Starvation is the norm for birds and mammals

It is difficult to find out whether animals in the wild have enough food, but research suggests that they are hungry most of the time, according to a new paper published in the American journal, Science, on Friday 22 July.

Researchers at the University of Reading also argue that this surprising conclusion could provide valuable new insights into animal behaviour.

The researchers undertook an analysis of a huge data base, the Global Population Dynamics Database (1), in which the changes over time in population densities of animals in many different environments are archived.

Using 1,780 of these ‘time series’ of mammals, birds, fish and insects, the Reading scientists have shown that populations are generally more abundant than their environments can support long term. Put together with earlier results that populations of birds and mammals are limited by their food supplies, it follows that these animals are generally short of food.

The first step of the analysis was to calculate the relationships between population’s growth rates and their densities. Natural populations of animals are expected to increase when their densities are less than the environment can support long term, because individuals then have abundant food for reproduction and maintenance. If their densities are higher, however, then population declines become inevitable.

“The precise relationship between a population’s growth rate and its density determines the way population density changes over time,” said Professor Richard Sibly, the lead researcher on the study. “The simplest possibility would be a straight-line relationship. This is not what was found.

“Rates of population growth are high at low population densities, as expected, but contrary to previous predictions, they decline rapidly with increasing population size, and then flatten out, for all four animal groups.

“This produces a strongly concave relationship between a population’s growth rate and its size. It is this that leads to the conclusion that animals will be found living at densities above those the environment can support long term.”

Professor Sibly suggests that such a conclusion could tell us a great deal about the ways animals compete with each other for food.

He said: “We are now investigating two theories. The first theory holds that the principal competition is between juveniles and adults. When population densities are low, reproduction is very successful and many juveniles are added to the population and many of them survive, so the population grows rapidly. On the other hand, when there are more animals than the environment can support, then it is the juveniles that suffer. The adults get what they need but most of the juveniles die. The result is that the population declines slowly as adults die, without any births to replace the deaths.

“The second theory applies when food is dispersed over territories. In the extreme case, the number of territories is fixed. When population densities are low, reproduction is very successful , so the population grows rapidly, as before. When there are more animals than the environment can support, then only some of the animals get territories. Since the number getting territories is fixed, the surplus population dies.”