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Very Small Ecosystems

  • Douglas J. SpielesEmail author
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Part of the Springer Series on Environmental Management book series (SSEM)

Abstract

A common criticism of the individualistic, nonequilibrium view of ever-changing ecosystems is that “natural” ecological change happens over great time scales – so great, in fact, that such change is irrelevant for our current ecosystem preservation and conservation efforts. Of course ecosystems change, the argument goes, but they change over millennia. The holistic equilibrium view is based on the scale of years to decades, and on this scale ecosystems may be treated as stable – progressing through succession to the domain of attraction – and would remain stable if it were not for human activities. Without a doubt, humans have accelerated ecosystem change by creating stressful ecological conditions and by introducing and unintentionally favoring invasive species. It is also clear that the dominant plant and animal communities of an ecosystem, given a regular disturbance regime and a constant stress regime, can remain relatively unchanged over time. Indeed, ecological changes on the scale of years to decades may be subtle; individual species may a respond to a gradual increase in regional temperature, perhaps, or slow processes of erosion and sedimentation. As we have seen, such responses can result in monumental ecosystem change over long periods of time, but it is true enough that these changes may not even be noticeable on an annual basis. Therefore, the argument may conclude, it is our duty to restore ecosystems to and maintain them in the appropriate stable state.

Keywords

Microbial Community Extrinsic Factor Community Resilience Microbial Ecosystem Microbial World 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

A common criticism of the individualistic, nonequilibrium view of ever-changing ecosystems is that “natural” ecological change happens over great time scales – so great, in fact, that such change is irrelevant for our current ecosystem preservation and conservation efforts. Of course ecosystems change, the argument goes, but they change over millennia. The holistic equilibrium view is based on the scale of years to decades, and on this scale ecosystems may be treated as stable – progressing through succession to the domain of attraction – and would remain stable if it were not for human activities. Without a doubt, humans have accelerated ecosystem change by creating stressful ecological conditions and by introducing and unintentionally favoring invasive species. It is also clear that the dominant plant and animal communities of an ecosystem, given a regular disturbance regime and a constant stress regime, can remain relatively unchanged over time. Indeed, ecological changes on the scale of years to decades may be subtle; individual species may a respond to a gradual increase in regional temperature, perhaps, or slow processes of erosion and sedimentation. As we have seen, such responses can result in monumental ecosystem change over long periods of time, but it is true enough that these changes may not even be noticeable on an annual basis. Therefore, the argument may conclude, it is our duty to restore ecosystems to and maintain them in the appropriate stable state.

A counter-argument may also be made on the basis of scale. The holistic, ideal ecosystem is stable and at equilibrium only on the temporal and spatial scale that is convenient for the human experience. The collections of species that we consider to have integrity, the ecosystem functions that we consider to be healthy, the response to disturbance that we consider to be resilient – all are products of chance that coincide with our own scale of perception. By evolutionary analogy, the “domain of ecosystem attraction” is only stable in the same sense that species appear to be immutable. And so, we may consider a certain ecosystem arrangement to be a part of our legacy, we may find it to be inspirational and culturally significant, and we may see that it provides humans with valuable products or functions. But it is quite another thing to say that this perceived stable state is what the ecosystem should be.

The case for the individualistic view of ecosystems over large spatial scales and long time frames has been demonstrated elegantly by paleoecologists. The following chapters of this book are devoted to the consideration of ecosystems on scales of time and space that are more relevant to humans. First, though, I wish to bracket the paleoecological evidence for individualistic ecosystems with evidence from the small-scale ecosystems of the microbial world.

