Pattern and process in wetlands of varying standing crop: The importance of scale.

Abstract

Predicting diversity is a central theme of ecology. In this thesis, two aspects of diversity were explored, number of vegetation types and species richness. In the first chapter, I examined the relationship between number of vegetation types and standing crop in wetlands. Fifteen individually homogeneous wetland sites which represented a broad standing crop gradient were sampled in eastern Canada. There appears to be an inverse relationship between number of vegetation types and standing crop. In chapter 2, I investigated the relationship between species richness and standing crop at two levels of organization: the among vegetation types level and the within vegetation type level. The results indicated that the model of species richness proposed by Grime had predictive power at a coarse-grained level of organization, among vegetation types, but did not survive the transition to a finer-grained level of organization, the within vegetation type level. Therefore, the higher level processes which structure species richness patterns among vegetation types are not the same processes that determine richness patterns within a vegetation type. Numerous forces, abiotic or biotic, can maintain or alter vegetation states and species richness at different levels of organization. In the third chapter, I tested whether these "assembly forces" had the potential to duplicate the patterns observed in chapters 1 and 2 in a large microcosm experiment. Chapter 4 investigated the effects of a disturbance involving the complete removal of above ground vegetation in five wetlands of varying standing crop found on the Ottawa River. The results indicated that the wetland standing crop gradient and the diversity patterns observed along this gradient were likely the result of an underlying natural disturbance gradient. In chapter 5, I considered the results from chapters 1 to 4 with regards to the conservation value, potential threats, and proper management techniques for wetlands, particularly for low standing crop wetlands. In addition to the fifteen wetlands surveyed in chapter 1, additional low standing crop wetland sites from the Georgian Bay area in Ontario and the Tusket River Valley in Nova Scotia were surveyed. The results revealed that low standing crop wetlands had many more nationally rare species than did high standing crop wetlands, in addition to the higher species richness and higher number of vegetation types discussed above. The results of the microcosm experiment revealed that low standing crop wetlands may be severely threatened by eutrophication and by an invasive Eurasian weed, Lythrum salicaria. In each of the five wetland sites examined in the field experiment, L. salicaria exhibited dramatic increases in abundance over the three year observation period. Further, the disturbance treatment at each site did not slow the invasion of L. salicaria, but instead increased the rate of invasion in at least one site. I conclude that low standing crop wetlands, despite their high conservation value, are severely threatened and must be protected from the effects of eutrophication and invasion by Lythrum salicaria. (Abstract shortened by UMI.)