No abstract

A high-resolution (2-9 year sampling interval) fossil pollen record from the Galápagos Islands, which spans the last 2690 years, reveals considerable ecosystem stability. Vegetation changes associated with independently derived histories of El Niño Southern Oscillation variability provided evidence of shifts in the relative abundance of individual species rather than immigration or extinction. Droughts associated with the Medieval Climate Anomaly induced rapid ecological change that was followed by a reversion to the previous state. The paleoecological data suggested nonneutral responses to climatic forcing in this ecosystem prior to the period of human influence. Human impacts on the islands are evident in the record. A marked decline in long-term codominants of the pollen record, Alternanthera and Acalypha, produced a flora without modern analogue before 1930. Intensified animal husbandry after ca. 1930 may have induced the local extinction of Acalypha and Alternanthera. Reductions in populations of grazing animals in the 1970s and 1980s did not result in the return of the native flora, but in invasions by exotic species. After ca. 1970 the trajectory of habitat change accelerated, continuously moving the ecosystem away from the observed range of variability in the previous 2690 years toward a novel ecosystem. The last 40 years of the record also suggest unprecedented transport of lowland pollen to the uplands, consistent with intensified convection and warmer wet seasons.

An ecological system that had been resilient to climatic change for thousands of years was transformed in decades by the introduction of livestock to the Galápagos. Habitat restoration at El Junco, San Cristobal Island, has included the exclusion of cattle, removal of exotic species, and replanting with the endemic shrub, Miconia robinsoniana. M. robinsoniana was known to form a characteristic zone of near mono‐dominant shrub cover at equivalent elevations on other islands. Analysis of modern and fossil pollen data from El Junco Crater Lake showed that two of the most common shrubs of the San Cristóbal uplands, Acalypha and Alternanthera, were the most common pollen types for more than two thousand years. In the 1930s, 30 years before the first scientific description of that habitat, the abundance of both taxa declined rapidly as livestock densities increased. The restoration target, based on the assumption that mono‐dominance by M. robinsoniana is the natural state, reflects a shifted baseline. Acalypha and Alternanthera should be considered for inclusion within future restoration efforts.