Alan G. Hildrew

Field Experiments on the Drifting, Colonization and Continuous Redistribution of Stream Benthos

Invertebrate drift is a phenomenon of prime importance to the benthic community of streams (see review papers by Elliott 1967; Hynes 1970a, b; Muller 1966, 1974; Ulfstrand 1968; Waters 1969, 1972). The downstream movement in stream currents of large numbers of invertebrates profoundly influences benthic community dynamics in two ways. Firstly, the continuous loss of animals into the water column reduces benthic density, and since some species and size classes are more prone to drift than others, the composition of the local community is affected. Secondly, the continuous settling out of animals from the drift plays an important colonizing role. Thus, the twin influences of drift contribute to a continuous redistribution of benthos. The extent and rate of these two dynamic processes are among the most basic of ecological parameters in the stream ecosystem, and yet very little study has been specifically directed to their measurement. The phenomenon of drift is one which lends itself to experimentation in the field, and it is surprising that so little has been made of this opportunity. Studies involving field experiments have, however, been carried out into the question of the distance of drift (Waters 1965; McLay 1970; Elliott 1971a), the relationship between drift and density of the benthos (Waters 1965; Dimond 1967), the influence of light on drift rates (e.g. Elliott 1965a; Holt & Waters 1967), and the influence of temperature on drift rates (Wojtalik & Waters 1970). In the present study, we distinguish between movements of invertebrates brought about by crawling over the substrate, as opposed to drifting, and evaluate the role that drift plays in the colonization of new areas of stream bed exposed both experimentally and naturally. Colonization of unpopulated areas of stream bed, whether exposed by erosion, excavation of new channels, or denuded by pollution, has often been reported to be initiated by the drifting of invertebrates from upstream (Moffett 1936; Surber 1937; Laurie & Ericksen Jones 1938; Leonard 1942; Muller 1954; Patrick 1959; Chutter 1968; Crisp & Gledhill 1970). Colonization of experimentally exposed substrates has been studied by Moon (1940), who never mentioned drift, and more quantitatively by Waters (1964), Cairns & Ruthven (1970), Dickson & Cairns (1972), Glime & Clemons (1972), Nilsen & Larimore (1973) and Ulfstrand, Nilsson & Stergar (1974). However, none of these workers specifically attempted to measure the role of drift in colonization as opposed to movements across the substrate. We also performed concurrent experiments on the spatial and temporal variability of drift in the stream, and estimated the rate and extent of drift mediated losses and gains by all the principal taxa of the benthic community.

Flow Refugia and the Microdistribution of Lotic Macroinvertebrates

Changes in the microdistribution of benthic macroinvertebrates were observed, between patches identified as refugia from high flow and the remaining benthic habitat, over a range of discharges in a small stream with prominent flow refugia. Total invertebrate abundances were similar across all patch types at baseflow, but local densities were higher in flow refugia after periods of high and fluctuating flow. Different species- and size-specific responses were observed: microdistributions of larval Chironomidae and small nymphs of the stonefly Leuctra nigra did not change with discharge. Conversely, the stonefly Nemurella pictetii and large nymphs of L. nigra were most abundant in refugia, relative to the remaining benthic habitat, during periods of high and fluctuating discharge. The interactive effects of leaf litter and local flow conditions on species microdistributions were investigated. In some seasons, some species may respond indirectly to hydraulic conditions through their direct response to litter mass. Observed microdistribution changes probably do not reflect very short-term reactions to individual flow events, but perhaps a longer-term response to seasonal flow conditions. These are some of the first field data implicating the role of in-stream flow refugia in macroinvertebrate community structure.

Quantification and Resolution of a Complex, Size-Structured Food Web

I. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 I

Biomonitoring of human impacts in freshwater ecosystems:the good, the bad and the ugly

