Ecological and functional consequences of coastal ocean acidification: Perspectives from the Baltic-Skagerrak System

. 2018 Dec 1;48(8):831–854. doi: 10.1007/s13280-018-1110-3

Box 4 Direct, indirect, and interactive effects
Anthropogenic drivers (including OA) rarely operate in isolation. Combinations of driver may have additive, synergistic, or antagonistic effects (Todgham and Stillman 2013). Additive effects are linear, and arise when the effects of multiple combined drivers equals the sum of their effects in isolation. In contrast, synergistic and antagonistic effects are non-linear and arise when the combined effects of multiple drivers is greater than (synergistic) or less than (antagonistic) the sum of those drivers in isolation (Todgham and Stillman 2013; Gunderson et al. 2016). Note that, these non-linear terms relate to the outcome of the interaction, not the effect itself: synergistic negative effects of temperature and OA on growth, for example, will create a greater decrease in growth than the sum of their single effects. Drivers also have direct, and/or indirect, effects on the focal organism or process. Direct effects (of single or multiple drivers) operate directly, such as the impacts of temperature on metabolic rate. Indirect effects operate through chains of interactions, e.g. decreased growth of a filter-feeder due to acidification-induced reduction in nutritional value of its phytoplankton food. Indirect effects typically operate through the food-web, and can be at least as great as direct effects (Alsterberg et al. 2013).

Box 4 Direct, indirect, and interactive effects

Anthropogenic drivers (including OA) rarely operate in isolation. Combinations of driver may have additive, synergistic, or antagonistic effects (Todgham and Stillman 2013). Additive effects are linear, and arise when the effects of multiple combined drivers equals the sum of their effects in isolation. In contrast, synergistic and antagonistic effects are non-linear and arise when the combined effects of multiple drivers is greater than (synergistic) or less than (antagonistic) the sum of those drivers in isolation (Todgham and Stillman 2013; Gunderson et al. 2016). Note that, these non-linear terms relate to the outcome of the interaction, not the effect itself: synergistic negative effects of temperature and OA on growth, for example, will create a greater decrease in growth than the sum of their single effects.

Drivers also have direct, and/or indirect, effects on the focal organism or process. Direct effects (of single or multiple drivers) operate directly, such as the impacts of temperature on metabolic rate. Indirect effects operate through chains of interactions, e.g. decreased growth of a filter-feeder due to acidification-induced reduction in nutritional value of its phytoplankton food. Indirect effects typically operate through the food-web, and can be at least as great as direct effects (Alsterberg et al. 2013).