ANIMAL DENSITY : A NEW QUANTITATIVE APPROACH Gary

We measured the summer diet of moose in two study areas at Isle Royale, Michigan, by counting recently browsed stems, inventorying the numbers of plants fed upon, and estimating intake rate from direct observation of feeding animals. Applying these techniques in two study-area forest types, we found that among species important as summer browse, Sorbus americana and Acer spicatum were preferred over BetMla alleghaniensis and ~. papyrifera; this was based on relative frequency in the diet compared to relative availability. An average adult moose consumed about 4.2 kg/day of browse, dry-wt; total consumption was 4.8 kg/day because moose were also eating aquatic vegetation. Based on intake per animal and removal of browse per unit area, summer density averaged 3.8 adult equivalents/km2 , a relatively high level. Current browsing pressure upon the plants appears responsible for a trend of decline in the abundance of preferred species. As part of a program to measure the flux of energy and minerals in the soil-vegetation-moose-wolf system at Isle Royale National Park, Michigan, we have been analyzing the quantity and species composition of food taken by moose. The island, maintained primarily as a wilderness park and lying 102 in the northern Lake Superior, is one of the few sites in the world today where one can observe interactions, undisturbed by man, among original, major components of a northern-forest community. We began this study by focusing on summer, the season of greatest nutritional demands in moose and the period of growth and reproduction in plants; soon we will be measuring foraging during other seasons. This is a partial report on work carried out during summer 1972; a more complete one is in preparation and will include results of aquatic-feeding studies (Belovsky et al.). Isle Royale, a 550-km 2 archipelago (referred to here as "the island"), is forested with boreal and northern-hardwood elements. Interspersed over the island are many poorly drained areas, beaver ponds, and inland lakes. Moose were not known at Isle Royale until the early 1900's when they appeared and, in the absence of wolves, displayed an irruptive population growth culminating in die-offs during the 1930's. Wolves arrived in the late 1940's, after which the moose population appeared to be more stable than before. In recent years, however, the population has increased without a concomitant increase in wolves (see discussion in Jordan et al. 1973, this volume). These points and other natural history aspects are covered in reports on ecological studies at Isle Royale dating back nearly 70 years: Adams 1909, Cooper 1913, Murie 1934, Aldous 103 and Krefting 1945, Krefting 1951, Mech 1966, Shelton 1966, Jordan, Shelton and Allen 1967, Jordan, Botkin and Wolfe 1971, and Allen et al. 1959-1973. All studies reported here were conducted during 1972 in two areas at the west end of the island. The study areas were delineated by forest type during 1971 when production and mineral content of available moose browse was measured (Jordan et al. in prep.). In 1972 the soils of these areas were surveyed (Dominiski in prep.). Production, mineral composition, and utilization by moose of aquatic vegetation in ponds adjacent to or, contained within these study areas is also under investigation. The study areas are known as "Yellow Birch" and "Coastal". The former is dominated by a partial canopy of mature Betula alleghaniensis with ~. papyrifera, Abies balsamea, Picea glauca, and Thuja occidentalis also in the canopy. Understory includes reproduction of all the canopy species plus Sorbus americana, Corylus cornuta, Lonicera canadensis, Diervilla lonicera, Acer spicatum, and ~. saccharum. The site lies several kilometers from the shore of Lake Superior. The Coastal area lies close to the lake shore and tends to have poorer growing conditions than the Yellow Birch. with its cool, often foggy weather, the Coastal supports a more boreal forest dominated by Abies and Pice~; intensity of lake storms 104 causes extensive wind-throw openings in the canopy. Betula papyrifera is the dominant deciduous tree in the coastal study area, occurring commonly in the understory along with Sorbus, Acer spicatum, and Prunus pennsylvanica. In both areas, moose browsing greatly suppresses tree reproduction, leading to canopy openings which in turn promote a productivity of shrubs not commonly observed in similar-aged stands of the region. This work was supported by National Science Foundation (u.S.) grant GB-29566, and field work was dependent upon the generous cooperation of personnel of Isle Royale National Park, particularly Superintendent Hugh P. Beattie and District Ranger Frank M. Deckert. Important contributions were made by our field assistants Daniel MacEachron, Marshall Tate, and Lawrence Stowe. METHODS Browse-utilization methods described elsewhere were not appropriate for our investigation. McMillan (1953) relied on direct observations of moose for all his data. We attempted this, but the lack of open vegetation at Isle Royale and the need for replication among a number of individual animals showed this to be inadequate. Krefting (vide Hosley 1949) used visual estimates of browsed vegetation, but his method is not sufficiently quantitative to permit statistical analysis. 