Laserfiche WebLink
<br />waste products of metabolism. Wallace (1980) <br />1157] reviewed the ecology of filter feeding in- <br />sects and stresses the importance of currents <br />bringing in allochthonous organic matter for <br />food. High spring runoff typically brings most of <br />this allochthonous material into a stream (Dance <br />et al.. 1979 13211. Macan (19631 1102] sug. <br />gests that current is not as important as food in <br />explaining the distribution of aquatic organisms <br />in the headwaters of streams. <br /> <br />The dorsa-ventral flattening of many aquatic in- <br />sects may be an adaptation to living in fast flow- <br />ing waters. but dorsa-ventral. flattened insects <br />may also be found in slow-water habitats; how- <br />ever, Harker (1954) 1691 suggests that flatten- <br />ing may be primarily an adaptation for crawling <br />under stones and boulders. <br /> <br />Drifting of organisms is another important <br />aspect related to currents. Drifting of macroin- <br />vertebrates in a lotic situation is important in the <br />distribution of these organisms, in avoiding an <br />unsuitable environment. and in providing easily <br />accessible food for fishes. Waters (19651 <br />11601 hypothesized that organisms may enter <br />drift by being swept from the tops of stones, <br />where the erosive action of currents is greatest. <br />Kovalak 119791 [921 evaluated the changes in <br />benthos density with current velocity and animal <br />size. He hypothesized that more of the larger <br />animals may drift because they are in search of <br />more rapid currents to satisfy oxygen require- <br />ments. Walton (19781 11581 found drift to be <br />more a function of behavior than morphology. <br />Drift may also relate to time of day or water <br />temperature. <br /> <br />Substrate <br /> <br />The substrate is an important factor in influenc- <br />ing the type and abundance of benthic fauna ex- <br />isting in a given area I Ruggiero and Marchant, <br />1979 [132] I. In areas of silt accumulation dur- <br />ing low flows and in pools, large numbers of <br />Chironomidae may be found IHynes, 1970 <br />18411. Sand, in general. is the poorest habitat <br />due to the limited organic content and the cons- <br />tant shifting caused by currents (Mikulski, 1961 <br />11111 ). Hynes (19701 1841 ranks habitats and <br />presents expected populations of organisms. <br />Scott and Rushforth (1959) 11351 stated that <br />rubble supports more animals than sand because <br />of its stability and space for habitation as well as <br /> <br />the presence of more organic material. Oetritus- <br />feeding animals such as Leuctra spp., Baetis <br />spp., and Rithrogena spp. increased in numbers <br />as increased food became available. as did the <br />stonefly Isoper/a spp., which is a carnivore (Eg- <br />glishaw. 1964 [451 I. <br /> <br />Resh 119791 1129] reports that riffles support <br />more production of a species of caddisfly. <br />Ceraclea ancy/us, than does a pool. Wise and <br />Molles (19791 [1721 found that small gravel <br />(10 to 25 mml supported more individuals and <br />more species than large substrate (> 75 mml. <br />Williams and Mundie (19781 11701 observed <br />that maximum numbers and biomass occurred in <br />medium gravel 24.2 mm mean diameter. <br />whereas diversity was greatest in large gravel <br />(40.8 mml. Heterogeneity of the substrate may <br />also affect the types of benthic invertebrates <br />colonizing. Williams 119801 [1691 reported that <br />26 percent of the species found showed definite <br />substrate preferences, 35 percent showed <br />slight preferences, and 39 percent showed no <br />preference. Predator aquatic insects tended to <br />have a definite substrate preference as <br />collectors-gatherers and scraper insects had <br />Iittle-to-no preference. <br /> <br />Diversity and Equitabi/ity <br /> <br />Dills and Rogers (1974) 1391 showed that <br />aquatic systems expo;:;ed to environmental <br />stress le.g., pollutionl have fewer species and <br />less diversity than naturally occurring com- <br />munities. An ecosystem containing a high <br />species diversity is usually considered to be <br />healthier and more stable. Many investigators <br />have attempted to describe community struc- <br />tures and diversity using mathematical formulas <br />(Wilhm and Dorris, 1 968 11 671 I; Cairns et aI., <br />(19681 1201; Gaufin, 119731 1611; and Keefe <br />and Bergerson, 1977 [911). Kaesler and <br />Herricks 119791 1901 have shown that diversity <br />index calculations at the generic level have been <br />highly correlated with calculations provided at <br />the species level. <br /> <br />Equitability is used as a measure to evaluate the <br />component of diversity due to the distribution of <br />individuals among the species. Weber 119731 <br />(161). however. concludes that estimates of <br />equitablility based on samples containing less <br />than 100 specimens should be evaluated with <br />caution. if at all. <br /> <br />8 <br />