Conceptual ecological models provide a means to describe how landscape and ecosystem dynamics likely occur. They assist with simplifying what can be an overwhelmingly complex dynamic process allowing natural resource managers to better understand the causes and potential responses to the conditions they observe in the field. Depending on the need for the assessment, conceptual models for the selected resource may vary from broad (e.g., covering all montane forests in an ecoregion) to more specific (e.g., pinyon-juniper woodland) to an individual vegetation patch at a local site.
Conceptual “state-and-transition” models (STMs) can be a primary source to describe dynamics associated with landscapes and the vegetation types they support. Documented models originating with LANDFIRE biophysical settings and with NRCS ecological site descriptions are two primary sources to consider (Keane et al. 2002) especially for upland ecosystems. These models aim to provide a quantitative or qualitative characterization of natural succession and disturbance dynamics for a given type as it occurs within a given regional landscape. For LANDFIRE models, a maximum of five “boxes” (or “states”) are used, typically to describe an early-succession state, two to three mid-succession states (e.g., open canopy to closed canopy mid-successional woodland), and one late-succession state that characterize dynamics unaltered by significant human influence.
The Ecological Site Descriptions are less quantitative than LANDFIRE models, but typically include more local geophysical characteristics, successional dynamics, and responses to common natural disturbances. They also might describe the effects of grazing regimes on vegetation in an area. These models form a foundation for ecological condition assessment at project scales. They initially highlight geophysical constraints and dynamics one should anticipate in unaltered conditions. They may also include effects of common ecological stressors or management practices. The model for pinyon-juniper woodland in Figure 8.3. illustrates how common stressors or management practices might be depicted graphically
While ecological models described in Box 8.3 get at more detailed dynamics of vegetation in different landscape setting, conceptual ecological models can also be used to characterize patterns and processes taking place across more generally defined landscapes that integrate the main resources and stressors that affect management of the DoD installation. Again, the goal of this model might be two-fold. First it can identify a limited number of ecological characteristics and interactions with species and ecosystems—along with the critical causal links among them. Some of these characteristics will be especially pivotal, influencing a host of other characteristics of the target and its long-term persistence. Their identification should lead to selecting a short list of “conservation targets” that if assessed and acted upon, will represent the primary biodiversity concerns at stake for management. Second, the model, or subsets of the broader landscape model, will be used to describe key ecosystem components, their driving ecological processes, and their natural variation over time and space, and typify an exemplary, or reference, occurrence (more detail in section 5 below). Such defining characteristics of a target are the “key ecological attributes” of that target. These models provide a framework for organizing information and thinking about the target, their impacts and stressors, and anticipated management responses. Effective models help answer the question, “What are the causes that result in our current condition?”
What is ecosystem stress?
The structure and species composition of any ecosystem naturally varies over time and across regions, and experiences varying disturbances from fire, drought, wind damage, or flooding. Natural resource managers often use the concept of a natural range of variability (NRV), essentially synonymous with historical range of variability (HRV), to describe these long-term historical characteristics of ecosystems (e.g., Landres et al. 1999, Romme et al. 2012). Our knowledge of NRV is based on historical information, paleoecological studies of past conditions, research on current conditions where relatively free of human impacts, and simulation models of ecosystem dynamics (Parrish et al. 2003, Stoddard et al. 2006, Brewer and Menzel 2009). This knowledge provides important clues about the long-term ecological processes and natural disturbances that shape ecosystems, the flux and succession of species, and even the relative role of humans in shaping the systems. This knowledge provides a reference for gauging the effects of current anthropogenic stressors (Landres et al. 1999).
For these reasons, understanding NRV is an important part of ecosystem assessment. In essence, where disturbances (large- and small-scale, exogenous and endogenous; repetitive and as random rare events) fall outside of NRV, they are likely causing some degree of ecosystem stress.
There is concern that current ecological conditions are changing so rapidly that natural and historical information is no longer relevant. However, there are several ways in which NRV remains an important guide for our conceptual models of ecological integrity (Higgs 2003, Higgs and Hobbs 2010):
- First, the purpose of understanding NRV is not to lock us in the past, but to ensure that we connect the historical ecological patterns and processes to the present and future.
- Second, to suggest that we can simply take over the management of complex natural ecosystems without understanding NRV is naive and problematic.
- Third, understanding NRV will ensure that we can anticipate change and emphasize resilience in the face of future changes.
- Our models and our understanding of the NRV, especially as related to disturbance, can also be informed by sites that represent reference conditions (Woodley 2010).
As described by Brooks et al. (2016), reference sites represent areas that are intact or have minimal human alteration; i.e., “reference standard” or “exemplary ecosystem occurrences.” In effect, they provide us with an understanding of the current range of conditions resulting from natural disturbance regimes. Typically, the initial approach to identifying reference sites is to rely on a combination of factors, including naturalness, apparent ecological integrity, and lack of evidence of human alteration. Naturalness and integrity are often judged by historical fidelity (connectivity in time), a full complement of native species, characteristic species dominance and productivity, presence of typical ecological processes such as fire, flooding, and windstorms, and minimal evidence of anthropogenic stressors (Woodley 2010). This information can be used to set levels of ecological integrity along a gradient from minimally disturbed conditions to severely impacted sites (Davies and Jackson 2006).
Given the extensive loss or alteration of ecosystems in many jurisdictions, current ecological conditions may only include conditions that are outside the NRV. And while management goals are not dictated by the NRV baseline in their evaluations, it is one source of information in guiding an assessment.
Not in isolation
The effects of natural disturbances cannot be considered in isolation. Disturbances may interact with one another, such that effects of an initial disturbance alter characteristics and effects of subsequent disturbances (Paine et al. 1998 Robertson and Platt 2001, Platt et al. 2002, Suding et al. 2004, Schroder et al. 2005). As a result, species may invade following sequences of disturbances, especially when de novo or rare and random disturbances are involved (Kercher and Zedler 2004, Zedler and Kercher 2004).
Natural landscapes can be greatly affected by human-caused alterations of natural disturbance regimes and by de novo anthropogenic disturbances. Altering disturbance regimes changes the environments to which species may have become adapted. Habitat fragmentation as a result of human activity is a major cause of indirect alteration of disturbance regimes, especially those of large-scale disturbances. Fires that otherwise might have swept across large regions of the southeastern U.S., for instance, are contained in much smaller areas by a fragmented landscape (Gilliam and Platt 2006). The result may be less frequent, but more intense fires that are now less dependent on global climate patterns and more dependent on fuel accumulation (Slocum et al. 2007). Similarly, floodplain communities once linked to natural flooding cycles are in altered hydrologic regimes (Sparks 1998, Sparks et al. 1990).
Human disturbances of natural ecosystems may reduce standing biomass and simplify community structure and composition (Menges and Quintana-Ascencio 2003)—or, on other occasions, they may increase biomass by interrupting normal burning cycles. Most significantly, human disturbance regimes typically deviate from historic ecological disturbance regimes and oftentimes result in radical shifts in the ecosystem, such as the introduction of exotic species (Menges and Quintana-Ascencio 2003).
Next Page: Military disturbances and associated ecosystem consequences
Author
Patrick Comer, Ph.D., Chief Ecologist
NatureServe
William Platt, Professor
Louisiana State University
Steve Orzell, Botanist/Ecologist
Avon Park Air Force Range, United States Air Force
Understanding landscape and ecosystem dynamics
8.1. Understanding landscape and ecosystem dynamics
Box 8.3: Conceptual ecological models to understand ecosystem dynamics