Print PageMonday, August 8, 2011
Exhibit Hall 3, Austin Convention Center
Background/Question/Methods The Intermediate Disturbance Hypothesis (IDH) has been frequently invoked over the last 30 years, with its simple prediction based on a tradeoff between colonization and competition. Despite many documented patterns of species diversity that appear consistent with the IDH, few studies actually test presumed underlying mechanisms. On the rocky slopes of the mountains in southwestern New Mexico, as many as 12 common species of saxicolous lichens co-occur. As shards of rock flake off due to weathering, lichens are removed and bare rock is made available for colonization; as a result, lichen cover decreases with increasing disturbance. One hundred 1 m2 plots were censused in 1978 to quantify the diversity of these lichen communities as a function of disturbance. Additionally, ten 10 x 10 cm paired plots were established, with one plot disturbed with a chisel to remove 100% of the rock face and the other plot left undisturbed. A 32-year photographic record of the paired plots allowed us to (1) determine if tradeoffs between colonization rates and competitive ability are consistent with the IDH, and (2) use this unique dataset to observe the underlying processes involved in competition and colonization.
Results/Conclusions The 1 m2 plots revealed that lichen diversity did, in fact, result in a unimodal relationship with percent lichen cover (i.e. disturbance), consistent with the IDH. Using the long-term photographic data, we found tradeoffs between competition and colonization ability, with some species having high colonization rates into cleared plots and relatively poor competitive ability in undisturbed plots. However, the strength of the trade-off for each species is inversely correlated with patterns of overall species occurrence. We observed that the best support for the putative mechanisms behind the IDH comes from relatively rare species and propose that this may be a feature of many other disturbed communities.
