Department of Wildland Resources and the Ecology Center, Utah State University, 5230 Old Main, Logan, Utah, USA.
Department of Biological Statistics and Computational Biology, Cornell University, 1198 Comstock Hall, Ithaca, New York, USA.
Ecology. 2018 Jul;99(7):1621-1632. doi: 10.1002/ecy.2363. Epub 2018 Jun 7.
Stable coexistence requires intraspecific limitations to be stronger than interspecific limitations. The greater the difference between intra- and interspecific limitations, the more stable the coexistence, and the weaker the competitive release any species should experience following removal of competitors. We conducted a removal experiment to test whether a previously estimated model, showing surprisingly weak interspecific competition for four dominant species in a sagebrush steppe, accurately predicts competitive release. Our treatments were (1) removal of all perennial grasses and (2) removal of the dominant shrub, Artemisia tripartita. We regressed survival, growth, and recruitment on the locations, sizes, and species identities of neighboring plants, along with an indicator variable for removal treatment. If our "baseline" regression model, which accounts for local plant-plant interactions, accurately explains the observed responses to removals, then the removal coefficient should be non-significant. For survival, the removal coefficients were never significantly different from zero, and only A. tripartita showed a (negative) response to removals at the recruitment stage. For growth, the removal treatment effect was significant and positive for two species, Poa secunda and Pseudoroegneria spicata, indicating that the baseline model underestimated interspecific competition. For all three grass species, population models based on the vital rate regressions that included removal effects projected 1.4- to 3-fold increases in equilibrium population size relative to the baseline model (no removal effects). However, we found no evidence of higher response to removal in quadrats with higher pretreatment cover of A. tripartita, or by plants experiencing higher pre-treatment crowding by A. tripartita, raising questions about the mechanisms driving the positive response to removal. While our results show the value of combining observations with a simple removal experiment, more tightly controlled experiments focused on underlying mechanisms may be required to conclusively validate or reject predictions from phenomenological models.
稳定共存需要种内限制强于种间限制。种内和种间限制之间的差异越大，共存越稳定，任何物种在去除竞争者后经历的竞争释放就越弱。我们进行了一项去除实验，以测试一个先前估计的模型，该模型显示在一个山艾灌木丛草原中，四个优势物种的种间竞争出乎意料地微弱，该模型是否准确预测竞争释放。我们的处理方法是：（1）去除所有多年生草；（2）去除优势灌木，蒿。我们将生存、生长和繁殖回归到邻接植物的位置、大小和物种身份上，以及去除处理的指示变量。如果我们的“基线”回归模型（解释局部植物-植物相互作用）准确地解释了对去除的观察响应，那么去除系数应该不显著。对于生存，去除系数从未显著不同于零，只有蒿在繁殖阶段对去除有（负）响应。对于生长，去除处理对两种物种，草地早熟禾和披碱草，有显著的正效应，表明基线模型低估了种间竞争。对于所有三种草本植物，基于包含去除效应的重要率回归的种群模型预测，与基线模型（无去除效应）相比，平衡种群大小增加了 1.4 到 3 倍。然而，我们没有发现证据表明，在预处理的蒿覆盖度较高的样方中，或者在预处理时被蒿挤得更紧的植物中，对去除的反应更高，这对驱动去除后正响应的机制提出了质疑。虽然我们的结果表明将观察与简单的去除实验相结合的价值，但可能需要更严格控制的实验，以确定或否定现象学模型的预测。