It is critical that the impacts of environmental stressors on natural systems are detected, monitored and assessed accurately in order to legislate effectively and to protect and restore ecosystems. Biomonitoring underpins much of modern resource management, especially in fresh waters, and has received significant sums of money and research effort during its development. Despite this, the incorporation of science has not been effective and the management tools developed are sometimes inappropriate and poorly designed. Much biomonitoring has developed largely in isolation from general ecological theory, despite the fact that many of its fundamental principles ultimately stem from basic concepts, such as niche theory, the habitat template and the r–K continuum. Consequently, biomonitoring has not kept pace with scientific advances, which has compromised its ability to deal with emerging environmental stressors such as climate change and habitat degradation. A reconnection with its ecological roots and the incorporation of robust statistical frameworks are key to progress and meeting future challenges. The vast amount of information already collected represents a potentially valuable, and largely untapped, resource that could be used more effectively in protecting ecosystems and in advancing general ecology. Biomonitoring programmes have often accumulated valuable long-term data series, which could be useful outside the scope of the original aims. However, it is timely to assess critically existing biomonitoring approaches to help ensure future programmes operate within a sound scientific framework and cost-effectively. Investing a small proportion of available budgets to review effectiveness would pay considerable dividends. Increasing activity has been stimulated by new legislation that carries the threat of penalties for non-compliance with environmental targets, as is proposed, for example, in the EUs Water Framework Directive. If biomonitoring produces poor-quality data and has a weak scientific basis, it may lead either to unjustified burdens placed on the users of water resources, or to undetected environmental damage. We present some examples of good practice and suggest new ways to strengthen the scientific rigour that underpins biomonitoring programmes, as well as highlighting potentially rewarding new approaches and technologies that could complement existing methods.

THE IMPORTANCE OF MEIOFAUNA IN FOOD WEBS: EVIDENCE FROM AN ACID STREAM

Seasonal food webs were constructed for the whole invertebrate assemblage (meio- and macrofauna) inhabiting Broadstone Stream (southeast England). High and uniform taxonomic resolution was applied in a dietary analysis, by resolving the complete benthic community to species, including the meiofauna, protozoa, and algae. Meiofauna accounted for 70% of all species in the summary web and for 73% and 63% of those in the summer/autumn and spring webs, respectively. The web structure changed between summer/autumn and winter/spring, due to differences in species composition. Many stream invertebrates fed on meiofauna and organic matter. Addition of meiofauna to the Broadstone web increased the percentage of intermediate species. Seasonal webs contained between 54 (spring 1997) and 86 (autumn 1996) interactive taxa and 229–378 trophic links. Marked differences in web complexity were found between the summer/autumn and winter/spring periods. Meiofauna accounted for most of the links in the web with a high proportion of intermediate–intermediate links in summer and autumn (0.421–0.440) and also of intermediate–basal links during winter and spring (0.509–0.628). In general, the summary web showed that intermediate species and basal resources were numerically dominant components in this stream. Web connectance rose slightly between summer (0.052) and winter (0.061) and increased further in spring (0.079), coinciding with a reduction in species number. A high fraction of detritivores was combined with omnivorous predators, many of which supplemented their diets with organic matter and, depending on season, with algae and invertebrate eggs. In addition, a wide range of feeding modes was found among meiofaunal species. The diversity of the Broadstone community suggests that the impact of top predators tends to dissipate. A low proportion of top predators in the web was combined with a low mean number of prey items, other than detritus, in their guts (large predators, 1.08–1.26 prey/individual gut; small-sized tanypods, 2.15–2.32 prey/individual gut). Dietary similarity was highest in autumn and winter 1996, and observed feeding links of the most common predatory species showed low overlap in their diets. The web architecture of this stream is reticulate and complex, and the patterns observed in these seasonal webs differed from previous stream webs, resulting in low connectance, high linkage density, long food chains, and a high proportion of intermediate species and of intermediate–intermediate links. The food web derived from Broadstone Stream clearly demonstrates that the meiofauna increases web complexity and thus, taking into account their functional diversity, may be crucial to the understanding of food web properties and ecosystems processes in streams.

Chapter 5 - Food Web Structure and Stability in 20 Streams Across a Wide pH Gradient

Summary Recent attempts to include more ecological detail in connectance food webs have revealed strong relationships between food web structure, species abundance and body size. Few studies, however, have assessed these and other macroecological patterns in food webs in order to examine how network structure, dynamics and their determinants change across environmental gradients. Here, we present 20 highly resolved, standardized stream food webs along a wide pH gradient (5.0–8.4). Our main goal of this study was to assess the influence of external environmental and internal biotic influences on community structure and stability. Many structural features of the food webs changed across the gradient, with web size, linkage density and complexity all increasing with pH. Chlorophyll- a concentrations in epilithic biofilms, as well as the biomass of macroinvertebrates and fish, were also positively correlated with pH. Directed connectance was not correlated with pH in our study, however, although some of the smallest food webs at the lowest pH displayed the highest connectance amongst the networks in this data set. The prevalence of generalism in such food webs, with many alternative food chains passing through any given species, might serve to confer a degree of stability upon these networks, if most feeding links are weak. We found clear differences between our allometrically inferred measures of per capita interaction strengths within the food web, with particularly strong links existing between herbivores and their algal resources and between fish and invertebrates, both of which also became more prevalent at high pH. Predatory interactions between invertebrates and between fishes were far weaker, with the weakest links of all being the few instances of invertebrates parasitizing fishes. Dynamic modelling simulations that ran for the equivalent of 10 years revealed that fewer species were lost from the more acid food webs than those at high pH, further confirming the suggestion of a negative relationship between stability and pH. This finding, which supports earlier ideas from empirical studies that have considered other types of stability, might account for the limited biological recovery in previously acidified freshwaters that are showing evidence of chemical recovery.

