Effects of herbivorous snails and macroalgal canopy on recruitment and early survivorship of the barnacle Semibalanus balanoides (L.)

This study investigated the effects of grazing by the herbivorous snail Littorina littorea (L.) and the presence of a macroalgal canopy on recruitment and early survivorship of the barnacle Semibalanus balanoides (L.) at four sites of various distances from the mouth of Toothacher Cove on Swans Island in the Gulf of Maine in 1998. Recruitment plates were attached to the substrate at each site in all possible treatment combinations, including either presence of absence of macroalgal canopy, presence or absence of the herbivore, and presence or absence of a cage. Significant differences in recruitment were observed among sites in April and May, but not in June. The average number of recruits also varied significantly among herbivore treatments in May and June, and in the May macroalgaexherbivore interaction. Survivorship did not differ among sites, but did vary significantly among herbivore treatments. These differences were due almost entirely to consistently low recruitment and survival in the uncaged or open controls. Comparisons of the caged and open treatments explained between 88 and 99% of the variation in the respective main effects tests. The presence or absence of L. littorea or of macroalgal canopy had little effect on S. balanoides recruitment or survivorship. Results support previous conclusions that recruitment is greater on more exposed shores and suggest that exclusion cages may protect barnacle recruits from factors such as desiccation and the whiplash effect of macroalgal fronds.     

Legumes mitigate ecological consequences of a topographic gradient in a northern Mongolian steppe

Topography should create spatial variation in water and nutrients and play an especially important role in the ecology of water-limited systems. We use stable isotopes to discern how plants respond both to ecological gradients associated with elevation and to neighboring legumes on a south-facing slope in the semi-arid, historically grazed steppe of northern Mongolia. Out of three target species, Potentilla acaulis, Potentilla sericea, and Festuca lenensis, when >30 cm from a legume, all showed a decrease in leaf δ¹⁵N with increasing elevation. This, together with measures of soil δ¹⁵N, suggests greater N processing at the moister, more productive, lower elevation, and more N fixation at the upper elevation, where cover of legumes and lichens and plant-available nitrate were greater. Total soil N was greater at the lower elevation, but not lichen biomass or root colonization by AMF. Leaf δ¹³C values for P. acaulis and F. lenensis are consistent with increasing water stress with elevation; δ¹³C values indicated the greatest intrinsic water use efficiency for P. sericea, which is more abundant at the upper elevation. Nearby legumes (<10 cm) moderate the effect of elevation on leaf δ¹⁵N, confirming legumes’ meaningful input of N, and affect leaf δ¹³C for two species, suggesting an influence on the efficiency of carbon fixation. Variation in leaf %N and %C as a function of elevation and proximity to a legume differs among species. Apparently, most N input is at upper elevations, pointing to the possible importance of grazers, in addition to hydrological processes, as transporters of N throughout this landscape.