Theoretical Ecology Virtual Issue

To start with, let me give you my understanding about what is theoretical ecology. I have a quite broad interpretation of this topic. For instance, although mathematical models are increasingly used to formulate ecological theories, I do not see that as a specific requirement. For example, in evolutionary biology, theories on adaptive sex ratio (proposed by Trivers & Willard (1973) in Science) and on equal investment between sexes (formulated by Fisher (1930) in his seminal book) are two well-known theories proposed verbally without any formulated model.

I would define theoretical ecology as the science that looks for explanations of ecological patterns that have been described so that we can predict, based on some more or less simple principle(s), how, when, where and/or why the focal pattern occurs. In a great introduction to theoretical ecology published in 1989, Peter Yodzis defined theory as follows:

Theory proceeds by making assumptions about how things work, assumptions that simplify the vast complexity of nature by abstracting out certain features that the theorist regards as essential. It then deduces the consequences of these assumptions - and comparing these consequences with observed data is a way of looking for patterning the data

This statement adds one key point to the definition I proposed above: a theory needs to be falsifiable. Two other statements by Yodzis are crucial from my viewpoint to realize fully that theoretical ecology and empirical ecology are complementary and strongly co-vary:

Most observers develop hypotheses about their systems and try to test these hypotheses. Often they start work on a system with some hypothesis already in mind. Now, one function of theory is to sharpen hypotheses. This process of sharpening hypotheses can take two forms: it can involve making the hypotheses themselves more precise, or it can involve pinpointing just exactly which measurements need to be made in order to test the hypotheses (perhaps most efficiently in some sense).

and

“a theory has a sort of life of its own, a course of development that flows from its inner logic. And in the course of this process the theory will often generate new hypotheses, which, often enough, can be tested in the field or lab.”

As theories evolve over time, there are a huge number of papers proposing ecological theories. Unsurprisingly, Journal of Animal Ecology has a long tradition of publishing high-impact papers in theoretical ecology, starting as early as the 1930s. It was thus far from an easy task to select a limited number of papers to include in this Virtual Issue. I used three main rules to make my selections: (1) restricting the search to papers proposing a new theory, (2) prioritising papers to cover the diversity of topics published in the journal, including community ecology, population dynamics, spatial ecology, behavioural ecology, and physiological ecology, and (3) representing the full history of the journal by selecting papers published over a period longer than 80 years.

The present Virtual Issue includes 25 papers that were published between 1933 and 2015. It starts with the first comprehensive formulation of a theory on density-dependence Nicholson proposed based on a “competition curve” as early as in 1933.

I hope this virtual issue will stimulate further works proposing new theories in ecology. A theoretical paper in Journal of Animal Ecology should establish, or substantially extend, our theoretical understanding of an issue of general importance for the field of animal ecology. The best theory papers will have a close relationship to considerations of how empirical analysis can be tensioned against the new theoretical developments.

Jean-Michel Gaillard
Senior Editor, Journal of Animal Ecology

Population dynamics

The Balance of animal populations (1933) Nicholson.
Proposed the first comprehensive formulation of a theory on density-dependence based on a competition curve.

The natural control of animal populations (1949) Solomon.
Asked for caution in accepting Nicholson’s conclusions (that were based on a deductive approach) as biological principles and proposed an inductive approach of density dependence to provide a more complete and coherent theory.

Stability in insect host-parasite models (1973) Hassell and May.
Discussed several models of host-parasite interactions, especially in the case of equilibrium levels of host and parasite populations.

Habitat, the templet for ecological strategies? (1977) Southwood.
Made an explicit link between ecological strategies arising from evolutionary trade-offs and natural habitats and identified two major axes of habitat variation (durational stability and resource level and constancy).

Regulation and stability of host-parasite population interactions: i. regulatory processes (1978a) Anderson and May.  
Regulation and stability of host-parasite population interactions: ii. destabilizing processes (1978b) Anderson and May.
Investigated the biological processes that have stabilizing or destabilizing influences on the dynamics of host-parasite associations.

