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A solid background in population ecology is valuable for anyone concerned with natural resources management. However, without the opportunity of field experience, it may be difficult for students to become familiar with some of the principles of population biology. Microcomputers and programmable pocket calculators allow the construction of simple simulation models, making it possible to carry out some ‘numerical experiments’ in the classroom which may play the same role in the learning process as in vivo experimentation This work introduces a mathematical model based on the Lotka—Volterra equations and a computer program developed for a programmable calculator. They can be used to assess the influence of environmental heterogeneity and disturbances on the results of competitive relationships between two plant populations. As an example, the interaction between two tussock grass species of the Patagonian steppe, Stipa speciosa Trin. et Rupr. and Festuca pallescens (St Yves) Parodi (the second being less x...

A model for estimating volumetric water content as a function of soil water potential and gravel content was developed. In the system studied gravel can withhold up to 67% of the amount held by the fine material. Water content at field capacity decreases 50% when gravel content (>11 mm) increases from 0% to 40% of total soil weight. For those values of gravel content, penetration depth of a rainfall of 20 mm increases from 38 cm to 59 cm when the initial water content is 60% of the field capacity.