Talley Theresa Sinicrope
Department of Environmental Science and Policy, University of California, One Shields Avenue, Davis, California 95616, USA.
Ecology. 2007 Jun;88(6):1476-89. doi: 10.1890/06-0555.
Patches, gradients, and hierarchies are three common organizational frameworks for assessing the effects of spatial heterogeneity on species distributions. Since these frameworks are often chosen a priori, without knowledge of study systems, they may not correspond to the empirical heterogeneity present and may result in partial or erroneous conclusions about the forces structuring species distributions. I tested the consequences of choosing particular frameworks and whether patch heterogeneity structured patchily distributed populations of the valley elderberry longhorn beetle (Desmocerus californicus dimorphus) along four rivers in California's Central Valley (USA). A comparison of the three approaches revealed that each led to incomplete conclusions about controls on the beetle's distribution and populations. Patch analysis revealed weak effects of patch size and quality, and high unexplained variance, which likely reveals large amounts of stochasticity since replication was high. The patch analysis therefore concluded that distributions consistent with patch dynamic structures like classic metapopulation, source-sink, and mainland-island models existed in the different rivers. Conversely, gradient analyses revealed a gradient-distribution pattern responding to continuous and often large-scale variables, such as host-plant age or size, water availability, and the presence of an invasive leguminous tree; again most variance in beetle occurrence remained unexplained. Hierarchical analysis identified the natural spatial patterns of the system but gave no indication of causal processes. The combination of all three approaches explained the maximum variance in beetle occurrence, through inclusion of a comprehensive list of explanatory variables, multiple spatial scales, various types of heterogeneity, and a focus on the scales at which beetle-environment interactions were strongest. Surprisingly, these results still supported the notion that the beetle exists as a metapopulation, a structure thought to be rare because it ignores habitat quality and landscape conditions. These analyses exemplify the simultaneous importance of local patch attributes and broad-scale and/or gradient variables that are commonly overlooked in patch studies. Importantly, some patch attributes acted over inter-patch scales, affecting the perception of patch distances and distributional extents. Only through the integration of frameworks was I able to decipher the system's complexity and see that all three types of heterogeneity were acting in the system, sometimes over unexpected scales.
斑块、梯度和层次结构是评估空间异质性对物种分布影响的三种常见组织框架。由于这些框架通常是在对研究系统缺乏了解的情况下预先选定的，它们可能与实际存在的异质性不相符，并可能导致关于构建物种分布的作用力的部分或错误结论。我测试了选择特定框架的后果，以及斑块异质性是否构建了美国加利福尼亚中央谷地四条河流沿岸分布零散的山谷接骨木长角甲虫（Desmocerus californicus dimorphus）种群。对这三种方法的比较表明，每种方法都得出了关于甲虫分布和种群控制的不完整结论。斑块分析显示斑块大小和质量的影响较弱，且存在大量无法解释的变异，由于重复度较高，这可能揭示了大量的随机性。因此，斑块分析得出结论，不同河流中存在与经典集合种群、源-汇和大陆-岛屿模型等斑块动态结构一致的分布。相反，梯度分析揭示了一种梯度分布模式，该模式响应连续且通常是大规模的变量，如寄主植物的年龄或大小、水可用性以及一种入侵豆科树的存在；同样，甲虫出现情况的大部分变异仍无法解释。层次分析确定了系统的自然空间模式，但未表明因果过程。所有三种方法的结合通过纳入全面的解释变量列表、多个空间尺度、各种类型的异质性以及关注甲虫与环境相互作用最强的尺度，解释了甲虫出现情况的最大变异。令人惊讶的是，这些结果仍然支持了甲虫以集合种群形式存在的观点，这种结构被认为很罕见，因为它忽略了栖息地质量和景观条件。这些分析例证了局部斑块属性以及在斑块研究中通常被忽视的广泛尺度和/或梯度变量的同时重要性。重要的是，一些斑块属性在斑块间尺度上起作用，影响对斑块距离和分布范围的认知。只有通过整合这些框架，我才能解读系统的复杂性，并看到所有三种类型的异质性都在系统中起作用，有时作用于意想不到的尺度。
