Contrasting effects of tree origin and urbanization on invertebrate abundance and tree phenology

Abstract

The ongoing wide‐scale introduction of nonnative plants across the world may negatively influence native invertebrate fauna, due to a lack of coevolved traits related to the novel plants, e.g., unique phytochemicals or shifted phenology. Nonnative plants, specifically trees, are common in urban environments, areas that already pose novel habitats to plants and wildlife through a wide array of anthropogenic factors. For example, impervious surfaces contribute to increased ambient temperatures, the so‐called urban heat island effect (UHI), which can affect local plant phenology. Yet, few studies have simultaneously studied the effects of urbanization and tree species origin on urban invertebrate communities. We measured the city‐level UHI and phenology of nine native and seven nonnative tree species in five city‐center parks in southern Sweden, as well as four common native species in a rural control forest. We quantified the abundance of invertebrates on a subset of native and nonnative tree species through shake sampling, sticky traps, and frass collection. In the urban environment, nonnative trees hosted significantly fewer invertebrates compared to native trees. Furthermore, the nonnative trees had a delayed phenology compared to native species, while the peak of caterpillars associated with the subset of trees surveyed for this measure was significantly earlier compared to that of the native species studied. The effect of tree species origin on urban invertebrate abundance was of a greater magnitude (effect size) than the effect of urbanization on invertebrate abundance in native tree hosts. Hence, the results indicate that the impact of nonnative vegetation may be a stronger driver of invertebrate declines in urban areas than other factors. As the effect of species origin on tree phenology was at a level comparable to the urban effect, increasing prevalence of nonnative vegetation can potentially obscure effects of urbanization on phenology in large‐scale studies, as well as induce mismatches to invertebrate populations. Since parks harbor a large proportion of urban biodiversity, native trees play a crucial role in such habitats and should not be considered replaceable by nonnative species in terms of conservation value.

Urbanisation impacts plumage colouration in a songbird across Europe: evidence from a correlational, experimental, and meta-analytical approach

“…However, lower carotenoid bioavailability does not necessarily reflect the direct action of pollution but the result of changes in phenology and invertebrate biomass in polluted habitats (Eeva et al, 2010; Sillanpää et al, 2008). At the European level, it has been shown that the dynamics of insect prey biomass, in particular caterpillars, differ in timing and spatial distribution between urban and non-urban populations (Jensen et al, 2022; Pollock et al, 2017; Seress et al, 2018). Therefore, a diet low in carotenoids, either due to diet quality or quantity, during plumage development (i.e., either during postnatal development in young or post-fledging/breeding moult in juveniles/adults) could contribute to the observed dullness pattern across urban great tits, especially for chicks.…”

“…Indeed, in the same population, we have previously found evidence for selective disappearance of “ low quality ” birds between fledging and recruitment (Salmón et al, 2017). Nonetheless, juvenile birds usually moult faster than adults and are less efficient in absorbing or utilising dietary carotenoids (Ferns & Hinsley, 2008; Hill, 2002); while this could lead to paler plumage in birds in their first year, compared with older birds, in both urban and forest habitats, the effect might be exacerbated in the urban environment where carotenoid availability is limited compared to the forest and resources often remain lower after the breeding season (Jensen et al, 2022; Pollock et al, 2017; Seress et al, 2018). Although we found habitat differences in colouration in 1 yr birds, these did not persist among birds of 2 yr or older.…”

Urban tree composition is associated with breeding success of a passerine bird, but effects vary within and between years

“…We included oak trees as they have been found to be a primary source of caterpillars and modulate the breeding phenology of great tits in forest populations (van Noordwijk et al 1995;Visser et al 2006). We included beech trees as this was by far the most common species within the parks (Jensen et al 2021).…”

“…A slight mismatch in caterpillar emergence can have detrimental effects at the population level for lepidopterans, because leaves quickly decrease in water and nitrogen content, while defense compounds increase (van Asch and Visser 2007). In urban areas, non-native tree species differ from natives in phenology; while urban native trees show advanced budburst in relation to their rural counterparts in the Northern hemisphere, urban non-native trees were found to have a delayed phenology compared to native trees within the city (Jensen et al 2021, Appendix S1). Combined with arti cial light at night, ambient temperature has been found to modulate egg-laying phenology of great tits (Dominoni et al 2020).…”

“…Furthermore, we expected increasing tree diversity to have a positive effect on breeding success and nestling weight, as we expected that it would increase the number of potential food sources within the territory, in addition to spreading out the food availability over a longer period. Because non-native trees host a lower abundance of invertebrates and caterpillars compared to native trees in our study system (Jensen et al 2021), we expected the number of non-native tree individuals in the territory to have a negative effect on breeding success and nestling weight (Narango et al 2018). Finally, we expected to nd annual variation in the magnitude and direction of effects, as yearly differences in temperature and budburst may result in different food sources being available, in addition to territories varying in their local peak of food abundance.…”