Flooding and Bugs – What are the impacts?

Friday 21st February 2014

Understandably attention has focussed on the plight of people, property and livestock during the recent floods. But what has it been like being a little bug during the recent weather? Has the flooding damaged our environment long-term?

There are 40,000 species of invertebrate in the UK and they vary enormously in their biology, ecology and lifestyle choices. During the winter some bug species are eggs, some are larvae or juveniles, some are pupae, some are adults and some are at a mixture of stages. Most, but not all, invertebrates hibernate or rest over winter and are therefore inactive.

One might think that the arrival of a sea of water hundreds of time deeper than you are tall would be curtains, finito, but that would be thinking like a human. Firstly, although bugs need oxygen like we do, they don’t need the rapid replenishment that we do, and can often slow down their metabolism so that they are using oxygen very slowly, eggs and pupae particularly don’t need much oxygen. Some bugs – particularly those with soft moist skin are able to absorb oxygen from water, others create watertight homes that retain their own air.

An excellent example of the latter is the Yellow-loosestrife bee (Macropis europaea). The mother bee digs a burrow in the ground and then, using an oil created by the Yellow-loosestrife and collected by her, she lines the walls with the water proofing oil. The larvae, in their little cells stocked with Yellow-loosestrife pollen and nectar, can then sit out the winter safe from floods.

Most bugs have longer than we would expect to get to air before they drown and most can sit out brief flooding. Insects and other arthropods are not as dense as people and can often float. Therefore an alternative strategy is to simply let go, float away, grab some flotsam and hopefully end up somewhere drier. Of course many insects can fly, but it takes energy and often body warmth to get the flight muscles working. Certainly most bugs can’t simply fly out of the way of winter floods.

The problem is not as simple as having to get oxygen from water instead of air, bugs are also competing for that oxygen with the bacteria that are decaying dead plant material. Before long, anoxic conditions start to prevail in the top layers of the soil. Not only is oxygen at a low concentration, the anaerobic conditions stimulate the production of toxic decomposition by-products such as hydrogen sulphide. Earthworms such as the Chestnut earthworm (Lumbricus castenaeus) will up-sticks and take their chances in the water rather than the soil.

Despite their tactics, a great many bugs will have drowned in the recent floods. Most at risk are soil and sward inhabiting invertebrates on land that has not been flooded frequently in recent decades. These species are unadapted to being submerged and most species of earthworm and fly larvae will drown if their soil is submerged for a few weeks. Hibernating queen bumblebees and wasps are also likely to drown if flooded.

Once the water retreats about 90% of the bugs will have died or left.

In contrast, land that has been regularly flooded is inhabited by species that are adapted to flooding. This specialist floodplain fauna is much less abundant than the fauna in unflooded soils. The weight of bugs in the soil of a floodplain grassland is probably less than a quarter that found in a comparable unflooded meadows. Floodplains contain species that are particularly tolerant of inundation, for example the Floodplain earthworm (Octolasion tyrtaeum). Regularly flooded floodplains have fewer bugs than unflooded habitats, but if both are flooded then afterwards there will be more bugs surviving in the regularly flooded land. Read more on flooding grasslands and effects on bugs here.

Where earthworm populations have been drowned it is likely to take several years for populations to build up. Healthy earthworm populations keep soil aerated and free draining. So the flood risks exacerbating soil compaction, with increased risk of further flooding as a result.

In addition to the water itself, flooding can bring with it toxic pollution that will threaten all aquatic and floodplain living species, everything from mayflies to mussels and beetles to earthworms. All it takes is for one container of concentrated insecticide to float up, get crushed and empty into floodwater and there would be death and destruction to aquatic life over many hectares of floodplain. Industrial chemicals and mining waste can also be released by the flooding and while perhaps not as devastatingly potent as a neat insecticide, they can carry the additional risk of being very persistent and difficult to remove from the land.

A floodplain is part of the river that is not always wet and this should be the premise for its management. Development on floodplains should be stopped, if insurers were able to refuse to insure future floodplain developments that would surely help.

Flooding should be managed to provide long term consistency so that the fauna and flora of the catchment can develop and thrive, switching an area from regular flooding to no flooding, or from grassland to arable and back again, does not help to develop the rich and robust communities of well adapted species that will help resist flood damage.

As well as common and widespread species, floodplains can also provide important habitats for some of our more threatened species – animals such as the highly endangered Tansy beetle (Chrysolina graminis) that now only survives on the floodplains around York.

Flood plains are great places to have flower-rich meadows full of bees, beetles and butterflies, a habitat that can cope with inundation and grow robustly on a diet of silt and water. In Yorkshire we are already working with landowners to turn arable land next to rivers into flower-rich grassland as part of our B-Lines programme.

In Conclusion….

While there will be much reduced abundances of a wide range of bugs in many of the flooded areas next year, these populations and communities should bounce back over a few years.

As the climate changes, we will probably get more frequent and dramatic flooding in the future. The very communities of animals will change with a shift towards flood tolerant species in many areas. If this can be done in a planned way then that will help wildlife. A habitat with a regular flooding regime will be healthier than one that is dramatically, but very infrequently, flooded.