R. T. Clarke

Long distance seed dispersal by wind : measuring and modelling the tail of the curve

Abstract The size and shape of the tail of the seed dispersal curve is important in determining the spatial dynamics of plants, but is difficult to quantify. We devised an experimental protocol to measure long-distance dispersal which involved measuring dispersal by wind from isolated individuals at a range of distances from the source, but maintaining a large and constant sampling intensity at each distance. Seeds were trapped up to 80 m from the plants, the furthest a dispersal curve for an individual plant has been measured for a non-tree species. Standard empirical negative exponential and inverse power models were fitted using likelihood methods. The latter always had a better fit than the former, but in most cases neither described the data well, and strongly under-estimated the tail of the dispersal curve. An alternative model formulation with two kernel components had a much better fit in most cases and described the tail data more accurately. Mechanistic models provide an alternative to direct measurement of dispersal. However, while a previous mechanistic model accurately predicted the modal dispersal distance, it always under-predicted the measured tail. Long-distance dispersal may be caused by rare extremes in horizontal wind speed or turbulence. Therefore, under-estimation of the tail by standard empirical models and mechanistic models may indicate a lack of flexibility to take account of such extremes. Future studies should examine carefully whether the widely used exponential and power models are, in fact, valid, and investigate alternative models.

RIVPACS models for predicting the expected macroinvertebrate fauna and assessing the ecological quality of rivers

Abstract The European Union Water Framework Directive recognises the need for and value of biological monitoring. This paper reviews the modelling approach known as River Invertebrate Prediction and Classification System (RIVPACS ) for assessing the ecological quality of river sites using macroinvertebrate sampling. The RIVPACS philosophy is to develop statistical relationships between the fauna and the environmental characteristics of a large set of high quality reference sites which can be used to predict the macroinvertebrate fauna to be expected at any site in the absence of pollution or other environmental stress. The observed fauna at new test sites can then be compared with their site-specific expected fauna to derive indices of ecological quality. All methodological decisions in any such model development have implications for the reliability, precision and robustness of any resulting indices for assessing the ecological quality and ecological grade (‘status’) of individual river stretches. The choice of reference sites and environmental predictor variables, the site classification and discrimination methods, the estimation of the expected fauna, and indices for comparing the agreement, or lack of it, between the observed and expected fauna, are all discussed. The indices are assessed on the reference sites and on a separate test set of 340 sites, which are subject to a wide range of types and degrees of impairment.

Successful Conservation of a Threatened Maculinea Butterfly

Bringing Back the Large Blue Flagship endangered species, such as the Large Blue butterfly have driven conservation programs worldwide. However, the Large Blue butterfly (Maculinea arion) became extinct in the United Kingdom. The apparent driver of this extinction was a complex set of events documented by Thomas et al. (p. 80, published online 18 June; see the Perspective by Settele and Kühn). Life-tables and modeling demonstrate how ecological changes, affecting multiple species, cascaded to negatively impact Large Blue populations. When the changes that cause these extinctions were addressed, reintroduction efforts proved successful. Other insects have experienced similar declines and, hopefully, on sites where their known resources remain abundant, a similar approach may be applied. Prediction of population dynamics in relation to habitat requirements has led to a conservation success in the UK. Globally threatened butterflies have prompted research-based approaches to insect conservation. Here, we describe the reversal of the decline of Maculinea arion (Large Blue), a charismatic specialist whose larvae parasitize Myrmica ant societies. M. arion larvae were more specialized than had previously been recognized, being adapted to a single host-ant species that inhabits a narrow niche in grassland. Inconspicuous changes in grazing and vegetation structure caused host ants to be replaced by similar but unsuitable congeners, explaining the extinction of European Maculinea populations. Once this problem was identified, UK ecosystems were perturbed appropriately, validating models predicting the recovery and subsequent dynamics of the butterfly and ants at 78 sites. The successful identification and reversal of the problem provides a paradigm for other insect conservation projects.

