Climate-related change in an intertidal community over short and long time scales. Sagarin, R. D.; Barry, J. P.; Gilman, S. E.; Baxter, C. H..
Changes in the abundance of macroinvertebrate species documented in a rocky intertidal community between surveys in 1931-1933 and 1993-1996 are consistent with the predicted effects of recent climate warming. We resampled 57 0.84-m(2) plots of an intertidal transect first surveyed by W. G. Hewatt at Hopkins Marine Station (HMS), Pacific Grove, California, between 1931 and 1933. Replicating precisely the location of the plots and methodology used by Hewatt, we documented changes in the abundances of 46 invertebrate species, indicating that this intertidal community changed significantly during the 60 yr between surveys. Changes in abundance were related to geographic ranges of species. Most southern species (10 of 11) increased in abundance, whereas most northern species (5 of 7) decreased. Cosmopolitan species showed no clear trend, with 12 increasing and 16 decreasing. Although Hewatt did not record algal species as thoroughly as invertebrates, we were able to document a massive decline in cover of Pelvetia compressa, a cosmopolitan fucoid alga that is typically more common in the southern part of its range. Shoreline ocean temperature, taken daily at HMS, warmed by 0.79 degrees C during this 60-yr period, with average summer temperatures up to 1.94 degrees C warmer in the 13 yr preceding our study than in the 13 yr preceding Hewatt's. The hypothesis that climatic warming drove the observed range-related community shifts is supported further by historical records and data from other investigators. Several alternative hypotheses to explain changes in the invertebrate community at HMS, including habitat changes, anthropogenic effects, indirect biological interactions, El Nino-Southern Oscillation (ENSO) events, and upwelling are considered to be less important than climate change. Changes in species' abundances over a short period (3 yr) were relatively small compared to large species shifts over 60 yr and were unrelated to geographic range of the species, indicating that short-term population fluctuations play a relatively minor role in the long-term community changes that we observed.
Grassland Responses to Three Years of Elevated Temperature, CO. Erika S. Zavaleta; M. Rebecca Shaw; Nona R. Chiariello; Brian D. Thomas; Elsa E. Cleland; Christopher B. Field; Harold A. Mooney.
Global climate and atmospheric changes may interact in their effects on the diversity and composition of natural communities. We followed responses of an annual grassland to three years of all possible combinations of experimentally elevated CO2 (+ 300 [iL/L), warming (+80 W/m2, +-1•C), nitrogen deposition (+7 g N.m-2.yr-1), and precip- itation (+50%). Responses of the 10 most common plant species to global changes and to interannual variability were weak but sufficiently consistent within functional groups to drive clearer responses at the functional group level. The dominant functional groups (annual grasses and forbs) showed distinct production and abundance responses to individual global changes. After three years, N deposition suppressed plant diversity, forb production, and forb abundance in association with enhanced grass production. Elevated precipitation en- hanced plant diversity, forb production, and forb abundance but affected grasses little. Warming increased forb production and abundance but did not strongly affect diversity or grass response. Elevated CO2 reduced diversity with little effect on relative abundance or production of forbs and grasses. Realistic combinations of global changes had small di- versity effects but more marked effects on the relative dominance of forbs and grasses. The largest change in relative functional group abundance (+50% forbs) occurred under the combination of elevated CO2 + warming + precipitation, which will likely affect much of California in the future. Strong interannual variability in diversity, individual species abundances, and functional group abundances indicated that in our system, (1) responses after three years were not constrained by lags in community response; (2) individual species were more sensitive to interannual variability and extremes than to mean changes in en- vironmental and resource conditions, and (3) simulated global changes interacted with interannual variability to produce responses of varying magnitude and even direction among years. Relative abundance of forbs, the most speciose group in the community, ranged after three years from >30% under elevated CO2 + warming + precipitation to <12% under N deposition. While opposing production responses at the ecosystem level by different func- tional groups may buffer responses such as net primary production (NPP) change, these shifts in relative dominance could influence ecosystem processes such as nutrient cycling and NPP via differences between grasses and forbs in tissue chemistry, allocation, phe- nology, and productivity.