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similar to that developed by Kohler
et al.
mentioned above, as well as demographic,
social and economic dimensions. The objective is to identify the conditions of
resilient or non-resilient developments of such a system, characterized by the
interconnection between variables with different dynamics, some slow (land
degradation), others fast (commercial connections and land use) and others,
intermediaries (demography and climate). The author writes explicitly that he is
looking for “candidate” characteristics to explain whether a system is or is not
resilient. The model is used as a laboratory experiment to better understand, on the
one hand, which dynamics have led to a dense and well interconnected population of
the Maya and, on the other hand, how this socio-ecological system has responded to
the change in environmental conditions.
The model is based on a combination of several methods: MAS, CA, GIS and a
network model. The space is divided into 20 km
2
cells. GIS is used to manage the
empirical data about the environment, which are used to estimate the agricultural
potential and the access to water resources at the cell level. The agents represent the
settlements and MAS handles their interactions with their environment (agricultural
activity and food consumption) as well as the consequences of these interactions in
terms of demography and migration. AC is used to formalize the development of the
forest. At last, the agents are connected in networks within which they trade. From
this trade they make profits. When a settlement exceeds a certain population
threshold, a mechanism allows to expand the road network to settlements located
within a radius of 40 km, facilitating trade with them.
The simulations are spread over 600 time steps, starting from an initial situation
without settlement, reaching a maximum expansion toward 400 iterations, followed
by a decline. The system shows great sensitivity to the interaction between the
growth rate of trade, which generates increasing profits, development of the
population and land degradation. Around the 400th iteration, the system is at its
peak: all the settlements are interconnected, the population is large, the agricultural
area occupies a large part of space (at the expense of the forest which has receded),
and the land is degraded. At this stage the system is very vulnerable and climate
variations will result in strong demographic effects. The decline of a major node in
the network will result in a cascade of declines. However, all the simulations do not
lead to a strong increase of the population and do not lead to a peak corresponding to
the estimate made from the observation of the archaeological remains. The more
determining parameters are the ones managing the trade growth and land
degradation.
The author concluded that his model helped to demonstrate that there is not just a
single causality to explain the decline of the Maya civilization (a drought has
sometimes been referred to as a possible explanation). It is the interactions between
several phenomena (climatic variation, population pressure, deforestation, soil
