Aquatic macroinvertebrate responses to drought

A critique submitted by Brett Moore for the Evolution class.

Aquatic macroinvertebrates have received and are continuing to receive considerable amounts of scientific attention. The large amount of diversity within macroinvertebrates allows them to be present in almost all natural freshwater environments (Resh and Rosenberg 1984). The evolutionary adaptations that created the diversity within the group also created the great variety of life histories and physiological requirements, which promote the use of macroinvertebrates in scientific studies and biomonitoring, the use of living organisms and their responses to measure the quality of the environment, (Merritt et al 2008).

Ecosystem disturbances, whether anthropogenic or natural phenomena such as drought, can create changes in aquatic macroinvertebrate communities by altering habitats and the physiochemical conditions they have adapted to live by. Thus, disturbances can have a large effect on the community structure of aquatic biota. However, many studies have shown that even after a single event disturbance that does not chronically alter habitat, such as a drought and the drying of aquatic habitats, aquatic macroinvertebrate taxa richness can make a rapid recovery (Boulton 2003, Wallace 1990). The fast recovery is most likely the result of the evolutionary history of drought in aquatic environments (Boulton 2003).

Boulton and others have done several studies addressing the effects of drought, drying, and intermittent streams on aquatic macroinvertebrates. In Parallels and contrasts in the effects of drought on stream macroinvertebrate assemblages, Boulton observed the effects of drought as an intermittent stream disturbance and the ecological changes that occurred. As riffle habitat dried and the stream turned into isolated pools, species assemblages changed favoring the species more adapted for lower oxygen levels, higher temperatures, and predation on other invertebrates. When flow commenced recovery of the species assemblages began to take place. Recovery rates were rapid for species that had strategies for surviving drying. Two main strategies were identified: 1. Species were adapted for finding refuge. 2. Species had a life history strategy for dry conditions (Boulton 2003). Strategies for finding refuge include being mobile, or being able to bury into damp substrates, while some life history strategies to survive dry conditions include having a terrestrial winged adult stage of the species existent during dry conditions, having desiccation resistant forms of eggs, or having tendencies for downstream drift once flow commences (Boulton 2003, Wallace 1990). Therefore, immobile invertebrates without life histories to survive drying should be heavily effected and take longer to recover from a stream drying disturbance. Several studies have been able to show this. For example, mollusks, which do not have a winged adult stage and are fairly immoblie compared to other invertebrates, have been shown to be among the last taxa to recover following disturbances (Wallace 1990).

Boulton (2003) acknowledges the linkages between the history of drought and the evolutionary adaptations of aquatic macroinvertebrates. However, there are still information gaps and questions that need to be answered. Boulton states that most of the data that exists about the effects of drought on aquatic biota is short term, and the lack of pre-impact data exists because studies are opportunistic and droughts are phenomenological events. Understanding the life histories, life cycles, behaviors, and the genetic structure of aquatic macroinvertebrates is also extremely important, however much of the information is still poorly known. Also, several questions still need to be clarified or answered such as: Does frequent drying select for life histories and certain community assemblages? Do the same responses occur across most geographical regions (Wallace 1990)? Once more information becomes available about the evolutionary adaptations of aquatic macroinvertebrates science may have a better understanding of evolutionary processes and what may happen to biota in the future if global climate changes occur.

Literature Cited:

Andrew J. Boulton (2003). Parallels and contrasts in the effects of drought on stream macroinvertebrate assemblages Freshwater Biology, 48 (7), 1173-1185 DOI: 10.1046/j.1365-2427.2003.01084.x

Merritt, R. W., K.W. Cummins, and, M.B. Berg (eds.). 2008. An introduction to the aquatic insects of
North America, 4th ed. Kendall/Hunt, Dubuque, Iowa.

Resh, V.H., and D.M. Rosenberg (eds.). 1984. The ecology of aquatic insects. Praeger
Publishers, New York, NY.

J. Bruce Wallace (1990). Recovery of lotic macroinvertebrate communities from disturbance Environmental Management, 14 (5), 605-620 DOI: 10.1007/BF02394712