Climate change ecology

Most organisms are affected by climate: where they occur, how their physiology is organized, or when they breed or migrate. Climate change is thus expected to affect organisms in many possible ways. Long-term data clearly show changes in species ranges or timing of breeding. Some species may profit from these changes, whereas many more are likely to suffer, because their populations are already under pressure through habitat destruction or other human related activities. Little is known how the projected climate change will affect population dynamics, let alone the functioning of entire ecosystems, and the potentially cascading effects must be a concern for society at large.

From a biologists’ perspective, climate change is an interesting experiment, because the process of adaptation to changing environments can be studied in detail. How quickly do species adapt, and how do species differ in speed of adaptation? Are evolutionary changes observed and required or can flexible responses suffice? What are the consequences for population dynamics if a species does adapt insufficiently? And how would this affect the stability of ecosystems? These are the kind of questions our research group is dealing with.

Phenology among trophic levels

In nature one of the most notable effects of climate change is the advance of timing of seasonal events, like flowering of plants and reproduction in animals. At first glance, this seems to be a good response, because when it is warmer organisms can make use of longer growing seasons. However, species often differ in the extent of such a response to climate change, which may cause problems.

Many birds breed in the spring when abundant food is present during a short period. Forest breeding song bird species mostly hatch their chicks when caterpillars are abundant. In the Netherlands, this caterpillar peak lasts for only three to four weeks, after which most caterpillar species pupate and become unavailable to the birds. A proper match between hatching and the caterpillar peak guarantees a high reproductive success.

As a result of climate change caterpillars in The Netherlands and the UK have advanced their peak dates about 20 days between 1985-2010^1,2. The birds that rely on these caterpillars for chick feeding have responded in diverse ways: in the UK great tits responded to a similar extent with their hatching dates and thus maintained their synchrony. However, in the Netherlands responses differed among trophic levels, and tits and flycatchers increasingly bred too late to fully profit from the caterpillar peak¹. Whereas tits and flycatchers did advance their hatching dates, sparrowhawks that rely on abundant passerine fledglings to feed their offspring did not at all advance their hatching dates. The responses to climate change we observe are thus not always sufficient to match the changes in timing of important food sources.

Pied flycatchers as model species

Most of our work is performed on a small insectivorous passerine, the pied flycatcher. This species can be easily studied, because it readily uses artificial nest boxes for breeding. We capture all parents and give them unique rings, as we do with their offspring. By creating such a pedigree we can study the inheritance of important ecological traits, like migration and breeding dates. Knowing whether traits have a genetic basis is important for the study of possible evolutionary responses to climate change. In our study population in SW-Drenthe (NL) we have annually more than 300 pairs breeding in nest boxes, and more than 100 young recruiting as breeders which we ringed locally.

Pied flycatchers can furthermore easily be manipulated during the nestling stage. We can study their diets by using nest box cameras. We can experimentally investigate the direct consequences of hatching early or late in the season by changing the hatching dates of nests. And we even managed to make Dutch pied flycatcher pairs to breed successfully in Sweden under natural conditions by translocating them.

Consequences of an insufficient timing response to climate change?

One consequence of the increasing mismatch between hatching date and the caterpillar peak is that late breeding individuals have increasingly lower reproductive success. This is most likely due to the change in food availability. With the Pied Flycatchers in the Netherlands we have been measuring chick diets with nest box cameras in different years. In cold springs like 2010 and 2012, both early and late breeders provide their chicks mostly with caterpillars, whereas in warm springs like 2007 and 2011 only the earliest nests get caterpillars, whereas the later nests are raised on a mixture of other insects.

Between 1980-2001 the reproductive advantage of breeding early increased; i.e. later breeders performed increasingly worse relative to early breeders³. Interestingly, this did not lead to a decline in the mean reproductive success of pied flycatchers. Preliminary data do suggest that in recent years selection for early breeding has diminished again, suggesting that the flycatchers are adapting to a certain extent to climate change.

Pied flycatcher populations were affected by the insufficient response in timing to climate change: local populations in the Netherlands that were breeding in rich oak habitats with an early food peak declined towards extinction. In habitats with later caterpillar peaks such a decline was not observed⁴.

PhD students:

• Jelmer Samplonius
• Janne Ouwehand
• Claudia Burger (former PhD)

References:

1. Both C et al (2009). J. Anim. Ecol. 78:73-83
2. Charmantier A et al (2008). Science 320:800-803
3. Both C & Visser ME et al (2001). Nature 411:296-298
4. Both C et al. (2006). Nature 441:81-83