Data from: Complex relationships between competing guilds along large-scale environmental gradients

Despite much research over the past 30 years, there is still little general understanding of how the outcomes of interactions vary along environmental gradients, particularly at large geographic scales. A simple expectation is that decreasing environmental quality should reduce densities of competitors and hence the effects of competition should weaken in poorer environments. A counterintuitive consequence is that associations between densities of competitors might change from negative to positive as environments decrease in quality. Here we test these predictions in a set of vascular plant communities where perennial species share space and resources with less competitive annuals. We surveyed nine gray dune communities annually for 5 years along a cross-European latitudinal gradient of habitat quality. We find that densities of annual and perennial species are negatively correlated at the high-quality end of the gradient, while at the low-quality end, guild densities are uncorrelated or positively correlated, consistent with a weakening of competition linked to increasing environmental limitations. Our results suggest that even simple interactions can give rise to nonobvious changes in species associations along environmental gradients. They highlight that understanding the outcome of species interactions may require explicit characterization of their changing intensity with environmental quality and that the factors limiting species’ codistribution can vary along environmental gradients.

尽管近30年来已有大量相关研究，但学界对生物间相互作用的结果如何沿环境梯度变化仍缺乏普遍认知，在大地理尺度下这一问题尤为突出。一个直观的预期是：生境质量下降会降低竞争者的种群密度，因此在环境较差的生境中，种间竞争的作用会随之减弱。而一个反直觉的结果是：随着生境质量下降，竞争者的种群密度之间的关联可能从负相关转为正相关。本文选取了多年生植物与竞争力较弱的一年生植物共享空间与资源的维管植物(vascular plant)群落，对上述预测展开验证。我们沿横跨欧洲的生境质量纬度梯度，对9个灰沙丘(gray dune)群落开展了为期5年的年度调查。结果显示：在生境质量较高的梯度区间，一年生与多年生植物的种群密度呈负相关；而在质量较低的区间，两类植物功能群的种群密度无关联或呈正相关，这与环境限制加剧导致竞争作用减弱的理论预期相符。本研究结果表明，即便简单的物种间相互作用，也可能沿环境梯度引发物种关联模式的非直观变化。研究同时强调，要理解物种相互作用的结果，需明确其作用强度随生境质量的变化情况；而限制物种共同分布的驱动因子，也会沿环境梯度发生改变。