Deborah J. Wilson

Limitation of collared lemming populations during a population cycle

Collared lemmings (Dicrostonyx groenlandicus) fluctuate cyclically in abundance on the Kent Peninsula, Northwest Territories, Canada. We tested whether predation was a necessary limiting factor of the lemming population in summer during the peak and decline phases (1996 and 1997) of the cycle. We reduced predation by means of a fence and an overhead mesh of monofilament line over 11 ha, completed late in the summer of the increase phase (1995). We used mark-recapture and radio-telemetry to investigate differences in demography between this predator exclosure and three control areas. Neither proportions of reproductive animals, nor sex ratio, nor net distances moved between trapping sessions differed significantly between the Exclosure and Controls. Adults were larger in the Exclosure than on Controls throughout the experiment; female juveniles born in the first summer litter of the peak were also larger within the Exclosure by August. Density was much higher in the Exclosure than on Controls in both 1996 and 1997. While density declined in 1997 on Controls, it first declined but then rose in the Exclosure; hence the population decline was reversed within the experimental area. Most mortality was due to predation in all three summers, but survival was significantly higher within the Exclosure only in the decline phase. We conclude that predation was necessary to limit the population in the peak and decline summers, but that the magnitude of the limitation was greater in the decline. Out results demonstrate that predation depresses peak and minimum densities of Dicrostonyx and further shapes the population cycle by extending the duration of the decline.

Nitrogen Mineralization, Plant Growth and Goose Herbivory in an Arctic Coastal Ecosystem

1 Lesser snow geese feed intensively on graminoid vegetation in intertidal salt marshes at La Perouse Bay, Manitoba. Relatively little grazing occurs in more inland sites, where the same vegetation persists but tidal inundations are infrequent. At some sites geese have grubbed vegetation exposing sediments. 2 Feeding preferences of geese may be linked to the availability of nitrogen for plant growth in the different areas. Total soil nitrogen, exchangeable inorganic nitrogen, net mineralization of nitrogen, together with soil properties, were measured in intertidal and inland sites (both grazed and ungrazed), and in intertidal grubbed sites. Where applicable, rates of net mineralization of nitrogen were compared with aboveground biomass and with the nitrogen content of shoots of graminoids in both grazed and ungrazed (exclosed) swards. 3 All soils were regosolic static cryosols with a thin Ah humus horizon. Most graminoid roots were confined to the top 2 cm of soil, few penetrated deeper than 5 cm. Bulk density of soil 1-2 cm below the surface was 0.87 g cm3 and 0.53 g respectively, in the intertidal and inland marsh, and water content and salinity of soil were higher in the inland marsh. 4 Total nitrogen in soils (0-2 cm below the surface) in the inland marsh was 118 g ? 6g N m-2 compared with 80 + 2 g N m2 in the intertidal marsh. Total soil nitrogen, exchangeable inorganic nitrogen and net mineralization of soil nitrogen were significantly greater beneath intact swards compared with grubbed swards in the intertidal marsh. 5 Seasonal net cumulative amount of nitrogen mineralized in the intertidal marsh (86 days) in 1991 was 0.53 g N m-2 at a soil depth of 1-2 cm. The comparable value for the inland marsh was 0.06 g N m2. In 1992 (54 days) the net amount of nitrogen mineralized in vegetated intertidal sites was 0.18g N m-2 (1-2cm) and in grubbed sites it was 0.06 g N m2. 6 Cumulative above-ground biomass of graminoid species in exclosed plots was greater in the intertidal marsh compared with that in the inland marsh. The amount of nitrogen (as percentage) in above-ground biomass was also higher in plants from the intertidal marsh. 7 Differences in the quantity and quality of vegetation are associated with variation in biogeochemical cycling in soils. Geese exploit patches of vegetation as their primary source of forage where net above-ground primary production is high and plant tissues are rich in nitrogen.

Primary and secondary resource pulses in an alpine ecosystem: snow tussock grass (Chionochloa spp.) flowering and house mouse (Mus musculus) populations in New Zealand

