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Theor Ecol (2008) 1:21–28
DOI 10.1007/s12080-007-0002-0
ORIGINAL PAPER
Ageneral theory of ecology
Samuel M. Scheiner & Michael R. Willig
Received: 9 January 2007 /Accepted: 22 May 2007 / Published online: 29 June 2007
#Springer Science + Business Media B.V. 2007
Abstract Ecologists bemoan the dearth of theory in ecolo- general theory establishes relationships among constituent
gy, in particular, the lack of an overarching, general theory. theories through shared fundamental principles. The next
Thesecomplaintslargelyare unjustified. The components of challenge is to develop and integrate unified, constituent
a general theory of ecology have existed for the past half theories and to establish the relationships among them within
century; ecologists simply have failed to explicitly recognize the framework established by the general theory.
them. We present a general theory of ecology and show how
it relates to ecology’s numerous constituent theories and Keywords Conceptualframework.law.model.theory
models. The general theory consists of a description of
the domain of ecology and a set of fundamental principles.
Thedomainofecologyisthespatialandtemporalpatternsof Introduction
the distribution and abundance of organisms, including
causes and consequences. Fundamental principles are broad Like the emperor who had no clothes, ecology often is
statements about the patterns that exist and the processes that criticized as lacking the theoretical adornments of the
operate within a domain. The seven fundamental principles physical sciences. Indeed, for many years ecologists have
of the theory of ecology are: the heterogeneous distribution decried the fact that we have no general theories. Often, this
of organisms, interactions of organisms, contingency, envi- debate is framed as whether ecology has laws (e.g., Lawton
ronmentalheterogeneity,finite and heterogeneous resources, 1999; Simberloff 2004), a necessary component of a well-
the mortality of organisms, and the evolutionary cause of developed theory. We contend that pessimism about the
ecological properties. These principles are the necessary and theoretical foundations of ecology is ill founded: ecology
sufficient elements for a general theory of ecology. The has had a robust theoretical framework for many years. We
propositions of any constituent theory of ecology can be ecologists simply have not recognized that fact, in part
shown to be a consequence of these fundamental principles because we have misunderstood the nature and form of a
along with principles from other science domains. The comprehensive theory. Given a proper understanding of the
nature and form of an overarching theory, exposing a
S. M. Scheiner (*) general theory of ecology becomes less challenging. The
Division of Environmental Biology, goal of this paper is to present the fundamental principles of
National Science Foundation, such a theory so as to further its development.
4201 Wilson Blvd.,
Arlington, VA 22230, USA
e-mail: sscheine@nsf.gov What is a theory?
M. R. Willig
Center for Environmental Sciences and Engineering A theory is a framework or system of concepts and
and Department of Ecology and Evolutionary Biology, propositions that provides causal explanations of phenom-
University of Connecticut, ena within a particular domain (Hempel 1965; Suppe 1977;
Storrs, CT 06269, USA
e-mail: michael.willig@uconn.edu Miller 1987; Giere 1988). The purpose of a theory is to
22 Theor Ecol (2008) 1:21–28
provide a set of linkages for observations and lower-level how an articulated theory can be used to guide model
models or theories of those observations. Confusion exists development.
because the term “theory” is used in reference to a variety The issue of prediction is separate from the existence
of kinds of systems at different levels of specificity. of a well-developed theory. A theory can provide
Moreover, the nature of theory differs at each level. In this understanding without prediction. Precise predictions
context, we recognize three levels of theory. may be difficult or impossible for many reasons, including
At the broadest level, a general theory consists of an context dependency, scale dependency, nonequilibrial
entire domain of science and a set of fundamental principles dynamics, and the importance of historical contingency.
