Competition and Predation

8 Key excerpts on "Competition and Predation"

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  The Political Economy of Predation

  Manhunting and the Economics of Escape

  • Mehrdad Vahabi(Author)
  • 2015(Publication Date)
  • Cambridge University Press

    (Publisher)

  Predation is also distin- guished from parasitism. In this sense, predation is confined to macropha- gous predatory animals eating animals. The Meaning of Predation 46 However, this definition is not universally accepted. A broader definition of predation (Levinton, 1982; Woodin, 1983; Menge, 1995; Abrams, 2000; Bengtson, 2002) extends ‘predation’ to organism interactions in general. ‘In this article, the term “predation” is used to describe an interaction in which individuals of one species kill and are capable of consuming a significant frac- tion of the biomass of individuals of another species. This definition includes finches that consume seeds, and the interaction between insect parasitoids and their hosts. However, it does not include most disease organisms and also does not include many herbivores’ (Abrams, 2000, p. 79). Hence, neither predator nor prey has to be an animal; it includes micro-organisms and organisms eat- ing other organisms that are smaller, of the same size or larger. It embraces grazing whatever the object of grazing may be (for example, grass, microbes in mats or plankton). The major point of this broad definition of ‘predation’ is that it kills the victim for nutritional purposes. Parasitism is not predation, since it does not kill the organism. In line with this definition, predation as a selective mechanism has played a major role in evolution since the Cambrian explosion (about 600 Ma). ‘The fossil record is generally silent with regard to animals older than about 600 million years, and only indirect evidence suggests that ani- mals or animal-like organisms and predatorial modes of life existed earlier’ (Bengtson, 2002, p. 290). Following a broad definition of interspecific pre- dation, three points should be underlined with regard to the dynamics of prey–predator relationships. First, the dynamical relationships between prey and predator can be explained in terms of coevolution.

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  The Ecological World View

  • Charles Krebs(Author)
  • 2008(Publication Date)
  • CSIRO PUBLISHING

    (Publisher)

  Populations of a single species are not found alone in nature. Rather, they exist in a matrix of many populations of other species. Although some species in an area will be unaffected by the presence or absence of one another, in some cases two or more species will interact. The evidence for such interactions is quite direct: A population of one species changes when a population of a second species changes. We are concerned here only with direct interactions between two species. Ecologists classify interactions on the basis of their effects, which may be negative or positive.

  In negative interactions, one or both of the species involved suffer some loss, either in population size, reproduction or mortality. There are five types of negative interactions between individuals of different species:

  • Predation: one animal species eats all or part of a second animal species.

  • Herbivory: one animal species eats all or part of a plant species.

  • Competition: two species use the same limited resource or harm each other while seeking a resource.

  • Infection: a microorganism lives in or on a host and impairs the physiological function of the host.

  • Parasitism: two species live in a physically close and obligatory association in which one (the parasite) depends metabolically on the other (the host).

  Positive interactions are grouped into two large categories:

  • Mutualism: Two species live in close association with each other to the benefit of both.

  • Commensalism: Two species are closely associated—one draws a benefit while the other is unharmed.

  We will discuss infection and parasitism in Chapter 8 and mutualism and commensalism in Chapter 9. In this chapter we will focus on how the first three negative interactions—predation, herbivory and competition—affect the distribution and abundance of the interacting species.

  7.2 PREDATION IS THE PRIMARY FACTOR LIMITING THE ABUNDANCE OF MANY POPULATIONS

  Predation is an important process from two points of view. Firstly, predation may restrict the distribution of a species (as we saw in Chapter 3) or reduce its abundance. If the prey is a pest, we may consider predation useful because it makes the pest less common. However, if the prey is a valuable resource, such as caribou or domestic sheep, we may regard predation as undesirable because it causes an economic loss or a conservation problem. Secondly, predation is a major selective force in evolution, and many adaptations we see in organisms, such as warning coloration, have their explanation in the evolution of predator–prey systems.

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  Interaction and Coevolution

  • John N. Thompson(Author)
  • 2014(Publication Date)
  • University of Chicago Press

    (Publisher)

  CHAPTER 2 PARASITISM, GRAZING, AND PREDATION

  Antagonistic interactions occur between species because living organisms are concentrated packages of energy and nutrients (trophic interactions) and because resources are limited (competition). That species respond evolutionarily to these interactions is evident immediately; Darwin used antagonistic interactions more than any other kind to illustrate in the Origin of Species how selection works. The problem in evolutionary ecology, however, is not simply to catalog how species respond evolutionarily to antagonistic interactions but rather to discern patterns in how species respond.

