Ecosystems as superorganisms: the neglected evolutionary implications.
A new conception of evolution is considered in the light of ecosystems as superorganisms, where competition takes place and selective pressures are transmitted throughout the fabric of an ecosystem via chains of interacting niches, and the end result is a coordinated evolution where the structure of the ecosystem exerts control over the direction of evolutionary change. Evidence is presented for this position in the form of observations on the role of species adaptation as a barrier to biological invasion within climax ecosystems. Finally, humanity, as a possible ecosystem level 'virus,' is considered in the context of its obligations to the whole ecosphere.
It would seem that individual ecosystems at a glance possess properties of whole organisms. The ability to "reproduce" their particular community assemblages in multiple locations throughout a given region, via displaced succession and their ability to seemingly "sense" their surroundings, collectively, and react appropriately could be taken as just two examples, though there are many others. The central issue is whether or not these characteristics are the product of a sum total of largely independent processes operating on the level of species or individuals--which is the current paradigm, ushered in by H. A. Gleason, or whether these processes are somehow linked and interdependent on one another, as F. E. Clements, and more recently J. E. Lovelock, have suggested. (1,2,3) This could imply the existence of a degree of networking within ecosystems that has profound implications for the neo-Darwinian concept of selection only for fitness.
Are Ecosystems Superorganisms?
Superorganism is a term used to describe a collection of organisms that exhibit individual division of labor and eusociality. (4)
Detractors claim that within true superorganisms, the individuals comprising the entity need to be in physical contact with one another and that the entity its self needs to be the unit of selection, which would seem to rule out ecosystems completely. (5)
Within the framework of these minimalist criteria, there are entities that must comprise true superorganisms, like Eukaryotic cells, which contain either Chloroplasts or Mitochondria. Both of these organelles posses genetic material of their own indicating a separate evolutionary origin from the rest of the cell they inhabit. Both are only found in nature as part of Eukaryotic cells, where they exist in an endosymbiotic relationship with the host cell, and said cell is generally regarded as the unit of selection. (6)
Controversially considered by some as superorganisms are hierarchical insects such as ants and bees. Colonies of these eusocial organisms seem to act as a unit of selection in their own right, as there exists sterility amongst the workers. Reproduction is the prerogative of the Queen, yet mutations beneficial or otherwise are only outwardly manifested in the "super-phenotype" of the colony; it is this that is subject to the pressures of selection, even though individual ants, wasps, bees, termites, etc. are free living as opposed to being in physical contact with one another. (4)
There is no general eusociality between components on the level of the ecosystem, although there is differential fitness, and this can be important at different stages of succession and community assembly. Most ecosystems also fail the "superorganism test" by the "physical contact" criteria, although there are bacterial biofilm ecosystems in which species do exist in close physical proximity to one another and do exhibit something akin to eusociality, but these are very much the exception rather than the rule as far as ecosystem organization is concerned.
Another approach to the question entirely, is to afford ecosystems the quasi-superorganismal status of "superconstruction" as do F. J. Odling-Smee and colleagues. (7) They argue that ecosystems posses some of the properties of whole organisms but not all of them.
Ecosystems, like hierarchical insect colonies, lack physical interconnectivity between parts, but could theoretically be considered as a unit of selection, and indeed are by some like R. Lewontin, who has argued forcefully for the idea that there are multiple levels of selection operating in nature. (8)
Ecosystems as units of selection could be effectively characterized by the type of biotic and abiotic interactions that allow them to possess certain types of communities, and perform certain collective functions (their assembly rules). Ecosystems possessing different assembly rules could compete with one another, and factors like differential efficiency could allow for one type of ecosystem to predominate over the other. Experiments involving artificial selection between ecosystems would seem to support this. (9,10)
Returning to the stringent definition of superorganism employed by detractors of the notion, a case could even be made for ecosystems possessing yet another of the major requisite properties, namely connectivity between components, but not necessarily of a physical nature. To understand this it is necessary to return to the species niche concept.
The Species Niche and a new conception of natural selection
Niches are variously (and often overly simplistically) described in textbooks as "feeding patterns" or "regions" within an ecosystem where a particular species can be found.
The father of the niche concept was J. Grinnell, who proposed that the niche was a recess within an ecosystem combining both a species living and feeding requirements that could potentially be vacated by one species and occupied by another. (11) C. Elton regarded the niche as being analogous to a species position in a food web but like Grinnell's definition it was purely qualitative and therefore had only limited descriptive power. (12)
G. E. Hutchinson revolutionized ecology by characterizing the niche as an n-dimensional hypervolume whose dimensions correspond to environmental gradients along which the species in question was differentially distributed. This niche has two aspects, the realized niche, which is the portion of the hypervolume that is not partitioned with any other species niche, and the fundamental niche, which is the totality of the hyper volume, the sum of both the partitioned and non-partitioned niche dimensions. (13)
More recent definitions have been focused on incorporating both a species impacts on other species and its environment, and its requirements--but like the Hutchinsonian niche, these tend to use the notion of multivariate space. (14)
The process of natural selection can be looked upon as the differential survival of phenotypes subject to a changing environment. However it is necessary to consider this process in the context of the species niche. Odling-Smee et al suggest an evolutionary niche concept, where the natural selection histories of species are its dimensions. (7) If we consider changes to the ranges of resource gradients in a Hutchinsonian niche to be analogous to these natural selection histories then a novel definition of evolution could be derived. Whereas traditionally, evolution has been seen as a change in allele frequencies in a population over time, it can now be visualized in the context of the ecosystem as a change in the ranges of dimensions comprising a species niche resulting in a corresponding, appropriate change in allele frequencies.
