Yosef Cohen

Moose Browsing and Soil Fertility in the Boreal Forests of Isle Royale National Park

Selective foraging by moose on hardwoods and avoidance of conifers alters community composition and structure, which in turn can affect nutrient cycles and pro- ductivity. The effect of moose browsing on the nutrient cycles of boreal forests was studied using three 40-yr-old enclosures on Isle Royale, Michigan. Two alternative mechanisms by which moose affect ecosystems were tested: (1) moose depress both the quantity and quality of litter return to the soil, and hence N mineralization and net primary productivity, by browsing on hardwoods and avoiding conifers; (2) moose stimulate N mineralization, and hence net primary productivity, by opening the canopy and by dropping fecal pellets. Soil nutrient availability and microbial activity, including exchangeable cations, total car- bon and nitrogen, nitrogen mineralization rates, and microbial respiration rates, were uniformly higher in exclosures than outside. These differences were more significant where browsing intensity was high and less often significant where browsing intensity was low. N mineralization in browsed plots declined with increasing moose consumption rates. Net primary production in enclosures and browsed plots was strongly correlated with N min- eralization. N mineralization in turn was positively correlated with litter N return and negatively correlated with litter cellulose content. These differences in litter quantity and quality were caused by an increased abundance of unbrowsed spruce outside the enclosures. Moose pellets alone mineralized less N but more C than soil alone, but pellets combined with soil stimulated N and C mineralization more than the sum of the two separately. However, this did not appear to be sufficient to offset the depression in nitrogen and carbon mineralization in soil resulting from the increased abundance of unbrowsed spruce. We conclude that, in the long term, high rates of moose browsing depress N mineralization and net primary production through the indirect effects on recruitment into the tree stratum, and subsequent depression of litter N return and litter quality. These results suggest that the effects of herbivores on ecosystems may be amplified by positive feedbacks between plant litter and soil nutrient availability.

Effects of moose browsing on vegetation and litter of the boreal forest, Isle Royale, Michigan, USA

Large mammalian herbivores can influence the dynamics and structure of ecosystems by selectively removing tissues of specific plant species. The plant community composition can be altered as animals feed on some species but not others, changing the biomass, production, and nutrient cycling of an entire ecosystem. We used four paired moose (Alces alces) exclosures and browsed plots (built between 1948 and 1950) on Isle Royale, Michigan, to examine the influence of moose on aboveground biomass, production, and annual litterfall of boreal vegetation in 1987. Tree biomass was significantly greater (X = 230 vs. 150 Mg/ha, df = 3, P < .05), shrub biomass was significantly less (X = 1.9 vs. 3.1 Mg/ha, P < .05), and herb biomass was significantly less (X = 0.2 vs. 0.8 Mg/ha, P < .05) in exclosures than in browsed plots. Tree production was greater in exclosures than in browsed plots (X = 7.9 vs. 5.0 Mg.ha—1.yr, P = .05), but there was no difference in the production per unit biomass between exclosures and browsed plots. Shrub production in exclosures was similar to that of browsed plots (X = 3.5 vs. 2.3 Mg.ha—1.yr—1, P < .05), despite total vegetation biomass differences between paired plots. There was significantly greater herb litter produced in the browsed plots than in the exclosures (X = 0.7 vs. 0.1 Mg.ha—1.yr—1, P < .05). Moose browsing prevented saplings of preferred species from growing into the tree canopy, resulting in a forest with fewer canopy trees and a well—developed understory of shrubs and herbs. In addition, browsing may have altered the eventual balance of white spruce (Picea glauca) was balsam fir (Abies balsamea), causing an increase in the former and a decrease in the latter. Thus, browsing by moose influences in long—term structure and dynamics of the boreal forest ecosystem, which has important implications for forest ecosystem management, especially where the population dynamics of moose are regulated.

Effects of moose movement and habitat use on GPS collar performance

We tested a radiotelemetry collar that uses a Global Positioning System (GPS) unit to calculate animal locations. We placed the collar in a range of cover types and compared locations reported by the collar to differentially-corrected GPS locations. We placed the collar on a free-ranging moose (Alces alces) and determined how selection of cover types, collar movement, and collar orientation affected GPS locations. On or off the moose, the GPS unit collected a location in >90% of location attempts in areas with no or thin canopy cover, including mature deciduous canopies in winter. Under a mature conifer canopy or a mature deciduous canopy in summer, 60 to 70% of location attempts were successful. Locations from the GPS unit in the collar were close to the expected precision of non-differentially corrected GPS (within 40 m 50% of the time and 100 m 95% of the time). Locations did not have a directional bias. Movement of the moose while a location was being attempted did not affect GPS locations. The moose occasionally laid down so the collar was horizontal. Although this decreased the success of location attempts, <1% of location attempts were so affected. GPS radiotelemetry has great promise for expanding our knowledge about hourly, daily, and annual patterns in animal movements and habitat selection.

