The founder space race: a response to Waters et al. Hannah L. Buckley, Adrian M. Paterson, and Robert H. Cruickshank Department of Ecology, Lincoln University, PO Box 84, Canterbury 7647, New Zealand We welcome the attempt by Waters et al. [1], in the spirit of the journal that they are publishing in, to address scales not typically dealt with by ecology (too long) or evolution (too short). The basic scenario that they ad- dress [i.e., the expansion of the range of a taxon (species, population, or genotype) into empty niche space], is a nonequilibrium situation in which many ecological pro- cesses occur. However, the timescales they refer to range from very short (a few generations) to very long (thou- sands to millions of generations). They suggest that a ‘founder takes all’ principle might explain reduced ge- netic diversity in recently colonised areas. We are con- cerned that their use of the term ‘density-dependent effects’ to describe how genetic structure within newly colonised areas impedes establishment of invading taxa misrepresents the process and does not address all the main mechanisms by which this can occur. The arrival of a taxon in an area can be broken down into three phases: (i) dispersal; (ii) colonisation; and (iii) estab- lishment. Dispersal occurs when individuals or propagules arrive in an area. Colonisation occurs when propagules grow and/or individuals survive long enough to breed. For establishment, the new population must overcome obsta- cles to expansion, such as strong Allee effects, and expand to a point where it is self-replacing. The ‘founder takes all’ principle, as referred to by Waters et al. [1], is primarily concerned with establishment. Although several taxa may enter the dispersal and/or colonisation phases in an area, it is not until establishment occurs that evolutionary pat- terns begin to form. The main mechanism that Waters et al. invoke for the generation of genetic structure could otherwise be called ‘space pre-emption’ [2], where different taxa dominate different parts of a newly occupied range, simply due to variation in arrival order. This process of differential space pre-emption leading to genetic structuring is a ‘priority effect’. Priority effects occur where there are intertaxon interactions whose outcome depends on the relative or absolute timing of arrival of the interacting taxa [3]. This allows for different establishment outcomes for taxa in similar areas, even with the same potential taxon pool. Priority effects are not, as implied by Waters et al. [1], density dependent. Density-dependent processes are those that change with an increase in population density, such as density-dependent competition [4]. Although the occupan- cy of space depends on the density of the resident popula- tion, the order or arrival of taxa depends on their dispersal characteristics and chance events, rather than on the density of the resident population. We argue that evolutionary biologists and ecologists should consider several additional, not mutually exclusive, mechanisms that maintain genetic structuring within populations. First, pre-existing variation, including beha- vioural variation, such as social behaviour and differing dispersal ability, may provide particular taxa with a com- petitive advantage that could be considered a preadapta- tion for establishment. Second, niche specialisation of established taxa could exclude invaders because these resident individuals perform better in their new environ- ment due to local adaptation, biasing the outcomes of competitive interactions. Third, genetic incompatibility or assortative mating may exclude invaders because their propagules do not produce fertile offspring. Fourth, tem- poral effects occur when there has been insufficient time for genetic heterogeneity to arise due to the rarity of colonisa- tion events (i.e., number of colonisations and success of interbreeding) relative to the length of time required for genetic material from invading taxa to become detectable. These processes are not necessarily density dependent. For example, niche specialisation could be density independent because the success of the established taxon depends on its relative fitness advantage in the local environment, rather than on its density. Ultimately, what is of interest in this discussion is how these ecological mechanisms scale with time, from a few generations to hundreds of thousands of years. Over longer timescales, changes in multiple ecological processes and environments are likely to occur. Conclusions about eco- logical causes of evolutionary patterns must be made with care to ensure that such complexities are accounted for. Opening a dialogue between ecologists and evolutionary biologists is a useful start. References 1 Waters, J.M. et al. (2013) Founder takes all. Density-dependent processes structure biodiversity. Trends Ecol. Evol. 28, 78–85 2 Connolly, S.R. and Muko, S. (2003) Space pre-emption, size-dependent competition and the coexistence of clonal growth forms. Ecology 84, 2979–2988 3 Lawler, S.P. and Morin, P.J. (1993) Temporal overlap, competition, and priority effects in larval anurans. Ecology 74, 174–182 4 Begon, M. et al. (1996) Ecology: Individuals, Populations and communities, Blackwell Scientific Publications 0169-5347/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tree.2013.01.005 Trends in Ecology & Evolution xx (2013) 1–1 Letter Corresponding author: Buckley, H.L. (Hannah.Buckley@lincoln.ac.nz, buckleyhannah@gmail.com). TREE-1653; No. of Pages 1 1