How Cities Are Literally Keeping Animals From Making It Big

Animals in warmer places will benefit from being smaller and leaner than their cousins who inhabit colder climes. And cities are heat traps. Put two and two together…

Cities are becoming bigger and are also growing in number, and these changes are happening faster every year, in the process reducing or entirely replacing native ecosystems. Much remains to be understood about how these profound changes affect biodiversity, both locally and globally. Analysing the impact can help us evaluate the effects of urbanisation and design effective and sustainable conservation measures.

For one, the effect of city dwelling on animals is seen in rapidly changing populations and in a variety of species, as well as in modified animal behaviour and altered physiological attributes – such as body shape.

In a recent study, an international collaboration of ecologists demonstrate that cities also affect the body size of animals.

Body size is a distinguishing trait of species. It affects their interactions within ecosystems, and is related to reproductive fitness and longevity. When it is warmer outside, size also determines metabolic costs: a larger body means more energy is needed to keep it going. So animals in warmer places will benefit from being smaller and leaner than their cousins who inhabit colder climes.

This pattern has been observed among many animals, chickadees, rabbits, dogs and monkeys, to name a few. For warm blooded animals, bigger, squatter bodies can generate and keep in more heat, making them better able to tolerate the cold.

It is not so surprising then, that scientists have predicted that a warming world will prefer smaller animals. And cities are notorious heat-traps that are significantly warmer than their surrounding areas. There are many reasons for this: lesser green cover, higher energy consumption, heat-absorbing materials lining buildings, tall structures that create heat-confining urban canyons.

Does this mean the birds, animals and insects in cities are smaller than they are outside?

The research consortium, led by Luc De Meester from Katholieke Universiteit Leuven, Belgium, sampled 95,001 individuals from 702 species to find the answer. The consortium included researchers from Belgium, France, Germany, Italy and Brazil.

Harini Nagendra, a professor of sustainability at Azim Premji University, Bengaluru, called the study “a massive and pretty unique effort”.

The researchers chose species from ten varied animal groups, or taxa – from microscopic rotifers within lichens, zooplankton in ponds and ground spiders to insect groups such as butterflies and moths.

The joint effort was crucial: “Each animal group needs sampling in a different way, an expertise only found within the diversity of research groups of the consortium, as is the identification expertise,” Thomas Merckx, lead author of the paper from Université catholique de Louvain, Belgium, told The Wire.

“Each individual has to be sized as well,” Nagendra said. “Body size varies by a factor of 400!”

All 10 taxa were sampled simultaneously using the same technique. They were divided into three classes: urban, semi-urban and non-urban. Each animal group was sampled in 81 local subplots within 27 larger plots spread across 8,140 sq. km in northern Belgium.

They found that smaller species dominated urban settings for a majority of animal groups, just as they’d expected. And the smaller sizes corresponded to higher ambient temperatures recorded at ponds, grasslands and woodlots in urban settings.

However, three taxa – butterflies, moths and grasshoppers – displayed the opposite pattern. In urban settings, the members of these groups were found to be larger. Between non-urban and highly urban sub-plots, the mean body size of these three taxa increased by 14%. On the other hand, creatures from other groups had displayed a 16% decrease (both numbers on average).

According to Merckx, his group is the first to show that urbanisation is causing animals’ body sizes to increase and decrease.

Why would these three groups go against the flow?

The scientists later found that insects of species belonging to these groups that were larger also moved around more. In the other groups, it was either that the smaller critters moved around more or that there was no such relationship at all.

Mobility is key in the face of severe habitat fragmentation – which is another bad thing that urbanisation does.

When a species is more mobile, it has a better chance of coping even when its habitat is severely fragmented. Hans Van Dyck, a professor of behavioural ecology and conservation at the Université Catholique de Louvain and one of the study’s authors, explained in a press release that for some animal groups, larger size results in better mobility. So in these taxa, urban fragmentation causes shifts towards increased body size.

While hotter pockets of the city encourage animals to shrink in size, habitat fragmentation encourages some animals to become bigger so that they can move around better.

Though both these pressures are strong, these three groups appear to be shaped mainly by the fact that their ecosystem was cut up, and less so by the fact that their environment is getting warmer.

De Meester, Van Dyck and Merckx urged urban planners to mitigate these drivers of size change. During the course of the study, they noted that larger species of water fleas were almost absent in urban ponds (in northern Belgium). This implies a much higher risk of algal blooms in city ponds because only larger fleas are efficient at filtering algae from the water.

For example, they recommend creating urban ponds, planting trees along streets and creating green spaces to provide more cool spots where local species can chill. They also advocate replacing exotic plants with more functionally important native species.

Such efforts are already underway in many places in India, including Chennai. For example, Nizhal, a Chennai-based non-profit organisation (whose name in Tamil means ‘shade’), has been trying to regenerate the city’s biodiversity with a focus on bringing back local, indigenous species, like the purasu (Butea monosperma) and the endangered Asoka (Saraca asoka) tress.

According to Nagendra, the study’s results should apply to all urban contexts, and the conclusions wouldn’t be very different should the study have been conducted in Indian cities.

Of course the specifics will vary, and we can’t predict which taxa and species will be smaller and which bigger. “Despite the fact that urbanisation is fast emerging as one of the most important global drivers of biodiversity change, very limited ecological research examines cities,” Nagendra said. “Urban ecological research needs to ramp up in response to the scale of the issue.”

Harini Barath is a freelance science writer based in New England, USA.