Effects of fertility control on populations of ungulates: General, stage-structured models

Regulating the abundance of ungulate populations using hunting can prevent populations from reaching levels that cause harm to natural and human domin ated systems. However, there are an increasing number of cases where huntin g is infeasible, and in such cases, fertility control has been widely advoc ated as an alternative means for controlling populations. Here, we develop simple analytical models offering general insight into the feasibility of u sing fertility control to regulate the abundance of ungulates. The models a re structured in stages to represent variation in the duration of effect of fertility control agents. Analysis of these models offers several predicti ons, amenable to testing in field studies. (1) More than 50% of fertile fem ales will need to be maintained infertile to achieve meaningful reductions in ungulate numbers even when fertility rates are low (2) The relationship between the proportion of females maintained infertile and the steady state density is highly nonlinear. This means that small errors in estimating le vels of infertility can lead to large errors in achieved density. It also m eans that managers should expect to see little change in steady-state densi ty across a broad range of delivery rates. (3) The efficacy of fertility co ntrol as a management technique depends strongly on the persistence of the effect of the fertility control agent and the ability of managers to recogn ize previously treated animals. (4) Fertility control using long-lived agen ts can be more efficient than culling in regulating ungulate numbers. (5) T reating small populations with irreversible agents magnifies the likelihood of population extinction relative to treatment by culling. As with all tec hniques, managing population fertility must extend from a sound understandi ng of the influence of management actions on the state and dynamics of the population.