DOI: 10.25338/b8vd1r
title: Riparian land-cover data and model code for: Multiple-region, N-mixture community models to assess associations of riparian area, fragmentation, and species richness

description: The associations of habitat area and fragmentation with species richness
 long have been major topics within community ecology. Recent discussion
 has focused on properly assessing fragmentation independent of habitat
 area (fragmentation per se), and on whether fragmentation has significant
 negative or positive associations with species richness. We created a
 novel, multiple-region, N-mixture community model (MNCM) to examine the
 relations of riparian area and fragmentation with species richness of
 breeding birds in mountain ranges within the Great Basin, Nevada, USA. Our
 MNCM accounts for imperfect detection in count data at the survey-point
 level while allowing comparisons of species richness among regions in
 which those points are embedded. We used individual canyons within
 mountain ranges as regions in our model and measured riparian area and the
 normalized landscape shape index, a metric of fragmentation that is
 independent of total riparian area. We found that riparian area, but not
 its fragmentation, was a primary predictor of canyon-level species
 richness of both riparian obligates and all species. The relation between
 riparian area and riparian-obligate species richness was nonlinear:
 canyons with ≥ 25 ha woody riparian vegetation had relatively high species
 richness, whereas species richness was considerably lower in canyons with
 &lt;25 ha. Our MNCM can be used to calculate other metrics of
 diversity that require abundance estimates. For example, Simpson’s
 evenness of riparian obligate species had a weak negative association with
 riparian area and was not associated with fragmentation. Projections of
 future riparian contraction suggested that decreases in species richness
 are likely to be greatest in canyons that currently have moderate (~10-25
 ha) amounts of riparian vegetation. Our results suggest that if a goal of
 management is to maximize the species richness of breeding birds in
 montane riparian areas in the Great Basin, it may be more effective to
 focus on total habitat area than on fragmentation of patches within
 canyons, and that canyons with at least moderate amounts of riparian
 vegetation should be prioritized.
description: We used data from 23 canyons in the Shoshone Mountains, Toiyabe
 Range, Toquima Range, and Monitor Range (central Great Basin, Nevada,
 USA).  We derived riparian area and fragmentation at
 the canyon level from 2013 National Agricultural Inventory Program (NAIP)
 images. We did not attempt to distinguish the cause of fragmentation or
 the historical extent of riparian cover in these canyons. There were no
 substantial disturbances or changes in land use in these canyons from
 2001-2018 that would drive changes in the extent of riparian area or its
 fragmentation. It is possible that changes occurred gradually, but we
 believe that the slow rate of growth of woody vegetation minimized these
 changes over the temporal extent of our work. To calculate riparian area,
 we delineated a buffer (500 meters from the center line of the canyon
 bottom) that was large enough to contain all riparian vegetation in most
 canyons. We mapped riparian cover within the buffer in QGIS version 3.0.2
 (QGIS.org 2018) and classified that cover as wet meadows (little to no
 perennial woody vegetation; dominated by grasses, sedges, and forbs) or
 woody riparian (extensive cover of perennials, primarily Betula
 occidentalis, Populus spp., Prunus virginiana, Salix spp., or Rosa
 woodsii). We selected these two classes because they are not used equally
 by riparian obligates. Although some riparian obligates forage in wet
 meadows, the meadows do not contain the woody vegetation in which most
 species nest. We were not confident in our ability to discern structural
 classes (e.g., trees versus shrubs) or individual woody species in the
 NAIP images and therefore did not further classify the images. Wet meadows
 were present in 9 of 23 canyons, and the cumulative area of those meadows
 in a given canyon was 0.3-13.1 ha. Woody riparian vegetation was present
 in all canyons, with a canyon-level mean cumulative area of 18.0 ha (range
 0.3-96.5 ha). Overall, riparian vegetation was a small percentage of the
 land cover in our canyons (mean 2.7%, range 0.1-21.9%).
 We selected the Normalized Landscape Shape Index (nLSI)
 (McGarigal et al. 2012) as our measure of fragmentation. This metric is
 not highly correlated with total area of patches of a given class (e.g.,
 riparian vegetation) when that total area is &lt;30% of the defined
 landscape (Wang et al. 2014), as is the case in our study system. nLSI for
 a sampling unit (canyons in our case) is equal to the cumulative perimeter
 of patches minus the minimum possible perimeter of those patches. nLSI
 then is standardized by dividing each value by the difference between the
 maximum and minimum possible cumulative perimeter of those patches in the
 sampling unit. The nLSI of a sampling unit with a single square patch
 would be 0, the nLSI of a sampling unit with a checkerboard distribution
 of patches would be 0.5, and the nLSI of a sampling unit with maximally
 disaggregated patches would be 1. Other fragmentation metrics that are not
 highly correlated with total area of a focal class of patches are less
 intuitive measures of variance, require a minimum number of patches, or
 require user delineation of core focal areas. We calculated nLSI of woody
 riparian in each canyon with the program FragStats (McGarigal et al.
 2012). We did not calculate nLSI for wet meadows because nLSI cannot be
 calculated if the area of the focal class is zero in some sampling units,
 as was the case for several of our canyons. The correlation between all
 pairwise combinations of nLSI, woody riparian area, and wet meadow area in
 our canyons was &lt;|0.56|, which was sufficiently low to include all
 three variables in a given model (Dormann et al. 2013).