Diversity and Abundance of Terrestrial Gastropods in Voyageurs National Park, Mn: Implications for the Risk of Moose Becoming Infected with Parelaphostrongylus Tenuis
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DIVERSITY AND ABUNDANCE OF TERRESTRIAL GASTROPODS IN VOYAGEURS NATIONAL PARK, MN: IMPLICATIONS FOR THE RISK OF MOOSE BECOMING INFECTED WITH PARELAPHOSTRONGYLUS TENUIS Tim Cyr1, Steve K. Windels2, Ron Moen3, and Jerry W. Warmbold2,4 1Integrated BioSciences Graduate Program and Natural Resources Research Institute, University of Minnesota Duluth, Duluth, Minnesota 55811; 2Voyageurs National Park, 360 Highway 11 East, International Falls MN, 56649; 3Natural Resources Research Institute, 5013 Miller Trunk Highway, Duluth, Minnesota 55811; 4Present address: University of South Dakota, 414 East Clark St, Vermillion, South Dakota 57069. ABSTRACT: Voyageurs National Park (VNP) has a stable population of about 40–50 moose (Alces alces). Recent declines in moose abundance in adjacent areas in northern Minnesota raise concerns about the long-term viability of moose in VNP. The parasitic nematode Parelaphostrongylus tenuis has been documented in moose in VNP and has been implicated in moose declines in other popula- tions. Terrestrial gastropods are the intermediate hosts for P. tenuis, and describing spatial and temporal differences in their abundance should increase understanding about the risk of P. tenuis infection for VNP moose at the individual and population levels. We used cardboard sheets to estimate species com- position and abundance of terrestrial gastropods in representative vegetation communities in VNP. We collected a total of 6,595 gastropods representing 25 species, 22 terrestrial snails and 3 slugs; 8 are known vectors of P. tenuis, including the slug Deroceras laeve, the most common species found. Gastropods were more abundant in September than July, and in upland forests (maximum = 555 gas- tropods/m2) more than in wetter lowlands (20 gastropods/m2). We used location data from GPS- collared moose in VNP to estimate the relative exposure of moose to gastropods that could be infected with P. tenuis larvae. The boreal hardwood forest and northern spruce-fir forest ecotypes had the high- est use by moose and high abundance of P. tenuis vectors in summer, and may pose the greatest risk for infection. Habitat use and the related risk of ingesting gastropod vectors varied by individual moose. Our method can be extended in moose range to estimate the relative risk of P. tenuis infection. ALCES VOL. 50: 121–132 (2014) Key Words: Alces, meningeal worm, Minnesota, moose, P. tenuis, parasite INTRODUCTION Infections can be lethal and cause mortality The parasitic nematode Parelaphostron- indirectly through increased risk of predation gylus tenuis can be fatal to moose (Alces or accidents (Lankester et al. 2007, Butler et al. alces) (Anderson 1964), and was the prob- 2009, Wünschmann et al. 2014). Voyageurs able cause of 5% of mortality of radio-col- National Park (VNP) in northern Minnesota lared moose in northwestern Minnesota and maintains a stable, low-density population of >20% of incidentally-recovered moose in about 40–50 moose, and P. tenuis infection northern Minnesota (Murray et al. 2006, and associated mortality has been documented Wünschmann et al. 2014). The infection in and surrounding VNP (Windels 2014). causes weakness in the hindquarters, circling, Though the effect on moose at the popula- tilting of the head, and increased fearlessness tion level in VNP is unknown, previous of humans (Anderson and Prestwood 1981). studies suggest it is unlikely to be a major 121 GASTROPOD VECTORS OF P. TENUIS IN VNP – CYR ET AL. ALCES VOL. 50, 2014 mortality source at the currently low white- regions of Minnesota and the surrounding tailed deer (Odocoileus virginianus) density areas is possible (e.g., from northwestern (3–6 deer/km2) (Whitlaw and Lankester Minnesota [Nekola et al. 1999] or rock out- 1994b). crops in northeastern Minnesota [Nekola The normal lifecycle of P. tenuis 2002]). Gastropods exhibit habitat preferences includes white-tailed deer as the definitive that result in variation in presence or density host and terrestrial gastropods as intermedi- across vegetation communities or other habitat ate hosts (Lankester and Anderson 1968). features, and few studies have examined their White-tailed deer ingest infected gastropods abundance and diversity at fine spatial scales while foraging and gastropods become (Moss and Hermanutz 2010). infected with P. tenuis by crawling over or The risk of P. tenuis infection is presum- near infected deer feces (Lankester 2001). ably influenced by vector density and could However, only 0.1–4.2% of gastropods col- vary within a population because individual lected in Minnesota and Ontario were moose demonstrate differential habitat use infected with P. tenuis larvae (Lankester (Gillingham and Parker 2008). Fine-scale and Anderson 1968, Lankester and Peterson habitat use derived from GPS collars can 1996). At those infection rates, a white-tailed help clarify the risk of