Whenever a pair of giant pandas have a baby in a zoo, the event is certain to attract hundreds of thousands of visitors — and millions of viewers on the TV. It’s like a baby born to royalty, signalling the continuation of a dynasty. Ensuring that pandas in the wild have babies — and continue their succession — is harder than it might appear. Panda populations are now more fragmented and isolated than they were 30 years ago. To continue a dynasty, one needs a mother and a father. And one or the other may be lacking — an event that gets more likely as populations become small and isolated.
The Fourth National Giant Panda census found pandas living in 33 isolated populations. Some are large populations, but one-seventh of the total — 259 animals — live in 25 populations. So what’s likely to be their fate?
Small populations suffer the unavoidable vagaries of sex and death. A population of two males and two females has a one in eight chance of producing either all males or all females in the next generation, for example. Even a population of ten individuals, equally balanced between males and females, has a one in 500 chance of having all the same sex in the next generation. That’s a game the population must play generation after generation. The odds of having a severe imbalance — say only three or fewer females in the next generation — are much higher. The odds of having the following year good for females are no different from another year good for males. Another bad year can follow a bad year.
It’s both better and worse than this, of course. Pandas may live 25 years, so even if all the animals born in a year are females, there may be males alive from previous years. But females don’t breed until there are five years old. It isn’t just sex; it’s death too. All the individuals in a generation may die without heirs from random, individual events by chance when a population is small.
These and other complications mean we must model individuals and follow their fate from birth to death — their sex, mates, and offspring. As the name suggests, in individual-based models, the individuals matter. One cannot use the assumptions of familiar techniques such as Leslie matrices to model population trends.
Some panda populations have very little chance of creating dynasties. Populations with fewer than 30 individuals have a one in five chance of going extinct in 100 years on average. Some may not be that lucky. Our models are the best-case scenarios, too.
In addition to demographic accidents, there can be environmental disasters too. Pandas depend on bamboos, which have episodic, massive flowering events, where after the bamboos all die. Large areas of the panda’s range are likely to have such events in the next decade or so.
The bad news keeps on coming. Climate disruption changes species ranges, forcing them to higher elevations. We modelled this too. The panda populations will become even more fragmented and vulnerable.
What can we do? Managers need to reconnect isolated populations. The best way to do that is to protect more habitat and restore habitat connections where required. The new Giant Panda National Park will do that. Our models show it will help, but not for all populations. We do provide specific examples of where habitats corridors would be possible.
The alternative is to introduce individuals either from captive populations or other wild populations. Introducing animals from elsewhere was the strategy for rescuing the increasingly inbred panther population in southern Florida. As the panther example showed, such efforts are often complex and controversial.
There are more than 600 captive giant pandas in breeding centres in China. This number should increase by more than 30 per year, providing the possibility for reintroducing animals to the wild. The main food source of giant pandas is bamboo. That makes reintroduction more likely than carnivorous animals. However, we have much to learn before we can rescue small, isolated populations with high risk of local extinction by reintroducing giant pandas from breeding centres.
The pandas’ problems are general ones. Human actions have left many species living in isolated populations — lions and elephants in Africa, tigers in Asia, are examples. These and other large-bodied species demand large areas of continuous habitat. Human needs for crops, cities, and roads makes this impossible. Smaller species have fragmented ranges too. Consider satellite images of land cover, for instance. They show how little forest is far from forest edges with land cleared for our use. Very many species have fragmented ranges. Although the exact demographic parameters will vary from species to species, our models provide insights into what other fragmented populations might encounter.
1 Nicholas School of the Environment, Duke University, Box 90328, Durham, NC 27708, USA