Abstract
BackgroundAlthough the species-urban green area relationship (SARu) has been analyzed worldwide, the global consistency of its parameters, such as the fit and the slope of models, remains unexplored. Moreover, the SARu can be explained by 20 different models. Therefore, our objective was to evaluate which models provide a better explanation of SARus and, focusing on the power model, to evaluate the global heterogeneity in its fit and slope.MethodsWe tested the performance of multiple statistical models in accounting for the way in which species richness increases with area, and examined whether variability in model form was associated with various methodological and environmental factors. Focusing on the power model, we analyzed the global heterogeneity in the fit and slope of the models through a meta-analysis.ResultsAmong 20 analyzed models, the linear model provided the best fit to the most datasets, was the top ranked model according to our efficiency criterion, and was the top overall ranked model. The Kobayashi and power models were the second and third overall ranked models, respectively. The number of green areas and the minimum number of species within a green area were the only significant variables explaining the variation in model form and performance, accounting for less than 10% of the variation. Based on the power model, there was a consistent overall fit (r2 = 0.50) and positive slope of 0.20 for the species richness increase with area worldwide.ConclusionsThe good fit of the linear model to our SARu datasets contrasts with the non-linear SAR frequently found in true and non-urban habitat island systems; however, this finding may be a result of the small sample size of many SARu datasets. The overall power model slope of 0.20 suggests low levels of isolation among urban green patches, or alternatively that habitat specialist and area sensitive species have already been extirpated from urban green areas.
Highlights
The species-urban green area relationship (SARu) has been analyzed worldwide, the global consistency of its parameters, such as the fit and the slope of models, remains unexplored
Of the 101 articles we found that analyzed bird communities in urban green areas, relevant information was found in 49 studies
It is relatively low compared with that observed in terrestrial habitats for several taxa, including birds (z = 0.25 for independent islands; see Fig. 1 in Drakare et al 2006), and in urban datasets for multiple taxa (z = 0.27, Matthews et al 2016a). This suggests that, whilst urban green areas can be considered as isolates (Szlavecz et al 2011; Matthews 2015; Fattorini et al 2018a), as far as birds are concerned, urban green areas are distinguishable from other true and habitat island systems due to the relatively gradual rate of increase in species richness with area. This could potentially be the result of low turnover rates of bird species among patches within systems of urban green areas, which is consistent with the species-fragmented area relationship perspective (Hanski et al 2013), but as we looked at island species-area relationships (SAR) as opposed to accumulation curves (Matthews et al 2016b), further tests are needed to evaluate this hypothesis
Summary
The species-urban green area relationship (SARu) has been analyzed worldwide, the global consistency of its parameters, such as the fit and the slope of models, remains unexplored. Whilst urban green areas can be regarded as habitat islands, and despite their relatively small size rendering them amenable study systems for testing island theory, few urban ecologists contextualize their work in terms of island biogeography (see Fattorini et al 2018a, b)
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