1 Unravelling the role of mate density and sex ratio in competition for pollen Felix J. J. A. Bianchi and Saul A. Cunningham F. J. J. A. Bianchi, CSIRO Ecosystem Sciences, PO Box 2583, QLD 4001, Australia. – S. A. Cunningham (saul.cunningham@csiro.au), CSIRO Ecosystem Sciences, PO Box 1700, Canberra ACT 2601, Australia. Mate density and sex ratio are commonly used concepts in pollination biology, but are not always clearly distinguished. Here we propose that mate density should only capture the number of male-phase lowers in a deined area and ignore female-phase lowers. Sex ratio is the ratio of male and female-phase lowers in a deined area and captures female–female competition. We use a spatially explicit simulation model to quantify the efect of variation in mate density and sex ratio for plant–pollinator systems characterized by combinations of high or low rates of pollen loss and deposition from pollinator to stigma and then assess the size of pollination neighbourhoods. In eicient systems with relatively little pollen loss, female– female competition is of overriding importance. In contrast, in wasteful systems with high pollen loss rates, mate density becomes the dominant factor and sex ratio is no longer consequential. hese patterns were observed at both landscape and local scales. Systems with low deposition and low pollen loss rates are associated with large pollination neighbourhoods, which decline with increasing deposition and pollen loss rates. hese results show that mate density and sex ratio should carefully be distinguished and highlight the complex way in which pollen loss interacts with deposition rate, which has not previously been appreciated. Many authors have shown that the probability of success- ful pollination for any given lower can be inluenced by the density of potential mates (Levin 1990, Kunin 1993, 1997, Aizen 1997, Duncan et al. 2004a, Kirchner et al. 2005, Dauber et al. 2010) or the sex ratio of lowers (Cunningham 1995, Carlsson-Granér et al. 1998, Aizen 2001). To date these two closely related ideas have not been clearly distinguished (but see Aizen 1997, Öster and Eriksson 2007, Elliott and Irwin 2009). We argue here that mate den- sity and sex ratio efects difer both in how they function and in the consequences in terms of pollination outcomes. We also show how diferent circumstances make either sex ratio or mate density more inluential. At the same time, the way that pollen movement responds to the spatial arrangement of plants also inluences the size of the pollination neigh- bourhood (White et al. 2002). We seek to more carefully deine these ideas and use a simulation model to explore the way in which mate density and sex ratio diferently afect the probability of pollination and the size of pollination neigh- bourhoods. In doing so, we aim to advance our theoretical understanding of pollen competition and pollen limitation of plant reproduction (Ashman et al. 2004) and determi- nants of gene low, considering consequences on ecological rather than evolutionary time scales. he importance and mode of pollen transfer difers across plant species according to many traits including mating sys- tem, genetic controls and loral structure, but regardless of this variation the majority of plant species derive a beneit from pollen transfer by animals (Regal 1982). To simplify our discussion here we consider a common situation: a plant with hermaphrodite lowers that attract animal visitors as pol- len vectors but with temporal separation of male and female function such that at any time a lower is in male or female phase (Bertin 1993). To further simplify we assume that all possible mate combinations are equally viable (e.g. there is no structured pattern of preference for particular mates). It is widely expected that the rate of compatible pollen receipt will be related to the number of potential pollen donors within a certain distance as the importance of pollen donors is likely to decrease with increasing distance from the focal recipient. he appropriate distance depends on the for- aging range and behaviour of the pollinators, being greater when pollinator specialisation is high and when foraging dis- tances are long. Mate density (or variations on this phrase) refers to the number of possible mates within that distance. It is not unusual for studies to leave the deinition of the appropriate distance or area undeined, but nevertheless relect a view that only the ‘local’ patch is relevant. Further, although it is not always made clear, we suggest that when referring to mate density there is the direct implication that the measure is not inluenced by the presence of other female phase lowers, because only male phase lowers are potential mates. In other words, mate density ignores female–female competition. Oikos 000: 001–009, 2011 doi: 10.1111/j.1600-0706.2011.19842.x © 2011 he Authors. Oikos © 2011 Nordic Society Oikos Subject Editor: Regino Zamora. Accepted 10 May 2011