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Insects in the city: The effects of urbanization on tree phenology and insect diversity on native and exotic trees

Jayousi, Sherin (2019) BION02 20191
Degree Projects in Biology
Abstract
Urbanization expands very quickly, and it is one of the major threats to biodiversity. The spread of exotic trees in urban sites negatively affect the distribution of native trees and thereby insects. Herbivorous insects are highly dependent on native trees as a host and for providing nutrition. Any impact on the distribution of native trees will hence potentially also alter the insect abundance and diversity in urban sites. The objectives of my study were to investigate the effect of urbanization on tree phenology and to assess the differences in insect and spider abundance and diversity between urban and rural sites, and also between native and exotic tree species. This study was carried out in Malmö, Sweden. The results indicate that... (More)
Urbanization expands very quickly, and it is one of the major threats to biodiversity. The spread of exotic trees in urban sites negatively affect the distribution of native trees and thereby insects. Herbivorous insects are highly dependent on native trees as a host and for providing nutrition. Any impact on the distribution of native trees will hence potentially also alter the insect abundance and diversity in urban sites. The objectives of my study were to investigate the effect of urbanization on tree phenology and to assess the differences in insect and spider abundance and diversity between urban and rural sites, and also between native and exotic tree species. This study was carried out in Malmö, Sweden. The results indicate that the higher temperature in urban parks, attributed to the urban heat island effect, causes an earlier bud burst of trees in urban sites. Also, an earlier bud burst of native, compared to exotic species, was indicated in urban parks. The results showed that the abundance of insects and spiders was significantly higher in native than exotic tree species, while there was no difference between native trees in urban and in rural sites. And more caterpillar species were collected in native tree species and in rural site. However, there were no significant differences in frass dry weight (indicating abundance of caterpillars) between urban and rural sites or between native and exotic tree species. Despite the earlier bud burst in urban areas, there were no significant differences in the peak date of frass (greatest weight) between urban and rural sites, which indicates that herbivorous insects do not really change their activity to match the changes in urban tree phenology.

The results showed that native trees have higher abundance and diversity of insects and spiders. Because urbanization may reduce the proportion of native tree species and replace them with exotic trees, this could lead to negative consequences for the insect and spider communities. Such an effect on lower trophic level can lead to larger effects higher up the food chain. More efforts should be directed to urban areas and the choice of tree species planted there, planting native tree species should be included in conservation plans in order to promote urban biodiversity. (Less)
Popular Abstract
Insects in the city: The effects of urbanization on tree phenology and insect diversity on native and exotic trees

Urbanization expands globally and it can affect biodiversity both by changing the land cover and by increasing the temperature of an urban area, called the Urban Heat Island effect (UHI). The purpose of this study was to assess the effect of UHI and how this affects both tree and insect phenology. Additionally, this study aims to address the differences in insect and spider abundance and diversity between trees in urban and rural sites, and also between native and exotic tree species. Also, I wanted to investigate if there are differences in caterpillar biomass by measuring frass (caterpillar droppings) between urban and... (More)
Insects in the city: The effects of urbanization on tree phenology and insect diversity on native and exotic trees

Urbanization expands globally and it can affect biodiversity both by changing the land cover and by increasing the temperature of an urban area, called the Urban Heat Island effect (UHI). The purpose of this study was to assess the effect of UHI and how this affects both tree and insect phenology. Additionally, this study aims to address the differences in insect and spider abundance and diversity between trees in urban and rural sites, and also between native and exotic tree species. Also, I wanted to investigate if there are differences in caterpillar biomass by measuring frass (caterpillar droppings) between urban and rural sites and between native and exotic tree species.

The study was carried out in Malmö, Sweden, where I studied trees and insects in five urban parks and one rural site (Skrylle). Nine native tree species (Acer platanoides, Betula pendula, Crataegus monogyna, Fagus sylvatica, Quercus robur, Salix caprea, Sorbus intermedia, Tilia x europaea, Prunus avium) and seven exotic species (Ailanthus altissima, Ginkgo biloba, Gleditsia triacanthos, Platanus x hispanica, Pterocarya fraxinifolia, Robinia pseudoacacia, Quercus cerris) were studied. I used temperature loggers to investigate the effect of UHI and tree phenology was observed by monitoring the date of bud burst. I collected insects and spiders in the canopy by shaking tree branches and collecting flying insects by sticky traps. Frass collectors (large funnels) were used to collect frass every 3-4 days and I used frass dry weight as a measurement of caterpillar biomass.

