SURVIVORSHIP CURVE DEFINITION:
It is a graph showing the number and proportion of individual surviving at each age for a given species or group.
TYPES OF SURVIVORSHIP CURVES:
- Convex curve (Type I): most individuals live to adulthood with mostly morality occuring during old age (heavy mortality within a short time at maturity) e.g humans, elephants, deer.
- Straight line curve (Type II): an individual’s chance of dying is independent of its age. That means the species has a constant mortality rate throughout the life span of the population e.g small birds and mammals.
- Concave curve (Type III): few individuals live to adulthood (high mortality in early life), with the chance of dying decreasing with age e.g insects, oysters, snapping turtles, redwood tree.
environmental resources such as food and space are not inexhaustible…. when population increases these factors become limited and therefore checks population growth. These factors are called Environmental Resistance.
The maximum number of organisms that the environmental resources can sustain at equilibrium (when death rate equals birth rate due to environmental resistance) is called Carrying Capacity.
WHAT IS A SURVIVORSHIP CURVE?
A survivorship curve is a graph showing the number or proportion of individuals surviving to each age for a given species or group (e.g. males or females). Survivorship curves can be constructed for a given cohort (a group of individuals of roughly the same age) based on a life table.
There are three generalized types of survivorship curves:
- Type I or convex curves are characterized by high age-specific survival probability in early and middle life, followed by a rapid decline in survival in later life.
- They are typical of species that produce few offspring but care for them well, including humans and many other large mammals.
- Type II or diagonal curves are an intermediate between Types I and III, where roughly constant mortality rate/survival probability is experienced regardless of age. Some birds and some lizards follow this pattern.
- Type III or concave curves have the greatest mortality (lowest age-specific survival) early in life, with relatively low rates of death (high probability of survival) for those surviving this bottleneck.
- This type of curve is characteristic of species that produce a large number of offspring (see r/K selection theory).
- This includes most marine invertebrates.
- For example, oysters produce millions of eggs, but most larvae die from predation or other causes; those that survive long enough to produce a hard shell live relatively long.