The continental hydrological cycle from the perspective of the organizational theory of ecological functions

10 April 2024

In a previous post, we discussed causal circularity in living systems using the continental hydrological cycle as an example (which can be consulted in the text “Do forests play a role in the continental hydrological cycle in tropical regions?”). We concluded that text by asking whether forests indeed play a role in maintaining the continental hydrological cycle in tropical regions such that they can be said to be involved in their own self-maintenance. But could the fact that the flow of water in forests is controlled by living systems be an indication that the forests have a function in the continental hydrological cycle?

According to the theory of biological autonomy, biological functions are always attributed to components that act as constraints within the organizational closure of a living system. To illustrate what it means to assign a function to a constraint in accordance with this theory, let’s consider a classic example from debates about functions: the heart. The heart is a constituent part of many animals and has the function of pumping blood. In terms of the theory of biological autonomy, we can assert that when the heart pumps blood, it is acting as a constraint. Firstly, blood pumping has the causal power to modify, for instance, the distribution of gases and nutrients in an animal body. In a specific sense, it reduces the degrees of freedom in this distribution process (which occurs, of course, with the contribution of several other constraints, such as the entire set of arteries, arterioles, capillaries, etc.). Secondly, if we take into account that it takes approximately a minute for blood to circulate throughout the body of a human being, for instance, we can observe that, within the timescale in which the heart, while pumping blood, constrains the distribution of gases and nutrients in the body, it remains conserved, along with the arteries, arterioles, and capillaries. In this precise sense, there are no changes in the properties relevant to its role as a constraint that are brought about by the blood circulation process within that timescale.

The attribution of a function to a constraint, according to this theory, is rooted in the causal role that a particular component of a system plays in vital processes within a living system, operating as a constraint within the closure of constraints that characterizes its organization. This implies that this component contributes to the maintenance of the body organization and, simultaneously, is sustained through the roles of other constraints and the organization itself. Let’s once again consider the example of the heart. By pumping blood and thus acting as a constraint on the distribution of gases and nutrients, the heart contributes to the maintenance and existence of all other body parts, as well as the organism itself, which, in turn, contributes to the maintenance and existence of the heart itself. As mentioned earlier, closure of constraints underscores a fundamental characteristic of biological systems: their constituent components and operations are interdependent in their maintenance and collectively determine the conditions necessary for the system’s existence.

In the theory of biological autonomy, constraints that arise under the influence of other constraints are termed dependent, while those that contribute to the production of other constraints are labeled as enabling. For a constraint to be considered part of the closure of constraints, it must fulfill both roles, i.e., act as both a dependent and an enabling constraint. However, what about situations where constraints are exclusively dependent or enabling? Constraints that are only enabling or dependent establish connections between the living system and other systems, whether living or not. These systems either form the living system’s environment or operate at the physicochemical levels of processes within living beings. This shows that characterizing a system as organizationally closed doesn’t require all constraints affecting its dynamics to be part of the closure. It also underscores that discussing organizational closure doesn’t imply advocating for independence of the living system from its environment. A system that exhibits organizational closure of constraints is, in fact, a physically open system intrinsically linked to its environment, with which it exchanges energy and matter. Without this connection and the exchange of matter and energy the living system cannot maintain itself.

The theory of biological autonomy began to take shape in the 1990s, primarily focusing on cells and organisms. Enzymes and organs, such as the heart, served as examples to illustrate how components of living systems play functions when they act as constraints. For instance, enzymes catalyze reactions within cells, reducing their degrees of freedom, and organs such as the heart constrain the degrees of freedom in processes such as the distribution of gases and nutrients in specific organisms. In 2014, three researchers published a paper extending the organizational explanation proposed by this theory to ecological systems. They argued that the functions of components in ecological systems, operating at the ecosystem level, can be seen as precise (differentiated) effects of both biotic (living) and abiotic (inanimate) components that act as constraints on the flow of matter and energy in ecosystems (Nunes-Neto et al., 2014; El-Hani and Nunes-Neto, 2020). This demands that ecosystems exhibit an organization framed in terms of a closure of constraints. Nunes-Neto, Moreno, and El-Hani employed the phytotelma of a bromeliad as a model of an organizationally closed ecological system to develop their theory (Nunes-Neto et al., 2014). They considered the intricate relationships of predation and decomposition within the phytotelma internal food web, involving a spider species, mosquito larvae, and microorganisms. This food web results in a reduction in the degrees of freedom in the flow of atoms such as those of nitrogen. The model was built with two hierarchical levels—one representing the flow of atoms and the other, the roles of the biotic components acting as constraints. This pioneering proposal marked the initial steps toward establishing an organizational theory of ecological functions, which has seen continuous refinement and development since.

Organizational theory of ecological functions and cloud formation in the oceans

A few years later, El-Hani and Nunes-Neto (2020) took the organizational theory as a starting point to address the transition from a pre-biotic world – composed of purely physicochemical systems – to a life-constrained world. They described how the cloud formation system in the oceans does not just result from a sequence of physicochemical events related to water evaporation and precipitation (i.e., it does not just result from a closure of processes). There is an active and decisive participation of a network of interactions between marine organisms, especially phytoplankton, which lead to the secretion of a sulfurous substance, dimethylsulfide (DMS). DMS contributes, in turn, to the formation of cloud condensation nuclei over the ocean, in a process similar to that which occurs on the continent involving aerosols released by forests, as explained above. When clouds precipitate, the rain carries precursors of dimethylsulfide to the ocean, making them available for the metabolism of marine organisms. This completes the cycle established between these organisms and the formation of clouds. As noted by these authors, it’s important to recognize the mutual dependence relationship between marine microbiota and clouds, which can be understood in terms of their roles as constraints acting on physicochemical processes at specific temporal scales. More precisely, they serve as both dependent and enabling constraints in the control of dimethylsulfide flow. The microbiota relies on sulfur deposited by cloud precipitation and transported by rivers, just as clouds depend on sulfur derived from the dimethylsulfide produced by the phytoplankton. Thus, the authors propose that the organizational theory of ecological functions offers a consistent foundation to explain the transition from a closure of processes, in which the sulfur and water cycles corresponded only to a closed sequence of physicochemical dynamic states, to a system characterized by a closure of constraints, in which life begins to exercise control over these two cycles. If we generalize this argument, we can then reach a central thesis of the Gaia theory, namely, that when living beings began to control an important part of the planetary physicochemical processes, we transitioned from a world controlled only by such processes to a life-constrained world.

