A positive relationship between abundance and occupancy of species (AOR) is commonly observed in ecological communities, but the mechanisms driving this pattern are elusive. Succession after disturbance is an important factor structuring many plant communities, yet little is known about how and if this may shape AORs, and if AORs change over time within and between plant communities. Do AORs change through time as a plant community recovers from disturbance? Do patterns in AOR differ between plant communities?Location:
Prince Albert National Park, Saskatchewan, Canada.Methods:
Changes in AOR were evaluated through time following prescribed burning of fescue grassland and adjacent forest transition plant communities invaded by Populus tremuloides Michx. using a 35-yr data set. Species presence data were used as a frequency measure of abundance and mean species cover was used as a relative measure for abundance. Species data used for the abundance and occupancy measures were collected in permanent sampling plots prior to burning in 1975, and after burning in 1983, 1995 and 2010.Results:
Changes in grassland AORs based on the frequency were shown through time since disturbance, as an increase in abundance relative to occupancy occurred in 1983 and 2010. There was no significant change in grassland AORs when the relative abundance data were used. The forest transition AOR based on frequency showed the 1983 post-burn AOR had lower abundance relative to occupancy, while the relative abundance measure showed the pre-burn 1975 AOR had lower abundance relative to occupancy. The removal of litter, changes in soil resources and increase in trembling aspen suckering after fire may have caused changes to plant community structure after burning, explaining differing AOR patterns in the grassland and forest transition plant communities.Conclusions:
Although exact mechanisms behind the observed changes in AOR are hard to determine, the phenomenon of change within and between communities over time since disturbance has not previously been documented. The observed differences in AOR between frequency and relative abundance indicate that measures of abundance should be chosen cautiously. Post-disturbance succession should be considered as a mechanism influencing AORs in communities impacted by disturbance.