I now turn briefly to further discussion of organism interactions, as within chemostats, and the consequences of these interactions. Organisms occupying the same environment can be described as sympatric. Sympatric organisms can interact in a variety of ways, both directly and indirectly, i.e., involving or not involving some form of physical or chemical contact. These interactions can also be cooperative versus competitive, or one organism can directly and negatively impact the fitness of another organism. Immediately below I focus on competitive, indirect interactions, with discussion of cooperative interactions presented in a subsequent chapter. I then return to discussion of quantitative chemostat theory, considering in particular competitive interactions as they can occur within a chemostat.
Competitive interactions among organisms can be direct (i.e. agonistic interactions) or instead can be indirect (i.e., exploitative competition). Competitive interactions also can occur between species (interspecific competition) or between conspecifics (intraspecific competition). Interspecific competition can result in competitive exclusion, where one species drives another species to extinction. More generally, competition among organisms will tend to result in negative-negative interactions, where the fitness of the individual organisms involved is reduced relative to a more competition-free state. That is, more resources may be available to one organism alone than would be available to each of two or more competitor organisms.
Intraspecific competition generally results in density-dependent effects such as those that give rise to logistic growth. When populations are clonal, however, then competition between individual genotypes as seen with periodic selection can come to resemble competition between the genotypes associated with different species. The reason is that gene pools are not mixing when populations are purely clonal, just as when two distinct species are in competition. This claim comes with the caveat, though, that phenotypes are likely to be more similar within species than between species. That latter point means, essentially, that competition among conspecifics can be more intense than that between different species. The point nonetheless still holds that more resources typically will be available to one organism alone than will be available to organisms that are competing.
Exploitative competition occurs when two organisms are utilizing the same resource or resources. Basically, if a quantity of resource is or was being exploited by one organism, then it is no longer available to a second organism. This is a form of competitive interaction between two (or more) organisms, but is indirect in the sense that the two organisms need not physically interact. Indeed, they need not even come into contact to have an impact on each other. Generally these indirect ecological interactions are studied as interspecific interactions, specifically interspecific competition. In actuality, however, two organisms would tend to have a much greater likelihood of employing exactly the same resources if they instead are both members of the same species. Indeed, the concept of species itself can be defined ecologically in this manner, known as the ecological species concept, where two individuals are more likely the same species if they are more likely to compete over the same resources within a given environment.
One can view this competition over resources as occurring between specific alleles. With different species, the alleles presumably are different, and, crucially, are not part of the same gene pool. Within the same species the alleles are not necessarily different and are part of the same gene pool. For clonal organisms, however, the potential for the alleles to move between individuals is either small or nonexistent. Therefore, with microorganisms especially, exploitative competition is a reasonable way of envisaging not just interspecific competition but intraspecific competition as well. Gaining of resource by one allele thus comes at the expense of access to that resource by another allele. All else held constant, selection therefore would tend to favor genotypes that have the greatest potential to get to resources first, though other issues such as resource utilization efficiency can be important as well.
Exploitative competition can be seen in chemostats as reduced densities of both competitors at steady state. Thus, for example, two equally matched species or genotypes may – at least hypothetically – both display half the steady state density than either would display without the competitor present. Often such coexistence will either not occur for competing species, however, nor necessarily persist given evolution and periodic selection for competing genotypes.
Especially in simpler environments, such as that represented by a chemostat, the consequence of exploitative competition typically is extinction of one competitor, that is, the inferior competitor. This extinction is equivalent to the fixation of the alleles or genotypes associated with the superior competitor, and this equivalence is particularly the case to the extent that it is intraspecific competition among clones rather than interspecific competition among species that is being considered. Overall, these ideas can be summarized as a rule, that two or more distinct genes pools cannot occupy the same niche in the same place at the same time, or at least be dependent on the same limiting resource, without one of the lineages coming to dominate the environment, including to the point of driving the other gene pools to extinction. This rule – more simply stated as two species cannot occupy the same niche without one of the species being driven to extinction – is known as the competitive exclusion principle.
Competitive exclusion is dependent on relative environmental simplicity since otherwise it can be less likely for two species to completely share a single niche. This is why it is a phenomenon that tends to be easily observed especially in chemostats. There, competition between organisms can be viewed from the perspective of saturation kinetics. At very high resource densities, which tend to be associated with low population densities (re: logistic growth), organism populations will tend to increase in density as a function of their inherent properties, such as the rapidity with which they convert these resources into progeny. Though one competitor may have a growth rate advantage over the other, actual exploitative competition doesn't play a significant role in the competition because resource densities are not (yet) greatly limiting. Competition instead is in terms of growth rates. When population densities become higher, however, then resource densities will be reduced. As a consequence, competition for resources can become especially relevant in terms of exploitative competition. The result, especially in simple ecosystems, can be a reduction in resources to a point that resource densities can be sufficient to sustain the superior competitor but insufficient to sustain the inferior competitor. The latter, given this relative absence of resources, decline in density until extinction stochastically occurs. This “exclusion” of one competitor by another is, as noted, called competitive exclusion.
