Habitat Suitability Indices / Predicted Species Distributions
Species distributions were modeled using Maxent
(http://www.cs.princeton.edu/~schapire/maxent/). The suitability value of any mapped pixel is the sum of
the probabilities of that pixel and all other pixels with equal or lower probability, multiplied by
100 to give a percentage. This value represents the % of pixels with a lower suitability value. The higher the value, the higher the probability of a species' occurrence.
The boundary of the Maxent distribution layers was created by merging the spatial extents of the selected California Central Valley hydrologic units and The Nature Conservancy's Great Central Valley ecoregion (see www.nature.org). We chose this merged boundary as it allowed complete overlap with the Central Valley's 12 largest hydrologic units while providing some coverage for the foothills to the east.
Additional information on our modeling methods can be found here.
A list of environmental data layers used in the models can be found here.Estimated Mean Densities
Density estimates (birds per hectare) and standard errors are given for each of
the focal bird species (see Table 1 below) examined in this study (format: density
+- standard error), for each Hydrologic Unit. These estimates are presented from point count surveys where at least 10 observation
records were available for a given Habitat Group (see Table 2 below) within
each Hydrologic Unit. Densities were calculated at each point count site based
on a 50 meter survey radius from the point count location.
Within a given Hydrologic Unit, the densities from all point count locations falling within specific Habitat Groups were averaged to calculate the estimates presented in the 'Densities by watershed' point layer. For riparian focal species, density estimates are separated into riparian and wetland habitats.
The Hydrologic Unit layer's boundaries are derived from polygons defined in the CalWater 2.2.1 product (the California Interagency Watershed Map of 1999; updated May 2004, "calw221"). CalWater 2.2.1 is the State of California's working definition of watershed boundaries (see www.gis.ca.gov).
| HABITAT GROUP |
SPECIES |
| Grassland | Burrowing Owl |
| Grasshopper Sparrow | |
| Northern Harrier | |
| Savannah Sparrow | |
| Western Meadowlark | |
| Oak Woodland | Acorn Woodpecker |
| Ash-throated Flycatcher | |
| Blue-gray Gnatcatcher | |
| Lark Sparrow | |
| Nuttall's Woodpecker | |
| Oak Titmouse | |
| Western Bluebird | |
| Yellow-billed Magpie | |
| Riparian and Wetland | Black-headed Grosbeak |
| Blue Grosbeak | |
| Common Yellowthroat | |
| Song Sparrow | |
| Spotted Towhee | |
| Swainson's Hawk | |
| Tricolored Blackbird | |
| Yellow-breasted Chat | |
| Yellow Warbler |
Table 2. Habitat Groups and Associated WHR Types
| HABITAT |
WHRNAME |
| Grassland | Perennial Grassland |
| Annual Grassland | |
| Pasture | |
| Oak Woodland | Blue Oak-Foothill Pine |
| Blue Oak Woodland | |
| Coastal Oak Woodland | |
| Valley Oak Woodland | |
| Montane Hardwood-Conifer | |
| Montane Hardwood | |
| Riparian | Aspen |
| Montane Riparian | |
| Valley Foothill Riparian | |
| Desert Riparian | |
| Wetland | Wet Meadow |
| Freshwater Emergent Wetland |