(e.g., the theory of evolution, Darwin 1859; Mayr 1982; Ecological theories—both general and constituent—are
Kutschera and Niklas 2004). That is the type of theory that often probabilistic rather than deterministic, identifying
we present, a general theory of ecology. A general theory the contexts within which particular patterns or process
does not make specific predictions. Rather, it provides the are more likely to manifest. To the extent that theory
scaffolding on which the components of more specific, drives research, constituent theories make clear where
constituent theories are assembled and integrated. It assumptions lie, the sorts of predictions that are possible,
exposes assumptions that are sometimes hidden at the level and criteria for evaluating the relative merits of different
of models or constituent theories and identifies areas that lines of inquiry.
are ripe for theoretical development. In addition, it provides The utility of a general theory may be less apparent
the interconnections that link constituent theories to each because it is two levels removed from the quantitative
other. A mature or fully developed general theory enhances predictions that rigorously evaluate theories and test models
understanding at the broadest of levels and facilitates (Ives and Agrawal 2005). Consequently, we address this
prediction and forecasting via well articulated models and issue at the end of this essay, once we have clarified the
constituent theories. nature of a general theory and have articulated a theory
At the intermediate level are constituent theories, which of ecology.
set boundaries and identify parameters for particular
interest, guiding model development. Constituent theories
can overlap in domain and differ in scope. Depending on Ageneral theory of ecology
the form and domain of a constituent theory, it may make
no predictions, or it may make qualitative predictions. A general theory consists of a domain plus a set of
Most important, a constituent theory unifies a set of fundamental principles. Fundamental principles are broad
interrelated models. For example, Scheiner and Willig statements about empirical patterns and the processes that
(2005) present a constituent theory in ecology that pertains operate within a domain. The domain and fundamental
to variation in species richness along environmental principles of a general theory of ecology are presented
gradients and show how a number of models are derived below. We do not present the fully amplified theory with all
from its propositions. of its components. Such an endeavor would take an entire
At the most specific level, models are instantiations of a book. Rather, our goal is to present its basic premises in
constituent theory. At this level, predictions are formalized sufficient detail so that others understand their meaning and
and causal understanding motivates the process. Most reasons for inclusion within the theory. Clarification of the
references to theory in ecology actually relate to one of nature of those premises results in a framework for theories
these instantiations (Ives and Agrawal 2005, and related in ecology. Having such a framework and articulating the
papers). We prefer the word “model” for these theories to theories within it will help to catalyze the unification of
indicate that the domain is relatively narrow and predictions theory in ecology.
are contingent on well-specified conditions. Models often We do not claim that the general theory of ecology is
are referred to as theories and are the theories that are most novel. Quite the contrary, its elements have existed for at
familiar to ecologists. We emphasize that this is not the least the past 50 years. However, it has never been
level of theory that we present here. described formally and concisely, although many of its
If quantitative predictions occur at the level of models, principles are implicit in the table of contents of most
what is the utility of the other levels? That question is easy ecology textbooks (see references in Table 1). Nor do we
to answer for constituent theories. Such theories identify the claim that the theory, as presented here, is the final version.
necessary structure for models to address a particular issue Rather, it should be considered provisional and evolving.
(e.g., diversity along gradients). Such guidelines make The list of fundamental principles may require additions,
model development easier and more complete, and can deletions, or refinements. Critically, this process can occur
reveal interrelationships among seemingly disparate mod- only after the theory has been formalized and discussed in
els. For example, Scheiner and Willig (2005) demonstrate an open scientific forum.
Theor Ecol (2008) 1:21–28 23
Table 1 Definitions of ecology as presented in various textbooks The rules governing fundamental principles
Source Definition
Fundamental principles can be determined by a simple set
Odum (1971) [T]he study of the structure and function of rules, although we recognize that the application of the
of nature rules is open to debate and disagreement. Here, we present
McNaughton [The] scientific study of the relationships between the rules in the abstract; they are made more concrete when
and Wolf organisms and their environments applied to the principles of the theory of ecology.
