Freshwater is one of the most threatened resources on Earth. Dragonflies can tell us what we need to know about the state of this precious resource
Viewed from space, our planet is a blue speck of mostly water in a seemingly endless expanse of darkness. It is this water that is vital for life as we know it. This wonderful life is amazingly complex, yet very fragile. Away from the sea, it is fresh water on which life depends, especially free running water and precious wetlands, all of which are teeming with life.
Yet fresh water is the most threatened habitat on Earth.
Several thousand species worldwide live in freshwater habitats, from the smallest ponds to the largest rivers. Some are highly sensitive to any human impact while others are real opportunists. They will inhabit the most artificial of habitats, like cattle troughs and even bird baths. It is this range of sensitivities that make them very useful as measures for the quality of fresh water.
When a water system becomes degraded through human impact like pollution or damming, there is a change in the species profile away from sensitive specialists towards insensitive generalists. We can quantify this and relate it to whether a fresh water system is deteriorating or improving.
A prominent group of species associated with water and that can tell us something about the state of our water resources is dragonflies – the collective term for true dragonflies and damselflies. When they are young they live in the water as larvae, then later emerge as flying adults that grace fresh waters throughout the world, except the ice caps. Both life stages are predatory.
So these beautiful insects are near the top of the food chain and have few natural enemies other than birds. These are occasionally frogs, spiders and robber flies. At times humans enjoy the larvae as a tasty addition to a side dish. For example, in Bali, larvae may be fried in coconut oil and served with vegetables. Indeed, dragonflies and humans are much more intimately linked than normally thought.
In South Africa, a water-scarce country, we have been conducting research on new ways for assessing the quality and ecological health of fresh water systems using dragonflies. There are 162 species of dragonfly in South Africa alone. Some are sensitive specialists, while others are hardy generalists. This and their two-staged lifestyle, with dependencies on both the water and land, make them excellent candidates for freshwater assessment.
We have developed an index that is based on three main features of each species in turn.
The general distribution of a species;
Its threat status (its rating on the International Union for Conservation of Nature and Natural Resources’ Red List); and
Its sensitivity to human modifications of the water system.
When a system deteriorates there is a shift in the total scores of all the species present from high to low. When systems are restored there is a shift in the other direction from low to high. Using dragonflies it is possible to determine whether there should be concern about a system that is going downhill or whether a system is improving, and how well it’s doing.
Using dragonflies to this end is incredibly simple. All you need is a good guide, a pair of close-focus binoculars and a sunny day.
Recently all our research has been synthesised into a user-friendly manual showing how to undertake fresh water assessments. As this index operates at the level of species, it is highly sensitive. And as dragonflies are relatively easy to identify, it is easy to use.
Dragonflies are pushed away from their normal habitats when invasive alien trees like eucalyptus, wattles and pines shade the water and bank. This can lead them to become locally extinct. This means that the removal of alien trees from the banks of rivers, in particular, is an important nature conservation exercise. It has been one of the great contributions to South Africa’s nature conservation through the governmental Working for Water Programme.
But not all human activities are harmful to dragonflies and other water, fauna and flora. Farm dams can encourage many species that would otherwise be very scarce in the area. Good nature conservation dams are those with constant water levels, much water weed and marginal vegetation, and no pollutants, especially fertilisers and pesticides used in agriculture.
Successful management of fresh water biodiversity depends on the quality of data on the species that these ecosystems support. Projects monitoring the health of fresh waters are a vital component of this. The process of fresh water assessment is very pleasant, like going bird watching. This new approach makes freshwater assessment so much easier than in the past and makes a major contribution to nature conservation.
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