Community Ecology
Disturbance and equilibrium
Disturbance and equilibrium
Outline:
1. Definitions of disturbance
a. Pulse vs. press (Bender et al. 1984)
2. Properties of disturbance (Pickett and White 1985)
3. What is natural?
a. State-shifts
5. Definition of equilibrium, including static equilibrium and dynamic equilibrium
a. Three aspects to equilibrium: resistance, resilience, and persistence
6. Equilibrium assumed because population numbers seem to remain constant over time
7. Nonequilibrium
a. Special case: chaos
8. Three alternatives to equilibrium in explaining why population numbers remain steady
9. Discussion of the usefulness of the equilibrium concept, despite its lack of realism
a. (Some) types of models: structural, conceptual, analytical, simulation
Disturbance is an incredibly complex and ill-defined concept. This leaves disturbance as quite possibly my least-favorite word in ecology. It is used by ecologists and non-ecologists alike to make fuzzy statements that sound authoritative and yet are hollow upon deeper reflection. It is an extremely important ecological process, however, so the purpose of today’s lecture is to introduce you to the concept (and also to the lack of precision that clings to it). In community ecology we seek general properties or patterns. By understanding what patterns exist (and why they occur), we can detect perturbations and predict the effects of particular events. Disturbance affects those properties or patterns.
Disturbance has been variously defined by ecologists, with little consensus. One definition that is commonly used is from Steward Pickett and Peter White’s classic book, The Ecology of Natural Disturbance and Patch Dynamics:
"any relatively discrete event in space and time that disrupts ecosystem, community, or population structure and changes resources, substrate, or the physical environment" (Pickett and White 1985)
The two key parts of this definition are that disturbances are:
Note also that there are different kinds of disturbances:
Pulse –
Press –
Not all changes are disturbances (some changes are merely natural variance). A disturbance falls outside natural variance (but since we often do not know what the natural variance of a system is, we may be misled into thinking that an event is a disturbance). For example, on a localized and short time scale, a forest fire is a disturbance. But taking a longer view, certain forests (e.g. longleaf pines in Florida, for example) are fire-adapted and require periodic fires for proper seedling regeneration (cones do not open and release the seeds contained therein except by high heat from a fire, a life-history adaptation to fire that is termed serotiny). So, are disturbances a part of the system itself? Are they 'inside' or 'outside' the system? The answer depends on your frame of reference. For example, Allen and Wyleto (1983) described a fire-driven prairie system at two levels of reference. Using species abundances and individual fires, they characterized the successional response to fire. At this level, a single fire is 'outside' the system and plant species respond to a fire as an extrinsic driver (i.e., a disturbance). Aggregating fires into a multi-year fire frequency and collapsing species abundances to presence/absence, they characterized the species assemblages induced by various fire frequencies. At this level, fire is ‘inside’ the system (i.e., within normal variance). That, is changing the nature of the observations/response variables measured also changes the role of the disturbance as being extrinsic or intrinsic to a system.
Properties of disturbance:
Disturbances (and their effects) can be described/quantified in terms of a few defining properties (Pickett and White 1985):
It is important when using the term disturbance that you quantify exactly what you mean. Using the above terms/properties allows you to do so.
What is “natural”?
Historically, a common perception of nature was that of an equilibrated and equilibrating system that, although occasionally perturbed by various disturbances, still tended toward some natural balance. While few professionals still subscribe to this model, it remains to be appreciated just how unusual such an equilibrium state might be in nature.
Sprugel (1991) reviewed several examples of systems thought to be exemplary of the balance of nature in a "natural" state, including the African savanna, the "Big Woods" of Minnesota, the lodgepole pine landscapes of the Yellowstone area, and old-growth forests in the Pacific Northwest. His conclusions were:
· "Natural" vegetation is far less stable than it may seem to be from our human perspective; in particular, all of the examples cited are transient or nonequilibrium over timescales measured in life-times of the dominant organisms.
· Vegetation may preserve small or transient effects for a very long time, especially in the case of forests of long-lived trees.
· "Every point in time is special" in that at any time, vegetation has some characteristics that distinguish it from the same system at any other time.
· Thus, it may be impossible (and irrelevant?) to define the "natural state of the system" for many if not most systems.
State-shifts
Implications
In terms of practical aspects, most conservation and management activities are designed to preserve “nature,” “the wild,” or “wilderness.” While managing for a natural, undisturbed pattern may seem appealing, the simple fact is that in most real landscapes, such a pattern is unrealistic.
How can disturbance actually increase diversity?
Hopefully by now you realize that the effects of disturbance depend upon the organism, system, and variable in question.
