The ability to accurately assess changes in species richness through time is fundamental to understanding the factors that influence community diversity. However, estimates of species richness (SR) often differ depending on the time frame of observation. At short time scales increases in SR through time are a consequence of incomplete sampling of the species pool, while at longer time scales the colonization and extinction of species (turnover) becomes important. In order to separate changes in SR produced by sampling processes from those reflective of true ecological dynamics White et al. (2004) developed a two-phased model of the species-time relationship (STR) that incorporates a short phase dominated by sampling effects followed by a longer phase reflecting species turnover.
For this study we use the two-phase STR to measure temporal turnover and changes in zooplankton SR for 15 boreal shield lakes that have been sampled continuously for 12-29 years. Zooplankton communities in 9 of these lakes have been damaged by anthropogenic acidification, while the remaining 6 are reference lakes. Our objectives were: 1) to determine the temporal resolution required to separate ecological changes from sampling effects; 2) to use the appropriate temporal resolution to calculate SR estimates and turnover rates; and 3) to compare rates of species turnover and temporal trends in SR experienced in acid-damaged lakes with those in reference lakes.
Results/Conclusions
The STR for 9 of the 15 communities was best fit by a two-phased model that incorporated a sampling phase ranging from 2-7 years (average=3.3). For the remaining 6 communities, a power model (SR=cTw, where c and w are fitted parameters and T is time) fit better than the two-phase model suggesting that census error may not have been a significant consideration when analyzing those datasets.
Temporal turnover rates, represented by the exponent (w) of the power function fitted to the ecological phase of the STR, were significantly lower in reference lakes (0.21±0.06) than in lakes recovering from acidification (0.37±0.11). Analysis of trends in SR indicated that 4 of the 9 communities recovering from acid-damage experienced significant increases in SR through time, likely contributing to the higher mean turnover rate for recovering lakes.
Overall, our results highlight the need to consider census error when analyzing changes in SR through time and suggest that the two-phase STR provides a valuable approach for reducing the influence of census error when assessing temporal patterns of SR in stressed communities.