813 Interplay between Allee effects and collective movement in metapopulations Aina Astudillo Fernandez, Thierry Hance and Jean Louis Deneubourg A. Astudillo Fernandez (aastudil@ulb.ac.be) and J. L. Deneubourg, Univ. Libre de Bruxelles, Campus Plaine, CP231, Boulevard du Triomphe, BE-1050 Brussels, Belgium. – T. Hance, Univ. Catholique de Louvain, Carnoy bte L7.07.04, Croix du Sud 4-5, BE-1348 Louvain-la-Neuve, Belgium. Population growth can be positively or negatively dependent on density. erefore, the distribution pattern of individuals in a patchy environment can greatly affect the growth of each subpopulation and thereby of the metapopulation. When population growth presents positive density-dependence (Allee effect), the distribution pattern becomes crucial, as small populations have an increased extinction risk. e way in which individuals move between patches largely determines the distribution pattern and thereby the population dynamics. Collective movement, in particular, should be expected to increase the potential number of colonisers and therefore the probability of colonising success. Here, we use mathematical modelling (differential equations and stochastic simulations) to study how collective movement can influence metapopu- lation dynamics when Allee effects are at stake. e models are inspired by the two-spotted spider mite, a phytophagous pest of recognised agricultural importance. is sub-social mite displays trail laying/following behaviour that can provoke collective movement. Moreover, experimental evidence suggests that it is subject to Allee effects. In the first part of this study we present a single-species population growth model incorporating Allee effects, and study its properties. In the second part, this growth model is integrated into a larger simulation model consisting of a set of interconnected patches, in which the individuals move from one patch to the other either independently or collectively. Our results show that col- lective movement is more advantageous than independent dispersal only when Allee effects are present and strong enough. Furthermore they provide a theoretical framework that allows the quantification of the interplay between Allee effects and collective movement. e way in which individuals distribute themselves in a patchy environment can greatly affect their population dynamics. is is mainly due to the fact that population growth is highly dependent on local population density, which in turn depends on the distribution of individuals between patches. e behavioural strategies that determine inter-patch movement and habitat selection play an impor- tant role in shaping distribution of the individuals among connected populations (Hanski 2001). However, the inter- play between such behaviour and metapopulation dynamics is poorly studied (Bowne and Bowers 2004), mainly because ethologists and ecologists often work at different time scales (Lima and Zollner 1996). e importance of dispersal behaviour is even greater for species that are subject to the Allee effect (Allee 1931, 1949, Stephens et al. 1999, Courchamp et al. 2008). e Allee effect arises when each individual benefits from the presence of conspecifics in a way that makes its own fitness increase with the population size or density (Stephens et al. 1999, Courchamp et al. 2008). Examples are numerous across living beings and range from plants suffering from lack of pollinators when at low densities (Groom 1998), to mongooses benefiting from the improved anti-predator behaviour of large groups (Clutton-Brock et al. 1999). In population dynamics, this can translate into a demographic Allee effect, that is when the per capita growth rate of the population is positively dependent on population size (or density) (Stephens et al. 1999, Courchamp et al. 2008). A direct consequence is that the growth rate of the popu- lation can become very low at low densities, a phenome- non known as depensation (Lidicker 2010). In the case of a strong Allee effect, the growth rate can become negative beyond what is called an Allee threshold and drive the population to extinction (Stephens et al. 1999, Courchamp et al. 2008). Several studies have recognised the importance of the Allee effect in shaping population and meta- population dynamics (Hanski and Gilpin 1991, 1996, Hanski and Gaggiotti 2004). is has been shown in spa- tially implicit models (Amarasekare 1998a, Zhou and Wang 2004) as well as spatially explicit models (Brassil 2001, Roy et al. 2008, Sato 2009). ese studies show that the Allee effect influences colonisation rates because small populations do not colonise successfully, and this can have a great impact on the resulting metapopulation dynamics. Oikos 121: 813–822, 2012 doi: 10.1111/j.1600-0706.2011.20181.x © 2012 e Authors. Oikos © 2012 Nordic Society Oikos Subject Editor: Justin Travis. Accepted 21 October 2011