Environmental Engineering Reference
In-Depth Information
(a)
(b)
FIGURE 9.8
Woodhouse's toads (a) have expanded their range at expense of Arizona toads (b) because of habitat alteration
associated with construction of impoundments.
wildlife species. Migratory songbirds use large water bodies (e.g., rivers) rather than small,
open-water guzzlers. Migrating birds travel in jet streams at high altitude and probably
seek large features such as rivers, lakes, and flowering trees as settling cues for necessary
resources. 46 Bats use artificial water sources but larger sources provide water for more
species than small guzzlers. Larger sources have more open water and fewer obstacles to
flight.47 47 Over these sources, bats expend more energy acquiring water because they use
more overflights and drinking. 48
Artificial water sources in arid regions also allow for interactions among species histori-
cally separated by habitat differences, such as Woodhouse's and Arizona toads in central
Arizona, and Mexican and plains spadefoots in southeastern Arizona; these amphib-
ians may cooccur and even hybridize to a much greater extent than they did in the past
(Figure 9.8). Availability of water sources for introduced grazing mammals also allows for
the spread of less arid-adapted invasive non-native species such as American bullfrogs and
Rio Grande leopard frogs known to negatively affect native amphibians of the Southwest. 49
Other sources of mortality (e.g., various pathogens) may also be increased as a result of
these more permanent sources of water. 50 Frogs and toads of the Southwest that use natural
temporary rain pools for breeding generally shun artificial water sources that typically
support a predatory and pathogenic fauna absent from ephemeral pools and have escaped
large-scale declines to date. 51 Nonetheless, by avoiding cattle tanks, green toads of the
Chihuahuan Desert were forced to breed in marginal aquatic habitats nearby and expe-
rienced high rates of mortality, presumably because water was channeled to the tanks for
livestock use. 52 It is increasingly clear that arid land grazing practices have far reaching
affects on desert flora and fauna.
9.4.2 Habitat Loss and Alteration along the Colorado and Rio Grande Rivers
Forty of the 1061 North American freshwater fishes went extinct in the last century. 53
Evidence suggests that extinction rates for North American freshwater fauna are five times
higher than those for terrestrial fauna and that these temperate freshwater systems are being
depleted of species as rapidly as tropical forests, some of the most stressed ecosystems on the
planet. 54 In the United States, the three leading threats to extinction of freshwater fauna are
habitat alteration in the form of nonpoint pollution, interactions with non-native species, and
Search WWH ::




Custom Search