Fecundity

Parity is the organization of fecundity into two distinct types, semelparity, and iteroparity.

Semelparity occurs when an organism reproduces only once in its lifetime, with death being a part of its reproductive strategy. These species produce many offspring during their one reproductive event, giving them a potential advantage when it comes to fecundity, as they are producing more offspring.

Iteroparity is when a species reproduces multiple times over its lifetime. This species' strategy is to protect against the unpredictable survivability of their offspring, in which if their first litter of offspring dies, they can reproduce again and replace the dead offspring. It also allows the organism to care for its offspring, as they will be alive during their development.[8]

There are a multitude of factors that potentially affect the rates of fecundity. For example: ontogeny, population density and latitude.

Ontogeny

Fecundity in iteroparous organisms often increases with age but can decline at older ages. Several hypotheses have been proposed to explain this relationship. For species with declining growth rates after maturity, the suggestion is that as the organism's growth rate decreases, more resources can be allocated to reproduction. Other possible explanations exist for this pattern for organisms that do not grow after maturity. These explanations include: increased competence of older individuals; less fit individuals have already died off; or since life expectancy decreases with age, older individuals may allocate more resources to reproduction at the expense of survival.[6] In semelparous species, age is frequently a poor predictor of fecundity. In these cases, size is likely a better predictor.[9]

Population density is often observed to negatively affect fecundity, making fecundity density-dependent. The reasoning behind this observation is that once an area is overcrowded, fewer resources are available for each individual. Thus there may be insufficient energy to reproduce in high numbers when offspring survival is low. Occasionally high density can stimulate the production of offspring, particularly in plant species, because if there are more plants, there is food to lure pollinators, who will then spread that plant's pollen and allow for more reproduction.[6]

There are many different hypotheses to explain the relationship between latitude and fecundity. One claimed that fecundity increases predictably with increasing latitude. Reginald Morean proposed this hypothesis, the explanation being that there is higher mortality in seasonal environments.

A different hypothesis by David Lack attributed the positive relationship to the change in daylight hours found with changing latitudes. These differing daylight hours, in turn, change the hours in which a parent can collect food. He also accounts for a drop in fecundity at the poles due to their extreme amounts of day lengths, which can exhaust the parent.[10]

Fecundity intensity due to seasonality is a hypothesis proposed by Phillip Ashmole. He suggests latitude affects fecundity due to seasonality increasing with increasing latitudes. This theory relies on the mortality concept proposed by Moreau but focuses on how seasonality affects mortality and, in turn, population densities. Thus in places with higher mortality, there is more food availability, leading to higher fecundity. Another hypothesis claims that seasonality affects fecundity due to varying lengths of breeding seasons. This idea suggests that shorter breeding seasons select a larger clutch size to compensate for the reduced reproduction frequency, thus increasing those species' fecundity.[10]

Fecundity is a significant component of fitness. Fecundity Selection builds on that idea. This idea claims that the genetic selection of traits that increase an organism's fecundity is, in turn, advantageous to an organism's fitness.[10]

Fecundity Schedule

Fecundity Schedules are data tables that display the patterns of birth amongst individuals of different ages in a population. These are typically found in life tables under the columns Fx and mx.

Fx lists the total number of young produced by each age class, and mx is the mean number of young produced, found by finding the number of young produced per surviving individual. For example, if you have 12 individuals in an age class and they produced 16 surviving young, the Fx is 16, and the mx is 1.336.[9]