Aggregation, interference and foraging by larvae of Plectrocnemia conspersa (Trichoptera: Polycentropodidae)

The predatory net-spinning caddis larva Plectrocnemia conspersa aggregates in patches of high density during most of the year. The behavioural basis of the aggregation depends on (a) the initial tendency to spin a net where prey have been captured and (b) the abandonment of sites where prey are not captured within a threshold time after a previous meal. There is a powerful behavioural mutual interference effect in laboratory experiments between larvae at densities similar to those in the field. Larvae contest ownership of nets and the outcome is determined mainly by body size, usually without injury to either contestant. The lack of an aggressive response in August may be due to this mutual interference or to aspects of environmental instability which make an optimal allocation of foraging effort between patches difficult to attain.

Chapter Two - Long-Term Dynamics of a Well-Characterised Food Web: Four Decades of Acidification and Recovery in the Broadstone Stream Model System

Summary An understanding of the consequences of long-term environmental change for higher levels of biological organisation is essential for both theoretical and applied ecology. Here, we present four decades of data from the well-characterised Broadstone Stream community, detailing biological responses to amelioration of acidification and the recent invasion of a top predator (brown trout, Salmo trutta L.) that was previously excluded by low pH. After several decades of reductions in acidifying emissions, species characteristic of less acid conditions have started to invade or recolonise Broadstone and other European freshwaters, but these signs of biological recovery are still patchy and have lagged behind chemical recovery. One possible explanation for slow recovery is ecological inertia arising from the internal dynamics of the food web, a hypothesis we investigate here using a combination of surveys, experiments and mathematical modelling. The invasion of this hitherto invertebrate-dominated system by a large, generalist vertebrate predator could be expected to alter the structure and stability of the food web. Long-term survey data revealed that the community has experienced waves of invasions or irruptions of progressively larger predators since the 1970s, as pH has risen. Intra-annual fluctuations in prey populations have become increasingly damped and the mean abundance of many species has declined, although none of the previously common taxa have been lost. This suggests that predation, rather than simple chemical tolerance, plays a key role in determining the trajectory of recovery, as the top-down effects of the generalist predators spread diffusely through the reticulate food web. Dynamical simulations indicate that the food web may have become less robust over time as pH has risen and larger predators have become dominant. These results suggest that, though none of the original suite of large invertebrate predators has been driven to local extinction, such an eventual outcome is feasible.

The predatory Chironomidae of an iron-rich stream: feeding ecology and food web structure

Abstract. 1. Three species of Tanypodinae (Chironomidae) were found in an acid and iron‐rich stream in southern England. Maximum abundance was achieved in summer and they were sparse at other times. Individuals were aggregated on the stream bed and were overrepresented in accumulations of leaf litter.

Genetic differentiation of a European caddisfly: past and present gene flow among fragmented larval habitats

We describe the genetic structure of a freshwater insect species and interpret it in terms of present‐day population dynamics and possible postglacial colonization history. The sampling regime represented a large area of the species range in northwest Europe, particularly focusing on Britain, a region relatively neglected in molecular population genetic studies. Plectrocnemia conspersa generally showed low levels of genetic variation across the sampled populations (Nei’s D = 0.0138) and subdivision was unrelated to the pattern of the drainage network. However, the results do suggest that populations across the region are not at equilibrium and that British populations still show effects of the recolonization of the species following the last glacial maximum. Levels of genetic diversity were lower in Britain than in mainland Europe. Two‐dimensional scaling showed genetic differentiation between major regions and the pattern of genetic diversity indicates a more recent origin of populations in the north and west of the area compared with the south and east. We argue that, despite the highly fragmented larval habitat, dispersal over tens of kilometres is frequent. Over longer distances, however, P. conspersa does still show evidence of founder effects and postglacial range expansion into Britain.