“A reference condition is the condition against which past, present and future ecosystem condition is compared to in order to measure relative change over time. It represents the condition of an ecosystem that is used for setting the high level (or one endpoint) of reference levels of the variables that reflect high ecological integrity. The reference condition corresponds to a state where all condition indicators have a (spatially averaged) value of 1 (100%).”
Source/reference: SEEA-EA 2021, para. 5.69.
Note: “For ecosystem accounting purposes, the reference condition is based on the principle of maintaining ecological integrity, stability and resilience (over ecological timeframes). In many ecosystem types, it refers best to the natural state (i.e., the ecological state of a natural ecosystem), in terms of ecosystem characteristics at their natural condition allowing for dynamic ranges. The metrics of condition represent the distance from natural irrespective of the characteristic, ecosystem type, or potential desired outcome from a human perspective. The reference condition of an ecosystem corresponds to the condition where the structure, composition and function are dominated by natural ecological and evolutionary processes including food chains, species populations, nutrient and hydrological cycles, self-regeneration and involving dynamic equilibria in response to natural disturbance regimes. An ecosystem at a natural reference condition exhibits an absence of major human modification. An ecosystem at its reference condition attains maximum ecological integrity… The natural state may not be related to supply of ecosystem services and may not be the target of current legislation, policy or ecosystem management objectives… Using the natural state as the reference condition is preferred and recommended. However, in many cases, it may not be possible to define a reference condition as ‘natural’ in absolute terms, since the environment may have changed due to both human and natural processes. In cases where a natural state does not represent a meaningful reference for condition accounts, particularly for anthropogenic ecosystems such as agricultural and urban ecosystems, alternative reference conditions, still characterised by integrity, stability and resilience, can be established and considered as anthropogenically-derived reference conditions.” (SEEA-EA 2021, paras. 5.70-5.72).

“A reference level is the value of a variable at the reference condition, against which it is meaningful to compare past, present or future measured values of the variable.”
Source/reference: SEEA-EA 2021, para. 5.65.

“Regulating and maintenance services are those ecosystem services resulting from the ability of ecosystems to regulate biological processes and to influence climate, hydrological and biochemical cycles, and thereby maintain environmental conditions beneficial to individuals and society”
Source/reference: SEEA-EA 2021, para. 6.51.