Factors controlling plant diversity in marshes

E. Geho and P. Keddy

Project Overview

            Because the chosen marshes around Lake Maurepas are anthropogenic in origin, they likely originated rapidly (over a few decades, probably from nearby seeds or as seeds dispersed by logging activities).  Thus, the few founders of these plant communities may have given rise to the existing marshes.  From this perspective, the low diversity of these marshes may be simply a founder effect, maintained by limited dispersal.  Further, the few founder species may have rapidly filled the clearings and provided a closed canopy within a few years. A closed canopy would further reduce opportunities for successful establishment by newly arrived propagules. 

The experimental introduction of plant species can test dispersal as a limiting factor, and explore the relative importance of specified factors as controls on plant distribution.  This large transplant experiment was conducted to measure the relative importance of

• competition,
• herbivory (mammalian herbivores, primarily nutria), and
• sedimentation

in controlling the number of species in the marsh. 

Sixteen species were introduced to these marshes (12 herbaceous species; Acorus calamus, Cladium mariscoides, Eleocharis sp., Juncus effusus, Panicum hemitomon, Peltandra virginica, Pontederia cordata, Rhynchospora corniculata, Rhynshospora inundata, Saururus cernuus, Schoenoplectus americanus, Typha domingensis and four woody species Acer rubrum, Cephalanthus occidentalis, Nyssa aquatica, Taxodium distichum).   The exclusion of herbivores resulted in significant biomass increases (2 to 26 times) for Typha domingensis and Taxodium distichum.  Removal of competition from neighbors resulted in significant biomass increases (2 to 10 times) for five species: Acorus calamus, Cephalanthus occidentalis, Panicum hemitomon, Pontederia cordata, and Rhynchospora corniculata. 

In summary, the effects of competition from established plants may be preventing establishment after dispersal.  Further, competition is apparently more important than herbivory, and, at least in the short term, added sediment, like that from a freshwater diversion, is unlikely to influence the number of species found.  Despite studies supporting the prevalence of flooding and/or salinity at reducing germination, recruitment and survival, the increase in elevation created by the sediment addition, which would ameliorate flooding and salinity pressures, did not increase plant diversity over the short termLonger-term experiments are needed to properly assess this hypothesis.

Citation: Geho, Ellen, D. Campbell and P. Keddy. 2007. Quantifying ecological filters: the relative impact of herbivory, neighbours, and sediment on an oligohaline marsh. Oikos 11 6r 1006-1016.