Community dynamics of the benthic fauna in a woodland springbrook

Summary

The benthic macrofauna of a woodland springbrook, Morgan's Creek, Kentucky, was studied between February 1963 and September 1964 at eight collecting sites. The basic habitat type of the stream is a rubble-riffle complex. Four stations (III, IV, VI, and VII) were located in riffle sections at different points along the stream and additional stations were established at the spring source (I), a cascade area (II), a pool (V), and a reach at the mouth of the stream (VIII).

A total of 33 taxa were studied in detail, 30 of which were at the species level. Gammarus minus was the most abundant benthic macroinvertebrate in the stream. It made up 82% of the 224,400 organisms collected during the first 12 months of the study. The next most abundant species were Asellus brevicaudus, 4.4%; Phagocata gracilis, 3.0%; Baetis amplus, 1.6%; Ochrotrichia unio, 1.1%; Epeorus pleuralis, 0.9%: Diplectrona modesta, 0.9% and Baetis herodes, 0.7%.

Although the largest number of animals per standard (5-minute) sample was collected at Station I, the greatest density (expressed as numbers per unit area covered by a sample) actually occurred at Station II (32,370 /m²). The remaining stations ranked as follows: I (13,160/m²), IV (11,050/m²), VI (10,820/m²), III (5890/m²), V (1260/m²), VII (490/m²), and VIII (120/m²), for the period 15 February 1963 through 1 February 1964.

Information on life histories, based on differences in seasonal abundance and the duration of the life cycle, is summarized for 32 species. Of these, 24 were seasonal in occurrence and had relatively long life cycles. Two-thirds of the seasonal forms were autumn-spring growers, rather than summer growing forms.

The community organization of Morgan's Creek was examined from the standpoint of species diversity and total numerical variations at each station. The situation at Stations I and II is typical of communities existing under strongly limiting conditions. The greatest diversity of invertebrates, as well as a relatively abundant number of individuals, was found at Stations III and IV, indicating a probable climax situation in this area. Both species diversity and total numbers of individuals showed marked decreases below Station IV; this was attributed to the removal of forest cover in this region. The most important effect of the removal was the alteration of temperature conditions in the stream. However, since terrestrial leaf detritus was shown to be the most important source of plant material eaten by the herbivores, removal of the riparian vegetation could, under more severe conditions, cause the depletion of primary food supplies essential to the maintenance of the woodland stream community.

The environmental factors: water movement, temperature, carbon dioxide, alkalinity, hydrogen ion concentration, and dissolved oxygen were measured throughout the study. The abundance and distribution of the animals are examined in relation to these components and to that of substrate. It is concluded that temperature, substrate, and water movement are among the most important factors in Morgan's Creek.

High discharge following a March spate severely devastated the fauna but had little lasting effect on the community. Species with short life cycles were able to recoup their losses quite rapidly and showed an increase in numbers soon after the flood; those with long life cycles were more seriously affected and decreased in numbers or only managed to maintain themselves following resumption of more normal discharge. The nonseasonal fauna showed the greatest decreases in numbers as a result of the spate, but their recovery was faster than for species with long seasonal cycles.

A number of closely related species, with the same basic food niche, apparently coexist in Morgan's Creek. Competition between these species is avoided through the interplay of differences in microhabitat preferences and spatial and temporal distribution patterns.