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HIGH-PROFILE efforts at reintroducing locally extinct species such as wolves
into Yellowstone National Park or condors into California may be intrinsically
flawed, according to a mathematical study by Finnish and Swedish
researchers.

Esa Ranta of the University of Helsinki and his colleagues modelled the
interactions between species in a community to find out what happens when
conservationists attempt to reintroduce a species that has died out. They found
that once a species vanishes from a habitat, the “doors” of that community may
shut permanently, preventing the species from ever coming back, says Ranta. A
proportion of the modelled reintroductions failed miserably. “And when
reintroduction was successful it often led to additional extinctions,” Ranta
says.

The group’s predictions appear to be backed by evidence in the wild. When
wolves from the Canadian wilderness were reintroduced into Yellowstone in 1995
and 1996, the local coyote population halved within two years. The wolves hunted
and killed their smaller rivals.

The researchers also found that once a species dies out, many others follow
in a cascade of extinctions. The larger the number of species in the original
community, the greater the effect. “This supports the notion that more
diverse communities are more fragile,” says Donald Waller, a conservation
biologist at the University of Wisconsin at Madison.

Plants also appear to be at risk from a cascade of extinctions. For instance,
studies have shown that Middlesex Fells, an isolated 400-hectare woodland park
in Boston, Massachusetts, lost nearly 40 per cent of its original species
between 1894 and 1993. “Wherever we have the data to look carefully at plant
communities, we are seeing a massive loss of species. In many cases we don’t
understand it. The process explored in this mathematical model may well be
important in ultimately trying to understand species loss,” says Waller.

Waller says that preserving existing species may be more cost-effective than
trying to reintroduce lost ones. “Captive breeding, habitat restoration, and
reintroduction are intensive and expensive. If we are serious about conserving
wild species, we need to maintain their interrelationships with other species,”
he says.

He does caution that the modelling study may not be all bad news. For
instance, only about 40 of the 450 species reintroduced in the theoretical model
failed. The model also does not incorporate a phenomenon called patch dynamics,
where in some local regions, competition among species is temporarily reduced,
making reintroduction more likely, he says.

Nonetheless, Waller says the maths gives us cause for concern. Ranta agrees:
“The take home message is that it is better to avoid extinction due to human
causes as much as possible,” he says.