Why don't we know more about ecological succession?

Background/Question/Methods Succession is a foundation concept in ecology that describes the changes in species composition through time in many different communities.  Despite a century of research documenting and attempting to explain such changes in communities through time, we argue that ecology lacks an accepted general theory of succession.  Further, any theory of succession would be difficult to test given the spatial and temporal scale required to follow well-replicated complete successional sequences in most communities.  Despite the lack of a general theory, are there general predictions that arise from previous successional studies? Most work assumes that succession progresses towards a stable climax state, so one might predict an increase in stability through succession.  Early colonizing species gradually give way to more competitive species—a balance between colonization and competitive exclusion should result in a mid-successional peak in diversity.  To test these predictions, we followed succession in replicate natural aquatic microcosm communities found within leaves of carnivorous pitcher plants (Sarracenia purpurea).  These container communities persist for up to 18 months, but allow us to follow succession for the complete lifetime of the community. 

Results/Conclusions Contrary to our expectations, community stability generally decreased through time.  This decrease in stability was largely due to a decline in the strength of canalizing top-down and bottom-up forces through succession.  Predation by mosquito larvae and resource-input (insects captured by the leaf) are highest in young leaves, but then decline, leaving communities subject to stochastic processes.  We did not detect a unimodal peak in diversity, but rather found a continuous change in diversity through succession.  However, the direction of the diversity change was dependent on trophic level.  Bacterial diversity decreased slowly through time, but consumer diversity increased throughout succession. These communities are similar to other detritus-based communities and the successional dynamics observed here may provide predictions about community structure in those communities.  More broadly though, the strengths of canalizing forces are likely to vary during succession in many other communities.  Understanding the balance between canalizing and stochastic forces during succession may reveal more similarities among different ecosystems and lead to a more general theory of succession.