Anne E. Magurran

Explaining the excess of rare species in natural species abundance distributions.

The observation that a few species in ecological communities are exceptionally abundant, whereas most are rare, prompted the development of species abundance models. Nevertheless, despite the large literature on the commonness and rarity of species inspired by these pioneering studies, some widespread empirical patterns of species abundance resist easy explanation. Notable among these is the observation that in large assemblages there are more rare species than the log normal model predicts. Here we use a long-term (21-year) data set, from an estuarine fish community, to show how an ecological community can be separated into two components. Core species, which are persistent, abundant and biologically associated with estuarine habitats, are log normally distributed. Occasional species occur infrequently in the record, are typically low in abundance and have different habitat requirements; they follow a log series distribution. These distributions are overlaid, producing the negative skew that characterizes real data sets.

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.

Fish in larger shoals find food faster

SummaryExperiments on shoaling cyprinids hunting for food on patches in tanks demonstrate and advantage of foraging in a group. Individual goldfish (Carassius auratus) and minnows (Phoxinus phoxinus) in a shoal of conspecifics located food more rapidly as shoal size increased from 2 to 20. although shoaling minnows form polarised schools more readily than goldfish, which rarely do so, both species benefited from the trend of speedier food location with increasing group size.

Assemblage time series reveal biodiversity change but not systematic loss

Changing Assemblages Although the rate of species extinction has increased markedly as a result of human activity across the biosphere, conservation has focused on endangered species rather than on shifts in assemblages. Dornelas et al. (p. 296; see the Perspective by Pandolfi and Lovelock), using an extensive set of biodiversity time series of species occurrences in both marine and terrestrial habitats from the past 150 years, find species turnover above expected but do not find evidence of systematic biodiversity loss. This result could be caused by homogenization of species assemblages by invasive species, shifting distributions induced by climate change, and asynchronous change across the planet. All of which indicates that it is time to review conservation priorities. Ecological communities are experiencing changes in species composition rather than unidirectional loss. [Also see Perspective by Pandolfi and Lovelock] The extent to which biodiversity change in local assemblages contributes to global biodiversity loss is poorly understood. We analyzed 100 time series from biomes across Earth to ask how diversity within assemblages is changing through time. We quantified patterns of temporal α diversity, measured as change in local diversity, and temporal β diversity, measured as change in community composition. Contrary to our expectations, we did not detect systematic loss of α diversity. However, community composition changed systematically through time, in excess of predictions from null models. Heterogeneous rates of environmental change, species range shifts associated with climate change, and biotic homogenization may explain the different patterns of temporal α and β diversity. Monitoring and understanding change in species composition should be a conservation priority.

Provenance, Shoal Size and the Sociobiology of Predator-Evasion Behaviour in Minnow Shoals

An investigation of the predator-evasion behaviour of minnow (Phoxinus phoxinus) shoals confronted with a pike (Esox lucius) showed that individual minnows generally chose the behaviour that minimized their chance of being eaten by the predator. As soon as the pike had been detected, minnows switched from dispersed small shoals to a single compact school. They then commenced inspection behaviour, during which individuals or groups approached the predator. This inspection served to confirm recognition of the pike and provide information on its behaviour. Avoidance and skittering behaviour took place when the pike began stalking. It was only when the predator escalated its attack and struck at the shoal that the minnows performed their most costly predator evasion tactics, such as flash expansion and fountain. After such tactics individuals often became separated from the shoal and as such were most vulnerable to capture. As a last resort, individual minnows hid among stones. Minnows from provenances with and without pike exhibited a similar repertoire of antipredator behaviour patterns, but those sympatric with the predator integrated their tactics more effectively and regained pre-exposure behaviour sooner after each encounter. Shoal size had an important effect on the execution of tactics. Minnows in shoals of 10 were more likely than minnows in shoals of 20 or 50 to abandon schooling behaviour and seek cover as individuals.

