Floods occur when water inundates land, typically due to heavy rainfall, overflowing rivers, or storm surges. These events drastically alter habitats, impacting diverse life forms across all trophic levels.
Terrestrial Organisms
Land animals face immediate threats during a flood. Many smaller mammals, such as voles, shrews, and mice, are vulnerable to drowning as their burrows and nests become submerged. Larger mammals and reptiles are often displaced from their territories, forcing them to seek higher ground, which can lead to increased stress and exposure to predators in unfamiliar environments. The rapid movement of floodwaters can also injure or trap animals, and debris poses additional physical dangers.
Birds can also be impacted, especially during nesting season, as breeding sites may be destroyed or submerged. While flying animals like birds and bats can often escape immediate danger by taking to the skies, severe storms can exhaust them or leave them vulnerable to predation once they seek shelter. For insects, a study in Oklahoma following a major flood revealed a 93% decrease in abundance and a 60% decrease in species presence among invertebrates nine months after the event. Many soil and sward-dwelling invertebrates, unadapted to prolonged submersion, are likely to drown if their habitat is underwater for several weeks.
Plants, the base of terrestrial food webs, are susceptible to flood damage. Prolonged waterlogging quickly depletes soil oxygen, a condition known as anoxia or hypoxia, which is harmful to most plant roots. This oxygen deprivation can impair a plant’s metabolism and growth, with many crops dying or being damaged within one to seven days depending on species and conditions. Floodwaters can also erode soil, exposing roots and making them vulnerable to disease, while deposited sediment can bury plants or create a crust that hinders seedling emergence.
Aquatic Organisms
Aquatic organisms, despite being water-adapted, are affected by the changes floods bring. Fish can be displaced from their usual habitats, with strong currents washing them out of rivers and potentially stranding them in isolated pools as floodwaters recede. Floods can alter the timing of fish migrations; for example, some fish species typically swim upstream to spawn as spring rains swell rivers, while others, like American eels, migrate to the ocean during fall floods.
Water chemistry in aquatic ecosystems can be altered during a flood. Floodwaters often carry large amounts of sediment, leading to increased turbidity, which reduces light penetration and can clog the gills of fish and invertebrates. The influx of organic matter can also lead to oxygen depletion as microbes decompose the material, creating hypoxic or anoxic conditions that can cause widespread fish kills. Additionally, floodwaters can introduce pollutants such as sewage, chemicals, and agricultural runoff, contaminating water sources and exposing aquatic organisms to toxins.
Aquatic invertebrates, including insects, respond to flooding in various ways. While some species, like certain midges, show resistance to disturbances, others, such as some caddisflies and crane flies, are resilient and can recover rapidly after a flood. Floods generally reduce the abundance and diversity of invertebrates due to increased shear stress from water flow, which can remove organisms or force them to seek refuge. Sediment movement can also scour invertebrates and their food sources, disrupting the aquatic food web.
Microscopic Life
Microorganisms, including bacteria, fungi, and protozoa, are fundamental to nutrient cycling and decomposition, and their communities are impacted by floods. Flooding quickly reduces oxygen levels in the soil, creating anaerobic conditions that favor certain microbial groups while being detrimental to others. This shift can lead to a decline in aerobic bacteria and fungi, while anaerobic bacteria, such as denitrifiers and methanogens, may thrive.
The altered oxygen levels and nutrient availability directly influence microbial community composition and function. For instance, populations of nitrogen-fixing rhizobial bacteria, essential for plant health, can decrease after prolonged soil saturation. Similarly, populations of beneficial mycorrhizal fungi, which aid plants in nutrient uptake, can also decline. Fungi tend to be more sensitive to flooding than bacteria, and their concentrations can remain elevated in flooded areas even after visible water recedes.
Floods can also introduce new microbial populations or alter the balance of existing ones. Floodwaters often carry pathogenic microbes, leading to potential outbreaks of waterborne diseases that can affect wildlife and humans. The decomposition of dead plant and animal biomass in flooded soils can promote the growth of specific microbial groups, further shifting the community structure and influencing gas exchange and nutrient dynamics within the ecosystem.