The Web of Life – Ecosystems

Ecosystems

An ecosystem is an environment with life. While ecosystem analysis emphasizes resident organisms – biota – in a biome, an ecosystem also includes the abiotic (non-living) elements within the area.

The biota in a biome invariably comprise a trophically-tiered community of energy producers and consumers. The health of a biome depends upon a balance of biota, characterized as a web of interrelations.

Geographically, the biosphere is the zone of life on Earth, bounded by space on the outside, and, on the inside, the mantle upon which the oceans and terrain ride. There are 4 elements (bioelements) of the biosphere: atmosphere (air), lithosphere (land), hydrosphere (water), and biota. With the arrival of the last bioelement – biota – the biosphere was born (biogenesis).

Each ecosystem has its own signature. There is a subtle geographic harmonic to a biome, defined at the baseline by abiotic elements, beginning with the geological and climactic characteristics of the region, which shape other bioelements. Biota are affected by the local geographic harmonic which is expressed upon them as an energetic resonance.

The relations between bioelements comprise ecology. As an academic discipline, ecology studies life’s relations as a subdiscipline of biology.

Biodiversity refers to the diversity of life at every level, from genes to ecosystems. Though commonly used as shorthand for species diversity, biodiversity also comprises the diversities of genomes, ecosystems, and ecologies.

Biodiversity is the primary factor in an ecosystem’s self-organized criticality. The greater the biodiversity, the more robust an ecosystem is. Conversely, decreasing biodiversity increases the propensity of an ecosystem to collapse under stress.

A habitat is the environment in which a specific species population lives. There are as many habitats within an ecosystem as there are species. Each habitat is characterized by the ecology of the species – that is, the various interactions a species may have during its life cycle.

Ecology Gyre

In Nature all is harmony, a consonance forever agreed on. ~ Russian poet Fyodor Tyutchev

Abiotic elements provide the substrate for life. Atmospheric and water qualities provide a baseline for an ecosystem. Everything that lives relies upon the air around it and water upon it.

Water quality is critical to life: purity, chemical composition, pH, temperature, salinity, and clarity. For an ecosystem to arise, life must adapt to the water supply.

Biogeochemical cycles are the wheels upon which ecology turns. The organic elements are the crucial characters: carbon, nitrogen, oxygen, phosphorous (essential to cellular structures and processes), and sulfur (a cardinal protein component). Then there is water.

While not alive in the organic sense, the atmosphere, lithosphere, and hydrosphere are all gyres: fluxing with productions, intake and output, and interactions with biota. The recycling of abiotic elements drives the ecology of every ecosystem.

Reproduction and recycling are comparable as a numbers game. To one looking at an ecosystem as a series of snapshots, water’s “population” changes appear mathematically quite like biotic populations in dynamic fluxes. The strong correlation exists because all life relies upon water.

Nitrogen is another dramatic example. Nitrogen’s reluctance to take a biologically-usable form makes nitrogen fixation a highly-prized skill. Balanced recycling is essential. More than any other element, nitrogen availability limits biotic growth potential.

While life certainly affects the water cycle and other abiotic gyres, the converse is not true. Albeit altered, even dramatically, abiotic dynamics continue regardless of life. This is the striking asymmetry between bioelements. In being heavily dependent on a tolerable confluence in other bioelements, biota is the odd element out. Life is the most fragile bioelement.

A 4 °C temperature difference between the Indian and Pacific Oceans 2 million years ago (MYA) shifted rainfall patterns across Africa, drying East Africa. Grasslands replaced woodlands. A gyre of speciation happened: adaptations of plants, grasses, grazing animals, and their predators. Loss of woodlands drove hominid development.

The dynamic interactions of each biospheric element are an enduring dance within each element and with the other elements. Ecology is constantly in flux.

 Soil

Soil lies at the heart of Earth’s ‘critical zone’ – the thin veneer extending from the top of the tree canopy to the bottom of aquifers. ~ American environmental engineer Steve Banwart

For terrestrial ecosystems, soil is the fundamental substrate. Its quality is a key resource for life aboveground.

Soil starts as relatively large lumps that are identical to parent rock in chemical composition. Young soils are variable in their nutritional base.

