Revisiting Elton's copepods: lake construction has altered the distribution and composition of calanoid copepods in the British Isles

Abstract

It is now widely accepted that the construction of new lakes, ponds and reservoirs facilitates the invasion of non-indigenous aquatic species, due largely to low biotic resistance from native communities. The role played by constructed waters appears to be a particularly frequent feature of zooplankton invasions. Charles Elton, in his classic 1927 book “Animal Ecology”, noted that the estuarine calanoid copepod Eurytemora velox had invaded constructed inland waters in Britain and highlighted the lack of a key species, Eudiaptomus gracilis, in allowing its establishment. At the time, Elton’s observations were dismissed and his findings largely consigned to obscurity. Using occurrence records gathered since this time and current knowledge of calanoid copepod ecology and invasion biology, we re-examined the distributions of three species of freshwater calanoid copepods in the British Isles to: 1) determine the legacy of lake and pond construction on their distributions, and 2) reassess the conclusions made by Elton in light of this knowledge. The lack of natural lakes in the south and east of England, and the subsequent widespread development of new lakes and ponds, has altered calanoid copepod distributions. The common E. gracilis occurs frequently in the north and west of the British Isles in natural lakes, and is found in the south and east in constructed waters. The estuarine E. velox was found only in 3 natural freshwater sites, all in close proximity to the coast, but has been recorded in 23 constructed sites, many of these well inland. Elton noted a general lack of co-existence between E. velox and E. gracilis, with the relatively slow establishment rates of E. gracilis thought key in allowing the estuarine species to invade. However, subsequent collections suggest long-term cooccurrence of these species at some sites. We suggest that E. velox has now successfully invaded freshwaters in the British Isles. Eudiaptomus vulgaris is not known from natural lakes, but has been recorded in several constructed waters, and appears to have invaded Britain facilitated by lake construction. Current knowledge supports Elton’s contention that constructed waters are more readily invaded than natural waters, and that biotic resistance due to the presence of key species is important. While some specific criticisms of Elton’s ideas were valid, we argue that Elton’s concepts regarding constructed waters and invasions were in many ways correct and prescient of current understanding.