Urbanization, Uncontrolled Growth
and Habitat Fragmentation
Urbanization ranks among the five most-frequently cited
pressures on threatened plants of the U.S./Mexico borderlands
(Nabhan et al. 1991). Between 1940 and 1990 the populations
of Arizona, Baja California Norte, and Sonora shifted from
being one half to two-thirds rural, to over three-quarters
urban (Table 4). This obviously
changes the degree to which the majority of the inhabitants
are "in touch" with natural resource conservation
issues, but it also poses profound threats for most land,
water, vegetation and wildlife resources within a half-hour's
drive of the largest metropolitan areas.
The actual effects of this urbanization on biodiversity are
many and mutually reinforcing, including the aggravation of
the "urban heat island effect" (Balling 1988; Nabhan
1990); the channelization or disruption of riverine corridors;
the proliferation of exotic species; the killing of wildlife
by automobiles, by toxics, and by pets; and the fragmentation
of remaining patches of natural vegetation into smaller and
smaller pieces that are unable to support viable populations
of native plants or animals.
One of the best-studied cases of the effects of urbanization
on natural habitat remnants within the metropolitan grid has
been summarized by Nabhan (1990) for Papago Park inside Phoenix,
Arizona. Known as Papago Cactus National Monument until 1929,
less than a thousand hectares of depauperate natural vegetation
remains, surrounded by golf courses, baseball fields, irrigation
canals, highways, and tract housing. Most of the park's native
carnivores have been killed by traffic, and in the absence
of predators other than humans, jackrabbits, cottontails,
gophers and ground squirrels have proliferated, particularly
as a result of out-migration from artificially watered golf
courses and parklands. In sample plots below Papago Buttes,
95% of the palo verde trees one meter tall or more have rabbit
droppings under them, and more than half of these "nurse
plants" have either rabbit holes or bedding areas cleared
beneath them, free of herbs or cacti. Sign of small mammals
have become ubiquitous, including gnawing marks on palo verde
shoots and cactus stems (McAuliffe 1990).
These unchecked herbivores have devastating effects on the
regeneration of the cactus-legume associations in the remnant
desert scrub vegetation below Papago Buttes. Jackrabbits and
cottontails quickly ravaged a cohort of 150 palo verde seedlings
which germinated with summer rains one year, and by the end
of the year, the last twelve surviving seedlings were all
scarred or clipped by herbivorous jackrabbits and cottontails.
Palo verdes are typically the most frequently-used nurse plants
by saguaro cacti, but there was no evidence of saguaro recruitment
in these plots since 1941. Of the saguaros aged 150 years
or younger, 60% had been gnawed at the base by small mammals.
Not only saguaro populations, but those of yuccas, chollas
and certain herbs have declined precipitously over the last
four decades as Papago Park has become a desert island in
an urban sea (Nabhan 1990).
During the last forty-five years, the urban heat island effect
has caused an increase of 3.9 degrees Centigrade in minimum
temperatures, as well as higher wind velocities and greater
local evaporation rates (Balling and Brazel 1987). Desert
bighorn, badgers and bobcats are no longer seen in the park
as they were earlier in the century. Mountain bike trails
have proliferated to the degree that not only vegetative cover
but topsoil has largely been stripped from the rocky flanks
of the Papago Buttes.
From:
Nabhan, Gary Paul and Andrew R. Holdsworth. 1998. State
of the Sonoran Desert Biome: Uniqueness, Biodiversity, Threats
and the Adequacy of Protection in the Sonoran Bioregion.
p.34-36. Tucson, Ariz.: The Wildlands Project.
Sources
(Balling and Brazel 1987)
(Balling 1988; Nabhan 1990);
(McAuliffe 1990).
(Nabhan 1990).
(Nabhan et al. 1991).
Tony L. Burgess and Martha Ames Burgess "Clouds, Spires
and Spines," in Tucson: A Short History (1986)
Alberto Búrquez and Angela Martínez-Yrízar. Conservation
and Landscape Transformation in Sonora, Mexico. (1997)
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