People have noticed and marveled at the variety of species that can be found in a prairie, pond, or forest, and also at how the same species seemed to occur at the same times every year. And so people have also wondered why there were these patterns of species co‑occurrences. Community ecology emerged as a scientific discipline at the end of the 19th century as an attempt to understand this repeatable pattern that seemed to be indicative of the "balance of nature." Thus the topic of equilibrium is foundational to CE.
equilibrium
resistance –
resilience –
persistence –
The evidence that is needed to show that equilibrium is present is difficult to obtain:
must show that following a perturbation, numbers will return to a value seen before the perturbation (but logistically and ethically difficult to perturb a population, and need to follow it for a long time)
2 forms of equilibrium:
Equilibrium began to be questioned in 1800s after extinctions became known (fossil evidence): how could equilibrium exist if some populations go extinct?
But it took until the late 20th century to really be examined.
nonequilibrium
noneq. does not imply dominance by stochastic factors!
DeAngelis and Waterhouse 1987 - excellent review
chaos theory = special form of nonequilibrium
characteristics of chaos -
origins with H. Poincaré, coined by J. Yorke
French mathematician Henri Poincaré in the 1880s pointed out that it is impossible to calculate the precise trajectories of the planets and stars of our solar system because they are continually pulling and pushing on each other via gravity, making their future positions impossible to determine with precision: this is contrary to the Newtonian view of the cosmos at that time that everything can be determined with mathematical precision
Robert May (Australian physicist turned biologist at Princeton and then Oxford) was examining population growth --> recall that a population was assumed to grow towards a stable value known as carrying capacity, at which point the population’s demand for resources will match the resources available, and population growth will level off to stability --> but May found that if he increased r (pop. growth rate) even more, there was no single stable value reached (instead, the pop. size alternated around several values) (May 1976) --> May showed these patterns to James Yorke (Univ. MD mathematician), who coined the term chaos
"butterfly effect" from Edward Lorenz (1972)
George Sugihara (Ph.D. student of May’s, 1983; now at the Scripps Inst. of Oceanography in La Jolla, CA) - although chaos precludes long-term prediction, chaos is not randomness (Sugihara and May 1990)
randomness is informationless noise; chaos, in contrast, contains information that can be used to predict the short-term future of a non-linear system (see also Hsieh et al. 2005)
James Gleick, a writer for the NY Times (not a scientist) wrote a best-selling popular-science book Chaos: Making a New Science (1987)
So with nonequilibrium and even chaos being so common, how do many communities remain so constant over time?? There are 3 potential reasons:
1)
2)
3)
So, is equilibrium a useful concept? In other words, why is it still around, now that we know about nonequilibrium?
Next lecture: what happens to communities following disturbance: succession
References:
Allen, T.F.H., and E.P. Wyleto. 1983. A hierarchical model for the complexity of plant communities. J. Theor. Biol. 101:529-540.
Bender, E.A., T.J. Case, and M.E. Gilpin. 1984. Perturbation experiments in community ecology: Theory and practice. Ecology 65:1-13.
DeAngelis, D.L., and J.C. Waterhouse. 1987. Equilibrium and nonequilibrium concepts in ecological models. Ecol. Monogr. 57:1-21. [excellent overview of the subject]
den Boer, P.J. 1968. Spreading of risk and stabilization of animal numbers. Acta Biotheor. 18:165-194.
Gleick, J. 1987. Chaos: Making a New Science. Viking Press, New York, NY.
Hsieh, C., S.M. Glaser, A.J. Lucas, and G. Sugihara. 2005. Distinguishing random environmental fluctuations from ecological catastrophes for the North Pacific Ocean. Nature 435:336-340.
Knight, D.H. 1987. Parasites, lightning, and the vegetative mosaic in wilderness landscapes. Pp. 59-83 in: Landscape Heterogeneity and Disturbance (M.G. Turner, ed.). Springer-Verlag, New York, NY.
Lorenz, E. 1972. "Predictability: Does the Flap of a Butterfly's Wings in Brazil Set Off a Tornado in Texas?" Presentation given at the annual meeting of AAAS, Boston, MA, 29 Dec.
May, R.M. 1976. Simple mathematical models with very complicated dynamics. Nature 261:459-467.
Pickett, S.T.A., and P.S. White. 1985. The Ecology of Natural Disturbance and Patch Dynamics. Academic Press, New York, NY.
Sprugel, D.G. 1991. Disturbance, equilibrium, and environmental variability: what is 'natural' vegetation in a changing environment? Biol. Conserv. 58:1-18.
Sugihara, G. and R.M. May. 1990. Nonlinear forecasting as a way of distinguishing chaos from measurement error in time series. Nature 344:734-741.