The Ecology Book

151

A thick green scum of algae covers

parts of Lonar Lake, in Maharashtra,

India. Algae thrive in high-nutrient

conditions, but decomposing algae

consume oxygen, and depleted levels of

oxygen lead to fewer fish surviving.

See also: The food chain 132–133 ■ The ecosystem 134–137 ■ Energy flow

through ecosystems 138–139 ■ T roph ic ca scades 14 0 –14 3

ECOSYSTEMS

suggested that the intense fishing

had progressively reduced the

resilience of the ecosystem.

Holling argued that ecological

resilience is not always positive.

If a freshwater lake experiences

a large input of nutrients from

agricultural fertilizers, for example,

it will become eutrophic: algae will

thrive, depleting the lake’s oxygen

and making it unsuitable for fish.

Such a lake may be resilient, but it

will become less biodiverse. Holling

claimed that three critical factors

determine resilience: the most a

system can be changed before

crossing a threshold that makes

total recovery impossible; the ease

or difficulty in making a big change

to the system; and how close to the

threshold a system is currently.

Changing states

According to Holling’s view,

resilience at the ecosystem level is

enhanced by its populations not

being too rigid—meaning that the

components of the ecosystem

can change. One example is the

disappearance of most American

chestnuts from forests in eastern

North America, which was largely

compensated for by the expansion

of oaks and hickories. For Holling,

this counted as resilience, because

although the exact mix of tree

species had changed, broad-leaved

forest still remained.

Ecologists now understand that

ecosystems can have more than

one stable state. In Australia, for

example, woodlands dominated by

mulga trees can exist in a grass-

rich environment that supports

sheep-farming, or in a shrub-

dominated environment that is

totally unsuitable for sheep. ■

The role of budworm

Spruce budworm caterpillars

have devastated balsam fir

forests in eastern North

America six times since

the 18th century. Holling

described this process as

having two very different

states: one with young,

fast-growing trees and few

budworms; and one with

mature trees and very large

numbers of budworms.

Between outbreaks of

budworms, young balsam fir

grow alongside spruce and

white birch trees. Eventually,

the fir becomes dominant.

A combination of this

dominance and a sequence

of very dry years stimulates

a huge increase in the

budworm population. The

mature fir is destroyed, giving

the spruce and birch an

opportunity to regenerate.

By keeping the balsam fir

in check, the budworm also

maintains the spruce and

birch. Without it, the fir trees

would crowd out the others.

So the system is unstable but

at the same time resilient.

Spruce budworm larvae in

Quebec, Canada, feed voraciously

on fir and spruce before they

pupate. Moths emerge about a

month later, ready to mate.

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