Fire is an agent of sudden change
in the physical characteristics of an ecosystem, and can lead
to dramatic or subtle changes in ecosystem function. As a disturbance
process, fire can influence the water cycle, erosion, biomass,
nutrient cycling, and productivity, as well as the availability
of features such as nesting or roosting sites, within an ecosystem.
This page illustrates a time series
of fire occurrences in the Jalisco-to-Gila migratory corridor
from the MODIS Land Rapid Response System Fire Detection Algorithm.
MODIS is a sensor on the TERRA satellite that was launched in
1999. It acquires data about light and heat energy from the Earth's
surface. The information that MODIS gathers about heat energy
is converted to an estimate of the temperature of each 1-km pixel
on the Earth. Thermal anomalies, or hot spots, that indicate
active fires are identified each day for almost the entire surface
of the Earth.
Fire locations are plotted in the
images below of the Jalisco-to-Gila (or Guadalajara to Phoenix)
migratory corridor. The time series shows monthly composites
of fire occurrence in 2001-2002.
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FIRE LOCATIONS
RED DOTS are locations of fires that burned some time during
the time frame represented by the image.
The background image is
a vegetation map made using data from the AVHRR, another remote
sensing instrument orbiting the Earth.
This image shows the locations
of all fires in May 2002. |
 |
Much of the Jalisco-to-Gila corridor
is affected by fire over the course of the year, especially the
thornscrub and tropical deciduous forests in the south. Some
of Arizona's Sky Islands, as well as parts of the semi-arid grasslands
in northern Sonora and southern Arizona, are also affected by
fire each year. Follow the link for May 2002, above, to view
a true-color MODIS image showing the locations of active fires
on May 22.
Most fires in the seasonally arid
regions of Mexico occur annually in the spring, prior to the
onset of the monsoonal rains. Some of these fires are part of
the agricultural crop cycle, some of them occur naturally, and
some of them are started deliberately when forest vegetation
is cleared for planting pasture or crops. Satellite data from
three years of observation (1998-2000) indicate that, on a 
national basis,
many of these fires occur in lands classified as fragmented forest,
with much smaller numbers in agricultural lands and protected
areas (data from Galindo et al. 2003 are shown in the graph at
right).
Furthermore, there is an important
relationship between interannual climate variability and both
the incidence of fires and sizes of burn areas. The very large
number of fires and larger fire sizes in 1998 were related to
a strong El Niño event associated with wintertime desiccation
in the tropics - at the same time that northern portions of the
migratory corridor received unusually high rainfall.
Even though most of the fires happen
while migratory creatures are further north, the fire cycle can
change the structure and productivity of vegetation resources
they need in their spring and fall migrations, and in their wintering
areas. Along with climatic variability, human uses of the land
can, collectively, affect the operation of the regional phenomenon
of migration.
News Item:
Mexico fires threaten Monarchs (BBC, 19 May 2000)
http://news.bbc.co.uk/hi/english/world/americas/newsid_754000/754853.stm
References:
MODIS Land Rapid Response System
- Web Fire Mapper
http://maps.geog.umd.edu/maps.asp
Galindo, I., P. López-Pérez,
& M. Evangelista-Salazar. 2003. Real-time AVHRR forest detection
in Mexico (1998-2000). International Journal of Remote Sensing
24: 9-22.
Hutto, R.L. 1989. The
effect of habitat alteration on migratory land birds in a West
Mexican tropical deciduous forest: A conservation perspective.
Conservation Biology 3 (2): 138-148.
Maass, J. M. 1995. Conversion
of tropical dry forest to pasture and agriculture. In Seasonally
Dry Tropical Forests, S. H. Bullock et al. (eds.), pp. 399-422.
Cambridge University Press, New York.