P. tenuis infection to deer would need to consume up to 1000 gas- individuals and populations of moose. Com- tropods to encounter a single larva (Lenarz bined, individual differences in habitat use 2009). However, Lankester and Peterson and variability among habitat types in gas- (1996) reasoned that even at such low rates tropod diversity and abundance may result of infection in gastropods, the high rates of in differential risk of moose and other cer- infection measured in white-tailed deer in vids to P. tenuis infection (VanderWaal et al. the region (≤91%, Slomke et al. 1995) is 2014). explained by the large volume of vegetation In this study we surveyed terrestrial gas- eaten on and near the ground over a few tropod species on the Kabetogama Peninsula months in the autumn. Infection rates in in VNP. Our objectives were to 1) estimate white-tailed deer derived from winter fecal the abundance and diversity of terrestrial samples have ranged from 67–90% from gastropods in different ecotypes, with parti- the 1970s to the present in VNP (Gogan et al. cular focus on known vectors of P. tenuis, 1997, VanderWaal et al. 2014). White-tailed 2) document changes in gastropod abun- deer are the definitive host of P. tenuis but dance over the growing season, and 3) com- moose, an aberrant host, also ingest infected pare the use of cover types by GPS-collared gastropods during foraging and become moose to density of P. tenuis vectors to infected. Initial signs of P. tenuis infection estimate the encounter risk of individual can appear in moose as early as 20 days after moose. experimental infection (Lankester 2002). Gastropods are necessary for P. tenuis to STUDY AREA complete its life cycle. Therefore, knowledge Voyageurs National Park (48.50° N, of gastropod populations in VNP may help 92.88° W) is an 882 km2 protected area com- managers better understand the role of prised of a mixture of forested land (61%) P. te nu is in local moose population dynamics. and large lakes (39%) along the U.S.-Canada The distribution and habitat preferences border. Moose are primarily restricted to of terrestrial gastropods in VNP have not the Kabetogama Peninsula (Windels 2014), been studied previously. Extrapolation from a 300 km2 roadless area in the center of studies of gastropod communities in different VNP, and have remained relatively stable 122 ALCES VOL. 50, 2014 CYR ET AL. – GASTROPOD VECTORS OF P. TENUIS IN VNP since the 1990s with density ranging from Table 1. Area (km2) and % total area covered by 0.14–0.19 moose/km2 (Windels 2014). each of 6 terrestrial vegetation ecotypes sampled White-tailed deer density in winter during on the Kabetogama Peninsula, Voyageurs the study ranged between 3–6/km2 (Gogan National Park (VNP), Minnesota, USA, June- et al. 1997, unpublished data of VNP). Vege- September 2011. Area calculations exclude lakes and ponds. Ecotype classifications are according tation is a mix of southern boreal and Laur- to the US-National Vegetation Classification entian mixed conifer-hardwood forests System applied to VNP (Hop et al. 2001). comprised primarily of a mosaic of quaking Ecotype Area (km2)% aspen (Populus tremuloides), paper birch (Betula papyrifera), balsam fir (Abies balsa- Northern Spruce-Fir Forest 66 23 mea), white spruce (Picea alba), white pine Boreal Hardwood Forest 62 21 (Pinus strobus), red pine (P. resinosa), jack Northern Pine Forest 52 18 pine (P. banksiana), and black spruce (Picea Treed Rock Barrens 39 13 mariana) (Faber-Landgendoen et al. 2007). Northern Shrub Swamp 8 3 Soils range from thin, sandy loams over bed- Rich Conifer Swamp 5 2 rock to poorly draining clays at lower eleva- Total 232 80 tions (Kurmis et al. 1986). Beaver-created wetlands and associated seral stages are abundant (Johnston and Naiman 1990). representative of those across the entire − Temperatures vary from 40 to 36 °C, with Peninsula. At each site we sampled during an average annual temperature of 1.4 °C. a single over-night period at approximately Mean annual precipitation is 62 cm, with 1-month intervals in each of 4 periods: 6– most precipitation falling between May and 20 June, 29 July–3 August, 18–25 August, September (Kallemeyn et al. 2003). and 9–14 September. We used 0.25 m2 cardboard sampling METHODS squares (50 Â 50 cm) placed on ground We used the “ecotype”-level vegetation vegetation to collect gastropods (Lankester classification derived from the USGS-NPS and Peterson 1996, Hawkins et al. 1998, Vegetation Map (Hop et al. 2001) to select Nankervis et al. 2000, Maskey 2008). We the 10 most common terrestrial ecotypes on randomly selected a starting sample point the Kabetogama Peninsula to sample for gas- tropods. We excluded 4 of these because and direction within each polygon such that they were too wet to sample with our metho- a 100-m sampling transect would fit entirely dology: poor conifer swamps, rich hardwood within the polygon. We placed 10 corrugated swamps, wet meadows, and shrub bogs. The cardboard squares on the 100-m transect and remaining 6 ecotypes comprised 80% of verified that all were in the same ecotype.