Usinger: Introduction 



Role of Insects in Aquatic Communities 



In 1887 Stephan A. Forbes wrote an essay, "The Lake 

 as a Microcosm," showing that a lake is essentially 

 a self-contained or closed community. This concept 

 is also applicable, though to a lesser extent, to ponds 

 and even to larger streams. Each community of plants 

 and animals is more or less attuned to its physical 

 and biotic environment, and the various elements of 

 which it is composed are integrated to form an eco- 

 system. Insects play an important but not a vital role 

 in such systems. As dominant members of the littoral 

 fauna, together with fishes, they are intermediate in 

 position between the autotrophic or constructive ele- 

 ments (green plants) and such heterotrophic or destruc- 

 tive elements as the bacteria (intro. fig. 1.) 



Zooplonkton 



Fishes 



Heterotrophic 

 Bacteria 



Littoral Fauna 



Autotrophic 

 Phytoplankton 



Autotrophic 

 Littoral Flora 



Nutritive Substances in Solution 

 or on the bottom 



Intro, fig. 1. Simplified diagram of the dynamics of an aquatic 

 community. Solid arrows represent constructive steps; dotted 

 lines, reductive steps. 



In the littoral fauna, insects such as mayfly nymphs 

 and midge larvae serve as primary converters of plant 

 materials into animal protoplasm. As pointed out by 

 Elton (1947) such basic herbivores are "key indus- 

 tries" in a community and are usually small in size 

 and large in numbers. Successive links in the food 

 chains are larger and scarcer and are usually carniv- 

 orous. Essentially, the food chain concept is simple 



Intro, .fig. 2. Diagram of the food chain in a pond. The continu- 

 ous arrows show the course of inorganic salts and the broken 

 lines indicate their course after they have been built up into 

 living matter, o, phytoplankton; b, zooplonkton; c, weed-dwelling 

 fauna; d, bottom fauna (Macan, Mortimer, and Worthington, 1942). 



, Dissekrd Mrtrimtt '■ 



Intro, fig. 3. Food-cycle relationships in a 

 lake (Lindeman, 1941). 



(intro. fig. 2) with relatively few links. In nature, how- 

 ever, the situation becomes much more complex, so much 

 so that even in diagrammatic form it has been termed 

 a food web or food cycle (intro. fig. 3). Insects enter 

 into such a cycle as "browsers" and as "swimming 

 predators" on plankton and on benthic organisms. 



Probably no two biotic communities are identical 

 in every respect, but there are certain types that are 

 characteristic of particular aquatic situations, and 

 each of these types has a distinctive insect fauna. 

 Three representative types are: 



1. Lakes — with Chironomid larvae in the bottom 

 ooze and Chaoborus larvae that prey on plankton and 

 perform diurnal migrations from the bottom mud to or 

 near the surface. A varied insect fauna also occurs 

 in the shallow littoral waters of lakes but such forms, 

 with few exceptions, are more characteristic of shallow 

 ponds than of lakes. 



2. Ponds — with a large and varied insect fauna 

 including midge larvae in the bottom and mayfly 

 nymphs, caddis larvae, and others as basic herbivores 

 together with numerous predatory beetles, bugs, and 

 odonatan nymphs. Although dependent on the plankton 

 and rooted vegetation, it can truly be said that pond 

 insects are dominant forms of life in their limited 

 environment. Furthermore, they are admirably suited 

 to the uncertain conditions of pond life, with short 

 life histories and ready means of dispersal. 



3. Streams — with stonefly nymphs, mayfly nymphs, 

 caddisfly larvae, and various midge larvae as basic 

 herbivores — sieve feeders or grazers — and a host of 

 predaceous forms. Here again, insects predominate 

 and form the staple diet of most fishes. 