(1973) Fundamental principles are of two general types: those
Ricklefs (1979) [T]he study of the natural environment,
particularly the interrelationships between unique to the domain of consideration, and those shared
organisms and their surroundings with other domains. If a fundamental principle is unique to
Colinvaux [T]he study of animals and plants in relation to a domain, it must meet one of two criteria, either the
(1986) their habits and habitats principle is shared by many constituent theories within the
Ehrlich and [T]he study of the relationship between organisms domain, or the principle is necessary for distinguishing
Roughgarden and their physical and biological environments competing general theories. We refer to the former criterion
(1987) as the inclusionary rule and the latter as the exclusionary
Stiling (1992) [T]he study of interactions between organisms and rule. Inclusionary principles must be broad, whereas
between organisms and their environments
Dodson et al. [T]he study of the relationships, distribution, and exclusionary principles may be narrow and can often be
(1998) abundance of organisms, or groups of organisms, the result of the history of debates about a theory. If a
in an environment fundamental principle is shared with another domain, it
Krebs (2001) [T]he scientific study of the interactions that must be a consequence of mechanisms from another
determine the distribution and abundance of domain and have domain-specific causal significance. We
organisms refer to this criterion as the causal rule.
Begon et al. [T]he scientific study of the interactions between These criteria mean that a principle taken unchanged
(2006) organisms and their environment
Gurevitch et al. [T]he study of the relationships between living from another domain need not be specified within a theory.
(2006) organisms and their environments, the We take as given the fundamental principles of any other
interactions of organisms with one another, and general theory. We recognize the general tenet of con-
the patterns and causes of the abundance and silience, that the entire set of scientific theories must be
distribution of organisms in nature consistent with each other (Whewell 1858). Theories may
This paper The study of the spatial and temporal patterns of clash, but such clashes indicate foci of research that
the distribution and abundance of organisms, advance understanding. In general, theories inhabiting
including causes and consequences different domains will not clash directly, although results
from one domain can point to problems with theories in
other domains. For example, studies of geographical
distributions of clades of organisms, within the domain of
The domain of ecology historical biogeography, became important evidence for the
theory of continental drift, a part of the domain of geology.
Before we synthesize and make explicit the structure of In that instance, the need for a causal mechanism to explain
this general theory, we need to define its domain. distribution patterns (the causal rule) was one factor that led
Although domains are artificial constructs, they serve as to the development of new fundamental principles in
foci for organizing theories into coherent entities. The another domain.
domain of ecology and its general theory is the spatial
and temporal patterns of the distribution and abundance The fundamental principles of ecology
of organisms, including causes and consequences.
Although our definition of the domain spans the The general theory of ecology consists of seven fundamen-
definitions found in most textbooks (Table 1), it is tal principles (Table 2). These principles were in place by
different in two respects. First, our definition includes the the 1950s and were widely accepted by most ecologists
phenomena to be explained (i.e., spatial and temporal after the coalescence of the field of ecosystem ecology and
patterns of abundance of organisms) and the causes of the cementing of the Modern Synthesis in evolutionary
those phenomena. Some definitions include only the latter biology. Of course, the roots of all of the principles go back
(i.e., interactions of organisms and environments). Second, much further.
and most strikingly, our definition explicitly includes the The first fundamental principle—the heterogeneous
study of the consequences of those phenomena, thereby distribution of organisms—is a refinement of the domain.
embracing much of ecosystem sciences. It encompasses the basic object of interest and its most
24 Theor Ecol (2008) 1:21–28
Table 2 The seven fundamental principles of the general theory of fundamental principle and the type of rule that is the basis
ecology and the rules that determine their inclusion for inclusion are not always clear and may be subject to
Seven fundamental principles debate by scientists, historians, and philosophers.
