Medical College Admission Test

Ecological patterns and processes are influenced by human activities in two main ways: directly, by the transformation of land into infrastructure and productive areas, or indirectly, through the byproducts of human activities that might disperse away from their causal source and degrade ecosystem functions. Direct modifications of the land through human infrastructure (human settlements, transportation pathways, and power lines) and productive areas (agriculture, aquaculture, forestry, and cattle ranching) have increased globally during the last century as a result of accelerated human population growth.

Several studies have analyzed the patterns of direct human modification of the land surface as a proxy measure of human influence on natural ecosystems. Although human modification indices do not convey the entire human effect expressed as changes historically accumulated over natural ecosystems, they are useful to infer the spatial pattern and extent of the capacity of humans to transform the earth through land use. Many studies analyze how diverse ecological regions have different capacity to respond to landscape transformations, but only a few of them analyze how the physical geography (defined, for example, as biomes or ecoregions) affects the spatial patterns of human modification.

Mexico is an ecologically heterogeneous country that hosts a diverse array of ecosystems ranging from hyper-arid deserts to tropical rainforests, which have evolved as a consequence of the country’s complex topography. Mexico is also one of the biologically megadiverse countries of the world, with high endemism for birds, mammals, and reptiles. In principle, it would be expected that human developments and land transformations in Mexico follow the country’s complex environmental mosaic, with regions where environmental conditions are more favorable for human settlement and occupation (given a particular level of technological development) showing a larger human footprint.

But land settlements and landscape transformations are not only the result of physical geography; there are also technological and historical components. The historic dimension is especially relevant in countries such as Mexico, with a long history of human occupation and well-documented civilization collapses. Indeed, despite the common misperception that Europeans found a New World that was largely unoccupied and wild, what Spaniards found in Mexico was a densely populated territory with well-developed agricultural settlements and large urban centers that heavily impacted their respective hinterlands. When Europeans arrived to Mesoamerica the population of the larger territory of what we now call Mexico was in the order of tens of millions of people. Although the native population was devastated by European diseases, the encomienda system, and 16th Century droughts, its geographical distribution at the time of Spanish conquest conditioned the subsequent land occupation and landscape transformations.

The ability of humans to transform the face of the earth has been referred to as the ¨human footprint.¨ The Human Footprint Index (HF) is calculated by adding all major large-scale anthropogenic transformations over the land surface. It uses four variables to summarize the effects of human modification: population density, land use change, access areas, and electric infrastructure. This index has been used and modified at different scales, but always following the main idea that the intensity of human influence is the result of the type of activity, the area that each activity occupies, and the accumulation of activities within large areas. Its values distributed on a map reveal the major patterns of human influence over the broad landscape. The advantages of the HF index lie in the fact that it uses publicly available geographic data for the majority of countries and hence it is easily reproducible by different researchers in different regions, and its calculations are statistically simple, with an explicative clarity that can be easily understood.