COS 128-4
Community dynamics along a disturbance gradient
The complexity of vegetation dynamics resides in species-specific responses, historical effects, propagule pressure, and change to the disturbance regime. Recent theory support a complex relationship among biotic and abiotic filters, relative fitness differences and stabilizing niche differences (i.e., interactions) that determine community assembly. However, it is also possible that groups of organisms may respond similarly due to their similar evolved strategies for particular disturbance regimes. Such knowledge is imperative for understanding vegetation shifts in response to climate and nested vegetation dynamics. If communities are a result of such convergent evolution, then several members of that community are likely to respond to changes in a similar manner. To examine this possibility, we used the flooding disturbance gradient of the Mississippi River, documenting communities along the elevational gradient of riverine islands and shifts in those communities between a low flood and a high flood year. We hypothesized that increased magnitude of disturbance would lead to a shift of communities along the gradient (communities could be forecasted to shift in response to the disturbance gradient). A Monte Carlo permutation test compared dissimilarity values among elevations by randomly permuting the elevations 5,000 times, recalculating the dissimilarity values and comparing to the actual dissimilarly value.
Results/Conclusions
A total of 83 species were found between the two years on the five islands, but just a few dominated. Our hypothesis was partially supported. Both frequency and duration of flood disturbance shifted 1m (at lower elevations) to 3m (elevations > 3) up the elevation gradient from 2007 to 2008. All three dominant species on both sides of the island shifted their peak distribution up 1m to 2m in elevation; 2007 low elevation plots were less similar than expected by chance to 2008 adjacent and nonadjacent elevation plots and upper elevations were more similar, especially 1-2 m up the gradient. Species richness shifted up the disturbance gradient at least 2 m. The latter suggest a shift of the community up the disturbance gradient that mirrors the shift in the disturbance gradient itself. While species are known to respond individualistically to biotic and abiotic cues, assemblages may also be found together repeatedly under similar conditions (i.e., communities), and may respond as an ‘assemblage’, albeit not really functional groups. We do not support reinvigorating a Clementsian view, but suggest vegetation dynamics in light of changes to disturbance regimes may be predicted at the community level as well as the species level.