The fourth fundamental principle—environmental
1. Organisms are distributed in space and time in a heterogeneous heterogeneity—is a consequence of processes from the
manner. [inclusionary rule] domains of the earth and space sciences. For example,
2. Organisms interact with their abiotic and biotic environments. seasonal variation in temperature is the result of orbital
[inclusionary rule]
3. The distributions of organisms and their interactions depend on properties of the Earth, whereas a variety of geophysical
contingencies. [exclusionary rule] processes create heterogeneity in environmental stressors
4. Environmental conditions are heterogeneous in space and time. like salt (e.g., wave action near shores) or heavy metals
[causal rule] (e.g., geologic processes that create differences in bed-
5. Resources are finite and heterogeneous in space and time. rocks). It is beyond the scope of this paper to detail all of
[causal rule] those processes and their domains. Indeed, this principle
6. All organisms are mortal. [causal rule] encompasses many constituent theories and contains a
7. The ecological properties of species are the result of evolution. broad class of underlying mechanisms for the heteroge-
[causal rule]
neous distribution of organisms. As with the second
principle, particular mechanisms pertain to particular
constituent theories. Again, the fundamental principle
important property. The heterogeneity of distribution is one captures a wide range of theories and mechanisms so as
of the most striking features of nature: all species have a to provide a unifying framework.
heterogeneous distribution at some, if not most, spatial Thefifthprinciple—finiteandheterogeneousresources—
scales. Arguably, the origins of ecology as a discipline and is again a consequence of processes from the domains of the
the first ecological theories can be traced to its recognition earth and space sciences. Although variation in resources is
(Forster 1778; von Humboldt 1808). This heterogeneous similar to variation in environmental conditions, a funda-
distributionisbothcausedbyandacauseofother mental distinction is the finite nature of these resources.
ecological processes. The fundamental principles are not Unlike an environmental condition, a resource is subject to
independent causal mechanisms, rather, the mechanisms competition. For example, seasonal variation in light and
that they encompass interact. temperature are caused by the same orbital mechanisms, but
The second fundamental principle—interactions of light is subject to competition (e.g., one plant shades
organisms—includes within it the vast majority of ecolog- another), whereas temperature is a condition and not subject
ical processes responsible for heterogeneity in time and to competition. This distinction in the nature of environ-
space. Many definitions of ecology are equivalent to this mental factors with regard to competitive processes can
principle (Table 1). Within this principle, particular inter- result in very different ecological outcomes (e.g., patterns of
actions that are part of constituent theories act to unpack the â diversity in plant communities, Scheiner and Rey-Benayas
general theory (see Constituent theories section). 1994). Whether a particular environmental factor is a
The third fundamental principle—contingency—represents condition or a resource can be context dependent. For
either the inclusionary rule or the exclusionary rule, depending example, water is sometimes a resource subject to compe-
on views of the history of theories in ecology. We discuss the tition (e.g., plants in a desert), and sometimes a condition
exclusionary nature of this principle at the time of its genesis (e.g., fish in the ocean).
(see Competing general theories section). Since that time, The sixth fundamental principle—the mortality of
recognition of the importance of contingency in all ecological organisms—is the result of processes that come from the
processes has increased steadily and now appears in a wide domain of organismal biology, physiology, and develop-
variety of constituent theories and models. Contingency is an ment. By “mortal” we mean that no organism is invulner-
important cause of the heterogeneous distribution of able, i.e., any organism might die as the result of predation,
organisms, both at very large extents of time and space stress, or trauma. We do not mean by this principle that all
(e.g., a particular species arose on a particular continent) and organisms senesce. The senescence of organisms is a more
at very small extents (e.g., a seed lands in one spot and not narrow version of this principle that would apply to
another). particular constituent theories. Although the majority of
Wespeculate that general theories of all domains include multicellular species apparently senesce, this has not been
some version of this principle (e.g., genetic drift in demonstrated for some multicellular species. We are not
evolution; quantum theory in physics). Thus, it might be aware of an articulated theory of development or physiol-
considered fundamental to all scientific theories. This ogy that predicts the necessity of or conditions for
principle demonstrates that the decision to include a senescence, but suspect that it could be accomplished given
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