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Review ‘FABI essay - Liza Moerman - Period 5 - 2024 - INTRODUCTION’ by Liza Moerman

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Insects And The City – How to provide for natural enemies in urban areas?
Fundamental and Applied Aspects of the Biology of Insects Supervisor: prof.dr.ir. Erik Poelman
ENT30806_2023_5 Student: Liza Moerman
Year 2023-2024
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Introduction
18 October 2017. The world woke up to a devastating newsflash: “More than 75 percent decline
over 27 years in total insect biomass in protected areas” [1]. Although it wasn’t the first paper
reporting on large-scale insect loss, it did have great impact on the whole research-community.
The major alarming factor in this paper, is the fact that the research was conducted in protected
areas. So the question arose: if we can’t even protect species in the areas specially picked out
for this purpose, what are the effects outside these reserves? The paper, written by Hallmann
et al. (2017), started a chain of research projects, publications, essays and reports, one by one
confirming the inconvenient truth: we are in the middle of an insect crisis.
But why should we consider this an urgent crisis? Aside from the intrinsic value that insects
have as a part of nature, they also fulfill important functions for us, like pollination,
decomposition, pest control, key role in the food web and many more [2]. In order to counter
the insect crisis, protecting, conserving and expanding their habitat is necessary, to say the
least. Among the guilty, large-scale habitat loss and transformation are two of the most
important for this loss in insect diversity and abundance [3], under which urbanization is
considered a major contributor. With more than half the people on this planet living in cities
and prospects being that this trend will continue [?], the impact of urbanization on insect
diversity and abundance cannot be ignored anymore. In this essay, I want to explore the
potential of cities to become save havens for insects, and in this way contribute to the race
against global biodiversity loss.
A growing body of research has tried to capture the main drivers of urbanization and their
direct and indirect effect on insect. Interestingly, results on this topic have been contradictory
[3]. Depending on the taxonomic group, urbanization could have both positive and negative
effects on certain species. This could be explained by the fact that lots of mechanisms are
involved. Aside from city-related features like impervious surfaces, urban heat island, habitat
fragmentation, pollution and invasive species that affect insects species, the specific way in
which they’re affected is really dependent on the species traits, making it even more difficult
to distinguish “the impact of urbanization” on insects [3].
Overall an interesting pattern came forward: higher-trophic insects (predators and parasitoids)
are particularly affected by urbanization in comparison with herbivore insect species [4; 5]. This
pattern could be explained by the fact that predators and parasitoids generally need a wider
range of resources to complete their life cycles than the less demanding herbivores species.
This can cause a shift in the trophic relations between herbivores and predators, altering the
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bottom-up and top-down effects, and thus destabilizing populations [4]. A well-known
example of a risk following this shift is the collapse of top-down control of natural enemies,
causing pest outbreaks [?].
In this essay, I want to focus on higher-trophic insects (predators and parasitoids) and dive
deeper into the reasons and mechanisms causing the disadvantage of these natural enemies
relative to herbivore species, ultimately looking for answers to the question: how can we design
urban green areas so that they better provide for all life stages of natural enemies?
First I’m looking for the main differences between herbivores and their natural enemies and
the implications this has for the top-down control. Next, I’m looking at the “urbanization filter”
that causes shifts in the trophic relations to be more pronounced in urbanized areas. I’ll be
supporting this by presenting the example of the interaction between aphids on urban trees
and of their most important natural enemies, ladybirds. Lastly I’ll connect everything into a
summary of hands-on management tools for designing urban green spaces in favor of
predators and parasitoids, since I believe it is really important for science to translate
knowledge directly into accessible advice for practice.
References
1. Hallmann, C. A., Sorg, M., Jongejans, E., Siepel, H., Hofland, N., Schwan, H., Stenmans, W.,
Müller, A., Sumser, H., Hörren, T., Goulson, D., & De Kroon, H. (2017). More than 75
percent decline over 27 years in total flying insect biomass in protected areas. PLOS
ONE, 12(10), e0185809. https://doi.org/10.1371/journal.pone.0185809
2. Kim, K. C. (1993). Biodiversity, conservation and inventory: why insects matter. Biodiversity
And Conservation, 2(3), 191–214. https://doi.org/10.1007/bf00056668
3. Fenoglio, M. S., Calviño, A., González, E., Salvo, A., & Videla, M. (2021). Urbanisation drivers
and underlying mechanisms of terrestrial insect diversity loss in cities. Ecological
Entomology, 46(4), 757–771. https://doi.org/10.1111/een.13041
4. Chatelain, M., Rüdisser, J., & Traugott, M. (2023). Urban-driven decrease in arthropod
richness and diversity associated with group-specific changes in arthropod
abundance. Frontiers in Ecology And Evolution, 11.
https://doi.org/10.3389/fevo.2023.980387
5. Corcos, D., Cerretti, P., Caruso, V., Mei, M., Falco, M., & Marini, L. (2019). Impact of
urbanization on predator and parasitoid insects at multiple spatial scales. PLOS ONE,
14(4), e0214068. https://doi.org/10.1371/journal.pone.0214068

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