Conceptual models help us to understand the world around us without becoming overwhelmed with its complexity. Societal use of conceptual models is pervasive. Corporate organizational charts offer means to understand and express relationships within the business entity. Road maps represent transportation systems and the built environment. Depictions of food webs give insights into ecosystem structure, function, and composition. Conceptual ecological models are a specialized subset of conceptual models generally intended to describe the environmental factors that affect an ecological community, a species, or a population. Conceptual ecological models are useful in a variety of contexts ranging from development of research proposals and monitoring schemes, to regulatory decision-making applied in the development of biological opinions and habitat conservation plans. In the implementation of the federal Endangered Species Act, and its California counterpart, wildlife agencies should make greater use of conceptual ecological models and adhere to best practices for their use.
The British statistician George Box famously noted in 1976 that “all models are wrong, some are useful.” This is as true for conceptual ecological models as it is true for other models. As the National Research Council explained in a volume Models in Environmental Regulatory Decision Making (2007), all models are a simplification of reality, which is of course both a weakness and a strength. On the one hand, models are inevitably inaccurate in some of their attributes, hence they are not comprehensive reflections of reality. On the other hand, models — especially parsimonious models — incentivize scientists and policy makers to focus on the environmental factors that have the greatest potential to influence the fate of an ecological community or a species.
The limitations of models acknowledged, agency experts and scholars agree that conceptual ecological models, developed using the best available scientific information and professional standards of practice, can aid conservation planning efforts. For example, experts at the Army Corps of Engineers have advocated for the use of conceptual models in ecosystem restoration (Fischenich 2008) and at the U.S. Geological Survey developed a set of conceptual ecological models describing the environmental drivers, ecological factors and pathways, responses to environmental change, and management actions in support of a Biological Opinion for the endangered pallid sturgeon (Jacobson et al. 2015). Below is one of those models that places the life cycle of the species in spatial context and identifies where and when management and restoration should be implemented.
Figure 1 — Generalized population-level CEM showing life stages, geographic context of pallid sturgeon reproductive cycle, and management actions being considered in the upper and lower Missouri River. Juveniles less than 1-year old fit into the “exogenously feeding larvae” life stage.
Conceptual ecological models are valuable because they describe the drivers and stressors that affect a species and its habitat, they facilitate hypothesis testing to improve the understanding of ecological relationships, and they assist with the development of monitoring regimes and interpretation of monitoring results, ideally, in an adaptive management framework.
That said, the substantive provisions of the Endangered Species Act, regulations that implement those provisions, and agency guidance regarding those provisions, are silent on the use of conceptual models in regulatory decision-making, therefore out of step with prevailing scientific practices. Decision documents generated during the process of determining whether to list species as threatened or endangered and recovery plans for species already listed typically make no use of conceptual ecological models.
Biological assessments and biological opinions that grow out of interagency consultation do, on occasion, include conceptual models. However, both the joint consultation regulations and the Endangered Species Consultation Handbook (1998) fail to make any reference to conceptual models or describe the circumstances where the use of such models is warranted. There are exceptions to the foregoing. For example, the U.S. Fish and Wildlife Service’s Habitat Conservation Planning and Incidental Take Permitting Processing Handbook (2016) emphasizes the value of conceptual models as “a key foundation upon which the integrated approach to development of goals and objectives, monitoring and evaluation, and adaptive management systems are built.” Ironically in doing so, the Handbook appears to hold non-federal entities seeking incidental take permits to a higher standard than federal wildlife agencies when deciding whether to list species, developing recovery plans for listed species, or consulting on the effects of federal agency actions on listed species.
Conceptual ecological models have the potential to add transparency to agency determinations. Well-developed conceptual models can summarize the most important ecosystem descriptors, describe the spatial and temporal scales of critical processes, and identify current and potential threats to the ecosystem supporting listed species and their habitats. They can provide help in formulating goals and objectives and identifying indicators, management strategies, results, and research needs. Importantly, conceptual models play essential roles in determining measurement indicators for the monitoring that accompanies adaptive management. Where conceptual models specify important physical, chemical, or biological processes that effect a listed species, they contribute to identifying ecosystem attributes that should be measured, interpreting monitoring results, and guiding consideration of alternative courses of management. By all those means conceptual models facilitate open discussion and debate about the nature of listed species and their habitats and the management agenda necessary for species recovery. A second pallid sturgeon model provides an excellent example of a thoughtfully constructed conceptual ecological model developed consistent with the best practices in conservation planning, including those we describe below.
Figure 2 — Example of a life-stage component CEM showing the cause-effect linkages leading to survival to the next pallid sturgeon life stage.
In some circumstances, the structure of conceptual ecological models should anticipate development of quantitative (numerical) models, which are used to assess the effects of alternative management actions and to predict habitat and species responses to environmental change. While conceptual models guide the identification of management actions, metrics for assessing performance, and decision-relevant uncertainties, quantification of conceptual models allows for scenario analyses and model projections for changes in environmental conditions. Quantitative models directly derived from conceptual models are applied in the evaluation phase of adaptive management to assess the implications of observed performance and determine management needs. In systems like California’s Bay-Delta, the absence of a structure decision-making process that links goal setting, harvesting and synthesis of the best available scientific information, development of conceptual ecological models, use of quantitative models, and selection from among alternative actions inhibits our ability to adopt and implement policies that will lead to the conservation of targeted species and ecological communities.
Consistent with best practices in the scientific literature, regulatory decision-making under the Endangered Species Act should incorporate conceptual models. Decisions with respect to listing and designation of critical habitat, as well as recovery plans, should incorporate conceptual ecological models and be informed by them. In addition, biological opinions and habitat conservation plans that have the potential to have population-level effects on listed species should incorporate conceptual models to inform their effects analyses. Importantly, conceptual ecological models intended to inform regulatory decision-making under the Act should have the following characteristics:
§ They can take the form of flowcharts, matrices, contributing factors diagrams, or narrative description, but are most effective when presented as diagrams, with boxes representing environmental drivers and stressors that affect species performance and habitat availability and quality over time, and arrows representing relationships between the variables in the boxes.
§ They are accompanied by a clear narrative that explains whether the relationships set forth are descriptive, that is, are intended to describe known relationships based on the best available scientific information, are prescriptive, are intended to posit relationships to guide data collection and/or hypothesis testing, or are a hybrid of both.§ They describe driver-stressor-response relationships that affect population dynamics.
§ They are developed using the best available scientific information.
§ They specify the magnitude of the relationships and the degree of certainty associated with those relationships or explain the basis for omitting that information.
While there is a robust literature that has developed over the past quarter century describing best practices in the development of conceptual models generally and conceptual ecological models specifically, too often conceptual ecological models do not reflect best practices including those described above.