1.2 Systems

Photo: Aurbina, 2004, public domain, via Wikimedia Commons.

Rapa Nui (Easter Island) is one of the most isolated inhabited places on Earth, over 3,500 km from the nearest continent. Pollen records show it was once covered in palm forest. By around 1650 CE, that forest was almost entirely gone, cleared for farming, fuel, canoes and moving the island's enormous stone moai statues.

The popular story says the islanders wiped out their own forest, their population then collapsed, and Europeans arrived in 1722 to find the ruins of a self-destroyed society. What does the current evidence actually show happened to Rapa Nui's population?

Map: Eric Gaba (Sting), Wikimedia Commons Graphic Lab, CC BY-SA 2.5.

A system is a set of interacting or interdependent components, organized so that together they create a functional whole. Rapa Nui's forest, soil, rainfall and human population were never independent of each other: change one and the others responded.

A systems approach is a holistic way of visualizing a complex set of interactions, applicable to ecological situations (a forest, a lake) or societal ones (an economy, a farming community). Every system has storages (also called stores), amounts of matter or energy held within the system at a point in time, and flows, which provide the inputs and outputs of matter or energy across the system's boundary.

In a system diagram, storages are usually drawn as rectangular boxes and flows as arrows, with the direction of each arrow showing the direction of that flow. The size of a box or the thickness of an arrow can represent the relative size or magnitude of that storage or flow.

Photo: Agnieszka Kwiecień, Nova, 2024, CC BY-SA 4.0, via Wikimedia Commons.

Every flow is either a transfer or a transformation. A transfer involves a change in location of energy or matter, with no change in its form. A transformation involves a change in the chemical nature, the physical state, or the energy type of what is moving.

Sort each Rapa Nui example into the right category.

Systems can be open or closed. An open system exchanges both energy and matter across its boundary; a closed system exchanges only energy, not matter. Almost every real system is open, including Rapa Nui's forest and every ecosystem on this site. Only the global geochemical cycles approximate closed systems. Biosphere 2, the sealed Arizona research facility built in the late 1980s to test whether humans could live inside an artificial closed ecological system, is the standard example of a near-closed system in practice.

The Earth itself can be modeled as a single integrated system encompassing the biosphere, hydrosphere, cryosphere, geosphere, atmosphere and anthroposphere (the part of the environment shaped by human activity). James Lovelock's Gaia hypothesis, developed further with Lynn Margulis, models the whole Earth as a self-regulating system in which atmospheric composition and temperature are held within life-sustaining limits through feedback mechanisms, in something like the way a body regulates its own temperature.

The systems concept applies at any scale, from a single bromeliad plant holding a pool of water in a rainforest canopy, to the rainforest that surrounds it, to the whole atmosphere's circulation pattern. Rapa Nui's forest system sits between these: much larger than a bromeliad, far smaller than global atmospheric circulation, but analyzed with exactly the same storages-and-flows toolkit either way.

Negative feedback occurs when the output of a process inhibits or reverses that same process, reducing change and counteracting deviation from equilibrium. A classic illustration is the Daisyworld model (Lovelock and Andrew Watson): an imaginary planet seeded only with white and black daisies. Black daisies absorb heat and warm their surroundings; white daisies reflect heat and cool theirs. As the planet warms, white daisies are favored and spread, reflecting more heat and cooling the planet back down; as it cools, black daisies spread and warm it back up. No daisy "intends" to regulate anything, but the population balance of the two colors stabilizes planetary temperature purely through feedback, illustrating how Gaia-style self-regulation could work without conscious control.

A real ecological example: as prey numbers increase, predator numbers increase in response; more predators then reduce prey numbers again, pulling the system back toward its starting point.

As an open system, an ecosystem will normally exist in a stable equilibrium, a tendency to return to its previous state after a disturbance, maintained by stabilizing negative feedback loops. One important form of this is a steady-state equilibrium: flows are still occurring, but inputs are constantly balanced against outputs, so the system's overall storages stay roughly constant over time. Rapa Nui's forest existed in something close to a steady state before around 1200 CE: growth and seed dispersal (inputs to the storage) roughly balanced natural losses (outputs). It was the balance itself, not the flows, that broke down as human land use intensified.

Positive feedback occurs when a disturbance leads to an amplification of that disturbance, destabilizing the system and driving it further from equilibrium rather than back toward it. A well-known example: melting ice caps expose darker ocean or land beneath them, which absorbs more solar radiation than the reflective ice did (lower albedo), which raises temperatures further and melts still more ice. Positive feedback can also amplify a decrease rather than an increase: as a small population declines, its reproductive potential falls too, which can drive the population down further still, a spiral rather than a self-correction.

Positive feedback loops tend to drive a system toward a tipping point, the minimum amount of change that will destabilize the system enough for it to shift to a new equilibrium or stable state altogether.

A tipping point is where a small alteration in one component can produce a disproportionately large overall change: a regime shift between alternative stable states. A well-documented example is eutrophication: rising nitrate and phosphate concentrations from agricultural runoff can push a lake or coastal zone past a threshold, triggering algal blooms whose decomposition strips the water of oxygen. The Gulf of Mexico's seasonal hypoxic "dead zone," driven by nutrient runoff carried down the Mississippi-Atchafalaya river system, measured about 4,402 square miles in 2025; the long-term federal target is to shrink it to 1,900 square miles by 2035.

A model is a simplified representation of reality, used to understand how a system works and to predict how it will respond to change. A model can take many forms: a graph, a diagram, an equation, a simulation, or even a description in words. The systems diagram earlier in this tab, the Daisyworld thought experiment, and every population-growth or climate model you meet elsewhere in this course are all models in exactly this sense.

Simplification is what makes a model usable, but it always involves approximation, and therefore some loss of accuracy. A predictive model of global population growth or future climate can give very different results depending on which factors it simplifies away or holds constant. A small classroom terrarium modeling a rainforest will only ever approximate the real thing, precisely because of everything it has had to leave out.

Interactions between components in a system can generate emergent properties, patterns that appear only because components are interacting, and that none of the individual components has on its own. Predator-prey population oscillations are an emergent property: no single predator or prey individual "oscillates," the cycling pattern only exists at the level of the interacting populations. Trophic cascades, where a change at one trophic level ripples through several others, are another.

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Graph: OpenStax, Concepts of Biology, Community Ecology, CC BY 4.0, via Wikimedia Commons.

The resilience of a system, ecological or social, is its tendency to avoid tipping points and maintain stability: its capacity to resist damage, and to recover from or adapt efficiently to disturbance.

Diversity and the size of storages within a system both contribute to its resilience, and affect how quickly it responds to change (its time lag). A system with many different components has more possible pathways to absorb a shock than one with few; the displacement of North America's diverse native prairie ecosystems by single-crop monocultures is a well-documented loss of exactly this kind of resilience. Storage size matters too: a large lake buffers a sudden pollution input or dry spell far better than a small puddle does, simply because the same disturbance is a much smaller fraction of a larger store.

Human activity can reduce a system's resilience by reducing its storages and its diversity. Deforestation is the standard example: it directly removes a storage, the forest itself, and the diversity that depended on it, while also reducing the buffering effect that vegetation cover has on soil and water storages.

A small ornamental pond has been stable for years: fish, pond weed, snails and a stable water level, topped up only by rain. A caretaker suddenly begins draining half the pond's water every summer for irrigation elsewhere, without replacing it.