Scaling Ecological Dynamics

Garry Peterson

University of Florida

Gainesville, Florida

One of the central aims of global change research is to understand how local actions can produce global changes, and how those global changes may impact specific, local sites. The translation of data or understanding from one scale to another involves a process of scaling. Scaling can take any number of forms ranging from simple averaging or interpolation, to the use of complex simulation models. Most global change research has used simple scaling methods that often inadequately map processes from one scale to another due to the complexity of ecological interactions.

Scaling difficulties arise for several reasons. Firstly, ecological processes often contain positive or negative feedbacks which make simple extrapolation difficult and/or inaccurate. Secondly, an ecological process that is important at one scale may be unimportant at another, so even if a process is understood, that understanding may prove meaningless at a different scale. Thirdly, processes at different scales interact, making it difficult to isolate objects for study, and limiting the generality of conclusions. Finally, cross-scale interaction may self-organize ecological processes, meaning that alternative ecological organizations may be possible that do not exist in the current ecosystem. This means that scaling ecological processes requires understanding not only existing organizational hierarchies, but also the ways in which these hierarchies can reorganize.

The complexities of scaling challenge global change researchers to develop methods that effectively transfer, or transform, measurements and information across spatial and temporal scales. As discussed above, translating understanding across scales using addition or integration only works over a limited scale range. For example, representing a forest as a group of average trees may work to predict aggregate properties of the forest under current conditions, but will not be very useful in understanding how the forest could change in the future. That would require assessing the responses of different tree species. However, attempting to scale processes by adding new processes and variables as the scale range of a model increases can work across small ranges of scale, but will become unmanageably complex across broader ranges of scale.

Ecological Self-Organization

One alternative approach to scaling is to focus on the self-organization of cross-scale structure. Systems theory proposes that systems organize around a few key variables, which suggests that identifying these variables and constraints under which they interact will allow a dynamic, parsimonious method of translating across scales.

Scaling ecological processes requires understanding not only existing organizational hierarchies, but also the ways in which these hierarchies can reorganize.

Ecological organization is the result of the interaction among structures and processes operating at different scales. Ecologists have typically used hierarchy theory to analyze the effects of scale on the organization of ecological systems. However, hierarchy theory focuses upon the consequences of hierarchical organization, not on the processes that build and destroy hierarchy. Translating information across spatial and temporal scales requires understanding both how hierarchical organization is constructed and when it breaks down. The range of conditions under which a specific hierarchical organization maintains itself bound the application of scaling relationships derived for that hierarchical organization. These boundaries can be assessed by analyzing self-organizing processes that define ecological organization.

Self-organization occurs as nonlinear processes interact with one another and environmental heterogeneity. As small fast processes repeatedly interact with one another, these interactions may produce a larger slower structure that constrains the behavior of the small processes in such a way that they mutually reinforce one another. The "bottom-up" generation of such self-reinforcing structures provides a means of understanding how alternative hierarchical ecological structures can develop.

Ecosystems consist of the interactions between biotic and abiotic processes. Historical and current species extinctions and invasions have focused ecological interest on the role of species in the functioning of ecosystems. The variety of functions that a species can perform is limited, and consequently ecologists frequently have proposed that increases in species richness increases functional diversity. Often the analysis of ecological function has been confounded by scaling effects. Peterson suggests that ecological function and ecological organization can more clearly be understood when they are considered as emerging from a process of self-organization.

Resilience as an Emergent Property

Species perform many different types of ecological functions. They may regulate biogeochemical cycles, alter disturbance regimes, or modify the physical environment. Alternatively, a species may regulate ecological processes indirectly, through trophic interactions such as predation or parasitism, or through functional interactions such as pollination and seed dispersal. On the basis of these ecological roles, species can be divided into functional groups. Ecologists have proposed that increasing the number of functional groups in an ecosystem increases the amount of change a system can experience before it reorganizes. This property can be described as a system's ecological resilience.

Ecological resilience emerges from the self-organized interaction of many different processes occurring at different scales.

Ecological resilience emerges from the self-organized interaction of many different processes occurring at different scales. Resilience is enhanced when ecological interactions that dampen disruption reinforce one another. Self-organization occurs as interactions that are mutually reinforcing tend to persist, while those that disrupt one another will tend to vanish. Such situations may arise due to compensation when a species with an ecological function similar to another species increases in abundance as the other declines, or as one species reduces the impact of a disruption on other species.

Different species operate at different temporal and spatial scales, as is clearly demonstrated by the scaling relationships that relate body size to behavioral ecology. While many species may inhabit a given area, if they live at different scales they will experience that area quite differently, since ecological structure and processes vary with scale. This cross-scale variation provides different opportunities and costs to species that live at different scales. For example, a wetland may be inhabited by both a mouse and a moose, but these species perceive and experience the wetland differently. A mouse may spend its entire life within a patch of land smaller than a hectare, while a moose may move among wetlands over more than a thousand hectares (Figure 1.29). Scale separation reduces the strength of interactions between mice and moose relative to interactions among animals that operate at similar scales.

