Conceptual Basis for Ecological Prosthetics as a Subclass of Technoecosystem Engineering

David M. Blersch, Dept. Biological Resources Engineering

University of Maryland, College Park, Maryland

ABSTRACT

The engineering and operational dynamics of technoecosystems (hybrids of technological and ecological systems) were investigated, focusing specifically on novel behavior exhibited by an ecosystem when given control over its own energy sources via artificial feedback control circuits. A technoecosystem was constructed based upon wetland soil microcosms using redox potential as an indicator of system metabolism and as the controlled parameter. The system design and behavior suggests the ecological role of technological feedback control circuitry. The idea of ecological prosthetics is proposed as a conceptual framework with which to understand certain ecological/technological interactions of engineered ecosystems.

INTRODUCTION

Given the impact of human activities on ecosystems at all scales, it is important to consider the ecological role of technology. The research described here focuses on the role of technology in ecosystems through observation of the dynamics of a complex system composed of both biological and technological components.

The current state of information technology allows the creation of technoecosystems--hybrid complex systems comprising biological and technological components (Table 1). Electronic sensors and computerized monitoring and control programming can be used to supply information feedback loops to an ecosystem, possibly allowing the ecosystem a level of autonomy not normally found in nature--for example, allowing the ecosystem to control its own sources of energy. The resulting technoecosystem organizes in ways different from its natural analog, but still along pathways dictated by thermodynamic laws of nature. Thus system development and behavior might be analyzed using principles of ecology.

Term (Authors)

Definition

Technoecosystem (Odum, 1993)

“Systems in which formerly wild components of ecosystems are incorporated into technological systems as hybrids of living units and hardware homeostatically coupled.”

Technoecosystem (Duffield, 1976)

“Large, complex, spatially or functionally distinguishable… industrial systems under conscious human control viewed as ecosystems.”

Ecocyborg (Clark, et al., 1999)

“Systems that consist of both biological and technological components that interact at the scale of an ecosystem, where the latter is defined as a community of organisms together with their abiotic surroundings.”

Table 1. Relevant definitions available in the literature.

METHODS

A data acquisition (DAQ) system was constructed in the laboratory using a data acquisition card installed in a Pentium computer. The computer was programmed using Labview v.4.0 to monitor voltage from a number of analog input channels. The program also controlled a relay-switched power outlet via digital signal.

Wetland soil microcosms were constructed in 1-L jars. Wetland soil was harvested from USDA land in Beltsville, MD. Large woody debris was removed from the soil, and 300 g of wet soil was placed in each jar. Then, 300 mL of distilled water was placed in each jar. The microcosms were sealed and allowed to settle for 1 hr.

Calibrated platinum redox probes were installed through ports in the lids of each microcosm with the platinum tips near the bottom of the soil layer. A salt bridge was also installed in the water column in each microcosm, connecting it to a calomel reference probe in a KCl salt bath. In addition, feed tubes were installed in the water column in the experimental unit.

The redox and calomel reference probes were connected to the analog input channels on the DAQ computer. The feed tubes for the experimental unit were connected to variable-flow peristaltic pumps, one of which drew from a carbon reservoir (2.0 M sodium acetate) and the other from a nitrate reservoir (1.0 M potassium nitrate). Each pump was calibrated to deliver approximately 2 mL/sec. A switched power outlet controlled by the digital output of the DAQ computer allowed automatic on/off control of the pumps. Figures 2 and 3 show a picture and schematic of the experimental setup.

Program settings were input into the computer to result in the logic flowchart detailed in Figure 1. Redox potential (Eh) readings were taken every 30 minutes. The lower and upper redox thresholds were set for +200 mV and +250 mV, respectively. The computer compared the measured Eh with the thresholds. If the measured Eh was greater than the upper threshold, the carbon pump was turned on for 1 sec; if it was less than the lower threshold, the nitrate pump was turned on for 1 sec. For each experiment, the entire system was turned on and allowed to run for a number of days. The DAQ system recorded the redox potential and the pump event for each timestep.

Figure 3. Schematic layout of the wetland soil microcosm techno-ecosystem setup with automatic carbon and nitrate feed control.

Figure 2. Photograph of laboratory scale wetland soil technoecosystem setup with automatic carbon and nitrate feed control.

Start

User inputs:T, t, path,Eh lo, Eh hi

Measure Eh(Analog Input

Ch. 0)

Is Eh > Eh hi?

Wait time T

Activate Carbonpump for time t

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Record Time, Eh,"0" for both

pump events.

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Record Time, Eh,"1" for C-pump event,"0" for N-pump event

Is Eh < Eh lo?

YES

Activate Nitratepump for time t

(Dig. pulse line 1)

NO

Record Time, Eh,"0" for C-pump event,"1" for N-pump event

Figure 1. Flow chart for redox potential control program with nitrate or carbon source selection.

