Lifestyle Strategist Theory (LONG MESSAGE) (fwd)
ajt at rri.sari.ac.uk
Wed Nov 10 05:31:06 EST 1993
I'm posting this for Ewen: please reply to <EWEN at whmain.uel.ac.uk>
> Dear Plantnetters,
> The following little spiel forms the bare bones of a paper that I'm
> in the process of preparing to submit to "Functional Ecology". Whilst
> the old green things aren't the principal players here, the
> application of strategist theory MAY be of interest to some plant
> ecologists out there somewhere. So, before submitting to the trials
> and tribulations of referees, I though it would be nice to give you
> lot first go at shooting me down, basically, because I feel that in
> the Global Boozer that is "bionet.plants", we can talk freely, which
> is something that I've found it's very hard to do to actual referees !
> All comments, criticisms and opportunities for discussion gratefully
> recieved and entered into.
> RESOURCE DYNAMICS IN THE SOIL MICROBIAL BIOMASS:
> A SIMPLE MODEL REFLECTING STRATEGIST THEORY.
> E.F. McPherson
> Department of Environmental Sciences,
> University of East London,
> Romford Road,
> London E15 4LZ.
> 1: Summary.
> A simple rule-base defining the flow of carbon through the soil
> microbial biomass has been derived, based on the assignment of Grime's
> rCS Strategist theory to various sections of the soil microbial
> community. The model demonstrates that, although there are phases of
> dominance and decline, the community (as defined by the amount of
> "active" carbon in each of the strategy "pools"), tends to be
> dominated by stress-tolerant or competitor species under conditions
> that one would expect to favour ruderal species. This may suggest
> that, although there is possible application of "raw" rCS theory
> to the processes of the soil microbial biomass, modifications may
> need to be considered, and perhaps a reconcilation of Grime's theory
> with Tilman's theory may be the most useful way forward.
> The concept of lifestyle strategies as a matter of theoretical
> biology generates much interest in plant, animal and microbial
> ecology, and, although r-K theory, (as perhaps best outlined by
> MacArthur and Wilson (1967) and Andrews and Harris (1984)) may find
> its champions, it could be considered that Grime's rCS theory (1977
> et sub), is that most usefully applied to the field of plant ecology.
> Colasanti and Grime (1993) have applied rules derived from the rCS
> theory in a deterministic model for a plant succession using two-
> dimensional cellular automata, in which rules governing the resource
> capture and utilisation by individuals determine the development of
> the complete system.
> In this paper, I suggest a possible mechanistic model for the
> dynamics of resource flow through the soil microbial biomass in an
> "open" system . The complexity of the interactions within the soil
> microbial biomass may best, I feel, be simulated using terms and
> definitions suggested by both r-K and rCS strategist theory. We have
> developed a simple model consisting of Three "strategy pools", each
> representing a section of the soil microbial community, distinguished
> by it's utilisation of two "types" of resource. I have called
> these two resource types "Low Molecular Weight Carbon Compounds",
> (which we suggest are easily assimilated) and "High Molecular Weight
> Carbon Compounds", (which, similarly, we suggest are more recalcitrant
> to assimilation by the soil microbial biomass.) Simple rules, based
> on metabolic processes, control the utilisation of the resource by
> the strategy pools. Application of enrichment, and of killing
> disturbances which favour different strategy pools, are incorporated
> into the fabric of the model.
> Methods and Materials.
> Three "strategy pools" have been defined within the model. These
> initial pools represent the three major strategies as suggested by
> rCS theory , and are characterised in the table below;-
> Table One: Characterisation of the Strategy Pools.
> Rate of Utilisation of Rate of Loss of
> Low MW Carbon High MW Carbon Carbon
> Lifestyle Strategy
> Ruderal High Low High
> Competitor Moderate Moderate Moderate
> Stress-Tolerator Low High Low
> Rate of "Cell" death
> Ruderal High
> Competitor Moderate
> Stress-Tolerator Low
> Rules of Resource Utilisation.
> In each iteration of the model, which may usefully be
> thought of in terms of one "generation" of the soil microbial
> biomass, both pools of resource are allocated according to simple
> rules, to maxima defined within the model. Although there are two
> resource pools, they both represent at the simplest level, units of
> carbon. Each strategy pool is expressed therefore after each
> iteration of the model in terms of total carbon units extant within
> that pool. Starting, therefore, with a number of units of carbon in
> each pool, we have a simple flow, by which the processes
> of assimilation, respiration, cell death and concomitant recycling
> follow. Allocation of resource units is governed by assimilation
> coefficients for the two types of carbon for each strategy pool in
> combination with simple proportion of the soil microbial community
> resident in each pool at the start of each generation.