Microbes are ever the initiators of every ecosystem. Their success and diversity determine the support base for other life. Soil quality reflects this. The smell of rich earth emanates from geosmin (C12H22O) produced by Streptomyces soil bacteria. As an evolutionary adaptation to appreciate soil conditions, the human nose is extremely sensitive to geosmin: able to detect it at concentrations as low as 5 parts per trillion.

The pioneer plants that invade a new soil must tolerate severe conditions. To ease the arduousness, these plants often associate with nitrogen-fixing prokaryotes. Many lichens harbor cyanobacteria which help them establish themselves. Some flowering plants have root nodules that nestle microbial assistance.

In establishing their residence pioneers significantly change the soil. Carbon dioxide from root respiration produces carbonic acid which accelerates chemical weathering.

Dead plants become a substrate for less hardy microbes than those that first landed. Humus forms, greatly increasing a soil’s capacity for holding water. As roots grow and penetrate bedrock, they create cracks, breaking rock apart.

Soil matures into 3 layers, or horizons, unimaginatively labeled by convention as A, B, and C. The uppermost horizon (A) is a leaching layer: organic debris breaks down and is washed downward into the middle layer by rainwater.

Nutrients accumulate in the middle horizon (B), which contains humus and clay. The deepest horizon (C) comprises parent rock and fragments, resembling soil’s earliest times.

Soils become chemically less diverse as they mature. As rock weathers, roots absorb essential elements which become trapped in the plant body. Plant parts slowly recycle these elements into the soil. Nonessential elements leach away.

In processing soil over time, plants are the drivers of ecological chemistry. The composition of the plant community is a harbinger for the fate of a terrestrial ecosystem.

In good soil that is a largely homogeneous substrate, vegetation patterns itself upon water need and availability. As rainfall decreases, plants become sparser until aridity collapses into desert.

Though winds often blow rain clouds from the ocean to deposit their liquid load on lands near shore, there is correlation between soil moisture and locally generated rainfall inland. The process is intricate, but the upshot is that areas naturally tend to retain their humidity level via a self-reinforcing feedback loop.

During the day, the Sun warms the Earth, evoking evaporation from inland lakes and rivers, and from the soil itself. Water vapor rises until it meets colder layers of air and condenses. Then it starts to rain.

The more moisture in the soil, the more water that evaporates, increasing the likelihood of an afternoon shower. In a humid region, areas with lower soil moisture produce the warmest air, facilitating water vapor to rise the highest and meet cooler air layers the soonest. It rains most frequently in these places.

Afternoon precipitation events tend to occur during wet and heterogeneous soil moisture conditions, while being located over comparatively drier patches. ~ Swiss climatologist Benoit Guillod et al

Soil quality and Earth’s atmospheric gas composition are intricately intertwined via several dynamics.

Nitrogen is the nutrient that most often limits rates of plant growth. ~ American ecologist Christine Goodale

Even during summer dry spells, some patches of forest soil remain waterlogged. These catchments are hot spots of microbial activity that removes nitrogen from groundwater and releases it into the atmosphere.

Denitrification (nitrogen removal) typically reduces plant growth and thereby lessens forest productivity. Water retention and drainage affect soil quality and the nitrogen cycle in a surprising way: too much soil moisture during dry times reduces nitrogen, and thereby limits plant vitality.

The acidity of soil determines how much nitrous acid outgasses into the atmosphere or is retained as nitrite. Via nitrification by soil microbes, nitrite (NO2) turns into nitrate (NO3), which is a usable form of nitrogen for plants.

Nitrous acid (HNO2) plays a key role in regulating atmospheric processes. Sunlight breaks nitrous acid down into nitric oxide (NO) and a hydroxyl radical (OH). OH controls the atmospheric lifetime of gases which affect air quality, including catalyzing the chemistry that forms ground-level ozone, a primary component of smog.

Although hydrogen is an abundant element throughout the universe, H2 is present in only trace amounts in Earth’s atmosphere. Certain soil bacteria scavenge it out of the skies for fuel. Soil actinobacteria are the main sink for atmospheric hydrogen. This in turn influences the concentrations of other atmospheric gases, including methane (CH4) and nitrous oxide.

