Parallel, urban-driven phenotypic changes
There is increasing evidence that, for a given species, individuals inhabiting cities often differ from their counterparts outside of cities. Such differences may result from individuals suffering from environmental conditions in cities. For instance, my colleague Michela Corsini showed that, in the great tit (Parus major) and the blue tit (Cyanistes caeruleus), nestlings are lighter, and their mortality increases along the rural-urban gradient. Alternatively, these differences may result from populations adapting to environmental conditions in cities. For example, feral pigeons (Columba livia) are darker in cities than in rural areas; in a previous study, I showed that such variation may result from darker individuals being more tolerant to metal pollution. In any case, characterising rural-urban gradients is a prerequisite to further understand how urbanisation drives differences between populations.
Metal pollution: a good candidate for driving phenotypic changes
Among the potential drivers of phenotypic variation between urban and rural areas, metal pollution is a good candidate. First, metallic/metalloid trace elements (MTEs; e.g., lead, cadmium, copper) are mainly emitted by human activities such as road traffic, residential heating, coal burning and industry – and urban areas are by definition characterised by high densities of humans and resulting human activities. Second, MTEs can have lethal or sublethal effects on organisms. In birds, individuals nesting close to metallurgic smelters show reduced reproductive outputs. I recently demonstrated a similar pattern in birds nesting in cities: copper, arsenic, lead and cadmium are associated with reduced mass and survival of young great tits and blue tits in the capital city of Warsaw (Poland). Last but not least, my previous studies suggest that metal pollution may be responsible for urban melanism in the feral pigeon. All in all, metal pollution, if proven to vary consistently along rural-urban gradients, could explain urban-driven phenotypic changes measured in several cities worldwide.
Measuring bird exposure to metal pollution in replicated rural-urban gradients
(in green) where blue tits and great tits were sampled.
In this study, we investigated variation in the concentrations of six MTEs: zinc, copper, lead, arsenic, mercury and cadmium. We measured MTEs in the feathers of two bird species – great tits and blue tits. The birds were sampled near-simultaneously in eight replicated rural-urban gradients and four protected forests across Poland. Importantly, we quantified urbanisation at a fine spatial scale using high-resolution environmental data extracted from satellite imagery; we inferred urbanisation as either the percentage of impervious surface (the percentage of soil sealing and built-up areas), the percentage of non-tree cover (similar to the percentage of impervious surface, but also includes green lawns without trees) or an urbanisation score computed from several environmental variables. We also measured urbanisation as five habitat categories : urban centre, residential area, urban park, suburban forest and protected forest. These habitats reflect different levels and types of human activity.
Bird exposure to copper, zinc and lead increases along rural-urban gradients
This study demonstrates that, across eight replicated rural-urban gradients, copper, zinc and lead concentrations in bird feathers increased along the rural-urban gradients. On the contrary, mercury concentrations decreased with increasing urbanisation. Finally, arsenic and cadmium concentrations did not significantly vary along the rural-urban gradients. Importantly, those relationships were similar for both great tits and blue tits, and for one-year-old and older individuals. This result confirms the pervasive impact of urbanisation on the presence of MTEs in wild organisms, irrespective of the fact that these eight cities differed in terms of size and population density. The replicated nature of the experimental design convincingly demonstrates that metal pollution is likely to explain urban-driven reproduction impairments measured in other studies. Thus, metal pollution may generate selection for increased tolerance to metal pollution in urban wildlife.
A nudge to move from the urban/rural dichotomy
This study also demonstrates that comparing bird populations between urban parks and forests, a still common study design in urban ecology, is not appropriate to study the ecology and evolution of animal populations. Indeed, zinc and lead concentrations were similar in individuals sampled in urban parks and in suburban or protected forests. This means that studies characterising urban populations from individuals sampled exclusively in urban parks fail to consider the diversity of habitats within the urban mosaic. Moreover, such studies may conclude that there is no association between urbanisation and the phenotypes and/or genotypes they are measuring. We hope that our results will convince readers that adequate sampling design in the urban space is key, and that there is a need for a better characterisation of rural-urban gradients.
Read the paper here:
- Urbanisation Exposes Birds to Metal Pollution - October 29, 2021
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