Human activities are transforming grassland biomass via changing climate, elemental nutrients, and herbivory. Theory predicts that food-limited herbivores will consume any additional biomass ...stimulated by nutrient inputs ('consumer-controlled'). Alternatively, nutrient supply is predicted to increase biomass where herbivores alter community composition or are limited by factors other than food ('resource-controlled'). Using an experiment replicated in 58 grasslands spanning six continents, we show that nutrient addition and vertebrate herbivore exclusion each caused sustained increases in aboveground live biomass over a decade, but consumer control was weak. However, at sites with high vertebrate grazing intensity or domestic livestock, herbivores consumed the additional fertilization-induced biomass, supporting the consumer-controlled prediction. Herbivores most effectively reduced the additional live biomass at sites with low precipitation or high ambient soil nitrogen. Overall, these experimental results suggest that grassland biomass will outstrip wild herbivore control as human activities increase elemental nutrient supply, with widespread consequences for grazing and fire risk.
It is often suggested that the relative importance of biotic processes, such as recruitment, competition and predation of marine benthic species, varies predictably along a gradient of exposure to ...wave action. Several established models of community dynamics on rocky shores predict that top-down processes are more important for structuring communities on sheltered than on exposed shores. To test the relative dominance of top-down processes, we first measured the establishment of key benthic species (mussels, barnacles and algae) on 3 sheltered and 3 exposed rocky shores in southwest Ireland over two 6 mo periods. We then manipulated the presence of consumers (e.g. grazing gastropods, crabs, whelks), using caged exclosures, on 2 sheltered and 2 exposed shores to test for an interaction between effects of consumers and shore exposure on the establishment of benthic species. In contrast to predictions, we found that consumers strongly affected establishment of all species regardless of shore exposure. We also found that shore exposure was not a reliable predictor for spatial and temporal variation in rates of establishment of sessile benthic species. Our findings provide experimental evidence which demonstrates the importance of consumers in early post-settlement stages of benthic species—essential for the development of benthic−pelagic models.
Remarkably little is known about the effectiveness or rates of recovery of aquatic ecosystems from reductions in human-associated pressures at landscape scales. The retention of anthropogenic ...contaminants within ecosystems can retard rates of recovery considerably, while the trajectories of recovery processes vary with the extent of disturbance and the resilience of biotic assemblages. The Great Irish Famine of 1845-1850 comprised one of the most significant human disasters of the 19th century, causing the death of approximately one million people and the emigration of a further two million from the country between 1845 and 1855. We found, through analysis of detailed historical census data combined with paleolimnological investigation of sedimentary nutrient concentrations, stable isotope ratios, and diatom assemblages, that the trophic level of Lough Carra, a largely shallow calcareous lake in the west of Ireland with no urban areas or point sources of any significance in its catchment, reduced considerably during and immediately after the Great Famine, shifting to new equilibria within just 2-10 years. Our results demonstrate that the reduction of human pressures from diffuse sources at landscape scales can result in the rapid and monotonic recovery of aquatic ecosystems. Moreover, the recovery of ecosystems from diffuse pollution need not necessarily take longer than recovery from pollution from point sources.
A large dataset from 1,077 lakes in Finland, Ireland, Sweden and the United Kingdom was collated to analyse the relationship between nutrient status and occurrence of different taxa, as well as ...between total phosphorus or chlorophyll and commonly used macroinvertebrate metrics developed for river assessment. We found that most taxa were associated with mesotrophic conditions (
sensu
OECD). Species associated with oligotrophic status included
Baetis rhodani
,
Gammarus lacustris
and plecopteran larvae, a group commonly associated with low nutrient status also in rivers. Species tolerant of eutrophic conditions were the chironomid larvae (
Chironomus plumosus
and
Cryptochironomus defectus
); and two species of tubificids (
Psammoryctides barbatus
and
Potamothrix hammoniensis
). For a number of taxa the associations of benthic invertebrates with nutrient state reported in the literature were not supported by analysis of the REBECCA data. The analysis indicated a variable response of littoral macroinvertebrates to eutrophication pressure when using common metrics developed for macroinvertebrates in rivers. Several metrics showed significantly different responses in lakes with different alkalinity, justifying the use of alkalinity for typing water bodies. These significant responses suggest that benthic invertebrates may be a useful component for classification of ecological status in lakes. The low amount of variance explained by the regressions (<30%), however, suggests that further harmonisation of sampling methods, as well as statistically more robust assessment tools are needed to increase the comparability of datasets and to improve the precision in the dose–response relationships.
Recent reports have emphasized shortcomings in routine and emergency care leading to adverse outcomes in patients with tracheostomies. This two-part article provides a guide to the principles of care ...for staff looking after adult patients with tracheostomies in the hospital. The second part looks at routine and emergency care.
Recent reports have emphasized shortcomings in routine and emergency care leading to adverse outcomes in patients with tracheostomies. This two-part article provides a guide to the principles of care ...for staff looking after adult patients with tracheostomies in the hospital. The first part looks at indications and techniques.
The mean annual transfer (loss and retention) of nitrogen in a river system was estimated using a conceptual approach based on water surface area and runoff. Two different approaches for the ...calculation of water surface area were applied to determine riverine nitrogen retention in four European catchments, ranging between 860-14,000 km2 in area, and differing considerably in the proportion and distribution of surface waters, specific runoff and specific nutrient emissions. The transfer rate was estimated sequentially as either the mean value for the total catchment, on a sub-catchment scale, or considering the distribution of water surface area within a sub-catchment. For the latter measure, nitrogen retention in larger lakes was calculated separately. Nitrogen emissions modelled with MONERIS and HBV-N were used to calculate nitrogen river loads and compare those with observed loads. Inclusion of the proportion of water area within a sub-catchment improved modelled results in catchment with large lakes in sub-catchments, but not where there was a homogenous distribution of surface waters among sub-catchments.
The compositional heterogeneity of biotic assemblages among sites, or β-diversity, regulates the relationship between local and regional species diversity across scales. Recent work has suggested ...that increased harshness of environmental conditions tends to reduce β-diversity by decreasing the importance of stochastic processes in structuring assemblages. We investigated the effect of nutrient enrichment on the compositional heterogeneity of lake benthic invertebrate assemblages in Ireland at both local (within-lake) and regional (among-lake) scales. At local scales, we found that the compositional heterogeneity of benthic assemblages was related inversely to the extent of nutrient enrichment (as indicated by measurements of water column total phosphorus, total nitrogen, and chlorophyll a), after effects of lake morphology (i.e., surface area, connectivity, and depth of sampling) and alkalinity were accounted for. At regional scales, we found that nutrient-rich lakes had significantly more homogenous benthic assemblages than nutrient-poor lakes, over and above the effect of alkalinity and across a similar range of lake morphologies. These findings have profound implications for global aquatic biodiversity, as the homogenization of benthic assemblages at both local and regional scales may have important and unpredictable effects on whole aquatic ecosystems, with potentially considerable ecological and evolutionary consequences.
The chronologies and sediment accumulation rates for a lake sediment sequence from Lough Carra (Co. Mayo, western Ireland) were established by applying the constant initial concentration (CIC) and ...constant rate of supply (CRS) hypotheses to the measured
210Pb
excess profile. The resulting chronologies were validated using the artificial fallout radionuclides
137Cs and
241Am, which provide independent chronostratigraphic markers for the second half of the 20th century. The validity of extrapolating the derived CIC and CRS dates below the
210Pb dating horizon using average sedimentation rates was investigated using supplementary paleolimnological information and historical data. Our data confirm that such an extrapolation is well justified at sites characterised by relatively stable sedimentation conditions.
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