Simon D. Rundle

Threats to the running water ecosystems of the world

Running waters are perhaps the most impacted ecosystem on the planet as they have been the focus for human settlement and are heavily exploited for water supplies, irrigation, electricity generation, and waste disposal. Lotic systems also have an intimate contact with their catchments and so land-use alterations affect them directly. Here long-term trends in the factors that currently impact running waters are reviewed with the aim of predicting what the main threats to rivers will be in the year 2025. The main ultimate factors forcing change in running waters (ecosystem destruction, physical habitat and water chemistry alteration, and the direct addition or removal of species) stem from proximate influences from urbanization, industry, land-use change and water-course alterations. Any one river is likely to be subjected to several types of impact, and the management of impacts on lotic systems is complicated by numerous links between different forms of anthropogenic effect. Long-term trends for different impacts vary. Concentrations of chemical pollutants such as toxins and nutrients have increased in rivers in developed countries over the past century, with recent reductions for some pollutants (e.g. metals, organic toxicants, acidification), and continued increases in others (e.g. nutrients); there are no long-term chemical data for developing countries. Dam construction increased rapidly during the twentieth century, peaking in the 1970s, and the number of reservoirs has stabilized since this time, whereas the transfer of exotic species between lotic systems continues to increase. Hence, there have been some success stories in the attempts to reduce the impacts from anthropogenic impacts in developed nations. Improvements in the pH status of running waters should continue with lower sulphurous emissions, although emissions of nitrous oxides are set to continue under current legislation and will continue to contribute to acidification and nutrient loadings. Climate change also will impact running waters through alterations in hydrology and thermal regimes, although precise predictions are problematic; effects are likely to vary between regions and to operate alongside rather than override those from other impacts. Effects from climate change may be more extreme over longer time scales (>50 years). The overriding pressure on running water ecosystems up to 2025 will stem from the predicted increase in the human population, with concomitant increases in urban development, industry, agricultural activities and water abstraction, diversion and damming. Future degradation could be substantial and rapid (c. 10 years) and will be concentrated in those areas of the world where resources for conservation are most limited and knowledge of lotic ecosystems most incomplete; damage will centre on lowland rivers, which are also relatively poorly studied. Changes in management practices and public awareness do appear to be benefiting running water ecosystems in developed countries, and could underpin conservation strategies in developing countries if they were implemented in a relevant way.

Ocean acidification disrupts induced defences in the intertidal gastropod Littorina littorea

Carbon dioxide-induced ocean acidification is predicted to have major implications for marine life, but the research focus to date has been on direct effects. We demonstrate that acidified seawater can have indirect biological effects by disrupting the capability of organisms to express induced defences, hence, increasing their vulnerability to predation. The intertidal gastropod Littorina littorea produced thicker shells in the presence of predation (crab) cues but this response was disrupted at low seawater pH. This response was accompanied by a marked depression in metabolic rate (hypometabolism) under the joint stress of high predation risk and reduced pH. However, snails in this treatment apparently compensated for a lack of morphological defence, by increasing their avoidance behaviour, which, in turn, could affect their interactions with other organisms. Together, these findings suggest that biological effects from ocean acidification may be complex and extend beyond simple direct effects.

Comparing the strength of behavioural plasticity and consistency across situations: animal personalities in the hermit crab Pagurus bernhardus

Many phenotypic traits show plasticity but behaviour is often considered the ‘most plastic’ aspect of phenotype as it is likely to show the quickest response to temporal changes in conditions or ‘situation’. However, it has also been noted that constraints on sensory acuity, cognitive structure and physiological capacities place limits on behavioural plasticity. Such limits to plasticity may generate consistent differences in behaviour between individuals from the same population. It has recently been suggested that these consistent differences in individual behaviour may be adaptive and the term ‘animal personalities’ has been used to describe them. In many cases, however, a degree of both behavioural plasticity and relative consistency is probable. To understand the possible functions of animal personalities, it is necessary to determine the relative strength of each tendency and this may be achieved by comparison of statistical effect sizes for tests of difference and concordance. Here, we describe a new statistical framework for making such comparisons and investigate cross-situational plasticity and consistency in the duration of startle responses in the European hermit crab Pagurus bernhardus, in the field and the laboratory. The effect sizes of tests for behavioural consistency were greater than for tests of behavioural plasticity, indicating for the first time the presence of animal personalities in a crustacean model.

