Population models age-structured

The models used above have all had simple population structures. In all the cases, the generations were not overlapping and each organism had the same reproductive potential. Of course this is not realistic. 

Barnthouse and colleagues (Barnthouse 1993 Bamthouse et al. 1990, 1989) have explored the use of conventional population models to explore the interactions among toxicity, predation, and harvesting pressure for fish populations. These studies are excellent illustrations of the use of population models in the estimation of toxicant impacts. These models employed the use of information concerning the life history and age structure of the organism being modeled. 

Life History diagram for an age-structured population. The numbers of organisms in the population at time t, is dependent on the numbers of the one-year younger-age class of the year before and the survivorship percentage from t0 to tj. The numbers are also dependent upon the number of offspring from the previous year surviving up to age 1. This is a general model for many plant and animal populations. 

A toxicant can affect the population at a number of stages. A toxicant at a concentration that can cause acute mortality can decimate the population at every life stage, but older organisms may be less affected because of relative size. Materials that bioaccumulate over time may differentially affect older organisms that have had the time to have a high tissue concentration. This increase in tissue concentration may cause decrease of survivorship of older organisms. Such an increase in tissue concentration may also decrease the reproductive success of these older age classes. 

Materials that are preferentially toxic to early life stages will cause a lack of age 1 organisms coming into the population, producing a population lacking the early life stages. As exposure to these toxicants persists, the population will become aged and then as all the adults become postreproductive, collapse. 

---