Ecology: The Biology of Interaction. 4.15. Ecological Strategies

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4.14. Amensalism and Neutralism

D. Shabanov, M. Kravchenko. Ecology: The Biology of Interaction Chapter 4. Population Ecology

4.16. Population size regulation

4.15. Ecological Strategies How is the value of an individual to the population determined? "Natural selection recognises only one currency — successful offspring" (E. Pianka, 1981). We have noted that a population is a potentially immortal entity composed of mortal individuals. To sustain the population, an individual must survive and leave descendants who will also be able to survive. Note the duality of this task. In all likelihood, the greatest chance of survival would belong to an individual that expended no resources whatsoever on producing offspring. Yet such an individual would soon vanish without trace from the population. At the opposite extreme stands a hypothetical individual that, immediately upon emerging, channels all its energy into reproduction. Such an organism would perish itself and, if its offspring inherited so inefficient a pattern of resource allocation, would produce descendants with no prospect of survival. Therefore, the individual of greatest value to the population must be one that allocates resources between its own survival and reproduction in an optimal combination. This optimality can be assessed by calculating which allocation, under given conditions, maximises an individual's contribution to future generations. The measure used for this purpose in mathematical population biology is called reproductive value. Reproductive value is a generalised measure of survival and fecundity that accounts for the relative contribution of an organism to future generations. "It is easy to describe a hypothetical organism possessing all the attributes required for high reproductive value. It reproduces almost immediately after birth; it produces numerous, large, well-protected offspring and cares for them; it reproduces repeatedly and frequently throughout a long life; it wins in competitive interactions, avoids predators, and forages efficiently. Such an organism is easy to describe, but difficult to imagine..." (Begon et al., 1989). You will appreciate that this impossibility arises from the inherent tension between self-maintenance and reproduction (Fig. 4.15.1). One of the first to recognise this was the English philosopher Herbert Spencer in 1870, who spoke of the alternatives facing an organism: maintaining its own existence versus perpetuating itself through offspring. In modern terminology, these parameters are linked by negative correlations — a relationship in which improvement of a system along one parameter must be accompanied by its deterioration along another. [IMG_1] Fig. 4.15.1. In the rotifer Asplanchna, survival probability decreases as fecundity increases (Pianka, 1981) Different species (and different populations) allocate energy between self-maintenance and reproduction in different ways. One may speak of a species strategy, expressed in how members of the species acquire resources and how they expend them. Only a strategy under which individuals obtain sufficient energy to grow, reproduce, and offset all losses to predators and various misfortunes can be successful. Traits associated with different adaptive strategies may be linked by trade-off relationships — that is, by inescapable negative correlations (either/or relationships). For instance, offspring number and offspring survival are linked by a trade-off, as are growth rate and stress tolerance, and so on. The American ecologists R. MacArthur and E. O. Wilson described in 1967 two types of species strategies that result from two distinct selection regimes and are linked by a trade-off relationship. The standard notation for these strategies (r- and K-) is derived from the logistic equation. According to the logistic model, population growth can be divided into two phases: a phase of accelerating growth and a phase of decelerating growth (Fig. 4.15.2). While N is small, the term rN is the primary determinant of population increase and growth is accelerating. During this phase (the r-phase) population growth is rapid, and population size is higher the greater the reproductive capacity of individuals. When N becomes sufficiently large, the term (K-N)/K exerts the dominant influence on population size. During this phase (the K-phase) growth decelerates. When N=K, (K-N)/K=0 and population growth ceases. During the K-phase, population size is higher the greater the parameter K — which in turn is higher the more competitive the individuals. [IMG_2] Fig. 4.15.2. The r- and K-phases of population growth according to the logistic model It can be supposed that the populations of some species spend the overwhelming majority of time in the r-phase. In such species, the highest reproductive value belongs to individuals capable of rapid reproduction and rapid colonisation of unoccupied habitat with their offspring. In other words, selection during this phase favours an increase in the parameter r — the intrinsic rate of increase. This type of selection is called r-selection, and the species it produces are termed r-strategists. In species whose populations spend the overwhelming majority of time in the K-phase, the situation is quite different. The highest reproductive value in these populations belongs to individuals so competitive that they can secure their share of resources even when those resources are scarce; only then can they reproduce and contribute to the next generation. A population composed of such individuals will have a higher value of the parameter K — the carrying capacity — than one composed of individuals unable to compete for limiting resources. During this phase, K-selection acts on the population, yielding K-strategist species. K-selection is directed toward increasing the investment in each individual's development and enhancing its competitive ability. Transitions between these strategies are possible, but they are intermediate in character and do not combine the typical expressions of both forms. "One cannot be a lettuce and a cactus simultaneously" (E. Pianka). Of considerable importance in determining which type of selection (r- or K-) acts on a species is the dynamics of resource availability and the intensity of competition for those resources. When population size is reduced sharply and non-selectively by an externally driven resource shortage, r-strategists gain the advantage; when competition for scarce resources prevails, K-strategists gain the advantage. The choice between an r-strategy (increasing fecundity) and a K-strategy (increasing competitive ability) appears relatively straightforward, yet it impinges upon a multitude of organismal and life-history parameters. A comparison of these strategies in their typical form is given in Table 4.15.1. Table 4.15.1. Characteristics of r- and K-selection and strategies

