Climate change brings with it growing concerns about large-scale ecosystem transformations that may occur after critical ecological thresholds are crossed. Ecological threshold is defined as the point at which a relatively small Environmental change triggers a sudden and disproportionately large ecological response. Ecosystem transformation represents a discrete and permanent change in the structure and function of an ecosystem, often involving the replacement of dominant species by others. Although the prevalence of ecological thresholds in nature is debated, experts working in ecosystems chronically exposed to high abiotic stress consider these concepts to be key to understanding the impacts of climate change.

Authors research Osland et al. (2025) present a conceptual framework that highlights the strong relationships between abiotic stress, climate-driven threshold responses, and the risk of ecosystem transformation. Their hypothesis is that climate-driven threshold responses are particularly influential in ecosystems with high abiotic stress, where there is low diversity of autotrophic organisms and basic types (which modulate ecosystem processes and support entire ecological communities) play a dominant ecological role. In these environments, abiotic conditions are often close to the physiological limits of tolerance of key species, meaning that small abiotic changes can trigger landscape-scale ecosystem transformations. Conversely, small relief of stress near these thresholds can allow key species to thrive and expand into previously inhospitable locations. In low-stress environments, where species and functional diversity are higher, ecosystems are generally less susceptible to abrupt transformations.

Quantifying ecological thresholds can be challenging without strategically collected data. The authors propose a simple approach using the sigmoid equation and its derivatives to quantify absolute threshold (inflection point) and threshold zoneData should be collected along the abiotic stress gradient to effectively capture these nonlinear responses. Climate change can lead to transformations in either by exceeding critical physiological thresholds (e.g. transition from A to B in Figure 4) or stress relief near these thresholds (transition from B to A in Figure 4), allowing the spread of core species into new areas.

The study provides examples of threshold reactions from various high-stress environments:

• Coastal wetlands: Threatened by flooding and salinity. For example, wetlands can be converted to subtidal systems if the rate of relative sea level rise will exceed 7 mm/year. The cooling off, in turn, allows for the expansion of mangrove forests, as the black mangrove (Avicennia germinans) has a mortality threshold close to −7°C.
• Coral reefs: They are critically affected by heat stress. Coral bleaching typically occurs when water temperatures more than 1°C higher than the historical maximum monthly average temperature. At 8°C Degree Heating Weeks are dominant core coral species at increased risk of mortality.
• Dry lands: Controlled by high aridity and limited water availability. There are thresholds for plant seed germination in response to soil moisture (e.g. −1 MPa) and for tree mortality due to hydraulic failure during drought (e.g. −8 MPa, −3 MPa for different species).
• Alpine ecosystems: Limited primarily by low temperatures. For example, the treeline globally occurs at a soil temperature isocline of approximately 6.7°C during snowless periods. The survival of alpine plant seedlings may be limited by temperatures below −2.5°CWarming is leading to the expansion of woody plants into these areas.

Understanding these thresholds is key to evaluating vulnerability of ecosystems to climate change and for informed decision-making in ecosystem management. This includes identifying and protecting resilient areas, mitigating abiotic stress during extreme events, and facilitating ecosystem transformation in anticipation of future conditions. Climate change is increasing the frequency and severity of extreme events, meaning that threshold responses will play an increasingly important role in ecological dynamics. Spring

Glossary of key terms

• Abiotic stress (Abiotic Stress): A significant deviation from optimal environmental conditions (e.g. drought, temperature, flooding, salinity) that induces aberrant changes in the physiological processes of organisms, leading to damage or death.
• Alpine Ecosystems: Ecosystems located at high altitudes above the tree line, characterized by harsh conditions such as low temperatures, strong winds, and short growing seasons.
• Asymptote: In the context of sigmoidal models, the plateau or steady state level to which an ecological response approaches at very low or very high levels of an abiotic stressor.
• Coral Bleaching: The response of corals to stress, typically high temperature, in which they expel symbiotic algae (zooxanthelasma), leading to loss of color and often death of the coral if the stress persists.
• Degree Heating Weeks (DHW): A metric developed by NOAA to quantify the accumulation of heat stress on coral reefs, which helps predict coral bleaching and mortality.
• Diversity: The degree of species diversity or genetic variability in a given ecosystem or community. In environments with high stress, local diversity tends to be low.
• Drylands: Regions with high aridity and limited water availability that cover a significant portion of the global land surface.
• Ecological threshold value (Ecological Threshold): A tipping point at which a small change in an environmental driver (stressor) will trigger a sudden, disproportionate, and often irreversible change in an ecosystem.
• Ecosystem Transformation: A discrete and permanent change in the structure and function of an ecosystem, often involving the replacement of dominant or core species by others.
• Ecotone: A transition zone between two different ecological communities or ecosystems.
• Foundation Species: Species that significantly influence the structure, function, and biodiversity of an ecosystem by creating or modulating environments for other species.
• Physiological Limits (Physiological Limits): The range of environmental conditions (e.g., temperature, salinity, humidity) within which an organism can survive and function. Exceeding these limits leads to physiological stress and potential death.
• Threshold Quantification: The process of measuring or determining the precise value or range at which an ecological threshold occurs, often using statistical models such as the sigmoidal equation.
• Treeline: The upper elevation or northern/southern geographic limit beyond which trees cannot survive or form extensive forest stands.
• Coastal Wetlands: Ecosystems located at the dynamic interface between land and water, often exposed to stress from flooding and salinity.
• Sigmoidal Equation: An S-shaped mathematical model used to describe nonlinear relationships, often with an initial slow change, a rapid transition, and a subsequent plateau. It is used to quantify threshold values.
• Xylem Cavitation: The formation of air bubbles in the vascular tissues of plants (xylem), which disrupts water transport and can lead to plant death, typically due to drought.