Insect roles in nature

Text and photographs Marlies Craig or as credited

Back in 2018 and 2019 Andrew Carter of the Durban Natural Science Museum and I put together a temporary exhibition on the roles of insects in nature. In January 2020 Andrew was still putting on the finishing touches when Covid-19 struck. Sadly, visits to the museum stopped or came at a slow trickle.

I would hereby like to present the contents of the exhibition to the readers of Leopard’s Echo, per chance to reach a new audience. The information came largely from the book What Insect Are You? and from educational events offered under its banner. Those who know me may have seen me talk about this topic at local conservation events.

The exhibition also featured specimens that came from the museums’ wonderful insect collection, see the first picture in this article (and it so happens that one of the entomologists who reviewed the book What Insect are You? was the curator of this collection at that time, Kirstin Williams.)

The exhibition was called: Insects: the silent extinction. Do we know what we are losing?

Insects are disappearing at alarming rates. But is this a problem? You probably already know the answer. Insects are not just pests – they also play important roles in nature. Sure, most people can think of a few good uses for insects (they pollinate flowers, they make honey), but for many people that is where the good opinion ends.

Even those who know that insects are important, tend not to realise just how critical, how fundamental, they are to the workings and the survival of nature – and therefore, the human race.

In this and the following issues, I will cover the most important insect roles in nature, and what implications this has for everyone else.

Pollination

Pollination is the transfer pollen from the male part to the female part of a plant. The sperm in the pollen fertilises the egg cells in the plant ovary. Without fertilisation there will be no sexual reproduction and genetic recombination, the plant can not grow seeds or fruit.

Some plants, such as grasses, and some trees, rely on the random movement of wind to carry their pollen to the right place. But the majority of flowering plants get animals to carry pollen directly from flower to flower: mainly insects, but also birds, bats and small mammals. These plants attract their animal pollinators with colourful petals or lovely scents, and often reward them with sweet nectar or nutrient-packed pollen. Some flowers even smell of decaying meat, thereby attracting flies. The smell is such a good mimic, that flies are sometimes even fooled into depositing their eggs on the flower.

Many flowers and insects have developed close partnerships. They evolved together and now they cannot live without each other.

See how perfectly this carpenter bee fits into the flower? And do you see that the flower has a special pollen brush, just in the right place to deposit the pollen onto the bee’s hairy back? As the bee visits the next flower, it takes the pollen exactly where the pollen needs to go.

Tip of a tulip stamen covered with pollen grains.

Photo: JJ Harrison

This blister beetle while eating the petals, is pollinating the flower. The flower petals are both the advertisement and the payment for his services.

Pollination by earwig on an arum lily.

A drone fly dusted in pollen.

Seed dispersal

Seed dispersal is the movement or transport of seeds away from the parent plant. Plants have very limited mobility and consequently rely upon a variety of dispersal vectors to transport their propagules, including both abiotic and biotic vectors.

Many animals feed on seeds. In the process of feeding, seeds can be dispersed by simply knocking the seed from the plant to the ground or by being carried great distances from the plant.

Harvester ants collect seeds as food and take them back to their underground nest. Along the way they drop some, or forget to eat some, so inadvertently they help the plants spread their seeds. How important is this role?

Photo: Donkey shot

The Cape Floral Kingdom (fynbos) is a biodiversity hotspot. Although it is the smallest Floral Kingdom in the world, it is home to 8 700 plant species, 68% of which do not occur anywhere else. 1700 plant species are threatened to some extent with extinction.

20% of fynbos plants need ants to take their seeds underground. Without the ants their seeds would never grow.

The invasive Argentine ant is a major threat to these plants. These aggressive ants kill and eat the local harvester ant species. They also eats the plant seeds, but do not bury them.

Several Argentine ants surround a single harvester ant during an aggressive interaction. Argentine ants produce chemicals that recruit their nestmates during these encounters.

Photo: Regents of the University of California

Recycling

Many insects feed on animal and plant waste, including dead material and faeces. In this way they recycle nutrients in ecosystems. If there were no insects to clean it up, it would just pile up.

One place you will never fail to find insects is in a pile of dung!

A single adult bull or cow produces approximately 30 kilograms of dung per day. That is over 10 tons per year. In South Africa there are approximately 14 million cattle, each producing tons of dung per year. Without insects recycling this, South Africa would be covered in dung.

