10.7. Insects of temporary waterbodies
In a geological time-scale, all waterbodies are temporary. Lakes fill with sediment, become marshes, and eventually dry out completely. Erosion reduces the catchments of rivers and their courses change. These historical changes are slow compared with the lifespan of insects and have little impact on the aquatic fauna, apart from a gradual alteration in environmental conditions. However, in certain parts of the world, water-bodies may fill and dry on a much shorter time-scale. This is particularly evident where rainfall is very seasonal or intermittent, or where high temperatures cause elevated evaporation rates. Rivers may run during periods of predictable seasonal rainfall, such as the “winterbournes” on chalk downland in southern England that flow only during, and immediately following, winter rainfall. Others may flow only intermittently after unpredictable heavy rains, such as streams of the arid zone of central Australia and deserts of the western USA. Temporary bodies of standing waters may last for as little as a few days, as in water-filled footprints of animals, rocky depressions, pools beside a falling river, or in impermeable clay-lined pools filled by flood or snow-melt.
Even though temporary, these habitats are very productive and teem with life. Aquatic organisms appear almost immediately after the formation of such habitats. Amongst the macroinvertebrates, crustaceans are numerous and many insects thrive in ephemeral waterbodies. Some insects lay eggs into a newly formed aquatic habitat within hours of its filling, and it seems that gravid females of these species are transported to such sites over long distances, associated with the frontal meteorological conditions that bring the rain- fall. An alternative to colonization by the adult is the deposition by the female of desiccation-resistant eggs into the dry site of a future pool. This behavior is seen in some odonates and many mosquitoes, especially of the genus Aedes. Development of the diapausing eggs is induced by environmental factors that include wetting, perhaps requiring several consecutive immersions (section 6.5).
A range of adaptations is shown amongst insects living in ephemeral habitats compared with their relatives in permanent waters. First, development to the adult often is more rapid, perhaps because of increased food quality and lowered interspecific competition. Second, development may be staggered or asynchronous, with some individuals reaching maturity very rapidly, thereby increasing the possibility of at least some adult emergence from a short-lived habitat. Associated with this is a greater variation in size of adult insects from ephemeral habitats — with metamorphosis hastened as a habitat diminishes. Certain larval midges (Diptera: Chironomidae and Ceratopogonidae) can survive drying of an ephemeral habitat by resting in silk- or mucus-lined cocoons amongst the debris at the bottom of a pool, or by complete dehydration (section 6.6.2). In a cocoon, desiccation of the body can be tolerated and development continues when the next rains fill the pool. In the dehydrated condition temperature extremes can be withstood.
Persistent temporary pools develop a fauna of predators, including immature beetles, bugs, and odonates, which are the offspring of aerial colonists. These colon- ization events are important in the genesis of faunas of newly flowing intermittent rivers and streams. In addition, immature stages present in remnant water beneath the streambed may move into the main channel, or colonists may be derived from permanent waters with which the temporary water connects. It is a frequent observation that novel flowing waters are colonized initially by a single species, often otherwise rare, that rapidly attains high population densities and then declines rapidly with the development of a more complex community, including predators.
Temporary waters are often saline, because evaporation concentrates salts, and this type of pool develops communities of specialist saline-tolerant organisms. However, few if any species of insect living in saline inland waters also occur in the marine zone — nearly all of the former have freshwater relatives.