ASM Workshop Summary: When and how does structure affect function? A discussion of structural constraints on process at different LTER sites (Sept. 20, 2003)
Organizers: Tamara Hochstrasser and Jin Yao, Jornada Basin LTER.
19 participants from JRN, SEV, LUQ, FCE, KNZ, VCR, HBR, University of Nebraska, Oregon State University, and two international LTER sites, Kiskun LTER in Hungary, and Ben Gurion University in Israel.
Background and Motivation for the workshop
In order to compare complex systems with multiple interacting scales it is important to have a conceptual framework that allows us to describe these systems in a comparable way. The fundamental distinction between the ‘structure’ and the ‘function’ of a system and the understanding of the interrelationship between ‘structure’ and ‘function’ have long guided the thoughts of ecologists and have been useful in constructing models of ecological systems. Similar to other concepts in ecology the use and understanding of ‘structure’ and ‘function’ in ecological systems is constantly evolving. The two-way interaction between the structure and the function of a system is now commonly recognized at multiple scales, and often biological as well as abiotic structure(s) and function(s) are studied. The extensive use of the words ‘structure’ and ‘function’ has created the need for ecologists to clarify and redefine the notions of ‘structure’ and ‘function’. The goal of this workshop was to provide an opportunity for workshop participants to discuss their understanding of ‘structure’ and ‘function’ in their study systems and to summarize how ‘structure’ and ‘function’ as well as the relationship between ‘structure’ and ‘function’ are commonly understood by ecologists nowadays.
In our discussion, we emphasized the importance of structural constraints on ecosystem function. This corresponds to an increased recognition of the importance of structural constraints on ecological processes in recent ecological research. It is now commonly understood that for a fundamental ecological process common among systems (e.g. photosynthesis, water and nutrient cycling), structural constraints on the process often differ widely among systems. Therefore, a good understanding of variation among structural constraints on a process is needed to build an appropriate framework for comparing among systems and scales as is planned for the synthesis effort within the LTER network. Furthermore, a strong theoretical framework on the interrelationship between structure and function may help us infer function from structure. This would be beneficial for both fundamental ecological research and management, because often ecological structure is easier to measure than function and it is also easier to manipulate for management goals.
Workshop Discussion
The workshop consisted mainly of two consecutive discussion sessions. The questions discussed in the second discussion session were inspired by the outcome of the first session. During the first session, we focused on the studies described in six abstracts selected by the workshop organizers (references of these abstracts can be found at the end of this summary). Some of these abstracts summarized studies that were clearly addressing structural constraints on function, others were clearly not, and for some of them the classification in either of these categories depended on a reader’s definition of structure, function, and structural constraints on function. The discussion of these abstracts helped us to identify how structure and function are distinguished in ecological studies and what kinds of structural constraints occur in different ecosystems. As a result of this discussion, we summarized ideas as to what terms define ‘structure’ and ‘function’ in different study systems, and we listed examples of structural constraints as found in the abstracts.
During the second session, we discussed aspects of ‘structure’, ‘function’ and ‘structural constraints’ more in depth and breath, based on any studies we conducted, conceived, or knew of. We focused on three main questions: 1) What structural constraints are occurring in your study systems? 2) What are elements of structure and how could we study structural constraints on function? and 3) Can function be something that can not be measured as a rate? (because it was proposed during the first session that function is usually measured as a rate.)
Summary of discussion outcome
It was hard to agree on a general definition of what ‘structure’ and ‘function’ represent in ecosystems. Workshop participants listed a number of elements, though. Usually there was some debate on what terms needed to be included in the definition, but participants could agree on examples. A rough definition of the terms ‘structure’, ‘function’ and ‘structural constraints on function’ and a number of examples emerged from the workshop:
Structure was defined as a feature of the study system characterized by spatial or temporal variability, i.e. there are subunits that can be distinguished/ delimited either in space or in time. Structure points to an arrangement of parts in a whole. This arrangement, spatial/ temporal variability can be described in different ways according to the perspective of the investigator. Structure maybe heterogeneous or dynamic, but the cause of its heterogeneity or dynamics is not the focus of a study that examines structural constraints on function (in statistical terms structure is the independent variable). In a system theoretical context, structure is often depicted by a state variable. There was also a common recognition that multiple structures are often found in study systems and that these structures were part of the system. However, structures external to the system could also play an important role in ecological function. Examples of structure include patches in a landscape, species composition, population age structure, food web organization, species morphology.
