Douglas C. Andersen

Population Dynamics and Bioenergetics of a Fossorial Herbivore, Thomomys talpoides (Rodentia: Geomyidae), in a Spruce‐Fir Sere

Studies of the bioenergetics of the northern pocket gopher, Thomomys talpoides, are coupled with data on demography, activity budgets, and microclimates to model the energy requirements of individuals and populations in the Wasatch Mountains of Utah during 1976—1979. Metabolic rates during rest increased linearly with decreasing ambient temperature, but burrowing metabolic rates (16.3 mL O₂°h— ¹°g— ⁰ . ⁷ ⁵) were independent of both temperature and physical properties of the soil. Radio—telemetry studies indicated that free—ranging gophers are active °50% of each day. Conservative estimates of true energy consumption were calculated using estimates of habitat—specific minimum daily burrowing requirements. Rates of burrowing measures in the laboratory were either °0.0 or °2.0 cm/min. The low burrowing rate was observed when the soil was frozen or saturated with water, as would occur in the field in early winter and in spring, respectively. Gophers burrowed through soil at the study site at an average rate of °1.5 cm/min. Belowground food energy densities at gopher foraging depth declined from 24.6 to 3.2 J/cm³ along a successional gradient (subalpine forb meadow to Engelmann spruce dominated forest). We conclude that individual gophers are food limited within the climax spruce seral stage. Further, daily energy costs associated with reproduction in females may exceed the belowground energy supply available in intermediate seral stage (aspen and subalpine fir). Reduction of burrowing rates for any reason will affect gophers in the late seral stages proportionately more than those resident in the meadow. The peak gopher densities recorded (from 62 individuals/ha in the meadow to 2 individuals/ha in spruce forest) support these inferences. Detailed demographic information was obtained only in the meadow seral stage. Adult survivorship was lower in winter than in summer and varied greatly between years (0.18—0.70 yr— ¹). Juvenile survivorship from weaning through the first winter was comparable to adult annual rates. The fertility rate was 3.75 young°female— ¹°yr— ¹. The energy supply and demand analyses indicate that the growth of Thomomys talpoides populations in the early seral stages is seldom directly limited by the amount of food present. From our demographic, environmental, and autecological studies we conclude that stochastic events associated with weather affect energy acquisition (burrowing) rates, and thus survivorship. In montane environments, such events may prevent populations from attaining sizes at which territorial behavior would hypothetically limit further increases. The energy flow through the meadow population at moderate to high (1976—1977) densities (at least 1100 MJ°ha— ¹°yr— ¹) indicates that pocket gophers are proficient energy movers relative to nonfossorial small mammals. Subalpine T. talpoides populations appear commonly to attain such densities. More than 30% of the annual primary productivity allocated to belowground parts of meadow forbs may be consumed by gophers.

An organism-centered approach to some community and ecosystem concepts.

Abstract We present a discussion of the ecological concept of the niche based on the perspective of the individual organism, rather than that of a population or species. This discussion is then expanded to include other related ecological concepts such as guild, environment, habitat and functional group. Using the individual as the focus permits the development of a system of concepts which, we believe, approximate the way that ecological interactions occur in nature.

Plant Succession Following the Mount St. Helens Volcanic Eruption: Facilitation by a Burrowing Rodent, Thomomys talpoides

The 18 May 1980 eruption of Mount St. Helens in the Cascade Range (USA) initiated plant succession over a broad region. Northern pocket gophers Thomomys talpoides survived in widespread subalpine and montane sites where up to 25 cm of tephra had buried dormant or only recently snow-free vegetation. Burrowing and consequent construction of mounds by this herbivorous rodent has both modified the physical structure of the tephra and led to the placement of pre-eruption soil material on the tephra surface. We compared the composition of the plant community and the pattern of seedling establishment on and off gopher mounds. More plant species and individuals survived burial on mounds than at off-mound sites. Seedlings are also most diverse on mounds. We conclude that pocket gophers, through their soil-disturbing activities, can strongly affect plant population dynamics in volcanically disturbed areas, and thus may be an important agent in determining the pathway of succession. Limited observations suggest that differential herbivory may further affect plant community composition. Possible mechanisms that enhance plant survivorship and seedling establishment on mounds are discussed.

