The Normality Of Disturbance

Up to this point, the focus of this book has been on growth and development processes in ecosystems. In fact, these are most important features of ecosystem dynamics and they provide the origins of various emergent ecosystem properties. But the picture remains incomplete if disturbance and decay are not taken into account. On the following pages we will try to include those "destructive" processes into the "new" ecosystem theory as elaborated in this book. As a starting point for these discussions we can refer to common knowledge and emotion, as it is described in the poem of Andreas Gryphius (see above) who outlines the transience of human and environmental structures: Nothing lasts forever, towns will turn into meadows, flourishing nature can easily be destroyed, our luck can turn into misfortune, and in the end, what remains is emptiness, shadow, dust and wind. Although the poet seems to be comprehensible concerning the significance of decay, we cannot agree with his pessimistic ultimate: In the end, the death of organisms and disturbance of ecosystems can be useful elements of the growth, development and survival of the whole structure, i.e. if they expire within suitable thresholds and if we observe their outcomes over multiple scales.

On a small scale, we can notice that the individual living components of ecosystems have limited life spans that range from minutes to millennia (see Table 7.1). Death and decay of organisms and their subsystems are integral elements of natural dynamics. From a functional viewpoint, these processes are advantageous, to replace highly loaded or exhausted components (e.g., short life expectancies of some animal cells), or to adjust physiologies to changing environmental conditions (e.g., leaf litter fall in autumn). As a consequence of these processes, energy and nutrients are provided for the saprophagous branches of food webs, which in many cases show higher turnover rates than the phytophagous branches of the energy and nutrient flow networks. In those situations of death self-organized units give up their autonomy and their ability to capture and actively transform exergy, their structures are subject to dissipation. Reactivity, self-regulation, and the ability for replication are desist, releasing the internal order and constituents which thus potentially become ingredients of the higher system-level self-organization (see Chapter 3 "Ecosystems have Ontic Openness").

Also populations have limited durations at certain places on earth. Operating in a hierarchy of constraints, populations break down, e.g., if the exterior conditions are modified, if imperative resources are depleted, if the living conditions are modified by human actions, or if competition processes result in a change of the community assemblage. Following the thermodynamic argumentation of this book (see Chapters 2 and 6), in these situations a modified collection of organisms will take over, being able to increase the internal flows

Table 7.1 Some data about life expectancies of cells and organisms

Example

Average life span

Generation time of E. coli Life spans of some human cells Small intestine White blood cells Stomach Liver

Life span of some animals Water flea Mouse Nightingale

20min

1-2 days

1-3 days

2-9 days

12-20years 20-40 years 177 years

0.2 years 3-4 years 4 years

10-20 days

Horse

Giant tortoise Life span of some plants Sun flower Corylus avellana Fagus sylvatica Pinus aristata

1 year 4-10 years 200-300 years 4900 years and to reduce the energetic, material, and structural losses into the environment in a greater quantity than the predecessors. During such processes, of course, only the very immediate conditions can be influential: The developmental direction is defined due to a short-term reaction, which increases orientor values at the moment the decision is made, on the basis of the disposable elements and the prevailing conditions. Thereafter, the structural fate of the system is predefined by new constraints; an irreversible reaction has taken place, and the sustainability of this pathway will be an object of the following successional processes.

Of course, such community dynamics have consequences for the abiotic processes and structures. Therefore, also ecosystems themselves exist for a limited period of time only. Their typical structural and organizational features are modified, not only if the external conditions change significantly, but also if due to internal competition processes certain elements attain dominance displacing other species. These processes can be observed on many different scales with distinct temporal characteristics—slow processes can occur as results of climatic changes (e.g., postglacial successions throughout the Holocene), shifts of biomes (e.g., Pleistocene dynamics of rain forests), or continuous invasions of new species. On the contrary, abrupt processes often modify ecosystems very efficiently within rather short periods of time.

The most commonly known extreme event has taken place at the end of the Cretaceous age, 65 million years ago, when—purportedly due to an asteroid impact—enormous changes of the global community structures took place, no organism bigger than 25 kg survived on land: planktonic foraminifera went extinct by 83%, the extinctions of ammonites reached 100%, marine reptiles were affected by 93%, and the nonavian dinosaurs were driven totally extinct. No doubt, this was a big loss of biodiversity, and many potential evolutionary pathways disappeared; but, as we know 65 million years later, this event was also a starting shot for new evolutionary traits and for the occupation of the niches by new species, e.g., for the rapid development of mammals or organisms which are able to read or write books (see Box 7.1).

Box 7.1 Creativity needs disturbance

Necessity is the mother of invention.

Constraints mean problems in the first hand, but problems require solutions, and (new) solutions require creativity. Let us exemplify this by evolutionary processes, the genetic code and language. The constraints in the chemical beginning of the evolution were that whenever a primitive but relatively well-functioning assemblage of organic molecules was formed, the composition that made the entity successful was forgotten with its breakdown. The next entity would have to start from scratch again. If at least the major part of the well-functioning composition could be remembered, then the entities would be able to improve their composition and processes generation by generation.

For organisms the problem is to survive. When new living conditions are emerging the accompanying problems for the phenotypes are solved by new properties of the genotypes or their interactions in the ecological networks. The survival based on the two

(continued)

growth forms "biomass growth" and "network growth" are ensured by adaptation to the currently changed prevailing conditions for life. But information growth is needed, too, because survival under new emergent conditions requires a system to transfer information to make sure that solutions are not lost. These problems on the need for information transfer have been solved by development of a genetic system that again put new constraints on survival. It is only possible to ensure survival in the light of the competition by use of the adopted genetic system. But the genes have also created new possibilities because mutations and later in the evolution sexual recombinations create new possible solutions. Therefore, as shown in Figure 7.1 what starts with constraints and new and better properties of the organisms or their ecological networks ends up as new possibilities through a coding system that also may be considered initially as constraints.

