Increased, Male Biased Sex Ratio leads to Decrease, Population growth rate

The plausibility that a male-biased sex ratio would affect population status of different species is strong. For any given population, a male-biased sex ratio suggests that the number of available breeding females is reduced. If the male-biased sex ratio persists and/or increases over time, the offspring production will decrease and population size would be reduced. Additionally, for certain species, an increasing number of males could cause negative behavioral responses, for example, a higher competition for mating leading to more aggressive behaviors that can result in reduced adult survival rates for both male and females.  A reduced effective population also affects genetic diversity, which can further reduce population viability. 

There have been limited examples of field evaluation of the consequences of male-biased sex ratios on population status, as well as several modeling efforts focused on aspects of population viability in situations where a male-skewed situation could occur. These analyses have focused on avian, reptile or fish species, several of which undergo at least some degree of environmental sex determination.

• Surveys and viability analyses of a Tuatara (Sphenodon punctatus) population by Grayson et al. (2014) showed that a current population of 56% males at hatching would result in a 12% probability of extinction within the timeframe of the analysis (60 of 500 simulated populations become extinct, mean time to extinction=1183.3 years). 
• Using a behavioral approach Le Galliard et al. (2005) looked at how male-biased sex ratios in the common lizard (Lacerta vivipara) can negatively impacted mating to reduce population viability.
• In Kakapo (Strigops habroptilus), an endangered parrot species, male-biased production was shown to result in a prolonged species recovery, which risks conservation efforts to build a sustainable population and prevent the species from going extinct (Clout et al 2002; Robertson et al. 2006). 
• A model-based viability analysis by Brown et al. (2015) showed that a male-biased population due to environmental stressors could lead to a sharp decline in zebrafish (Danio rerio) population levels.
• Miller et al. (2022) developed a matrix model for fathead minnow (Pimephales promelas) that demonstrated how even minor increases in the proportion of males in this species could substantially affect population status over time due to a loss of breeding females.

Studies at the population level can be quite challenging in terms of required resources and, given the number of variables that might simultaneously influence a population, interpretation of results. Consequently, evaluation of population status in the context of adverse outcome pathways often relies upon model predictions that almost always are applicable only to a limited number of--sometimes one--species because of requirements associated with model parameterization. Given this, although it is entirely reasonable from an evolutionary perspective that male-biased sex ratios will negatively impact populations of a given species, it can be difficult to fully assess what this impact may be.  

Brown et al. (2015) and Miller et al. (2022) provide examples for zebrafish and fathead minnows, respectively, of approaches used to establish quantitative response-response relationships between male-biased sex ratios and population size/trends. In general, however, population models almost always rely on female productivity rather than male contributions to forecast population status.

The time-scale for this KER is entirely dependent on the life-cyle of the organism of interest. Small, short-lived animal species could experience population-level alterations due to biased sex ratios in days to weeks, while impacts on larger, long-lived species may take years to decades. 

It is difficult to define what form a feedforward/feedback loop might take for this KER. This would likely largely be a function of the stressor causing a male-biased population. If the stressor was short-term (e.g., affecting one age cohort) the situation might be self-correcting, as opposed to a longer-term stressor that continually causes a male-biased sex ratio, which theoretically should usually result in population extirpation.