James A. MacMahon

The Sustainable Biosphere Initiative: An Ecological Research Agenda: A Report from the Ecological Society of America

In this document, the Ecological Society of America proposes the Sustainable Biosphere Initiative (SBI), an initiative that focuses on the necessary role of ecological science in the wise management of Earths resources and the maintenance of Earths life support systems. This document is intended as a call to arms for all ecologists, but it will also serve as a means to communicate with individuals in other disciplines with whom ecologists must join forces to address a common predicament. This document focuses primarily on the acquisition of ecological knowledge. It identifies the ecological research programs of highest priority and recommends steps required to pursue research objectives. The document also lays the groundwork for improving the communication and application of ecological knowledge. The SBI proposes three research priorities: global change; biological diversity; and sustainable ecological systems.

COMPETITION AND SPACING PATTERNS IN DESERT SHRUBS

SUMMARY (1) Spacing patterns of shrubs were studied on a series of sites in the Mojave and Sonoran Deserts. Both aggregation and regularity in dispersion of individual shrubs were fairly common. Aggregation may result from vegetative reproduction or environmental heterogeneity, and regularity from competition among plants. (2) Small shrubs tend to be clumped, medium-sized ones tend to a random arrangement, and large shrubs tend to a regular pattern. This suggests the increasing importance of competition as the plants grow. (3) Further evidence of interference between plants was provided by the correlations of plant size with the distance to their neighbours. (4) Root systems were extensive enough to abut or overlap each other in the interplant spaces. (5) Most plants tended to have neighbours of the same species rather than other species. (6) None of these results depended on position along the considerable climatic gradients across the Mojave and Sonoran Deserts.

DISPERSAL AGENTS OF VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI IN A DISTURBED ARID ECOSYSTEM

An evaluation of wind, small mammals and grasshoppers as dispersal agents of vesicular-arbuscular mycorrhizal (VAM) fungi (Endogonaceae) was conducted in a shrub-steppe community disturbed by strip-mining in southwestern Wyoming. Wind-dispersed VAM spores were collected on the study site with spore traps arranged in transects and in snow drifts across the site. Small mammals and grasshoppers were captured using traps and nets, respectively, and examined for the presence of VAM spores. To determine the source of the spores blown onto the study site, deposition patterns of spore mimics released from two potential source areas were assessed. Wind accounted for the movement of large numbers of spores onto the site from distances up to 2 km. Small mammals also appeared to move spores but at a lower magnitude. The importance of these vectors to the establishment of the VAM symbiosis remains to be elucidated.

Alpine Seedling Establishment: The Influence of Disturbance Type

The effects of disturbance type on seedling environment and establishment of alpine species with different physiological and life history traits were examined during a 2-yr study on the Beartooth Plateau in southwestern Montana, USA. We compared soil temperatures, water potentials, and nutrients on mineral soils of a gravel borrow area with those on highly organic soils of a Geum turf area. Seedling emergence, growth, and survival of six seeded species (Geum rossii, Artemisia scopulorum, Potentilla diversifolia, Sibbaldia procumbens, Deschampsia cespitosa, and Festuca idahoensis) and emergence and survival of five unseeded species (Draba crassifolia, Draba incerta, Cerastium arvense, Arenaria rubella, and Androsace septentrionalis) were evaluated on both areas. The effects of N and P nutrient addition and surface organic mulch on the soil environment and seedling es- tablishment were evaluated on the borrow area, while differences between uncleared turf and turf cleared of vegetation were compared on the Geum turf area. Plots cleared of vegetation on the Geum turf area had higher levels of soil N (NO3-) and P than uncleared turf and both higher levels of N (NO3- + NH4+) and P and higher soil temperatures (surface, 5, and 15 cm depths) than fertilized or not-fertilized borrow area treatments. Fertilization increased N and P on borrow area soils, but after 2 yr N had decreased significantly. Soil water potentials (5 and 15 cm depths) did not differ between cleared plots on the Geum turf area or any of the borrow area treatments and were never low enough to cause plant stress. Vegetated Geum turf had significantly lower water po- tentials than cleared plots, especially late in the growing season. Mulch had no effect on soil water potential or nutrients on the borrow area and increased soil temperatures only on clear days during the first growing season. Wind removed or redistributed the mulch over time, thus decreasing potential effects. Seedling emergence was highly dependent on soil surface stabilization and reflected species life history traits. Growth of seedlings was slow, and varied among species and treatments: 0.005-0.04 and 0.02-0.20 g total mass after the first and second growing seasons, respectively. Significantly higher total seedling mass was observed on cleared Geum turf plots than on any of the borrow area treatments, and on fertilized than on not-fertilized plots on the borrow area. Seedling mortality of most species was much lower than previously found for alpine ecosystems, rarely exceeding 50% even after 2 yr. On the borrow area mulch increased survival, probably through microenvironmental amelioration. The nu- trient pulse from fertilization increased mortality of several species, presumably by creating plant nutrient demands in excess of availability during year 2. Both disturbance characteristics and species life history and physiological traits affected seedling establishment. Pretreatment soil properties of the two disturbance types had the greatest effects on soil temperatures and nutrients and, consequently, on seedling growth and survival. Soil surface characteristics had the largest effects on seedling emergence; surface stabilization was essential for holding both soil and seed in place. Single species responses varied in magnitude but were similar on both disturbance types. In general, there were larger differences among species in emergence and growth than in survival. Thus, successful seedling establishment on different alpine disturbance types may depend more

