Geomys Bursarius) in the Grassland Physiographic Regions of Indiana
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2010. Proceedings of the Indiana Academy of Science 119(1):87–94 DISTRIBUTION OF THE PLAINS POCKET GOPHER (GEOMYS BURSARIUS) IN THE GRASSLAND PHYSIOGRAPHIC REGIONS OF INDIANA Vanessa S. Quinn: Department of Biology/Chemistry, Purdue University-North Central, Westville, Indiana 46391 USA Chia-Chun Tsai and Patrick A. Zollner: Department of Forestry and Natural Resources, Purdue University, West Lafayette ABSTRACT. We conducted surveys for plains pocket gophers (Geomys bursarius) in nine counties in central and northern Indiana. The estimated mean (6 1 SD) population from road surveys was 512 6 440 individuals which is lower than previous surveys. Gophers were not found in Warren and Tippecanoe counties where they were historically present and showed a decline in Benton and northern Jasper counties. The decline in northern Jasper County may have been in part due to the timing and intensity of our survey work. Northern Newton County shows mounding activity presumably indicating healthy populations. More than a dozen clusters of mounds were observed along the eastern edge of White County in an area where mounding activity was not previously detected. Finally, we analyzed the spatial pattern of gopher mounds detected during this and three previous surveys in Indiana to delineate eight putative Indiana subpopulations. The geographic extent of these subpopulations incorporates 404 km of roads within suitable habitat. Keywords: Density, distribution, Geomys bursarius, plains pocket gopher, program DISTANCE The plains pocket gophers (Geomys bursar- ways and line-transects on selected public lands ius) is a species of special concern in Indiana with suitable habitat; (2) to determine burrow- due to a limited distribution in grassland ing activity using the sign-count inventory physiographic regions of northwest Indiana method (Reid et al. 1966); and (3) to combine (Fig. 1; Heaney & Timm 1983; Thorne 1989). data on observed mound locations from all This species was likely distributed throughout gophers surveys within Indiana (Conaway northwest Indiana prior to 1900 (Evermann & 1947; Tuszynski 1971; Thorne 1989) to define Butler 1894). Three surveys conducted since putative subpopulations based upon geograph- 1947 have shown declining populations. Con- ic proximity of mounds. away (1947) conducted a limited survey show- ing discontinuous and scattered Indiana popu- METHODS lations within the range of pre-settlement Study area.—We searched portions of nine populations. The second, and more thorough, Indiana counties within the Iroquois Till Plains survey found gophers in several northwestern region of northwestern Indiana (Gray 2000; Indiana counties with the majority of popula- Benton, Carroll, Cass, Jasper, Newton, Pulaski, tions located in Jasper and Newton counties Tippecanoe, Warren and White counties; (Tuszynski 1971). The most recent survey Fig. 1) for gopher mounding activity from identified population distribution as declining May–August 2008. The grassland physiograph- relative to previous surveys (Thorne 1989). We ic regions of Indiana are the only portions of report results of a survey conducted in the the state where gophers have been located summer of 2008 in the grassland physiographic (Conaway 1947; Tuszynski 1971; Thorne 1989). regions of northwest Indiana in addition to Within this region we searched areas occurring those sites where gophers were previously within potentially suitable habitat (cover class- identified. Our objectives were (1) to determine es of 21 {open}, 31 {barren}, 52 {shrub}, 71 the current distribution of gophers in Indiana {grassland/herbaceous}, or 81 {pasture/hay}) using surveys of roadside and railroad right-of- from USGS Indiana landcover data (USGS 2002). These designations corresponded to Correspondence: e-mail: [email protected] vegetative conditions in which gophers were 87 88 PROCEEDINGS OF THE INDIANA ACADEMY OF SCIENCE Figure 1.—Location of Iroquois Till Plains region of northwestern Indiana (inset) is the shaded area. The potential plains pocket gopher habitat is indicated by dark shading within the Iroquois Till Plains region. QUINN ET AL.—PLAINS POCKET GOPHER 89 documented to occur during the 1988 survey survey was completed). The number of mounds (Thorne 1989). observed was recorded, and the perpendicular Road survey.—During May–June 2008 we distance from the foot transects to each mound surveyed roadsides located within suitable was recorded in meters. cover-types in the grassland physiographic Data analyses.