Featured Image Caption: European garden spiders build smaller, fuller webs in cities where there are fewer, smaller insects to eat. (Source: Mike Pennington via Creative Commons)
Reference: Dahirel, M., De Cock, M., Vantieghem, P., & Bonte, D. (2019). Urbanization-driven changes in web building and body size in an orb web spider. Journal of Animal Ecology, 88, 79-91. DOI: 10.1111/1365-2656.12909
Bright Lights, Big City
If you have ever visited or lived in a big city, you are probably familiar with the hustle and bustle, massive buildings, honking cars, strong odors, and crowds of people. More than half of the people in the world now live in cities, but this was not always the case. After the Industrial Revolution, many people left their rural lifestyles behind to build new industries in cities. The move from rural to urban areas has continued to increase as global human populations grow exponentially. Much like humans who have made the move to big cities, urban spiders still work every day, but the work might be a little different from what they did in rural areas.
Life in the big city can be much harder for animals than it is for us. Compared to the natural habitats of animals, cities can make it more difficult for animals to travel, be hotter, lack sufficient food and shelter, and be disorienting because of light and air pollution. These challenges can be detrimental for many animal species, but some are able to adapt their body structure and behaviors so they can better survive these harsh environments. Some spiders – such as Araneus diadematus, or the European garden spider – do really well in both urban and rural areas.
Spider Webs to the Rescue!
Even with all of the challenges that living in a city brings, many animals have been able to adapt to city-living, including pigeons, rats, and – you guessed it – spiders! Some spiders build webs that they use to capture and eat small insects. Web size (capture area) and fullness (mesh width) are two characteristics that spiders can change depending on the food available in their environment.
Think of a web like a fishing net. If your goal is to catch more fish, you will need a bigger net. If you need to catch smaller fish, you are going to need a net with smaller holes in it, so that the fish don’t escape. A spider can build a bigger web with a larger capture area if it needs to catch more insects, and a fuller web with a smaller mesh width if it needs to catch smaller insects. Larger, less-full webs take the same amount of energy to make as small, full webs, but the large, full webs that are needed to catch many, small insects require much more of the spider’s energy.
The European garden spider tears down its web each day and rebuilds it the next day changing the structure of the web based on the insects the spider encountered the day before. So, the question is, can spiders adapt to city-living by changing their web-building behaviors? This is exactly what a group of researchers at Ghent University in Belgium wanted to find out.
Big City, Smaller Webs
The team of researchers visited 63 sites throughout Belgium with high, medium, or low urbanization levels and measured the webs of European garden spiders at each location. They found that highly urbanized sites had fewer, smaller insects than less-urban sites, and as a result, the spiders built smaller, fuller webs. Although the number of these spiders between more-urban and less-urban environments were the same, spiders in the more-urban sites were smaller due to increased temperatures in cities. Even though spiders would do better if they made larger, fuller webs in cities, they might lack the energy it takes to make these webs.
This study shows that web-building spiders might be able to adapt to our urban ways by changing the size and fullness of their webs to catch more insects. However, not all spiders have this ability since some build more permanent webs and others build no webs at all. Spiders are frequently studied for their role in keeping insect populations under control. Without spiders, agricultural systems could decline, and insect-borne diseases might be more prevalent. Therefore, it is crucial that we make strides to improve the quality of cities for animals by implementing more green spaces and reducing air and light pollution.