Vegetated roofs in boreal climate support mobile open habitat arthropods, with differentiation between meadow and succulent roofs

Abstract

Vegetated roofs are hoped to benefit urban wildlife, yet there are few empirical results regarding the conservation potential of such roofs. In this paper, we focus on arthropods on vegetated roofs. We vacuum sampled 17 succulent, meadow or succulent-meadow roofs, in Helsinki, Finland, and used order to species level information together with trait data to describe the communities. We evaluated the importance of biophysical roof characteristics on shaping arthropod assemblages to provide information concerning roof designs that promote rich arthropod fauna. Arthropod communities differed between the three roof types and the influence of roof variables varied between and within arthropod orders. The main local drivers of arthropod abundance across the individually analysed taxa were roof height and vegetation, with mainly positive effects of height (up to 11 m) and litter cover, and mainly negative effects of grass cover. Based on trait data from true bugs, spiders and ants, the roofs consisted mainly of common dispersive species that are generalist feeders and associated with dry open habitats or have wide habitat tolerance. We found one true bug species new to the country and assume that it arrived with imported vegetation. Based on these findings, vegetated roofs of varying height and size benefit common generalists and fauna of open dry habitats, but seem to lack rare native specialists and may introduce non-natives if imported plant material is used. Because the responses to vegetation characteristics are taxon-specific, high diversity of roof vegetation types would benefit arthropod conservation.

Arthropod Communities on Young Vegetated Roofs Are More Similar to Each Other Than to Communities at Ground Level

“…It is highly likely that species with poor drought tolerance will fail to establish viable populations on roofs that have easily drying substrates. Observations that the arthropod fauna of such roofs typically consists of species associated with dry and xeric habitats and of tolerant habitat generalists (Brenneisen, 2006;Madre et al, 2013;Kyrö et al, 2018Kyrö et al, , 2020Pétremand et al, 2018) supports this assumption. As vegetation was also more similar between roofs than between roof and ground level plots, we consider it as another likely important filter for the roof fauna (Siemann et al, 1999;Langellotto and Denno, 2004;Haddad et al, 2009).…”

“…Yet, suction sampling is biased toward smaller and lighter species, (Doxon et al, 2011) many of which may also be readily dispersed by air currents. Furthermore, we assume that species that were positively associated with roofs and negatively with time in the roof vs. ground model, arrived with roof materials (see Kyrö et al, 2020), but disappeared later from these roofs. Later on, some species started to thrive on the roofs (taxa responding positively to the interaction between roof and time) and their parasitoids became common.…”

“…The low number of replicates and limited building height range may explain, at least partly, why we failed to find support for predictions derived from the classical Island Biogeography Theory (IBT) (MacArthur and Wilson, 1967) that predicts species richness to increase with patch size (here roof size) and connectivity to source habitat (here building height). Yet, as suggested by Kyrö et al (2020), the low utility of predictions derived from IBT on vegetated roofs may also relate to the smallisland effect: in small habitat patches variation in species richness is no longer connected to patch size, but e.g., effects of habitat quality are more important (Lomolino, 2000;Wang et al, 2018). Yet, roof size may be important in shaping temporal patterns in the vegetated roof arthropod fauna.…”

“…We only sampled the vegetation plots that were established with mats to avoid vacuuming loose substrate in the plots established with plug plants and seeds. Similarly to Kyrö et al (2020), the number of samples was adjusted to vegetation plot size following the formula n = √ a x 0.5 (a = vegetation plot area, m 2 ), resulting in one to eight samples per plot and 2-38 samples per roof (Table 1). Close to each roof we chose two ground level habitat patches with low open vegetation, where we took four reference samples.…”

“…Similar to fragments, permeability of the matrix between roofs varies, but analogous to oceanic islands, roofs are habitat patches that lack initial contact with arthropod source populations. Newly established vegetated roofs are still not empty from arthropods, because the construction materials carry soil microfauna onto the roofs (Rumble et al, 2018;McKinney et al, 2019), and are likely to also introduce epigeal arthropods (Kadas, 2006;Páll-Gergely et al, 2014;Kyrö et al, 2020). However, new roofs have high amounts of ecologically open space compared to habitat fragments, and are ecologically very different from the landscape within which they are placed, thus natural colonization is expected to play a key role in shaping communities on new roofs (Gillespie and Roderick, 2002).…”