Long‐term experimental loss of foundation species: consequences for dynamics at ecotones across heterogeneous landscapes

Biome transition zones where species co‐exist near their geographic limits are expected to be among the most responsive to changes in climate that result in mortality of foundation species. Long‐term (>13 years) patterns in dominance and cover of functional types were examined following the annual experimental removal of one of three foundation species at an arid‐semiarid biome transition zone. Objectives were to identify key processes influencing these patterns, and to predict future landscape‐scale dynamics following mortality of different foundation species. Aboveground biomass of dominant species was removed from 3 × 4 m plots. Cover by species on control and removal plots was estimated annually in communities with a single dominant species, and in ecotonal communities with two dominant species across two types of ecotones (semiarid grassland‐arid grassland, arid grassland‐arid shrubland). For most species and functional types, trends in cover through time were related to the remaining species assemblages rather than to variable climatic conditions. In ecotonal communities, resident co‐dominant species increased to dominance following the loss of a foundation species with little change in community composition. In mono‐dominated communities, dominant species loss resulted in a shift to species that are currently sub‐dominants of the same functional type as the species that was removed. Subdominant perennial grass and shrub species increased rapidly following removals in most grassland locations. Annuals and perennial forbs had variable cover values through time that were not clearly related to pulses in precipitation and were not synchronized across locations. The most common response regardless of the identity of the foundation species removed or the community type was an increase in cover by sub‐dominant grasses and by the arid grass species ( Bouteloua eriopoda ). The rate of increase by B. eriopoda depended on initial cover, and resulted from both vegetative growth and seedling recruitment from other locations. Seed dispersal and seedling recruitment from other locations were not found for the semiarid grass, Bouteloua gracilis or the arid shrub, Larrea tridentata . Because B. eriopoda is more susceptible to shrub invasion, drought, and livestock grazing than the other two foundation species, widespread loss of either of the other species and subsequent increase in B. eriopoda is expected to result in an increase in landscape‐scale vulnerability to these environmental drivers. The extent and magnitude of this change in vulnerability will depend on the ability of B. eriopoda to respond based on the micro‐environmental conditions of the site regardless of which species ( B. gracilis , L. tridentata ) is lost.