The role of diversity in the maintenance of ecosystems has been studied widely in the past decade.By correlating richness and diversity with basic ecosystem processes, these investigations lend support to the hypothesis that species diversity significantly influences ecosystem functioning and, in turn, provide support for the conservation of biodiversity. Nonetheless, the majority of these investigations demonstrate that conservation of a relatively small number of generally dominant species is sufficient to maintain most processes. Indeed, there is remarkably little evidence to support the contention that less common species, those likely of highest conservation concern, are important in the maintenance of ecosystem functioning. Here we summarize studies, most employing alternative methodological strategies, wherein less common and rare species are demonstrated to make significant contributions to ecosystem functioning. Evidence exists among studies of keystone species, aggregate effects of less common species, and species turnover. Our findings suggest that (1) less common species can make significant ecosystem contributions; (2) further investigation into the effects of rare and less common species on ecosystem maintenance is sorely needed; (3) further investigation should embrace a variety of approaches; and (4) until further research is conducted a prudent conservation approach is warranted wherein the contribution of less common species to ecosystem functioning is assumed. Especies Raras y Funcionamiento del EcosistemaResumen: El papel de la diversidad en el mantenimiento de los ecosistemas ha sido ampliamente estudiado en laúltima década. Al correlacionar riqueza y diversidad con procesos ecosistémicos básicos, estas investigaciones dan sustento a la hipótesis de que la diversidad de especies influye significativamente en el funcionamiento del ecosistema y, a su vez, proporciona soporte para la conservación de biodiversidad. Sin embargo, la mayoría de estas investigaciones demuestran que la conservación de un número relativamente pequeño de especies generalmente dominantes es suficiente para mantener a la mayoría de los procesos. Ciertamente, es notable la escasa evidencia para sustentar el argumento de que las especies menos comunes, probablemente las de mayor preocupación para la conservación, son importantes para el mantenimiento del funcionamiento de los ecosistemas. Aquí resumimos estudios, la mayoría de los cuales emplearon estrategias metodológicas alternativas, que demuestran que las especies menos comunes y raras contribuyen significativamente al funcionamiento del ecosistema. Existe evidencia en estudios de especies clave, de los efectos de especies menos comunes y de recambio de especies. Nuestros hallazgos sugieren que (1) las especies menos comunes pueden hacer contribuciones significativas al ecosistema; (2) se requiere más investigación del efecto de especies menos comunes y raras; (3) la investigación futura debería incluir una variedad de estrategias y † †Current address:

Summary1 Ecotones, the transition zones between adjacent ecological systems, may intensify or concentrate the flow and processing of materials and organisms between systems. 2 To determine whether the salt marsh/upland transition zone is an ecotone, both in functioning as a habitat, as well as concentrating materials and organisms, I conducted an extensive survey of 12 marsh /upland transition zones in the Pt Reyes area of California. 3 The high saltmarsh was identified as an ecotone, with biological and physical conditions distinct from the adjacent marsh plain and upland. It supports a unique plant assemblage and greater total soil nitrogen than the adjacent upland. 4 Taxonomic richness patterns did not reflect changes in habitat spatial characteristics (e.g. ecotone area, ecotone area/perimeter), instead showing a quadratic correlation along the gradient, with increased diversity of both plants and spiders in the ecotone. Spider richness was enhanced with increased vegetation complexity, which was greatest in the ecotone. 5 This study presents empirical evidence that suggests coastal marsh/upland ecotones harbour increased diversity and may concentrate nitrogen pools, thereby highlighting the need for further research investigating the relationship between landscape connections and the coupling of species and biogeochemical processes.

Abstract. Salt marshes currently play an important role as filters for upslope nitrogen (N) inputs. This could change in the future with sea level rise, warming and eutrophication, which are expected to favor monocultures over diverse plant communities. We explored patterns in gross N cycling, dissimilatory nitrate (NO 3 À ) reduction to ammonium (NH 4 þ ) (DNRA), and denitrification in a salt marsh soil under two typical redox conditions (aerobic and anaerobic), and in soils under plant communities manipulated to simulate potential future composition (forb and graminoid monocultures). Natural salt marsh soils exhibited high potential gross N mineralization rates, averaging 50.4 6 5.7 lg N g À1 d À1 under aerobic conditions; rates declined to 23.6 6 3.4 lg N g À1 d À1 under an N 2 headspace. Microbial NH 4 þ uptake and gross nitrification together accounted for only 14 % of gross N mineralization. Nitrogen retention via DNRA and microbial uptake greatly exceeded N losses via denitrification. Gross nitrification rates were greater in the forb and graminoid monocultures than in the control. This effect may be mediated by the lower plant biomass in the monocultures than in the control, which may have reduced competition between plants and nitrifiers for NH 4 þ . Soil NO 3 À concentrations and net nitrous oxide (N 2 O) fluxes were greatest for the forb monoculture, likely due to higher soil oxygen (O 2 ) concentrations in these plots. Our results suggest that salt marsh soils with a diverse plant community have high potential rates of N mineralization and microbial N retention, and the establishment of forb monocultures could lead to greater ecosystem N losses.

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