Social Predation

How Group Living Benefits Predators and Prey

Guy Beauchamp

• Publisher: Academic Press
• Year: 2013
• Print ISBN: 9780124072282
• eBook ISBN: 9780124076549
• Topic: Scienze biologiche
• Subtopic: Zoologia

The classic literature on predation dealt almost exclusively with solitary predators and their prey. Going back to Lotka-Volterra and optimal foraging theory, the theory about predation, including predator-prey population dynamics, was developed for solitary species. Various consequences of sociality for predators have been considered only recently. Similarly, while it was long recognized that prey species can benefit from living in groups, research on the adaptive value of sociality for prey species mostly emerged in the 1970s. The main theme of this book is the various ways that predators and prey may benefit from living in groups. The first part focusses on predators and explores how group membership influences predation success rate, from searching to subduing prey. The second part focusses on how prey in groups can detect and escape predators. The final section explores group size and composition and how individuals respond over evolutionary times to the challenges posed by chasing or being chased by animals in groups. This book will help the reader understand current issues in social predation theory and provide a synthesis of the literature across a broad range of animal taxa.- Includes the whole taxonomical range rather than limiting it to a select few- Features in-depth analysis that allows a better understanding of many subtleties surrounding the issues related to social predation- Presents both models and empirical results while covering the extensive predator and prey literature- Contains extensive illustrations and separate boxes that cover more technical features, i.e., to present models and review results

Part B

Prey

Outline

Chapter 3 Antipredator Ploys

Chapter 4 Antipredator Vigilance: Theory and Testing the Assumptions

Chapter 5 Antipredator Vigilance: Detection and the Group-Size Effect

Chapter 6 The Selfish Herd

Chapter 3

Antipredator Ploys

Abstract

Various lines of defences against predators are available to a prey species living in groups. Some defences are deployed early in the predation sequence while others come into play later when the predator attempts to capture fleeing prey. Living in groups may allow prey animals to detect predators sooner. Predators may also encounter groups less often, and if predators can capture only one individual during an attack, prey in groups will also benefit from a dilution of predation risk. When pursuing fleeing animals in a group, predators may be less able to focus on a particular prey and suffer a reduction in capture rate through the confusion effect. The confusion effect may favour homogeneity in the appearance and behaviour of prey group members. Animals in groups may also gain protection against predators by adopting defensive formation or by actually chasing predators away. Signals aimed at dissuading predators from attacking also appear quite common in prey that live in groups. Despite these various lines of defences, not all prey animals appear to benefit from living in groups, suggesting that other forces keep animals together.

Keywords

Aposematic displays; Collective detection; Confusion; Encounter-dilution; Oddity; Swamping

Chapter Outline

3.1. Introduction

3.2. Antipredator Ploys

3.2.1. Collective Detection

3.2.2. Encounter-Dilution

3.2.3. Confusion

3.2.4. Swamping

3.2.5. Group Defence

3.2.6. Aposematic Displays

3.3. Are Antipredator Ploys Effective?

3.4. Concluding Remarks

3.1 Introduction

In the upper Bay of Fundy, on Canada’s eastern coast, semipalmated sandpipers, a small shorebird, aggregate in large numbers during fall migration to feed on abundant mudflat amphipods. Individuals spend two to three weeks in the Bay area to accumulate the fat needed to fuel their nonstop flight to coastal areas of South America (Hicklin, 1987). Roosting flocks on the shore during high tide can exceed 100,000, and feeding flocks in the thousands are common during low tide. Such aggregations attract the attention of many birds of prey, including peregrine falcons, which nest on nearby cliffs. Roosting flocks are harassed frequently by falcons, perhaps explaining why sandpipers often stay on the wing over the ocean during high tide, investing considerable energy (Dekker et al., 2011).

Sandpipers deploy several lines of defences against predation attempts by falcons. The sheer number of anxious eyes scanning the horizon certainly increases the chances of locating falcons before they come fatally close. Indeed, the ability of sandpipers to detect an approaching falcon is uncanny. In addition to falcons, sandpiper flocks also attract many bird watchers who, despite all their fancy equipment, always spot a fast approaching falcon long after the sandpipers raise the alarm. Even though this interrupts feeding at a crucial time of the year, sandpipers allocate a sizable proportion of time to vigilance against predators, highlighting the importance of early detection.

Not all birds in the flock detect the approaching predator, and yet the whole flock becomes alerted very rapidly. The benefits of early detection through vigilance would not translate into an advantage for the flock if information about a detected threat failed to pass quickly through the flock. Rapid information transfer ensures that individuals that fail to detect the predator directly can flee sooner than if they waited to detect the predator on their own. In fact, birds are so attuned to fright reactions by companions in the flock that false alarms are quite common (Beauchamp, 2010b). Detectors sometimes make mistakes in identifying a threat like, say, an inoffensive crow that from afar looks like a falcon, and their quick departure triggers a wave of escape across the whole group, suggesting that individuals react to the alarm cues of their companions rather than to the alleged threat itself.

The sheer number of targets for the predator to choose from represents another line of defence available to sandpipers. Falcons can only capture one prey at a time during an attack. Therefore, the risk of predation for any individual can be effectively diluted by the mere presence of many alternative targets. Obviously, if larger groups attracted proportionately more falcons, the dilution advantage would be somewhat reduced. If predators attack the first individual in the group that they encounter during an attack, then the presence of companions can also reduce risk by adding layers of protection between an individual and the predator. In this case, risk can be manipulated by pursuing more advantageous positions in the group. For example, positions that are not in the direct line of attack of a falcon approaching from the outside of the group offer more protection.

