Srivastava D S, Trzcinski M K, Richardson B A, Gilbert B
Department of Zoology, University of British Columbia, 6270 University Boulevard, Vancouver, British Columbia V6T 1Z4, Canada.
Am Nat. 2008 Dec;172(6):761-71. doi: 10.1086/592868.
Ecologists have hypothesized that the exponent of species-area power functions (z value) should increase with trophic level. The main explanation for this pattern has been that specialist predators require prior colonization of a patch by their prey, resulting in a compounding of the effects of area up trophic levels. We propose two novel explanations, neither of which assumes trophic coupling between species. First, sampling effects can result in different z values if the abundances of species differ (in mean or evenness) between trophic levels. Second, when body size increases between trophic levels, effects of body size on z values may appear as differences between trophic levels. We test these alternative explanations using invertebrate food webs in 280 bromeliads from three countries. The z value of predators was higher than that of prey. Much of the difference in z values could be explained by sampling effects but not by body size effects. When damselflies occurred in the species pool, predator z values were even higher than predicted, as damselflies avoid small, drought-prone bromeliads. In one habitat, dwarf forests, detrital biomass became decoupled from bromeliad size, which also caused large trophic differences in z values. We argue that there are often simpler explanations than trophic coupling to explain differences in z values between trophic levels.
生态学家们推测，物种 - 面积幂函数的指数（z值）应随营养级升高而增加。对这种模式的主要解释是，特化捕食者需要其猎物先在斑块中定殖，从而导致面积效应在营养级上的叠加。我们提出了两种新的解释，这两种解释均不假定物种间存在营养级联。其一，如果不同营养级的物种丰度（均值或均匀度）存在差异，抽样效应会导致不同的z值。其二，当营养级间的体型增大时，体型对z值的影响可能表现为营养级间的差异。我们利用来自三个国家的280个凤梨科植物中的无脊椎动物食物网对这些替代解释进行了检验。捕食者的z值高于猎物的z值。z值的大部分差异可以用抽样效应来解释，而不能用体型效应来解释。当豆娘出现在物种库中时，捕食者的z值甚至高于预期，因为豆娘会避开小型、易干旱的凤梨科植物。在一个生境，即矮林中，碎屑生物量与凤梨科植物大小脱钩，这也导致了z值在营养级上的巨大差异。我们认为，通常有比重级联更简单的解释来阐释营养级间z值的差异。