RESULTS

• All experimental units actively controlled inputs of nitrate and carbon as they became limiting as indicated by the Eh in relation to the threshold setpoints.

• Generally, all experiments exhibited oscillatory variation in Eh as it converged on a quasi-steady state within the upper and lower threshold setpoints (Figure 4).

• Significant variability existed between experimental replicates (Figure 5), but mean of all experimental units exhibits strong influence of the controlled inputs on the Eh as compared to the controls receiving no input (Figure 6).

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Trial 14: Carbon/Nitrate selection (modified repeat of 13).USDA Soil (2 weeks old).Data collection every 15 minutes, Pump every 2 sample period (30 minutes).

Upper threshold: +250 mV

Lower threshold: +200 mV

Figure 4. Typical results of redox potential vs. time for wetland soil techno-ecosystems receiving 2.0 M sodium acetate solution and 1.0 M potassium nitrate solution added via controlling computer.

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Figure 5. Redox potential vs. time for all nitrate/carbon experimental trials, showing the variability of results among the set of trials.

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Figure 6. Mean values of redox potential averaged for each time step for controls group and nitrate/carbon addition groups vs. time. Error bars represent standard error.

IMPLICATIONS

• Viewed from the perspective of the microbial ecosystem in the soil microcosm, the measurement and control circuitry comprise an artificial information pathway that allows access to additional sources of energy.

• The components within the ecosystem that can take advantage of the technological feedback pathways are favored, and the system thus self-organizes to harness the pathways to the fullest extent.

• Coupling of the technological components to the ecological system causes a translation of system boundaries to include previously external sources of energy (Figure 7).

A.

Microcosm

B.

Microcosm

Technoecosystem

Electricity

Computer

Figure 7. Translation of energy sources by the addition of artificial feedback: (A) original microcosm functioning off internal energy reservoirs; (B) microcosm accessing previously external energy sources, now internalized.

ECOLOGICAL PROSTHETICS

Although the physical materials used to construct the technological feedback are artificial to the natural system, the feedback loop conceptually may not be:

In a flooded wetland, redox potential drops as electron acceptors are used up. Reduction of electron acceptors lower on the redox scale can result in products detrimental to wetland plant survival--for example, hydrogen sulfide from sulfate reduction (Pezeshki, 1994). Decomposition of plant material releases organic matter and nitrogen which, in turn, affect the redox potential. Over time, the rates of these processes will balance to a quasi-steady state of redox potential.

Thus redox potential acts as a control on plant survivorship which, in turn, acts as a control on redox potential. The technological feedback control circuit in these experiments may in fact be interpreted as a technological substitution of an existing control mechanism (Figure 8)—in other words, an ecological prosthesis, where:

PROSTHESIS: The artificial replacement of a functional biological part.

Characteristics of the prosthetic ecological circuit:

It has the potential to operate at a substantially different rate than the natural analog.

It has the potential to be networked simultaneously to multiple levels of control hierarchy.

It allows access to outside sources of previously unavailable energy.

CONCLUSIONS

• Construction of technoecosystems may be accomplished via a process akin to prosthetic engineering by the substitution of natural feedback circuits with technological counterparts.

• Conceptualizing technological control circuitry as ecoprosthetics may aid in the engineering of technoecosystems through analogic thinking, e.g., using engineering control theory and hierarchical analyses.

• Ecoprosthetics is one of a number of possible sub-categories that may be defined as part of a classification scheme for technoecosystem engineering.

Microcosm

TechnoecosystemElectricity

Computer

Org.Matter

Reduced

Oxidized

Microbes

RedoxPotential

A.

B.

Figure 8. Feedback control circuit substitution in the construction of wetland soil technoecosystems that forms the conceptual basis of ecological prosthetics. (A) Natural wetland ecosystem showing candidate feedback circuit (red) and energy source (blue); (B) wetland soil technoecosystem with technological feedback circuit substitution (red) and novel energy source (blue).

References for this poster:

Clark, O.G., Kok, R., Lacroix, R. 1999. Mind and autonomy in engineered biosystems. Engineering Applications of Artificial Intelligence 12(3): 389-399.

Duffield, C. 1976. Geothermal Technoecosystems and Water Cycles in Arid Lands. Arid Lands Resource Information Paper No. 8. University of Arizona Office of Arid Lands Studies, Tucson, Arizona.

Odum, H.T. 1993. Ecological and General Systems: An Introduction to Systems Ecology. University Press of Colorado.

Pezeshki, S.R. 1994. Plant response to flooding. Plant-Environment Interactions (R.E. Wilkinson, ed.), pp. 289-321. Marcel Dekker, Inc., New York.