Additive effects of exotic plant abundance and land-use intensity on plant–pollinator interactions
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The continuing spread of exotic plants and increasing human land-use are two major drivers of global change threatening ecosystems, species and their interactions. Separate effects of these two drivers on plant–pollinator interactions have been thoroughly studied, but we still lack an understanding of combined and potential interactive effects. In a subtropical South African landscape, we studied 17 plant–pollinator networks along two gradients of relative abundance of exotics and land-use intensity. In general, pollinator visitation rates were lower on exotic plants than on native ones. Surprisingly, while visitation rates on native plants increased with relative abundance of exotics and land-use intensity, pollinator visitation on exotic plants decreased along the same gradients. There was a decrease in the specialization of plants on pollinators and vice versa with both drivers, regardless of plant origin. Decreases in pollinator specialization thereby seemed to be mediated by a species turnover towards habitat generalists. However, contrary to expectations, we detected no interactive effects between the two drivers. Our results suggest that exotic plants and land-use promote generalist plants and pollinators, while negatively affecting specialized plant–pollinator interactions. Weak integration and high specialization of exotic plants may have prevented interactive effects between exotic plants and land-use. Still, the additive effects of exotic plants and land-use on specialized plant–pollinator interactions would have been overlooked in a single-factor study. We therefore highlight the need to consider multiple drivers of global change in ecological research and conservation management.
KeywordsGlobal change Alien plants Agricultural intensification Pollination Specialization
We thank Ezemvelo KZN Wildlife for permission to work within the Oribi Gorge Nature Reserve, and all South African farmers who granted us access to their land. We are grateful to S.-L. Steenhuisen and S. Johnson for providing advice and field equipment, and to H. and M. Neethling, P. Pillay and F. Voigt for their manifold support. We thank J. Albrecht for valuable discussions on the statistical analyses, and R. Brandl, K. Fiedler and two anonymous reviewers for insightful comments that substantially improved the manuscript. Funding was provided by the Robert Bosch Stiftung. Field work complied with the current laws of the Republic of South Africa.
- Baayen RH (2011) languageR: data sets and functions with “Analyzing linguistic data: a practical introduction to statistics”. R package version 1.4Google Scholar
- Bates D, Maechler M, Bolker B (2012) lme4: linear mixed-effects models using S4 classes: R package version 0.999999-0. Available at: http://cran.R-project.org/package=lme4.
- Boon R (2010) Pooley’s trees of eastern South Africa, 2nd edn. Flora and Fauna Publications Trust, DurbanGoogle Scholar
- Cooper KH (1985) The conservation status of indigenous forests in Transvaal, Natal and OFS, South Africa. Wildlife Society of Southern Africa, DurbanGoogle Scholar
- Dray S, Legendre P, Blanchet FG (2011) packfor: forward selection with permutation (Canoco p.46). R package version 0.0-8/r100. Available at: http://R-Forge.R-project.org/projects/sedar
- Ghazoul J (2004) Alien abduction: disruption of native plant–pollinator interactions by invasive species. Biotropica 36:156–164Google Scholar
- Henderson L (2007) Invasive, naturalized and casual alien plants in southern Africa: a summary based on the Southern African Plant Invaders Atlas (SAPIA). Bothalia 37:215–248Google Scholar
- Oksanen J, Blanchet FG, Kindt R, Legendre P, Minchin PR, O’Hara RB, Simpson GL, Solymos P, Stevens MHH, Wagner H (2011) vegan: community ecology package. R package version 2.0–2. Available at: http://vegan.r-forge.r-project.org/
- Picker M, Griffiths CL, Weaving A (2004) Field guide to insects of South Africa, 2nd edn. Struik, Cape TownGoogle Scholar
- Pooley E (1998) A field guide to wild flowers: KwaZulu-Natal and the eastern region, 1st edn. Natal Flora Publications Trust, DurbanGoogle Scholar
- Potts SG, Biesmeijer JC, Kremen C, Neumann P, Schweiger O, Kunin WE (2010) Global pollinator declines: trends, impacts and drivers. Trends Ecol Evol 25:345–353Google Scholar
- R Development Core Team (2012) R: a language and environment for statistical computing. R Foundation for Statistical Computing, ViennaGoogle Scholar
- Sala OE, Chapin FS, Armesto JJ, Berlow E, Bloomfield J, Dirzo R,Huber-Sanwald E, Huenneke LF, Jackson RB, Kinzig A,Leemans R, Lodge DM, Mooney HA, Oesterheld M, Poff NL,Sykes MT, Walker BH, Walker M, Wall DH (2000) Global biodiversity scenarios for the year 2100. Science 287:1770–1774Google Scholar
- Scholtz C, Holm E (2008) Insects of southern Africa, 2nd edn. Protea Boekhuis, PretoriaGoogle Scholar
- von Maltitz G (2003) Classification system for South African indigenous forests: an objective classification for the Department of Water Affairs and Forestry. Environmentek report ENV-P-C 2003-017, CSIR, PretoriaGoogle Scholar
- Ezemvelo KZN Wildlife (2011) KwaZulu-Natal land cover 2008 V1.1. Unpublished GIS coverage. Biodiversity Conservation Planning Division, Ezemvelo KZN Wildlife, Cascades, PietermaritzburgGoogle Scholar
- Woodhall S (2005) Field guide to butterflies of South Africa, 1st edn. Struik, Cape TownGoogle Scholar