Can't live with them, can't live without them? Balancing mating and competition in two-sex populations

Abstract

Two-sex populations are usually studied through frequency-dependent models that describe how sex ratio affects mating, recruitment and population growth. However, in two-sex populations, mating and recruitment should also be affected by density and by its interactions with the sex ratio. Density may have positive effects on mating (Allee effects) but negative effects on other demographic processes. In this study, we quantified how positive and negative inter-sexual interactions balance in two-sex populations. Using a dioecious grass (), we established experimental field populations that varied in density and sex ratio. We then quantified mating success (seed fertilization) and non-mating demographic performance, and integrated these responses to project population-level recruitment. Female mating success was positively density-dependent, especially at female-biased sex ratios. Other demographic processes were negatively density-dependent and, in some cases, frequency-dependent. Integrating our experimental results showed that mate-finding Allee effects dominated other types of density-dependence, giving rise to recruitment that increased with increasing density and peaked at intermediate sex ratios, reflecting tension between seed initiation (greater with more females) and seed viability (greater with more males). Our results reveal, for the first time, the balance of positive and negative inter-sexual interactions in sex-structured populations. Models that account for both density- and sex ratio dependence, particularly in mating, may be necessary for understanding and predicting two-sex population dynamics.

Sex‐specific selection on plant architecture through “budget” and “direct” effects in experimental populations of the wind‐pollinated herb, Mercurialis annua

“…It is also noteworthy that our two common gardens differed in which component of plant architecture was most strongly associated with the distance of pollen dispersal. Although our experimental design does not allow us to attribute this difference to density with any confidence, an effect of density seems most likely, given what is known about the sensitivity of mating to density in wind-pollinated plants in general (Compagnoni et al 2017), including M. annua (Eppley and Pannell 2007a;Hesse and Pannell 2011;Labouche et al 2017). At low-density, canopies are sparser, with correspondingly fewer obstacles to pollen dispersal.…”

“…Although our experimental design does not allow us to attribute this difference to density with any confidence, an effect of density seems most likely, given what is known about the sensitivity of mating to density in wind‐pollinated plants in general (Compagnoni et al. ), including M. annua (Eppley and Pannell ; Hesse and Pannell ; Labouche et al. ).…”

Influence of local density and sex ratio on pollination in an ambophilous flowering plant

“…Although it was not technically feasible to quantify the relative contributions of insect and wind pollination to reproduction, our results suggest that they respond differently to changes in density and sex ratio. Assuming that wind pollination was more effective at high than at low density, as reported for many dioecious species (e.g., Allison, ; Knapp et al., ; Steven and Waller, ; Hesse and Pannell, ; Compagnoni et al., ), the net contribution of insect pollination to seed production must have been positive at low density, given that seed set was relatively consistent among density treatments (Fig. ).…”

“…Insufficient pollination may limit reproductive success in dioecious species (e.g., de Jong et al., ; Knight et al., ; but see Sutherland and Delph, ) and can be related to the demographic context of pollination in diverse ways, depending on the type of pollen vector (Wilcock and Neiland, ; Knight et al., ). For wind‐pollinated species, pollination success may be influenced by density and sex ratio because pollen receipt depends on proximity to neighboring male plants (Steven and Waller, ; Hesse and Pannell, ; Compagnoni et al., ). This is caused by distance‐dependent dilution of pollen due to advection‐diffusion dynamics of wind, gravitational settling of pollen, and filtration onto vegetative structures (Whitehead, ).…”

“…Despite the potential importance of density and sex ratio for the reproductive success of dioecious plants, their combined effects have rarely been investigated experimentally (but see Van Drunen and Dorken, ; Compagnoni et al., ). In particular, it is unclear how such demographic factors might influence pollination dynamics in dioecious species that are ambophilous in which some populations may rely on both animals and wind for pollination.…”