Rapid ecosystem reorganization events, like regime shifts, can result from rapid changes in the state of the drivers or from non-linear responses to gradual changes in drivers. By their nature, rapid reorganization events are difficult to study because they are often unpredictable, restricting our ability to monitor the entirety of the transition. Experimentally induced reorganization events are needed to separate the true cause and timing of the underlying driver change from other factors that may be also changing during the same period of time and from background stochasticity. Understanding how communities respond when drivers shift between states, if these changes are reversible, and what influences how quickly a community transitions from one state to another all require a clear knowledge of when a driver shift occurred.
We used an existing long-term experiment in the Chihuahuan Desert to ask how the presence of an existing community at the time of a driver shift – in our case colonization - influences how quickly it transitions from one community state to another. Our study site consists of three manipulations of small mammals: i) open-access control plots, ii) kangaroo rat removals where only a dominant genus of rodents is excluded and all other species are allowed access, and iii) rodent removals where all rodents are removed. In March 2015, we opened the kangaroo rat and rodent removals to unfettered colonization and monitored their transition to their new control plot state.
We found that the rodent removal plots quickly became indistinguishable from the reference control plots in rodent composition, species richness, and energy use. Kangaroo rat removal plots, however, still exhibited detectable differences from control plots over a year later. Our results suggest that the existing rodent community on the kangaroo rat removals is slowing the transition to the control state, potentially through processes such as priority effects (i.e. through prior arrival, a species can retard or prevent colonization of subsequent species). Thus, the rapidity with which a community reorganizes in response to a change in the state of the system may depend on whether an established community is retained during the state change. Changes in drivers that also reduce the existing community to low abundance may trigger more rapid transitions that those that leave the existing community intact.