Just as species can be divided into functional groups, species can be grouped based upon the specific scales that they exploit. The ecological scales at which species operate often strongly correspond with average species body mass, making this measure a useful proxy variable for determining the scales of animals' perception and influence. The resilience of ecological processes, and therefore of the ecosystems they maintain, may depend upon the organization of functional groups within and across scales.

If species in a functional group operate at different scales, they provide mutual reinforcement that contributes to the resilience of a function, while at the same time minimizing competition among species within the functional group. This cross-scale resilience complements a within-scale resilience produced by overlap of ecological function among species of different functional groups that operate at the same scales. Since ecological resilience is constructed by processes interacting across a range of scales, it also varies with scale and by process.

The ecological scales at which species operate often strongly correspond with average species body mass, making this measure a useful proxy variable for determining the scales of animals' perception and influence.

From this perspective, ecological resilience does not derive from redundancy in the traditional engineering sense of repeated function; rather it emerges from overlapping function within scales and reinforcement of function across scales. The apparent redundancy of similar function replicated at different scales adds resilience to an ecosystem, because disturbances are limited to specific scales, while if functions continue to operate at other scales, they are able to maintain a function. An example of this interaction between disturbance and resilience was described in South Florida. During Hurricane Andrew in 1992, mature mangrove trees were killed by wind damage, however many young mangroves survived. Many of these young mangroves were located in gaps caused by lightning. Local lightning disturbances provided the mangrove population with increased resilience to the large scale disturbance produced by Hurricane Andrew.

Different species operate at different temporal and spatial scales, as is clearly demonstrated by the scaling relationships that relate body size to behavioral ecology.

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Figure 1.29

Time and space scales of the boreal forest, and their relationship to some of the processes which structure the forest. These processes include insect outbreaks, fire, atmospheric processes, and the rapid CO₂ increase in modern times. Contagious mesoscale disturbance processes provide a linkage between macro-scale atmospheric processes and micro-scale landscape processes. Scales at which deer mouse, beaver and moose choose food items, occupy a home range and disperse to locate suitable home ranges vary with their body size.

The analysis of ecological functions from a cross-scale perspective allows one to identify the scales over which an ecological process is robust, and over which it is brittle. For example, avian regulation of spruce budworm populations breaks down after budworm density exceed a critical level. This suggests that it is possible to identify scales of landscape change that may be most sensitive to changes in ecosystem function, or alternatively, at what scales critical ecological functions are in danger of being eliminated. Species turnover or process modification may not have immediately visible consequences, but they decrease ecological resilience to disturbance or disruption. For example, the loss of large birds from the forests of New Brunswick may reduce the scale range over which budworm predation is resilient, producing outbreaks at lower budworm densities.

The loss of species or processes produces ecosystems that are more vulnerable to ecological collapse, and reduces the variety of possible alternative ecological organizations . The loss of species that represent functional groups, especially large species that generate mesoscale vegetative patterns such as elephants, moose or tapirs, may eliminate possible types of ecological organization. There are suggestions that this may have occurred during the Pleistocene extinctions of mega-herbivores, and such losses appear to be particularly difficult to reverse, even if large scale ecological engineering projects are undertaken.

Conclusions

Translating either data or understanding across scales is often difficult. Peterson argues that moving across scales is difficult because the cross-scale interaction of ecological processes produces dynamic, self-organized hierarchical structures. Viewing ecological organization as being composed of self-organized systems rather than being constrained by a complex set of top-down controlling processes allows one to identify opportunities for cross-scale ecological re-organization. It shifts the focus of scaling from looking solely at how change is transmitted across scales, to looking at what are the situations in which reorganization within a scale can occur, or entirely new levels of organization may form. Doing this provides a framework to identify what types of qualitative change are possible and plausible.

Moving across scales is difficult because the cross-scale interaction of ecological processes produces dynamic, self-organized hierarchical structures.

Focusing upon the self-organized dynamics of cross-scale interaction lays the ground work for developing a theory of novelty. One thing that is virtually certain about the consequences of global change is that, both to citizens and to scientists, its consequences are going to be surprising. When the resilience of existing systems is overwhelmed, processes and events that have never happened before will happen. Formerly unimportant processes or species will become important, and formerly fundamental things will lose their importance. This departure from the past suggests that theories that assume stable cross-scale organization are inadequate. Science requires a better understanding of how novel organization arises, and self-organization provides a framework upon which to build such a theory.