428 



S. F. MacLean, Jr. 



Taken over a number of years and cohorts, mean emergence of 

 adults must reflect population size. P. hannai are clearly most abundant 

 in wet meadows and polygon troughs (Table 11-4). Tipula carinifrons 

 adults emerged at greatest density from mesic meadows. The overall den- 

 sity of emerging T. carinifrons (3.0 individuals m"^) was less than that of 

 P. hannai (11.5 individuals m"^); however, because of the difference in 

 size of adults, the biomass of emerging T. carinifrons (35.3 mg m"^) was 

 greater than that of P. hannai (19.6 mg m'^). 



Enchytraeidae 



The life cycles of the three dominant enchytraeid species last from 

 one to two years, and may include one or two distinct periods of recruit- 

 ment each season (Figure 11-4). In both Cernosvitoviella atrata and 

 Mesenchytraeus sp. early season recruits come from eggs that were de- 

 posited in cocoons the prior season, while the late season recruits hatch 

 from eggs deposited in the same season. Thus, Enchytraeidae can over- 

 winter successfully as eggs, immature worms, or mature worms. 



Growth rates of these species are reflected in the ratio of production 

 to average biomass, P/B. Mesenchytraeus sp. grows to the largest size, 

 over 300 \xg dry weight over the two-year life cycle, and the P/B (annual 

 production divided by average biomass, both in mg m"^) ratio is quite 

 high, 3.22. C atrata is much smaller with a maximum size of about 20 

 pig, but growth is accomplished in one year, and P/B is 2.89. The two- 

 year growth period and modest size (maximum = 65 ^g) of Henlea per- 

 pusilla gives rise to a P/B of 1 .49. Although the average biomass of C 

 atrata (131 mg m"^) is half that of H. perpusilla (260 mg m"^), the 

 estimated annual production of the two species is nearly equal. This em- 

 phasizes the danger of basing estimates of ecological importance upon 

 density and biomass data alone. The relative growth rate of the three 



a> 



5 



D^ 



Cernosvitoviella atrata 



j_ 



_L 



nz 



Henlea perpusilla 



X 



^ Ih 

 a: 



EirE 



Mesenchytraeus sp A 

 1 — I 1 



FIGURE 11-4. A schematic view of the life cycles of the 

 three dominant species of Enchytraeidae. 



.am 



,4^' 