> Rules of Application of Stress and Disturbance within the model.
> The model relies on the introduction of additional carbon
> to the system, which occurs every five iterations, as the model is an
> open one as opposed to a closed system. Three types of "killing"
> event are applied. A "ruderal" event, which favours the r-pool, is
> simply the immediate reallocation of 50% of the C-pool carbon and 90%
> of the S-pool carbon from the strategy pools to the resource pools.
> Similarly, a "stress" event involves reallocation of 50% of the C-
> pool and 90% of the r-pool to the resource pools. The Third type of
> event affects all of the population, an "80%" event which immediately
> reallocates 80% of the carbon from all three strategy pools to the
> resource pools. Although this does not directly correspond to the
> definitions of stress and disturbance discussed below, it is simply a
> device of the model intended to simulate such conditions. There is no
> other direct interaction between the strategy pools.
> Four different groups of simulation are described
> The first group of simulations shows the development of all 3
> strategy pools under conditions of "steady-state" enrichment and what
> may be usefully considered as "monoculture".
> The second group of simulations show all 3 strategy-pools extant
> within the model at the start, with initial levels of 50 units
> allocated to each pool. The 5-iteration resource spilt is 5 units
> High Availability Carbon and 95 units Low Availability Carbon. This
> second group shows a "steady- state" development of the community,
> and the application of the three killing events as outlined above.
> The third group of simulations shows the development of the 3
> strategy-pools under different "qualities" of 5-iteration nutrient
> enrichment, but again, with 50 units initially allocated to each
> The final group of simulations is from an extended version of
> the model. This version suggests that, in a way which may be similar
> to the activities of the soil microbial biomass, "ruderal" members of
> the community may reproduce by sporulation. This version of the model
> simulates the re-appearance of the ruderal community under conditions
> of high levels of High availability carbon, which may be usefully
> compared to an "improvement" in the state of the environment during a
> successional process.
> Simulation Group One.
> Little deserves comment here. All three sections of the
> community reach steady state levels which reflect the quality of the
> nutrient enrichment and the lack of inter-section competition.
> Simulation Group Two.
> "Steady State"; The ruderal section of the community, as may be
> expected, rapidly utilises the High-Availability carbon, and quickly
> exhausts the supply. The initial surge of the ruderal section causes
> a temporary dominance of the community, and a concomitant allocation
> of the majority of the resource to the development of the ruderal
> section. As the level of High- Availability carbon in the simulation
> falls, we see the development of the competitor and stress-tolerator
> communities, which are more efficient at the utilisation of the Low-
> Availability carbon. As the ruderal community dies out, there is a
> rise in the proportion of the community that is in the competitor
> pool, as the competitors are far more efficient at the utilisation of
> the High-Availability carbon than the stress-tolerators. However, the
> lower metabolic activity and lower death and respiration rates of the
> stress tolerators eventually lead to the dominance of the community
> by the stress- tolerator component. This is apparently in agreement
> with the results of the similar situation in the Colasanti/Grime
> cellular automata model.
> "Ruderal Kill Event": It is interesting that in this simulation,
> under comparatively low "quality" of nutrient enrichment, that the
> ruderal community does not dominate under the event designed to
> favour it, and that instead, the competitors seem to quickly reach a
> steady, dominant state within the community. However, under
> increasing quality of enrichment, and given the density-dependant
> nature of the allocation of the nutrient resource, the ruderals do
> show an increasing dominance of the community.
> "Stress Kill Event": The stress-tolerator section of the
> community comes quickly to dominate the community, and indeed exists
> in effective monoculture after few iterations. The stress-tolerator
> reaches a steady-state after some 200 iterations. However, under
> increasing quality of enrichment, this overall pattern changes
> little, and indeed, the point where the stress-tolerators reach their
> maximum appears to come earlier as the quality increases.
> "80% Kill Event": The 80% Kill Event affects all sections of the
> population equally, and, therefore, it might be expected that the
> eventual result may parallel the "steady-state" situation. However,
> the density-dependent nature of the resource allocation, and the
> initial higher level of the competitors as compared to the stress
> tolerators combine to allow competitor dominance of the community.