Microbial Ecosystems

Ecologists have often noted that we know very little about the species with which we share this planet. About two million species have been identified and named by taxonomists; the actual total may be anywhere between four and 100 million species (Wilson 2002). This alone is a daunting thought, but consider that identification work to date has preferentially focused on animals and plants. Comparatively, almost nothing is known about microorganisms. Of the identified two million species, only a few thousand are species of bacteria, and about 100,000 are fungi and algae. And yet it is microorganisms that truly dominate the planet. By one estimate, there are about 5 × 1030 living prokaryotic cells on earth (Whitman et al. 1998). The number of species to which these cells belong is “widely held to be unknown and unknowable,” and in fact the whole species concept, which is largely based on sexual reproduction, is problematic for microorganisms (Curtis et al. 2002). Even so, bacterial diversity can be estimated based on the variety of nucleic acids extracted from a particular environment. This can give us a ballpark approximation of what must be a staggering planetary diversity: 160 bacterial species per milliliter of seawater, and approximately 20,000 species per gram of soil. These rough estimates give rise to more questions about the unseen hoards: “Who are they?, and What are they doing?” (Curtis et al. 2002; Ward 2002).

In certain respects, the microbial ecosystem is analogous to macroscopic ecosystems. There are producers, both photoautotrophs that use solar energy and chemoautotrophs that use chemical energy to fix carbon. There are consumers – chemoheterotrophs, which must ingest and obtain energy from carbon like we humans, and photoheterotrophs that metabolize carbon with light as an energy source. There are predators, parasites, pathogens, scavengers, and symbionts. There are even life strategies that correspond with r- and K-strategists: those that grow explosively when nutrients are plentiful and conditions optimal and revert to periods of latency or dormancy when conditions are less than ideal, and others that are superior competitors of slow, steady growth in a low-nutrient environment. Once established, the various species compete fiercely for resources. There are stressors in the microbial world; some, like desiccation, hypoxia, and osmotic stress are the same factors that stress macroscopic ecosystems. There are also disturbances. Sudden inundation, rapid nutrient influx, or agitation might disrupt the microbial ecosystem and send it into a reorganization phase. So the components of an ecosystem are all here. But do microorganisms behave as a community the way we understand the communities of forests, lakes and coral reefs?

Indeed they do. While it is true that microorganisms can and do occur singularly, it is now becoming clear that prokaryotic organisms predominantly exist in communities that respond to and effect changes upon their environment. In a word, they are ecosystems. Commonly called biofilms, these microbial communities are recognizable to humans as the slime that grows on slippery rocks in a stream, on a long-submerged boat hull, or on a contact lens. The ability of microbes to form such communities is an ancient and widespread survival mechanism for prokaryotic life on earth (Hall-Stoodley et al. 2004). Microbial colonization and growth is reminiscent of macroscopic ecosystem succession (Fig. 6.1).
Fig. 6.1

Successional development of a biofilm community. Stage 1: initial attachment of cells to the surface. Stage 2: production of extracellular polymeric substances resulting in more firmly adhered “irreversible” attachment. Stage 3: early development of biofilm architecture. Stage 4: maturation of biofilm architecture. Stage 5: dispersion of single cells from the biofilm. Republished with permission of Annual Reviews, Inc. from Biofilms as complex differentiated communities by Stoodley et al. 2002; permission conveyed through Copyright Clearance Center, Inc

Biofilm colonizers are individual bacteria, fungi, protists, or algae that, while drifting through their aquatic environment, collide with a solid surface with which they share a weak molecular attraction. These are the pioneer species of the biofilm ecosystem. Successful colonizers are able to secure their purchase with sticky, hair-like appendages and then by secreting a kind of biological glue to secure their hold amidst the flotsam of the microscopic world. This may include cellular debris, inorganic material, and even other organisms, all of which may or may not become attached to the fledgling community. Just like windblown seeds that land in soil, germinate, and begin the struggle for life, colonizing microorganisms compete for space and resources. One way in which microbial pioneers protect their advantage is by secreting prodigious amounts of an extracellular polymer that forms the physical barrier of a slime coat around their point of attachment. But the defenses are not only physical. Bacteria in marine biofilms have been shown to produce a potent toxin which paralyzes and kills the amoeba that otherwise threaten to consume them (Matz et al. 2008). If this were succession in an abandoned field, we would call such action inhibition.

