Effects of compound stressors on marine ecosystems

At present, a wide array of factors are detrimentally affecting ocean health, and predicting their combined effects on marine ecosystems remains a difficult challenge.

There exists a mixture of more immediate, short-term, localised threats, such as habitat modification and overfishing, alongside broader, slower changes in underlying global ocean conditions (e.g. water temperature and pH, due to increased atmospheric carbon dioxide). There is concern within the scientific community that the combination fo local/regional/global drivers has the potential to push marine systems to new, and potentially dysfunctional, states.

Individual stressors can affect biological community composition, ecosystem function, and ecosystem  services in significant ways, and much work to date on specific species, or species groups (e.g. corals) provides a compelling body of evidence. Empirical observations of pairwise stressors in combination have, however, shown their effects are not always additive (in which the total effect is the sum of the individual effects). The total effect may be ANTAGONISTIC (total effect < sum of individual effects) or SYNERGISTIC (total effect > sum of individual effects). Incorporating this range of responses in to ecosystem-wide models allows the potential amplifying effects of larger systems both local and global forcings to be explored - this is the reason OSIRIS was created. If synergistic effects prove to have large ecosystem consequences, an additional level of caution may be required in how marine ecosystems are managed and protected. The aim of this work is to provide an analytical framework which allows the effects of multiple simultaneous stressors to be investigated, and the risks to ecosystems to be evaluated.

Otters playing in Monterey Bay, during a visit to Hopkins Marine Station in the early days of OSIRIS development

OSIRIS model

The OSIRIS (Ocean System Interactions, Risks, Instabilities and Synergies) model represents ecological systems as a network of interconnected nodes. It is a coupled-ODE (ordinary differential equation) model, and the basic outputs are time-series of the state of each node. Nodes are loosely defined as representations of groups of elements with shared characteristics, and are either biotic nodes, which are biological populations associated with species groups, or abiotic nodes, which are typically nutrient/chemical concentrations. Time series of multiple external forcings can be defined (e.g. water temperature, pH, salinity) which affect specified nodes. The core components of the model are indicated in the diagram to the right.

The state of each node is locally stable around its equilibrium, and the dynamics of the node states are determined by two main components: (i) the rate of relaxation towards the local equilibrium; the equilibrium itself can be affected by external forcing;  (ii) the combined direct influences of other nodes and direct influences of external forcings - as indicated below.

Core components of the OSIRIS model

The OSIRIS (Ocean System Interactions, Risks, Instabilities and Synergies) model represents ecological systems as a network of interconnected nodes. It is a coupled-ODE (ordinary differential equation) model, and the basic outputs are time-series of the state of each node. Nodes are loosely defined as representations of groups of elements with shared characteristics, and are either biotic nodes, which are biological populations associated with species groups, or abiotic nodes, which are typically nutrient/chemical concentrations. Time series of multiple external forcings can be defined (e.g. water temperature, pH, salinity) which affect specified nodes. The core components of the model are indicated in the diagram to the right.

The state of each node is locally stable around its equilibrium, and the dynamics of the node states are determined by two main components: (i) the rate of relaxation towards the local equilibrium; the equilibrium itself can be affected by external forcing;  (ii) the combined direct influences of other nodes and direct influences of external forcings - as indicated below.

Model applications and findings

The applications described below involved close collaborations with Ocean Conservancy and the Centre for Ocean Solutions at Stanford University, who also both funded this work. 

We have parameterized OSIRIS so far for three cases:

  1. A generic temperate marine ecosystem, with 16 nodes representing the common major functional groups (plus a detritus node)

  2. A western boundary upwelling current ecosystem based on the California Current System in the eastern Pacific;

  3. An Arctic marine ecosystem based on the Chukchi and Beaufort Seas in the Pacific Arctic.

 

Common themes running through these projects include: combinations of local and global forcings; effects of synergies between forcings; impacts on specific nodes (species groups); resilience to changes in forcings at species- and system-level. Example findings are shown below.

Increased variability in external forcings, combined with greater levels of synergy between forcings (within empirical uncertainties) produces an increasingly negative ecosystem impact. [2]

The ecosystem can be held in a temporarily-stable novel configuration due to external pressures (here fishing pressure), and return to their original stable state, so long as the perturbations are not too large so as to create a new stable configuration. [1]

Climate-related stressors in the Arctic have a larger impact on animal populations than do acute stressors like increased shipping and subsistence harvesting. Neglecting interactions between stressors vastly underestimates the risk of population crashes. [3] 

Ongoing work

Currently on-going work is using OSIRIS to generate data for developments in Ecosystem service valuation. Results will be presented when the work is complete.

Outputs

[1] Bailey, R. M. & van der Grient, J. M. A. (2020) OSIRIS: A model for integrating the effects of multiple stressors on marine ecosystems. J. Theor. Biol. https://doi.org/10.1016/j.jtbi.2020.110211

[2] J.M.A. van der Grient, R.M. Bailey, G.H. Leonard, A. Zivian, A. Merkl. The effects of forcing strength, forcing variability and interaction between stressors in a modelled California Current-like marine ecosystem (under review)

[3]  K. R. Arrigo, G. L. van Dijken, M. A. Cameron, J. van der Grient, L. M. Wedding, L. Hazen, J. Leape, G. Leonard, A. Merkl, F. Micheli, M. M. Mills, S. Monismith, N. T. Ouellette, A.Zivian, M.Levi 7 & R.M.Bailey (2020) Synergistic interactions among growing stressors increase risk to an Arctic ecosystem. NATURE COMMUNICATIONS, 11, 6255. https://doi.org/10.1038/s41467-020-19899-z

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