> Simulation Group Three.
> The third group of simulations shows the development of the
> "steady-state" under different qualities of nutrient enrichment. All
> four situations follow in broad the pattern set out under low quality
> enrichment, but, as the quality of the enrichment increases, the
> point where the stress- tolerators dominate the community occurs
> earlier and earlier in the simulation.
> Simulation Group Four.
> This fourth group of simulations involves a slightly different
> application of the rule-base in an attempt to simulate re-appearance
> of the ruderal community from, for example, a spore bank. A 50-
> iteration High-Level Enrichment was applied. As may be expected,
> there is a surge in the ruderal population, and a concomitant
> decrease as the processes simulated in the model take effect, and the
> ability of the environment to support a ruderal community
> There is an intrinsic difference in the complexity of micro-
> biological systems, such as the one which is modelled here, and macro-
> biological systems, as modelled by Colasanti. (Andrews and Harris,
> 1982). This leads to obvious limitations with respect to modelling
> approaches that are based on the rCS theory. The complexity of the
> micro-biological system makes it difficult to deal with other than
> perhaps the most gross phenomena, and this is why the flux
> of carbon in the microbial biomass was chosen. It can be, eventually,
> easily experimentally followed and the model modified accordingly, in
> much the same vein as Jenkinson and Parry's (1989) model of soil
> Nitrogen flux.
> The basic theories, terms and conflicts of and between
> application of various theories of lifestyle strategies should be
> familiar to most readers of this group. However, Grace (1991), is a
> valuable resource in understanding the differences between the
> approaches taken in application of (most notably) Grime's rCS theory,
> and Tilman's theories to plant ecology. Grace states, usefully, that
> the two theories are less in conflict than we may previously imagine,
> and the development of the model presented in this paper draws
> heavily from both; Grime's, in the construction of the community and
> the allocation of distinct lifestyle strategies to sections of the
> soil microbial biomass, and Tilman's (1982) in that the dynamics of
> the population are (partially) a function of resource concentration
> and the concentration of the resource as a function of the supply
> rate and the uptake rate.
> It may be the case that there are useful parallels between
> Tilman's theory and the classic r-K theory, in that Boyce (1984)
> attempts a restitution of r-K selection as a model of density-
> dependant natural selection. Boyce states that density-dependence is
> only one factor that shapes the evolution of lifestyle strategies.
> The model demonstrated here suggests that this may to a certain
> degree be the case, although the semantics applied may differ
> somewhat;- That as the proportion of one section of the community
> changes with time, we see an evolution within the community structure
> of the dominance of one lifestyle strategy. The strategies are already
> an inherent part of the community;- They do not evolve de novo.
> In discussing the simulations that are presented here, there
> appears nothing to contradict the overall conclusions of the
> Colasanti/Grime model, and consequently, the general possibility of
> application of Grime's theory to the processes of the development of
> the soil microbial community.
> It falls, however, that there are terminologies in Grime's theory
> that we cannot usefully apply to the field of microbial ecology, and
> Pugh (1980) makes a useful extension of the stress-competition
> Grime describes R- (Ruderal), C- (Competitors) and S- (Stress-
> tolerant) strategies. Ruderal plants equate with the r-strategy of
> Macarthur and Wilson, i.e., being representative of low stress and
> high disturbance. C- competitors equate to Low stress and Low
> disturbance, being incapable of responding to stress or disturbance
> but merely there in competition for resources with other plants, and
> the S- Stress tolerators, being capable of withstanding high stress
> but little or no disturbance. Pugh (1980) goes on to add a fourth
> category, SE- Survivor-Escapers, which then fall capable of
> withstanding both high levels of stress and disturbance.
> However, in dealing with Grime's theory, there still exists
> within the literature an uncertainty, not even resolved by Grime
> himself, about what constitutes a stress and what a disturbance in a
> system. Killham (1985) describes "environmental disturbance caused by
> pollution is a 'stress'", whereas Grime (1977,1979) deals only very
> vaguely with definitions, beyond stress being "external constraint
> that limits production of dry plant matter, and Disturbance as
> consisting of mechanisms which limit the biomass causing
> its destruction. Sousa (1984) considers disturbance as " a discrete,
> punctuated killing, displacement or damaging of one or more
> individuals that directly creates an opportunity for new individuals
> to become established", whilst Rykiel (1985) defines it as " a cause,
> physiological force, agent or process, either biotic or abiotic,
> causing perturbation in an ecological component or system".