  Some evolutionary patterns may result from the ways organisms feed on other species. Organisms differ greatly in how they attack their victims, including whether they kill their victims, how long they remain to feed on a single victim before killing it or leaving it, and how many victims they feed upon during their lifetimes. These differences in modes of feeding influence how (1) organisms specialize on their victims, (2) victims defend themselves against enemies, and (3) coevolution proceeds between enemies and their victims. This chapter considers patterns in specialization, defense, and coevolution between parasites, grazers, and predators and their victims in an effort to find patterns in interaction and coevolution that transcend taxonomic boundaries.

  MODES OF FEEDING

  The number of ways of categorizing interactions between species is probably unlimited. Terms such as grazer, browser, predator, parasite, herbivore, and carnivore vary among researchers in the breadth or narrowness of their usage (Dindal, 1975; Starr, 1975). For example, Harper (1977) uses the term predation in a general way to describe all the different ways that herbivores feed on plants, whereas Lubchenko (1979) restricts the use of the term to consumers that kill their hosts. All classifications of interactions are necessarily artificial, and their functional purpose can be only as aids to grouping some of the differences in how species interact. Here I separate modes of feeding into parasitism, grazing, and predation because these categories seem useful in assessing some patterns in how selection acts on interacting species. These categories are no more discrete than the concepts of population or community, but like these latter concepts they provide a useful tool for comparative studies in ecology.

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  Introduction to Population Biology

  • Dick Neal(Author)
  • 2018(Publication Date)
  • Cambridge University Press

    (Publisher)

  There are two components of this competition: exploitation and interference (see the introduction to Chapter 18). Exploitation means that prey consumed by one predator are unavailable to another, and similarly parasites and parasitoids find less unexploited prey to attack as the predator density increases. Interference competition can have stronger effects and includes such activ- ities as predators fighting for the same prey item, or predators establishing feeding territories to defend rich sources of food. Such behaviour reduces the time available for searching for prey with the result that fewer prey are eaten as the density of the predator increases. There are some cases in which the number of prey eaten per predator increases at low predator densities but then decreases once the predator density increases beyond a certain density because of the increasing competition among predators (Figure 19.9b). The initial increase probably occurs in most of the social carnivores that hunt prey that are difficult to capture and kill. For example, the adult females in a pride of lions cooperate with one another when they hunt large ungulates. They may attack the same individual from different directions so that the prey is less able to defend itself, and in some cases a lion may steer potential prey towards an area where other members of her pride are lying in ambush. In this way, a large pride will be more successful than a small pride when attacking animals like buffalo, wildebeest or kudu. Wolves also cooperate when hunting for caribou, moose and elk by taking turns at (a) (b) No. prey eaten per predator No. prey eaten per predator Predator density Predator density Figure 19.9 Functional responses of predators to changes in predator density showing (a) the effects of competition between predators, and (b) the added effect of cooperation between predators or group stimulation of predators. Functional Responses of Predators 311

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  Population Ecology

  First Principles - Second Edition

  • John H. Vandermeer, Deborah E. Goldberg(Authors)
  • 2013(Publication Date)
  • Princeton University Press

    (Publisher)

  152 6 Predator–Prey (Consumer–Resource) Interactions T he first five chapters of this book have dealt with the situation in which all interactions are among individuals within a single species. Individuals interact in order to reproduce, thus creating a birth rate. They interact indirectly when they use the same resources, thus creating the phenomenon of competition. They interact in complicated ways to create nonlinear effects, especially in structured populations. This somewhat extensive introduction to population ecology examined many of these sorts of interactions but within the context of individuals interacting with one another in a single population or that of subpopulations interacting with one another in a larger popula-tion. Yet in most real ecosystems, critical interactions occur among species. A fir forest in northern Finland may be an excellent example of a population in which the dominant interactions are individual to individual within a single population of one species. But in a tropical rain forest in Borneo, hundreds of species of trees interact with one another in a multispecies context. There it is a case of populations interacting with populations in the context of com-petition among species of trees, which is the critical force structuring the eco- system. Even the simple density-dependent models we examined were implic-itly cases of populations interacting with one another. When two individuals of the same species consume the same resource, if that resource is actually another organism, as it frequently is, the critical interaction is between the population of the consuming species and the population of the species that is consumed — a consumer–resource or predator–prey interaction. Depending on one’s intentions in developing a model, it is frequently use-ful and certainly justifiable to take a phenomenological approach and simply model a single population as having density dependence without stipulating what is causing that effect.

