THE ASSEMBLY OF EXPERIMENTAL WETLAND PLANT-COMMUNITIES

The ability to predict the composition of communities from environment al factors is a central goal of community ecology. We carefully select ed a pool of species and subjected it to a range of environmental fact ors to determine which factors were able to filter out subsets of spec ies. We began with a pool of 20 species and sowed them into 120 wetlan d microcosms representing 24 different habitat treatments and monitore d them for 5 yr. The treatments were fertility, water depth, fluctuati ons in water depth, soil texture, leaf litter, length of the initial g rowing season, and invasion by Typha. After 5 yr 14 species persisted; no rare species survived. The experimental communities differed from random expectation and were assembled by rules that constrained their organization. There were strong and consistent effects of fertility, w ater level, and leaf litter on community composition. Community assemb ly was modeled as a series of environmental filters. Some aspects of a ssembly were deterministic: trajectories were constrained within two ' 'pathway basins'' and species rank abundances were significantly conco rdant within treatments. Other factors indicated that assembly has a s trong stochastic component: 50% of species were present only occasiona lly and we cannot accurately predict species ranks. Community stochast icity did not show any clear patterns among treatments.