Small invertebrates – especially earthworms, ants, and termites – help keep soil healthy, allowing soil to absorb decayed plant organic matter and thus nourish vegetation. These critters also maintain an ecological balance which limits the incursion of detrimental species.

In contrast, monoculture agriculture destroys natural biodiversity, reducing soil productivity, and allowing infestation of unwelcome pests. The typical response by modern farmers has been to apply chemicals which further degrade soil quality. From a perspective of maintaining a sustainable or depletable resource, human agriculture is stunningly inept.

 Termites

In many parched ecosystems, termite nests serve as oases in the desert. Termites improve soil quality, enhancing plant growth.

We tend to think about large mammals as being the big dominant driver of what’s happening in the savanna, but the more we look at the termite mounds the more they seem to be driving what’s going on. ~ American ecologist Robert Pringle

As termites dig through the ground, the plethora of holes they create allow rain to soak deep into the soil rather than running off or evaporating.

Termites artfully mix particles of sand, stone, and clay with organic bits, including their own feces and other bodily excretions. That helps soil cohere, retain nutrients, and resist erosion.

Termite gut bacteria are avid nitrogen-fixers: able to extract this vital element from the air and convert it into usable fertilizer.

Termites are the ultimate soil engineers. ~ English entomologist David Bignell

 Community Ecology

Ecological networks are complex, with each species typically closely linked to all others, either directly or indirectly. ~ Spanish biologist José Montoya et al

From the base up, the trophic pyramid encapsulates the process by which an ecosystem arises. Producers create opportunities for the initial consumers, and so on up the ladder of predation, to top predators.

A multitude of factors determines the gyre of an ecosystem’s evolution, which is displayed like a moving picture, by a biome’s biodiversity. These factors affect habitat viability: climate, environmental heterogeneity, the arrangement and patchiness of resources, and the nature, distribution, and interactions of biota in their multifarious relationships.

The most abundant few species often numerically dominate communities and play a disproportionately large role in community and ecosystem processes. ~ Australian ecologist Sean Connolly et al

Biodiversity is a complex product of ecosystem ecology. At every scale, from microbial to floral to animal, species differ in their importance to a community.

Ecological balance extends throughout the food web. For instance, loss of apex predators can have an outsized impact on an ecosystem.

Top consumers in the food web are enormous influencers of the structure, function, and biodiversity of most natural ecosystems. ~ American ecologist James Estes

Little is known about ecosystem ecology: neither what makes for optimal dynamics, nor how much stress a community can take before it collapses (self-organized criticality).

 Plant Stress

Plants play a key role in terrestrial ecosystems. Beyond herbivores that directly consume plant matter, 85% of plant production ends up as debris that feeds insects, fungi, and microbial decomposers. Plant death is almost as important as its life.

Plants have defensive responses to various stresses. These include adding toxic chemicals to their leaves and redistributing nutrients, such as lowering the nitrogen content of plant parts that herbivores prefer. The upshot of plant stress is lowering the nutrient content of leaves and branches that comprise the bulk of debris that feeds the minute masses on the ground. Thus, the nutritional quality of leaf litter directly affects the bottom of the food web in nearby land and streams, and indirectly influences the entire heterotrophic food chain in the area.

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Ecosystems exhibit self-organized criticality: able to recover from damaging events, but subject to devastation from conditions that are too adverse for too long.

Rapidity of adaptation has its limits, particularly for more descendant life forms, which are dependent upon the ecological landscape defined by less-elevated life. The fate of the later-evolved relies upon the well-being of more primitive organisms – the evolutionary irony of biotic entanglement.

Though the effect is melodramatic, catastrophic events are rare. More commonly, thriving ecosystems are diminished by incremental quality changes in fundamental abiotic elements: degraded air, water, and/or soil.

With their different natures, the ecologies of microbes, fungi, plants, and animals are dissimilar, even as the dynamic fecundity of an ecosystem is intertwined among all biota. Numerous concepts relating to biotic ecology are known: food webs, tropic chains and loops, resource competition, keystone species, and interspecies relations, in forms both nonsymbiotic and symbiotic. But the holistic gyre is greater than the mere sum of species populations in sustaining a healthy ecosystem.

