Geography Reference
In-Depth Information
threshold as connections between remnants of native land are severed, inva-
sions of nonnative species increase, erosion carries away fertile soil, and tem-
peratures rise.
The 30 percent threshold represents an ecological tipping point for su-
birdia. This was evident in my studies of Seattle's birds, but theory suggests
that this threshold will characterize changing landscapes wherever they exist,
from the tropics to the tundra. As with many tipping points, detecting the
impending change and enlisting civic concern are dii cult because all seems
good right up to the clif . In Seattle's subirdia, bird diversity remains high as
forest cover drops from 60 percent to 40 percent of its former extent. Diversity
declines slowly as forest cover erodes to 30 percent, but then the decline ac-
celerates with each additional, though slight, drop.
Social and ecological systems change after tipping points are crossed,
often in ways that naturally increase their resilience to future change. In this
way banks are restructured after economic meltdown. Building codes are re-
written after earthquakes. Security measures are reevaluated after attacks or
abuses of trust. In the face of disaster, human systems frequently reorganize
by increasing our connection to nature. Community gardening has increased
in postconl ict Bosnia; park projects blossomed in Berlin where a wall once
isolated residents; and after the earthquake that leveled much of Port-au-
Prince, Haiti, Martissant Park was built for those who needed a positive space
within which to gather, grieve, and reconnect. These responses give me hope
that the benei ts of our ecological resolve might engender more proactive re-
sponses in the future. Our existing civic organizations can push their urban
ecosystems back from the tipping point by increasing, improving, and restor-
ing the green spaces that prove so valuable in our most trying times.