Harvester Ant Foraging and Shrub-Steppe Seeds: Interactions of Seed Resources and Seed Use

Granivore—seed interactions involve a feedback between granivore seed selectivity and seed availability. We examined this feedback to determine how seed preferences by the western harvester ant, Pogonomyrmex occidentalis, related to seed availability and, in turn, affected the soil seed pool. Preferences were estimated from natural diets as well as from experiments that controlled seed size, relative availability, and distance from ant nests. Seed availability to ants varied with season and over 2 yr. Colony activity and seed intake rates were correlated with seed availability. Seed preference by ants was correlated with the seasonal availability of preferred species, but not with unpreferred seeds. From the soil seed pool, ants preferentially harvested small, sound seeds. They removed 9—26% of the potentially viable seed pool each year, and as much as 100% of available preferred species. Seed densities were lower 2—7 m from nests, where foraging activity was concentrated, than 7—12 m from nests. In contr...

Ecosystem Management in the Context of Large, Infrequent Disturbances

ABSTRACT Large, infrequent disturbances (LIDs) can have significant impacts yet seldom are included in management plans. Although this neglect may stem from relative unfamiliarity with a kind of event that rarely occurs in the experience or jurisdiction of individual managers, it may also reflect the assumption that LIDs are so large and powerful as to be beyond the ability of managers to affect. However, some LIDs can be affected by management, and for many of those that cannot be affected, the resilience or recovery of the system disrupted by the disturbance can be influenced to meet management goals. Such results can be achieved through advanced planning that allows for LIDs, whether caused by natural events, human activities, or a combination of the two. Management plans for LIDs may adopt a variety of goals, depending on the nature of the system and the nature of the anticipated disturbance regime. Managers can choose to influence (a) the system prior to the disturbance, (b) the disturbance itself, (c) the system after the disturbance, or (d) the recovery process. Prior to the disturbance, the system can be managed in ways that alter its vulnerability or change how it will respond to a disturbance. The disturbance can be managed through no action, preventive measures, or manipulations that can affect the intensity or frequency of the disturbance. Recovery efforts can focus on either managing the state of the system immediately after the disturbance or managing the ongoing process of recovery. This review of the management implications of LIDs suggests that management actions should be tailored to particular disturbance characteristics and management goals. Management actions should foster survival of residuals and spatial heterogeneity that promote the desired recovery pattern and process. Most importantly, however, management plans need to recognize LIDs and include the potential for such disturbances to occur.

Ten Suggestions to Strengthen the Science of Ecology

Abstract There are few well-documented, general ecological principles that can be applied to pressing environmental issues. When they discuss them at all, ecologists often disagree about the relative importance of different aspects of the sciences original and still important issues. It may be that the sum of ecological science is not open to universal statements because of the wide range of organizational, spatial, and temporal phenomena, as well as the sheer number of possible interactions. We believe, however, that the search for general principles has been inadequate to establish the extent to which generalities are possible. We suggest that ecologists may need to reconsider how we view our science. This article lists 10 suggestions for ecology, recognizing the many impediments to finding generalizations in this field, imposed in part by the complexity of the subject and in part by limits to funding for the study of ecology.

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.

Carrion decomposition and nutrient cycling in a semiarid shrub–steppe ecosystem

Vertebrate carrion decomposition and nutrient cycling have both direct and indirect effects on the soil properties, fauna, and flora associated with an animals carcass. While few comprehensive quantitative studies have been undertaken, those that have show considerable variability in decomposition processes and rates, their regulating variables, and the resultant ecosystem effects. In this two-part study, decomposition rates of vertebrate species were measured in a semiarid, shrub–steppe environment (Wyoming, USA). First, decomposition loss rates of mass, energy, and nutrients were measured for rat carcasses (Rattus norvegicus) in four seasons and two microsites (surface and underground burrows). Decomposition rates varied significantly between microsites (burrow > surface in spring and summer) and among seasons (spring > summer > autumn ≈ winter), with mass loss amounts linearly correlated with ambient air temperatures. Energy and nutrient losses were related to phased carcass organ/tissue losses, with ...

Individual foraging components of harvester ants: movement patterns and seed patch fidelity

SummaryWe investigated individual foraging components of the western harvester ant,Pogonomyrmex occidentalis, in the native seed background of a shrub-steppe environment. Our study identified factors affecting foraging movements and seed selection by individual ants. Some assumptions and predictions of central-place foraging theory and a correlated random walk were evaluated for individual foragers. Results showed that ant size was only weakly correlated with the seed sizes harvested; seed size was a more important constraint than a predictor of seed selection. Individual ants spent more time in localized search behavior than traveling between search areas and nests.P. occidentalis foragers encountered seeds randomly with respect to time, and handled a mean of 1.7 seeds/trip. A correlation of increased search effort with greater travel distances was consistent with central-place foraging theory but, contrary to it, search and travel effort were not associated with energetic reward.Individual ants exhibited fidelity in both search site and native seed species. Spatial analyses of foraging movements showed a highly oriented travel path while running, and an area-restricted path while searching. Searching ants moved in a manner consistent with a correlated random walk. The deterministic component of patch fidelity and the stochastic component of search may override energetic foraging decisions in individualP. occidentalis ants.