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This study shows two aspects: the predator-prey relations and the preconceptions held by children (9)(10) years old) on the construction of food chains. The subjects of the study were 506 fourth-, fifth-, and sixth-grade children from the Mexican education curriculum. The conclusions are based on a single application of an instrument after a lecture in the classroom. The instrument consists of three tasks. The outcome showed that the classification of herbivorous and carnivorous is based in children's preconceptions of size and ferocity. These preconceptions guide the children's selection of the higher-level predator in the construction of a food chain. Another element used by children is the predator-prey relation in the construction of a food chain. The results shed light on the difficulties students have at higher education levels in the resolution of food webs and the relationships with ecosystems.One of the most important issues in science teaching at all educational levels is the relationship between living organisms and their environments. This is why there is interest in studying the construction and conceptualization of food webs and food chains, understood as relationships between populations of producers and consumers from different trophic levels. Food webs and food chains are important for the comprehension of more complex concepts in ecology. Such concepts are considered by science teachers to be an important and difficult part of teaching biology (Finley, Stewart, & Yarroch, 1982).An investigation of food webs by Griffiths and Grant (1985) found a set of preconceptions using a model of a learning hierarchy employed to assess problem solving of food webs. From these preconceptions, it is worth stressing the following points:I . The interpretation of food web dynamics is made in terms of a single food chain. 2 . In a food web, a change in one population will only affect another population if the two 3. The change in size of a prey population has no effect on its predator population.populations are directly related as predator and prey. Brumby (1982) has also reported the student's understanding of a food web in terms of food chains, which supports the above-mentioned points.Further, Webb and Boltt (1990) showed that the predator-prey relation is the principal component in the student's construction of a food chain. This relationship is not translated to other routes in a food web, if it has no effect on contiguous populations.

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En el presente artículo se planteó como objetivo, analizar el estrés académico en estudiantes universitarios y su relación con la práctica de ejercicio físico o deporte. La presente investigación pertenece a un estudio descriptivo transversal y comparativo; donde participaron 102 estudiantes universitarios. El instrumento que se utilizó para evaluar el nivel de estrés académico fue el Inventario de Estrés Académico. Los resultados mostraron que los estudiantes que no realizan ejercicio físico o deporte presentan significativamente mayores reacciones de preocupado o estresado [F(1,100)= 4.767, p<.031]; las mujeres presentan significativamente mayor estrés académico, específicamente en situaciones inquietantes [F(1,100)= 4.818, p<.030] y los estudiantes de la carrera de Motricidad Humana presentan significativamente mayores reacciones de preocupados o estresados [F(2,99)= 6.653, p<.002] que las demás carreras participantes. Por lo que se concluye, que el estrés académico existe en los estudiantes universitarios, mostrándose con mayor frecuencia en los estudiantes que pertenecen a la carrera de Motricidad Humana o del área de salud; sobre todo en aquellos estudiantes que no realizan actividad física o deporte y que los estresores suelen presentarse en situaciones inquietantes y en las reacciones de preocupado o estresado; en el exceso de tareas y trabajos; evaluaciones de los profesores como los exámenes, ensayos, prácticas, trabajos de investigación, entre otros.

The educational system in Mexico has undergone important changes in recent decades. One of the most important took place in 1993, when the compulsory character of secondary school (ages 12-15) was established. National In-service Courses (NICs) were established as a way to bring curricular changes proposed in the reform near to the teachers. The results, according to the National Certification Exams, have not been satisfactory. In this paper, we describe the general context of education in Mexico, putting emphasis on teacher initial preparation and the reform that took place as well as the way in-service courses are implemented, analyzing their possibilities to bring about real changes in science teaching and learning.