28.5 Spatial Models of Human Impacts on Ecosystems
an official objective of assessing the outcomes of
the policies formulated in 1994, and reducing the
debate over contentious issues (http://www.unfpa.
org!).
The implications of population growth on
ecosystems can be assessed through the application
of migration models that examine redistribution of
population. The impacts of the distribution of a
growing population can be used as inputs to the
economic models discussed previously and in rural
settings have been examined through approaches
that employ concepts of carrying capacity and intensification of land use.
28.5.3 Migration Models
Three classes of migration models can be identified: behavioral, economic, and spatial. Behavioral
models are fundamentally descriptive. Theyexamine the decision-making processes that an individual engages to decide whether to migrate. Essentially, the decision to migrate is based on an
individual's assessment of satisfaction in the existing location when compared to alternatives. If alternatives offer sufficient incentive, then migration
will occur. IT not, the person remains in the present
location, and issues that led a person to consider
moving will have to be managed.
A wide range of variables contributes to the decision to move. While the weighing of these will
be unique to each individual, the push-pull model
of migration categorizes the most common factors
that attract or repulse people to and from origins
and destinations. The model was elaborated by
Caldwell (1975) and has been applied most frequently in the context of rural to urban migration
in countries of the south.
Studies have shown that the people involved in
migration and the relative importance of rural-rural
and rural-urban migration changes over time. Initially, rural to rural migration is the most frequent
response to pressure on resources. However, with
declining rural opportunities and an expansion of
perceived opportunities in towns and cities,
rural-urban migration increases. Age, gender, and
level of education of migrants also change over
time. The first rural-urban migrants tend to be
young men, between 15 and 35 years old, with
some education. These are the people most likely
to succeed in finding urban employment. Later, as
unskilled jobs are more available, the level of education of the migrant is less important. Relatively
few women are involved at the early phases of
rural-urban migration. However, recent evidence
from South America suggests that as urbanization
proceeds women come to predominate.
425
Empirical testing of these behavioral constructs
has shown that, among the variety of variables that
influence migrants, economic circumstances are
fundamental (Todaro, 1969). Once economic concerns are specified, then other factors modify the
process, including relatives or friends at origin and
destination, environmental circumstances, and cultural values. Todaro's research shows that what is
critical is not the reality of economic opportunity,
but rather the opportunities that people in rural areas perceive to exist in the cities. He developed a
numerical model that focuses on perceived employment opportunities and includes other factors
that migrants consider, including many of those
mentioned previously.
The push-pull and perceived employment opportunity models were developed in the countries
of the south. In the north, these models would have
had greater relevance in the past. Important contemporary redistributions of population are the continued pattern of suburbanization; in metropolitan
areas, the movement of workers' residences to formerly rural communities with good transportation
links to the metropoles; and migration of retirees
to areas with attractive environments. For example,
the migration trends in Michigan include the
growth of towns located near Detroit and migration to recreational areas in the northern portions
of the state (Manson and Groop, 1996). Contemporary patterns of international migration, for example, those of people from Mexico to the United
States and from southern Europe and Africa to the
cities of the European Union countries, are driven
by economic circumstances and by social and
political circumstances, such as war and political
unrest. Recent studies indicate that the latter will
become dominant instigators of international migration in future decades.
While migration models aid in understanding the
redistribution of popUlation from rural to urban areas, the growth of population within rural areas,
particularly in countries of the south, has received
considerable attention in studies of carrying capacity and of intensification of land use.
Carrying capacity studies of the relationship between societies and their land resources borrow
from the ecological concept that a particular land
area can support a finite population. The concept
has been elaborated by scientists such as Hardin
(1968, 1992, 1993) and Ehrlich and Ehrlich (1990)
and is commonly discussed in journals such as Population and Development Review, Population and
Environment, and Population Bulletin. This perspective has been challenged by a number of economists, including Boserup (1965, 1981) and Simon
(1986, 1992), by anthropologists concerned with
an official objective of assessing the outcomes of
the policies formulated in 1994, and reducing the
debate over contentious issues (http://www.unfpa.
org!).