Facilitation also occurs in biofilms, though it is more commonly known as recruitment. The established colonizers provide attachment sites that enable other microbes to join the growing colony. While some organisms can apparently attach themselves to the extracellular slime coat, others attach to specific binding sites provided by the pioneer organisms. By both inhibition and recruitment, then, the pioneer organisms set the stage for biofilm development and growth. Maturation of the community is a function of the physicochemical environment, but there is clear evidence for abundant cell-to-cell communication (Molin et al. 2000). Communication may play a role in the structural formation of the community and in resource use, for constituent organisms of the biofilm are to a certain extent interdependent. Organisms with similar functions – sulfur reduction, for example – might constitute a guild. The biofilm is a collection of guilds, much as we consider a macroscopic ecosystem to be comprised of trophic guilds. For certain functions, guilds in a biofilm community are syntrophic with other guilds, meaning that each guild metabolizes the waste products of another. Thus the microbial community can be a series of interlocking syntrophic relationships, such that a given guild is exchanging resources and products with several other guilds and with the surface itself. Altogether it is a picture of an adapted and highly organized biofilm assemblage.

The organization even extends to three-dimensional structure, for as the community grows it develops into a mushroom-like matrix that is not uniform within, but rather comprised of aggregates of cells – patches, one might say – interspersed with channels for water flow. According to Mary Ellen Davey and George O’toole (2000), “Numerous conditions, such as surface and interface properties, nutrient availability, the composition of the microbial community, and hydrodynamics, can affect biofilm structure.” Biofilm structure is affected by perturbations in the environment, such as flow rate and turbulence and changes in nutrient availability, but it is partially driven by the microorganisms themselves. The constituent organisms’ range of motility, access to particular resources, and density all play a role in biofilm architecture. Through chemical quorum-sensing systems, the rate of growth and differentiation is regulated and the biofilm structure takes shape (Karatan and Watnick 2009). In this way the community grows in complexity and size, and as its patches differentiate into interdependent guilds it develops what might be thought of as a set of aggregate functions. Some biofilm functions are extraordinarily useful to humans while others are a scourge, but of course from the microbial perspective, humans might be a source of nutrients, a point of attachment, or irrelevant altogether.

Biofilm communities are surprisingly beautiful in arrangement and function. They are also impermanent. After a certain period of growth through recruitment and cell division, guild differentiation, and development of mature synergistic (and perhaps antagonistic) function, the biofilm community reaches a stage of dispersion. Driven by some combination of environmental cues and internal communication, organisms of the biofilm community secrete enzymes that degrade the slime-coat matrix. The microbes are then released. By individual motility or as clumped aggregates swept away by flowing water, they leave the former biofilm structure (Hall-Stoodley et al. 2004). Upon release, some cells may perish by predator or parasite. Others may be swept away to die in an unfortunate encounter with inhospitable conditions. Some, though, disperse and find new points of colonization. These are sites of new biofilm ecosystems – just as the site that they vacated is a site for the potential colonization of new pioneers. And all of this, from pioneer colonization through maturation and dispersion to colonization again, may take place over the span of a few hundred micrometers and a few days, weeks, or months (Picioreanu et al. 2000).

Biofilms as Model Ecosystems

These collections of organized and specialized cells, capable of chemical communication and interspersed with a network of circulating water, bear obvious resemblance to multicellular organisms (Molin et al. 2000). But these are not multicellular organisms, they are communities. Highly organized though it is, the biofilm is not populated only by compatible species performing their functions in harmony. Julian Wimpenny (2000) has suggested that the biofilm is more like an ecosystem than a tissue, and as such the “microbial community will consist of a mélange of types. These will include primary resource converters; secondary and subsequent species relying on products of a food chain; scavengers that do not contribute to the efficiency of the community or may even detract from it; parasites, predators, and competitors, none of which represent added value for the association. What is more, as time goes by, other species will be imported or exported so that the community will change in ways that may or may not be energetically favourable.” In fact, the biofilm-as-superorganism hypothesis has not been supported by experimental evidence. For instance, organisms that form a coherent community in one nutrient environment have been shown to separate when subjected to a different condition. Even biofilms consisting of a single species are not fixed to a particular structural organization; rather, the structure is a complex result of environmental condition and organismal response (Molin et al. 2000).

