Information to review
To begin to understand the balance of nature, a few concepts we have not yet considered are useful. However, we will start with a question. In the previous submodule on growth we saw that organisms are endowed with the capacity for exponential growth. How can nature achieve balance if each living organism is capable of taking over the environment? In this respect, balance and stability are similar concepts. Some kind of restraint is necessary to orchestrate the balance among all the different organism making up the ecosystem. We considered negative feedbacks, a general concept about restraint, without going into more detail. Certainly negative feedbacks play an important role in achieving balance, but an evolved "self-restraint"is also part of the balance of nature. As you might expect, this simple idea of the balance of nature, is a very complex process. We look at some aspects of this complexity below.
Stability and metastability
In thinking about nature being balanced, we need to understand what this might actually mean in the temporal dynamics of an ecosystem (and landscape, but we have not discussed landscapes yet). In chemistry or physics, a stable state reflects the balance between processes (think of two for simplicity). An example might be the balance between the precipitation of a substance and its dissolution. We can also think about this as the equilibrium condition. In the case of an organism, stability might refer to the balance between the input of food and the output of energy (work), leaving the organism healthy. Too little input would lead to starvation. However, as soon as life gets involved, this idea of stability begins to get complex. An organism can survive within a wide range of food input by regulating the output of energy. In addition, this idea of stability has a temporal dynamic as well. A bear could be gaining weight (out of balance) in the summer and fall, in order to lose weight during the winter hibernation (also out of balance). This short-term lack of stability is no threat to the organism and is a normal part of its existence. This longer term condition implies a different kind of stability, which could be disrupted if Fall season food supplies changed or hibernation habitats decreased in quality (e.g. were colder requiring more stored energy). The longer term "stability" could be labeled "metastability." However, be forewarned that different disciplines, and even different people in the same discipline, use this term inconsistently. See multiple definitions <here >.
The key to thinking about biological or ecological stability then may lie in thinking about how adapted the organism or ecosystem is to change, especially regular or predictable change. The "steady as she goes" stability in ecological systems is related to a concept of "resistance." Thus the oak tree is resistant to strong winds because of the strength of its dense wood fibers. The stability imparted through "resilience" is conveyed through adaptations that rapidly bring back the system into balance. Thus a trembling aspen may lose many of its light weight small branches and twigs in a windstorm, but these can reappear rapidly, even in the same season, due to both a rapid rate of growth and an indeterminate pattern of branch growth. As you might expect, given the million years of evolution, organisms and ecosystems have discovered many strategies to achieve both "resistance" and "resilience." Both impart stability. An interesting example to read about involves the globally threatened coral reefs:
In some cases departures from ecological stability may be easy to observe, in other cases a bit more difficult to detect, especially in the short term. Thinking aabout resistance and resilience can provide some insight into these differences. If strategies for resistance are overcome by changing conditions, you may be able to easily observe this. The fallen oak tree may take many centuries to recover. The shallow water, heat tolerant, coral may die when exposed to extreme heat, and then take many years to recolonize. Ecosystems are often very resilient to seeming catastrophic disturbance. For example, when in 1998, a severe ice-storm hit the Northeast and Canada, many trees lost many of their main branches.
The ground was covered with downed debris and it looked like the aftermath of a tree harvest in the forest. Three years later you could detect the storm by careful inspection of the forest floor, but the canopy was filling nicely in most forests. 10 years later, only those familiar with the normal forest floor or tree branching structure could see the signs of this disturbance. Humans have taken advantage of ecosystem resilience in many ways, Tree harvest is usually followed by vigorous regrowth, with full canopy coverage often taking only 3-5 years (optimizing photosynthesis and renewed productivity). This resilience, however, can be misleading over a period of frequent harvests, with repeated recovery not reaching original levels of productivity. Depletion of the nutrient pool, especially nitrogen, could cause the initial resilient response, to "run out of steam." Thus, the signs of this deficit may not be obvious for many years after the tree harvest.
