Melbourne nature strips planted with native gardens held seven times the insects of mowed lawn

The control sites are the reason to trust the direction of this result. The researchers counted insects on strips before and after planting ^[2], and the paper's title names a before-after-control-impact design ^[3]. In that design, comparison sites that are left alone get surveyed over the same period. They absorb whatever else changed in those seasons, such as a wet spring or a good year for bees, so the gardens are credited only with the gap between changed and unchanged sites. For the strips studied, I'd treat the gain as caused by the planting.

Bell's explanation for the low lawn baseline is short. "Traditional lawn nature strips offer little in the way of shelter, and frequently mowed lawns rarely allow plants to flower," she said ^[7]. Divide the sevenfold rise in abundance by the doubling in species ^[1] and you get roughly 3.5 times as many insects per species on average ^[1].

The bee result matters most for anyone choosing plants. "Bees showed the strongest affinity for these gardens and were positively associated with the number of flowers and the number of different plants in flower," Bell said ^[5]. That link comes from comparing gardens with one another, so it is a correlation. The before-after design supports the claim that planting raised insect counts. On its own, it does not show that adding flowers to a particular garden brings more bees.

The study cannot say whether the suburb gained insects or the gardens pulled them in from nearby. A strip in flower is a feeding site, and a count taken there records where insects were during the survey. Bell makes the city-scale case in terms of many gardens. "While individual nature-strip gardens are a great start, collective urban-greening actions on nature strips have the capacity to effectively support pollinators in cities," she said ^[8]. That city-wide claim goes beyond the study, which, as described, measured individual strips.

Bell aims the evidence at local government. "This research provides evidence that can support council investment in nature-strip gardening and help inform future biodiversity strategies," she said ^[10]. She also argued that the gardens have social benefits. "People who are more connected to nature do more to protect it," she said ^[11]. The release does not give the number of strips surveyed, the number of seasons, what a garden costs to plant and maintain, or whether any social outcome was measured.