COMMUNITY ECOLOGY - ORIGINAL PAPER Productivity, herbivory, and species traits rather than diversity influence invasibility of experimental phytoplankton communities Erik Sperfeld • Andrea Schmidtke • Ursula Gaedke • Guntram Weithoff Received: 10 July 2009 / Accepted: 16 February 2010 / Published online: 7 March 2010 Ó Springer-Verlag 2010 Abstract Biological invasions are a major threat to nat- ural biodiversity; hence, understanding the mechanisms underlying invasibility (i.e., the susceptibility of a com- munity to invasions by new species) is crucial. Invasibility of a resident community may be affected by a complex but hitherto hardly understood interplay of (1) productivity of the habitat, (2) diversity, (3) herbivory, and (4) the char- acteristics of both invasive and resident species. Using experimental phytoplankton microcosms, we investigated the effect of nutrient supply and species diversity on the invasibility of resident communities for two functionally different invaders in the presence or absence of an herbi- vore. With increasing nutrient supply, increased herbivore abundance indicated enhanced phytoplankton biomass production, and the invasion success of both invaders showed a unimodal pattern. At low nutrient supply (i.e., low influence of herbivory), the invasibility depended mainly on the competitive abilities of the invaders, whereas at high nutrient supply, the susceptibility to herbivory dominated. This resulted in different optimum nutrient levels for invasion success of the two species due to their individual functional traits. To test the effect of diversity on invasibility, a species richness gradient was generated by random selection from a resident species pool at an intermediate nutrient level. Invasibility was not affected by species richness; instead, it was driven by the functional traits of the resident and/or invasive species mediated by herbivore density. Overall, herbivory was the driving factor for invasibility of phytoplankton communities, which implies that other factors affecting the intensity of her- bivory (e.g., productivity or edibility of primary producers) indirectly influence invasions. Keywords Invasion  Biodiversity  Experimental microcosms  Community ecology  Cylindrospermopsis raciborskii Introduction Biological invasions are an important aspect of human- caused global environmental change (Vitousek et al. 1997). Invasive species are among the important threats on bio- diversity (Millennium Ecosystem Assessment 2005) with potentially high negative impacts, both environmentally and economically (Mack et al. 2000). Communities differ in their susceptibility to invasions for reasons which have been intensely investigated during the past decade (Levine et al. 2004; Fridley et al. 2007). Understanding the underlying mechanisms is crucial for the development of strategies to control invasions (Huston 2004), particularly for lakes, where the greatest impact of introduced species on diversity is expected (Sala et al. 2000). Three main factors have been described to influence invasions of species: (1) the number of propagules arriving Communicated by Ulrich Sommer. Electronic supplementary material The online version of this article (doi:10.1007/s00442-010-1594-4) contains supplementary material, which is available to authorized users. E. Sperfeld (&)  A. Schmidtke  U. Gaedke  G. Weithoff Department of Ecology and Ecosystem Modeling, Institute of Biochemistry and Biology, University of Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany e-mail: eriksperfeld@googlemail.com Present Address: A. Schmidtke Department of Applied Plant Ecology, Biocenter Klein Flottbek, University of Hamburg, Ohnhorststr. 18, 22609 Hamburg, Germany 123 Oecologia (2010) 163:997–1010 DOI 10.1007/s00442-010-1594-4