Characteristics | r-selection and r-strategies | K-selection and K-strategies --- | --- | --- Climate | Variable, unpredictable | Constant, predictable Mortality | Catastrophic, density-independent | Caused by competition, density-dependent Mortality curve | Usually type III | Usually type I or II Population size | Variable, non-equilibrium | Constant, close to carrying capacity of the environment Free resources | Appearance of free resources, filling of "ecological vacuum" | Almost no free resources, occupied by competitors Intraspecific and interspecific competition | Weak | Acute Body size | Relatively small | Relatively large Development | Rapid | Slow Sexual maturity | Early | Late Reproduction rate | High | Low Reproduction during lifetime | Often semelparous | Iteroparous Number of offspring per brood | Many | Few, often one Amount of resource per offspring | Low | High Lifespan | Short | Long Adaptations | Simple | Sophisticated Optimized | Productivity | Efficiency

One might be surprised that semelparity is characteristic of r-strategists, while iteroparity is characteristic of K-strategists. This feature is most easily explained by example. Imagine mice colonising a grain storehouse (resources are abundant, competition is absent). Consider the strategies of two species. Species No. 1. Sexual maturity at 3 months; litter size 10; the female lives for one year and is capable of reproducing every three months. Species No. 2. Sexual maturity at 3 months; litter size 15; after rearing the litter, the female dies of exhaustion. In the first case, after three months 10 offspring plus their parents (12 individuals in total) will begin to reproduce; in the second case, 15 offspring will do so. The second species can achieve a higher rate of colonisation of available resources. The typical r-strategy compels individuals to invest maximally in reproduction as early and as intensively as possible; consequently, r-strategists are often limited to a single breeding season. On the other hand, it is easy to understand why typical K-strategists reproduce many times. In a competitive environment, only offspring in which substantial resources have been invested are likely to survive. Furthermore, to survive and reproduce, an adult must expend considerable energy on its own maintenance and development. Therefore, at the extreme, K-strategists produce a single offspring per reproductive event — as do elephants, whales, and in most cases humans. Yet however well-adapted these animals may be, a parental pair will eventually die. For the population not to decline to extinction, the pair must leave two surviving offspring and must therefore produce more than two. This being so, iteroparity is a necessary condition for the survival of K-strategist populations. In 1935, the Soviet botanist L. G. Ramensky distinguished three groups of plants, which he called cenotypes (the concept of strategies had not yet been formulated): violents, patients, and explerents. In 1979, the same three groups were independently rediscovered (under different names) by the British ecologist J. P. Grime (Fig. 4.15.3). These strategies are as follows. [IMG_3] Fig. 4.15.3. The "Grime triangle" — a classification of species strategies — Type C (competitor; violent in Ramensky's terminology): expends the greater part of its energy on maintaining adult organisms; dominates in stable communities. Among plants, this type most commonly includes trees, shrubs, or robust herbs (e.g., oak, common reed). — Type S (stress-tolerant; patient in Ramensky's terminology): tolerates unfavourable conditions through specialised adaptations; exploits resources in habitats where competition for them is minimal. These are generally slow-growing organisms (e.g., Sphagnum, lichens). — Type R (from Latin ruderis, ruderal; explerent in Ramensky's terminology): replaces violents in disturbed communities or exploits resources temporarily unused by other species. Among plants, these are annuals or biennials that produce large numbers of seeds. Such seeds either form persistent soil seed banks or are capable of effective long-distance dispersal (e.g., dandelion, rosebay willowherb). This enables such plants to await the release of resources or to colonise open patches in a timely manner. Many species are capable of combining different strategy types. Scots pine belongs to the CS category, as it grows well on poor sandy soils. Common nettle is a CR strategist, as it dominates in disturbed habitats. A species strategy may be plastic. Pedunculate oak is a violent in the broadleaved forest zone and a patient in the southern steppe. The Japanese art of bonsai (the cultivation of dwarf trees in containers) can be viewed as a method of converting violents into patients. An interesting exercise is to compare the MacArthur-Wilson and the Ramensky-Grime classification schemes. It is clear that r-strategists correspond to R-type organisms, the explerents. K-strategists, however, correspond not only to C-type organisms (violents) but also to S-type organisms (patients). Violents maximise their competitive ability (and carrying capacity) in conditions of intense competition for readily utilisable resources, while patients do so under conditions of difficult resource acquisition. In other words, the ecological problems faced by an oak competing for light in a dense forest and by a fern surviving under dim illumination in the depths of a cave have much in common: both require optimisation of resource acquisition and refinement of individual fitness.

4.14. Amensalism and Neutralism

D. Shabanov, M. Kravchenko. Ecology: The Biology of Interaction Chapter 4. Population Ecology

4.16. Population size regulation