Once a change in the ranges of niche dimensions occurs there would be a time delay before the appropriate mutant genes emerge in the species gene pool. This phenomena represents a form of "Evolutionary inertia" or "catch-up time" between an ecological change and an evolutionary one, implying that perhaps some short term aspects of a species evolutionary development could be "pre-determined" by the super structure of the Ecosystem, so to speak. The orthogenetic implications are obvious, however the "force" that is driving the evolutionary change is not intrinsic or "vital" but extrinsic--originating from the structure of the Ecosystem it's self.
It is not simply changes in the ranges of environmental gradients that could generate selective pressures, but also changes in the ways in which species niches interact with one another. Niche construction occurs where in the absence of adaptations, species "adapt" the environment to their own requirements. (7)
By engineering environments, niche constructing organisms regulate environmental gradients in profound ways, which could effect the distribution of other species. Although, as has been mentioned, there may be little physical interconnectivity between the components of an ecosystem, there is a tremendous degree of abstract interconnectivity where species niches are in constant contact with one another, reciprocally imposing conditions for existence.
Given this "hyper-connectivity," it is possible that evolutionary changes effecting one species could be felt throughout the whole ecosystem. It is unlikely that such an interconnected structure would have emerged without the ability to regulate its self. Within an ecosystem "network," evolutionary events could be coordinated cybernetically, where chains of interacting niches transmit selective pressures throughout the whole ecosystem, like a nervous system. The ultimate goal of the selection of a successful trait in this model of evolution should be to optimize the stability of the whole ecosystem, in addition to improving fitness. Theoretically then the stability benefits to an ecosystem with an increasing biodiversity are additive and cumulative.
"Species loading" as an ecosystem defense mechanism
It is thought that highly biodiverse ecosystems containing many species with specialized adaptations posses a great degree of functional redundancy, which confers structural versatility; this being the diversity-stability hypothesis. (15) A by product of increasing the diversity within an ecosystem would be the loss of successively greater amounts of unutilized niche space.
There is evidence suggesting a positive correlation between functional diversity and resistance to invasions within ecosystems. (15,16) When invading species attempt to establish themselves within ecosystems, they have the potential to disrupt them, as there is a chance that there will exist no predator/prey feedback to regulate the invaders proliferation.
When an ecosystem moves towards greater component functional diversity it becomes successively more difficult for invading species to establish themselves. If an invading species attempts such, it will likely encounter extremes of competitive exclusion as many of its requisite niche dimensions will be heavily partitioned with more adapted species, reducing the invaders fitness.
Similarly when new species evolve within ecosystems exhibiting high degrees of "niche-packing," they are very rarely successful, but when they are, they usually have very highly specialized adaptations, like polyploid plants. Naturally occurring polyploid plants are very rare, but are occasionally observed. (17) This would be expected as the likelihood of such a large scale mutation being beneficial is very slim, but the end result, if successful, is macroevolution within one generation. (18) As we can see, this is a particularly strong instance where the structure of the ecosystem plays a dominant role in the direction and rate of the evolution of its components.
The correlation between functional diversity and stability is currently a subject of intense discussion amongst Ecologists, but perhaps the most important interpretation that can be drawn from this is that ecosystems resist invasions on the level of ecosystems, rather than on the level of their components. This tendency would seem to be indicative of the functional integration and coordination needed to combat a systemic threat, which would be expected of a superorganism.
Perhaps the networking arrangements and assembly rules, which promote increases in functional diversity within ecosystems, were selected for in early, competing ecosystems. Perhaps also, being able to "evolve" particularly pernicious invasive species would have benefited ecosystems, as they could have used them to "attack" rival ecosystems by probing for weaknesses.
Such speculations would be very difficult to test, but perhaps through a combination of mathematical modeling and some kind of paleo-ecological investigation into the compositions of primordial ecosystems, some evidence could be obtained, either supporting or rejecting these suppositions.
Humans, who don't seem to fit into the picture the same way as all other organisms do, could be considered as the ecosystem level equivalent of a virus, an "obligate--ecoparasite" so to speak. Humans co-opt the mechanisms of existing ecosystems, and through niche construction, engineer those ecosystems in some desirable manner, whether it be to create a living space, or to create a means of producing nutrition. This is analogous to a virus, which can engineer a cell to produce more copies of its self, but is ultimately completely reliant on the mechanisms of the host cell for survival.