In this paper on the relationship between marine life and cloud formation over the oceans, El-Hani and Nunes-Neto explore the attribution of ecological functions to inanimate or abiotic components. However, for these components to be included as functional elements, they must, like biotic elements, satisfy a fundamental criterion of the theory, namely, they must be subject to closure. This means they should act as internal constraints in the organization of the system and, as a result, be under its control. As previously discussed, being an internal constraint on the organization of the system implies, according to this theory, that the component (in this case, an abiotic item) must act as both a dependent and an enabling constraint. If it does not fulfill this role as an internal constraint in the organizational closure, ecological functions cannot be ascribed to these components. It’s important to note, however, that they can still be viewed as relevant to the dynamics of the ecological system, as they can act as external constraints that influence its processes, even though they are not part of its internal organization.

Exploring another case in a book chapter published this year (El-Hani, Lima & Nunes-Neto, 2024), proponents of this approach explain that in savanna ecosystems, the control exerted by fire-adapted plant species on fire, through characteristics related to their flammability, i.e., how easily they enter into combustion, serves as an example of how a constraint that was once external to the system (the fire itself) can become an integral part of its internal dynamics when placed under the control of its organization. When fire becomes integrated into the ecosystem dynamics through plant species adapted to it, which exhibit flammability characteristics that influence its frequency, it takes on a constructive role in the ecosystem dynamics. Fire is an enabling constraint in savannah ecosystems, contributing to plant regrowth processes, while also being a dependent constraint because its occurrence relies, to a certain degree, on the plant species adapted to it. In these instances, one can even talk about coevolution of fire and biota (McLauchlan et al., 2020). When fire is not under the control of the system, it acts just as an external constraint, influencing ecosystem dynamics but without being governed by its organization. This lack of control may be a reason for fire having a destructive role in this case.

But beyond the relationship between marine organisms, water and sulfur in the oceans, as well as plants and fire in savannah environments, how could the organizational theory of ecological functions explain the relationship between forests and rain on continents?

Organizational theory of ecological functions and the continental hydrological cycle

Based on the arguments presented above, we can reflect on the case of the continental hydrological cycle, which we discussed at the beginning of this text. Could this also be a case of organizational closure in an ecological system, in which trees play a crucial functional role in exchanges between continent and atmosphere? Just like in the example of the heart, according to the organizational explanation we can understand that trees are a constitutive part of the continental hydrological system and play the function of controlling exchanges between continents and the atmosphere in the hydrological cycle. More specifically, they constrain the flow of water, energy, carbon, and other elements between these two environments. In doing so, they contribute to the maintenance and existence of other parts (e.g., clouds, water) of the hydrological system on the continents, as well as to the maintenance and existence of the system itself, which ultimately contributes to the maintenance of the trees.

Furthermore, can we say that water in humid forests, initially an external constraint, has been recruited as an enabling and dependent constraint to form the internal organization of these ecosystems, similar to the case of fire in savannah ecosystems? Firstly, we can consider that water is one of the primary shaping agents in humid forests, influencing the distribution, physiognomy, and species diversity characteristic of these ecosystems (Ellison et al., 2017, 2012). Water can play crucial roles in the ecosystem processes of a humid forest while being under the control of internal constraints within its organization, such as plants. For example, plant characteristics can determine varying rates of evapotranspiration and modulate the emission of cloud-nucleating aerosols in these ecosystems. These factors interact with other climatic elements, determining the volume of water circulating in the system and influencing the continental rainfall patterns. Once water precipitates, its role in seed germination becomes crucial. The plants that germinate from these seeds will subsequently evapotranspirate and emit aerosols, contributing to further rainfall, thus closing the cycle.

If the Amazon rainforest and water are constraints within the organizational closure of the continental hydrological system, we can claim that they not only determine each other’s maintenance and existence but also of the system itself. Therefore, according to the organizational theory of ecological functions, we can assert that forests grow where it rains, and it rains where forests grow. Similarly to the hydrological cycles in oceans, at some point in the Earth evolutionary history, the hydrological cycles in continents underwent a transition from a closure of processes to a system characterized by a closure of constraints. This was also part of the transition to a world constrained by life, where plants became facilitators while also depending on the continental hydrological system.


El-Hani, C.N., Nunes-Neto, N., 2020. Life on Earth Is Not a Passenger, but a Driver: Explaining the Transition from a Physicochemical to a Life-Constrained World from an Organizational Perspective, in: Baravalle, L., Zaterka, L. (Eds.), Life and Evolution, History, Philosophy and Theory of the Life Sciences. Springer International Publishing, Cham, pp. 69–84.

El-Hani, C. N., Lima, F. R. G. & Nunes-Neto, N. F., 2024. From the organizational theory of ecological functions to a new notion of sustainability. In: Mossio, M. (Ed.). Organization in Biology. Springer International Publishing, Cham, pp.  285-328.

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