Inter– and intraspecific trait compensation of defence mechanisms in freshwater snails

Trait compensation occurs when mechanically independent adaptations are negatively correlated. Here, we report the first study to demonstrate trait compensation in predator–defence adaptations across several species. Freshwater pulmonate snails exposed experimentally to predation chemical cues from fishes and crushed conspecifics showed clear interspecific differences in their behavioural avoidance responses, which were negatively correlated with shell crush resistance. The type of avoidance response varied between species: thin–shelled species (Lymnaea stagnalis and Physa fontinalis) moved to the water–line or out of the water, while those with thick shells moved under cover or showed a mixed response. There were also intraspecific size–linked differences, with an ontogenetic increase in shell strength accompanied by a decrease in behavioural avoidance. Such trait compensation in response to predation has important implications for interspecific interactions and food–web dynamics.

Nutrient Incluence on a Stream Grazer: Orthocladius Microcommunities Respond to Nutrient Input

A whole—stream enrichment experiment of phosphorus and, further down—stream, of phosphorus and nitrogen, allowed us to examine the growth and density responses of the tube—building larval chironomid Orthocladius rivulorum to nutrient enrichment of the Kuparuk River in arctic Alaska, and to evaluate nutrient effects on the tube microbial community. The larva feeds by grazing a diatom monoculture of Hannaea arcus from the tube exterior, thus direct nutrient effects on the tube microbiota may translate into indirect nutrient effects on the larva. Electron microscopy indicated that tube silk was formed into a sheet, with a filamentous substructure that repeated at 50—nm intervals. Bacterial micro—colonies occurred at the points where the erect diatoms were attached to the silk. Microbial activity of Orthocladius tubes in the P—fertilized section was 2—3 times that of the control section of the river, and total microbial biomass in the P—fertilized section was 3—4 times that of the control section. Chlorophyll a was also higher on Orthocladius tubes downstream of both P and N + P fertilization sites. However, the rate of biomass accumulation on tubes was more rapid downstream of N + P addition, suggesting primary P and secondary N limitation of the rate of primary production in the river. Chlorophyll a was higher on tubes than on rocks or experimental tiles, which indicated that tubes were a more favorable algal habitat for Hannaea. Pupal tubes had less chlorophyll a than larval tubes, suggesting that larval activity may have contributed to the higher algal biomass on tubes. Orthocladius benefitted from the enhanced tube flora; larvae grew larger in the fertilized sections of the stream than in the upstream sections. The results suggest that Orthocladius with its tube and associated biota function as microcommunities that respond directly and indirectly to the surrounding nutrient regime, but have considerable trophic independence from surrounding portions of the epilithon. They may constitute 12—43% of total epilithic algal biomass.

Interactions between freshwater snails and tadpoles: competition and facilitation

SummaryFreshwater snails and anuran tadpoles have been suggested to have their highest population densities in ponds of intermediate size where abiotic disturbance (e.g. desiccation) is low and large predators absent. Both snails and tadpoles feed on periphytic algae and, thus, there should be a large potential for competitive interactions to occur between these two distantly related taxa. In a field experiment we examined the relative strength of competition between two closely related snail species, Lymnaea stagnalis and L. peregra, and between L. stagnalis and tadpoles of the common frog, Rana temporaria. Snail growth and egg production and tadpole size at and time to metamorphosis were determined. Effects on the common food source, periphyton, were monitored with the aid of artificial substrates. Periphyton dry weight was dramatically reduced in the presence of snails and/or tadpoles. There were no competitive effects on growth or egg production of the two snail species when they were coexisting. Mortality of L. peregra was high (95%) after reproduction, but independent of treatment. Growth of L. stagnalis was reduced only at the highest tadpole densities, whereas egg production was reduced both by intraspecific competition and by competition with tadpoles. Differences in egg production were retained after tadpole metamorphosis. Tadpole larval period increased, weight of metamorphosing frogs decreased and growth rate was reduced as a function of increasing tadpole density. However, contrary to expectation, snails had a positive effect on tadpole larval period, weight and growth rate. Further, in experimental containers without snails there was a dense growth of the filamentous green alga Cladophora sp. We suggest that the facilitative effects of snails on tadpoles are due to an “indirect mutualistic” mechanism, involving competition between food sources of different quality (microalgae and Cladophora sp.) and tadpoles being competitively dominant over snails for the preferred food source (microalgae). In the presence of tadpoles snails will be forced to feed on low-quality Cladophora, increasing nutrient turnover rates, which results in enhanced productivity of microalgae, increasing tadpole food resources. Thus, tadpoles have a negative effect on snails through resource depression, while snails facilitate tadpole growth through an indirect enhancement of food availability.

