Smaller families pose extinction risk

A safer environment leads to smaller families with larger offspring, but places species at greater risk of extinction, according to Australian research.

Doctoral student Daniel Falster from Sydney's Macquarie University, and his colleagues, report their findings in this month's edition of The American Naturalist.

They have developed a model that mimics the evolutionary process across the plant and animal kingdoms from butterflies to fish, to gorillas and even humans.

By estimating the evolutionary pressure on a particular characteristic, such as the amount of parental time and energy spent on each offspring, it can predict how that character will change.

"Our model was able to explain some of the major offspring size patterns seen across species, such as why larger parents have proportionately smaller offspring," Falster says.
Populate or perish

One of the findings from the model is that the more dangerous or threatening the environment is, the more offspring you need to produce, but the smaller they will be.

Larger animals and plants, which are "more comfortable" with their surroundings, can reduce the number of offspring they have and spend more energy nurturing their growth, Falster says.

"(Some) mammals live in a relatively safe environment and can therefore have fewer offspring, and that works well, provided those conditions continue," he says.
Bigger not best

Despite being larger, the reduced number of offspring places many species in danger of becoming extinct if their environment changes.

"If conditions change or new predators enter the system, it places them in a precarious situation," Flaster says.

"A lot of endangered species also have very large offspring. They have low population growth rates, and this makes it hard for them to endure, or recover from, any sort of harvesting or loss."

Falster hopes that his model will help biologists struggling to understand what makes some species vulnerable to extinction.
Human connection

Falster also thinks his model helps explain aspects of human nature.

Patterns in human demography are consistent with patterns seen among wild species, he says.

"As humans become richer and their environments become safer, we see parents investing more resources into their offspring and family sizes decreasing," Falster says.

He adds that the model helps explain differences in family size between developed and developing nations.

"Predictions from the model are exactly what we've observed when comparing across countries," Falster says.