Evidence that patch size affects productivity and invasion in grassland ecosystems

Background/Question/Methods

The influence that species have on ecosystem functions depends on their spatial extent. For plants the spatial arrangement of species greatly determine the balance of inter-/intraspecific competition, and highly influences the degree of complementarity effects. Ultimately, this can affect productivity, resistance to invasion, and biodiversity.  In 2010 at the Guelph Turfgrass Institute, 20 4x4m experimental plots were planted with 5 treatment levels. Sixteen selected species were planted in a total area of 1x1m per plot. Species selection was based on the typical functional diversity of tallgrass prairies, and availability for restoration projects. For each species, the 1x1m total area was divided into the following treatment levels: 1-1x1 m patch, 4-0.5x0.5 m patches, 16-0.25x0.25 m patches, or 32-0.125x0.125 m patches, and species locations were randomly assigned. The fifth treatment level was a uniformly mixed plot of the sixteen species.

Results/Conclusions

Positive biodiversity effects were evaluated by calculating overyielding, which occurs when a polycuture of species produces significantly more biomass than expected based on average monoculture yields of component species. Resistance to invasion was evaluated by comparing proportions of invader biomass to planted species biomass. Overyielding and resistance to invasion were greatest in plots with the two smallest planted patch sizes (0.25x0.25 m and 0.125x0.125 m) for both 2010 and 2011, with no effect of year. The uniformly mixed plots had higher aggregation of more dominant species, indicating greater inter- to intraspecific competition at very fine patch size. Smaller patch size plots may experience greater functional facilitation and resource exploitation between species, which could explain the increase in ecosystem functions. These results are not only significant to experimental ecology, but restoration efforts as well.