Quantifying positive and negative interactions between the sexes
In dioecious organisms, the relative frequency and absolute density of the sexes (sex structure) can have important effects on demographic processes. The majority of the existing literature studies the response of fecundity to the relative frequency of the sexes (sex ratio). However, there are two additional ways in which sex structure can affect population dynamics. First, sex ratio could affect other demographic rates, such as growth and survival, as a result of competitive hierarchies between the sexes. Second, the absolute density of the population should modify the effects of sex ratio. In other words, there should be an interaction between sex ratio and the absolute density of the population. We carried out a field experiment on a native perennial grass, Poa arachnifera, to address two questions: 1) what are the effects of sex ratio and of the densities of each sex on the fecundity, survival, and growth of Poa arachnifera? 2) Do sex ratio and absolute density interact? We established the experiment in Fall 2013 by replicating a gradient of sex ratios across a range of absolute densities. Starting in Spring 2015, we collected data on survival and growth, and performed seed viability assays in the lab to quantify fecundity.
Density was the strongest factor controlling growth and survival, while sex ratio was the main determinant of fecundity. The best models on growth indicate an interaction between absolute density and sex ratio. In particular, we found evidence that females exert a stronger competitive effect than males, and that this competitive effect increases in female biased treatments. The models on fecundity show that plots without males did not produce viable seeds. This result suggests that in nature P. arachnifera can be mate limited, because isolated female clones are common in field populations. These results highlight the importance of competitive interactions among the sexes. To study these interactions, it is crucial to consider a response surface that crosses a range of sex ratios across a range of absolute densities.