The results show an increase in average daily temperature of around 1.7°C in urban parks compared to the rural site, showing that there is an UHI effect. I also found that the date of bud burst was earlier in urban than rural sites, and also earlier in native than exotic tree species. These results support the prediction that the UHI has an effect on plant phenology. However, I did not find a difference in insect and spider abundance between urban and rural sites. This might be due to the method of collecting insects and spiders, because most of the trees were high and do not have low branches that I could reach and shake to get good estimates of insect abundance. Nevertheless, since I did not find any differences in frass dry weight between urban and rural sites, it may actually be that urban and rural trees provide equally good habitat for arthropods.

I found a higher abundance of insects and spiders in native than in exotic tree species, and more caterpillar species were also found in native than in exotic tree species. This result shows how native trees better support the abundance and diversity of arthropods, and native trees are well known in supporting insects and provide a good habitat to feed on. Oak, Quercus robur, showed the highest amount of frass and this species is an especially good food source for caterpillars. Finally, despite the earlier bud-burst in urban sites, there was no difference in the peak date of caterpillar biomass between urban and rural sites. This could possibly be that UHI effect does not modify caterpillar phenology. However, there was an earlier peak date of caterpillar biomass in exotic tree species. Therefore, exotic and native trees have very different dynamics, both when it comes to tree phenology and the dynamics of the insects that feed on them.

Overall, this study shows that more efforts should be directed to urban areas and the choice of tree species planted there, and planting of native tree species should be included in conservation plans in order to promote urban biodiversity.


Master’s Degree Project in Biology/Conservation Biology/45credits 2019
Department of Biology, Lund University

Supervisor: Anna Persson (CEC)
Co-supervisor: Maria von Post (CEC) (Less)
Please use this url to cite or link to this publication:
author
Jayousi, Sherin
supervisor
organization
course
BION02 20191
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
9002768
date added to LUP
2020-01-24 09:44:14
date last changed
2020-01-25 03:39:11
@misc{9002768,
  abstract     = {Urbanization expands very quickly, and it is one of the major threats to biodiversity. The spread of exotic trees in urban sites negatively affect the distribution of native trees and thereby insects. Herbivorous insects are highly dependent on native trees as a host and for providing nutrition. Any impact on the distribution of native trees will hence potentially also alter the insect abundance and diversity in urban sites. The objectives of my study were to investigate the effect of urbanization on tree phenology and to assess the differences in insect and spider abundance and diversity between urban and rural sites, and also between native and exotic tree species. This study was carried out in Malmö, Sweden. The results indicate that the higher temperature in urban parks, attributed to the urban heat island effect, causes an earlier bud burst of trees in urban sites. Also, an earlier bud burst of native, compared to exotic species, was indicated in urban parks. The results showed that the abundance of insects and spiders was significantly higher in native than exotic tree species, while there was no difference between native trees in urban and in rural sites. And more caterpillar species were collected in native tree species and in rural site. However, there were no significant differences in frass dry weight (indicating abundance of caterpillars) between urban and rural sites or between native and exotic tree species. Despite the earlier bud burst in urban areas, there were no significant differences in the peak date of frass (greatest weight) between urban and rural sites, which indicates that herbivorous insects do not really change their activity to match the changes in urban tree phenology.

The results showed that native trees have higher abundance and diversity of insects and spiders. Because urbanization may reduce the proportion of native tree species and replace them with exotic trees, this could lead to negative consequences for the insect and spider communities. Such an effect on lower trophic level can lead to larger effects higher up the food chain. More efforts should be directed to urban areas and the choice of tree species planted there, planting native tree species should be included in conservation plans in order to promote urban biodiversity.},
  author       = {Jayousi, Sherin},
  language     = {eng},
  note         = {Student Paper},
  title        = {Insects in the city: The effects of urbanization on tree phenology and insect diversity on native and exotic trees},
  year         = {2019},
}