Vulnerability of Acorn Weevils (Coleoptera: Curculionidae) and Attractiveness of Weevils and Infested Quercus alba Acorns to Peromyscus leucopus and Blarina brevicauda

Both Peromyscus leucopus white-footed mice and Blarina brevicauda shorttailed shrews prey upon the curculionid weevils that attack Quercus alba acorns. Peromyscus leucopus also feeds on Q. alba acorns, but B. brevicauda does not. We conducted feeding experiments using weevil larvae or pupae (1) to determine the relative preference of P leucopus for infested and noninfested acorns; (2) to examine consumption rates of P leucopus and B. brevicauda when larvae are freely available, and (3) to determine whether weevils that have exited acorns and burrowed into the soil are differentially vulnerable to these two predators. No preference by P leucopus was detected in either field or laboratory studies where entire acorns were offered, but P leucopus showed a significant preference for sound over infested when only nutmeat was presented. No significant changes in the rate of consumption of larvae were detected over a 3-day trial period, nor did consumption rate change as the number of exposed larvae was increased from 30 to 50. Curculio pardalis (89 %) and Conotrachelus naso (6%) were the most prevalent weevil species encountered. Overwintering weevils burrowed up to 21 cm into the soil. Blarina brevicauda preyed upon pupae at depths to 15 cm, but P leucopus foraged efficiently to only 3 cm. The null hypothesis that the ratio of sound to infested acorns has no affect on acorn mortality due to granivory by P leucopus could not be rejected on the basis of field experiments using exclosures. Although neither species clearly regulates weevil abundance, B. brevicauda seems a likely determinant of adult weevil abundance in years following low acorn production.

Herbivorous mammals along a montane sere: Community structure and energetics

All common herbivorous mammals were censused along a successional gradient in northern Utah in order to assess some of the changes in ecosystem attributes predicted to occur by Odum (1969). Biomass (B) and energy flow [Production (P), Respiration (R), and P + R (e)] through each of nine species in each of four serai stages (montane meadow, Populus -dominated forest, Abies -dominated forest, and Picea -dominated forest) were estimated. Tests of eight predicted trends were supportive in five cases (P/R ratio, P/B ratio, B/e ratio, net community production, and species richness), inconsistent in two cases (total organic matter and species evenness/general diversity) and ambiguous in one (size of organism). Community stability, as measured by changes in average community biomass between years of relatively normal and extremely low precipitation, increased with ecosystem maturity. Individual species, however, fluctuated greatly in biomass between the 2 years. Our results are consistent with the hypothesis that mammals are determinants of successional patterns only insofar as they affect plant colonization. However, a continuum probably exists between completely passive species and those that strongly influence successional patterns in plant communities through their effects on plant recruitment and/or mortality. Changes in the plant community in turn affect the composition of the herbivorous mammal community.

EFFECTS OF RIVER FLOW REGIME ON COTTONWOOD LEAF LITTER DYNAMICS IN SEMI-ARID NORTHWESTERN COLORADO

Abstract We compared production and breakdown of Fremont cottonwood (Populus deltoides wislizenii) leaf litter at matched floodplain sites on the regulated Green River and unregulated Yampa River in semi-arid northwestern Colorado. Litter production under trees was similar at sites in 1999 (250 g/m2, oven-dry) but lower in 2000 (215 and 130 g/m2), a drought year that also featured an outbreak of defoliating beetles at the Yampa River site. Our production values were similar to the few others reported for riparian forests within semi-arid or arid areas. Leaf litter in portions of the floodplain not inundated during the spring flood lost organic matter at the same rate as leaves placed in upland sites in 1998 and 2000: 35 to 50% of organic matter during an approximately 160-day spring and summer period. Inundated litter lost 55 to 90% of its organic matter during the same period. Organic matter loss from inundated leaves increased with duration of inundation and with deposition of fine sediment. Pooled across locations, leafpack data suggested that nitrogen concentration (mg N/kg organic matter) increased until about 65% of the initial organic matter was lost. This increase likely reflected the buildup of microbial decomposer populations. The role of insects and other macroinvertebrates in litter breakdown apparently was minor at both sites. Large spatial and temporal variation in litter dynamics in aridland floodplain settings is ensured by microtopographic variation in the alluvial surface coupled with year-to-year variation associated with most natural flood regimes. Factors reducing flood flow frequency or magnitude will reduce overall breakdown rates on the floodplain towards those found in drier upland environments.

Effects of Cottonwood Leaf Beetle Chrysomela Scripta (Coleoptera: Chrysomelidae) on Survival and Growth of Fremont Cottonwood (Populus Fremontii) in Northwest Colorado

Abstract Greenhouse and field studies were used to assess the effects of a cottonwood leaf beetle (Chrysomela scripta) outbreak on growth and survivorship of Fremont cottonwood (Populus fremontii) along a segment of the Yampa River in northwestern Colorado. Exposure of greenhouse-reared seedlings to either 1 or 3 larvae reduced survivorship and the shoot and root masses of surviving seedlings. In the field sapling survivorship in unmanipulated, naturally established patches of saplings was inversely related to the intensity of beetle damage noted the previous year in both 1998–1999 and 1999–2000. Floodplain saplings treated with the insecticide esfenvalerate during two growing seasons added more height and radial growth than untreated saplings. Field observations in riparian stands along the Yampa River indicated that beetles preferentially oviposited and fed on cottonwood seedlings over saplings and mature trees during late summer. Taken together, the greenhouse and field studies suggest herbivory by C. scripta is a major determinant of Fremont cottonwood distribution and abundance at the Yampa River site. Repeated surveys detected few or no beetles in stands along the neighboring but regulated Green River. We hypothesize that flow regulation, by reducing P. fremontii seedling abundance, precludes C. scripta from reaching outbreak status along the Green River.

