Geomatics and Landscape Ecology Laboratory, Department of Biology, Carleton University, 1125 Colonel By Drive, Ottawa, ON, Canada.
John Carroll University, 1 John Carroll Blvd., University Heights, OH, U.S.A.
Biol Rev Camb Philos Soc. 2022 Feb;97(1):99-114. doi: 10.1111/brv.12792. Epub 2021 Aug 28.
The legacy of the 'SL > SS principle', that a single or a few large habitat patches (SL) conserve more species than several small patches (SS), is evident in decisions to protect large patches while down-weighting small ones. However, empirical support for this principle is lacking, and most studies find either no difference or the opposite pattern (SS > SL). To resolve this dilemma, we propose a research agenda by asking, 'are there consistent, empirically demonstrated conditions leading to SL > SS?' We first review and summarize 'single large or several small' (SLOSS) theory and predictions. We found that most predictions of SL > SS assume that between-patch variation in extinction rate dominates the outcome of the extinction-colonization dynamic. This is predicted to occur when populations in separate patches are largely independent of each other due to low between-patch movements, and when species differ in minimum patch size requirements, leading to strong nestedness in species composition along the patch size gradient. However, even when between-patch variation in extinction rate dominates the outcome of the extinction-colonization dynamic, theory can predict SS > SL. This occurs if extinctions are caused by antagonistic species interactions or disturbances, leading to spreading-of-risk of landscape-scale extinction across SS. SS > SL is also predicted when variation in colonization dominates the outcome of the extinction-colonization dynamic, due to higher immigration rates for SS than SL, and larger species pools in proximity to SS than SL. Theory that considers change in species composition among patches also predicts SS > SL because of higher beta diversity across SS than SL. This results mainly from greater environmental heterogeneity in SS due to greater variation in micro-habitats within and across SS habitat patches ('across-habitat heterogeneity'), and/or more heterogeneous successional trajectories across SS than SL. Based on our review of the relevant theory, we develop the 'SLOSS cube hypothesis', where the combination of three variables - between-patch movement, the role of spreading-of-risk in landscape-scale population persistence, and across-habitat heterogeneity - predict the SLOSS outcome. We use the SLOSS cube hypothesis and existing SLOSS empirical evidence, to predict SL > SS only when all of the following are true: low between-patch movement, low importance of spreading-of-risk for landscape-scale population persistence, and low across-habitat heterogeneity. Testing this prediction will be challenging, as it will require many studies of species groups and regions where these conditions hold. Each such study would compare gamma diversity across multiple landscapes varying in number and sizes of patches. If the prediction is not generally supported across such tests, then the mechanisms leading to SL > SS are extremely rare in nature and the SL > SS principle should be abandoned.
“大而少（SL）胜过小而多（SS）”原则的影响在保护大斑块而忽视小斑块的决策中显而易见。该原则认为，一个或几个大生境斑块（SL）比几个小生境斑块（SS）能保护更多的物种。然而，该原则缺乏经验证据的支持，并且大多数研究发现两者之间没有差异，或者结果相反（SS 比 SL 更有利）。为了解决这个困境，我们提出了一个研究议程，提出了一个问题：“是否存在一致的、经验验证的条件导致 SL 比 SS 更有利？”我们首先回顾和总结了“单一大或多个小”（SLOSS）理论和预测。我们发现，“大而少（SL）胜过小而多（SS）”的大多数预测假设，斑块间灭绝率的差异决定了灭绝-扩散动态的结果。当由于斑块间的运动较少，因此各个斑块中的种群彼此之间在很大程度上相互独立，并且由于物种对最小斑块大小的要求不同，导致物种组成沿着斑块大小梯度具有强烈的嵌套性时，这种情况预计会发生。然而，即使斑块间灭绝率的差异决定了灭绝-扩散动态的结果，理论也可以预测 SS 比 SL 更有利。如果灭绝是由拮抗物种相互作用或干扰引起的，导致景观尺度的灭绝风险在 SS 中传播，就会发生这种情况。如果由于 SS 的移民率高于 SL，并且 SS 比 SL 更接近物种库，因此 SS 的扩散-殖民动态结果决定了 SS 比 SL 更有利。考虑到斑块间物种组成变化的理论也预测 SS 比 SL 更有利，因为 SS 比 SL 具有更高的 beta 多样性。这主要是由于 SS 中环境异质性更大，因为 SS 内和跨 SS 生境斑块的微生境变化更大（“跨生境异质性”），或者 SS 比 SL 具有更多异质的演替轨迹。基于我们对相关理论的回顾，我们提出了“SLOSS 立方假说”，其中三个变量 - 斑块间运动、扩散风险在景观尺度种群维持中的作用以及跨生境异质性 - 组合在一起预测 SLOSS 的结果。我们使用 SLOSS 立方假说和现有的 SLOSS 经验证据，仅当以下所有条件都成立时，才预测 SL 比 SS 更有利：斑块间运动较少，扩散风险对景观尺度种群维持的重要性较低，以及跨生境异质性较低。测试这个预测将是具有挑战性的，因为它需要对具有这些条件的物种组和区域进行许多研究。每个这样的研究将比较多个景观的 gamma 多样性，这些景观的斑块数量和大小不同。如果这个预测在这样的测试中没有得到普遍支持，那么导致 SL 比 SS 更有利的机制在自然界中就极为罕见，“大而少（SL）胜过小而多（SS）”原则应该被摒弃。