Context. Rodent populations in many parts of the world fluctuate in response to resource pulses generated by periodic high seed production (masting) by forest trees, with cascading effects on predation risk to other forest species. In New Zealand forests, populations of exotic house mice (Mus musculus) irrupt after periodic heavy beech (Nothofagus spp.) seedfall. However, in alpine grasslands, where snow tussock grasses (Chionochloa spp.) also flower and set seeds periodically, little is known about house mouse population dynamics. Aims. Our primary objective was to test for an increase in alpine mouse density following a summer when snow tussocks flowered profusely. We also estimated mouse density in adjacent montane forest over 2 years, and assessed mouse diet, to predict their potential impacts on native species. Methods. Flowering intensity of Chionochloa was assessed by counting flowering tillers on permanent transects (2003–06). Mouse density was estimated with capture–mark–recapture trapping in alpine (2003–07) and forest (2003–04) habitats. Mice were also collected and their stomach contents analysed. Flowering or fruiting of alpine shrubs and herbs, and beech seedfall at forest sites, were also measured. Key results. Chionochloa flowered profusely in austral summer 2005/06. Between autumn (May) and spring (November) 2006, mean alpine mouse density increased from 4 ha–1 to 39 ha–1, then declined to 8 ha–1 by autumn (May 2007). No mice were captured in 768 trap-nights during the following spring (November 2007). Prior to the mouse irruption, mouse density was consistently higher at alpine (0.4–4.0 mice ha–1) than at montane forest (0.02–1.8 mice ha–1) sites (in 2003–04). Alpine mouse diet was dominated by arthropods before mast flowering, and by seeds during it. Conclusions. The density and dynamics of alpine mice in relation to intensive snow-tussock flowering were similar to those in New Zealand beech forest in relation to beech masts. Implications. We predict the timing and duration of periods of heightened predation risk to native alpine fauna, as the result of pulses in mouse density and likely associated pulses in the density of stoats (Mustela erminea), a key exotic predator.

Century-scale effects of invasive deer and rodents on the dynamics of forests growing on soils of contrasting fertility

Understanding the long-term impacts of invasive mammalian browsers and granivores in mixed forests is difficult due to the many processes potentially affecting the demography of long-lived trees. We constructed individual-based spatially explicit simulation models of two mixed conifer–angiosperm forests, growing on soils of contrasting phosphorus (P) availability, to investigate how browsing by invasive red deer (Cervus elaphus scoticus) and granivory by invasive rodents (primarily house mouse Mus musculus) might alter forest dynamics. Models were parameterized with field data. Seedling growth and survival rates were estimated inside and outside deer exclosures. Seed predation rates were estimated at high and low rodent densities in mast and non-mast seeding years. For the alluvial terrace forest, which grew on P-rich soil, our model contained 15 tree species dominated by angiosperms; our model of the P-poor marine terrace forest contained seven species dominated by conifers. The two forest models were us...

Are lemming winter nest counts a good index of population density

Abstract Lemmings construct nests of grass and moss under the snow during winter, and counting these nests in spring is 1 method of obtaining an index of winter density and habitat use. We counted winter nests after snow melt on fixed grids on 5 areas scattered across the Canadian Arctic and compared these nest counts to population density estimated by mark–recapture on the same areas in spring and during the previous autumn. Collared lemmings were a common species in most areas, some sites had an abundance of brown lemmings, and only 2 sites had tundra voles. Winter nest counts were correlated with lemming densities estimated in the following spring (rs  =  0.80, P < 0.001), but less well correlated with densities the previous autumn (rs  =  0.55, P < 0.001). Winter nest counts can be used to predict spring lemming densities with a log-log regression that explains 64% of the observed variation. Winter nest counts are best treated as an approximate index and should not be used when precise, quantitative lemming density estimates are required. Nest counts also can be used to provide general information about habitat-use in winter, predation rates by weasels, and the extent of winter breeding.

Using artificial nests to explore predation by introduced predators inhabiting alpine areas in New Zealand

Abstract Many bird species endemic to alpine New Zealand are now at critically low densities and restricted in range, making predator‐prey research difficult. We used artificial nests in the Borland Valley, Fiordland National Park, to investigate (1) which introduced species is the most frequent nest predator in the two habitats, (2) whether nest survival differs between habitats, and (3) the utility of artificial nests for guiding conservation management. We used different types of artificial nest in 2 different years and undertook a calibration study of the two types. In 2003, survival of artificial nests containing wax eggs and chicken eggs was high in both habitats. In 2004, survival of artificial nests containing plastilina eggs and chicken eggs was low in both habitats, but was higher in alpine grassland compared with beech forest. Stoats and possums were the most frequent predators (36 and 22% respectively of artificial plastilina nests in alpine grassland and high‐altitude beech forest combined); these percentages did not vary significantly between habitats. Given the low density and sparse distribution of vulnerable species in much of New Zealand, data from artificial nests can be a useful tool for studying predation in these remote and difficult habitats, or at least, preferable to ignorance. However, the type of artificial nest used can strongly affect the rate at which they are destroyed.

