The Adaptive Cycle /
*EXCERPT FROM WORKING DRAFT 2/23/2019*
ENVIRONMENTAL SCIENCE: A GLOBAL CONCERN 15TH EDITION
4.4 THE NATURE OF CHANGE & RESILIENCE
The adaptive cycle explains disturbance and change in a complex system.
Large, abrupt, non-linear change results in a fundamental restructuring of the biological community.
The risk of ecological systems tipping into undesirable states is significant for people and their well-being.
How much can you change a system and have it remain recognizable or deliver something you have come to rely on? This is a question of resilience. It’s of the utmost importance right now because humans are rapidly changing natural systems through things like deforestation, habitat disruption, pollution, and climate change. We are introducing change at a scale, speed, and level of connectivity that was unthinkable a hundred years ago. What might happen? Can natural systems remain stable and keep delivering the ecosystem services we all depend on?
In this section, we will use our knowledge of diversity, redundancy, and connectivity in biological communities to increase our understanding of system dynamics. When we learn how ecosystems respond to change, we may acquire insight into how we might adapt our own behavior to avoid unintended ecosystem changes in the future.
The adaptive cycle explains a system’s response to disurbance
In 1973, the Canadian ecologist C.S. “Buzz” Holling introduced a model that gave ecologists a more complete picture of change and disturbance. Through his observations of natural systems, Holling confirmed that the pioneer species of primary succession and the sustaining species of secondary succession were indeed important, but he didn’t think they told the whole story. Holling proposed that most complex systems move through cycles of change according to a cyclical pattern, the Adaptive Cycle (fig 4.24). He identified four phases that typically begin with rapid growth (primary succession; box “1”) which moves to a conservation phase (secondary succession; box “2”). These phases were well-recognized by scientists at the time. Holling’s innovation was the introduction of a release phase (box “3”) that was brought about by a disturbance, and a reorganization phase (box “4”) in which the system recovers. Remarkably, adaptive cycles have been documented in people, social communities, ecosystems, businesses, nations, global systems, and paradigms of thought.
Figure 4.24 The Adaptive cycle
The first phase of the Adaptive Cycle resembles primary succession. This is the is the rapid growth phase where early pioneers rush in to exploit newly available resources. In natural systems, this may consist of fast-growing grasses and shrubs on newly cleared land; in economic systems, it may be entrepreneurs and innovators who a rush in to seize on opportunity.
The system then moves up to the top right conservation phase, which resembles secondary succession. In this phase, the community switches from opportunists to specialists. A small steel-making business that grows from a local producer to a national, and then global competitor serves as an example. In the conservation phase, the future seems more certain & determined, and there is a loss of flexibility because different ways of doing things (redundancy) are eliminated for efficiency.
The release phase is brought about by disturbance or a shock to the system, and it happens quickly. In ecosystems, the disturbance might take the form of fire, drought, insects, pests, or disease. In an economy, it might be a new technology or market shock. Things change quickly in the release phase; dynamics are chaotic, and historic connections are broken.
The reorganization phase is one of renewal and growth, where novelty can thrive. Invention, experimentation, and creative ideas are the order of the day. In ecosystems, we might see buried seeds germinating or the surviving trees of a forest fire begin to regenerate. In economic & social systems, we would see many new groups appear to try and take control of the system. This phase begins to sort out and capitalize on the chaos of the release phase.
Holling observed that when a disturbance appears in an ecosystem, the system could stay in its prior path (continuing again into box 1, the rapid growth phase) or it can leave its historical path and transform into something fundamentally different (“possible exit” in fig 4.24). For example, a clear-water lake could survive a disturbance like flooding or an invasive species and recover to a state that resembled its historical clear-water structure. Alternatively, the clear-water system could suddenly shift into a turbid-water system that behaves in an entirely different way.
There are numerous examples of unexpected and abrupt shifts in nature, including kelp forests that transform into systems dominated by sea urchins, or coral-dominated reefs that transition into algae-dominated reefs (figure 4.25). In ecology, we refer to the movement from one ecologically stable state to another as a regime shift. Regime shifts are usually sudden, abrupt, and nonlinear. They occur over a period of weeks to decades, which are relatively rapid when compared to the long time cycles of the conservation (K) phase, which may last hundreds of years or more. These transitions have significant implications for human well-being because they bring surprising, expansive, long-lasting, and frequently negative changes to the benefits and services we receive from nature. You may be familiar with this idea through the popular notion of “tipping points,” which have been documented in financial markets, climate systems, the brain, and social networks.
