UNDERSTANDING THE DYNAMICS OF NATURE:  Cumulative Effects, Lag Periods, and Thresholds
Chris Maser

Nature, which has only intrinsic value, allows each component of an ecosystem to develop its prescribed structure, carry out its ecological function, and interact with other components through their evolved, interdependent processes and self-reinforcing feedback loops. No component is more or less important than another; each may differ from the other in form, but all are complementary in function.


A vine and epiphytic ferns in southern Malaysia, each serving a different function.

Our intellectual challenge is recognizing that no given factor can be singled out as the sole cause of anything. All things operate synergistically as cumulative effects that exhibit a lag period before fully manifesting themselves. Cumulative effects, which encompass many little, inherent novelties, cannot be understood statistically because ecological relationships are far more complex and far less predictable than our statistical models lead us to believe. Our inability to recognize cumulative effects arises, in part, because we live in the invisible present.

The invisible present is our inability to stand at a given point in time and see the small, seemingly innocuous effects of our actions as they accumulate over weeks, months, and years.1 Consider that all of us can sense change—day becoming night, night turning into day, a hot summer changing into a cold winter, and so on. Some people who live for a long time in one place can see longer-term events and remember the year of the exceptionally deep snow when pronghorn antelope starved to death on the prairie or the year of the intense grass fires.

In spite of such a gift, only unusual people can sense, with any degree of precision, the changes that occur over the decades of their lives. At this scale of time we tend to think of the world as being in some sort of steady state (with the exception of technology), and we typically underestimate the degree to which change has occurred—like the gradually growing din of traffic as your town's population increases over time. We are unable to directly sense slow changes, and we are even more limited in our abilities to interpret the relationships of cause and effect in these changes. Hence, the subtle processes that act quietly and unobtrusively over decades reside cloaked in the invisible present, such as gradual declines in habitat quality.

Changes in land-use patterns, which alter habitats, often have a delayed, negative effect on the species occupying the habitat; an effect may not be recognized for years, as demonstrated by a study of the cooperatively breeding acorn woodpecker. The study was conducted in Water Canyon, which is located in the Magdalena Mountains of south-central New Mexico.

Acorn woodpeckers rely on self-constructed sites in "granary trees" to store acorns for use during winter and spring. Most granaries, which consist of precisely made holes, are in dead trunks and limbs of narrow-leafed cottonwood trees. Groups of woodpeckers with large storage facilities, which equate to high-quality territories, have greater reproductive success and better rates of survival than do pairs or groups with poorly developed storage sites.

In the study, most territories, which had contained birds in 1985, were unoccupied by 1995. This drastic decline was associated with the gradual collapse of individual granary trees in the invisible present, a collapse that ultimately resulted in the loss of nearly all the large storage facilities. The lack of new, high-quality granaries from 1975 to 1995 was probably due in part to the distinctly bimodal age structure of the cottonwood trees—nearly all of which were either very young or old. The scarcity of middle-aged trees reflected a period of intensive grazing by cattle, during which production of young cottonwoods was suppressed because cattle tend to eat them.2

Hidden within this scale of time (1975 to 1995) are the interwoven threads of cumulative effects, lag periods, thresholds, and the various degrees of irreversibility. To understand the dynamics of cause and effect, we will visit the Serengeti-Mara ecosystem, which is a component of the Serengeti Plain in Africa. Here, long-term ecosystem monitoring has highlighted the following complex of interactions.

The limitation of food has clear effects on regulating the populations of wildlife, particularly the migratory wildebeest and the non-migratory cape buffalo. In turn, predation limits populations of smaller resident ungulates and small carnivores. Thus, systems can be self-regulating through the dynamics of food availability and predator-prey interactions. Interactions between African elephants and their food, for example, both allow and maintain the coexistence of savanna and grassland communities. However, with increased woodland vegetation, predators' success in capturing their prey increases. Under these circumstances, artificially regulating a population's size may not be required. In addition, periodic physical events, such as droughts and floods, create disturbances that affect the survivorship of ungulates and birds through feedbacks among the three spheres—atmosphere (air), hydro-lithosphere (the continents and the water surrounding them), and biosphere (the area inhabitated by living organisims).

