Greening urban areas with indigenous plants can offer insects a refuge from city threats, dramatically boosting species numbers, a new Australian study shows.
Bees and dung beetles may lack the popular appeal of endangered mammals such as the rhino or shy pangolin, but they play a vital role in ecosystems and are also increasingly vulnerable to human threats in Southern Africa.
University of Pretoria entomologist Prof Clarke Scholtz says several of some 5,000 named insect species in Southern Africa — more than twice the number of plant species — are “considered to be rare or threatened”.
Worldwide about one third of insect species is threatened with extinction and about 41% of those species are in decline, he noted.
“In SA about 70% of the indigenous plants and a large percentage of crops and fruit trees depend on honeybees for pollination. Their populations are crashing in North America and Europe,” he said.
“Insects are also essential to recycling the dung produced by livestock and wildlife (helping to control flies), aerating soil and providing a food source for many species of reptiles, birds and mammals, including humans,” said the co-author of Pollinators, Predators and Parasites: The Ecological Roles of Insects in Southern Africa.
About 70% of the indigenous plants and a large percentage of crops and fruit trees depend on honeybees for pollination.
— University of Pretoria entomologist Professor Clarke Scholtz
Yet pesticides and bug zappers kill insects indiscriminately. Just as shark nets kill an array of marine life, bug zappers annihilate any insect attracted by their light and noise.
“Those electric shock devices are supposed to kill mosquitoes, but they kill just about anything. I’ve seen geckos and bats caught up in them. They are nasty,” said Scholtz of the bug zappers which electrocute thousands of insects a night.
Urbanites have a way to reduce the spiralling loss of insect species by planting indigenous vegetation, however small the space (or pot).
University of Melbourne researchers demonstrated that increasing the diversity of plant species inside a city boosted the number of insect species by seven times “confirming the ecologist benefits of urban greening projects”.
Lead author Dr Luis Mata, from Melbourne University’s school of agriculture, food and ecosystem sciences, said: “An increase in the diversity and complexity of the plant community led to, after only three years, a large increase in insect species richness ... and a higher number and diversity of interactions between insects and plant species.
“Most importantly, the indigenous insect species we documented spanned a diverse array of functional groups: detritivores that recycle nutrients; herbivores that provide food for reptiles and birds; predators and parasitoids that keep pest species in check,” Mata stated.
Greening projects like rooftop gardens or urban wildlife meadows also boost people’s wellbeing and mitigate urban warming.
The researchers identified a small space — a grass lawn and two trees — for their four-year study, which began in April 2016. They transformed the green space by planting 12 indigenous plant species.
They conducted 14 insect surveys, sampling each plant species for ants, bees, wasps, beetles and other insects. In total, 91 species indigenous to Victoria, Australia, were identified there out of 94 species.
After one year, the area had attracted 4.9 times more species than two that were found there at the study launch. By the third year, this had risen to 7.3 times more insect species, according to the findings, published in the British Ecological Society journal, Ecological Solutions and Evidence.
“We hope our study will serve as a catalyst for a new way to demonstrate how urban greening may effect positive ecological changes,” said Mata.
Habitat destruction or transformation, and construction, are among the main threats to insects, said Scholtz, along with pesticides, veld fires, alien invasive plants and light pollution.
“Most insect species in Southern Africa, however, are not sufficiently studied to enable scientists to comment on their rarity or level of endangerment,” said Scholtz, but butterflies are the exception.
Many of the species identified as “critically endangered” are butterflies, which are the most studied group of insects, he said.
Internationally two studies — one in temperate Germany and another in tropical Puerto Rico — have reported massive declines in species, Scholtz noted.
“The German study reported a 76% decline in the biomass of flying insects and the Puerto Rican study a decline of 98% in ground-living fauna and 78% for canopy species.”
One in six bee species across Europe has vanished and more than 60% of dung beetle species have disappeared, he reported.
Scholtz, who lived in Pretoria most of his adult life, said they had dung beetles in their garden. “Did you know there are 600 species of dung beetles?” he asked.
“Planting more indigenous and butterfly-friendly plants instead of lawns would attract many more species of insects which are important to gardens and crops,” said Scholtz, urging South Africans to support insect conservation.
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