Department of Zoology, University of British Columbia, Vancouver, BC, Canada.
Applied Freshwater Ecology Research Unit, British Columbia Ministry of the Environment, Vancouver, BC, Canada.
J Anim Ecol. 2018 Sep;87(5):1383-1394. doi: 10.1111/1365-2656.12845. Epub 2018 May 29.
Increasing habitat availability (i.e. habitat suitable for occupancy) is often assumed to elevate the abundance or production of mobile consumers; however, this relationship is often nonlinear (threshold or unimodal). Identifying the mechanisms underlying these nonlinearities is essential for predicting the ecological impacts of habitat change, yet the functional forms and ultimate causation of consumer-habitat relationships are often poorly understood. Nonlinear effects of habitat on animal abundance may manifest through physical constraints on foraging that restrict consumers from accessing their resources. Subsequent spatial incongruence between consumers and resources should lead to unimodal or saturating effects of habitat availability on consumer production if increasing the area of habitat suitable for consumer occupancy comes at the expense of habitats that generate resources. However, the shape of this relationship could be sensitive to cross-ecosystem prey subsidies, which may be unrelated to recipient habitat structure and result in more linear habitat effects on consumer production. We investigated habitat-production relationships for juveniles of stream-rearing Pacific salmon and trout (Oncorhynchus spp.), which typically forage in low-velocity pool habitats, while their prey (drifting benthic invertebrates) are produced upstream in high-velocity riffles. However, juvenile salmonids also consume subsidies of terrestrial invertebrates that may be independent of pool-riffle structure. We measured salmonid biomass production in 13 experimental enclosures each containing a downstream pool and upstream riffle, spanning a gradient of relative pool area (14%-80% pool). Increasing pool relative to riffle habitat area decreased prey abundance, leading to a nonlinear saturating effect on fish production. We then used bioenergetics model simulations to examine how the relationship between pool area and salmonid biomass is affected by varying levels of terrestrial subsidy. Simulations indicated that increasing terrestrial prey inputs linearized the effect of habitat availability on salmonid biomass, while decreasing terrestrial inputs exaggerated a "hump-shaped" effect. Our results imply that nonlinear effects of habitat availability on consumer production can arise from trade-offs between habitat suitable for consumer occupancy and habitat that generates prey. However, cross-ecosystem prey subsidies can effectively decouple this trade-off and modify consumer-habitat relationships in recipient systems.
增加栖息地的可利用性（即适合占据的栖息地）通常被认为会提高移动消费者的丰度或产量；然而，这种关系往往是非线性的（阈值或单峰）。确定这些非线性背后的机制对于预测栖息地变化的生态影响至关重要，但消费者-栖息地关系的功能形式和最终原因往往理解不足。栖息地对动物丰度的非线性影响可能表现为觅食的物理限制，限制消费者获取资源。如果增加适合消费者占据的栖息地面积以牺牲产生资源的栖息地为代价，那么消费者和资源之间随后的空间不匹配应该导致栖息地可用性对消费者生产的单峰或饱和效应。然而，这种关系的形状可能对跨生态系统猎物补贴敏感，这些补贴可能与受体栖息地结构无关，并导致消费者生产对栖息地的影响更加线性。我们调查了溪流养殖的太平洋鲑鱼和鳟鱼（Oncorhynchus spp.）幼鱼的栖息地-产量关系，这些幼鱼通常在低流速的水池栖息地觅食，而它们的猎物（漂流的底栖无脊椎动物）则在流速较高的急流上游产生。然而，幼年鲑鱼也会消耗与水池-急流结构无关的陆地无脊椎动物的补贴。我们在 13 个实验围栏中测量了鲑鱼的生物量产量，每个围栏都包含一个下游水池和一个上游急流，范围涵盖相对水池面积（14%-80%水池）的梯度。与急流栖息地相比，增加水池的相对面积会降低猎物的丰度，从而导致鱼类产量的非线性饱和效应。然后，我们使用生物能量模型模拟来研究水池面积与鲑鱼生物量之间的关系如何受到陆地补贴水平的变化的影响。模拟表明，增加陆地猎物输入使栖息地可利用性对鲑鱼生物量的影响线性化，而减少陆地输入则夸大了“驼峰状”效应。我们的结果表明，消费者生产的栖息地可利用性的非线性效应可能来自于适合消费者占据的栖息地和产生猎物的栖息地之间的权衡。然而，跨生态系统的猎物补贴可以有效地消除这种权衡，并改变受体系统中的消费者-栖息地关系。
