Ecology, 89(10), 2008, pp. 2760–2769 Ó 2008 by the Ecological Society of America ALLEE EFFECT LIMITS COLONIZATION SUCCESS OF SEXUALLY REPRODUCING ZOOPLANKTON ANDREW M. KRAMER, 1,3 ORLANDO SARNELLE, 1 AND ROLAND A. KNAPP 2 1 Department of Fisheries and Wildlife, Michigan State University, East Lansing, Michigan 48824 USA 2 Sierra Nevada Aquatic Research Laboratory, University of California, Mammoth Lakes, California 93546 USA Abstract. Understanding the dynamics of populations at low density and the role of Allee effects is a priority due to concern about the decline of rare species and interest in colonization/invasion dynamics. Despite well-developed theory and observational support, experimental examinations of the Allee effect in natural systems are rare, partly because of logistical difficulties associated with experiments at low population density. We took advantage of fish introduction and removal in alpine lakes to experimentally test for the Allee effect at the whole-ecosystem scale. The large copepod Hesperodiaptomus shoshone is often extirpated from the water column by fish and sometimes fails to recover following fish disappearance, despite the presence of a long-lived egg bank. Population growth rate of this dioecious species may be limited by mate encounter rate, such that below some critical density a colonizing population will fail to establish. We conducted a multi-lake experiment in which H. shoshone was stocked at densities that bracketed our hypothesized critical density of 0.5–5 copoepods/m 3 . Successful recovery by the copepod was observed only in the lake with the highest initial density (3 copepods/m 3 ). Copepods stocked into small cages at 3000 copepods/m 3 survived and reproduced at rates comparable to natural populations, confirming that the lakes were suitable habitat for this species. In support of mate limitation as the mechanism underlying recovery failure, we found a significant positive relationship between mating success and density across experimental and natural H. shoshone populations. Furthermore, a mesocosm experiment provided evidence of increased per capita population growth rate with increasing population density in another diaptomid species, Skistodiaptomus pallidus. Together, these lines of evidence support the importance of the Allee effect to population recovery of H. shoshone in the Sierra Nevada, and to diaptomid copepods in general. Key words: Allee effect; alpine lakes; copepod; Hesperodiaptomus shoshone; inverse density dependence; mesocosm experiment; reintroduction; Sierra Nevada, California, USA. INTRODUCTION Understanding the dynamics of populations at low density has become a priority, due to concern about the viability of populations of rare and endangered species, and the importance of such dynamics to more general population processes such as colonization. As a result, inverse density dependence, commonly referred to as the Allee effect (Allee et al. 1949, Courchamp et al. 1999), is increasingly recognized as an important phenomenon in natural systems (Courchamp et al. 1999, Stephens and Sutherland 1999). Several mechanisms can result in the Allee effect, including mate limitation and obligate cooperation (Allee et al. 1949, Odum 1959, Courchamp et al. 1999), with two related consequences. The first is a decrease in population growth rate as population density declines to low densities. When severe, this decrease in growth rate can result in the second consequence, a minimum (critical) density below which the population declines to extinction (Courchamp et al. 1999). Evidence from observational studies supports both decreased growth rate and the existence of critical densities in natural populations (Veit and Lewis 1996, Kuussaari et al. 1998, Morris 2002, Serrano et al. 2005, Stoner and Ray-Culp 2005), and theoretical models have been developed to explore the impact that the Allee effect can have on population persistence (Boukal and Berec 2002, Dennis 2002, Liebhold and Bascompte 2003, Calabrese and Fagan 2004), metapopulation dynamics (Amarasekare 1998, Brassil 2001, Martcheva and Bolker 2007), species invasions (Cruikshank et al. 1999, Taylor and Hastings 2005, Drake and Lodge 2006), and predator–prey dynamics (Kent et al. 2003). Despite long-standing interest in Allee effects, manip- ulative experiments in natural systems are rare. A few researchers have examined the Allee effect in plants, finding that low density can threaten population persistence due to insufficient pollination (Lamont et al. 1993, Hackney and McGraw 2001) or reduced competitive ability (Cappuccino 2004). We are aware of only two field experiments on animals that have tested Manuscript received 12 September 2007; revised 15 January 2008; accepted 12 February 2008. Corresponding Editor: D. E. Schindler. 3 Present address: Odum School of Ecology, University of Georgia, Athens, GA 30602. E-mail: kramera3@uga.edu 2760