Accuracy of GPS telemetry collar locations with differential correction

Global Positioning System (GPS) units in telemetry collars provide an unbiased and precise estimate of animal locations. Under ideal conditions at least 50% of locations are expected to be within 40 m in uncorrected mode GPS, and within 5 m in differential mode GPS. When the collar was placed under open sky, most locations were 3-dimensional locations that could be differentially corrected. Under hardwood canopies with leaves on, the frequency of 3-dimensional locations decreased, the frequencies of failed location attempts and 2-dimensional locations increased, and the precision of GPS locations decreased. We compared the precision of each GPS mode by calculating uncorrected mode and differential mode locations from the same pseudo-range and ephemeris data. We varied the number of satellites used in the location solution to simulate the effect of decreased satellite acquisition due to canopy cover on precision of locations. Precision of locations increased if signals from >4 satellites were used to calculate the location in uncorrected mode and in differential mode. We found that 2-dimensional locations were almost as precise as 3-dimensional positions if the altitude of the GPS unit was known. If the altitude used to calculate a 2-dimensional location was within 50 m of the actual collar altitude, the precision of 2-dimensional differential mode locations was better than 3-dimensional uncorrected mode locations. If the error in altitude was 100 or 150 m, then 50% of 2-dimensional differential mode locations were within 70 m and 95% were within 185 m of the true location. We used GPS locations from collars placed in different cover types and on free-ranging moose (Alces alces) to determine the effect of season, time of day, rainfall, and cover type on GPS performance. On free-ranging moose the collar GPS unit found ≥4 satellites on 52% of location attempts, >50% of locations were 3-dimensional, and >24% of locations were 2-dimensional. Precise tracking of individual animals in all weather throughout the year is possible with GPS telemetry.

A SPATIALLY EXPLICIT MODEL OF MOOSE FORAGINGAND ENERGETICS

Herbivores contend with spatial and temporal variation in the quality, quantity, and availability of their food resource. The energy that female moose (Alces alces) store during the summer as body fat is used to meet energy needs in winter. The foraging strategy used by an animal affects its daily and annual energy balance. Consequently, foraging strategies and the distribution of food in the landscape can affect individual fitness and population growth through their effects on the energy balance of reproductive females. Models that unify landscape structure, foraging theory, and animal energy metabolism can be used to investigate the effects of foraging strategies on survival and reproduction. We developed an energy and activity simulation environment (EASE) model that predicts the seasonal changes in energy requirements of a female moose foraging in a spatially explicit landscape with a resolution of 1 m2. We validated EASE for both moose and deer (Odocoileus spp.) with respect to body mass changes repo...

SPATIAL PATTERNS IN THE MOOSE–FOREST–SOIL ECOSYSTEM ON ISLE ROYALE, MICHIGAN, USA

The effects of herbivores on landscape patterns and ecosystem processes have generally been inferred only from small-plot or exclosure experiments. However, it is important to directly determine the interactions between herbivores and landscape pat- terns, because herbivores range over large portions of the landscape to meet requirements for food and shelter. In two valleys on Isle Royale, Michigan, USA, soil nitrogen availability and its temporal variance decreased rapidly as consumption of browse by moose (Alces alces) increased up to 2 g·m 22 ·yr 21 ; with greater amounts of consumption, nitrogen avail- ability was uniformly low and constant from year to year. We tested three geostatistical models of the spatial distribution of available browse, annual browse consumption, conifer basal area, and soil nitrogen availability across the landscape: (1) no spatial autocorrelation (random spatial distribution); (2) short-range spatial autocorrelation within a patch, but random distribution of patches at larger scales (spherical model); and (3) both short-range autocorrelation within a patch and regular arrangement of patches at larger scales (harmonic oscillator model). Conifer basal area and soil nitrogen availability fit the harmonic oscillator model in both valleys. Annual consumption and available browse showed oscillations in one of the valleys and only short-range autocorrelation in the other. In both valleys, however, the spatial pattern of annual consumption followed that of available browse. The predom- inance of spatially oscillatory patterns suggests that the interactions of moose with the forest ecosystem cause the development of both local patches of vegetation and associated nitrogen cycling rates, as well as the development of higher order patterns across the larger landscape. We suggest a coupled diffusion model of herbivore foraging and plant seed dispersal that may account for these patterns.

Spatial heterogeneities, carrying capacity, and feedbacks in animal-landscape interactions

We review the interactions between foraging animals and the spatial distribution of ecosystem properties, such as productivity and nutrient cycling rates, across the landscape. Such interactions are non-linear, and require re-examination of the uncritical application of the concept of carrying capacity. Spatial heterogeneity of the animal-landscape system has three major components: heterogeneity of resource distributions in the physical environment, heterogeneity of plant tissue chemistry, such as concentrations of secondary compounds or nutrients and associated plant growth traits such as evergreeness, heterogeneity of movement modes by the animal. Furthermore, all three types of heterogeneity interact and can either reinforce or offset one another, thereby affecting system stability and dynamics. We conclude by discussing the applications of these concepts to management objectives.