In one view, the organism in the ecosystem have co-evolved over many years to create a "stable" system. This stability, as discussed above, is in part a steady-state (somewhat related to resistance), and in part a "metastable state" (somewhat related to resilience). The
net result at the level of the ecosystem is a complex and dynamic balance. This includes a co-evolved balance among producers, all competing for light, water, nutrients, and space; a balance between producers and consumers, e.g. avoiding overconsumption which would lead to the decline and/or elimination of the producer; a balance between predators and prey, again, without eliminating prey, which would in turn starve out the predator; a balance between host and parasite, without the host being completely resistant and the parasite not being too harmful to the fitness of the host; a balance between flowers and pollinators, where there are not too many pollinators and not enough food; a balance between seed production and consumer/dispersers, which avoids the complete consumption of all the seeds produced and also avoids the starvation of the consumer. This collective stability includes r - selection strategies as well as K - selection strategies. The patterns of stability encompass a complex food web where there is overlap in plant/herbivore, predator/prey, and resource competition.
Bottom-up and top-down "control"
Ecologist wonder how this balance of nature works. What, if any, are the primary mechanisms that can impart stability to the system? The evolution of stability clearly involves genetic selection at the level of the individual species at its local populations, but how does this translate to adaptations for stability at the level of the ecosystem? How can this information imparting stability be stored, for example after a catastrophe or human clear-cut, to the subsequent ecosystem. In contrast, some might argue that there is no inherent stability at all at the ecosystem level. For example, a forest could be seen just a slowly responding system given the slow rates of change in trees, shrubs, and perennial undergrowth. Given a human perception of time, this slow and unstable change, is interpreted as stability. The longer-term outcomes in species composition are just the cumulative result of individual species' response to the abiotic and biotic environment, and these outcomes can be quite variable (unstable) as a result of small changes.
Source: Thirteen Years of Shifting Top-Down and Bottom-Up Control
PETER L. MESERVE, DOUGLAS A. KELT, W. BRYAN MILSTEAD, AND JULIO R. GUTIЙRREZ
2003. BioScience (July) Vol.53 No.7 pp. 633-646.
Given the diversity of ecosystems on the planet, it might be naive to think that the "balance of nature" has a single (or few) universal mechanistic explanations. Two alternative views are that the ecosystem is controlled "bottom-up" or conversely, "top-down." Perhaps more than explanation of how the ecosystem works, these two concepts suggest examining ecosystem dynamics in two ways. As an example of bottom-up control, we might envision an aquatic ecosystem (its productivity and species composition) being controlled by the algal community. If it is in a season with optimal light and nutrients, it stimulates a bloom which can feed a healthy and diverse food chain. However, if either becomes limiting, the food chain drastically simplifies and loses significant productivity. With top-down control, a predator like a wolf, may control the density of deer, which in turn controls the differential seedling success of trees in the forest. Because feeding at the top (wolf on deer) has consequences for feeding at the bottom (deer on saplings), this phenomena is also referred to as a "trophic cascade ." EoE is a resource for further explanation of these concepts and the stability that they might (or might not) impart to the ecosystem.
Co-evolution of organism and environment, Gaia
At the broadest scale, that of the entire Earth, concepts have emerged in the last few decades that attempt to understand the balance of nature for the whole Earth. The Gaia Hypothesis captures this notion in an ecological context, but it also transcends ecology into philosophy and ethics. Sticking with ecological aspects of the notion of Gaia, this concepts suggests the whole Earth achieves biogeochemical balance through system feedbacks that have evolved over the billion+ year lifespan of the planet. This planetary evolution has resulted in a stable oxidative environment with relatively stable atmospheric constituents. Given the chemically unstable state of an oxidative environment, this should not be occurring unless there is some kind of planetary mechanism to maintain atmospheric stability over these extremely long periods of time. Hence the invocation of Gaia as the planetary control system. At this point in the development of the theory, there are only many hypotheses as to how the Earth actually maintains homeostatic control over important global processes. Very active work on understanding global carbon cycling driven by growth concern about climate change, may soon improve our insights into planetary stability and the processes governing it.
Study questions to respond to on the class blog
1) Think about a local system (e.g. some aspect of your household, your bank account, the city coffers, your backyard, your compost pile, a nearby stream, etc.) and draw a diagram that will help you describe it as a dynamic system, with inputs and outputs, storage, and controls and feedbacks on each part. Then use words to describe the stability or instability in the system and how it works, and where it will be at some future point in time.
2) Reflect on controls that may operate to govern human stability on the planet.
Please go to the class blog to consider these questions.