David A. Elston

Long-term datasets in biodiversity research and monitoring: assessing change in ecological communities through time

The growing need for baseline data against which efforts to reduce the rate of biodiversity loss can be judged highlights the importance of long-term datasets, some of which are as old as ecology itself. We review methods of evaluating change in biodiversity at the community level using these datasets, and contrast whole-community approaches with those that combine information from different species and habitats. As all communities experience temporal turnover, one of the biggest challenges is distinguishing change that can be attributed to external factors, such as anthropogenic activities, from underlying natural change. We also discuss methodological issues, such as false alerts and modifications in design, of which users of these data sets need to be aware.

Regression analysis of spatial data

Many of the most interesting questions ecologists ask lead to analyses of spatial data. Yet, perhaps confused by the large number of statistical models and fitting methods available, many ecologists seem to believe this is best left to specialists. Here, we describe the issues that need consideration when analysing spatial data and illustrate these using simulation studies. Our comparative analysis involves using methods including generalized least squares, spatial filters, wavelet revised models, conditional autoregressive models and generalized additive mixed models to estimate regression coefficients from synthetic but realistic data sets, including some which violate standard regression assumptions. We assess the performance of each method using two measures and using statistical error rates for model selection. Methods that performed well included generalized least squares family of models and a Bayesian implementation of the conditional auto-regressive model. Ordinary least squares also performed adequately in the absence of model selection, but had poorly controlled Type I error rates and so did not show the improvements in performance under model selection when using the above methods. Removing large-scale spatial trends in the response led to poor performance. These are empirical results; hence extrapolation of these findings to other situations should be performed cautiously. Nevertheless, our simulation-based approach provides much stronger evidence for comparative analysis than assessments based on single or small numbers of data sets, and should be considered a necessary foundation for statements of this type in future.

Analysis of aggregation, a worked example: numbers of ticks on red grouse chicks.

The statistical aggregation of parasites among hosts is often described empirically by the negative binomial (Poisson-gamma) distribution. Alternatively, the Poisson-lognormal model can be used. This has the advantage that it can be fitted as a generalized linear mixed model, thereby quantifying the sources of aggregation in terms of both fixed and random effects. We give a worked example, assigning aggregation in the distribution of sheep ticks Ixodes ricinus on red grouse Lagopus lagopus scoticus chicks to temporal (year), spatial (altitude and location), brood and individual effects. Apparent aggregation among random individuals in random broods fell 8-fold when spatial and temporal effects had been accounted for.

Empirical models for the spatial distribution of wildlife

Empirical models for spatial distribution of wildlife, given data from a complete census or a random sample of sites, are reviewed briefly. 2. The use of covariates, recorded at different resolutions, for modelling spatial distribution is explored. Presentation of model predictions in map form is discussed. A framework of models for change in spatial distribution, given data from successive surveys, is developed. Methods for quantifying and presenting precision and bias are described. The methods are illustrated for two bird species (green woodpecker Picus viridis and redstart Phoenicurus phoenicurus) and for red deer Cervus elaphus, using data from north-east Scotland

Spatial asynchrony and periodic travelling waves in cyclic populations of field voles

We demonstrate evidence for the presence of travelling waves in a cyclic population of field voles in northern Britain by fitting simple, empirical models to spatially referenced time series data. Population cycles were broadly synchronous at all sites, but use of Mantel correlations suggested a strong spatial pattern along one axis at a projection line 72° from North. We then fitted a generalized additive model to log population density assuming a fixed–form travelling wave in one spatial dimension for which the density at each site was offset in time by a constant amount from a standard density–time curve. We assumed that the magnitude of this offset would be proportional to the spatial separation between any given site and the centroid of the sampling sites, where separation is the distance between sites in a fixed direction. After fitting this model, we estimated that the wave moved at an average speed of 19km yr-1, heading from West to East at an angle of 78° from North. Nomadic avian predators which could synchronize populations over large areas are scarce and the travelling wave may be caused by density–dependent dispersal by field voles and/or predation by weasels, both of which act at a suitably small spatial scale.

Europe-Wide Dampening of Population Cycles in Keystone Herbivores

Cycling in Unison Many small mammals, especially voles, display semi-regular cycles of population boom and bust. Given the fundamental importance of small mammals as basal consumers and prey, such cycles can have cascading effects in trophic food webs. Cornulier et al. (p. 63) collated raw data from vole populations across Europe collected over the past 18 years. Reduction in winter growth rate was common across a wide variety of habitats with very different local climates, suggesting the presence of a continental-scale climatic driver of vole populations. Synchronicity in vole population fluctuation across Europe suggests a common climatic driver. Suggestions of collapse in small herbivore cycles since the 1980s have raised concerns about the loss of essential ecosystem functions. Whether such phenomena are general and result from extrinsic environmental changes or from intrinsic process stochasticity is currently unknown. Using a large compilation of time series of vole abundances, we demonstrate consistent cycle amplitude dampening associated with a reduction in winter population growth, although regulatory processes responsible for cyclicity have not been lost. The underlying syndrome of change throughout Europe and grass-eating vole species suggests a common climatic driver. Increasing intervals of low-amplitude small herbivore population fluctuations are expected in the future, and these may have cascading impacts on trophic webs across ecosystems.

