The biosphere
It is time to switch gears and once again, to cross from the micro scale of cells and organisms to the mega scale, in this case the biosphere.
Biomass and its evolution
The biosphere is often considered in terms of an abstract system of interactions (between populations, between life and its environment...); in terms of its direct physical imprint, it is simply the sum-total of all living organisms’ bodies: the biomass,
Quantitavely, the total biomass stands at roughly 560 billion tons of carbon (\(\text{Gt C}\) ). We have previously noted that eukaryotes curently represent a 480 \(\text{Gt C}\) biomass, which only leaves about 15% to prokaryotes. The eukaryotic biomass is completely dominated by plants (450 \(\text{Gt C}\)), of which land plants represent more than 99%. Fungi and protists bring respectively 12 \(\text{Gt C}\) and 4 \(\text{Gt C}\). For all their ecological prominence and cultural salience, animals contribute only 3 \(\text{Gt C}\) (around 0.5% of the earth’s biomass), among which we find 1.4 from insects, 1.1 from fish and less than 0.3 from mammals. From that paltry fraction, only 5% come from wild mammals — the remaining 95% can be found in humans and their livestock).
This distribution is not constant of course. On the shot term, global extinction events have sometimes reduced the biomass by as much as 80% on a relatively short timeframe (from a geological perspective). There are also long-term trends, since the resounding evolutionary success of land plants has led to their current dominance.
From populations to the biosphere
The biosphere is not uniform. The living world is incredibly diverse, and can be organized into a hierarchy of nested levels, each with its own characteristic dynamics.
A population is a group of individuals of the same species living in the same area. Individuals are considered part of the same species if they can interbreed and produce fertile offspring. In genetic and evolutionary terms, this normally implies having a vast share of the gene pool in common, and a common ancestor that is extremely proximate. Members of the same species are then phenotypically close and tend to occupy the same ecological niche, which makes populations the basic units of ecological systems.
A biocenosis is the set of all populations living in a particular area and interacting with one another. The nature of these interactions is worth expounding with a bit more details. We can classify them based on relative benefit/detriment:
Mutualism designates interactions in which both species benefit. An ubiquitous example is mycorrhizae: associations between fungi and plant roots. Over 90% of land plants form this type of association, where they receive water and mineral nutrients from fungi, whose mycelium is much more efficient at extracting them from the soil, in exchange from sugars produced through photosynthesis. Another striking case, taking the concept to its limit is lichen, which is in fact the composite of fungi and small photosynthetic partnera like algae or cyanobacteria. Lichens are extremely successful survivors, very often the first and last living entities to occupy hostile environments like deserts, tundra, or the exterior of human buildings, whereas either of their components alone could not make it.
Parasitism is an interaction in which one species (the parasite) benefits at the expense of the other (the host). Parasites include organisms from virtually every major group, as do hosts, and the evolutionary arms race between them is one of the most potent drivers of evolution.
Often, parasitism does not accelerate death, since the parasite benefits from the host staying alive. However, there is an important case when it does: predation, which in a broad sense means the consumption of an organism by another: it includes herbivory (the consumption of plants by animals) as well as carnivory (that of animals by animals). The need for prey to espace predators through speed, camouflage, toxins, armor, mimicry, and countless other tactics exercises a formidable selective pressure on the relevant species.Competition occurs when two species use the same limited resource. Competition can be direct (interference competition: one species actively excludes the other) or indirect (both species consume the resource, and the more efficient one gradually outcompetes the other). The exclusion principle states that two species cannot coexist indefinitely on the same limiting resource; one will eventually drive the other to extinction, or force it into a different ecological niche.
Commensalism (resp. amensalism) can designate an interaction where one of the parties benefit (resp. suffer) while the other is unaffected. These are somewhat unstable equilibria, or perhaps examples of Sorites problem, since on closer examination, it must always be possible to find subtle costs or benefits in ostensibly neutral impacts.
We should also mention symbiosis, which if orthogonal to the distinctions above and denotes the case of organism living in close physical association. Endosymbionts live inside a larger organism, while ectosymbiont live on their surface.
Lastly, we should add that interactions are not isolated vertices; they form dense, interconnected webs that belie the apparent simplicity of the few examples mentioned here. The zoocenosis is more colloquially called fauna and the phytocenisis, flora.