Once an alarm has been raised in the group, individuals in some species simply flee as fast as possible to safety. In other species, individuals adopt defensive formations aimed at dissuading predators. Sandpipers cannot hide anywhere on the open mudflats. In addition, such small birds cannot put up much of a fight against a much larger predator like the falcon. However, sandpipers cluster in tight airborne formations that move in erratic fashion, alternatively flashing their upper darker parts and their white belly. From a distance, waves of flashing pass through the group very rapidly, and such displays may make it more difficult for the predator to target any single prey in the group. That such formations serve as a line of defence is readily evident since the chances of capture increase dramatically for any individual that becomes separated from the swirling flock.

This example with sandpipers illustrates the various lines of defences available to a prey species that lives in groups. Some defences are deployed early in the predation sequence, like detecting the predator at the onset of attack. Other defences come into play later in the sequence when the predator attempts to capture fleeing prey. Caro (2005) describes the many ways by which birds and mammals may decrease their risk of predation at different stages of the predation sequence. Several of these mechanisms apply only to prey in groups, such as the dilution and confusion effects. I will focus here on the antipredator ploys available to animals that live in groups. Several of the ploys described above for sandpipers are also used frequently in other species, and I will describe these, including collective detection, dilution, and confusion. I will leave two aspects to separate chapters in view of the large literature devoted to them: antipredator vigilance (Chapters 4 and 5) and the selfish-herd hypothesis (Chapter 6). Most of the themes developed here apply to both single- and mixed-species groups. However, group formation may provide unique benefits to participants in mixed-species groups, and these will be covered in Chapter 8.

3.2 Antipredator Ploys

3.2.1 Collective Detection

The concept of collective detection is an old one: more than 100 years ago, Galton noted that in a herd of cattle many eyes and ears are available to detect predators, and crucially that an alarmed individual soon warns all the others (Galton, 1883). Detecting a predation threat can be divided into two aspects: vigilance and then the response in the group once a threat is detected. Here, I am concerned with the latter. The role of vigilance in early detection of predation threats will be discussed in the following chapter.

In influential models of antipredator vigilance, detection by one individual in the group is expected to alert all remaining group members instantaneously, which means that early detection allows everyone in the group to escape before the predator closes in (Lima, 1990). This information transfer about predation threats from detectors to non-detectors in the same group is called collective detection. In terms of survival, the benefit to collective detection is the difference between the probability of surviving an attack for a non-detector, when at least one other companion detects the predator, and the probability of surviving when no group member detects the predator soon enough. Without collective detection, early detection only benefits early detectors, and the remaining group members must rely on other means to evade predation.

3.2.1.1 Signals Used for Collective Detection

What cues can be used to transfer information about predation threats from detectors to non-detectors within the same group? Chemical alarm substances are known to be produced in many taxa, including fish (Krause, 1993) and invertebrates (Machado et al., 2002), in which they are known to provide information about predation risk. However, the release of these compounds requires mechanical damage of the skin by a predator, and it is therefore unlikely that chemical communication can work before the predator captures an individual, as is required by collective detection. Nonetheless, such cues might alert prey to recent local successful predation, which might usefully alert them to high-risk areas. This is somewhat similar to collective detection, except here prey that have recently been in the same area can warn others about heterogeneous predation risk in space rather than about specific impending attacks.

Acoustic cues have many desirable properties for communicating alarm within a group. Such cues are typically loud and rapidly transmit over large distances. In many species of birds and mammals, detectors of predation threats emit alarm calls that very quickly alert non-detectors in the same group (Hoogland, 1979; van der Veen, 2002). Another example of an auditory cue that communicates alarm is the plop produced by Iberian green frogs (Rana perezi) jumping into the water in response to a predation threat (Martín et al., 2006).

Noises produced by an escaping bird, including whistles or whirring sounds produced by a rapid take-off, are also used to transfer information about threats (Coleman, 2008; Hingee and Magrath, 2009). The intensity of the sounds produced during an alarm probably acts as a cue. But how do we know that it is not simply a visual cue associated with the rapid departure, rather than an auditory cue, that is triggering the subsequent alarm? In these two studies, the authors played back sounds emitted by alarmed birds and observed escape responses by foraging individuals in the absence of visual signals. The relative contribution of visual and auditory cues of alarm has also been examined in common voles (Gerkema and Verhulst, 1990). Voles that were prevented from hearing escape noises from alarmed companions often failed to react to an alarm raised. Furthermore, at large distances between detectors and non-detectors, only auditory cues were effective in transmitting alarm.

Visual cues can also be used to communicate alarm. In contrast to sound production, however, visual cues have a shorter range, especially in cluttered habitats, and they can only be perceived when the receiver of information is facing in the general direction of the sender. Changes in locomotor behaviour in response to predation threats are thought to visually communicate alarm in many species. For instance, in mammals, the rapid movements associated with flight contrast with slow foraging movements and provide information about danger (Berger, 1978; Caro, 1986). In some bird species, rapid alarm flights not associated with intention movements, which are commonly displayed in many species prior to casual departures, communicate alarm to others (Davis, 1975). In a marine insect, the arrival of a fish predator leads to changes in velocity and frequency of turning for peripheral foragers, which act as an alarm signal for the remaining group members (Treherne and Foster, 1981). A similar effect was found for prey fish foraging in a habitat that precludes visual detection of the predator for most individuals in the group (Handegard et al., 2012).

Animals can respond to both the quality and number of visual alarm cues. Although changes in locomotor activity in only one group member may be ambiguous with respect to causation, several flight reactions in rapid succession can strongly signal a threat (Cresswell et al., 2000; Lima, 1995b; Marras et al., 2012).

Mechanical perception plays an important role in some species. Alarm signals can propagate through the substrate and influence companions independently of correlated visual or auditory cues. Agitated movements in the wat...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Part A: Predators
• Part B: Prey
• Part C: General Considerations
• Conclusion
• References
• Index
• Colour Plates