Gulls and crabs: numbers soar as waters warm

Lesser black-backed gulls

ResearchBlogging.orgRising temperatures in the North Sea are leading to greater numbers of lesser black-backed gulls as their favourite food source, swimming crabs, become ever more plentiful. 

The lesser black-backed gull (Larus fuscus) is one of the most familiar bird species around the coastlines of Europe, and now it appears that we’ll be seeing even more of them. The gull population has rocketed over the last few years in breeding colonies around the North Sea, particularly along the shorelines of Belgium, France, Holland and Germany. The gulls usually arrive at these nesting colonies around mid-April after travelling from their wintering grounds in the Iberian Peninsula and North Africa. Once there, the female lays up to three eggs which are incubated in turn by both parents for around 28 days.

While at their breeding grounds, the gulls mostly feed on several different species of swimming crab (subfamily Polybiinae). These swimming crabs spend the early part of their lives as small larvae that float along with the plankton in the sea. In the last few years, the numbers of these swimming crab larvae have exploded, while at the same time, a new species of warm-water swimming crab (Polybius henslowii) has arrived in the southern part of the North Sea. Increased sea surface temperatures appear to have triggered these changes in larval abundance, and have consequently led to greater numbers of adult swimming crabs.

A paper published earlier this year in Biology Letters clarifies the link between warming sea surface temperature, swimming crab numbers and changes in the abundance of lesser black-backed gulls. The authors show that continued warming since 1997 has led to higher water temperatures throughout the year in the North Sea. This is particularly significant as temperature changes can have a huge effect on marine organisms, altering where they live, their survival rates and even their ability to reproduce. More importantly, any changes in the abundance of one species can have a knock-on effect for many other organisms. In this case, consistently higher temperatures have created more favourable conditions for swimming crab larvae, so that many more individuals are now able to survive to adulthood. And fortunately for the lesser back-backed gulls, this means that there is a glut of swimming crabs for them to feed upon during the breeding season. The crabs not only provide a reliable supply of food for the gulls, but they also offer an important source of calcium for the developing chicks. Thus, many more chicks are able to reach adulthood, and this is reflected three to four years later in the doubling in size of several gull colonies around the North Sea.

These findings highlight the massive effect that climatic changes can have on natural ecosystems. Although the temperature increase in the North Sea may seem small (1-1.5°C), many marine organisms are actually incredibly sensitive to such changes, and, as this study shows, the effects may even extend out of the oceanic environment to terrestrial habitats. And so, even now, we are beginning to see how climate change is having actual measurable effects on animal populations.

These findings were only revealed because the study was able to make use of long-term datasets going back as far as the late 1970s. Such information is extremely valuable as it allows researchers to see patterns in animal populations that are only noticeable over long time periods. It is especially important in determining the response of species to changes in the climate. This study used several long-term data sources, including the Extended Reconstructed Sea Surface Temperature, the North Sea Benthos Survey and the Continuous Plankton Recorder. Unfortunately, there are few other places in the world where sufficient data exist to investigate changes in populations over long periods of time. This scarcity makes studies like this even more important as we try to find out more about how ecosystems cope in a rapidly changing world.

Reference: Luczak C, Beaugrand G, Lindley JA, Dewarumez JM, Dubois PJ, & Kirby RR (2012). North Sea ecosystem change from swimming crabs to seagulls. Biology Letters, 8 (5), 821-824 PMID: 22764111

Image: Flickr

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