02_stressor_response.qmd

## Stressor-Response Functions

Stressor-response functions describe the relationship between a specific stressor (such as habitat loss, temperature, or a specific pollutant) and the response of a target species, where responses can include abundance, growth rate, reproduction, or mortality ([@rosenfeld2024determinants]; [@jarvis2023process]). Stressor-response functions are used to predict how a population (or study system) will respond to changes in the environment and to help identify thresholds or "critical levels" at which a stressor becomes harmful. Stressor-response functions are often used to inform environmental policy and management decisions, for example, by identifying risk levels of pollution or temperature change for a particular species or ecosystem[^02_stressor_response-1]. Stressor-response functions are generally developed through primary research (i.e., mechanistic, empirical, experimental etc.) and expert opinion ([@pirotta2022understanding]; [@jarvis2023process]). Note, however, that stressor-response functions are often continuous empirical or mechanistic relationships and identification of specific thresholds as harmful or benign will often be a subjective user-defined activity for stressors and responses that are without direct regulatory guidance (e.g., habitat area, population size).

[^02_stressor_response-1]: https://www2.gov.bc.ca/assets/gov/environment/air-land-water/water/waterquality/water-quality-guidelines/approved-wqgs/wqg_summary_aquaticlife_wildlife_agri.pdf.

![An example of a stressor-response function for Pacific salmon from @jensen2009impact shows the relationship between a stressor (percent fine sediment) on the x-axis and the biological response (percent egg-to-fry survivorship) on the y-axis.](images/image005.png){#fig-1}

There are many types of stressor-response functions, including linear, threshold, and non-linear ([@rosenfeld2017developing]; [@larned2019stressor]). Linear functions describe a simple, linear relationship between the stressor and the response, with the response increasing or decreasing at a constant rate as the stressor increases. Threshold functions describe a breakpoint at which a stressor becomes harmful, beyond which the response increases rapidly. Non-linear functions describe more complex relationships, with the response changing at different rates as the stressor increases. The example provided in @fig-1 shows a customized non-linear stressor-response function fit to empirical data (reference points). Stressors do not always act independently ([@schafer2018advancing]; [@jarvis2023process]), and it is also possible to include interactions among variables in stressor-response functions, such as the risk of exposure to a harmful pathogen being temperature dependent.

For a more in-depth discussion on the foundations of stressor-response functions, refer to the following resources:

-   [@rosenfeld2024determinants]**; [@jarvis2023process]**: Conceptual overviews of stressor-response functions as a generalizable model for context dependence. This paper provides a valuable overview to conceptualize stressors as a mechanism to characterize the state of a system and ecological process. [@jarvis2023process] also outline common forms of stressor-response functions and key considerations for the creation of a stressor-response function from empirical data.
-   [@piet2021roadmap]: *A roadmap towards quantitative cumulative impact assessments: Every step of the way*. Provides an important roadmap for working groups to consider linkages between land-use activities, resulting ecosystem pressures, functional linkages in space and time (exposure) and the consideration of endpoints to target study systems.
-   **Incorporating Indigenous Knowledge**: In many instances, stressor-response functions may be developed through expert opinion from local communities based on value systems. Where appropriate, working groups may include a customized stressor-response function to represent potential risks and values based on traditional knowledge systems and expert opinion. Refer to [@houde2007six] and [@alexander2019bridging] for further discussion. Examples of many other values-based Indigenous-led cumulative effects management programs exist across Canada.