8172 - Laserfiche WebLink

Home Browse Search

sites, the assemblages- for reservoir fishes, large river fishes, and warmer water fishes should become more apparent. Species-Environment Relations Fish Analysis of fish data by CCA acid "DCCA indicated a strong association of fish species with elevation. One cautionary note is important regarding interpretation of the biplot from DCCA (Figure 18). A causal relationship between an environmental variable and a species that is plotted near the end of an environmental axis is often inferred. However, an example from Figure 18 demonstrates how this interpretation can be misleading. Rio Grande sucker (Cat ple in Figure 18) is plotted near the end of the potassium axis. One might be tempted to conclude that high potassium is somehow necessary for Rio Grande suckers to be in high relative abundance. However, this interpretation is incorrect; there is no causal relationship. Rather, Rio Grande suckers were in high relative abundance throughout the lower Jemez River. It happened that two of the sites, identified as outliers in ordination, were downstream from Soda Dam, an artesian spring source with high potassium and sodium content. Thus, the association between potassium and Rio Grande sucker is coincidental. Rio Grande suckers were generally absent or in low abundance at sites that had white sucker in high relative abundance. This suggested that introduced white suckers may have had a deleterious effect on Rio Grande sucker populations in the upper Rio Grande drainage. Alternatively, the explanation may lie in contrasting degrees of habitat specificity. Both species are open substrate spawners that prefer gravel/cobble substates with open interstitial spaces in in which the eggs can incubate and the photophobic larvae can escape predation. 'The larvae of each species have only moderately developed respiratory structures, and therefore require well-oxygenated waters in which to survive. Whereas the white sucker is adapted to habitat conditions over a wide range of elevations, the Rio Grande sucker is more specialized, being confined to a lower and narrower range of elevations in graded chanaelsa. Any change within the watershed that may impact the balance between erosion and deposition would be relatively more damaging to populations of Rio Grande sucker due to its habitat specialization. Chimnondds and Bend* Macminvertebrates Investigation of species-environment relations for chironomids and for benthic macroinvertebrates was limited in the present study to examination of the relation between number of species and elevation. No significant relation was found between the number of chironomid species and elevation. This result is in contrast to the finding of Ward (1986), who found that the number of chironomid taxa decreased as elevation decreased. It also contrasts with the finding that the number of benthic ° A stream is considered graded when there is a balance between erosional and depositional processes. Grade implies, both stability and grbdual change with continual adjustments of slope, depth, and width maintaining the balance between erosion and deposition. ?.L ?c