Certainly, biofilms and macroscopic ecosystems are not perfectly alike. Forests and prairies do not have a stalk-like matrix of slime holding them together, nor do they fly apart at some chemical signal. Likewise, biofilms do not exhibit the same sort of gradual interspersion with one another that we see in larger ecosystems. Nonetheless, the similarities are striking. A biofilm’s ecosystem-like attributes and rapid pace of succession makes it a potentially useful model for the evaluation of our ecosystem concepts. Do integrity, health, resilience, and stability make any sense for biofilms? Stability would naturally have to be considered on a much smaller scale; even so, it is clearly ephemeral in microbial communities. Some biofilms are apparently remarkably persistent amidst treatment of detergents and antibiotics, but stability suggests not only persistence but also constancy of structure and function (Molin et al. 2000). Biofilm structure changes with maturation; species and individuals arrive and depart; the relative density of species changes as the community grows; predominant function changes with resource availability. Furthermore, biofilm systems regularly collapse and disperse. None of these characteristics would seem to indicate inherent stability. But as we have seen, stability is a relative concept. If we choose to define the system as the interaction of symbiotic organisms over a discrete period, I have no doubt that we could find instances of stability.

How about ecosystem health and integrity? The terms we use them to describe macro-ecosystems can really only be understood in terms of a native reference system. For biofilms, and really microorganisms in general, we have no such understanding. It is possible, though, for a sort of invasion to happen in biofilm communities. For example, despite physical and chemical barriers to invasion, both viruses and bacteria have been shown to invade with the potential to alter the structure and function of the biofilm community. It is also possible that a shift in environmental conditions, like hydrodynamics or nutrient availability, can change the selection pressure and consequently alter the structure of the biofilm. As a consequence of environmental conditions and genetic responses, biofilms occur in a variety of structures, ranging from gangly, spatially diverse bulbs to flat homogenous structures – each unique structure presumably with its own functional character (Doolittle et al. 1995; Burmolle et al. 2006). So invasion and environmental variation can alter the biofilm community, but we have no basis for determining whether one biofilm structure has more integrity than another, or whether one function is healthier.

Actually, we do have a basis for judging the health and integrity of some biofilm systems – these are the microbial ecosystems that we actively manage. Two examples will illustrate the point. We use biofilm communities in the treatment of our wastewater, primarily to convert solid organic waste to gases like carbon dioxide and methane. These communities have health and integrity when their metabolic and reproductive rates are high and steady. Erratic substrate, inadequate oxygen, or toxins like bleach in the waste stream can all reduce the health and integrity of this type of biofilm. I also actively manage the biofilm on my teeth by brushing, flossing, and trying to maintain a reasonably healthy diet. Should I neglect any of these for an extended period, my unhealthy oral biofilm would soon be obvious to both my wife and my dentist. The point is this: like macroscopic ecosystems, the only way we can evaluate the integrity and health of biofilm ecosystems is if, by some function or appearance, they are of benefit to humans.

How about resilience? For an ecosystem to be resilient, as we have seen, it must maintain its basic structure and function after disturbance. In the case of biofilm ecosystems there might be small perturbations during the maturation process – causing a few cells to be sheared off or killed here and there – but dispersion is the major disturbance event. Upon dissolution of the matrix, the biofilm organisms, nutrient gradients, chemical signaling networks, and flow channels all dissipate; this would appear to be parallel to the destruction of biomass structure and organization in the Release Phase of the adaptive cycle. To be resilient, then, the biofilm components would be expected to reform communities of similar structure and function at a new point of colonization. Put another way, we might ask: is the post-dispersion reorganization of structure and function predictable?

