Biological Changes: Animals
Between 36% and 73% of the native fish species have been completely eliminated from the Middle Rio Grande, while over 20 nonnative species have been introduced. Nonnative fish will likely continue to increase as long as the natural hydrological cycle of the river is disrupted. Considerable focus has been placed recently on the decline of the Rio Grande silvery minnow. This minnow was once abundant and widespread, occurring along the Rio Grande from approximately Espanola to the Gulf of Mexico as well as along the Pecos River. It is now restricted to about 5% of its former historic range, occurring only along part of the Middle Rio Grande. Although several factors have led to the decline of silvery minnows, most significant has been the alteration of their habitat by river channelization, water diversion and impoundment. Silvery minnows spawn in response to the increase in stream flow resulting from the spring runoff or summer thunderstorms, and their eggs drift in the water. Eggs and larvae are typically carried downstream, and the presence of diversion dams along the river now prevents adults from returning upstream. This means that the minnows tend to be concentrated along the lower-most portion of their current range, south of San Acacia Diversion Dam. This portion of the river often goes dry during the summer when water is diverted for agricultural use, thus leaving the minnows without suitable habitat. Although the river may have periodically dried up prior to river regulation, the combination of the presence of diversions dams, which prevent up-stream movement by adult minnows, and the rapid removal of water for irrigation, now typically traps minnows in the dewatered zones. Even when complete desiccation is avoided, minnows are concentrated into small pools where they suffer high predation rates from native and exotic fish, birds and mammals, and where the outbreak of pathogenic diseases is high. The struggle over the survival of silvery minnows represents more than just the survival of this one species, as the hydrological conditions required by minnows benefit a variety of other riverine and riparian organisms.
Native amphibians, particularly leopard frogs and chorus frogs, are declining with the loss of the wetlands and predation by, and competition with non-native bullfrogs. Bosque songbird populations face may threats; for example, species such as the southwest willow flycatcher are rapidly losing appropriate breeding habitat and many species of warblers and vireos suffer heavily from nest parasitism by brown-headed cowbirds. The Rio Grande turkey was eliminated from the valley due to hunting, while sandhill crane populations were significantly reduced. Fortunately, reintroduction programs for these species appear to be successful. Rodents such as the meadow jumping mouse and tawny-bellied cotton rat, which favor wet meadows, have declined with the loss of these habitats. Beaver were largely eliminated near the end of the 19th century, mainly due to trapping and habitat depletion, but have been reintroduced and have largely recovered. The jaguar, wolf, grizzly bear and mink have disappeared from the valley. Livestock, including cattle, sheep, goats, horses, donkeys and mules, heavily impact the riparian trees, shrubs, grasses and forbs, while feral cats and dogs negatively affect native animal populations. The invertebrate communities have changed as well; introduced crustacean isopods (pillbugs and woodlice) are now the most abundant macrodetritivores (invertebrates that chew on fallen organic material) on the forest floor.
Follow this link to learn more about the impact of Biological Changes on Plants.
