Grazing Loss Research Articles

Factors Regulating in situ Filtering Rates of Cladocera

Individual filtering rates of four species of Daphnia, as well as Holopedium, Diaphanosoma, Ceriodaphnia, and Bosmina, were measured in 10 lakes under various field conditions to identify important factors regulating in situ filter-feeding for these cladocerans. Linear regression analysis indicated that carapace length accounted for almost 60% of the total variance in individual filtering rates when data from all 10 lakes were pooled. We suggest that zooplankton in these lakes filter-feed in proportion to their volume, since the functional slope of this regression approximated 3. Taxonomic affiliation had generally little effect on filtering rates. Temperature, ranging from 12 to 22 °C, had no statistically significant effect on filtering rate; there were also no significant differences between rates measured at dawn and dusk compared with midday rates for animals ranging in size between 0.30 and 1.30 mm. Addition of phytoplankton concentration as a second independent variable to the regression relating filtering rate to length further increased explained variance to 64%. Since the two main determinants of in situ filtering rates were zooplankton carapace length and concentration of edible phytoplankton, both of which are independent of taxonomic affiliation, investigators interested in assessing grazing losses should seriously consider measuring zooplankton in terms of size distribution rather than taxonomic affiliation.

Species‐specific rates of growth and grazing loss among freshwater algae1

Recent investigations into the population dynamics of phytoplankton communities have emphasized the variabilities of loss rates rather than growth rates in governing the changes that occur. Many freshwater phytoplankton, however, grow at rates that are measurably less than their maximum physiological capability because of nutrient limitation. At any moment the nutrient most limiting in situ division rates varies from species to species within an assemblage and the extents of limitation change with time. Rates of mortality inflicted by herbivorous zooplankton are likewise species‐specific, but the size of algal particles is a good predictor of whether the particle will be grazed at all. We manipulated grazer abundances and nutrient concentrations in enclosures to examine the effects on algal net growth rates. Grazed species were usually unicellular organisms. Some colonial species showed increased growth rates in the presence of grazers; in all cases these species were limited by N or P in situ. The variability of algal growth rates was much greater than that of measured loss rates due to grazing. Alternate loss rates, primarily sinking, seem to exhibit variability similar to growth rates in comparisons among all species in the assemblage.

Productivity and mineral cycling in an oak coppice forest 2. Annual net production of the forest

Annual net production was estimated in the secondary coppice forest near Tokyo, which was dominated by a deciduous oak,Quercus serrata Thunb. Lateral growth of stems and old branches was directly estimated by examining the annual rings for 35 shoots in a clear-cut quadrat of 10m×10m. Phytomasses of current organs were also weighed in the quadrat. Preharvest losses of current organs were determined by twelve 0.5 m2 litter traps for fine litter and twelve 6 m2 quadrats for woody litter. Branch production was also assessed indirectly by use of the stem-branch allometry and death of branches. The results of the indirect method were in sufficient agreement with the result of the direct one. Grazing loss of leaves from the canopy was estimated directly from the loss in leaf area and indirectly from the animal faeces caught by the litter traps.

Summer-growing components of a pasture system in a subtropical environment. 2. Relations between pasture and animal production

SummaryRelations were developed between the carrying capacity or production of fat-corrected milk (FCM)/ha and various measures of pasture growth for six pasture treatments grown in a subtropical environment. The pasture and animal production variables were obtained from pastures grazed on a weekly rotation using stocking densities which were varied so that a target weight of leaf material remained on the pasture at the conclusion of each grazing.Within a treatment, there was a curvilinear relation between the rate of leaf growth and carrying capacity, with the greatest efficiency of conversion at the lowest leaf growth rates. This relation generally varied between pasture treatments because of differences in grazing intensity. There was little variation in the production of FCM per cow between grazing periods within a treatment, reflecting the experimental technique of matching the stocking density to the amount of forage. Consequently FCM/ha largely mirrored the stocking density with some adjustment for differences in production per cow caused by differences in grazing intensity between treatments. Production of FCM/ha was affected more by a unit change in residue leaf yield than by a unit change in leaf yield before grazing.Using the rate of leaf removal (intake plus grazing losses) per cow as an inverse index of grazing intensity, it was apparent that the use of variable stocking densities to obtain specified residue yields after grazing did not necessarily equalize the grazing intensity because the rate of leaf removal per cow varied according to the before-grazing leaf yield. Also soiling by dung caused differences in the acceptability of forage for grazing, resulting in differences in grazing intensity between pastures with the same yield and stocking density.