Humans with access to modern health care have a Type I curve, although plots of actual life table data for humans always include the aforementioned first year dip, reflecting the great vulnerability of infants to disease and accidents. In populations described by a Type II curve, animals die at a constant rate, regardless of age. Deaths typically occur as a result of accidents and disease, with relatively few from old age. This reflects the vulnerability of small animals to many different types of environmental perturbations. In populations described by Type III curves, many animals dies very early in life.
Later deaths occur at a constant, age-independent rate.
Often many, many small young are produced, and they are given little or no care. Many invertebrate species are described by this type of curve, particularly aquatic invertebrates, but fishes also often follow this pattern. Another way to think about survivorship curves is to impose a set of life stages on the x-axis of the curves. I population will have a chance to reproduce, while most members of a Type II population will die during the pre-reproductive stage. A commonly used measure is r, the rate of natural increase .