DIET SELECTION IN GOATS: A TEST OF INTAKE‐RATE MAXIMIZATION

The mechanisms of diet choice by herbivores are poorly understood. We tested whether the preference of goats among five grass species was accounted for by differences in intake rate or differences in species-specific attributes of the grasses. When offered a choice between grass species, the goats selected diets that tended to maximize intake rate. Only a small amount of the residual variation was explained by the individual preference of goats for each grass species. The animals exhibited partial preferences, only leaving the lower intake-rate alternative ungrazed in a quarter of the trials. It is argued that the most likely explanation of this is the difficulty of discriminating between alternatives of similar intake rate. Discrimination error is likely to be a general explanation of mixed diets in herbivores, regardless of whether other explanations, such as rate maximization subject to nutrient constraints, also apply.

HIGH POTENTIAL FOR COMPETITION BETWEEN GUANACOS AND SHEEP IN PATAGONIA

Abstract Guanacos (Lama guanicoe) are the largest native Artiodactyl in South America and the most widely distributed. In arid Patagonia, densities are low and negatively related to domestic sheep numbers in space and time consistent with interspecific competition theory. Although guanacos and domestic sheep have been described as intermediate feeders sharing food resources, no studies have been conducted to compare their diets in sympatric conditions and explore whether the potential exists for direct interspecific competition. We assessed the diet of both species across 9 different sites and 2 seasons by microhistological analysis of fecal samples. We found that (1) guanacos and sheep are generalist herbivores feeding on a wide range of plant species; (2) both are intermediate feeders able to include both monocotyledoneous and dicotyledoneous plants in their diet; (3) both are able to change their diets seasonally; and (4) food niche overlap is high, particularly in summer when food resources are more scarce than in spring. We conclude that the potential for competition between guanacos and sheep is high and could have played a major role in the demise of guanacos. Consequently, current management practices focused on maximizing sheep numbers are not compatible with the recovery of guanaco populations.

THE PERILS OF HAVING TASTY NEIGHBORS: GRAZING IMPACTS OF LARGE HERBIVORES AT VEGETATION BOUNDARIES

The boundaries between vegetation patches are focal points for dynamic interactions between plant communities, particularly in grazed ecosystems where vegetation types may differ in their acceptability to herbivores. Here we show that key vegetation resources attract herbivores, and the surrounding vegetation receives a higher impact than if it is associated with patches of less preferred vegetation (an example of apparent competition). We studied the influence of proximity to preferred grass patches on utilization of the less preferred dwarf shrub, heather (Calluna vulgaris) by red deer (Cervus elaphus) and sheep (Ovis aries) at a range of spatial scales in the Cairngorm Mountains of Scotland, UK. There was a sharp decline in heather utilization with increasing distance from the edges of grass patches. The proportion of grass in the local landscape (within 1 km) had a significant positive effect on heather utilization both at the grass–heather boundary and beyond 5 m from the grass patch. There was also a significant effect of dominant grass species on the utilization of heather within 50 cm of the grass-patch edge, with utilization around Agrostis/Festuca patches (most preferred) being greater than around Nardus-dominated patches, and lowest around patches of Molinia. The greatest contribution to variation in heather utilization was at the smallest scale, and variance components decreased as spatial scale increased, making it impossible to predict local heather utilization (i.e., at the scale of individual plants and of individual bites by foraging ungulates) from large-scale parameters alone, such as herbivore density. These findings emphasize that vegetation–herbivore interactions are localized within the landscape, and that it is these hot spots which are the key fulcrum for vegetation dynamics.

The evolution of phylogenetic differences in the efficiency of digestion in ruminants.

This study investigates, for the first time (to our knowledge) for any animal group, the evolution of phylogenetic differences in fibre digestibility across a wide range of feeds that differ in potential fibre digestibility (fibre to lignin ratio) in ruminants. Data, collated from the literature, were analysed using a linear mixed model that allows for different sources of random variability, covariates and fixed effects, as well as controlling for phylogenetic relatedness. This approach overcomes the problem of defining boundaries to separate different ruminant feeding styles (browsers, mixed feeders and grazers) by using two covariates that describe the browser–grazer continuum (proportion of grass and proportion of browse in the natural diet of a species). The results indicate that closely related species are more likely to have similar values of fibre digestibility than species that are more distant in the phylogenetic tree. Body mass did not have any significant effect on fibre digestibility. Fibre digestibility is estimated to increase with the proportion of grass and to decrease with the proportion of browse in the natural diet that characterizes the species. We applied an evolutionary model to infer rates of evolution and ancestral states of fibre digestibility; the model indicates that the rate of evolution of fibre digestibility accelerated across time. We suggest that this could be caused by a combination of increasing competition among ruminant species and adaptation to diets rich in fibre, both related to climatically driven environmental changes in the past few million years.