Measuring the ‘urbanness’ of a bird community

Harnessing the power of global citizen science data sets to improve local understanding, Corey T. Callaghan (Centre for Ecosystem Science, UNSW Sydney) introduces the Urban Greenspace Integrity Index as a means to track restoration efforts in urban areas.

Restoring urban biodiversity has many benefits (for examples, see here, here, or here), but what should we actually be focusing on in our restoration efforts? If we restore habitats but only common pigeons (for example) take up residence, is the project truly a success? Many people have relied on species richness as the mainstay for measuring the success of a restoration project, and indeed this is a critical component of restoration performance. But we anecdotally know that some animals are ‘better’ than others in urban greenspaces. In a global sense, we want to avoid biotic homogenization, as in having the same 10 species in urban greenspaces throughout the world. We have proposed a solution to this problem, which uses broad-scale empirical data. Here, we introduce the Urban Greenspace Integrity Index.

Local-level urban index
The local-level urban index (on an eBird checklist) is negatively correlated with the distance from the city centre, demonstrating that it appropriately measures the urbanness of a bird community. Credit: Corey Callaghan.

The Urban Greenspace Integrity Index (UGII) is a simple, robust, and tractable method to judge whether urban restoration efforts are transitioning towards an improved ecosystem, without the need for a priori reference states. Our method quantifies improved diversity, incorporating whether desired species reenter the community. In short, it measures the ‘urbanness’ of a given community. UGII has two fundamental steps: (1) it assigns continental-scale urban scores that are species specific (see here for more detail); and (2) it collapses these species-specific urban scores to community-level metrics.

It works by first calculating a local-level urban index, based on the species’ urban scores in a local community – with many samples. To do so, we take the 0.25 quantile of a local-level sampling unit. The average of the local-level sampling units is then defined as the local-level urban index. As in community ecology, we also define a scaler, which measures the potential urbanness of the species pool, and this is the regional urban index. This is done similarly to the local-level sampling index: taking the 0.25 quantile of the potential list of species urban-scores in the regional pool. To get the Urban Greenspace Integrity Index, we divide the local-level urban index by the regional-level urban index. Et voilá! A measure of a local community’s bird urbanness that is comparable across many regions.

A potential workflow using the UGII to monitor restoration in urban greenspaces. Credit: Corey Callaghan.

We demonstrated the applicability of the tool using eBird, a broad-scale citizen science project with more than 600 million bird observations, globally. We first assigned species-specific urban scores (explore these here) for all bird species, using night-time lights data. We then used ten example urban greenspaces to show that the urban index on an eBird checklist – a distinct sampling unit in the eBird database – was empirically related to the species richness on an eBird checklist. We also found that the urban index on an eBird checklist was empirically related to the distance from the city centre for Melbourne, Brisbane, and Sydney. These results are critical because they confirm that our measure makes intuitive sense in relation to two separate measures of urbanization.

The Urban Greenspace Integrity Index can be used to compare parks, or to measure changes through time to track restoration success.  The Index has wide applicability in ecology and although we highlight its applicability with broad-scale citizen science data, other local-scale research datasets can also be used.  With >50% of humans now living in cities and growing pressure for urban consolidation, it’s high time that we up our urban greenspace restoration efforts, and UGII will help to make these efforts more effective.

Read the full Commentary, Using citizen science to define and track restoration targets in urban areas, in Journal of Applied Ecology.

Read more on this topic in the Oikos article, Generalists are the most urban-tolerant birds: an analysis of ecological and life history traits using a novel continuous measure of bird responses to urbanization.


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