Cue association and antipredator behaviour in a pulmonate snail, Lymnaea stagnalis

Associative learning may help to offset costs of unnecessary escape behaviour by providing accurate information about the current risk to potential prey. We investigated innate antipredator behaviour and cue association learning in naive gastropods. Juvenile laboratory-reared great pond snails, Lymnaea stagnalis (L.), were exposed to odour cues from a natural predator, tench, Tinca tinca (L.), and crushed conspecifics. The snails showed an innate antipredator behaviour to odour from T. tinca, by crawling above the water line (crawl-out response). This crawl-out response was significantly increased in the presence of alarm cues (crushed conspecifics). In a second experiment, juvenile L. stagnalis were exposed to tench odour and alarm cues in aquaria before being tested in behavioural assays. The behavioural responses to tench cue alone were similar to those seen in response to tench plus alarm cues presented together during the first experiment. Hence, L. stagnalis is apparently capable of relating potential predation risk to recent experience. In a final experiment snails were removed from pre-exposure cues for periods of 1, 4 and 8 days prior to behavioural assays. A raised level of avoidance persisted for at least 8 days, suggesting that this behaviour may be retained over timescales relevant to predation risk in the natural environment. The ability of organisms to modify antipredator behaviour based on recent experience, as found in L. stagnalis, would allow costs associated with unnecessary responses to be reduced while still allowing the organisms to avoid active predators.

TRAIT COMPENSATION IN MARINE GASTROPODS: SHELL SHAPE, AVOIDANCE BEHAVIOR, AND SUSCEPTIBILITY TO PREDATION

Many organisms have evolved morphological and behavioral traits that reduce their susceptibility to predation. However, few studies have explicitly investigated the relationships between defensive traits and susceptibility. Here we demonstrate a negative correlation between morphological defenses and behavioral avoidance across several species of marine gastropod that is linked to vulnerability to crab predation. Snails that had relatively taller shell spires (high aspect ratio) showed greater responsiveness when exposed to predation cues than did species with disc-like shells (low aspect ratio). Our results suggest that the snail species most vulnerable to predation compensated by showing the highest levels of behavioral avoidance, and hence may be at a disadvantage in competition with less vulnerable species. This has important implications because the behavioral response of herbivorous gastropods to predation cues may play a central role in structuring rocky intertidal communities through trait-mediated indirect effects.

Exposure to Elevated Temperature and Pco2 Reduces Respiration Rate and Energy Status in the Periwinkle Littorina littorea

In the future, marine organisms will face the challenge of coping with multiple environmental changes associated with increased levels of atmospheric Pco2, such as ocean warming and acidification. To predict how organisms may or may not meet these challenges, an in-depth understanding of the physiological and biochemical mechanisms underpinning organismal responses to climate change is needed. Here, we investigate the effects of elevated Pco2 and temperature on the whole-organism and cellular physiology of the periwinkle Littorina littorea. Metabolic rates (measured as respiration rates), adenylate energy nucleotide concentrations and indexes, and end-product metabolite concentrations were measured. Compared with values for control conditions, snails decreased their respiration rate by 31% in response to elevated Pco2 and by 15% in response to a combination of increased Pco2 and temperature. Decreased respiration rates were associated with metabolic reduction and an increase in end-product metabolites in acidified treatments, indicating an increased reliance on anaerobic metabolism. There was also an interactive effect of elevated Pco2 and temperature on total adenylate nucleotides, which was apparently compensated for by the maintenance of adenylate energy charge via AMP deaminase activity. Our findings suggest that marine intertidal organisms are likely to exhibit complex physiological responses to future environmental drivers, with likely negative effects on growth, population dynamics, and, ultimately, ecosystem processes.

The influence of drought-induced low freshwater flow on an upper-estuarine macroinvertebrate community

Severe drought in the Thames catchment during 1989 and 1990 resulted in increased water abstraction from the river Thames, reducing the amount of freshwater flowing into the Thames estuary over Teddington weir. Increased salinity levels were recorded in the upper estuary, related to the decrease in flow, which in turn affected the community of freshwater invertebrates inhabiting a site below Teddington weir. A large decrease in BMWP score was recorded following increased abstraction in both 1989 and 1990. However, the disappearance of taxa was not directly related to their perceived sensitivity to organic pollution, with Asellidae, Caenidae and Unionidae being amongst the first families to be affected. Multidimensional scaling ordination indicated that the community demonstrated a cyclical pattern of disturbance and recovery related to variations in salinity and flow-dependent influx of organisms via invertebrate drift. The study indicates that reducing freshwater flow below a critical level can have detrimental effects on the diversity of macroinvertebrate communities in certain sections of tidal rivers.