Landscape context affects both capture probability and abundance of solitary bees in cities

It is intuitive to hypothesize that urban landscapes are broadly hostile
to insects, but responses of wild bees to urbanization are inconsistent
and often positive. We used a mark-recapture study of the bicolored sweat
bee (Agapostemon virescens) to separate two possible causes of this
pattern: differences in capture probability (bees are more attractive to
sampling sites in low-resource areas) from patterns of true abundance
(bees are equally abundant in high- and low-resource landscapes, as long
as resources are present locally). To distinguish these possibilities, wee
placed experimental patches of Echinacea purpurea, a
favored floral resource, in and near community gardens along a landscape
gradient of % impervious surface. We collected mark-recapture data
of A. virescens visitation to these patches, and
analyzed capture probability and abundance in relation to % impervious
surface in the surrounding landscape, and local resource context, measured
as inside or outside the community garden in low-resource settings
(pavement, mulch, or turf grass).  This study took place
in the cities of Cambridge, Somerville, and Medford, Massachusetts, USA,
which are part of the greater Boston metropolitan
area.  We found that capture probability was higher in
low-resource contexts (outside of gardens and/or in landscapes with a high
% impervious surface).  Fewer individual bees visited
patches in non-garden sites and landscapes with high impervious surface,
but those bees visited the patches more often. In addition, all of our
experimental patches were used by A. virescens. These data
include the capture histories of individual bees, and the characteristics
of the sites and community gardens.

直观而言，我们可提出假说：城市景观整体对昆虫存在不利影响，但野生蜂类对城市化的响应却并不统一，且常呈现正向趋势。本研究以双色隧蜂（bicolored sweat bee，学名*Agapostemon virescens*）为对象开展标记重捕实验，旨在解析该现象的两种潜在成因：一是捕获概率差异（低资源区域的采样点对蜂类更具吸引力），二是真实种群丰度格局（只要局地存在食物资源，高、低资源景观中的蜂类丰度并无差异）。为区分这两种可能性，我们沿不透水面占比的景观梯度，在社区花园内部及其周边布设了作为优质蜜源资源的紫锥花（*Echinacea purpurea*）实验样地。我们收集了双色隧蜂造访这些样地的标记重捕数据，并围绕周边景观的不透水面占比，以及局地资源背景（低资源环境下的社区花园内外，即铺装路面、护根覆盖物或草坪），分析了捕获概率与种群丰度的变化规律。本实验在美国马萨诸塞州剑桥市、萨默维尔市以及梅德福市开展，这些区域隶属于大波士顿都会区。研究结果显示：低资源环境（花园外及/或不透水面占比高的景观）中的捕获概率更高。非花园样地及不透水面占比高的景观中，造访样地的蜂类个体数量更少，但这些个体的造访频次更高。此外，所有实验样地均有双色隧蜂造访。本数据集包含蜂类个体的捕获历史，以及样地与社区花园的相关特征信息。