Final answer:
Survivorship curves are used to express the differences in survival rates of species throughout their lifespan. These curves are graphical representations that show the mortality rates associated with different ages and are classified into three types: Type I, Type II, and Type III, each corresponding to specific mortalities patterns related to age.
Step-by-step explanation:
Differences between the arms of survivorship curves are expressed by illustrating the varying patterns of survival rates for different species over their lifespan. These curves graph the number of individuals alive at successive age intervals. There are three primary types of survivorship curves. Type I is characterized by low mortality in early and middle life with an increase in mortality in older ages, typical of humans and many larger mammals. Type II shows relatively constant mortality rates throughout the life of the species, which is typical for many birds. Lastly, Type III represents species that experience high mortality rates in early life, but individuals that reach maturity have a higher probability of survival; this is common in plants and some marine invertebrates.
To compare the life histories of different populations and their survival strategies, ecologists use these curves as they depict immensely important ecological information on how species interact with their environment.