Pathways of Effects - National Guidelines
Illustrating the links between human activity and its potential impact on aquatic ecosystems

Table of Contents


The type of PoE model used, and therefore its complexity, will differ depending on the geographical unit or scale at which the model is built and the degree of detail to be included. The geographical or spatial unit can be as broad as a bioregion or as small as a marine protected area (MPA), a single estuary or a species-specific habitat.

Three categories of PoE models exist within DFO to address different analysis goals and objectives:

  • Holistic models;
  • Endpoint models; and
  • Activity/action and sector-based models.

4.1 Holistic models

The holistic PoE model is the starting point for IM and provides a foundation for future detailed PoE development. A holistic model is a high-level one that illustrates all human activities taking place in the same spatial unit and that could collectively create cumulative impacts on the ecosystem. The holistic model is designed to be read from the bottom up, whereas subsequent model types are read from the top down.

Holistic models are effective planning tools for the management of a particular spatial unit to ensure appropriate measures are taken to avoid detrimental effects. They can help identify:

  1. the legal authorities responsible for managing activities that result in/create a specific pressure (e.g., nutrient loading);
  2. the existing management responses to that pressure (e.g., municipal laws); and
  3. gaps in legislation, policies and regulations (e.g., no existing regulation on fertilizer).

The holistic model example provided in Figure 3 illustrates the multitude of pathways between all human activities (e.g., agriculture, fisheries); potential pressures (physical, chemical and biological stressors) stemming from these activities (e.g., nutrient regime changes, biota changes); and their impacts on endpoints (ecosystem components or attributes such as spawning ground) and aquatic ecosystem goods and services (e.g., water quality, fish harvest). The model depicts the socio-cultural and economic activities that rely upon those goods and services. The model is built to be circular in nature, in that the human activities and the socio-cultural and economic activities, dependencies or values are both the initiators and the objects of the environmental changes.

Figure 3: Example of a holistic PoE model

4.2 Endpoint PoE models

Once the holistic model is complete, one or more endpoint models can be developed. Endpoint models are more detailed than holistic PoEs as they illustrate all potential impacts on selected endpoints (the ecosystem components, functions or social, cultural or economic values that need to be maintained or protected). Endpoints are directly linked to specific objectives as derived from legislative mandates (e.g., Species at Risk Act recovery objectives) or international agreements (e.g., MPA network objectives that conform to Convention on Biological Diversity guidance). Alternatively, they may be linked to management or conservation objectives (e.g., conservation objectives for Ecologically and Biologically Significant Areas (EBSAs)). Other endpoints may be derived from special socio-economic or cultural objectives (e.g., protection of clams and salmon that are important sources of food, income and cultural activities for Pacific Coast First Nations (GESAMP, 2008). Objectives need to be defined prior to the development of an endpoint PoE.

Figure 4 is an example of a comprehensive PoE linked to a holistic model that provides significant detail regarding the pressures and impacts generated by specific sectors, activities or actions.

Figure 4: Example of a holistic model with common pressures generated by aquaculture and marine transportation sectors, and associated detailed PoE for aquaculture

There are three types of endpoint models, depending on the objective of the analysis:

  • Species-based model;
  • Pressure-based model;
  • Socio-cultural and economic-based model.

4.2.1 Species-based model

Figure 5: Example of a species-based model

In a species-based model, the endpoint can be an ecologically significant species, a species at risk or other species of management concern. This type of model identifies the pathways between human activities, their sub-activities, the pressures and the potential cumulative impacts on an ecosystem component/attribute that a species depends upon for survival or recovery.

Figure 5 illustrates the specific relationships among two human activities (marine transport and oil and gas), their sub-activities (shipping and drilling), the pressures (introduction of contaminants, etc.) and the potential impacts (contamination of water column and bottom sediment, etc.) on the habitat and ultimately on the bowhead whale population.

4.2.2. Pressure-based model

The pressure-based model in Figure 6 illustrates the pathways from one pressure (e.g., noise) to one or more measurable endpoints (e.g., beluga whale migration corridor, fish health). Such a model shows how a single pressure may affect many endpoints and how the mitigation of this pressure would contribute to the protection of multiple ecosystem components and functions. The combination of several pressure-based models can also illustrate the cumulative effects of various pressures on specific ecological components.

4.2.3 Socio-cultural and economic-based model

A socio-cultural and economic-based model illustrates the potential consequences of an altered ecological component or the loss of an ecosystem goods or service on a specific social, cultural or economic user (e.g., industry).

Figure 7 shows the linkages between a measurable endpoint (e.g., capelin spawning and rearing area) and the potential impacts on ecosystem goods and services (e.g., provisioning and cultural services) which may eventually affect economic activities (e.g., tourism) and cultural values (e.g., recreational), and ultimately social demographics. Some activities and values can be impacted directly—for example, by losing capelin spawning ground, there would be less capelin for marine mammals and the whale watching industry would become less lucrative (leading to loss of jobs). Alternatively, impacts may be indirect – for example, a regulation that forbids the building of a retaining wall on a coastal beach could affect, in turn, other economic sectors such as transportation (e.g., relocation of a road).

Figure 6: Example of a pressure-based model (generic)
Figure 7: Example of a socio-cultural and economic model

4.3 Activity/action and sector-based models

Figure 8: Example of an activity or action-based model

Activity/action models and sector-based models are two types of PoEs that form the building blocks of a holistic model. They may be used individually or in combination with other such models (e.g., to review environmental assessment proposals).

4.3.1 Activity or action-based models

Activity or action-based models illustrate the potential stressors and effects of a specific action (e.g., Figure 8 illustrates the effects that the removal of an in-water structure may have on fish or fish habitat). Such models are used by DFO Habitat Program to evaluate development proposals in terms of the activities that are involved, the known cause-effect relationships, and the mechanisms by which stressors ultimately lead to effects in the aquatic environment (DFO, 2006). Each pathway represents an area where mitigation measures can be applied to reduce or eliminate a potential effect. When mitigation cannot be applied or fully address a stressor, the remaining effect is referred to as a “residual effect.” To support habitat program practitioners, a series of PoE diagrams were developed for common activities associated with a broad range of development proposals. The purpose of these PoE templates is to enable habitat managers to have a common reference tool to highlight aquatic effects of specific concern.

4.3.2 Sector-based models

Sector-based PoEs demonstrate the potential impacts derived from a specific sector, in order to inform IM and other planning processes in various spatial units. Sector-based models can be created for each sector operating within an area and can also be combined to identify cumulative effects. For example, Figure 9 is a PoE of the marine transport sector that focuses on activities related to shipping. The sub-activities can generate several pressures (e.g., oils and contaminants; collision; noise). The pressures may cause changes in water quality (from contaminants), injury or mortality (from collision) and increased stress (from noise). In turn, all these impacts may harm ecosystem components, as well as the ecosystem goods and services these components provide to other socio-economic and cultural activities (such as fisheries, tourism and aquaculture).

Figure 9: Example of a sector-based model
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