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

Abstract

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.