An international team of scientists shows for the first time that urbanization is changing the metabolism of animal communities. The increase in temperatures associated with urban environments generates higher costs for its fauna, which in most cases becomes smaller and smaller to survive. For butterflies, moths and grasshoppers, the effect is the opposite: their size is increasing.
The rapid growth of cities entails a series of changes that do not only affect the landscape. The small living beings that inhabit buildings, sidewalks and asphalt must cope with the loss and fragmentation of habitat, the proliferation of invasive species, light and noise pollution and climate change. Their adaptation to the drastic changes generated by humans occurs in record time.
But in order to survive in cities, both aquatic and terrestrial species reduce their body size. This is revealed for the first time by a group of ecologists, led by the Catholic University of Leuven in Belgium.
According to the study, published in the journalNature, the increase in temperatures in cities causes the ‘heat island’ effect due to the accumulation generated by concrete and other absorbent materials. The fragmentation of the habitats also influences this thermal anticyclone, so that the species spend more energy in maintaining their metabolism and invest less in their growth.
"Higher temperatures generally accelerate development rates, and this may be faster than growth rates," Thomas Merckx, lead author of the work and researcher at the Belgian university, tells Sinc. But the changes are not the same in all species.
While in most body sizes are reduced, for other groups the opposite effect occurs. This is the case of animals that tend to move and disperse –such as butterflies, moths and grasshoppers–, their size increases to ensure mobility and thus find new habitats.
To arrive at these results, Merckx and his colleagues studied 95,001 individuals from 702 species and 10 different taxonomic groups living north of Belgium in urban and non-urban settings. In total, they collected samples from each group at up to 81 different locations.
The most surprising changes
"Among the 10 taxa we analyzed, the greatest change in size occurred in those known as water fleas (cladocerans), a suborder of small crustaceans, generally freshwater, that do not exceed 3 mm", says the scientist. In this group, urban communities were 44% smaller than non-urban site species.
“In fact, the largest species almost completely disappeared from urban ponds. This means that these water reservoirs in cities are more prone to toxic algal blooms, as large species are the ones that most effectively feed on these algae, ”says Merckx.
The body size of animals is a key feature in relationships between various groups, such as food chains or pollination networks.
“These changes are likely to have a strong impact on the function of the urban ecosystem. And this, in turn, will translate into an altered supply of the services that the ecosystem offers to humans ”, warns the scientist.
The research provides the recipe for mitigating the observed changes: more and better green infrastructure at different spatial scales within cities. "These measures will combat the‘ heat island ’effect, as well as the fragmentation of the urban habitat," concludes the expert.
Thomas Merckx et al. "Body-size shifts in aquatic and terrestrial urban Communities"Nature 23 May 2018
Body size is intrinsically related to metabolic rate and life history traits, and is a crucial determinant of food webs and community dynamics. Rising temperatures associated with the urban heat island effect results in increased metabolic costs and is expected to drive changes to smaller body sizes. Urban environments, however, are also characterized by substantial habitat fragmentation, favoring mobile species. Here, using a replicated sampling design, spatially nested in ten taxonomic groups of animals, we show that urban communities generally consist of smaller species. Furthermore, although we show urban warming for three habitat types and community-associated reduced mean body sizes for four taxa, three taxa show a shift toward larger species along urbanization gradients. Our results show that the general trend towards smaller species is canceled out by filtering out larger species when there is a positive covariation between size and dispersal, a process that can mitigate the low connectivity of ecological resources in urban settings. Therefore, we show that the urban heat island effect and urban habitat fragmentation are associated with changes in body level that are critically dependent on the association between body size and dispersion. Because body size determines the structure and dynamics of ecological networks, these changes can affect the function of the urban ecosystem.