Servicio de Información Comunitario sobre Investigación y Desarrollo - CORDIS

Matrix and LTRE analyses

Population demographic field data provide a wealthy source of information on the status of the population. Specific data give immediate insight in aspects of the life cycle, such as the structure of individual plant sizes telling about the survival and growth opportunities, and seed germination giving the opportunities for sexual recruitment. However, only by combining the population data the different aspects of the life cycle can be evaluated together and provide an integrated measure of the viability of the population.

Models are powerful tools for integrating complex datasets. For the purpose of population viability studies, matrix projection models in particular have proven their usefulness. Matrix models logically sort the demographic parameters obtained in the field, categorized in age or size classes comprising groups of individuals with similar demographic fates. The major output variable that the model computes is the population growth rate, an appropriate measure of the viability of the population. In addition, sensitivity analyses may be carried out by which one may quantify what aspects of the life cycle contribute most to the growth rate of the population. The measures that the analyses generate (so called elasticity) have proven their usefulness in conservation ecology.

Parameters with high elasticity are usually the most important targets for conservation management, as their improvement will boost the growth rate of the population most. A final tool within the context of the matrix modelling technique is Life Table Response Experiment (LTRE) analysis. This recently developed analysis gives insight in the factors that determine differences in the population growth rates between species, populations, types of management, and alike. It is a very powerful tool for unravelling what aspects of the life cycle are critically affected by certain habitat conditions and specific management actions. Elasticities and LTRE together thus comprise the matrix analysis toolbox that is indispensable for a proper evaluation of the viability of populations.

We have applied these techniques to the populations monitored, as described under the result ¿Demography¿. The design of our European-wide monitoring programme on field demography allows systematic comparisons to be made between species, differing in elemental traits such as longevity and dispersal ability, between regions within the distribution range of species, in particular central vs. marginal populations, and population within regions, differing in size and in the degree of isolation. All individuals were classified on the basis of size criteria (number of leaves or leaf size, depending on the species), and flowering individuals were placed in a separate class from vegetative non-flowering individuals. To allow comparisons, the criteria were identical for a given species across its entire range.

The results of the analyses show large differences between the populations, with a major part of the variation expressed at the level between the regions. Even in situations in which the population growth rates are similar (i.e. all populations are slightly increasing), the contributions of different life cycle stages can markedly differ. This indicates that the populations in different regions survive in radically different ways, and may respond to management actions in equally different ways. Our analyses identified the short-lived species Carlina vulgaris as the most variable within our species group. This small thistle can either grow fast and flower rapidly or extend its flowering and grow bigger but only in habitats and climates in which the survival chances are good. Carlina indicates that populations of the same species in different regions can behave almost as different species, underscoring the importance of conservation and a diversification of management actions at the regional level.

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Institute of Water and Wetland Research
6525 ED Nijmegen
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