. Bulletin. Natural history; Natural history. Aug. 1979 Bellrose et al.: Waterfowl and the Chancing Illinois Valley 37 resulted in the eradication of aquatic plants by 1969 (as previously described) and the consequent collapse of wigeon and coot populations. Aquatic plants had not recovered by 1976 and neither had the local wigeon and coot popula- tions (Fig. 20). SUMMER WATER LEVELS AND FALL WATERFOWL ABUNDANCE We have previously shown the cor- relation between summer water levels and wetland plants and that between wetland plants and the fall abundance of certain species of waterfowl. How- e
. Bulletin. Natural history; Natural history. Aug. 1979 Bellrose et al.: Waterfowl and the Chancing Illinois Valley 37 resulted in the eradication of aquatic plants by 1969 (as previously described) and the consequent collapse of wigeon and coot populations. Aquatic plants had not recovered by 1976 and neither had the local wigeon and coot popula- tions (Fig. 20). SUMMER WATER LEVELS AND FALL WATERFOWL ABUNDANCE We have previously shown the cor- relation between summer water levels and wetland plants and that between wetland plants and the fall abundance of certain species of waterfowl. How- ever, our most precise and extensive data are on waterfowl abundance and water levels. Our evaluation of these factors is made on the assumption that summer water levels, used to calculate the Moist-Soil Water-Level Index, gov- ern the growth of moist-soil food plants and that they in turn affect the abun- dance of fall waterfowl. Fig. 21 demonstrates that the fall populations of several species of dab- bling ducks are reflected in the Moist- Soil Water-Level Index. In the upper Illinois Valley, above Peoria, only the pintail sliowed a significant correlation between the Moist-Soil Water-Level Index and subsequent fall abundance. However, four species of dabbling ducks in the lower Illinois Valley (be- low Peoria) had significant correlations l)etween fall population levels and the Moist-Soil Water-Level Index (Fig. 21). The slopes of the linear regressions indicate that pintail numbers re- sponded the most dramatically when a high Moist-Soil Water-Level Index occurred. (The higher the index, the longer the duration of low water.) Wigeon and green-winged teal popula- tions responded less to these condi- tions, and fall blue-winged teal num- bers responded only slightly to a high Moist-Soil Water-Level Index. Fall populations of the mallard, the most abundant duck in the Illinois Valley, did not show a significant cor- relation with the Moist-Soil Water- Level Index for two reaso
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