An organism's biochemistry is determined by the composition of a series of enzymes that again are determined by the genes. Successful organisms will be able to get more offspring than less successful organisms and as the gene composition is inherited, the successful properties will be more and more represented generation after generation. This explains that the evolution has been directed toward more and more complex organisms that have new and emerging properties.

The genetic code is a language or an alphabet. It is a constraint on the living organisms that have to follow the biochemical code embodied in the genes. The sequence of three amino bases with four possibilities determines the sequence of amino acids in

Figure 7.1 Life conditions are currently changed and have a high variability in time and space. This creates new challenges (problems) to survival. Organisms adapt or a shift to other species takes place. This requires an information system that is able to transfer the information about good solutions to the coming generations of organisms. Consequently, an information system is very beneficial, but it has to be considered as a new source of constraints that however can open up for new possibilities.

Figure 7.1 Life conditions are currently changed and have a high variability in time and space. This creates new challenges (problems) to survival. Organisms adapt or a shift to other species takes place. This requires an information system that is able to transfer the information about good solutions to the coming generations of organisms. Consequently, an information system is very beneficial, but it has to be considered as a new source of constraints that however can open up for new possibilities.

the proteins. There are, in other words, 4 X 4 X 4 = 64 different codings of the three amino bases; but as there are only 20 amino acids to select from, it contains amino base coding redundant amino base coding combinations in the sense that for some amino acids two or more combinations of amino bases are valid. As an alphabet is a constraint for an author (he has to learn it and he is forced to use it if he wants to express his thoughts), the genetic code is a constraint for the living organism. But as the alphabet gives a writer almost unlimited opportunities to express thoughts and feelings, so the genetic code has given the living organisms opportunity to evolve, becoming more and more complex, more and more creative, having more and more connectivity among the components and becoming more and more adaptive to the constraints that are steadily varying in time and space. The genetic code, however, has not only solved the problem associated with these constraints, but it has also been able to give the living organisms new emergent properties and has enhanced the evolution.

When the human language was created a couple of millions years ago, it first provided new constraints for humans. They had to learn the language and use it, but once they have mastered the language it also gave new opportunities because it made it possible to discuss cooperation and a detailed better hunting strategy, e.g., which would increase the possibility for survival. The written language was developed to solve the problem of making the message transfer more independent of time and space. To learn to write and read were new constraints to humans that also open up many new possibilities of expressing new ideas and thoughts and thereby move further away from thermodynamic equilibrium.

Animals also communicate through sounds or chemicals for warnings, for instance by marking of hunting territories by urine. The use of these signals has most likely been a factor that has reduced the mortality and increased the change of survival.

We will use a numeric example to illustrate the enormous evolutionary power of the genes to transfer information from generation to generation. If a chimpanzee would try to write this book by randomly using a computer key board, the chimpanzee would not have been able to write the book even if he started at the big bang 15 billion years ago, but if we could save the signs that were correct for the second round and so on, then 1/40 of the volume would be correct in the first round (assuming 40 different signs), (39 X 39)/(40 X 40) would still be incorrect after the second round, (39 X 39 X 39)/(40 X 40 X 40) after the third round and so on. After 500 rounds, which may take a few years, there would only be 5 "printed" errors left, if we presume that this book contains 500,000 signs. To write one round of the volume would probably require 500,000 s or about a week. To make 500 rounds would there take about 500 weeks or about 9 years.

The variation in time and space of the conditions for living organism has been an enormous challenge to life because it has required the development of a wide range of organisms. The living nature has met the challenge by creation of an enormous differentiation. There are five million known species on earth and we are currently finding new species. It is estimated that the earth has about 107 species. We see the same pattern as we have seen for the genetic constraints: The constraints are a challenge for the living nature, but the solution gives new emergent possibilities with an unexpected creative power.

Table 7.2 shows that there have been several extinction events during the history of the Earth. An interesting hypothesis concerning global extinction rates was published by Raup and Sepkoski (1986). The authors have observed the development of families of marine animals during the last 250 million years. The result, which is still discussed very critically in paleontology, was that mass extinction events seem to have occurred at rather regular temporal intervals of approximately 26 million years. Explanations were discussed as astronomic forces that might operate with rather precise schedules, as well as terrestrial events (e.g., volcanism, glaciation, sea level change). We will have to wait for further investigations to see whether this hypothesis has been too daring.

Today we can use these ideas to rank the risk of perturbations in relation to their temporal characteristics. While mass extinctions seem to be rather rare (Table 7.2), smaller perturbations can appear more frequently (Figure 7.2). In hydrology, floods are distinguished due the temporal probability of their occurrence: 10-, 100-, and 1000-year events are not only characterized by their typical probabilities (translated into typical frequencies), but also by their extents. The rarer the event is, the higher is the risk of the provoked damages. A 100-year flood will result in bigger disturbances than a 10-year event. Also the effects of other disturbance types can be ordered due to their "typical frequencies" (Table 7.3). An often discussed example is fire. The longer the period between two

Table 7.2 Five significant mass extinctions

Geological period

Million years bp

Families lost (%)

Potential reason

Ordovician

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