效应随海拔呈现协同变化，进而对生物多样性产生往往较为复杂的影响。