The Ecology Book

279

A Great Tit feeds its chicks. If

breeding takes place after the peak

period for spring caterpillars, there

will be less food for the young birds,

and fewer will survive to breed.

could seriously upset the balance

of ecosystems. If flowers appear

earlier, the bees that pollinate them

can respond in one of two ways:

they can emerge earlier; or they can

move to a higher latitude to match

later flower emergence farther from

the equator. Studies of 10 wild bee

species in northeast North America

have shown that their behavior

has changed in line with earlier

flowering. However, bumblebees in

Colorado have not matched the

changes and their population has

fallen. If pollinators decline, so may

the plants that they pollinate.

There is evidence that many

primary consumers have adjusted

to changed natural phenomena, but

species higher in the food chain

seem to find it harder to make the

change. Although birds are now

nesting earlier than they once did,

the timing of insect emergence has

advanced more rapidly. This is

a problem for birds that depend

on peaks in insect abundance. For

example, Pied Flycatchers and

Great Tits feed their chicks on

caterpillars that are abundant for a

short period in spring. Due to

climate change, the caterpillar

peak is now earlier, but the birds

have not been able to advance

their egg-laying dates enough to

take advantage of the glut of food.

Studies show that fewer Pied

Flycatcher and Great Tit chicks

are surviving. Pied Flycatcher

numbers have declined in Dutch

woodlands, possibly as a result of

climate change.

Taking action

All of this disturbing evidence

has prompted climate scientists

worldwide to lobby governments

and demand policy change. Spring

creep has been used by scientists

as a definitive piece of proof that

climate change is occurring, and

researchers have called upon policy

makers to fight global warming to

save the familiar species that find

their very existence threatened

by phenological changes. ■

THE HUMAN FACTOR

Wall butterflies and

climate change

Climate change sometimes

produces unexpected results.

For example, in the UK, the life

cycle of the wall butterfly has

been disrupted by changing

climatic conditions. Previously,

the butterfly produced two

generations every summer.

The late-summer adults would

mate, the females lay eggs,

and the eggs then developed

into caterpillars. In September,

these caterpillars found

sufficient food to grow large

and sustain themselves in

hibernation through winter.

In spring, the caterpillars

metamorphosed into pupae,

and then became adults.

Warmer weather has allowed

a third generation to develop

in fall, with adults flying as

late as mid-October. By

the time the third generation

caterpillars hatch there is

little food, so most starve

and die. Scientists call this

a “developmental trap” and it

is probably responsible for the

decline in wall butterflies.

This butterfly I was

studying shifted its

entire range across

half a continent—

I said this is big ...

Everything since then

has just confirmed it.

Camille Parmesan

US_274-279_Spring_creep.indd 279 12/11/18 6:26 PM