All known life on earth resides in the thin layer enveloping the globe known as the ecosphere. This region extends from sea level to ~10km into the ocean depths and approximately the same distance up into the atmosphere. It is so thin that if an apple were enlarged to the size of the earth the ecosphere would be thinner than the peel. Yet a vast and complex biodiversity has arisen in this region. Furthermore, the ecosphere acts as integrator of abiotic factors on the planet accumulating in disproportionate quantities particular elements favored by the biosphere (Table 6.1). In particular, note that carbon is not readily abundant in the abiotic spheres yet is highly concentrated in the biosphere, where nitrogen, silicon, and aluminum, while largely available, are mostly unincorporated.
However, even in this limited domain the conditions for living organisms may vary enormously in time and space. The climatic conditions:
(1) The temperature can vary from--70 to ~55 centigrade.
(2) The wind speed can vary from 0km/h to several hundred km/h.
(3) The humidity may vary from almost 0-100 percent.
(4) The precipitation from a few millimeter in average per year to several meter per year, which may or may not be seasonally aligned.
(5) Annual variation in day length according to latitude.
(6) Unpredictable extreme events such as tornadoes, hurricanes, earthquakes, tsunamis, and volcanic eruptions.
Table 6.1 Percent composition spheres for five most important elements
Oxygen 62.5 Nitrogen 78.3
Silicon 21.22 Oxygen 21.0
Aluminum 6.47 Argon 0.93
Hydrogen 2.92 Carbon 0.03
Sodium 2.64 Neon 0.002
Sodium 0.28 Magnesium 0.03
The physical-chemical environmental conditions:
(2) Salt concentrations (it is important both for terrestrial and aquatic ecosystems)
(3) Presence or absence of toxic compounds, whether they are natural or anthropogenic in origin
(4) Rate of currents in aquatic ecosystems and hydraulic conductivity for soil
(5) Space requirements
The biological conditions:
(1) The concentrations of food for herbivore, carnivore, and omnivore organisms
(2) The density of predators
(3) The density of competitors for the resources (food, space, etc.)
(4) The concentrations of pollinators, symbiants, and mutualists
(5) The density of decomposers
The human impact on natural ecosystems today adds to this complexity.
The list of factors determining the life conditions is much longer—we have only mentioned the most important factors. In addition, the ecosystems have history or path dependency (see Chapter 5), meaning that the initial conditions determine the possibilities of development. If we modestly assume that 100 factors are defining the life conditions and each of these 100 factors may be on 100 different levels, then 10200 different life conditions are possible, which can be compared with the number of elementary particle in the Universe 1081 (see also Chapter 3). The confluence of path dependency and an astronomical number of combinations affirms that the ecosphere could not experience the entire range of possible states, otherwise known as non-ergodicity. Furthermore, its irreversibility ensures that it cannot track back to other possible configurations. In addition to these combinations, the formation of ecological networks (see Chapter 5) means that the number of indirect effects are magnitudes higher than the direct ones and they are not negligible, on the contrary, they are often more significant than the direct ones, as discussed in Chapter 5.
What is the result of this enormous variability in the natural life conditions? We have found ~0.5 X 107 species on earth and it is presumed that the number of species is double or 107. They have developed all types of mechanisms to live under the most varied life conditions including ones at the margin of their physiological limits. They have developed defense mechanisms. For example, some plants are toxic to avoid grazing, others have thorns, etc. Animals have developed horns, camouflage pattern, well-developed auditory sense, fast escaping rate, etc. They have furthermore developed integration mechanisms; fitting into their local web of life, often complementing and creating their environmental niche. The multiplicity of the life forms is inconceivable.
The number of species may be 107, but living organisms are all different. An ecosystem has normally from 1015 to 1020 individual organisms that are all different, which although it is a lot, makes ecosystems middle number systems. This means that the number of organisms is magnitudes less than the number of atoms in a room, but all the organisms, opposite the atoms in the rooms, have individual characteristics. Whereas large number systems such as the number of atoms in a room are amenable to statistical mechanics and small number problems such as planetary systems to classical mechanics or individual based modeling, middle number problems contain their own set of challenges. For one thing this variation, within and among species, provides diversity through co-adaptation and co-evolution, which is central both to Darwinian selection and network aggradation.
The competitive exclusion principle (Gause, 1934) claims that when two or more species are competing about the same limited resource only the best one will survive. The contrast between this principle and the number of species has for long time been a paradox. The explanation is rooted in the enormous variability in time and space of the conditions and in the variability of a wide spectrum of species' properties. A competition model, where three or more resources are limiting gives a result very different from the case where one or two resources are limiting. Due to significant fluctuations in the different resources it is prevented that one species would be dominant and the model demonstrates that many species competing about the same spectrum of resources can coexist. It is, therefore, not surprising that there exists many species in an environment characterized by an enormous variation of abiotic and biotic factors.
To summarize the number of different life forms is enormous because there are a great number of both challenges and opportunities. The complexity of ecosystem dynamics is rooted in these two incomprehensible types of variability.
Was this article helpful?