This is a difficult question to answer with precision, for biofilm structure can vary in ways subtle to the human observer but critical to the microbial world. The geometric structure is important, but so are the concentration and variety of solutes, the types and abundance of species and their distribution, arrangement, and activity, and the density, permeability, and viscosity of the matrix (Picioreanu et al. 2000). To some degree, these characteristics are controlled by the genetic expression of the colonizing organisms themselves: they are known as intrinsic factors. In theory, a complete understanding of the genome of each constituent organism would provide us with a limited predictive power for ecosystem assembly. At present, we lack such a complete understanding. Even if we had it, we would still be poor predictors without knowledge of the extrinsic factors of the microbial environment. Extrinsic factors might even be less knowable that intrinsic; they are the stochastic fluctuations of the microclimate (Wimpenny 2000).

Not surprisingly, microbial response to environmental change is still poorly understood, and so is the mechanism of biofilm assembly. The best attempts at understanding biofilm geometric structure have been simulations that account for the effects of extrinsic factors like hydrodynamics, substrate form and solute concentration on growth, attachment, and detachment. Assuming that microbes would respond to these variables in a uniform and consistent manner (which in real life they would not), one might simulate the growth biofilm ecosystems over space and time to test the hypothesis that biofilm ecosystems are structurally resilient. In fact, the simulations show quite the opposite – biofilm growth has no fidelity to geometric structure over space or time. Instead, the structures of post-dispersion biofilm communities are strongly influenced by micro-variation in the physical and chemical environment (Picioreanu et al. 2000). True, these are only simulations of extrinsic factors. And of course, the “real world” response to environmental conditions is driven in a specific way by the organisms of the biofilm ecosystem. But assuming that the colonizing organisms in each reorganization may well be different in type, number, and arrangement, the intrinsic effect would make it even less likely that biofilm ecosystems are resilient and mature in the same way time after time. On the contrary, it’s all contingent upon the peculiarities of place, moment, and constituent organisms.

Perhaps the minute details of biofilm structure are unimportant for resilience. Maybe resilience should be framed in broader terms of function. Does a biofilm community that functions by metabolizing ammonium to nitrous oxide, upon dispersion, re-form communities that also metabolize ammonium to nitrous oxide? Do the biofilm communities on my teeth spawn similar communities that eat away at my enamel and taint my breath? If they do, then biofilm ecosystems are clearly resilient; but if this is all we mean by resilience, then it is definitely a property of macroscopic ecosystems as well. A native forest replaced by nonnative shrubs and vines still does photosynthesis and respiration. An algae-choked lake still has trophic function. No, this is not what we mean by resilience in our macroscopic ecosystems. For an ecosystem to be resilient in the “strong” sense it must be some reincarnation of the species list and three-dimensional structure of the pre-disturbance state. If biofilm ecosystems are a useful model, it would seem that “strong” resilience is only likely under the most stable environmental conditions.

Such stable conditions may exist at certain places on earth. There are, for example, microbial communities in the water-filled pore spaces of rock deep below the planet’s surface. They are known as SLiMEs – subsurface lithoautotrophic microbial ecosystems. The environment is extremely nutrient poor, and consequently the metabolic rates of SLiMEs are among the lowest ever recorded (Stevens and McKinley 1995). The ecosystem appears to be based entirely on geochemically produced hydrogen, without access to any product of photosynthesis, past or present. Relatively protected from fluctuations of climate, nutrient availability, temperature, pressure, and flow rate, these must be some of the most stable ecosystems in existence. Though SLiME succession has not been studied, I can imagine a dispersion-reorganization scenario that is remarkably resilient. I also imagine that such a stable environment is the exception to the rule of earth’s ecosystems.

