RESILIENCE THEORY
Ecosystem services are ultimately determined by the functional state of an ecosystem . The foundations of this understanding were provided by ecologist C . S . Hollings in 1973 who brought forth the initial concept of ecological resilience . He suggested the ability of a system to adapt to a disturbance , as well as persist while sustaining its functions and productivity , determines the resilience of the system ( Hollings , 1973 ; Folke , 2006 ). Expanding upon this , he states “ resilience determines the persistence of relationships within a system and is a measure of the ability of these systems to absorb changes of state variables and still persist ” ( Hollings , 1973 ). While the concepts put forth by resilience theory were initially focused on ecosystems , it has been applied to other types of systems across different disciplines as well .
Resilience theory asserts that a natural system contains various states of equilibrium with different levels of ecosystem service provision . In other words , an ecosystem in its current , stable state can be improved or degraded depending on the situations and influences it is exposed to . Ecosystems are not perpetually within a single steady state , but only one ecosystem state is present at any given time . For example , a forest can transition into marshland and vice versa . In either situation , the ecosystem services provided are different following that transition from one state to another . This concept can also be applied to the entirety of a region , such as a barrier island or watershed . Likewise , pressures on these systems do not remain fixed over time , with small changes often not producing impacts until they reach a certain threshold and produce a disproportionately large effect ( Allen and Twidwell , 2018 ). Once this threshold is reached and impacts occur to that system , it collapses and reorganizes into a stable state . This stable state could be the return to its original state or to an alternative state .
Impacts caused by disturbances on a system are not always predictable , which produces uncertainties in the resulting state of a system . In some instances , this also produces unpredictable impacts and outcomes within downstream systems . Resilience theory emphasizes the interdependent , complex , adaptive nature of humanenvironment systems and their nonlinear behavior , uncertainty , and surprise ( Berkes and Folke 1998 , Folke et al . 2016 ).
Disturbances , which manifest as press or pulse perturbations , affect the state and function of a system . Press perturbations are sustained alterations to the ecosystem ( e . g ., drought , human development , sea-level rise ) while pulse perturbations are intense , instantaneous events that alter ecosystem function ( e . g ., hurricanes , flood events , wildfires ). When a certain amount of negative disturbances are introduced to a system , the functioning of the system will decline and subsequently reorganize into a state of equilibrium .
The Ball and Cup Model developed by C . S . Hollings is typically used to illustrate the concept of disturbances and alternative states . The ball represents the system in a specific state , with the location of the ball in a trough representing a stable state and its biological productivity as energy . As disturbances are placed on the ball , the energy in this system changes and causes the ball to move uphill into an unsteady state . Naturally , the ball will roll downhill back to the stable state . If enough energy is placed on the ball through a disturbance , the ball will pass over a threshold where it will roll downhill away from its initial position . Eventually , the ball will stabilize into a degraded alternative state .
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