Subalpine forests: a world perspective with emphasis on western North America

This paper represents an attempt to outline the general characteristics of the biota and some common abiotic features of subalpine forests in the northern hemisphere. We emphasize North American, temperate, subalpine forests and compare these to subalpine forests occurring between 30 and 60°N latitude in Europe and Asia. The northern hemisphere limit is based on the biogeographic history of the Earth. Because conifer evolution occurred primarily after the first division of Pangea (Daubenmire, 1978), modem conifer genera are generally restricted to either the northern (e.g., Abies, Picea and Pinus) or southern hemisphere (e.g., Podocarpus and Dacrydium), albeit with wide distributions within their respective hemispheres. Consequently, the two hemispheres lack forests composed of closely related genera. The three northern genera, plus Tsuga and Larix, commonly occur as dominants in temperate North America, Europe and Asia (Figure 1 a) and form forests that are physiognomically similar across all three continents. In fact, the tree, shrub and herb strata of northern hemisphere conifer forests contain many common genera of plants and animals. Our general restriction to the latitudes between 30 and 60° is based on the latitudinal vs altitudinal temperature compensation and its effect on the distribution of biotas. Air temperature decreases by about 6°C for a 1000 m elevation increase or 800 km (500 mi) increase in latitude. The result is that species and ecosystem types that are montane at mid-latitudes can occur at sea level at high latitudes. This phenomenon is part of the reason for a common confusion regarding the term ’subalpine’ in the literature. To some workers, subalpine is an explicit topographic term signifying the zone of montane vegetation directly below the true treeless

Snowpack and variation in reproductive ecology of a montane ground-nesting passerine, Junco hyemalis

Effects of snow depth and rate of snowmelt on reproduction of a montane groundnesting passerine were examined in a 5-year study of Dark-eyed Juncos Junco hyemalis in northern Utah, USA. Distribution of clutch sizes differed significantly among years. Although most clutches contained four eggs, 3-egg clutches, due primarily to second nestings, were more common during a year of early snowmelt and 5-egg clutches were most common during two years of late snowmelt. Average clutch size was lowest in an early snowmelt year and average clutch size and date at which meadows became snow-free were significantly positively correlated. Average hatching date of 4-egg clutches was also significantly positively correlated with date at which meadows became snow-free demonstrating that most birds tracked the pattern of snowmelt. Early snowmelt may allow more pairs to attempt second nesting, but late-lying snow causes breeding to be delayed, allowing time for only one nesting attempt. During this delay, female juncos continue to feed and some may gain enough reserves to produce larger clutches, accounting for the increase in average clutch size in years of late snowmelt. Three female juncos examined in June 1982, a period of late snowmelt, had significantly more lipid reserves than did six males collected at the same time, suggesting that females are not physiologically stressed while awaiting snowmelt. By tracking snowmelt patterns, juncos synchronize production of young with peak summer insect abundance and potentially decrease risk of predation. Snow depth and rate of snowmelt are thus proximate environmental factors that may influence the reproductive ecology of ground-nesting passerines.

VARIABLE ROLE OF AQUATIC MACROINVERTEBRATES IN INITIAL BREAKDOWN OF SEASONAL LEAF LITTER INPUTS TO A COLD-DESERT RIVER

Abstract We used coarse-mesh and fine-mesh leafpacks to examine the importance of aquatic macroinvertebrates in the breakdown of floodplain tree leaf litter that seasonally entered a sand-bedded reach of the sixth-order Yampa River in semiarid Colorado. Leafpacks were positioned off the easily mobilized channel bed, mimicking litter trapped in debris piles. Organic matter (OM) loss was fastest for leaves collected from the floodplain and placed in the river in spring (k = 0.029/day) and slowest for leaves collected and placed in the river in winter (0.006/day). Macroinvertebrates were most abundant in winter and spring leaves, but seemed important to processing only in spring, when exclusion by fine mesh reduced OM loss by 25% and nitrogen loss by 65% in spring leaves. Macroinvertebrates seemed to have little role in processing of autumn, winter, or summer leaves over the 50-day to 104-day monitoring periods. Desiccation during bouts of low discharge and sediment deposition on leaves limited invertebrate processing in summer and autumn, whereas processing of winter leaves, which supported relatively large numbers of shredders, might have been restricted by ice formation and low water temperatures. These results were consistent with the concept that microbial processing dominates in higher-order rivers, but suggested that macroinvertebrate processing can be locally important in higher-order desert rivers in seasons or years with favorable discharge and water quality conditions.