Aquatic Toxicology 106–107 (2012) 25–31 Contents lists available at SciVerse ScienceDirect Aquatic Toxicology jou rn al h om epa ge: www.elsevier.com/locate/aquatox Competition increases toxicant sensitivity and delays the recovery of two interacting populations Kaarina Foit ∗ , Oliver Kaske, Matthias Liess Helmholtz Centre for Environmental Research – UFZ, Department of System Ecotoxicology, Permoserstrasse 15, D-04318 Leipzig, Germany a r t i c l e i n f o Article history: Received 14 February 2011 Received in revised form 16 September 2011 Accepted 20 September 2011 Keywords: Indirect effect Intraspecific competition Interspecific competition Community Daphnia magna Culex pipiens a b s t r a c t We investigated how persistent competitive pressure alters toxicant sensitivity and recovery from a pesticide pulse at community level. Interacting populations of Daphnia (Daphnia magna) and Culex lar- vae (Culex pipiens molestus) were pulse-exposed (48 h) to the pyrethroid fenvalerate. The abundance and biomass of the populations were monitored by non-invasive image analysis. Shortly after exposure, Daphnia showed a concentration–response relationship with the toxicant with an LC 50 of 0.9 g/L. Culex larvae were slightly less sensitive with an LC 50 of 1.7 g/L. For both species, toxicant sensitivity increased with the population biomass of the respective species before exposure, which is explained by intraspe- cific competition. Several weeks after exposure to the highest treatment concentration of 1 g/L, the slight differences in sensitivity between the two species were amplified to contrasting long-term effects due to interspecific competition: high interspecific competition impaired the recovery of Daphnia. Sub- sequently, Culex larvae profited from the slow recovery of Daphnia and showed an increased success of emergence. We conclude that, in natural systems where competition is present, such competitive pro- cesses might prolong the recovery of the community structure. Hence, natural communities might be disturbed for a longer period by toxic exposure than predicted from single-species tests alone. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Risk assessment of toxicants is often based on single-species tests under optimal laboratory conditions for growth and repro- duction. In contrast, natural conditions are often non-optimal and the actual sensitivity and recovery of species might be substantially different (Heugens et al., 2001). As a consequence, the extrapolation of toxicity data from the individual to the ecosystem level involves uncertainties that are usually compensated for by safety factors (European Commission, 2003). These uncertainties can be reduced by the identification of general patterns of context-dependent sen- sitivity and recovery of species after exposure to toxic stress. At the individual level, the toxicant sensitivity of a species is known to be affected by various abiotic stressors. For example, acute sensitivity might depend on temperature (Song et al., 1997), salinity (Wildgust and Jones, 1998), oxygen level (Van der Geest et al., 2002), UV radiation (Liess et al., 2001), and the acquisition of food (Mommaerts et al., 2010). At the population and community levels, species also experience biological interactions. One important biological interaction that affects the coexistence of species is competition (Gordon, 2000). ∗ Corresponding author. Tel.: +49 341 235 1494. E-mail addresses: kaarina.foit@ufz.de (K. Foit), oliver.kaske@ufz.de (O. Kaske), matthias.liess@ufz.de (M. Liess). Toxicant-induced mortality is frequently observed to reduce com- petitive pressure, which has positive effects on the subsequent recovery of affected species. At the population level, disturbed species may benefit from the release from intraspecific competi- tion and recover rapidly (Beketov and Liess, 2005; Forbes et al., 2003; Liess, 2002; Linke-Gamenick et al., 1999; Moe et al., 2002; Muturi et al., 2010; Postma et al., 1994). At the community level, species may benefit from the release from interspecific compe- tition that is caused by the mortality of more sensitive species (Hanazato, 1998; Johnston and Keough, 2003; Kesavaraju et al., 2010; Relyea, 2009). However, competition at the community level might also persist at a high level, especially when some dominant species are not affected by the toxicant. To the authors’ knowledge, the process of species recovery in the context of persistently high competitive pressure has been shown only rarely, in two cases of phytoplankton communities (Fisher et al., 1974; Wang et al., 2011), and has not been investigated for heterotrophic species such as invertebrates or insects. In the present study, we investigated the effects of, and recovery from, a pesticide pulse in a simple model community with two competing species. We used Nanocosm, a test system composed of populations of the cladoceran Daphnia (Daphnia magna) and the mosquito Culex (Culex pipiens molestus). The Nanocosm system focuses on one important interaction of the field, namely competition. Both species are primarily filter feed- ers with a strong overlap of natural diets (DeMott, 1982; Merritt et al., 1992). Hence, Daphnia and Culex larvae compete for the same 0166-445X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aquatox.2011.09.012