Up or down in space? Uniting the bottom-up versus top-down paradigm and spatial ecology

Why is the World green what keeps herbivores, and herbivorous insects in particular, from consuming all of their food? When this question was first posed, the relative importance of top-down and bottom-up effects was hotly disputed. While modern ecologists may agree that impacts from several different directions will affect local insect densities, the bottom-up vs top-down jargon seems to be stuck in a unidimensional world. Here, we argue that the strength of almost every bottom-up and top-down force is likely to vary in space, and that in itself, spatial structure invokes new processes which defy classification in the traditional bottom-up top-down scheme. To understand the relative importance of different forces keeping herbivore numbers in check, we feel that we need a fresh synthesis between the novel paradigm of spatial ecology and the classical paradigms of top-down and bottom-up studies. This synthesis requires a consideration of forces beyond the standard framework of top-down vs bottom-up effects, and should be based on comparing the relative strength of such forces at several sites in a spatially explicit framework. Overall, we should switch our focus from whether the relative strength of top-down and bottom-up factors vary in space to why there is variation, how much variation there is, and at what spatial scale(s) it occurs.