Spatial and temporal variation in tree seed production and dispersal in a New Zealand temperate rainforest

Spatial and temporal variation in tree seed production is an important driver of the population dynamics of trees and of mammalian and avian seed consumers. Many studies have documented strong synchrony in production of intermittent large tree seed crops (masting), with cascading effects on the food webs of seed consumers and their predators. We used inverse modeling to characterize spatial and temporal variation in seed production and dispersal by four dominant tree species (two angiosperms and two conifers) over 8 years in southern temperate rainforests of New Zealand. In contrast to expectations from masting theory, there was little evidence of synchrony across species in years of high seed production, and only weak evidence in support of the expectation that temporal variation in seed production within species was strongly bimodal. Contrary to expectation from allometric scaling rules, there was no increase in reproductive effort once tree size (DBH) exceeded a minimum threshold (22–29 cm DBH) in the two angiosperm species. In the conifers, the minimum estimated size threshold for seed production was much higher (42–56 cm DBH), and in one of the species increased faster than linearly with biomass above the threshold size, indicating that the very largest trees in the conifer populations dominated seed production. Of the two species that occurred commonly on both fertile alluvial sites and less fertile uplifted marine terraces, the angiosperm species had higher per capita seed production on the fertile sites, while the conifer had higher per capita seed production (during seed years) on the less fertile sites. Local seed rain from all four species declined steeply with increasing distance from a parent, with peak local dispersal within 6 m of parent trees. This is not surprising given that the species are predominately gravity or animal dispersed with poor adaptations for wind dispersal. A substantial fraction of the input of seeds of all four species could not be attributed to local parent trees, consistent with longer distance dispersal by animals.

Landscapes of fear or competition? Predation did not alter habitat choice by Arctic rodents

In systems where predation plays a key role in the dynamics of prey populations, such as in Arctic rodents, it is reasonable to assume that differential patterns of habitat use by prey species represent adaptive responses to spatial variation in predation. However, habitat selection by collared (Dicrostonyx groenlandicus) and brown (Lemmus trimucronatus) lemmings depends on intra- and inter-specific densities, and there has been little agreement on the respective influences of food abundance, predators, and competition for habitat on lemming dynamics. Thus, we investigated whether predation affected selection of sedge-meadow versus upland tundra by collared lemmings in the central Canadian Arctic. We first controlled for the effects of competition on lemming habitat selection. We then searched for an additional signal of predation by comparing habitat selection patterns between 12 control plots and one large grid where lemmings were protected from predators by fencing in 1996 and 1997, but not during 5 subsequent years when we monitored habitat use in the grid as well as in the control plots. Dicrostonyx used upland preferentially over meadows and was more numerous in 1996 and 2011 than in other sample years. Lemmus was also more abundant in 1996 than in subsequent years, but its abundance was too low in the exclosure to assess whether exclusion of predators influenced its habitat selection. Contrary to the effects of competition, predation had a negligible impact on the spatial dynamics of Dicrostonyx, at least during summer. These results suggest that any differences in predation risk between the two habitats have little direct influence on the temporal dynamics of Dicrostonyx even if induced through predator–prey cycles.

Distribution, status and conservation measures for lizards in limestone areas of South Canterbury, New Zealand

Abstract The limestone outcrops of South Canterbury are outstanding landscape features, and many contain remnant native flora and fauna. Three skink species and four gecko species have been found there in subfossil remains. In 2008 the limestone habitats were surveyed to find which lizard species were present and to index their abundance, based on visual observation, artificial cover objects, pitfall traps, and searches of retreat sites. Common skinks (Oligosoma polychroma), McCanns skinks (O. maccanni) and Southern Alps geckos (Hoplodactylus ‘Southern Alps’) were found. Though the jewelled gecko (Naultinus gemmeus) was not found in limestone areas, three small populations were located elsewhere in South Canterbury. The species with the widest distribution and the highest number of sightings was the Southern Alps gecko. The two skink species were not observed living sympatrically. Both were found in low numbers in isolated pockets of suitable habitat. Conservation measures that might benefit these native lizards include reduced grazing as well as retention or addition of rock piles.

Prey switching by stoats (Mustela erminea): a supplemental food experiment

Context Prey switching by invasive carnivorans to changing food supply could severely impact on endemic prey of conservation importance, but experimental evidence for prey switching in carnivorans is rare. Stoats (Mustela erminea) were introduced to New Zealand and now threaten survival of many native birds, reptiles and invertebrates. Aim Our primary objective was to see whether abundant food caused stoats inhabiting an alpine grassland site to alter the rate at which they preyed upon weta (Orthoptera : Hemiandrus sp.), hares (Lepus europeus), birds and mice (Mus musculus). Methods We used dead rabbits as supplemental food in a before-after-control-impact experiment. Stoat scats were collected from a treatment and non-treatment site before and following food supplementation. Percentage frequency occurrence of the different prey types was assessed for the two sites during each experimental phase. Conclusions Stoats ate fewer ground weta and hares, the two most abundant prey types, when supplemental food was added. In contrast, consumption of mice remained relatively stable at both sites throughout the experiment, and the consumption of birds declined at both sites. Implications Our experiment suggests that stoats may continue to eat scarce endemic prey at similar per capita rates even when alternative prey are available. However, endemic prey that are locally or regionally abundant may be indirectly impacted by fluctuations in alternative prey.