Alternative dynamic regimes and trophic control of plant succession
Ecological succession has been the subject of intense study and debate throughout the history of ecology as conceptualizations of process were proposed and refined. Modern concepts view ecological succession as largely driven by bottom-up resource competition for light and nutrients. However, growing evidence shows that top-down effects of consumers can govern succession. These contrasting perspectives require synthesis. We offer such a synthesis by revitalizing the hypothesis that succession proceeds by abrupt transitions to alternative states or dynamic regimes. We present evidence from field sampling along two successional gradients in a New England old field aimed at identifying pattern, and from experimentation in the same field aimed at identifying process. Field sampling revealed that a competitive dominant plant existed in a mosaic with two distinct patch types of relative abundances (≤ 25% and > 30%). Competitive dominant plant abundance varied systematically with plant species diversity (evenness), and resource supply (light and soil nitrogen). The six-year field experiment tested for alternative regimes by systematic removal and staggered reintroduction of top predators. Long-term predator removal caused an abrupt and irreversible shift from a top-down to a bottom-up controlled regime with a breakpoint at approximately 25% relative abundance of the competitive dominant plant. This caused significant shifts in plant species evenness and resource supply (solar radiation and N mineralization rate). Moreover, the competitive dominant abundance, species evenness and resource supply in the two dynamic regimes matched levels in the different patch types in the field. We conclude that a single ecosystem can display both top-down and bottom-up control. Abrupt shifts in trophic control lead to abrupt changes in the rate of development of ecosystems consistent with a working hypothesis that succession proceeds via abrupt regime shifts. © 2006 Springer Science+Business Media, Inc.
Schmitz, OJ; Kalies, EL; Booth, MG
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