Data and code from: Support for an area–heterogeneity tradeoff for biodiversity in croplands

Tab-delimited, plain-text data files and R scripts. Please see "Metadata.txt" for details. <br> Abstract from associated paper: Rapid expansion of the human population poses a challenge for wildlife conservation in agricultural landscapes. One proposition is that we could increase biodiversity in such landscapes by increasing crop diversity. However, studies report both positive and negative effects of crop diversity on biodiversity. One possible explanation, derived from the “area–heterogeneity tradeoff hypothesis,” is that the effect of crop diversity on biodiversity depends on a tradeoff between increasing the number of crop types in a landscape and decreasing the amount of each single crop type. This should cause positive effects of increasing crop diversity at low to intermediate crop diversity and negative effects at intermediate to high crop diversity. We also proposed two factors that could change the point at which the effect of increasing crop diversity shifts from positive to negative. First, we predicted that this shift occurs at a lower crop diversity when the surrounding landscape contains less semi-natural habitat, and at a higher crop diversity when the landscape contains more semi-natural habitat. This should increase the likelihood of detecting negative effects of crop diversity when semi-natural cover is low and positive effects when it is high. Second, we predicted that the shift from a positive to negative effect occurs at a lower crop diversity when it is measured locally than when it is measured at greater distances from the site, making detection of negative crop diversity effects more likely when measurements are at local extents. We tested these predictions using data on the biodiversity of herbaceous plants, butterflies, syrphid flies, woody plants, bees, carabid beetles, spiders, and birds at 221 crop field edges in Eastern Ontario, Canada. We found support for an area–crop diversity tradeoff. Semi-natural cover and measurement extent influenced the biodiversity–crop diversity relationship, with positive effects when semi-natural cover was high, and negative effects when semi-natural cover was low and when crop diversity was measured at local extents. The results suggest that policies/guidelines designed to increase crop diversity will not benefit biodiversity in the landscapes where conservation action is most urgently needed, i.e. in landscapes with high agricultural use and low semi-natural cover.

本数据集包含制表符分隔的纯文本数据文件与R脚本，详情请查阅Metadata.txt文件。
相关论文摘要如下：
人口的快速增长给农业景观中的野生动物保护工作带来了严峻挑战。有观点提出，可通过提升作物多样性来改善此类景观中的生物多样性。但现有研究表明，作物多样性对生物多样性的影响存在正负双向性。
其中一个源自「面积-异质性权衡假说（area–heterogeneity tradeoff hypothesis）」的可能解释为：作物多样性对生物多样性的影响，取决于景观内作物类型数量提升与单一作物种植面积缩减之间的权衡关系。这意味着，在作物多样性处于低至中等水平时，提升作物多样性会产生正向影响；而在中等至高水平时，则会产生负向影响。
本研究还提出了两类可改变作物多样性效应由正转负临界点的因素。其一，我们预测：当周边景观中的半自然生境占比较低时，该临界点会出现在更低的作物多样性水平；而当半自然生境占比较高时，临界点则会出现在更高的作物多样性水平。这将使得在半自然覆被率较低的景观中更易检测到作物多样性的负向影响，而在半自然覆被率较高的景观中更易检测到正向影响。其二，我们预测：若在局地尺度开展测量，作物多样性效应由正转负的临界点会比在距样点更远的尺度下更低，因此在局地尺度开展测量时更易检测到作物多样性的负向影响。
本研究依托加拿大安大略省东部221处农田边缘的调查数据，对上述预测进行了验证，所调查的生物类群涵盖草本植物、蝴蝶、食蚜蝇、木本植物、蜂类、步甲科甲虫、蜘蛛与鸟类。研究结果支持了作物多样性与面积间的权衡关系假说。半自然覆被率与测量尺度会影响生物多样性与作物多样性之间的关系：当半自然覆被率较高时，二者呈正向关系；而当半自然覆被率较低且测量尺度为局地尺度时，二者呈负向关系。
研究结果表明，旨在提升作物多样性的政策或指南，无法在保护需求最为迫切的景观（即农业利用强度高、半自然覆被率低的景观）中对生物多样性产生增益效果。