Individual differences between animals and the natural regulation of their numbers (1978) Lomnicki.
Draw the attention of ecologists to the crucial role played by individual differences in population dynamics.

The descriptive properties of some models for density dependence (1981) Bellows.
Examined density-dependent models and compared their ability to describe observed density-dependent mortality.

Models of open populations with space-limited recruitment: extension of theory and application to the barnacle Chthamalus montagui (2001) Hyder et al.
Included space and size-specific survival in iopen-population models to improve predictions of population size and stability.

Parasite transmission: reconciling theory and reality (2002) Fenton et al.
Developed a model framework to assess the consequences of transmission rates in host-parasite systems.

Using evolutionary demography to link life history theory, quantitative genetics and population ecology (2010) Coulson et al.
Proposed an integrative approach to link population dynamics, life history, and quantitative characters.

Community ecology

Interspecific competition (1947) Crombie.
Proposed a general model of interspecific competition that can be used to explain the distribution and abundance of many species and the structure of natural communities.

Some problems in ecological theory and their relation to conservation (1964) Raup.
Reviewed the theory and concept in forest ecology and conservation in the light of several main assumptions involved in ecological theories.

Non-metabolic explanations for the relationship between body size and animal abundance (1993) Blackburn et al.
Produced a model showing that patterns of abundance vs. body size in natal assemblages do not necessarily require metabolic arguments to be explained.

Aggregation and coexistence 1. Theory and analysis (1996) Sevenster.
Reviewed the aggregation model of coexistence, with a special focus on methods of analysis.

Different but equal: the implausible assumption at the heart of neutral theory (2010) Purves and Turnbull.
Developed a model to assess whether the main assumption of neutral theory that all individuals in a community have the same fitness holds.

Linking niche theory to ecological impacts of successful invaders: insights from resource fluctuation-specialist herbivore interactions (2015) Gidouin et al.
Provided a mechanistic-statistical approach to identify the factors shaping novel species assemblages.

Dynamics of metapopulations: habitat destruction and competitive coexistence (1992) Nee and May.
Investigated the effects of removing patches on competitive coexistence between two species.

A practical model of metapopulation dynamics (1994) Hanski.
Developed a new approach to model metapopulation dynamics based on a generalized incidence function that performed well when applied to three butterfly metapopulations.

Spatial patterns of depletion imposed by foraging vertebrates: Theory, review and meta-analysis (1997) Dolman and Sutherland.
Used a simulation model to assess the spatial patterns of resource depletion in vertebrates and pointed out the significance of individual variation.

Behavioural ecology

The behavioural basis of redistribution i. the delta -model concept (1981) Taylor.
Developed a conceptual model of density-dependent spatial behaviour.

Deriving population parameters from individual variations in foraging behavior 1. empirical game-theory distribution model of oystercatchers Haematopus ostralegus feeding on mussels Mytilus edulis (1995) Goss-Custard.
Proposed a game theory model to assess the distribution of individual birds and explore how the proportion of birds failing to achieve enough food responds to changes in population size.

Foraging for intermittently refuged prey: theory and field observations of a parasitoid (2007) White and Andow.
Explored the effect of including accessible prey into an Optimal Foraging model in a host-parasitoid system.

Physiological ecology

An ecological theory on foraging time and energetics and choice of optimal food-searching method (1977) Norberg.
Developed a model to calculate the minimum energy budget  and the time budget for foraging in bird and mammal species.

Reconciling theories for metabolic scaling (2013) Maino et al.
Showed how the use of the Dynamic Energy Budget theory contributes to unify intra- and inter-specific metabolic scaling relationships.

References

Fisher, R. A. (1930). The Genetical Theory of Natural Selection. Clarendon Press.

Trivers, R. L., & Willard, D. E. (1973). Natural selection of parental ability to vary the sex ratio of offspring. Science, 179(4068), 90-92.

Yodzis, P. (1989). Introduction to theoretical ecology. Harper Collins.