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  Essentials of Ecology

  • Michael Begon, Robert W. Howarth, Colin R. Townsend(Authors)
  • 2014(Publication Date)
  • Wiley

    (Publisher)

  In fact, many of the effects of predation on com- munity structure (and of other disturbances) are the result of its interaction with competitive exclusion (taking up a theme introduced in Section 6.2). In an undisturbed world, the most competitive species might be expected to drive less competitive species to extinction. However, this assumes that the organ- isms are actually competing. Yet there are many situ- ations where predation may hold down the densities of competitors, so that resources are not limiting and individuals do not compete for them. When predation promotes the coexistence of species that might other- wise exclude one another, this is known as predator- mediated coexistence. For example, in a study of nine Scandinavian islands, pigmy owls (Glaucidium passerinum) occurred on only four of the islands, and the pattern of occurrence of three species of tit had a striking relationship with this distribution. The five islands without the predatory owl were home to only one species, the coal tit (Parus ater). However, in the presence of the owl, the coal tit was always joined by two larger tit species, the willow tit (P. montanus) and the crested tit (P. cristatus). Kullberg and Ekman (2000) argue that the coal tit is the superior competi- tor for food; but the two larger species are less affected than the coal tit by predation from the owl. It seems that the owl is responsible for predator- mediated coexistence, by reducing the competitive dominance enjoyed by coat tits in its absence. In another example, grazing by local zebu cattle in natural pas- ture in the Ethiopian highlands was manipulated to provide a no-grazing control and four grazing intensity treatments at two sites. Figure 7.23 shows how the mean number of plant species varied in the sites in October, the period when plant productivity was high- est. Significantly more species occurred at intermediate levels of grazing than where there was no grazing or heavier grazing (P < 0.05).

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  Ecology

  From Individuals to Ecosystems

  • Michael Begon, Colin R. Townsend(Authors)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  One obvious omission from the predator–prey interactions in our basic models was any acknowledgement that prey abundance may be limited by other prey, and predator abundance by other predators. Prey are bound to be increasingly affected by intraspecific competition as their abundance increases. Predators, too, are likely to be competing for, not simply preying upon, their prey, as well as being limited at high densities by the availability of resting places or safe refuges of their own.

  mutual interference

  More specifically, predators have been assumed in the models discussed so far to consume prey at a rate that depends only on prey abundance. In the Lotka–Volterra model, for example, the consumption rate per predator is simply aN, and consumption rate with a type 2 functional response is aN/(1 + ahN). Typically, however, consumption rate will also often depend on the abundance of the predators themselves. Most obviously, food shortage – the abundance of prey per predator – will commonly result in a reduction in the consumption rate per individual as predator density increases. However, even when food is not limited, the consumption rate can be reduced by a number of processes known collectively as mutual interference (Hassell, 1978 ). For example, many consumers interact behaviourally with other members of their population, leaving less time for feeding and therefore depressing the overall feeding rate. For instance, hummingbirds actively and aggressively defend rich sources of nectar and spend more time doing so the more competitor hummingbirds there are. Alternatively, an increase in consumer density may lead to an increased rate of emigration, or of consumers stealing food from one another, as do many gulls. All of these mechanisms give rise to a decline in predator consumption rate with predator density. Figure 10.9 a, for example, shows this when the crab Carcinus aestuarii forages for the mussel Musculista senhousia. Figure 10.9 b shows that the kill rate of wolves, Canis lupus, preying on moose, Alces alces

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  Community Ecology

  • Peter J. Morin(Author)
  • 2009(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  The main interactions among Tribolium involve interspecific consumption of eggs, larvae, and pupae (Park et al. 1965). Including cases of mutual predation as examples of competition potentially blurs the important distinctions between Competition and Predation and runs the risk of including all predator-prey interactions as just another kind of competition. Regardless of the mechanism involved, species often compete asymmetri- cally, in the sense that one species exerts considerably stronger per capita effects than another. Some of the earlier experimental evidence cited in support of strongly asymmetric interactions probably confounded asymmet- ric per capita effects with initial differences in the densities of manipulated species (Lawton and Hassell 1981). An unequal response to the removal of interspecific competitors may reflect different per capita effects of removed species or different initial densities of species of similar per capita competitive ability. Underwood (1986) has outlined the kinds of careful experimental C ompetition : M echanisms , M odels , and N iches 33 designs that are required to separate differences in per capita competitive effects from differences in density. Such approaches are feasible only where it is possible to exercise tight control over the densities of competitors. Extreme cases of asymmetric competition, in which one species has a strong negative effect on a second species and the second species has no detectable negative effect on the first, are sometimes called amensalisms (Burkholder 1952). In most experimental settings, it is unclear whether the complete absence of a reciprocal effect is real or just a statistical artifact of the small sample sizes associated with most field experiments. DESCRIPTIVE MODELS OF COMPETITION Models of interspecific competition can yield important predictions about the conditions promoting the coexistence or exclusion of competitors.

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