Assisted colonization: how far is too far? Results of a futuristic transplant garden experiment

Background/Question/Methods

Success of assisted colonization efforts will depend, in part, on the degree to which movement of species is synchronized with climate change.  Migrating the leading edge of a species distribution in advance of climate change effects could be useful where rate of change is predicted to rapidly increase.   Along the northern Gulf of Mexico, sea level is increasing and the rate of rise is projected to increase.  Species limited by dispersal and establishment sites are likely to become locally extirpated under rapid environmental change.  Those species occupying the seaward end of coastal gradients may be capable of establishing landward with the removal of biological filters, consistent with natural hurricane disturbance.  I hypothesized that species from seaward assemblages could colonize more inland sites if dispersal and competition filters were eliminated.  In August 2011, I initiated a reciprocal, futuristic transplant experiment.  A series of 4 m² plots (n=135) was established that spanned coastal assemblages across the East River estuary in northwestern Florida.  Existing vegetation was removed (treatment) or left undisturbed (control).  Individuals of the dominant graminoids representing the coastal gradient from salt marsh to upland pine savanna assemblages (Spartina alterniflora, Juncus roemerianus, Cladium mariscus, and Aristida stricta) were harvested.  Each plot was subdivided into four 1m² subplots, which were randomly assigned to a species.  Ten individuals of the assigned species were planted into each of the subplots (total plants = 5400).  Initial survival and plant condition were assessed in February 2012.          

Results/Conclusions

All species successfully established in plots located in their source assemblages.  At seaward plots, the upland grass A. stricta was the only species that did not survive and the fresh marsh dominant C. mariscus had low survival, suggesting that abiotic stress limited these salt intolerant species.  In all other parts of the coastal transition, the four species successfully established and were surviving.  The most dramatic colonization result was that of S. alterniflora, the salt marsh dominant, which survived in all freshwater wetland and upland habitats and appeared quite healthy.   Although I expected lower overall survival in control plots, results indicated that initial survival of several species was enhanced in some cases, possibly due to protection from herbivory.  These results suggest that assisted colonization of downslope species into assemblages much farther inland and upslope of their original source populations is feasible when biological filters are relaxed.