Insect inhabitants collapse: new proof hyperlinks it to dams

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(MENAFN – The Conversation)

Insects are the most numerous group of animals on the planet. There are an estimated 5.5 million species, 80% of which are still to be discovered. Yet insects are experiencing a steep, widespread decline worldwide: a “death from a thousand cuts” due to human activity.

Insects play almost every role imaginable in an ecosystem, including pollinating plants, controlling pests, and feeding other animals. The potential consequences of its demise are so dire that it has been dubbed the “Insect Apocalypse”.

After the flood of attention this impending environmental disaster generated, a more complex picture has emerged – with a gap in our understanding blatantly. Although tropical and subtropical regions are estimated to be home to 85% of the world’s insects, what happens in these regions is critically underestimated.

Dams and slopes

Understanding insect decline requires long-term data sets, which are rare, especially from the global south. In our new study we present one of the most comprehensive known datasets of subtropical freshwater insects over a period of 20 years. What we found was a profound decrease in insect numbers in all aquatic insect groups studied, including mosquitoes, mayflies, and dragonflies.

In canals, lakes, rivers and backwaters, there was a decline in one of the largest freshwater systems in South America, the floodplain of the Paraná River. At the same time, we found the numbers of invasive fish increasing and water chemistry becoming more unbalanced – environmental changes all linked to dam construction.

Itaipu Dam on the Paraná River. Jonas de Carvalho / Flickr

There are over 130 dams along the Paraná and its tributaries. The most important is Itaipu, the second largest hydropower plant in the world. Located in Brazil and Paraguay, the reservoir is so large that when it was filled, it flooded one of the largest waterfalls on earth, the Guaíra Falls. The removal of such a natural geographic barrier between the Lower and Upper Paraná Rivers has led to mass invasions of fish: many of them are insect predators.

At the same time, dams block the outflow of sediments and nutrients, disrupt water chemistry and make the water more transparent. Most aquatic insects are dark or speckled to camouflage themselves in the murky water. The increased water permeability weakened their ability to hide, making them even more susceptible to being eaten by the invading fish.

Some flying insects, such as mosquitoes, have aquatic larvae that are worse off in dammed rivers. Kathy2408 / Pixabay

Around 70% of Brazil’s electricity comes from hydropower, and dams will be essential in moving away from fossil fuels. Still, damming can have serious environmental and social impacts. Our study shows that dams can have negative impacts long after forests are flooded and local communities are displaced.

Tropical data gap

While the tropics and subtropics are the most biodiverse regions on earth, they are also among the most threatened. Their abundant natural resources are under immense pressure to provide food, water and energy to some of the world’s fastest growing human populations and developing countries.

Nonetheless, the logistical challenges of studying insects in such a biodiverse region, combined with persistent historical inequality in terms of the location of surveillance, mean that the tropics are underrepresented in studies of insect decline.

Insect decline is particularly little researched in tropical and subtropical zones. Hans / Pixabay

The lack of long-term datasets from the tropics and subtropics can distort the already complicated picture of the decline in insects around the world. One of the most comprehensive studies to date on the worldwide decline in insects compared 166 surveys from over ten years on five continents.

It found that land-based insects were actually in decline, but water-based insects were on the rise. However, of the 68 freshwater insect records in their analysis, only 7% came from the tropics. This apparent success is skewed by a plethora of studies from Europe and North America in which improved water quality and effective measures have increased aquatic insect numbers.

Read more: Get to know the insects that defy the leap into biodiversity – new knowledge

Our results contradict the conclusions of this research. Aquatic insects are in decline in the Paraná River system, which drains a significant portion of southern South America – underscoring the importance of better tropical data. Tropical and subtropical aquatic insects can be more at risk from human activities than their counterparts in more northerly regions. Freshwater regions are among the most threatened ecosystems in the world and must be the target of global conservation efforts.

Successes in protecting aquatic insects in some parts of the world should be celebrated – but without obscuring the challenges elsewhere. Tropical insects have been little researched and are not unimportant.

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