Geoscience Reference
In-Depth Information
Chapter 2
Modeling Post-Eruption Habitat Changes
for Deer at Mount St. Helens using Remote
Sensing and GIS
Ronald W. Davis, Louis C. Bender, Paul W. Mausel, Leonardo Chapa-Vargas,
and Richard E. Warner
Abstract
GIS and remote sensing are essential tools in evaluating wildlife habitat
quality. Ultimately, habitat evaluations should relate quality measures to popula-
tion dynamics particularly when habitat conditions are changing. Following the
1980 eruption, populations of Columbian black-tailed deer (
Odocoileus hemionus
columbianus
) (hereafter BTD) increased rapidly but quickly declined. Similar
declines have been historically attributed to a predominance of even-aged closed
canopy forest and low forage biomass. The link between forage and BTD popula-
tion dynamics has been shown but to our knowledge no study has applied geospatial
tools to quantify landscape forage conditions and relate this to population changes
for BTD. We used 6 dates of post-eruption Landsat imagery (1984, 1988, 1991,
1996, 1999, and 2002) to map forest successional patterns and estimate subsequent
changes in winter forage for a portion of the Mount St. Helens blast zone. We used
forest maps as inputs into a GIS model estimating the supportable density of deer as
an indicator of nutritional carrying capacity (NCC). To simulate potential road and
cover effects on habitat use, we reduced habitat values based upon published effects
of distance to roads and the ratio of forage to cover vegetation. We created 1000
randomly placed simulated home ranges to provide a bootstrap data set for model
comparisons and to characterize potential uncertainty around model estimates. We
compared model estimates to actual deer population estimates in the study area.
Habitat models indicated a pattern of rapid forest succession and decrease in forage
which closely followed the estimated population trends for the study area. Closed
canopy forest dominated the landscape after 1991, and by 1996 comprised 70% of
the winter range. The combined effects of roads and cover reduced estimated habi-
tat values by over 70% (
P
< 0.02) in all years although NCC models alone were
closer to actual population estimates in 1984 and 1988. By 1991, NCC reduced by
road and cover effects was closer to actual trends although NCC was most highly