Detection of ecological change using multiple organism groups: metrics and uncertainty

A number of biological approaches are commonly used to assess the ecological integrity of stream ecosystems. Recently, it is becoming increasingly common to use multiple organism groups in bioassessment. Advocates of the multiple organism approach argue that the use of different organism groups should strengthen inference-based models and ultimately result in lower assessment error, while opponents argue that organism groups often respond similarly to stress implying a high degree of redundancy. Using fish, macroinvertebrate, macrophyte and benthic diatom data, site-specific parameters (e.g., water chemistry and substratum) and catchment variables from European mountain (n = 77) and lowland (n = 85) streams we evaluated the discriminatory power and uncertainty associated with the use of a number of biological metrics commonly used in stream assessment. The primary environmental gradient for both streams types was land use and nutrient enrichment. Secondary and tertiary gradients were related to habitat quality or alterations in hydromorphology. Benthic diatom and macroinvertebrate metrics showed high discriminatory power (R2 values often >0.50) and low error (<30%) with the primary (nutrient) gradient, while both fish and macrophyte metrics performed relatively poorly. Conversely, both fish and macrophyte metrics showed higher response (high coefficients of determination) than either benthic diatom or macroinvertebrate metrics to the second (e.g., alteration in habitat/hydromorphology) gradient. However, the discriminatory power and error associated with individual metrics varied markedly, indicating that caution should be exercised when selecting the ‘best’ organism group or metric to monitor stress.

National-scale vegetation change across Britain; an analysis of sample-based surveillance data from the Countryside Surveys of 1990 and 1998

Patterns of vegetation across Great Britain (GB) between 1990 and 1998 were quantified based on an analysis of plant species data from a total of 9596 fixed plots. Plots were established on a stratified random basis within 501 1 km sample squares located as part of the Countryside Survey of GB. Results are primarily conveyed in terms of a classification of national land-cover into 22 mutually exclusive Broad Habitat types. Each of the fixed vegetation plots could be assigned to the Broad Habitat in which they were located in either year. Two types of analysis are reported, both based on changes in plant species composition within monitoring plots. The first examined turnover and net change between Broad Habitat types. The second quantified more subtle changes that had occurred within each Broad Habitat using a series of condition measures that summarized multivariate plant species data as a single scalar value for each plot at each time. There are major difficulties in using uncontrolled, large-scale surveillance data to unravel causal linkages and no attempt was made to quantitatively partition variation among competing causes. However, it was clear that results were broadly consistent with environmental drivers known to have operated prior to and during the survey interval. Large-scale vegetation changes could be summarized in terms of shifts along gradients of substrate fertility and disturbance. Changes implied increased nutrient availability across upland and lowland ecosystems while, in lowland landscapes, linear features and small biotope fragments saw a marked shift to species compositions associated with greater shade and less disturbance.

Sediment characteristics, invertebrate densities and shorebird densities on the inner banks of the Wash

This paper tests the possibility that, primarily through their effect on invertebrate prey densities, sediment characteristics can be used to predict the densities of shorebirds (Charadrii), on the inner banks of the Wash, east England. 192 quadrats were established with, on most shores, a transect of nine quadrats from the marsh edge to the Low Water Mark (LWM). Shorebirds were counted in winter in each quadrat. The densities of the main invertebrate prey species were sampled in early winter/late autumn at the mid-point of each quadrat, together with the sediment particle size and organic content. The time for which each quadrat was inundated over high water spring tides was also measured

Deriving population parameters from individual variations in foraging behaviour. I: Empirical game theory distribution model of oystercatchers Haematopus ostralegus feeding on mussels Mytilus edulis

