6a book chapter (2005) food webs kokkoris

7/30/2019 6A Book Chapter (2005) Food Webs Kokkoris

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--------3.0 I POPULATION

DYNAMICS AND FOOD

WEBS: DRIFTING

AWAY FROM THE

LOTKA- VOLTERRA

PARADIGM

Giorgos D. Kokkoris

Food webs are networks that depict consumer-resource interactions

(links) among species or trophic species (nodes). This approach has

been given a mathematical representation by the use of Lotka-Volterra

population and community dynamics, and may including basic biotic

relationships as intraspecific competition and predation, including par-

asitism. The central theme of food web research is the understanding ofstructure, function, dynamics, and complexity. In order to be able to pre-

dict food web behavior under external and internal effects, biotic, and

abiotic influences we need to be able to answer the question, "What

drives food web dynamics?" (see Scharler et al., Chapter 8.3). The suc-

cess in fulfilling this task will determine in part the management of our

ecosystems towards sustainability (see Section 7). Unfortunately, there is

little use of food web models in environmental management. Reasons

for that stem from the fact that theoretical ecologists rarely care toaddress practical problems and managers are reluctant to use food web

models to predict.

Do we really need to consider a new view of food webs and biotic com-

munities in the dawn of the twenty-first century? Are there any impor-

tant factors that have not been included in the study of natural food 71


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webs so far? Forces that act on food webs actually affect both the nodes

and the links of the graphical representation of a food web. There is now

enough evidence that both components of food webs need to be recon-

sidered and utilized approaches to be revised in future work.Populations are usually described only through reproduction and mor-

tality. But populations consist of individuals that grow and develop and

not all predator and prey individuals are identical (see De Roos and

Persson, Chapter 3.2). Life history variation among species, expressed as

different generation times influences population growth (see Scharler

et al., Chapter 8.3). Predation pressure may induce defences to some of

the individuals of the population of their prey, creating heterogeneity in

the prey population (see Vos et al., Chapter 3.4). Traditional approaches

ignore the dynamics resulting from the previously described aspects of

heterogeneity within the populations. This missing heterogeneity may

be an important determinant of the observed pattern and processes on

food webs and community level properties such stability, resilience, and

persistence.

One of the valid criticisms that have been developed is that interaction

between species can be described by a linear function of their densities

(Pimm, 1982). This is the ecological equivalent of the Law of Mass Action

that has been inherited to younger Ecology from older Chemistry.

Applied to community processes, this law states that if the individuals in

populations mix homogeneously, the rate of interaction between two

species is proportional to the product of the numbers of individuals in

each of the species concerned. As a result, predators for instance keep

consuming their prey independently of their density, which certainly

cannot be true in real systems. Responding to the call for more mecha-

nistic models, Fretwell (1977) and Oksanen et al. (1981) studied food

chains representing interactions that accounted for functional andnumerical response of predators. Integration of non-linear dynamics

into food web models has taken place recently as a result of significant

advances in computing power that is a sine qua non of such approaches

(Drossel et al., 2004, see also Dell et al., Chapter 8.1).

Food web models that follow the dominant Lotka-Volterra paradigm

use emergent food web properties such as diversity (species richness)

and connectance to determine other food web characteristics (Jansen

and Kokkoris, 2003). Traditional community assembly models ignoreadaptation processes (Kokkoris et al., 1999). But food webs evolve and

these models fail to provide clear mechanisms explaining how these

characteristics and structure emerge. The ecological interactions among

species in a community and the role they play in adaptation (behavioral,

developmental, or evolutionary) of species traits such as body size are


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usually left out (B0hn and Amundsen, 2004). Ecological interactions are

the structuring links in all food webs and the patterning of them is themajor factor in the stability, resilience, and persistence of biotic com-

munities (for a review see Berlow et al., 2004).

Kondoh (see Chapter 3.3) criticizes this static representation of nature.

For instance, if a trait that influences the strength of trophic interactions

is controlled by adaptation, then the food web architecture should

change in a way that is favored by this adaptation. These changes influ-

ence population dynamics and consequently the stability of the food

web (Kondoh, 2003a). Few pioneering studies also have recently investi-gated how complex food webs emerge from evolutionary community

assembly processes (Drossel et al., 2004; Loeuille and Loreau, 2005;

McKane and Drossel, Chapter 3.1). These studies provide useful insights

on the evolution of food webs and should be developed further to allow

invasions (or speciation) of species that possess characteristics that

may be quite different from those that already exist in the community

under study.

The points previously laid out are motivated from the chapters in thissection of the volume are part of the challenge in developing testable

predictions from food web studies. The chapters of this section clearly

justify the new approaches needed in the face of global change and

extinction crisis (Lawton and May, 1995). Food web ecologists move

gradually away from the dominant paradigm in the discipline and if this

is combined with a willingness to address practical issues, their models

will be of good service to environmental management and biodiversity

conservation.

6a book chapter (2005) food webs kokkoris

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