Wednesday, August 4, 2010 - 3:25 PM

SYMP 14-7: A Connell legacy of paradigm shifts in community ecology: what are the most important determinants of local biodiversity?

W. J. Platt and Kyle E. Harms. Louisiana State University


In 1978, Joe Connell proposed his “Intermediate Disturbance Hypothesis”, providing uniformitarian explanation for local changes in biodiversity over time as a function of rates of disturbance. The IDH was based on extensive empirical data regarding changes in species composition over time in communities of sessile organisms, together with life history concepts that indigenous species in any landscape are adapted for some range of environmental conditions that change over time following catastrophic disruption. A key component was condensation of empirical data from many studies into basic life histories that could be envisioned for any landscape (early species adapted for conditions occurring shortly after disturbance and late species adapted for conditions occurring some time after disturbance). The IDH catalyzed development of new concepts for community ecology. Over subsequent decades, hypotheses have emerged that involve: 1) recurrent non-equilibrium conditions rather than extended equilibrium punctuated by periodic disturbances, 2) disturbance, rather than interactions among species, being postulated to drive biodiversity in non-static landscapes, 3) disturbances as agents structuring ecological systems rather than specialization related to food webs, energetics, key species, etc., and 4) life histories originating from tradeoffs in colonization and persistence on sites rather than niche specialization relative to finite, invariant limiting resources.

Results/Conclusions These alternative concepts have stimulated critical examination of premises that once formed the basis of community ecology and development of new hypotheses that relate disturbance to biodiversity. The IDH is based in assumptions that disturbances are catastrophic, leaving no survivors. Post-IDH studies of a variety of natural disturbances have revealed that virtually all disturbances are non-catastrophic, leaving some survivors. These disturbances vary spatially and temporally in the types and magnitudes of effects, and they typically produce a wide variety of post-disturbance environments and opportunities. Moreover, they affect indigenous species differently. In addition, recurrent disturbances with predictable characteristics act as selection pressures, influencing life histories of indigenous species. Hence, classic species replacement of early by late species is but one of a multitude of responses that follow natural disturbances. Recognition of this variation in responses has contributed to new models relating biodiversity to effects and consequences of disturbances (e.g., survival of disturbances and responses to post-disturbance environments, extent of dispersal limitation, functional reassembly) rather than to niche differences among species. The disturbance-local biodiversity connection initiated by Joe Connell’s IDH thus has produced a major paradigm shift in community ecology over the past several decades regarding the dynamic nature of ecological systems.