JOEL E. COHEN

Vita Publications Laboratory of Populations

Human Population Studies

The human population doubled between 1955 and 1995 while awareness of environmental problems and economic inequalities increased.Though global average fertility rates have declined haltingly since about 1970, rapid population growth continues in many of the economically less developed countries. A question that arises naturally, and the title of our book, published in November 1995, is: How Many People Can the Earth Support?

Numerical answers to this question over the last half century have ranged from fewer than 1 billion to more than 1,000 billion. These answers range so widely because they are based on vastly different concepts of human carrying capacity and vastly different assumptions. In fact, how many people the Earth can support depends on the interaction of natural constraints with human choices that are being made now and yet to be made. Under any reasonable assumptions, human population growth on Earth must end, and the end is very likely to come within the next century. A major choice is whether population growth will end through lower global birth rates or increased global death rates.

Infectious Disease

Chagas' disease is an insect-borne infectious disease, a New World relative of African sleeping sickness. It afflicts millions of people in Latin America. After the short acute phase of the disease, no cure is known. Infection can be avoided by keeping the insect vectors out of human homes and by keeping domestic dogs and cats, which are reservoirs of infection, out of bedrooms. In collaboration with Argentine colleagues Ricardo Gürtlerand Roberto Chuit, a field study of the control of Chagas' disease is under way in rural northwest Argentina. We are trying to develop a mathematical model ofthe risk of transmission to humans based on data collected for that purpose.The aim of the model is to improve field interventions and disease control.

Food Webs The function of ecological communities of non-human species strongly affects human well-being. One approach to understanding an ecological community focuses on its food web, a flow chart of who eats whom. Food webs are important to human health and well-being because they describe the major pathways of food energy and of chemical and biological toxins. In collaboration with the International Rice Research Institute in the Philippines, we are describing and analyzing the food web of rice fields. This study provides an opportunity to test basic scientific generalizations about food web structure and may suggest possible improvements in biological control.

Mathematical Studies A substantial fraction of our effort is devoted to answering new questions of mathematics, statistics, and computation raised by populations. Recent work dealt with relative entropy, nonnegative stochastic matrices, and nonlinear mappings.