Re-examining the drift paradox

Abstract

Why do the upper reaches of streams remain populated by aquatic insects, other invertebrates and algae if many thousands of these organisms are swept downstream by the current every day? Invertebrate drift – basically, any invertebrate animal big enough to see with the naked eye that lives on the streambed but which is found entrained in the water column of a stream or river – provides the most observable example of this ‘drift paradox’.The idea of the drift paradox itself has been the subject of ecological angst since it was first noticed by Karl Muller nearly half a century ago. Surprisingly, of the hundreds of papers concerning invertebrate drift produced over the past five decades, only two have tried to formalize the problem and address it using theory, and both of these papers were published in the last six years 1xDensity dependence resolves the stream drift paradox. Anholt, B.R. Ecology. 1995; 76: 2235–2239Crossref | Scopus (74)See all References, 2xExact compensation of stream drift as an evolutionarily stable strategy. Kopp, M. et al. Oikos. 2001; 92: 522–530CrossrefSee all References. In a recent paper 3xPopulation persistence in rivers and estuaries. Speirs, D. and Gurney, W.S.C. Ecology. 2001; 85: 1219–1237CrossrefSee all References, Douglas Speirs and William Gurney present a treatment of this problem that uses equations describing advection and diffusion in molecular systems to approximate movement of animals entrained in the water column, as well as randomly directed movements of animals travelling along the streambed by their own volition. Their results explain the persistence of several real animals in real systems and suggest that these are cases where either advection is minimized or other forms of movement are very prevalent. These conclusions themselves are a fascinating take on an old problem that deserves much more investigation.However, one aspect of this work that is not elaborated on in their paper could, if true, have profound effects on our preconceptions regarding the management of streams and rivers. Speirs and Gurney suggest that population persistence is a delicate balance between advection by water flow and other movements acting to counter this downstream drift. However, a speculative implication of their conclusions is that small reductions in stream or river flow, not large enough to affect oxygen exchange, food availability, or other niche parameters for a given species, might actually increase population size and stability through an increase in the probability of population persistence.However, this speculation rests on two assumptions. The first, and more obvious, is that the species involved is not so specifically adapted that a small change in mean water velocity will make the habitat unsuitable. However, the key assumption is whether we believe that displacement owing to drift is adaptively compensated for and, therefore, whether reduced water velocities will result in reduced compensatory movements.