Monitoring change in biodiversity through composite indices

The need to monitor trends in biodiversity raises many technical issues. What are the features of a good biodiversity index? How should trends in abundance of individual species be estimated? How should composite indices, possibly spanning very diverse taxa, be formed? At what spatial scale should composite indices be applied? How might change-points—points at which the underlying trend changes—be identified? We address some of the technical issues underlying composite indices, including survey design, weighting of the constituent indices, identification of change-points and estimation of spatially varying time trends. We suggest some criteria that biodiversity measures for use in monitoring surveys should satisfy, and we discuss the problems of implementing rigorous methods. We illustrate the properties of different composite indices using UK farmland bird data. We conclude that no single index can capture all aspects of biodiversity change, but that a modified Shannon index and the geometric mean of relative abundance have useful properties.

Development of predator defences in fishes

SummaryA variety of development characteristics, morphological, behavioural, and experiential, contribute to the extreme vulnerability of young fishes to predation. The influence of these characteristics is complicated by the fact that the larval period is one of substantial and rapid change. Yet survival is the ultimate goal;-it is only by reaching maturity that individual fish have the opportunity to reproduce. With such high stakes it is not surprising that predator defences are of major importance during all phases of life. Developmental constraints may limit the defensive options for young fishes. Avoidance behaviours, which reduce the likelihood of encountering a predator or of being attacked by it, are particulaly evident in the youngest stages. Here size, coloration and dispersal are used to help elude the predators attention. As fishes grow and acquire greater morphological and behavioural sophistication, there is more scope for predator evasion when avoidance fails. Older fishes are increasingly able to respond to external stimuli and can detect and react to predators or join conspecifics in common defence (schooling). Behavioural development is not simply a consequence of growth and the concomitant physical alterations of the body; it is also mediated by experience that comes through interaction with the physical and biotic environment. Predispositions to respond to experience may be a product of evolutionary history. Although mortality rates decline markedly with development and maturity, changes in size or behaviour can render fishes vulnerable to new suites of predators. Effective predator avoidance can compromise other activities, such as foraging, and individuals may be forced to reconcile conflicting demands. Developmental niche shifts that occur, for example, when certain size classes take refuge in less profitable feeding habitats, represent one such trade-off. Niche shifts may also be mediated by the influence of the programme for morphological development on sensory or behavioural capabilities. In addition to all of these developmental consderations, natural variations in environmental conditions — such as temperature, photoperiod, predator density and variety, and presence of alternative prey — represent additional challenges to predator defences during the rite of passage from birth to reproduction.

Sexual Conflict as a Consequence of Ecology: Evidence from Guppy, Poecilia reticulata, Populations in Trinidad

An investigation of the behaviour of guppies, Poecilia reticulata, in the wild reveals that sexual conflict varies markedly in different habitats. Males from high-predation sites in Trinidad (where the pike cichlid Crenicichla alta occurs) engage in less antipredator behaviour than females, have lower foraging rates than males from low-risk sites and are able to devote a greater proportion of their time to pursuing females and attempting sneaky matings. As a result, females in such locations receive approximately one sneaky mating attempt per minute. Female behaviour in high-predation sites is thus constrained not only by their own predator avoidance but also by sexual harassment which is itself an indirect consequence of risk. Sexual harassment is likely to compromise female choice and may have significant evolutionary consequences.

The inheritance and development of minnow anti-predator behaviour

Abstract European minnow, Phoxinus phoxinus, populations that occur sympatrically with pike, Esox lucius, a piscivorous predator, form more cohesive schools and have better integrated evasion tactics than fish from pike-free waters. The present study, which used two laboratory-reared populations of minnows, showed that anti-predator behaviour is inherited, but that it is modified on the basis of early experience (of a simulated predator attack). The greatest adjustment in anti-predator behaviour occurred in the minnows naturally sympatric with pike, suggesting a genetic predisposition to respond to early experience that varies between populations.

A Cost of Sexual Harassment in the Guppy, Poecilia reticulata

Male guppies in wild Trinidadian populations devote a large proportion of their time to pursuing females, and females, as a result, are frequent targets of sneaky mating attempts. In this paper we demonstrate a cost, in terms of lost feeding opportunities, to these female recipients of sexual harassment. An experiment in pools of a Trinidadian stream manipulated sex ratio and fish density within the ranges naturally occurring in the system. We found that sexual harassment (from males) led to a 25% decrease in foraging beyond that which occurred as a result of female competition. Because the fecundity of female fish is a product of their feeding success, reductions in food intake have potentially serious fitness consequences.