In any case, slow and rapid changes of different spatial scales that initiate and maintain multiple conditions within an ecosystem become apparent only over several decades; hence there may be no a priori need to maintain one particular state. Beyond that, anthropogenic disturbances have direct (hunting) effects and indirect (transfer of disease) effects on wildlife. Therefore, conservation must accommodate both infrequent and unpredictable events and long-term trends by planning for the time scale of those events—without aiming to maintain the status quo.3

At length, cumulative effects, gathering themselves below our level of conscious awareness, suddenly become visible. By then, it is too late to retract our decisions and actions—even if the outcome they cause is decidedly negative with respect to our intentions. So it is that the cumulative effects of our activities multiply unnoticed until something in the environment shifts dramatically enough for us to see the effects through casual observation. That shift is defined by a threshold of tolerance in the ecosystem, beyond which the system as we knew it, suddenly, visibly, becomes something else.

In Africa, for example, the decline in the geographical distribution and numbers of elephants, as a result of expanding human activity, is recognized as one of the continent's serious conservation problems. Elephants and people coexist at various levels of human density, but when a threshold in human numbers and activity is reached, elephant populations disappear. This threshold is apparently related to a particular stage in the process of land being converted to agriculture, a situation in which farms become spatially dominant over the woodland that constitutes elephant habitat.4

However, if the African people had wanted to protect the elephants from extirpation in a particular area, repair of the elephant's habitat would need to have been initiated long before the threshold of vulnerability was reached. If, at this stage, habitat loss has seriously eroded the demographic potential of the species, halting the decline in population is limited more by demographic factors than by the amount of available habitat. Under this circumstance, it is not enough to conclude that mending the habitat will be sufficient to rescue a declining population.5 Why? One reason has to do with our limited understanding of the effects of spatial scale on habitat quality.

Grassland outside of Steamboat Springs, Colorado.

Consider, for example, that although habitat edges are a ubiquitous feature of modern, fragmented landscapes (such as remnants forests, grassland, prairies, and coastal marshes), researchers have an abiding tendency to focus their sampling designs on relatively small spatial scales. Their findings, therefore, can elucidate the influence of edge effects on animal communities over distances of only 66-820 feet (20-250 meters), whereas edge effects can penetrate as far as six-tenths of a mile (1 kilometer) into a habitat patch. Such large-scale edge effects can lead to an 80 percent reduction in the population size of interior species, even in very large habitat fragments.

In fact, edge effects of this magnitude can drive the interior-dwelling species to local extinction, whether in a remnant of prairie or forest. With respect to forests, a global analysis of protected areas suggests that edge effects of six-tenths of a mile may compromise the ability of more than three-quarters of the world's forested reserves to maintain the uniqueness of their interior community structures.6

If, therefore, we are to have a landscape with a desirable legacy to pass forward to the next generation as a proxy for all generations, we must protect existing biological, genetic, and functional diversity—including habitats and plant-community types—to foster the long-term ecological wholeness and biological richness of the patterns we create. Thus, to repair the commons, or anything else for that matter, we have to understand and honor the integration of it components.


  1. John J. Magnuson. Long-term ecological research and the invisible present. BioScience, 40 (1990):495-501.

  2. The preceding three paragraphs are based on: J. David Ligon and Peter B. Stacey. Land Use, Lag Times and the Detection of Demographic Change: The Case of the Acorn Woodpecker. Conservation Biology,10 (1996): 840-846.

  3. The previous two paragraphs are based on A.R.E. Sinclair, Simon A. R. Mduma, J. Grant, and others. Long-Term Ecosystem Dynamics in the Serengeti: Lessons for Conservation. Conservation Biology,21 (2007): 580-590.

  4. Richard E. Hoare and Johan T. Du Toit. Coexistence between People and Elephants in African Savannas. Conservation Biology,13 (1999): 633-639.

  5. Gregory R. Schrott, Kimberly A. With, and Anthony W. King. Demographic Limitations of the Ability of Habitat Restoration to Rescue Declining Populations. Conservation Biology,19 (2005): 1181-1193.

  6. The preceding two paragraphs are based on Robert M. Ewers and Raphael K. Didham. Pervasive Impact of Large-Scale Edge Effects on a Beetle Community. Proceedings of the National Academy of Sciences, 105 (2008): 5426-5429.

Sunrise is a magnificent symbol of the commons and a promise for tomorrow.

©Chris Maser 2009. All rights reserved.

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