Climate Change & Phenology

Nothing on this page goes extinct because the temperature is too high. Everything on this page goes wrong because two events that used to happen at the same time have stopped happening at the same time. Timing is the whole topic.

What phenology is

One way to measure the impact of climate change is to study events that happen out in nature, year after year, and record when they happen. That field of study is phenology.

The biological events in question are the seasonal ones: migration, egg laying, flowering and hibernation. Each of these has to happen at roughly the right moment, and organisms judge that moment using signals from the environment rather than a calendar.

Trees

In deciduous trees the events studied are bud setting, bud bursting and flowering. It is worth being precise about them, because they are easy to muddle:

Timing here is crucial to reproductive success. A tree pollinated by insects needs its flowering to coincide with the emergence of its pollinators – flowers that open before the insects are about get pollinated by nobody. Trees that use wind or water instead may depend on the weather, tides or river volume, all of which are themselves seasonal.

Birds

Birds migrate to different parts of the world at different times of year, which keeps them in suitable temperatures with enough resources all year round. Getting the journey right matters in both directions:

• The weather during the journey has to allow them to arrive at all – storms at the wrong time of year can blow migrating birds off course.
• Birds that migrate too early may arrive to find conditions unsuitable or resources not yet available.
• Birds that migrate too late may find all the nesting sites taken, or no mates left.

Nesting and egg laying have to be timed so that chick hatching coincides with resource availability. Birds often depend on insect larvae such as caterpillars to feed their young, and those caterpillars in turn depend on leaf bud bursting for their own food. Three levels of a food web, all needing to line up.

How climate change disrupts the timings

Climate change disrupts the timing of biological events because the environmental cues stop working the way they should:

• Temperature may be different from previous years, so a temperature-based cue arrives earlier or later than it used to.
• Photoperiod – day length – does not change at all, so a photoperiod-based cue may no longer coincide with the events it used to predict.

That second point is the key to this whole page, and it is subtle enough to be worth stating plainly. Day length is fixed by astronomy and cannot shift. Temperature is shifting. So any two species that use different cues will gradually drift apart, even though neither of them has done anything wrong.

If events no longer happen at the right time, species are left without the resources they need and food chains are disrupted. A trophic level can effectively go missing from the chain at the moment it is needed. This is a trophic mismatch.

Case study: reindeer and Arctic mouse-ear chickweed

Reindeer are migratory mammals and they use day length as the environmental cue for their seasonal migration. Arctic mouse-ear chickweed is a plant that forms part of the reindeer’s diet, and its peak productivity is determined by temperature.

Two species, two different cues. Warming moves the plant’s peak earlier while leaving the reindeer’s schedule exactly where it was.

Be careful how you phrase this one in an exam. The reindeer migration is not disrupted – it happens exactly as it always has. What is disrupted is the availability of the food that allows them to breed and raise young, and this mismatch between migration timing and resource availability can reduce their breeding success.

Case study: great tits and caterpillars

Great tits are small birds that often feed on insects, and they are especially dependent on caterpillars for feeding their young during the breeding season. The demand is not spread out – the birds need the greatest biomass of caterpillars around 9 days after their chicks hatch, when the chicks are growing fastest.

Here both events are cued by temperature, so you might expect them to shift together and stay in step. They do not, because warming has affected them by different amounts. The disruptive effect on caterpillars appears to be stronger than the effect on the birds:

• Caterpillar biomass now peaks around 2 weeks earlier.
• Great tit egg hatching now peaks around 1 week earlier.

Two weeks against one week leaves a gap of about a week between when the food is most abundant and when the chicks most need it. That gap is a temporal mismatch – temporal simply means related to time – and it reduces the breeding success of the great tits.

Case study: the spruce bark beetle

Not every phenology effect is a mismatch. Sometimes warming changes how many times something can happen in a year, and that is enough on its own.

Spruce bark beetles lay their eggs under the bark of spruce trees and other conifers, and they occur across Europe and North America. The beetles feed and mature under the bark, causing damage and sometimes killing the tree when large numbers of larvae are present. The cycle runs: eggs hatch into larvae that feed on tree phloem and other tissues, the larvae pupate into adults, the adults emerge from the bark, and it starts again.

The important detail is that this life cycle can be completed once or twice each year, and which of those happens is determined by temperature. Global warming has increased the number of cycles completed per year, because higher temperatures make two life cycles more likely.

The consequence is straightforward: the beetle population grows, and the trees are subject to damaging larvae twice each year rather than once, which increases the chance of tree death.

Worked examples