The length of productive life can be modified through selection: an experimental demonstration in the rabbit.

The objective of this study was to assess the feasibility of selecting for functional longevity in rabbits, defined as an ability to delay involuntary culling. Functional longevity was measured as the total number of AI performed after the first kindling. Breeding values were estimated using a discrete survival model. Male parents were selected on the basis of their progeny test results, and the efficiency of selection was estimated in the second generation, as was the correlated response on reproduction traits. A total of 48 males were progeny tested, based on the longevity of 10 daughters bred in 2 different farms. Based on their estimated genetic merit, 5 "high longevity" (HL) and 5 "low longevity" (LL) males were selected divergently and produced a new generation (5 bucks/sires and 10 daughters/bucks). A difference in longevity (+0.75 AI, i.e., 32 d) was observed between the 2 lines. In farm 1, the differences were mainly due to culling (26% in the LL line vs. 14% in the HL line) whereas mortality was similar in the 2 lines. In farm 2, mortality and culling were both higher in the LL line than in the HL line (33 vs. 15% and 19 vs. 7%, respectively). There was no difference between the 2 lines in terms of the reproduction traits recorded for each kindling. Nevertheless, because of the difference in the litter number between the 2 lines, the sum of young rabbits born alive per doe over her lifetime and the sum of young rabbits weaned per doe were higher in the HL line (+5 kits; P < 0.01). Selection for functional longevity using survival analysis is feasible for modifying lifetime reproduction traits.

[Population and environment. Requests for interdisciplinary analysis]. / Poblacion y medio ambiente. Requerimientos del analisis interdisciplinario.

PIP: Serious difficulties impede interdisciplinary research involving demographers, ecologists, and other students of the environment. The 1st problem concerns definitions of the different subject areas. Demographers have focused on the dynamics of some indicators that reflect complex and heterogeneous population processes. The relative autonomy of demography as a discipline was gained through an empirical orientation reflected in the statistical treatment of causality. But the traditional demographic paradigm is insufficient for untangling the causal mechanisms underlying population dynamics. Environmental disciplines on the other hand face methodologic difficulties in transcending a strictly biological focus to incorporate aspects of cultural and social influence on ecological processes. "Human ecology", a possible meeting ground for ecological and demographic studies, is more of an ambitious program of transdisciplinary research than an independent discipline. Relations between the environment and development processes, including population aspects, are of increasing international concern. A conceptual base has developed in Latin America which emphasizes the global and structural aspects of the environment and of development styles. It has been extremely difficult to apply the entire conceptualization to the concrete environmental problems that are of current interest to both civil society and governments. It may be time to replace the umbrella term "environment", defining it in more specific, systemic, and operational terms. It is time to delimit study topics in terms of concrete problems. A good example would be the situation of Lake Chapala, the largest lake in Mexico. Damage caused to it cannot be assessed by referring to the "population explosion" or an "overall development style". Environmental, economic, and sociodemographic aspects will however necessarily enter the analysis. Fragile and unstable situations are of special interest in the study of relations between population and the biophysical environment. Such areas can transform the structural conditions of population processes. A combined analysis of spatial distribution of populations and natural resources is urgently needed. For such an analysis, it would be useful to organize demographic data into large ecological zones.^ieng

[Population and environment: the challenges of complexity]. / Poblacion y medio ambiente: los desafios de la complejidad.

PIP: This work argues that the emerging science of complexity could provide a systemic and interdisciplinary approach to problems of population and the environment. The concept of complexity in everyday speech is first distinguished from the more specific scientific notion of complexity that has gained popularity in the past two decades. There is as yet no consensus on the definition of complexity, but it is provisionally identified as a characteristic of systems that meet certain criteria, including networks of internal and external relationships and nonlinearity of relations. Complex systems are human constructs; they are inventions or models including elements that establish nonlinear relations between themselves and with the total system. Construction and analysis of such models aids in understanding the functional relationships of a specific situation and opens up the possibility of interdisciplinary studies. Since the most important initial contribution to scientific recognition of complexity came from the hard sciences, chaos, a particular form of complexity, has been found to be ubiquitous and is no longer regarded as a curious exception in a world ruled by regularity. Four examples of situations in which the relationships between population and environmental pressures have been altered illustrate the uses of complex systems. The decimation of the New World population following the Spanish conquest, the scarcity of rubber plants in the Brazilian Amazon during World War II, the collapse of banana plantations in the Mexican state of Tabasco in 1941, and the drought and famine in the Sahel in the early 1970s are discussed in this light. Some of the most important methodological issues that may arise in applying the perspective of complexity are identified and discussed, including delimitation of the system and its surrounding conditions, identification of scales and levels of analysis, structural characterization of the system, trends and changes in complex systems, conditions of control, and conditions of observation.^ieng