Jessica Kurylo, the University of California Los Angeles and the University of Melbourne
Whether they represent the good, the bad, the ugly, or the prettiest little winged gems you ever did see, urban insects surround and greatly outnumber us. This isn’t a bad thing, though I concede big spiders on your bedroom wall are not pleasant and we don’t want more mozzies living in our gardens. On the plus side, many insects provide ecosystem services that directly benefit us, so maybe we should be better neighbours by giving a little more care and attention to understanding them.
Most of us live in urbanised areas, thus it is where most of us will have our first encounter (if not most encounters) with inchworms, spiders, beetles, flies, bees and—most everyone’s favourite—butterflies. However, our recent study failed to detect about 75% of southeast Melbourne’s regional butterfly fauna within its neighbourhoods. Expectedly, both butterfly species richness and abundance decreased with increasing impervious surface cover. But while abundance made a noticeable decline, starting at about 25% impervious surface cover, richness made a more linear decline. Butterflies are no different to other wildlife; their communities depend on the availability of different resources across the landscape. As adults, butterflies need food resources such as floral nectar/pollen, sap, or rotting fruit. To reproduce, they require larval host plants, with needs ranging from specific (only able to use 1 or 2 species), to generalist (able to use a range of plants from different families). In both larval and adult phases, butterflies also need places to rest and/or shelter.
Similar to the butterflies, floral and larval host plant resources decreased with increasing impervious surface cover ( see Kurylo et al. 2020, and Kurylo 2018 for further details). Across southeast Melbourne’s neighbourhoods, nectar providing floral abundance (native and non-native) had a small positive effect on total butterfly abundance as did larval host plant species richness and cover. However, exotic flowers had a negative effect on the abundance of the less common butterflies within the community throughout the year. Meanwhile, butterfly species richness was positively affected by the availability of remnant vegetation, tree cover, and larval host plant species richness (see Kurylo 2018, Kurylo et al. 2020 for further details). These results would seem to indicate that we can make our urban areas more friendly to butterflies by providing resources that can increase both their species richness and overall abundances with just some easy management and planting choices, such as installing wildlife gardens. But if you build it, will they come?
While the literature indicates an overall benefit of wildlife gardening to biodiversity, our study showed little effect on the local butterfly community. We found no difference in the butterfly community in wildlife gardens, compared to traditional gardens, though native floral abundance was higher in wildlife gardens. What is important to note here though, is that this gardening program was in areas consisting of between 30 and 50% impervious cover. Thus, the butterfly community they were serving was already fairly filtered by the gardens’ landscape context. Generalist butterflies (in terms of larval host plants specialization) dominated the butterfly community above 20% impervious surface cover. There is nothing wrong with this community, they are still pretty and a delight to watch in the garden. But they are not the butterflies most in need of our being better neighbours, nor the ones most often used to engage the public in biodiversity-oriented programs.
What to do? Urban areas already have some of the resources our insect neighbours need; if they didn’t, insects wouldn’t be here. But with a better understanding of how the spatial and temporal availability of their resource needs affects their distribution, we can make better informed management choices to not only help the ones present, but perhaps start to improve resource availability for those less common species with more specialized needs in a genuine or targeted manner—whichever is necessary. Allocating limited financial resources for urban conservation-oriented programs would also be helped by having more subtle information, like that presented here, for Melbourne’s butterflies.
Big and colourful, butterflies can be a gateway to appreciating insects. While they make some limited contributions to pollination, there are loads of other insects such as ants, flies, beetles, bees, and even lawn shrimp (yes, you read that right) doing far more for your veggie patch. They are pollinating the fruit and veg, recycling your organic manures (lawn clippings, fallen leaves, etc.) back into the soil to improve its health, and they are keeping other insects and pest species in check. Most of what they do for us goes wholly unnoticed. Our blind actions have led to general declines in insect richness and abundance in urban areas. They need us to be better neighbours. We can start by trying to understand their needs better, so we can improve our management choices.
For more information, contact Jessica (firstname.lastname@example.org) or follow her on Twitter (@SoilsFlutterBy).
Read more from the September 2020 edition of the ESA Bulletin.