Chemostats are simple to model mathematically, which is one of the great strengths of employing chemostats as ecological and evolutionary model systems. Factors in chemostats that influence competitive ability include death rates (which, effectively, can be assumed to be determined solely in terms of rates of washout) and birth rates which are determined, for example, by assuming saturation kinetics. In fact there are three factors involved in determining these kinetics. These are the density of the limiting resource, the intrinsic organism growth rate, and the constant employed in the Resource Density Function (RDF, above). This constant is the resource density that supports half-maximal organism growth rates.
In chemostats operating at steady state, the best competitor specifically is the organism that requires the lowest resource density to keep its population from declining. For two competing organisms, if the density of limiting resource is reduced to a level that sustains the population of one competitor (i.e., at the steady state density that represent its carrying capacity) but the other organism is unable to sustain its population at that same resource density, then the latter organism will decline in density to the point of extinction. The minimum resource density necessary to sustain population growth for a given organism can be defined in terms of the variables listed in the previous paragraph. That is,
7.11: Minimum substrate density = Washout × Constant / (Maximal rate – Washout),
where "Constant" is the half-maximal resource density (i.e., that supporting half-maximal population growth) and "Maximal rate" is the intrinsic growth rate of populations as seen under low population density conditions (that is, r). Recall that the lower the value of the "Constant" the better since this means that organisms are able to replicate at a given rate at a lower resource density whereas for obvious reasons the higher the "Maximal rate", the better in terms of competition between organisms in terms of growth rates.
What does the above equation mean? First, it means that the higher the rate of washout then the higher the required Minimum substrate density needed to keep a species from going extinct within a chemostat. This makes sense since an organism will have to display higher birth rates to achieve a steady state involving higher death rates (i.e., as a consequence of higher washout rates), and with saturation kinetics birth rates are assumed to vary as a function of resource density. More "Deaths" in other words requires a compensating more "Births" which in turn requires more resources and thus a higher minimum substrate density to maintain a population's density.
Equation 7.11 also makes sense mathematically since the lower the value of the denominator of a fraction then the larger the value of that fraction (numerator held constant). Since the washout rate is subtracted within the denominator, then the greater the rate of washout, the larger its negative value there. The larger its negative value, then the smaller the denominator, and therefore the larger the resulting minimum substrate density required to maintain an organism's population (in the face of washout). Again, higher washout rates therefore will give rise to higher minimum substrate densities. Note that "Washout" is also found in the numerator, but as a positive number. Therefore the numerator gets larger as a function of washout rate, just as the denominator gets smaller. Both have the effect of increasing the overall magnitude of the fraction, here the minimum substrate density required to maintain a population's size. The equation thus efficiently captures the idea that the greater the rate of washout then the more substrate an organism needs in order to sustain its population.
The second thing that the equation tells is that the higher half-maximal resource density ("Constant"), then the more resource that is are required by an organism to sustain its population. Biologically, this means that the organism either has less affinity for the limiting substrate or it utilizes that substrate less efficiently. This too makes sense in light of the equation since higher half-maximal densities mean that an organism requires more resource in order to sustain replication at a given, e.g., half-maximal rate. For organisms with the same intrinsic growth rate, this would translate directly into greater resource densities required to sustain a given rate of population growth. Similarly, mathematically, since the "Constant" is found in the numerator as a positive number, the larger it gets then the larger the resulting minimal substrate density required to sustain an organism's population in the face of washout: The more resource that an organism needs to sustain a necessary level of replication, then the greater will be its minimum substrate density and as a consequence the lower its competitive ability at carrying capacity.
Lastly, the higher the intrinsic growth rate ("Maximal rate") then the lower the substrate density required to sustain populations. If two organisms are identical except for their intrinsic growth rates, then the intrinsically faster growing organism will tend to out compete the intrinsically slower growing organism. Again mathematically, the larger "Maximal rate" then the larger the denominator, resulting in smaller value for the overall fraction, where the better competitor is the one that possesses smallest value for this fraction, i.e., the lowest minimum substrate density. Biologically this means that the better competitor at steady state within a chemostat is that which is able to continue to maintain its population size at resource densities that are below that necessary for its competitor to maintain its population size.
How might organisms become better competitors? The answer is simple, either they reduce their half-maximal resource density & ndash; i.e., either increase their affinity for limiting substrate or increase the efficiency with which they utilize the limiting substrate, or both – or they increase their intrinsic growth rate. Interestingly, it has been argued that it is thermodynamically impossible to simultaneously maximize efficiency and expediency. Here those concepts are described in part by the half-maximal resource density (efficiency) and the intrinsic rate of replication (expediency), respectively. Maximizing affinity for substrate through greater organism complexity or increasing rapidity of replication through reduced organism complexity presumably would come into conflict. Note then that the better competitor will be the one that most effectively addresses this tradeoff between efficiency and expediency, which are concepts that we will return to again in the chapter titled, "Virulence".
There are yet additional ways to achieve better-competitor status within chemostats. One way is to establish wall populations, which at a minimum may allow an organism to stave off competitive exclusion. Another approach is to evolve competitor-antagonistic mechanisms such as poisons that affect one's competitor but to which the producing organism is immune. These phenomena, too, can be viewed from a perspective of virulence evolution.