current human impacts on the ecosystem and supportive data for assessment

Yifan Yu

Overview

The current human impacts on the Mexican tropical dry forest is really bad.The following table showed the estimated conservation status of Mexican tropical dry forest in the early 1990s. 

source : Trejo& Dirzo, 2000 

Intact tropical dry forests refer to those whose native features are largely conserved without much human disturbance. Altered tropical dry forests refer to those where human activities have already caused measurable impacts while relatively large area of this vegetation type still remain. Degraded tropical dry forests refer to those where only few and isolated vegetations remain, showing the previous existence of large area of tropical dry forests. Converted tropical dry forests refer to those which once were tropical dry forests while now have completely been converted into other land types for other uses. (Trejo & Dirzo, 2000)

It is estimated that at that time, around 73% of the Mexican tropical dry forest were more or less altered, degraded or converted under negative impacts from human activities. (Trejo & Dirzo, 2000) 

A Specific Example in Morelos

The following figure shows the distribution change of tropical dry forests in Morelos, a state in the south of Mexico. It is estimated that in its primitive state, about 60% of this area was occupied by tropical dry forests, which was equal to 2842.9 km² in area. While in 1973, nearly one half of these vegetation type disappeared in the state and between 1973 and 1989, the tropical dry forest was lost at a dramatic rate of 1.4% per year, resulting in only 38% of original cover left in 1989. (Trejo & Dirzo, 2000)

 source:  Trejo & Dirzo, 2000

Original estimated area, lost area and deforestation rate of SDTF in the state of Morelos, Mexico (1973–1989)

source: Trejo & Dirzo, 2000

Several ways people make use of tropical dry forests in Mexico

In Mexico, conversion of tropical dry forests into agriculture fields, pastures and timber extraction are the three main human uses of this ecosystem. (Trejo & Dirzo, 2000; Castillo, Magana, Pujadas, Martinez and Godinez, 2005; Stoleson et al. 2005)

The history of Mexican people transforming tropical dry forests into agriculture fields can trace back thousands of years. According to Toledo et al, Maass and Challenger, some crops that we are familiar nowadays such as maize, beans and squashes were initially cultivated in such areas in Mexico (as cited in Maass et al., 2005). In the 20th century, such conversion of tropical dry forests into agriculture fields was greatly accelerated since Mexico government launched several supportive policies to encourage people to clear forests, establish farms, and grow more crops for self-consumption and trades. (Maass et al. 2005) Pastures for livestock ranching are also in great need in Mexico. Perramond claims that land for livestock grazing covers the largest area in northern Mexico (as cited in Stoleson et al. 2005). People there introduce exotic grasses, among which buffelgrass is one common type, conversing tropical dry forests into ranches(Stoleson et al., 2005).Timber resource is also a major benefit people get from tropical dry forests. According to Guerrero et al., many wood products are not only used for domestic production but also exported to other countries, such as the United States(as cited in Stoleson et al., 2005) Besides, Felger at al. states that the exploitation of mineral resources, which is rich in Mexico, indirectly affects tropical dry forests since in some areas, this ecosystem is cleared for building tunnels which are essential to mining ( as cited in Stoleson et al., 2005). In addition, Simon   claims that historically some tropical dry forests in Mexico were also cleared for production of such drugs as marijuana and opium. ( as cited in Stoleson et al., 2005)

In conclusion, tropical dry forests in Mexico are extensively used in many ways to support human life and production, resulting in really terrible conservation status.