Biofilms as Patches

Biofilm research is still in its infancy, and clearly there is still much to learn. Even so, these little slimy blobs of cells are useful as model ecosystems. The rapid turnover allows us to see ecosystem formation and reformation on a scale that we don’t often consider. The interplay of intrinsic and extrinsic factors mirrors the assembly of macroscopic ecosystems that we attempt to protect. Finally, the relationship between structure and function is a beautiful example of species interaction in a stochastic world.

The best thing about the biofilm as a model ecosystem is that it isn’t an abstraction at all. Biofilm systems are a critical component of every macroscopic ecosystem on earth. When we talk about nutrient cycling, decomposition, or infectious disease we are referring to the action of biofilms. Ubiquitous in astounding numbers, these micro-ecosystems are the basis for all of the higher-order ecosystem functions.

To connect individual biofilm systems with the macroscopic ecosystem, it may be useful to think of biofilms in patchy clusters. Consider a patch of swampland, for example, that is slightly elevated from the inundated forest floor that surrounds it. It is the site of a massive tree that has long since fallen over and decomposed, leaving only this small rise where the base of the trunk and rootmass once rested. The patch has a great deal of organic matter in the soil, ample light from the gap in the canopy, and periodic flooding from the fluctuating waters. In the moist soil and on the decaying vegetation there are biofilm communities, with perhaps a wide array of constituent organisms and structures. Despite the microbial diversity, there may be some similarity of function among biofilm aggregates, for within the patch the predominant resources and stressors are somewhat uniform. For example, many of the biofilm communities may be attached to – and well-adapted to mineralize – organic substrate, limited by nitrogen availability, and subjected to periodic oxygen stress when the water levels rise. As such, we may think of this collection of biofilm communities as a sort of functional patch – a meta-ecosystem, if you like.

The connections of this patch with the greater ecosystem are obvious. This little hummock may be an important habitat for invertebrates, and it is likely a site of seed germination, decomposition and mineralization. The meta-ecosystem, like the individual biofilm communities that exist within it, is at once a complete ecosystem in itself and a component of the larger swamp. Therefore, we can ask the same questions about this hummock that we ask about the individual biofilm system. Again, questions of health and integrity hold little meaning, and so I will focus on questions of fidelity. Is the microbial community of the swamp hummock stable over time? Presented with a disturbance, will it return to its former state?

Recent advances in microbiological methods allow for an unprecedented view of the microbial community within an ecosystem. By extraction, amplification, and analysis of nucleic acids and phospholipids, the diversity and genetic structure of a particular microbial community can be characterized. One such study analyzed the stability of a soil microbial community from a wet tropical forest (Pett-Ridge and Firestone 2005). Much like our hypothetical hummock, the soils of this forest were subject to periodic saturation, and thus hypoxia is one of the main stressors of the microbial environment. In the study, forest soils were incubated under semi-saturated conditions and treated with air, nitrogen, or fluctuating air and nitrogen on short- (12 h) and long-term (4 day) intervals. After 3 weeks of treatment, microbial activity was compared among treatments and with a control field sample. In effect, this design is a suitable test for stability in the microbial community. With fluctuating stress, can the microbial community maintain its diversity and function?

The answer appears to be a qualified yes, provided that the fluctuation mimics field conditions. The 4-day fluctuation selected for a microbial community that was similar to field conditions, while the aerobic, anoxic and 12-h fluctuation produced communities that were dramatically different from field conditions. Furthermore, it was shown that the forest soil includes organisms with a wide range of survival strategies regarding oxygen availability, and considerable redundancy for each life strategy. In this case, the axiom that diversity means stability appears to be supported: the bacterial community of this soil is well-adapted to periodic oxygen stress, and thus was able to cope with the 4 day fluctuation treatment. The caveat, though, is that any substantial departure from the field stress regime – including removal of oxygen stress altogether – resulted in a significantly different microbial community. This suggests that a community’s stability is relative to the selection forces acting upon it.