105 In 1971 our group initiated measurements of production and utilization by protecting sets of plants within randomly placed small exclosures (ca 4-m diam.); plants were selected outside to match each one inside, and utilization was measured by the difference in current growth, clipped and weighed, at the end of summer as well as at the end of winter. This technique has proven unsatisfactory, both economically and statistically. In summer 1972, we developed an integrated technique involving transects, plots, and direct observation upon feeding animals. Current browse removals were counted along transects; plant densities and biomass removed were tabulated in plots; and feeding rates by plant species were determined from close observations of moose. These measures were then combined to estimate composition of diets, daily food intake, total herbivory, impact upon vegetation, and the density of moose. Several transects of up to 1500 m were established in both study areas and were run every 2 to 3 weeks. Transects comprised straight lines along a given bearing; essentially the same ground was covered on each run because the observer was guided by flagging put out during his first run. The summer's total, combining 12 separate runs of individual transects, covered some 16.5 km of sampling. The observer walked the transect, counting within 2 m on either side of him all individual instances of leaf removal by moose during the past 10 days 106 or so. We restricted this counting to recent removals because of our interest in comparing dietary composition in different periods of summer. criteria of recent removal, a critical aspect of the technique, were developed through study of the cut surfaces of petioles or new leaders. A scab-like structure soon forms on newly cut surfaces, and, after 10 days or so, it becomes recognizably different in colour and thickness. From observation of control specimens, we conclude our criteria of freshness, under the existing environmental conditions, provide a reliable means for distinguishing recent removals. For inventorying browse plants and the total forage removed during summer, we established 52 known-area plots, spacing them regularly along the transects. Plants of all species used by moose and having current growth within reach of moose « 2.8 m high) were counted -those 0.5 m high within a l-m radius and taller ones within a 2-m radius of the plot center. Each evidence of a leaf being removed since the beginning of summer was counted; from this a total was extrapolated based on a summer browsing period of 124 days and the assumption that post-browsing regrowth is negligible. These data provide estimates of biomass removed per plant and per unit area as well as the density of browse plants. To determine individual intake by feeding moose, we observed animals through 7 x 35 binoculars at distances no 107 greater than 30 m. Two sets of data were collected, not necessarily simultaneously: bites per unit time and leaves per bite. To determine activity cycles in moose we made a series of 24-hr activity observations from fixed vantage points in the forest and at ponds. A pair of observers recorded all feeding, moving and resting of moose within their sight or hearing. Because moose are noisy when moving and feeding, one can monitor activity during hours of darkness. Density of moose is high in these forests, and the animals make frequent trips to ponds: several 24-hr records from a point allowing radial coverage of 100-150 m provided a large number of data on individual animals, and we believe the sample was adequate for us to reconstruct the daily activity cycle from it. From the direct observations we calculated the average number of leaves ingested during periods of feeding. Combined with our measurements of biomass/day and feeding hrs/day, we calculated an estimate of biomass-intake/day accounting for each browse species. We also measured aquatic feeding, using similar techniques, and thus were able to estimate total food consumption by the moose of the study area during summer. Total time required for the summer browse survey was 20 man-days of data collecting in the field plus several days for establishing study sites and a number more for analyzing data. lOB RESULTS AND DISCUSSION Moose Diets in Relation to Available Browse Composition of browse species available differed between the Coastal and Yellow Birch study areas (Table 1), with the relative abundance of 10 of the 16 species encountered being significantly different (chi-square p < 0.05). These vegetation differences no doubt reflect the sharp climatic differences between coastal and inland zones. The summer diets of moose are, we believe, well represented by the relative proportions of leaf removals accounted for on the "transects: Table 2 shows the averages of all transect runs in the two study-area forest types. Percentages are based on the proportion (dry-wt basis) of each species removed; this was determined by weighting the percent removals counted in each time period by the respective species' mean leaf-weight for that period. Table 1. Percentage of Individuals of Browse Species in TWo Study Areas. Browse plants are of species eaten by moose in summer and having current growth below 2.B m.