Generation of Spatial Patterns in Boreal Forest Landscapes

ABSTRACT Boreal forests are composed of a few plant species with contrasting traits with respect to ecosystem functioning and spatial patterning. Early successional deciduous species, such as birch and aspen, disperse seeds widely, do not tolerate low light and nitrogen availabilities, have rapidly decaying litter, and are highly preferred by herbivores. These later succeed to conifers, such as spruce and fir, which disperse seeds locally, tolerate low light levels and low nitrogen availability, have litter that decays slowly, and are unpalatable to most mammalian herbivores. Although there are also early successional conifers, such as jack pine and Scots pine, the aspen-birch-spruce-fir successional sequence is the most common over much of North America, and (without fir) in Fennoscandia and Siberia. The course of succession in these forests is controlled partly by seed dispersal and selective foraging by mammalian herbivores. Both of these processes are spatially dynamic, but little is known about how their spatial dynamics may affect ecosystem processes, such as nitrogen cycling or productivity. We present spatially explicit models that demonstrate the following: (a) Spatially explicit seed dispersal results in more clumped distribution of tree species and persistence of greater paper birch biomass than uniform seed rain across the landscape. Such results are consistent with current spatially explicit population models of dispersal and coexistence. (b) With localized seed dispersal, the concentrations of available soil nitrogen are distributed in larger patches with sharp transitions from low to high nitrogen availability near patch edges. In contrast, with a uniform seed rain, the distribution of soil nitrogen availability was more uniform and “hotspots” were more localized. Thus, the spatial pattern of an ecosystem process (nitrogen cycling) is determined by seed dispersal and competition for light among competing populations. (c) A dispersing herbivore, such as moose, that selectively forages on early successional deciduous species with high quality litter, such as aspen or birch, and discriminates against late successional conifers, such as spruce or fir, imposes higher-order repeated patterns of plant species and biomass distribution on the landscape. Thus, seed dispersal and herbivore foraging correlate properties in adjacent patches but in different ways, and different spatial patterns emerge. Other processes, such as insect outbreaks, fire, and water flow, also may correlate properties between adjacent patches and result in additional patterns.

Effects of Walleye Stocking on Population Abundance and Fish Size

Abstract Data from Minnesota Department of Natural Resources lake surveys and stocking records were used to examine the effects of stocking programs involving walleye Stizostedion vitreum on the abundance and size of fish in walleye populations and to determine to what extent and under what conditions these effects occurred. We used data from 4,470 surveys in 1,924 lakes and from 20,634 walleye stocking records in 1,716 lakes. The analyses were conducted for various sizes of stocked fish and stocking frequencies and for lake groups with different walleye reproductive status. We reached the following conclusions. (1) Stocking in lakes without natural walleye reproduction increased walleye population abundance, but stocking in lakes where reproduction occurred had no effect on population abundance, (2) In lakes without walleye reproduction, walleye mean weight was reduced by stocking. In lakes with walleye reproduction, fry stocking decreased the mean weight of fish, but fingerling stocking had no effect on...

Linking moose population and plant growth models with a moose energetics model

ABSTRACT Selective foraging by large mammals can change ecosystem properties such as plant species composition, nutrient cycling rates, and soil fertility. These changes, in turn, alter the availability of forage and could affect the relative efficiencies of foraging strategies used by these animals. We used a simulation model to predict how alternate foraging strategies affected the net annual energy balance of moose (Alces alces), moose density, and distribution of browse across the landscape. The model simulates the spatial distribution of vegetation in an 8-ha landscape of 1-m2 cells with seasonal changes in the energetic needs of free-ranging moose and plant phenology. The energetics model was integrated with a moose population model and a plant-growth model for long-term simulations. Changes in bite density in each feeding station are predicted with height and biomass logistic curves modified by a quadratic response to browsing. We tested foraging strategies using random, fractional, and marginal value theorem (MVT) algorithms on landscapes with a range of bite densities and differing spatial distributions. Small-scale disturbances (that is, tree-fall gaps) were required to maintain browse supply and prevent moose population extinction under all foraging strategies. Populations using a fractional stopping rule survived the 100-year simulations because moose browsed across much of the landscape and did not overbrowse patches with high bite density. Populations using random and MVT stopping rules became extinct in about 25 and about 50 years, respectively. Moose using a random stopping rule were in negative energy balance because travel time was high and the net energy intake rate was low on an annual basis. Moose using the MVT stopping rule were initially in positive energy balance, but as the high-density browse patches were overbrowsed and low-density unbrowsed patches grew out of reach, bite density decreased, and energy balance became negative in subsequent years. Thus, the foraging strategy used by individual moose resulted in creation of landscapes that strongly affected browse density, browse distribution, moose population density, and moose survival.