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What does the diversity of freshwater invertebrates tell us about ecosystem health?

Diversity of freshwater invertebrates can reveal a great deal about the health of aquatic ecosystems around the world. Prof Cathy Yule explains.
Hi, I’m Professor Cathy Yule, and today we will be learning about life in fresh waters. What does the diversity of freshwater invertebrates tell us about ecosystem health? Although 70 % of the planet is covered with water. Most of it is salty, and less than 3% of this water is fresh, drinkable water. And most of this freshwater is either frozen as ice at the poles and in glaciers, or it is under the surface in underground aquifers and in soil moisture. So only about half a percent of the Earth’s freshwater is present in rivers, lakes and wetlands, as habitats for animals and plants and readily available for human use for drinking, washing, recreation, agriculture, and industrial uses.
The freshwater habitats cover only about 1% of the Earth’s surface. But despite the small area of freshwater environments, they are extremely rich in biodiversity. Freshwaters are home to 10% of all described flora and fauna. About one in three of all vertebrate species. Most freshwater vertebrates are fish - there are almost 20,000 species of freshwater fish, and also turtles and frogs, and water birds, and about 70 species of specialised aquatic mammals, like platypuses, beavers, otters, and river dolphins. All river dolphins are endangered and some have recently become extinct. But the most diverse and least known of all the freshwater animals, are the freshwater invertebrates. Here you can see some examples of common invertebrates.
They are mostly quite tiny and you need to magnify them to see them clearly. The caddis fly you see here, builds a retreat made from tiny sticks and sand grains with a silk net at the front to catch its prey. It’s from a really diverse group of almost 15,000 species. You can also see some different spaces of mayfly nymphs - a highly diverse and abundant group that lives in a wide range of freshwater habitats. Here’s a mayfly that lives on stones and eats the algal biofilm. And here’s a stonefly nymph. Stoneflies need well oxygenated waters. And here is an adult stonefly. Stoneflies are an ancient group that have been on the planet since the Permian era.
Some freshwater invertebrates such as crabs have evolved from marine spaces as have prawns. These dragonflies have terrestrial adults but aquatic nymphs. There are nearly 6000 species of dragonflies globally. Here you can see mosquito larvae poking their syphons up into the air to breathe. And you can see the pupae that are about to turn into adults and fly away. There are about 3500 species of mosquitoes. Some invertebrates live on the surface like these water beetles which have two sets of eyes for catching prey both under the water and above the surface. And these water striders which are bugs that walk on the surface tension catching prey that get trapped there. Freshwater snails are fully aquatic.
There are about 5000 species of them. New species are being discovered all the time. Many years ago, I studied a small tropical mountain stream on Bougainville Island in Papua New Guinea for my PhD - it was in the forest not far from this volcano. I sampled the stream using a fine mesh net every month for a year, and I collected over 100,000 tiny invertebrates. I identified 182 different species. Only five of these species were previously known to science and had been given names. So 177 species were completely new and undescribed. The mountain forest catchment was intact. There were no towns, no roads, no dams, no pollution.
It was in pristine condition And that is the main reason why the diversity of invertebrates in the stream was so high. In comparison, I studied this urban river in Kuala lumper, the capital of Malaysia. I only found 27 species of invertebrates, all pollution tolerant animals like leeches, exotic snails and midge larvae. All the different species of aquatic invertebrates vary in their tolerance to different environments. Some can only tolerate the cool temperatures of streams and lakes in the mountains or at high latitudes. This is because cool water contains more oxygen than warm water. Some animals need flowing water to oxygenate their gills or to bring them their food.
Some can only live in the still waters of pools, lakes or swamps, as they’re washed away by flowing water. Some only eat leaf litter, so they only occur in forested streams, where leaves fall or are washed into the water. You can see these caddisfly larvae that build their homes from leaves, which are also their food. Some animals, like most freshwater snails, only eat algae, which grows best in open well lit waters. Almost always, the cleanest most natural water bodies have the highest biodiversity. Once freshwater ecosystems are disturbed by humans, the numbers of species decrease dramatically.
One way we can determine that a river or lake has been impacted by humans is by sampling the aquatic invertebrates and seeing how many and which species are there and by counting their abundance. Many of our rivers, lakes and swamps have been completely eliminated by drainage, or else severely impacted by competing human uses. They’re increasingly damaged and destroyed by deforestation of the catchments; urbanisation - building cities and towns and roads. These create impervious surfaces of concrete and asphalt, so the water quickly runs off into drains bringing with it pollutants. Urban water is typically polluted with heavy metals, sediment, rubbish, oil, and countless other toxic substances.
Rivers and streams are constrained in concrete channels and pipes, where water rushes long with no place for animals to live and hide and no foods such as leaves for them. Fresh waters are used for disposal of sewage and other human waste. Agriculture extracts enormous amounts of water, sometimes draining rivers and lakes dry. Agriculture also causes pollution with herbicides, pesticides and animal waste. Rivers are dammed destroying crucial habitats, drowning rivers and streams and forest catchments, and stopping migrating species like salmon and prawns. We have problems due to overharvesting of species, especially fish, prawns and freshwater mussels. And of course, now we have the impacts of climate change, which is increasing the severity and frequency of droughts and floods.
It is causing some aquatic habitats to flood and others to completely dry up killing all the animals and plants that used to live there. So what does the diversity of freshwater invertebrates tell us about the health of aquatic ecosystems around the world? Sadly, multiple studies have shown that freshwater ecosystems are among the most threatened on the planet. For the animals that we know well, the freshwater fish, birds, mammals, reptiles, and amphibians, we have seen a decline of 75% over the past 40 years. For the freshwater invertebrates, they have been so poorly studied, and most are still unknown. So often we don’t even know what we have lost. But we can turn this around.
Rivers and lakes can be rehabilitated, and plants and animals can return.

Most of the world’s fresh waters are trapped in ice, underground aquifers and soil moisture. Only about 0.5% of the Earth’s fresh water is present in rivers, lakes and wetlands which cover merely 1% of the Earth’s surface.

Despite this small area, freshwater habitats are extremely rich in biodiversity, harbouring 10% of all described flora and fauna. These include about 1/3 of all vertebrate species, particularly fish, but most aquatic animals are invertebrates such as snails, prawns and aquatic insects (like dragonfly and mayfly nymphs, beetles and bugs).

All the different species of aquatic invertebrates vary in their tolerance to different environments for example to still or flowing water, to different temperatures, and to various pollutants.

In this video we explore how pristine environments, such as forested streams, exhibit the greatest biodiversity, while human impacts such as urbanization, deforestation, dams, and agriculture cause dramatic decreases in the diversity of fauna of rivers, lakes and swamps.

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