Tree origin or traits? : what makes a tree suitable as an urban habitat for insect fauna?

Sammanfattning: A global decline in diversity of plants and animals, has led to an increased focus on the topic of biodiversity. The trend of urbanisation can be a facilitator for biodiversity loss, and cities generally have lower densities of species compared to non-urban areas. This has made biodiversity in cities an important concept. Cities can support both people and biodiversity, but knowledge is required by the professions working with planning and managing urban parks and green spaces. Trees are a vital component of urban parks and green spaces and have great importance for biodiversity in cities. Knowledge of how plants and trees interact with other species is thus important when aiming to improve biodiversity, and some research on the subject suggests that native plants are better for biodiversity while other research claims the benefits of non-native plants in urban areas for both biodiversity and other ecosystem-services. Most trees, regardless of species, share similar characteristics such as bark, branches, flowers, and leaves that could be important habitats for insects, an invertebrate group that is a prerequisite for biodiversity on higher trophic levels. This thesis is a part of a longer research project from SLU regarding biodiversity in urban environments. The data collected is intended for further studies regarding biodiversity in urban trees and greenspaces. The research question for this thesis is: Does geographical origin or tree morphological traits affect a tree’s suitability to act as a host for local insect fauna? The aim is to gain an understanding of the complex interplay between species interactions regarding trees and insects. Such understanding could be used in different fields and disciplines for planning and maintaining a biodiversity-friendly urban forest. The main method has been a field collection of insects from trees in the central Malmö area with a supporting literature study of trees as insect habitats. The criteria for trees inventoried was: old age, geographical origin and presence in both park and street environments. This resulted in 42 trees evenly divided on 6 species, Acer platanoides, Acer pseudoplatanus, Acer tataricum subsp. ginnala, Crataegus monogyna, Crataegus intricata and Crataegus orientalis. The result from literature and field method indicated that insect abundance varies between native and non-native trees without any correlation regarding geographical origin or morphology. Crataegus monogyna had a significantly higher number of larvae than other Crataegus species and Acer pseudoplatanus had a higher number of aphids than other Acer species. According to literature and field observations, non-native trees have the same morphological prerequisites as native trees to act as habitat, which indicates that more factors than geographical origin or morphological components affects the insect abundance in a tree. These findings may give an insight into the understanding of trees as urban habitats. The information might prove useful to greenspace planners, managers, urban ecologists, and urban foresters working to improve biodiversity in cities.