Risk and Opportunity in Wildland Ecosystems: Pests, Patches, and Panarchy

Brian W. Geils and Geral I. McDonald

Rocky Mountain Research Station

Managers of wildland ecosystems attempt to mitigate perceived risks, vulnerabilities and costs in the search for ecosystem sustainability. The science of risk analysis provides conceptual frameworks and assessment procedures for describing failure probabilities and consequences—usually with statistics and social-economic valuation. Risk analysis consists of three fundamental activities. On the basis of experience or expectation, a yet unrealized event is recognized as a potential threat to some management goal or value. When a warning from monitoring or research is received, event probability, extent, and severity are projected (e.g., a pest risk map). Concurrently, assessments are made of the values at risk and alternative costs of mitigation. For wildland managers values include such abstract goals as sustainability, forest health, resource productivity, and ecosystem integrity. The first challenge is to specified quantifiable objectives. Important considerations are estimation of uncertainty and the realization that intervention can modify but not completely determine the behavior of complex, natural ecosystems.

Forest and rangeland ecosystems enmesh with the realms of the physical, biotic, and cultural systems. The physical environment consists of an atmospheric-oceanic system that determines climate and weather, a dynamic geology of tectonics, orogeny, and volcanics. The physical system is responsive to biotic and cultural activities (anthropogenic climate change). The biota consists of the living components of the ecosystem that vary over time in distribution, abundance, ecological roles in response to their dynamic physical environment and evolutionary history. Evolutionary legacy involves the generational transmission of genetic information and phenotypic expression resulting from gene-environment interactions (developmental plasticity). Humans are included in the biotic system as a mobile, global, dominant species with prodigious capability to restructure ecosystems. However, transcending limits of biotic legacies, human culture is able to rapidly transmit experience and knowledge throughout populations and across generations. With an extended ability to plan and organize cooperative effort, social concepts of value, ethics, and responsibility emerge to influence decisions over environmental risks. The panarchy model for the behaviors of natural and cultural systems describes a progression through stages of exploitation, conservation, release (or crisis), and reorganization and a system evolution of potential, connectedness, and resilience. Wildland ecosystems change over time as does our interest in affecting what might instigate that change (threats) and their consequences (mitigation).

Although a diversity of users and context provides a rich source of ideas, all analysis is constrained by the world view of analysts and colored by their specific terminology (paradigm). Regarding management of wildland ecosystems, the extant world view favors equilibrium, historic conditions (or a range of reference conditions) and certainty of outcome. Ecological threats are viewed as those causing departures from the desired equilibrium and correction requires return the previous “natural” state. Management is perceived as an engineering problem of local and immediate control. An alternative view, however, recognizes that ecosystems are dynamic and connected—entrained by larger and slower systems of the hierarchy and resilient due to legacy structures and processes at the internal and lower levels. Ecosystem response after disturbance can be contingent on initial conditions and only predictable within a general (chaotic) or new (catastrophic) range. Ecological threats of special interest are those from eruptive populations (e.g., bark beetles and defoliators) and immigrant or emergent invasives (e.g., stem rusts or root diseases). The challenge of managers is how to facilitate the ecological and genetic accommodation of host and parasite populations with desirable outcomes. Promising new approaches for assessing risk are suggested from new multidisciplinary efforts under titles such as landscape genetics and landscape pathology.

In this synthesis, we present and support a conceptual framework for risk analysis based on the premise that system behavior (ecosystems or pathosystem) is determined by local conditions and specific historic and spatial connections. Spatial domains of various sizes are characterized by common environmental drivers such as temperature, rainfall, soil type, and insolation, Spatial domains are repeated across the landscape; their location can move with shifts of the environmental drivers (are sensitive to climate and land use changes). The biotic communities inhabiting these spatial domains have evolutionary histories influenced by varying time and spatial scales; and when perturbed, they exhibit the potential for multiple stable states. This new world view of ecosystems places a premium on an informed understanding of local dynamics and constraints. Certain forest insects, fungi, and other parasites or symbionts have an enduring, intimate association with their forest tree hosts (dominant vegetation and spatial domain partners). These associations have significant ecological and evolutionary significance that is meaningful for projecting the responses of wildland ecosystems to various disturbances.

We present evidence for the validity and utility of a model of evolutionarily significant units (ESU). These units represent long-term biotic interactions in a geographical location affecting ecological functions, resulting from persistent genetic discontinuities. We suggest these discontinuities are congruent over taxa, often form at geographic barriers, but can form in the absence of such barriers, and often have pervasive effects on host–parasite interactions. These zones may be demonstrated by changes in adaptive traits using techniques of landscape genetics (molecular makers). The ESU model provides an opportunity to more realistically map and project threats to wildland ecosystems from native and introduced forest pests. Specific examples are illustrated with white pine blister rust, fusiform rust, Armillaria root rot, budworm, and other disturbance agents.

Tuesday Morning Plenary

corresponding author:

Brian W. Geils
USDA Forest Service
Rocky Mountain Research Station
2500 South Pine Knoll Drive
Flagstaff, AZ 86001
928-556-2076
bgeils@fs.fed.us