> Grime has equated ruderal strategy with selection by disturbance.
> Habitats for ruderal organisms are characterized by obvious physical
> disturbances. However, application of, for example, a fertilizer may
> regarded as a disturbance, which does not in itself alter the gross
> physical environment. This model attempts to avoid this particular
> problem of application of semantics by combining both Rykiel and
> Sousa's definitions.
> It may be, therefore, that both Rykiel's and Sousa's definitions
> are more applicable to microbial ecology, although they should
> perhaps be qualified that perturbance, whilst it may cause a period
> with few active organisms, may not eliminate all of the biomass -
> resting stages or inactive forms incapable of exploiting the
> situation that arises during and after the disturbance will
> inevitably survive, as we suggest in the adaptation of the model
> shown in Simulation Group 4.
> It is suggested that the utility of most current strategy
> theories is contested by an emerging view that a multitude of
> interactions of varying intensities will exist within each ecosystem.
> Connell and Stayler (1977), and Pickett (1987) effectively contend
> that neither unit or individualistic strategies adequately describe
> the observed patterns of temporal change in protist communities.
> Although they report species replacement as being directional,it was
> not randomly distributed throughout time as predicted by the
> individualistic model, and that species did not occur in groups with
> distinct temporal boundaries as indicated by the unit hypothesis.
> Rather, that their results indicate that both species-specific and
> interspecific effects contribute to the observed successional
> patterns. If, as would appear to be logical, we assume that species
> are perhaps limited in their (initial) capabilities to show gross
> changes in strategy, the results shown by this model would
> bear their work out.
> However, outwith the limits of this very basic model, expressing
> this in terms of the autotrophic community within a system, we suggest
> that changes in the species composition related to the resource
> dynamics of the environment can be considered as an interactive
> successional process, and that applying strategy theory to this, it
> may be the case that there is a change in the strategy of sections of
> the soil microbial community as the process of succession continues.
> That is to say, that as the environment changes, the change in
> environment encourages an alteration in the strategy of species.
> Grime's theory does not apparently take into account that there
> may be more than one evolutionary approach to a particular
> environmental challenge (Southwood, 1988). For example, individuals
> within a particular bacterial species could conceivably respond to an
> environmental challenge in either of two ways, which could in
> themselves be viewed as either stress-tolerant or competitive. If the
> bacteria responds by reductive division of its chromosome,
> to eliminate unessential nuclear material, this could be viewed as a
> response to stress, initiated to reduce the energy required for the
> maintenance of the organism, or, alternatively, could be viewed at
> the same time as a competitively-driven response, allowing the
> organism to increase its competitiveness by allowing it to be able to
> reproduce rapidly and without the extra demands that the reproduction
> of unessential DNA would replace on what may be an initially limited
> situation for exploitation.
> Similarly, for a bacteria to incorporate plasmid DNA into its
> chromosome, could be viewed in either light; - as a stress-response
> or a competitively-driven response.
> In terms of competition, therefore, the relative competitive
> abilities of all sections of the community will be continually in
> flux, thus resulting in changes in species dominance. It may also
> suggest that the "natural" state of any community is one of continual
> "competition", the term here being used not in sensu stricto in that
> we envisage competition both within as well as between strategies
> (i.e. between species).It may be the case therefore that all
> successional processes will therefore tend towards a re-establishment
> of this state.
> Concluding Remarks.
> Following the flux of carbon is vital to the understanding of the
> processes which govern the development of the soil microbial
> community, and indeed interactions as a whole within the soil
> ecosystem. This mechanistic model may be a useful basis for the
> further development and application of lifestyle strategist theory in
> the soil microbial biomass.
> Thanks for taking the time to read this. As I say, I'ld appreciate
> any views, constructive tips, destructive tips. Shoot me down, Blow
> me up, see if I care, as long as you know what you're talking about.
> Or even if you don't I'ld still be glad to hear from you.
> My regards to you all,
> * "Beam Me Up, Scotty, *
> * This Planet Sucks !" *
> * Ewen McPherson, Research Assistant *
> * e-Mail: EWEN at whmain.uel.ac.uk *
> * Snail : Environment and Industry Research Unit *
> * Department of Environmental Sciences *
> * University of East London *
> * Romford Road, Stratford, *
> * London E15 4LZ *
> * United Kingdom *
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