1. The basic structure is described of a game theory model of the distribution of a wintering shorebird, the oystercatcher Haematopus ostralegus, foraging on 12 mussel Mytilus edulis beds whose quality as feeding areas for the birds differed. Model parameters were derived from field studies made over 15 years. A companion paper tests model predictions and illustrates how the model can be used to derive survival curves and predict estuary carrying capacity. 2. Mussel-bed quality was measured as the intake rate achieved by an oystercatcher of average competance foraging at random over the bed, and varied threefold across beds. Each individual in the model was given its own foraging efficiency, drawn at random from a normal distribution whose mean and variance depended on the birds age and feeding method. Its susceptibility to interference from other foraging birds was calculated from its local dominance score, the percentage of wins it had over the other individuals currently on the same bed. A birds local dominance score on a particular mussel bed was calculated from the proportion of birds on that bed having a lower global dominance rank, a measure of a birds competitive ability relative to those of all other birds on the estuary. Across individuals, foraging efficiency and susceptibility to interference were unrelated. 3. In every iteration, the model calculated a birds putative intake rate on each bed by subtracting the reduction in intake rate due to interference from the intake rate the bird could achieve there in the absence of interference. Each individual could then move to the mussel bed which, at that time, gave it the highest intake rate, provided the differences were sufficiently large (3%) for it to discriminate. 4. The main model limitations were the absence of: (i) a real-time base, so time-lags due to learning were not included; (ii) the energy costs associated with moving between beds; (iii) the opportunity for birds to increase their dominance score or foraging efficiency through familiarity with a mussel bed; (iv) the opportunity for individuals to control the amount of competition to which they are exposed by selecting particular times in the tidal cycle at which to feed; and (v) factors known to affect bed attractiveness to oystercatchers, such as the consistency of the substrate. None the less, the model was regarded as a suitable starting point for exploring how the proportion failing to acquire enough food, and either emigrating or starving, is affected by population size

Changes on the heathlands in Dorset, England, between 1987 and 1996

In 1996 all of the heathland in south-east Dorset, southern England, was surveyed using the same recording protocols as those used in surveys in 1978 and 1987. This approach enabled the extent of the heathlands, the degree of fragmentation, and the composition of the vegetation to be compared at a landscape scale over a period of 18 years. Between 1987 and 1996 the number of heathland patches increased from 142 to 151 and the total area of heathland decreased by 552 ha from 7925 ha in 1987. The principal cause of this loss is succession to scrub and woody vegetation, which continues at a rate of 1.7% per year despite conservation management. The most significant vegetation change was a decline in the area of the vegetation types, wet heath and peatland. These had remained constant between 1978 and 1987, but declined by 376 ha (45%) and 150 ha (25%) respectively between 1987 and 1996. Although there is no direct evidence, it is speculated that changing weather patterns or pollution might be the cause. These heaths, like many in Western Europe, are subject to a wide range of national and international legislative instruments designed to ensure their conservation. These measures call for the monitoring of the status of the conserved areas. This study, which has extended over almost 20 years, provides an insight in to the problems of monitoring change in a biotope of high conservation status.

Confidence limits for regression relationships between distance matrices: Estimating gene flow with distance

There is growing interest in assessing relation ships between two or more distance matrices, where distances are based on genetic, geographical, and/or environmental measures of dissimilarity for all pairwise combinations of n populations. Methods are developed and assessed for estimating confidence limits for the regression relationship between dependent matrix Y and matrix X and for estimating the value of x given critical y. Methods include a regression mixed model that incorporates an additional population effects variance and a jackknife-by-population regression method that omits the (n −1) distance observations for each population in turn. The approaches are illustrated using data to quantify rates of gene flow with distance between wild plant populations of sea beet and are assessed using simulations.

Effects of latitude, altitude and climate on the habitat and conservation of the endangered butterfly Maculinea arion and its Myrmica ant hosts

Analyses were made of the habitats of the endangered myrmecophilous butterfly, Maculinea arion, and its host ant, Myrmica sabuleti, in five regions, spanning most of the climatic range of the butterfly in west Europe. Near their altitudinal or north latitudinal limits, both species are confined to warm south-facing slopes where the turf is grazed <3 cm tall, but at low altitudes further south, where the summer climate is 4°C warmer, the butterfly and ant inhabit ground with any aspect other than south-facing, and the butterfly is restricted to swards >20 cm tall. Intermediate types of habitat were used in regions with intermediate summer climates. The implications for conservation management are discussed. The fact that M. arion has a narrow niche and occupies very early successional stages near its range-edge makes it difficult to conserve in regions with cool climates. A successful example is described from one site in the UK. During 20 years of intensive management the sward structure was altered from being tall and dense to that predicted as optimal for M. arion at its range-edge. The species of Myrmica changed greatly during this period, with thermophilous ants, including M. sabuleti, supplanting cool-loving non-host species. A population of M. arion now inhabits this site. We predict that the conservation of M. arion will be easier and cheaper to achieve under the warm climates of central lowland Europe where different, less intensive, management is required.