Due to the limited availability of suitable habitat, red-cockaded woodpeckers live in loose family groups that employ a cooperative breeding mating system (Jackson 1994). Red-cockaded woodpecker home ranges are large (average = 76.1 ha) but highly variable (17.2–159.5 ha; reviewed in Doster and James 1998).
Suitable habitat is defined by two main habitat components. The first is the presence of large pines. Pines ≥35 cm d.b.h. are generally required for a stand to be occupied by red-cockaded woodpeckers (Davenport and others 2000, James and others 2001, and Walters and others 2002). However, once large pine density exceeds 80 per ha, family group size (a demographic parameter related to productivity; Heppell and others 1994) declines (Walters and others 2002). Similarly, as the average d.b.h. of overstory pines increases above 35.0 cm, habitat quality declines (Davenport and others 2000). Theses declines, though, are likely linked to the maturation of the forests rather than the negative effects of large trees directly. Similar patterns have been observed for overstory pine basal area and small pine tree density in occupied stands, where values for these habitat attributes are lower than local maxima (James and others 2001, Rudolph and others 2002, Walters and others 2002).
The second notable habitat feature of high-quality red-cockaded woodpecker habitat is the open midstory. Hardwood midstory trees should be <3.26 m tall and ideally <1.8 m (Davenport and others 2002, Walters and others 2002). The open midstory is typically maintained through periodic fire (1-3 year burn interval), which also facilitates a wiregrass understory (James and others 2001). Because this species is non-migratory and suitable habitat is disjunct, connectivity of patches is critical for this species’ long-term persistence across the landscape.
The habitat suitability model for red-cockaded woodpeckers contains eight variables:
- successional age class
- forest patch size
- pine basal area
- hardwood basal area
- large (>35 cm d.b.h.) pine density
The first suitability function combines landform, landcover, and successional age class into a single matrix (SI1) that defines unique combinations of these classes
. We then directly assigned habitat suitability index scores to these combinations based on the relative rankings of vegetation types and successional age classes for red-cockaded woodpeckers reported in Hamel (1992).
We included forest patch size (SI2) as a variable because of the large home ranges of red-cockaded woodpeckers. We used data on home range sizes within the WGCP to define a logarithmic function that predicts suitability index scores from forest patch size
. The minimum home range size (17 ha) was assigned a score of 0.000, the largest home range size (170 ha) was assigned a 1.000, and the average home range size (49 ha) was assigned a 0.500
Pine basal area (SI3) is a key component of red-cockaded woodpecker habitat, and sites with pine basal areas either too low or too high are of poor quality. We fit a quadratic function
to data from Conner and others (1995) and Walters and others (2001;
on the relative abundance of red-cockaded woodpeckers in habitats with varying levels of pine basal area.
Mid- and overstory hardwoods reduce red-cockaded woodpecker habitat suitability. We fit an inverse logistic function
to data from Kelly and others (1993;
on the amount of hardwood basal area (SI4) around woodpecker nest cavities to predict habitat suitability based on this habitat feature.
As a resident species occupying disjunct habitat patches, red-cockaded woodpeckers occur as metapopulations. Therefore, dispersal between suitable forest patches is critical for persistence of this species on the landscape. Isolated patches lacking a breeding female have no productivity, so we used the median dispersal distance for females (3.2 km; Jackson 1994) to define average habitat suitability index scores (0.500). However, long-distance dispersal does occur (Hedrick, pers. comm.), so we assigned patches isolated up to 20 km from any other suitable sites at least some residual suitability (0.010). We fit an exponential relationship
through these data points
to describe how connectivity of patches influences habitat suitability.
Large pines (SI6) are a necessary component of red-cockaded woodpecker habitat because birds disproportionately forage and nest in large pines. However, a threshold exists above which habitat suitability declines as increasingly large trees reduce the preferred open character of the forest. We fit a quadratic function
to data from Walters and others (2002), who identified this threshold at 60–90 large pines/ha
To calculate the overall suitability index score, we determined the geometric mean of SI scores for forest structure (SI1, SI3, SI4, and SI6) and landscape composition (SI2 and SI5) separately and then the geometric mean of these means together.
Overall SI = ((SI1 * SI3 * SI4 * SI6)0.250 * (SI2 * SI5)0.500)0.500