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

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.

Assessing habitat quality of farm-dwelling house sparrows in different agricultural landscapes.

Having historically been abundant throughout Europe, the house sparrow (Passer domesticus) has in recent decades suffered severe population declines in many urban and rural areas. The decline in rural environments is believed to be caused by agricultural intensification, which has resulted in landscape simplification. We used giving-up densities (GUDs) of house sparrows feeding in artificial food patches placed in farmlands of southern Sweden to determine habitat quality during the breeding season at two different spatial scales: the landscape and the patch scale. At the landscape scale, GUDs were lower on farms in homogeneous landscapes dominated by crop production compared to more heterogeneous landscapes with mixed farming or animal husbandry. At the patch level, feeding patches with a higher predation risk (caused by fitting a wall to the patch to obstruct vigilance) had higher GUDs. In addition, GUDs were positively related to population size, which strongly implies that GUDs reflect habitat quality. However, the increase followed different patterns in homogeneous and heterogeneous landscapes, indicating differing population limiting mechanisms in these two environments. We found no effect of the interaction between patch type and landscape type, suggesting that predation risk was similar in both landscape types. Thus, our study suggests that simplified landscapes constitute a poorer feeding environment for house sparrows during breeding, that the population-regulating mechanisms in the landscapes differ, but that predation risk is the same across the landscape types.

Oxidative stress in birds along a NOx and urbanisation gradient: An interspecific approach.

Urbanisation is regarded as one of the most threatening global issues for wildlife, however, measuring its impact is not always straight forward. Oxidative stress physiology has been suggested to be a useful biomarker of health and therefore, a potentially important indicator of the impact that urban environmental stressors, especially air pollution, can have on wildlife. For example, nitrogen oxides (NOx), released during incomplete combustion of fossil fuels, are highly potent pro-oxidants, thus predicted to affect either the protective antioxidants and/or cause oxidative damage to bio-molecules. To date, epidemiological modelling of the predicted association between oxidative stress and NOx exposure has not been performed in wild animals. Here, we address this short-coming, by investigating multiple oxidative stress markers in four common passerine bird species, the blue tit (Cyanistes caeruleus), great tit (Parus major), house sparrow (Passer domesticus) and tree sparrow (Passer montanus), living along a gradient of NOx and urbanisation levels in southern Sweden. First of all, the results revealed that long- and medium-term (one month and one week, respectively) NOx levels were highly correlated with the level of urbanisation. This confirms that the commonly used urbanisation index is a reliable proxy for urban air pollution. Furthermore, in accordance to our prediction, individuals exposed to higher long- and medium-term NOx levels/urbanisation had higher plasma antioxidant capacity. However, only tree sparrows showed higher oxidative damage (protein carbonyls) in relation to NOx levels and this association was absent with urbanisation. Lipid peroxidation, glutathione and superoxide dismutase levels did not co-vary with NOx/urbanisation. Given that most oxidative stress biomarkers showed strong species-specificity, independent of variation in NOx/urbanisation, the present study highlights the need to study variation in oxidative stress across contexts, seasons and life-stages in order to understand how the ecology and phylogeny of species interact to affect species resilience to urban environmental stress.