Ecosystem service (ES) trade-offs arise from management choices made by humans, which can change the type, magnitude, and relative mix of services provided by ecosystems. Trade-offs occur when the provision of one ES is reduced as a consequence of increased use of another ES. In some cases, a trade-off may be an explicit choice; but in others, trade-offs arise without premeditation or even awareness that they are taking place. Trade-offs in ES can be classified along three axes: spatial scale, temporal scale, and reversibility. Spatial scale refers to whether the effects of the trade-off are felt locally or at a distant location. Temporal scale refers to whether the effects take place relatively rapidly or slowly. Reversibility expresses the likelihood that the perturbed ES may return to its original state if the perturbation ceases. Across all four Millennium Ecosystem Assessment scenarios and selected case study examples, trade-off decisions show a preference for provisioning, regulating, or cultural services (in that order). Supporting services are more likely to be "taken for granted." Cultural ES are almost entirely unquantified in scenario modeling; therefore, the calculated model results do not fully capture losses of these services that occur in the scenarios. The quantitative scenario models primarily capture the services that are perceived by society as more important-provisioning and regulating ecosystem services-and thus do not fully capture tradeoffs of cultural and supporting services. Successful management policies will be those that incorporate lessons learned from prior decisions into future management actions. Managers should complement their actions with monitoring programs that, in addition to monitoring the short-term provisions of services, also monitor the long-term evolution of slowly changing variables. Policies can then be developed to take into account ES trade-offs at multiple spatial and temporal scales. Successful strategies will recognize the inherent complexities of ecosystem management and will work to develop policies that minimize the effects of ES trade-offs.Ecology and Society 11(1): 28 http://www.ecologyandsociety.org/vol11/iss1/art28/ Ecology and Society 11(1): 28 two anonymous reviewers, and the many others who commented on previous versions of the "trade-offs working group" documents. Figures 2 and 4 were kindly prepared by Kathryn M. Rodríguez-Clark.

Seedlings of Eucalyptus tereticornis (Smith) were grown under two levels of availability each of CO (352 and 793 µmol mol), soil nutrients (1/24 and 1/4 Hoagland's solution) and light (full and 30% sunlight). Low soil nutrient availability or high light increased the C:N ratio of leaves, leading to lower leaf nitrogen concentrations, higher leaf specific weights and higher levels of both total phenolics and condensed tannins. These results were consistent with other studies of the effect of environmental resource availability on foliage composition. Similar results were observed when the C:N ratio of leaves was increased under elevated CO. The changes in leaf chemistry induced by the treatments affected the performance of 4th-instar larvae of Chrysophtharta flaveola (Chapuis) fed on the leaves. Increased C:N ratios of leaves reduced digestive efficiencies and pupal body sizes and increased mortality. Below a threshold nitrogen concentration of approximately 1% dry mass, severe reductions in the performance of larvae were recorded. Such changes may have significant consequences for herbivores of Eucalyptus, particularly in view of projected increases in atmospheric CO.

Regulation of acid-base homeostasis is essential for mammals and birds. Biotransformation and metabolism of absorbed plant secondary metabolites (PSMs) results in the production of organic acids that threaten acid-base homeostasis. Consequently these acids must be buffered and excreted from the body. The production of an acid load from detoxified PSMs should occur in herbivorous mammals and birds and with most PSMs and so may provide a unifying theme to explain many effects of PSMs on animal metabolism. Since the organic acids will be largely ionized at physiological pH, disposal of the hydrogen ion and the organic anion may proceed independently. Most hydrogen ions (H(+)) from organic acids are eliminated by one or more of three ways: (1) when they react with bicarbonate in the extracellular fluid to form carbon dioxide and the carbon dioxide is exhaled, (2) when they bind to dibasic phosphate and are excreted by the kidney as monobasic phosphate, and (3) when they are buffered and retained in the skeletal system. The secretion of phosphate ions and ammonium excretion are two ways in which the kidney replaces bicarbonate ions that have been eliminated as carbon dioxide. Secretion in the kidney tubule is an important means of excreting excessive organic anions rapidly. This process is saturable and may be subject to competition from a variety of different metabolites. Lagomorphs have limited capacity to form new bicarbonate from ammonium excretion and may therefore be obliged to excrete other cations such as sodium to balance the excretion of organic anions from PSMs. Acidemia has wide-ranging impacts on animals but browsing mammals and birds may have to break down muscle tissues to provide for urinary ammonium in order to generate bicarbonate for buffering. Acidemia also can affect the extent of urea recycling. Animals consuming browse diets may have to regulate feeding so that the rate of formation of hydrogen ions does not exceed the rate of disposal. The mechanisms by which this could occur are unknown.

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Hypogeous fungi often contain high concentrations of nitrogenous compounds, vitamins, and minerals and are major dietary items for many rodents and other small mammals in North American forests. For these reasons ecologists have presumed hypogeous fungi to be of high nutritional value for small mammals. In a series of feeding experiments we investigated the nutritional quality of sporocarps (fruiting bodies) of the hypogeous fungus Elaphomeyces granulatus in the diet of the golden—mantled ground squirrel Spermophilus saturates, which is highly mycophagogus throughout its season of activity. We compared the digestive utilization of the fungus by this small mammal with digestibility of a range of plant foods available in nature. The sporocarps of E. granulatus had a high concentration of nitrogen, but 80% of this nitrogen was in the totally indigestible spores and the relatively indigestible cell walls of the peridium, and only half of the remaining 20% was protein nitrogen. Even though the squirrels ate the peridium in preference to the spore—laden core, the digestibility of the ingested nitrogen was only °50%, which is much lower than the digestibility of most plants. The digestible energy content of the fungus was low compared with that of conifer seeds and similar to that of grasses and legumes that are naturally eaten by the ground squirrels in small amounts. The overall (dry—matter) digestibility of E. granulatus sporocarps (60%) appears to be near the minimum digestibililty on which golden—mantled ground squirrels are able to maintain themselves. Therefore, sporocarps of E. granulatus cannot be regarded as a high—quality dietary items in terms of the availability of nutrients. We suggest that this is true for hypogeous fungi in general and that their value as a food resource derives mainly from their great abundance during the active season of small mammals such as ground squirrels and from their emission of strong odors, which makes mature sporocarps highly detectable. These features should maximize the yield of energy and nutrients to small mammalian mycophagists in relation to foraging effort.