Size-selective harvest of larger fish favors maturation at smaller sizes and reduces yield. In terrestrial systems, trophy hunting can artificially increase mortality of individuals with large horns, tusks, or antlers, which can induce selective pressures and lead to evolutionary changes in heritable morphological traits that cannot be quickly reversed by natural selection.
Although it is easily conceivable that size-selective harvest regimes can alter patterns of natural selection and cause demographic changes in wildlife populations, such effects are usually less expected and thereby less studied in harvest regimes that are not size selective.
However, theoretical models show that size-independent harvest can also induce selective pressures on life history traits , and recent empirical work has documented harvest selectivity for age, sex, and behavioral traits in the wild.
Despite its importance for management, we still know very little about the consequences of such selectivity on population processes.
Fitness is maximized by allocating resources to survival or reproduction and the function over which allocation will be biased depends on levels of extrinsic mortality.
Selectivity on female reproductive tactics
Here, we test whether a hunting regulation that prohibits the killing of females with dependent offspring can induce selectivity on female reproductive tactics at the individual level and evaluate the effect of such selectivity on population processes.
We use more than 20 years of exceptionally detailed individual-based data on survival and reproduction in a heavily hunted population of brown bears (Ursus arctos) in Sweden, where two distinct maternal care tactics have been documented.
We start by documenting the temporal trend in the duration of maternal care and contrasting survival probabilities between females providing either short (1.5-year tactic) or long (2.5-year tactic) maternal care.
Longer maternal care entails a loss of reproductive opportunities in species where breeding is not resumed until current offspring are weaned.
Therefore, we compare two demographically and evolutionary meaningful proxies of fitness that integrate information on survival and reproduction, i.e., asymptotic population growth rate and net reproductive rate, between the two maternal care tactics to quantify the difference in fitness between these tactics.
Finally, we complement this analysis by evaluating the fitness pay-off of each maternal care tactic under various plausible scenarios of hunting pressure to determine if hunting can drive the relative occurrence of maternal care tactics in the population.
Occurrence of maternal care tactics in the population
We found that from 1987 to 2015, the odds of a litter being weaned after 2.5 years of maternal care increased by a factor 1.17 annually in the population.
The first documented litter in our study population that was raised with the 2.5-year tactic was born in 1993 and weaned in 1995, and since then 24.8% of the litters have received 2.5 years of maternal care; the rest received maternal care for 1.5 years.
Protective effect of maternal care
Adult (≥4 years-old (y.o.)) and yearling female brown bears in a family group during the hunting season have a survival advantage compared to when solitary.
For adult females, being solitary during the hunting season significantly reduces survival probabilit.
The annual finite mortality rate of solitary adult females was 0.16 ) and the hunting-induced annual finite mortality rate was 0.14.
In contrast, the annual finite mortality rate of adult females accompanied by dependent offspring (i.e. member of a family group) was only 0.06 and the hunting-induced annual finite mortality rate was 0.04, 9 cases during the study period 1993–2015, where a hunter accidentally shot the female before observing the dependent offspring).
Therefore, the odds of dying from hunting were 3.91 times higher for solitary females compared to adult females accompanied by dependent offspring.