Figure 1.
A map of Ring-necked Pheasant survey sites distributed throughout Nebraska.
Ring-necked Pheasant abundance was recorded at 405 survey sites distributed throughout 45 state Wildlife Management Areas and private property enrolled in the Open Fields and Waters program located in southern Nebraska (red points). Survey data was used to fit statistical models, which were evaluated using an independent testing dataset consisting of 150 survey sites evenly distributed across 10 road-transects (green points).
Table 1.
The range, mean, standard deviation and median values indicating the proportion of a land cover type within a spatial scale relevant to habitat management (1 km radius) and the surround landscape (5 km radius).
Table 2.
Pairwise Spearman’s ranked correlation Rho statistics for land cover variables.
Figure 2.
A directed acyclic graph describing the hierarchical Bayesian binomial-Poisson model used to assess the relationships between various land cover variables and Ring-necked Pheasant abundance.
Black nodes represent the non-covariate structure and the gray nodes represent the covariate structure. Notation: yij is the number of pheasants detected at survey site i during the jth survey and represents the product of a binomial distribution given the probability of detecting an individual (pij) and the number of individuals truly present was Ni. The detection probability, pij, at site i during the jth survey is a logit-linear function of covariates and parameter estimates
. It is assumed that Ni is Poisson distributed with a mean of λ. Mean abundance at site i is a function of site-specific covariates
with a random intercept
and a slope of
.
Table 3.
Parameter estimates of habitat and topographic variables measured at the management (1 km radius) and landscape scales (5 km radius), and the mixed-scale model with habitat variables measured at both the management and landscape spatial scales.
Figure 3.
The fitted and corrected relationships between Ring-necked Pheasant abundance and crop types in the surrounding landscape.
Fitted relationships for Ring-necked Pheasant counts indicated a positive response to small grains and row crops in the landscape (dark line), but failed to predict pheasant response in areas containing a higher proportion of either cover class located outside of the study region. The range of data values used to fit the relationship between Ring-necked Pheasant abundance and row crop is 0.00–0.75 and a mean of 0.25. The range of data values used to fit relationship between Ring-necked Pheasant abundance and small grains is 0.0–0.45 and a mean of 0.08. Assuming that too much row crop or small grains in the landscape is detrimental to pheasants, dashed lines represent the corrected relationships used to create the final spatial model of Ring-necked Pheasant abundance in Nebraska.
Figure 4.
The relationships between Ring-necked Pheasant abundance and the proportion of land cover types within a1 km radius.
Ring-necked Pheasant populations respond positively to the proportion of CRP (a) and grassland habitat (b) at the local management level (1 km radius). Solid line represents land cover relationships and the dashed lines represent the 95% credible intervals predicted out to the maximum range we observed during the study.
Figure 5.
The relationships between Ring-necked Pheasant abundance and the proportion of land cover types within a 5 km radius.
Ring-necked Pheasant populations respond positively to the proportion of row crop agriculture and small grains within the landscape (5 km radius), but negatively to the proportion of trees in the landscape. Solid line represents land cover relationships and the dashed lines represent the 95% credible intervals predicted out to the maximum range we observed during the study.
Figure 6.
The change in Ring-necked Pheasant response to CRP enrollment as the saturation of trees or small grains varies in the surrounding landscape (5 km radius).
CRP enrollment increases pheasant abundance; however the benefits of CRP are inhibited by trees (a) in the surrounding landscape while aided by small grains (b). Solid line represents null relationship of CRP and pheasant counts. Dotted lines represent additive effects of the second cover type in the landscape.
Figure 7.
The final 30×30-m resolution predicted Ring-necked Pheasant species distribution model for Nebraska based on the corrected fitted land cover and topographic variables.
The range of predicted values was divided into ten categories based on an equal area approach, whereas each color class represents 10% of the area within the entire species distribution model. Classifying the relative predicted abundance values using this approach allows users to pinpoint the top 10% of the areas within the Nebraska that contain the highest predicted abundance (bright red), which is useful in management planning and implementation.
Figure 8.
The evaluation of the predictive performance of the Ring-necked Pheasant fitted and the corrected species distribution models.
Standardized predicted values of Ring-necked Pheasant abundance compared to observed abundance values from an independent dataset collected in 2012 indicated that both the original spatial model (A) and the corrected spatial model (B) perform well. Data points are identified in blue, where the intensity of points is reflected by the color shade (dark blue = high intensity, and light blue = low intensity). The solid black line represents the fitted least-squares regression line and the two dashed lines represent the 95% confidence intervals. The dotted line identifies where a perfect fit would occur between predicted pheasant abundance and observed abundance.