Question

3) What are “age structured” population models and how do they inform toxicological impacts on populations? Why are such models better indicators of toxicant effects compared to the theoretical models of exponential and logistic growth?

Answer 1

Age Structure: In most populations, individuals are of different ages. The proportions of individuals in each age group are called the age structure of that population. For instance, an understory palm tree population (Astrocayum mexicanum ) in an evergreen forest of Mexico, had 50% individuals as a seedling (less than 2-year-old), 19% as saplings (8-year-old), 5% as 30-year-old adults and so until 70-year-old trees made up less than 2% of the population.

Age distribution is important as it influences both the natality and mortality of the population. The ratio of various age groups in a population determines the current reproductive status of the population , thus anticipating the future.

Populations in nature often consist of a mixture of stages and ages, yet toxicological studies even demographic studies, usually evaluate one starting life stage. In this study it was asked whether the starting age/stage structure of a population at the time of initial pesticide exposure influenced the impact that pesticides have on population growth rates. This question was answered by exposing differently structured populations of two terrestrial arthropod species, the two-spotted spider mite, Tetranychus urticae (Koch), and the pea aphid, Acrythosiphon pisum (Harris), to pesticides. The three structured populations tested were (1) eggs or neonates for A. pisum and T. urticae, respectively, (2) stable age distribution, and (3) young adult females only.

Instantaneous rates of population increase (ri) for the three structured populations were determined over time without exposure to pesticides (control) and after exposure to pesticides. Populations of T. urticae were exposed to 100 ppm of the pesticide dicofol; populations of A. pisum were exposed to 200 ppm Neemix. The ri for the three control populations of T. urticae and A. pisum converged in a closed system 16 and 17 days after the start of the study, respectively. Unlike the control populations, the ri of the three treated populations did not converge by Day 16 for the mite species or Day 17 for the aphid species after exposure to pesticides. Growth rates of populations started as eggs (mites) or neonates (aphids) remained significantly lower than those of the adult or mixed-age populations (P</=0.05). Acute mortality data indicated that exposure to 100 ppm dicofol was equivalent to the LC21 for the egg stage, the LC59 for immatures, and the LC69 for adult T. urticae. Thus, even though the egg stage was the least susceptible stage of T. urticae, populations started as eggs were significantly more susceptible than populations started as the stable age distribution or as adults. It was concluded that the initial structure of a population does have an influence on the impact that pesticides will have on populations and that age/stage structure should be given serious consideration when evaluating toxicant effects.