In fact, that is exactly what happened in Australia when they brought cattle into the country. The local dung beetles weren’t used to those soft cow pats and slowly the country was getting covered in faeces.

They had to bring in dung beetles from Africa and Europe to clean up their farm lands.

Dung beetles feed partly or exclusively on faeces (dung). A dung beetle can bury dung 250 times heavier than itself in one night.

Photo: Rafael Brix

Many dung beetles, known as rollers, roll dung into round balls, which are used as a food source or breeding chambers. Others, known as tunnelers, bury the dung wherever they find it. A third group, the dwellers, neither roll nor burrow: they simply live in manure.

Dung beetles play a remarkable role in agriculture and tropical forests. By burying and consuming dung, they improve nutrient recycling and soil structure.

Insects are important decomposers of plant material.

Insects, in particular flies and beetles, play a pivotal role in the decomposition of dead animals.

Beetle and fly larvae on a dead porcupine

Soldier fly larvae for example are so good at recycling compost, farm waste, offal from abattoirs, manure, even human sewage.

Soldier flies not only help in waste management, the maggots and pupae can also be harvested for fish and chicken food – 100% natural bird and fish food, there’s nothing better! The flies can even be ground up and added to animal feedstock to increase the protein content (instead of feeding minced cow brain to other cows and causing mad cow disease). China is already doing this on a large scale. This is the future.

Soil quality

Bacteria, fungi, earthworms etc. are the final decomposers that return nutrients from dead organic matter back to the soil. But they would take a long time to do the job alone. Insects play a huge role, by breaking down dead plant and animal tissue into smaller parts that other decomposers can work with, and by actively burrowing and burying.

Soil is not just some dead dirt, it is an entire ecosystem, teeming with life of varying sizes, that all process organic matter in various ways. If this ecosystem is destroyed, soil fertility drops and the balance of the food web is in danger.

Termites are significant detrivores, particularly in the subtropical and tropical regions, where they feed on dead plant material, generally in the form of wood, leaf litter or and other plant matter, and many grow underground fungus gardens. This recycling is of considerable ecological importance.

Ants also act as decomposers by feeding on organic waste, insects or other dead animals (like these Saharan silver ants devouring a camel tick.) They help keep the environment clean. Carpenter ants, which make their nests in dead or diseased wood, considerably accelerate the decomposition process of timber.

By some estimates, termites and ants alone, could account for 25% of all animal biomass on the planet! They are essential part of ecosystems, they actually build ecosystem structure, they build pipes deep into the soil, they bring minerals to the surface, mixed with their own waste products and saliva.

Insects sometimes act as “taxis” for many decomposers. Since fungus and other decomposers cannot move around on their own, some hitch a ride with flying insects.

Ambrosia beetles, for instance, have a close relationship with ambrosia fungi. The beetles are fungus farmers, growing the fungus in galleries within stressed or dead trees. The beetles spread the fungus as they move on to the next tree.

There are many other soil dwelling insects, apart from termites and ants: billions of springtails, earwigs, bark lice, silverfish, roaches, gazillion beetles and their larvae, various fly maggots, caterpillars, and so on. The food web in the soil is extremely complex. Soil fauna eat compost and fungi, bacteria and each other, and the soil itself. Without them, and the many other soil organisms (mites, earthworms, nematodes, centipedes, slugs, etc.), soils become dead and sterile and compacted.

In the next issue we cover some more roles, and then we consider the consequences.

About the author

Marlies Craig is an epidemiologist by training. She has worked for the Medical Research Council, then more recently for the United Nations’ Intergovernmental Panel on Climate Change, and is now employed at the WITS Reproductive Health Institute in the Climate Change and Health unit. Though she did originally study Biology and Entomology, her love affair with insects is very personal. In her book What Insect Are You? – Entomology for Everyone, she shares that passion with young and old, see whatinsectareyou.com. She hopes to kindle in people of all ages enthusiasm and a deeper appreciation of nature and show them why and how they can make a difference. She also started a non-profit organisation called EASTER Action which hopes to promote awareness and action on biodiversity, climate change, and sustainable living, see EASTERaction.org.

About Andrew Carter

Andrew Carter worked as Museum Officer at eThekwini Municipality for 34 years and was responsible for many of the displays at the Durban Natural Science Museum.