The implications of population growth on
ecosystems can be assessed through the application
of migration models that examine redistribution of
population. The impacts of the distribution of a
growing population can be used as inputs to the
economic models discussed previously and in rural
settings have been examined through approaches
that employ concepts of carrying capacity and intensification of land use.
28.5.3 Migration Models
Three classes of migration models can be identified: behavioral, economic, and spatial. Behavioral
models are fundamentally descriptive. Theyexamine the decision-making processes that an individual engages to decide whether to migrate. Essentially, the decision to migrate is based on an
individual's assessment of satisfaction in the existing location when compared to alternatives. If alternatives offer sufficient incentive, then migration
will occur. IT not, the person remains in the present
location, and issues that led a person to consider
moving will have to be managed.
A wide range of variables contributes to the decision to move. While the weighing of these will
be unique to each individual, the push-pull model
of migration categorizes the most common factors
that attract or repulse people to and from origins
and destinations. The model was elaborated by
Caldwell (1975) and has been applied most frequently in the context of rural to urban migration
in countries of the south.
Studies have shown that the people involved in
migration and the relative importance of rural-rural
and rural-urban migration changes over time. Initially, rural to rural migration is the most frequent
response to pressure on resources. However, with
declining rural opportunities and an expansion of
perceived opportunities in towns and cities,
rural-urban migration increases. Age, gender, and
level of education of migrants also change over
time. The first rural-urban migrants tend to be
young men, between 15 and 35 years old, with
some education. These are the people most likely
to succeed in finding urban employment. Later, as
unskilled jobs are more available, the level of education of the migrant is less important. Relatively
few women are involved at the early phases of
rural-urban migration. However, recent evidence
from South America suggests that as urbanization
proceeds women come to predominate.
425
Empirical testing of these behavioral constructs
has shown that, among the variety of variables that
influence migrants, economic circumstances are
fundamental (Todaro, 1969). Once economic concerns are specified, then other factors modify the
process, including relatives or friends at origin and
destination, environmental circumstances, and cultural values. Todaro's research shows that what is
critical is not the reality of economic opportunity,
but rather the opportunities that people in rural areas perceive to exist in the cities. He developed a
numerical model that focuses on perceived employment opportunities and includes other factors
that migrants consider, including many of those
mentioned previously.
The push-pull and perceived employment opportunity models were developed in the countries
of the south. In the north, these models would have
had greater relevance in the past. Important contemporary redistributions of population are the continued pattern of suburbanization; in metropolitan
areas, the movement of workers' residences to formerly rural communities with good transportation
links to the metropoles; and migration of retirees
to areas with attractive environments. For example,
the migration trends in Michigan include the
growth of towns located near Detroit and migration to recreational areas in the northern portions
of the state (Manson and Groop, 1996). Contemporary patterns of international migration, for example, those of people from Mexico to the United
States and from southern Europe and Africa to the
cities of the European Union countries, are driven
by economic circumstances and by social and
political circumstances, such as war and political
unrest. Recent studies indicate that the latter will
become dominant instigators of international migration in future decades.
While migration models aid in understanding the
redistribution of popUlation from rural to urban areas, the growth of population within rural areas,
particularly in countries of the south, has received
considerable attention in studies of carrying capacity and of intensification of land use.
Carrying capacity studies of the relationship between societies and their land resources borrow
from the ecological concept that a particular land
area can support a finite population. The concept
has been elaborated by scientists such as Hardin
(1968, 1992, 1993) and Ehrlich and Ehrlich (1990)
and is commonly discussed in journals such as Population and Development Review, Population and
Environment, and Population Bulletin. This perspective has been challenged by a number of economists, including Boserup (1965, 1981) and Simon
(1986, 1992), by anthropologists concerned with