Biodiversity is an abstract measure of the health of an ecosystem. The baseline for a biome’s potential biodiversity depends upon climate, especially for terrestrial biomes.

Whereas tropical forests are species-rich, polar regions support less diversity. For marine environments, sustained nutrient flow engenders biodiversity. Most notable are the biotic anchor of coral reefs and near cold-water upwellings. Unlike terrestrial life, marine ecosystems thrive near the poles. Vast areas of the ocean near the equator are sparsely populated.

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Different plants have their niches in a forest. Tall trees dominate a competitively created canopy; each tree vying for sunlight.

The understory is another plant community, with similar competition, albeit on a different scale. Plants in the understory are a mixture of ground cover, shrubs, and canopy tree saplings. Young canopy trees may stay as juveniles for decades, awaiting an opening in the forest overstory.

With less direct sunlight, the forest understory is more humid than exposed areas. The higher humidity allows fungi and other decomposers to flourish.

The rich earth drives nutrient cycling, providing a favorable environment for all kinds of organisms, beginning with the microbes and plants which directly rely upon soil quality.

A portion of the understory becoming exposed to direct sunlight from an opening in the canopy creates a cascade of local changes. More generally, a local event, such as fewer herbivores, can have a butterfly effect, and alter the dynamics of the biotic community.

Plants illustrate the difficulty in holistically comprehending the often subtle ecology gyre. For every combination of climate, soil, and altitude, there should be, hypothetically, a single plant species that grows better than others: occupying a greater vegetative spread and producing more seeds. This Darwinian concept is the competitive exclusion principle, which leads to the expectation that plant species richness should be uniformly low.

Instead, plant communities typically comprise numerous species, living side by side, that are nominally competitive. The logic of the competitive exclusion principle is impeccable, but its assumptions are unrealistic.

Environments are neither spatially uniform nor temporally constant. Soil qualities differ. Exposure to weather and other disturbances are not the same. Impacts from pathogens, herbivores, and pollinating agents vary. Then factor in that each plant has different tolerances and strategies.

So, the ecology gyre constantly fluxes for a microbiome from minor events. And spillovers from one microbiome to neighboring ones change conditions in a cascade.

While plants are rooted to a spot at any one time, animals wander about; creating damage to certain plants and opportunities for others; altering the vegetation mix, which affects herbivore foraging in seasons to come. As herbivore populations flux, so too the prospects for predators.

The nuanced balance is illustrated by grasshopper life and death. Grasshoppers constantly stressed by the prospect of predation alter their diet: eating more carbohydrates to meet their heightened energy needs. Consuming less protein means a grasshopper with less nitrogen. A nitrogen-poor corpse is less attractive to soil microbes, which require nitrogen to produce their decomposing enzymes.

The cascade effects of an unbalanced ecosystem can be considerable. Soil fertility affects plant growth, which impacts plant nutrient value to herbivores, on up the food chain.

 Island Biomes

Continental landmasses seem like ecological slowpokes compared to the biotic drama of oceanic islands, which are spawned by volcanoes and die by subduction. These islands sprout, grow, erode, and sink beneath the sea in geological time. Island biomes provide concise character studies of ecology gyres.

In the millions of years between volcanic cradle and watery grave, islands transform, and their tenants turn over time and again. Species ecologically adapt, occupy empty niches, specialize, and become exclusive.

Young islands with high mountains enjoy an increase in endemic ecosystems, with greater biodiversity and more unique species. The more diverse the lithosphere, the more diverse the biota.

The classic example of a speciation surfeit is the Galápagos Islands, owing to the rapid transformation of the volcanic islands there. The Galápagos gained notoriety as the inspirational terrain for English naturalist Charles Darwin, who concocted serial theories of evolutionary descent after visiting, but never credibly explained how evolution works. (See Spokes 3: The Elements of Evolution.)

An important influence in making the Galápagos an evolutionary hotspot is the cold Cromwell current coming from the west. The current brings an upwelling of nutrients that feeds the trophic web in the waters about the islands, cascading to the animals that forage in the sea.