An 11-yr exclosure experiment in a high-elevation island ecosystem: introduced herbivore impact on shrub species richness, seedling recruitment and population dynamics

Questions Do introduced herbivores and fire explain the mono-dominance of one legume shrub (Adenocarpus viscosus subsp. spartioides) above the tree line on an oceanic island, given the fact that a number of other legume shrub species are potentially present? What drives the observed landscape-scale lifedeath pattern within the mono-dominant shrub species population? Location The sub-alpine scrub vegetation of La Palma (Canary Islands, Atlantic Ocean). Methods An 11-yr exclosure experiment with sites distributed along an elevation and orientation gradient was used to identify the influence of introduced herbivore pressure on four endemic shrub species and their seedling recruitment. Further, we assessed the population dynamics and spatial pattern of the dominant shrub species A. viscosus subsp. spartioides. Habitat and vitality characteristics were investigated, assessing spatial topographic features and tree ring-based age estimates. Linear mixed models, ANOVAs, linear regression and variation partitioning were used as statistical analysis tools. Results Outside of the exclosures A. viscosus subsp. spartioides was virtually mono-dominant in the study area, even though other shrub species seem better suited in the absence of introduced herbivores. The presence of introduced herbivores significantly reduced seedling recruitment within all target species, except for A. viscosus subsp. spartioides. Mean age of A. viscosus subsp. spartioides increased with elevation, although vitality analyses indicated that the sub-alpine scrub is elevated above its growth optimum. Three out of four investigated shrub species showed differences in growth height depending on elevation and island orientation. Conclusion Introduced herbivores and fire are identified as key disturbances enhancing the occurrence of A. viscosus subsp. spartioides, a commonly less competitive species. However, Genista benehoavensis, a single island endemic shrub species, seems to be better adapted to the harsh climate conditions of the sub-alpine scrub in the absence of introduced herbivores than any other shrub species.