Lifestyle Strategist Theory (LONG MESSAGE) (fwd)

Tony Travis 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>

EWEN said:
> 
> 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.
> 
> Introduction.
> 
>      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 
>                                                       (Respiration)
> 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. 
> 
> 
> Simulations.
> 
>      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 
> pool.
> 
>      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.
> 
> 
> 
> Results.
>      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
> falls.
> 
> 
> Discussion
>      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 
> template.
>      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,
> 
> 
> EWEN
> ***************************************************
> *         "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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