—Distance sampling analysis: region of northwestern Indiana (Fig. 1) for Data on mound occurrence from both roadside signs of above-ground activity indicated by surveys and walking transects were geo-refer- mounding (Andersen 1988; Jones et al. 1983, enced. We used the program DISTANCE 5.0 1985). Mounds are visible from a vehicle and (Burnham et al. 1980; Buckland et al. 2000; have a unique shape and texture (Thorne 1989). Thomas et al. 2002) to estimate mound density Fresh mounds can be distinguished by their (accounting for decreasing detection probabil- characteristic triangular, or deltaic, shape with ity as a function of distance from observer) for a round patch of soil located at the apex roadside and foot transect surveys. DIS- (Macdonald 2006). Examination of the USGS TANCE estimated density based on the detec- cover types (USGS 2002) within Iroquois Till tion probability and the effective strip width. Plain physiographic province indicated approx- The effective strip width is the distance from imately 1341 km of targeted survey roads the transect at which the number of animals within the Iroquios till plain (see Study area detected beyond this point exactly equals the above) indicating suitable habitat. We searched number of animals missed within this distance the right of way and surrounding habitat for (Buckland et al. 2000). We fit data with mounds on all road segments within our study different combinations of four types of key area that were not on primary high speed roads functions and three types of series expansion (greater than 55 mph). Searches were conduct- with no constraints on monotonocity. We first ed by a passenger while the driver maintained a used the complete data set of all mounds (n 5 speed of 30–40 km/hr. Once mounds were 544) for analysis. None of the models had a sighted a TrimbleH GeoXTTM handheld GPS good fit to the distribution as indicated by a unit from the GeoExplorerH 2004 series (ETrim- high P-value (P , 0.00001; Zar 2009). We ble Navigation Limited) was used to georefer- pooled the data by grouping observations on ence the position of the vehicle with a precision the same transect as a cluster (n 5 135). We less than one meter. A laser range finder also truncated our data set by deleting all (Bushnell Yardage Pro Sport 450, in yards) or observations closer than three yards (n 5 3) as a measuring tape (in feet and inches) was used to well as the 10% of observations furthest from measure distance from the center of the road to the transect (n 5 14; suggested by Swann et al. the center of the mounds. We recorded date, 2004). The pooled data were analyzed using distance, name of the road, land use type, and ‘‘cluster’’ option in DISTANCE and gained took photos of mounds to confirm any ques- better fits (P 5 0.79). After selecting the best tionable mounding activity (i.e., mounds made combination of key functions and series expan- by gophers, moles, woodchucks, or coyotes). sions, we compared densities of clusters and Mounds without the characteristic shape (Mac- densities of mounds estimated by different donald 2006) were omitted. types of size-biased regression methods provid- Transect survey.—We implemented an addi- ed by DISTANCE. tional 30 walking transects within polygons of Subpopulation definition: First, we estimated suitable habitat on public lands. Within these gopher maximum dispersal distance as the public lands we selected the 30 largest polygons square root of home range size multiplied by (ranging from 0.7–0.9 km2) of suitable habitat 40 (Bowman et al. 2002). We used the upper from the USGS landcover which indicated bound of spring and summer average home these public lands contained polygons of range size in Minnesota (66 6 23 m2; Zinnel potentially suitable habitat ranging in size from 1992) and found a maximum dispersal distance 0.2 6 0.1 km2. Within each of the 30 selected of 378 m. polygons transects were located by intersecting Next, we combined all known mound loca- the centroid of the polygon with a random tions from all survey periods into a single point along its perimeter. The total length of coverage (Fig. 2). Using this coverage we the 30 transects was 16 km (0.5 6 0.2 km). We calculated the distance (299 6 539 m) to the walked each transect once in July (after road nearest occurrence of another mound in any 90 PROCEEDINGS OF THE INDIANA ACADEMY OF SCIENCE Figure 2.—Location of all plains pocket gopher mounds observed within the Iroquois Till Plains region and northern Newton County while conducting road, transect, and historic location surveys during the summer of 2008. other survey for each observed mound. We surveys. Where these buffers overlapped (indi- calculated the mean 6 one standard deviation cating a neighboring mound nearer than (944 6 655 m) distance to the nearest mound 2254 m) and included at least three distinct for those mounds further than estimated mound observations we classified those dispersal distance from their nearest neighbor. mounds as belonging to a putative subpopula- We used the upper 95% confidence interval tion of gophers in Indiana. Finally, we took all (2254 m) of this latter distribution to create a of the mound locations defining putative buffer around each mound location from all subpopulations and generated the minimum QUINN ET AL.—PLAINS POCKET GOPHER 91 convex polygon that encompassed all of those multiplied this density estimate by the area of points. We buffered that polygon by 2 km potential habitat and estimated 512 6 440 (approximate estimated distances between geo- gophers in the surveyed area.