The implication that Humans operate along para-viral lines within ecosystems has serious ramifications for Humanities obligations to the ecosphere at large. Humanity has the ability to niche construct to a far greater degree than any other species on the planet.
This ability, in many respects, obliges Humanity as a species to take an interventionist as opposed to a "hands off" approach to issues of conservation and restoration.
The reasoning for this rests in the fact that niche construction can be looked upon as a second order cybernetic feedback effect which operates in response to a change in the environment, where that environment or aspects of it are modified to counter the initial change.
Humans are capable of observing a change in the ecosphere, like the decline in species diversity, for example, and by virtue of their intelligence, react with appropriate remedial agency to the situation. Intelligence in this case operates simply as a sophisticated feedback mechanism, which can be utilized to both facilitate and resist changes in the environment.
The para-viral nature of the Humanity/ecosphere interaction does not allow humanity to be placed within the context of conventional ecology; instead it has been argued that the Human species is moving towards the status of a social superorganism in its own right, where sophisticated communications channels function analogously to a nervous system in a global super-brain. (19) In this respect, we as a species should care for ecosystems like we as individuals may care for flowers in a garden--through the ecosystem level equivalent of good horticultural practice, the Human social superorganism will be able to co-exist with nature in its capacity as "Global gardener."
I would like to thank Kenneth Bruce and Paul Devlin for their insights that went into this paper and their tremendous encouragement.
(1.) Gleason, H. A. (1939). "The individualistic concept of the plant association." American Midland Naturalist, 21, 92-110.
(2.) Clements, F. E. (1916). Plant Succession: An Analysis of the Development of Vegetation. Carnegie Institution of Washington DC, publication 242.
(3.) Lovelock, J. E. (2001). Gaia: A New Look at Life on Earth. United Kingdom: Oxford University Press.
(4.) Moritz, R. F. A., and Southwick, E. E. (1992). Bees as Superorganisms: An Evolutionary Reality. Germany: Springer-Velag.
(5.) Keller, L., and Reeve, H. K (edited by Keller, L.) (1999). "Dynamics of conflict within insect societies" in Levels of Selection in Evolution. Princeton, NJ: Princeton University Press.
(6.) Margulis, L. (1970). Origin of Eukaryotic Cells. New Haven, Connecticut: Yale University Press.
(7.) Odling-Smee, F. J., Laland, K. N., and Feldman, M.W. (2003). Niche Construction: The Neglected Process in Evolution. Princeton, NJ: Princeton University Press.
(8.) Lewontin, R. C. (1970). "The Units of Selection." An. Rev. Ecol. Syst, 1, 1-18.
(9.) Goodnight, C. J. (2000). "Heritability at the Ecosystem Level." Proceedings of the National Academy of Sciences, 97(17), 9365-9366.
(10.) Swenson, W., Wilson, D. S., and Elias, R. (2000). "Artificial ecosystem selection." Proceedings of the National Academy of Sciences, 97(16), 9110-9114.
(11.) Grinnell, J. (1917). "The Niche Relationship of the California Thrasher." Auk, 34, 427-433.
(12.) Elton, C. (1927). Animal Ecology. London, UK: Sidgwick and Jackson.
(13.) Hutchinson, G. E. (1959). "Homage to Santa Rosalina or Why are There so Many Kinds of Animals?" American Naturalist, 93, 145-159.
(14.) Chase, J. M., and Leibold, M. A. (2003). Ecological Niches: Linking Classical and Contemporary Approaches. University of Chicago Press.
(15.) Goodman. D. (1975). "The theory of diversity-stability relationships in ecology". Quarterly Review of Biology, 50, 237-266.
(16.) Naeem, S. (2000). "Plant diversity increases resistance to invasion in the absence of covarying extrinsic factors." Oikos, 91, 97-108.
(17.) Pokorny, M. L., Sheley, R. L., Zabinski, C. A., Engel, R. E., Svejcar, T. J., and Borkowski, J. J. (2005). "Plant Functional Group Diversity as a Mechanism for Invasion Resistance." Restoration Ecology, 13(3), 448.
(18.) Theissen, G. (2006). "The proper place of hopeful monsters in evolutionary biology." Theory in Biosciences, 124(3-4), 349-369.
(19.) Heylighen, F., Rosseel, E., and Demeyese, F. (1989). Self-Steering and Cognition in Complex Systems: Toward a New Cybernetics. Studies in Cybernetics Vol 22. Gordon & Breach Science Publishers.
Michael A. Woodley.
Department of Evolution, Ecology and Environmental Biology.
600 West 113th Street, Morningside Heights, New York, NY 10027.
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|Title Annotation:||News & Views|
|Author:||Woodley, Michael A.|
|Date:||Sep 22, 2006|
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