A biotope is the non-living physical environment that a biocenosis inhabits: the topography, hydrology, climate, chemistry, of a reasonably uniform area. Together, a biocenosis and its biotope constitute an ecosystem. It is more than the sum of these two parts; ecosystems are characterized by the energy and matter flowing between its inhabitants and the environment. One key network is an ecosystem’s trophic web, which describes, to put it bluntly, "who eats whom". Primary producers occupy the basis of a trophic pyramid and include autotrophs like plants or photosynthetic microbes. Primary consumers (roughly, herbivores), eat them, and are eaten by secondary consumers (carnivores), and so on. Some predators are on the fifth trophic level, like in this chain: plants -> ants -> frogs -> snakes -> eagles. A more precise nomenclature uses three dichotomies:
eaters of plants vs animals.
eaters of live beings vs dead matter (the latter are decomposers).
gatherers (who move from place to place to find food sources) vs miners (who become stationary once they have found a food source, that can sometimes last for their entire lifetime).
Ecosystems have no definite scale; definitions abound, they are mostly delineated in relatively homogeneous groups. Depending on the level of granularity, an old rotting log could be an ecosystem, or a large island could be.
At a larger end of that range, ecosystems can be assembled in groups with similar characteristics according to diverse criteria:
available resources (water, light, carbon, oxygen, nutrients...)
environmental features (climate, soil, geomorphology, radiation, geothermal flux, seasonality...)
disturbance factors (fires, floods, storms, volcanism, quakes...)
biocenosis characteristics (species interactions, trophic structure...)
In turn, ecosystem groups that share major traits like climate, latitude, altitude, soil and vegetation are said to be part of the same biomes. We could classify them as such:
Terrestrial biomes:
Tropical forests
Boreal forests & woodlands
Shrublands & heathlands
Savannas & grasslands
Deserts & steppes
Alpine outcrops, tundras, snowfields & glaciers
Subterranean biomes:
Caves & mineral interiors
Freshwater biomes:
Rivers & streams
Lakes & pools
Marine biomes:
Reefs, sand beds & mud plains
Ocean waters & ocean surface
Deep sea floors & abyss
Transitional biomes:
: wetlands, marshes, bogs & fens
: shores & littorals
: groundwaters & underground waters
: anchialine systems & sea caves
: coastal inlets, estuaries & lagoons
The biosphere is the sum of all and biocenoses: the totality of life on earth. It extends from the deepest ocean trenches, where abyssal creatures eke out a living to the upper atmosphere (which is not connected to any biome, but where bacterial spores drift). It is a thin film of organized matter in the geosphere. The net of interactions between biosphere and geosphere, the sum of all biomes and ecosystems is called the ecosphere.
Life’s impact on the earth system
The biosphere is not a passive passenger on an inorganic planet. It is being continuously shaped by the planet, but in turn it also shapes it:
The atmosphere has been transformed. Photosynthesis from cyanobacteriae, and later from plants, have pumped it full of oxygen, and the resulting ozone layer shields the surface from UV radiation.
The hydrosphere has been altered too. Oceanic chemistry (pH, oxygen content, nutrient concentrations) is largely determined by biological activity, and the precipitation of calcium carbonate by marine organisms has created vast deposits of limestone. On land,terrestrial plants shape the hydrological cycle through transpiration, root-mediated infiltration, and erosion control.
The lithosphere itself bears the marks of life. Soils are a biological product, formed by the acidic weathering of rock by plant roots, fungi and bacteria. Vast deposits of chalk, limestone, coal, and petroleum are purely biological in origin.
More broadly, ife is integrated into the geosphere through the great biogeochemical cycles: the circulation of elements between living organisms and the physical environment:
The carbon cycle: we have seen that via plant dominance in the biomass, life is largely autotrophic: carbon is moved from the atmosphere (\(\text{CO}_2\)) into organisms via photosynthesis, and is sent back to the atmosphere via respiration, decomposition, or combustion. On longer timescales, it can be buried in sediments, forming fossil fuels and carbonate rocks, with a possible return to the atmosphere via volcanic outgassing (or, now, human activities).
The nitrogen cycle: atmospheric \(\text{N}_2\) is fixed into biologically available forms (ammonia, nitrate) by certain prokaryotes, incorporated into essential biomolecules (amino acids, nucleotides), recycled through food webs and general biological interactions, eventually returning to the atmosphere through denitrifying bacteria.
The phosphorus cycle: unlike carbon and nitrogen, phosphorus has no significant atmospheric presence. It is weathered from rocks, absorbed by organisms, cycled through food webs, and eventually deposited in oceanic sediments.
The sulfur cycle: sulfur rotates between the atmosphere (as \(\text{SO}_2\) from volcanoes and biological emissions), the ocean (as sulfate), sediments (as pyrite and other minerals), and organisms (in amino acids like cysteine and methionine).