U.S. Geological Survey, Great Lakes Science Center, 1100 North Mineral Springs Road, Porter, Indiana 46304, USA.
Ecol Appl. 2010 Sep;20(6):1678-92. doi: 10.1890/08-1792.1.
Given bees' central effect on vegetation communities, it is important to understand how and why bee distributions vary across ecological gradients. We examined how plant community composition, plant diversity, nesting suitability, canopy cover, land use, and fire history affected bee distribution across an open-forest gradient in northwest Indiana, USA, a gradient similar to the historic Midwest United States landscape mosaic. When considered with the other predictors, plant community composition was not a significant predictor of bee community composition. Bee abundance was negatively related to canopy cover and positively to recent fire frequency, bee richness was positively related to plant richness and abundance of potential nesting resources, and bee community composition was significantly related to plant richness, soil characteristics potentially related to nesting suitability, and canopy cover. Thus, bee abundance was predicted by a different set of environmental characteristics than was bee species richness, and bee community composition was predicted, in large part, by a combination of the significant predictors of bee abundance and richness. Differences in bee community composition along the woody vegetation gradient were correlated with relative abundance of oligolectic, or diet specialist, bees. Because oligoleges were rarer than diet generalists and were associated with open habitats, their populations may be especially affected by degradation of open habitats. More habitat-specialist bees were documented for open and forest/scrub habitats than for savanna/woodland habitats, consistent with bees responding to habitats of intermediate woody vegetation density, such as savannas, as ecotones rather than as distinct habitat types. Similarity of bee community composition, similarity of bee abundance, and similarity of bee richness between sites were not significantly related to proximity of sites to each other. Nestedness analysis indicated that species composition in species-poor sites was not merely a subset of species composition at richer sites. The lack of significant proximity or nestedness effects suggests that factors at a small spatial scale strongly influence bees' use of sites. The findings indicate that patterns of plant diversity, nesting resource availability, recent fire, and habitat shading, present at the scale of a few hundred meters, are key determinants of bee community patterns in the mosaic open-savanna-forest landscape.
鉴于蜜蜂对植被群落的核心作用，了解蜜蜂的分布如何以及为何随生态梯度变化至关重要。我们研究了植物群落组成、植物多样性、筑巢适宜性、冠层覆盖、土地利用和火灾历史如何影响美国印第安纳州西北部开阔森林梯度上的蜜蜂分布，这一梯度类似于美国中西部历史上的景观镶嵌体。当与其他预测因子一起考虑时，植物群落组成并不是蜜蜂群落组成的重要预测因子。蜜蜂丰度与冠层覆盖呈负相关，与近期火灾频率呈正相关；蜜蜂丰富度与潜在筑巢资源的植物丰富度和丰度呈正相关；而蜜蜂群落组成与植物丰富度、与潜在筑巢适宜性相关的土壤特性以及冠层覆盖显著相关。因此，蜜蜂丰度受不同的环境特征预测，而蜜蜂物种丰富度则受一个组合的环境特征预测，这个组合包括了对蜜蜂丰度和丰富度有显著影响的预测因子。在木质植被梯度上，蜜蜂群落组成的差异与寡食性或专食性蜜蜂的相对丰度相关。由于寡食性比杂食性更罕见，且与开阔生境相关，因此它们的种群可能特别容易受到开阔生境退化的影响。在开阔和森林/灌丛生境中记录到的栖息地专化性蜜蜂比在稀树草原/林地生境中多，这与蜜蜂对中等木质植被密度的生境（如稀树草原）的反应一致，即作为生态过渡带，而不是作为截然不同的生境类型。各站点之间的蜜蜂群落组成相似性、蜜蜂丰度相似性和蜜蜂丰富度相似性与站点之间的接近程度没有显著关系。嵌套分析表明，在物种较少的地点，物种组成不仅仅是在较丰富地点的物种组成的子集。没有显著的接近或嵌套效应表明，在小空间尺度上的因素强烈影响着蜜蜂对地点的利用。这些发现表明，在几百米的尺度上，植物多样性、筑巢资源可用性、近期火灾和栖息地遮荫等模式是镶嵌开阔稀树草原-森林景观中蜜蜂群落模式的关键决定因素。
