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differently to the same characteristic, and some effects may be of a compensating nature, such as the influence of altered snow depth on the distribution of the pocket gopher. Some of the ecosystem complexity removed by the compilation of Figure 1 is taken into account in Table 1, which shows the nature of multiple responses, temporal dimensions, and magnitudes of ancitipated effects, although none of these has great precision. A distinction has been made between short-term and long-term effects, with the division at 5 years (i.e. the duration of the San ,Juan Pilot Project). Estimating the magnitude of impacts is particularly subjective but an attempt has been made to maintain comparabilit;y by using "detectability" as one criterion...!.! Thus, "No Effect" means "no detectable effect;" "Low" means that any effect would be barely detectable; "Medium" means that effects should be observable; and "High" means that changes should be clearly observed. Two other criteria used in Table 1 are based on the guidelines used by the Bureau of Reclamation impact evaluation reproduced in Appendix A. These guidelines define: (1) the strength of the link between the winter snowpack and the attributes we have studied; and (2) the potential value of the defined impact. For the first of these, our evaluation is based OR the snow-ecosystem link rather than the precipitation management-ecosystem link. This change was necessary because our studies have been of processes, not actual impacts. It is notable that few gross changes are predicted in Table 1 but that the magnitude of most effects is ex- pected to increase with the druation of the seeding program. This suggests maintenance of the general stability of the San Juan ecosystems in the face of minor environmental changes that are neither catastrophic nor maintained for decades. Table 1 also includes brief notes which explain the mechanism of any predicted impact and references to the specific subproject report in Chapter IV where further details can be found. A further simplication of ecosystem effects is made in Figure 2, which shows the linked effects of snowpack augmentation that have been demonstrated by work in the San Juan Ecology Project. Although the chains of influence shown in Figure 2 have not been corroborated in our work, in almost every case the individual links making them up have been derived from the results reported in Chapter IV. One such chain of processes and responses may be traced from the influence of an increased winter snowpack on the vegetation cover, either directly or indirectly through a change in the distribution or size of small mammal populations, to increased rates of soil erosion. Increased erosion affects the surficial layers of soil, and silver used in cloud seeding tends to concentrate in those layers. Thus, a mechanism may be provided whereby accumu- lated silver is removed from relatively large areas and concentrated in sedimentation areas. It is doubtful if this concentrating mechanism would be sufficient to induce a reduction in the decomposi- tion rates of plant waste because silver would be moving with soil material and the link to de- composition rates has only been demonstrated at high silver concentrations (Klein, this vol. p.12S). A further possibility that is not demonstrated in this study is that reduced rates of decomposition would be reflected in lower rates of nutrient cycling. This could produce either a decrease in primary productivity, or an increase in productivity due to greater moisture retention in surface soilF. ..!/ "Detectability" as measured by procedures used in the subprojects in this report. CLOUD SEEDING II II _ ADDITIONA L _ ....SNOWPACK ~POS~2?~, -.1 INCREASED . VEGETATION SOIL EROSION # ~ ~. ANIMALS ...; SI LVER ... ACCUMULATION - i ... ~ TERRESTRIAL ECOSYSTEM ~ P EXCHANGES WITH OTHER ECOSYSTEMS Figure 2. Chained Responses in the Terrestrial Ecosystem. All response links except those between Decomposition and Vegetation and'between the terrestrial and other ecosystems are described in Table 1 and in the relevant parts of Chapter IV. Modifications to this cycle of responses are also shown in Figure 2. These include the obvious feed- back loops: (1) between the vegetation of an area and the animal consumers who are dependent upon it; (2) the influence of animal populations directly on soil erosion rates as by trampling; and (3) a direct link between an augmented snowpack and plant de- composition. The latter seems to be a response to additional water fluxes through the system and should be reflected in increased rates of leaching and decomposition. As such, this process should tend to counteract any retardation in decomposition due to additional silver, although this may not be so in the sedimentation areas where the possibility of silver concentration has been suggested. Other responses are likely to involve ecological systems not studied in the San Juan Ecology Project. In, view of the nature of water, sediment, and silver movement which has been emphasized here, the transfer of all of these substances into the aquatic ecosystems of a mountain area is likely. Most of the sedi- mentation basins into which silver may be transported with surficial soil are likely to be lakes. The potential impact of silver in lake systems has not been estimated for the San Juan Mountains although a recent study (Freeman 1975) suggests that effects in mountain lakes would be slight. CONCLUSIONS The results of the.San Juan Ecology Project suggest that there should be no immediate, large-scale impacts on the terrestrial ecosystems of these mountains following an addition of up to 30 percent of the normal snowpack, but with no addition to maximum snowpacks. Further, much of the work re- ported here suggests that compensating mechanisms within the studied ecosystems are such that any impacts would be buffered, at least for short periods 12