Figure 4.25. Regime shifts happen when a complex adaptive system suddenly transforms into a different type of system, as when a coral dominated reef (above) shifts to an algae-dominated reef (below). These shifts may take people by surprise, impacting fisheries, livelihoods, recreation, economic activities, or food availability.
Ecologists study regime shifts in nature by using models, paleo-observation, contemporary observation, and experiments to compare and learn from marine, terrestrial, and polar systems worldwide. They assert that regime shifts occur because of two changes working in combination: (1) a significant disturbance like a storm, fire, or pest outbreak and (2) a background change in the biological community, like overfishing, disease, or the introduction of an invasive species that has left the system vulnerable.
Oftentimes, the new regime is less ecologically complex and diverse than the original regime, and it delivers fewer ecosystem benefits. For instance, seagrass meadows can undergo sudden and abrupt change when a background variable like overfishing combines with a shock or disturbance like a heat wave. The seagrass meadow regime can abruptly transform into an algae-dominated regime or a barren-sediment regime. The new degraded system can be persistent and difficult, if not impossible, to reverse.
Resilience is the ability of a system absorb disturbance and maintain its historic identity
Knowledge of tipping points and regime shifts is key to our understanding of resilience. Resilience is the capacity of a complex system to absorb a spectrum of shocks or disturbances and retain its fundamental function, structure, and identity. Even when shocks and disturbances occur, a resilient system absorbs these shocks and reorganizes itself to continue along in its predetermined or traditional cycle that occurred before the disturbance (it “stays in its loop.”) In a resilient system, things may seem chaotic after a disturbance, but over time community structure and productivity get back to normal, species diversity is preserved, and nature seems to reach a dynamic balance.
Many biological communities are resilient and remain relatively stable and constant over time. An oak forest tends to remain an oak forest, for example, because the species that make it up have self-perpetuating mechanisms. Through coevolution, the system developed ways to adjust to historic changes through mechanisms called feedback loops. Feedback loops can reinforce or amplify change (a positive feedback loop), or they can reduce or stabilize the force of a change (a negative feedback loop) (see section 2.2).
What helps a system be resilient? Our study of the structure of biological communities (section 4.3) may be helpful here. While the relationship between diversity and resilience is complex, research indicates that the simplification of systems (the reduction of diversity at all scales) often contributes to a loss of resilience. To illustrate, we will return to our example of a tropical rainforest (see fig 4.21) with several structural layers, including an emergent layer, canopy, understory, shrub layer, herb layer, and ground layer, each consisting of multiple species. In a complex, highly connected forest like this, each layer is working in its own way to trap moisture in the system and to maintain a hydrological balance. If the forest is simplified to include only a canopy, shrub, and ground layer, it may be more susceptible to a disturbance like drought because there are fewer sub-systems working to trap moisture, and the water more easily evaporates.
Resilience analysis is bringing new insight to how we manage natural systems.
Traditionally, people taking an organismal view of biological communities thought that disturbance was harmful. In the early 1900s, this view merged with the desire to protect timber supplies from ubiquitous wildfires, and to store water behind dams while also controlling floods. Fire suppression and flood control became the central policies in American natural resource management (along with predator control) for most of the twentieth century.
Recent concepts like the Adaptive Cycle are changing our understanding of disturbance and are informing our land management policies. Resilience thinking brings us the insight that change is a normal and natural process, and not something to be avoided at all costs. Grasslands and some forests are now considered “fire-adapted,” and fires are allowed to burn in them if weather conditions are appropriate. Floods also are seen as crucial for maintaining floodplain and river health. It may be that preserving species diversity by allowing in natural disturbances (or judiciously applied human disturbances) actually ensures stability over the long run, just as diverse prairies managed with fire recover after drought.
This is especially important today because many places on earth are experiencing an increase in disturbances like wildfires, flooding, droughts, storms, and pest outbreaks that are made worse by climate change. Managers can use their knowledge of resilience in specific ways to help prevent systems from moving into undesirable configurations (fig 4.26). For example, administrators can work to foster resilience by maintaining diversity and redundancy, managing connections between elements, and enhancing (or reducing) the strength of feedback loops. Large regime shifts are frequently preceded by changes in early-warning indicators, and the monitoring of these signals can provide cost-effective early action options for managers. Early action is a more practical, affordable, and effective strategy than trying to reverse a tipping point shift.