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Concepts and Principles of Population Dynamics

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Concepts and Principles of Population Dynamics Reprinted from Vkislas on Nemtology Concepts and Principles of Population Dynamics H. FERRIS AND L. T. WILSON Professor, Division of Nematology, University of California, Davis, CA 956 16. Associate Professor, Department of Entomology, University of California, Davis, CA 956 16. A population is a collection of individu- tion of a single plant root system, however, als of a single species of organisms spatially there are clearly islands of high aggrega- or temporally isolated from other such tion (multiple females in single galls of the groups. In some cases populations are root producing egg masses) and large por- geographically contiguous but exhibit tions of the root and rhizosphere unoc- gradual genetic variation across their cupied by the nematode. As the physical range (clinal variation). In a qualitative resolution is defined at a still finer level, sense, a population can be considered at the measurement of aggregation may be various levels of spatial resolution and will reversed towards randomness or further exhibit different characteristics in terms of to uniformity (for example, distribution of the means and variances of parameters de- eggs within a single egg mass). This change scriptive of its morphology, behavior, in perception of aggregation with chang- biological attributes, and distribution. In ing physical resolution is relevant in that it comparing two or more populations within parallels the information on population a species, such parameters can be used to dynamics of a species obtained by experi- quantify degrees of difference or similar- mental techniques varying in their resolu- ity. For example, if the thermal optimum tion. In other words, many of the quantita- for development of Meloidogyne incognita tive measures of populations are a function (Kofoid and White) Chitwood, is calcu- of the magnitude of the system included lated from the mean for samples of the in, or described by, these measures. For population taken from each state in which the purpose of this chapter, the boundary it occurs, the mean of all the populations of the system under consideration is at will have a large associated variance. If the the level of the population of a single mean is determined for the population in species. Consequently, all factors within a single state, it will probably differ from this boundary are considered intrinsic the national average and have a much properties of the population. Factors out- smaller variance. Similarly, if the mean is side the boundary, including food re- determined for the population in a single sources, environmental conditions, and field, it will vary from both state and na- other biotic agencies are considered ex- tional averages and have a still smaller trinsic to the population and to the system. variance. The term population dynamics is used to Similarly, the spatial characteristics and convey changes in the numbers, age class distributional measures of a population distribution, sex ratio, and behavior of a vary with the level of resolution at which population through time and space, deter- they are measured. As the physical size of mined by inherent characteristics of the in- the sample unit decreases, the perception dividuals and mediated by environmental of aggregation in the population increases. conditions, food resources, and interacting M. incognita may appear relatively uni- biotic agents. Populations have characteris- formly distributed across the southern tics that are the basis of their dynamics and United States. Within a single field, the ne- that are definable and measurable. They matode may appear to have an aggregated exhibit age-specific rates of development, distribution with regions of high popula- mortality, and reproduction. They migrate tion density, regions of low population into or out of an area at a definable rate. density, and regions of apparent absence They have a measurable sex and age com- of the population. At the level of resolu- position which may or may not be stable, 372 Population Dynamics: Ferris, Wilson 373 and which is also determinant of the netic recombination; parthenogenesis as dynamics of change in the population, an adaptation to the parasitic mode; dia- Intrinsic features of populations: Nema- pause, aestivation, and cryptobiosis as todes, like other organisms, have definable mechanisms of survival of adverse condi- life history strategies. These strategies can tions; and special mechanisms including be defined in terms of the number of life host recognition and width of host range. stages, the duration of the life stages, At the population level, variability among fecundity rates, life expectancy, sex ratios, individuals is important. The variability at and functionality of males. They can also the genetic level provides the basis for be defined in terms of the feeding habits selection and adaptation to environmental and mode of parasitism in the case of plant change. It mediates and allows the evolu- parasites. The strategies may be impacted tion of the organism. Plasticity, in response by density-dependent and density-inde- to environmental shifts can be measured pendent factors. Thus, resource limitation in terms of tactical variation in life history may impact the survival of organisms, strategies and genetic breadth to provide their reproductive potential and their germplasm adapted to changed condi- longevity. Temperature will affect the tions. rates of metabolic processes and con- Interaction urith extrin,,ic regulators: Re- sequently duration of life stages and source availability is a primary density- fecundity rates. We categorize organisms dependent regulator of the population in terms of their life history strategy to aid dynamics of nematodes and other or- in our understanding of the biology and ganisms. The abundance and state of the in the development of concepts of the food source (its physiological activity and evolutionary and adaptive significance of health), its distribution in the soil matrix these strategies. (horizontal and vertical), and its host status Life history strategies may undergo to the nematode all impact rates of popula- tactical shifts in response to the impact of tion change. Resource availability affects both density-dependent and density-inde- and mediates intraspecific and interspecific pendent conditions. Such alterations of the competition among individuals. The status general strategies are examples of the plas- of the environment in terms of tempera- ticity and adaptability of the organism and ture, soil moisture, aeration, pH, salinity are factors that promote its survival and or osmotic potential, particle or pore size success in its environment. affect and constrain developmental rates, Life history strategies of plant-parasitic activity, mortality, and fecundity. nematodes, in terms of their feeding habits Unfying conceptual theories: Over the last and mode of parasitism, range from ecto- century and a half, theories on the growth parasitism through various intermediate and dynamics of populations have been categories to sedentary endoparasitism, in- formalized into analytical models. Most volving varying levels of sophistication of such models are based on an adaptation of the relationship between parasite and host. the logistic growth curve: Life history attributes of plant-parasitic ne- K matodes involve an egg stage, fourjuvenile N, = stages, and male and female adults, al- 1 + (1 -NJK)e-rt though males are frequently rare or may However, most models of population pro- be absent. In many cases, there is indica- cess are more complex versions of these tion of a tendency to shortening or elimi- embodied concepts. The familiar deriva- nation of life stages. Thus, the first tive form of the logistic equation juvenile stage is frequently passed in the egg and some developmental stages may dN/dt = rN [(K-N)/K] be relatively short. Tactical shift capabil- ities in life history strategies in response to indicates the expected rate of change in resource limitation or adverse density in- the population (dN/dt) at any point in time dependent conditions include: sex ratio given the current size of the population alteration, which impacts the fecundity of (N), the carrying capacity of the environ- the organism and which may promote ge- ment (K), and the intrinsic rate of increase 374 Vistas on Neniutology of the population (r). The intrinsic rate of pendent may in fact be density-dependent increase of the population is the change if their affect is determined by nutritional per individual under conditions of unlim- status. As an example, if the population is ited resources. As such, it is the integral at a high density there may be insufficient result of births, deaths, immigration, and shelters or refuges for the organism to es- emigration for the extrinsic environment cape inclement conditions, or they may be of the system at low population densities. more accessible to predation and parasitism. The logistic equation as a descriptor of The general concensus of current population growth was developed inde- theory is evolving to a recognition of a pendently by Verhulst (1 1) and Pearl and midground in that density-dependent fac- Reed (9). A logistic model has been widely tors are obviously not always operating or used for both animal and plant popula- may not be as important, especially at low tions and has also been adapted to plant densities, and density-independent factors disease epidemiology (10), competition, are usually, but not exclusively, operating. and predator-prey interactions
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