Species that are perceived to be a nuisance are species that humans generally want to manage in terms of population growth and population size. If a species is to be managed to reduce population size and curb population growth, at what stage in their life cycle should we focus our efforts. Does targeting mature individuals always have the best outcome or are there instances where we can reduce birth rates to have a significant impact? Understanding when a species experiences the greatest rates of mortality is a key factor in determining when to target them for population management.
In ecology, survivorship curves provide information about the probability of survival throughout the entire life cycle. Ecologists recognize three primary patterns referred to as Type I, Type II, and Type II.
Type I Survivorship: Species with a Type I survivorship curve have a high probability of survival throughout their life with a sudden drop at a specific age. These species tend to have specific characteristics such as parental care, low birth rates, and extended social networks (herds, packs, or colonies) that improve survival rates. Examples of species with a Type I survivorship curve include species such as Elk, White-tailed Deer, Gray Wolves, and Humans.
Type II Survivorship: Species with a Type II survivorship curve have a steadily declining probability of survival throughout their life without any pronounced drop in probability of survival at a specific age. These species may have specific characteristics such as parental care, moderate to low birth rates, and extended social networks (herds, packs, or colonies) that moderate survival rates. However, they tend to be species that are small to moderate in size and are a key prey source for other species throughout their life. Examples of species with a Type II survivorship curve include species such as Sandhill Cranes and Coyotes.
Type III Survivorship: Species with a Type III survivorship curve have a very rapid decline in probability of survival early in life and then a good probability of survival late in life. These species may have specific characteristics such as large body size or physical/behaviour defense mechanisms that improve their survival as adults. Examples of species with a Type III survivorship curve include species such as Northern Pike, Water Hyacinth, and Autumn Olive.
Targeting Populations for Management: If the goal is to reduce population size and curb population growth, the best age to target is the age with the highest survival rate. Therefore, for species with a Type I survivorship curve, the young individuals should be targeted while for a Type III survivorship curve the older individuals should be targeted. This may seem counter intuitive but the key is to increase mortality (reduce survivorship) significantly to reduce population size and curb population growth. If we focus on a point in the life cycle when survivorship is very low it is difficult to have an impact. Using dandelions as a specific example, we can easily understand that we do not manage dandelions by trying to catch all of the seeds that are floating in the air. These plants literally produce thousands of seeds and only a few survive to become reproducing plants under normal conditions. If we use a survival rate of about 10%, we would have to capture 2,000 seeds to reduce the future population by 200 plants. However, if 90% of the plants that have germinated survive to reproduce and each plant produces 1,000 seeds, killing 200 dandelions will reduce the future population by 2,000 plants! By targeting the correct life stage, we had a significant impact on the size of the future population with less effort.