Resilience has also been experimentally tested in a community of soil microorganisms. In this study, sandy vineyard soil was amended with a compost of grape processing waste, and the microbial community was assessed at intervals over the next 6 months (Saison et al. 2006). The subject was the soil microbial community’s capacity for response to the abrupt change in environmental condition and its ability to return to its former size, structure and activity after the disturbance. To test the effects of disturbance strength, the researchers used compost treatments at both low and high levels, in which the high level was ten times greater than the low level treatment. Upon addition of a high level of compost, the soil microbial biomass and heterotrophic activity increased dramatically within a few days and remained high for the duration of the experiment. The low compost level had a weak effect on community structure and activity after 4 days; within a few more days, the low-level community was indistinguishable from the control. The authors conclude that community resilience was observed in the low level treatment (which we may think of as a minor disturbance), and that the same community subjected to a high level (major disturbance) did not show resilience within 6 months. Perhaps the high level community would ultimately have returned to the pre-disturbance state – the experiment was not carried out long enough to tell. But a disproportionate effect is evident: the high level community, treated with ten times the compost, changed and held the change for much longer than ten times the low level effect. The take-home messages that apply to ecosystems of all shapes and sizes are (1) that community resilience is a real phenomenon, and (2) that resilience is not necessarily a function of community type; rather it is a function of the extrinsic world.

Micro- and Macro-Ecosystems

Why take this trip through the microbial world in a book about ecosystem management? The utility of such an excursion is by now, I hope, abundantly clear. Size is really irrelevant; microbial ecosystems have the same basic characteristics and dynamics as the ecosystems in which we hike, fish, and hunt. And though I have compared one to the other as though they are separate systems, they truly are not. Microecosystems are linked to macroecosystems through countless hierarchical cycles. But there are some advantages to the isolation of microbial ecosystems in the study of ecosystem assembly, structure and function. Microbial systems respond rapidly to their environment and their succession is swift. Their genomic potential is vast and highly adaptable. They can be replicated and experimented with in ways that macroecosystems cannot. In short, these little ecosystems are wonderfully instructive, and they will only become more so as new molecular techniques increase the clarity with which we see their world.

And what can we learn from these small ecosystems that might be applicable to ecosystem management? I will highlight six lessons. (1) Both intrinsic and extrinsic factors influence ecosystem assembly and maturation. Neither can be ignored, for they are integrated: intrinsic factors are dependent upon the stochastic nature of colonization and on the stochastic constraints of the extrinsic world. And yet, even with obvious intrinsic factors in biofilms (recruitment and quorum sensing) and in macroscopic ecosystems (symbioses and feedbacks), neither can be characterized as a coherent and recurring unit. (2) Ecosystem structure and composition are directly associated with ecosystem function, but there are many structures and compositions that will allow for function. (3) Ecosystem stability may exist at certain well-defined periods, but stability is entirely dependent on the regularity of extrinsic factors like stress and disturbance. (4) Resilience likewise may occur over the course of ecosystem release and reorganization, but only if the prevailing extrinsic factors favor it. If the extrinsic factors are strong enough to select for a different structure, different complement of dominant organisms, or different function, the community will change. (5) There is nothing inherently wrong with the loss of stability or lack of resilience described in (3) and (4). (6) There is no more a normal community than there is a normal set of environmental variables.

Let me suggest that these six points apply not only to microbial ecosystems but also to ecosystem change on a continental scale over millennia, just as they apply to the wetlands, forest, and streams that we aspire to protect today. As we set the microscope aside and return our sights to the larger world, it may be useful to keep the smallest ecosystems in mind.

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© Springer Science+Business Media, LLC 2010

Authors and Affiliations

  1. 1.Denison UniversityGranvilleUSA

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