Function is a process/ dynamic change of interest for the comparison between study systems. In a study on structural constraints on function, we would like to explain the variability of this process among systems (in statistical terms function is the dependent variable). Ecological function is different from mathematical function. Ecological function is temporal not spatial; it often manifests itself as a flow (e.g. flow of water, wind) and/or can be measured by its rate. Examples of function include growth, recruitment, mortality, nitrogen cycling, carbon sequestration, primary productivity, respiration, reproduction.
Given this definition can function be something that can not be measured as a rate? – It was proposed that this is possible, that function could consist of a series of events or manifestations that would be more appropriately measured in a categorical/ordinal way (e.g. the behavior of animals, phenological stages of plants).
Structural constraints on function are therefore constraints exerted by a structure on a function, and the constraints can be either positive or negative (i.e. if a structure has an identifiable effect on function). This can be distinguished from constraints by resources. The latter are constraints on the function, but when the temporal/spatial variability of resource is not taken into consideration for the study, such constraints are not structural. Examples of structural constraints on function include the stand structure of a forest constraining primary productivity, seagrass patchiness affecting survivorship of the scallop, earth-dikes in arid lands influencing surface-flow of water.
Upon further examination structural constraints were found in almost all the study systems of workshop participants. For example, availability of buds limiting grass plants for tillering, and different levels of salinity in Florida salt marshes affecting population dynamics of invertebrates.
How could we study structural constraints? Workshop participants proposed two experiments. Experiment 1 was designed to study how spatial arrangement of individual plants affects productivity, given the same species richness and density. For Experiment 2 it was proposed to study the influence of forest architecture on primary productivity: e.g. by removing the vines in a tropical forest.
Brandon Bestelmeyer summarized the discussion outcomes. Again, he emphasized the two-way relationship between the structure and function of a system. He stated that functions generate other potential structures that in turn affect the same or other functions. Furthermore, feedbacks between structure and function may occur, or functions may determine structures outside of the focal study domain, which leads to new questions. These alternating structure-function relationships often occur across scales.
The workshop participants showed great interest in clarifying the definitions of structure, function, and structural constraints on function; it is clear that this type of discussion is worth pursuing. Two main considerations may be of interest in future discussion. First, most workshop participants were quite comfortable with the idea that a variable can be considered a ‘structure’ in a particular context and a ‘function’ in another context. Therefore, it is important to find the criteria for determining under what circumstances a variable should be considered a ‘structure’ or a ‘function’. Second, there was also a strong interest in finding better ways to study structural constraints more effectively and explicitly in ecosystems. Given such interest and importance, we encourage everybody to develop research in this field, as it seems promising for the comparison of LTER sites. The workshop organizers will make an effort to clarify remaining questions and publish a review paper. If you would like to engage in further discussion on this topic please contact Tamara Hochstrasser (tamara.hochstrasser@ucd.ie).
Cater, TC, and Chapin, FS. 2000. Differential effects of competition or microenvironment on boreal tree seedling establishment after fire. Ecology 81(4): 1086-1099.
Huenneke, LF, Anderson, JP, Remmenga, M, and Schlesinger, WH. 2002. Desertification alters patterns of aboveground net primary production in Chihuahuan ecosystems. Global Change Biology 8(3):247-264.
Irlandi, EA, Ambrose WG, and Orlando, BA. 1995. Landscape ecology and the marine-environment – how spatial configuration of seagrass habitat influences growth and survival of the bay scallop. Oikos 72(3):307-313.
Ludwig, JA, Tongway, DJ, Eager, RW, Williams, RJ, and Cook, GD. 1999. Fine-scale vegetation patches decline in size and cover with increasing rainfall in Australian savannas. Landscape Ecology 14(6):557-566.
Quetin, LB and Ross, RM. 2001. Environmental variability and its impact on the reproductive cycle of Antarctic krill. American Zoologist 41(1):74-89.
Sala, OE; Parton WJ, Joyce LA, and Lauenroth, WK. 1988. Primary production of the central grassland region of the United States. Ecology 69(1): 40-45.