Figure 1. Location of the Mintzita wetland complex to the Southwest of the City of Morelia.
Wetlands provide many ecosystem services and play a significant role in ameliorating environmental impacts of agriculture and other disturbances in watersheds (Zedler 2003). Wetland ecosystem services associated with water quality are of particular importance in developing countries, because technological alternatives to water treatment can represent a considerable share of limited public funds.
The city of Morelia is the capital of the state of Michoacán, Mexico, with a population close to one million inhabitants (Figure 1). Morelia is located in the western section of the central region of México known as the Bajío, an area dominated by semi-arid climate with temperate ecosystems at high elevations. In the region, access to water has always been limited and all major water bodies as well as many aquifers are over-used and over-exploited. Morelia depends heavily on one spring system, the Mintzita wetland complex, that provides more than 40 percent of the water used by the population of the city. The Mintzita wetlands are a highly biodiverse system that according to our surveys harbor more than 60 species of native wetlands plants (Escutia-Lara et al. 2009), and several species of endemic fish (Dominguez Dominguez and Perez once de Leon 2007); in addition several endangered species have been reported.
Figure 2. Objects made with Typha leaves, utilitarian objects are intended for local markets.
Although the Mintzita wetlands and the springs are public property, local residents make intensive use of wetland resources. Some species are used as forage, in particular a bulrush (Schoenoplectus americanus) dominant near the edge of the wetlands. Other species, such as cattail (Typha domingensis), are useful for handcrafting several products used by locals or for sale as souvenirs for tourists (Figure 2). Agricultural practices at the watershed level can impact the wetlands increasing run-off and nutrient levels. However, research on the effects of different uses of wetland plants and other human disturbances of the Mintzita wetlands, as well as other wetlands in the region, suggests that biodiversity and ecosystem services depend on maintaining the traditional use of some resources. Within the wetlands, the main human activities include the harvest of cattail, fires that propagate from nearby fields accidentally or are started to burn the wetlands (Figure 3), and the introduction of cattle, particularly during the dry season.
Cattail harvest experiments by Steven Hall (Hall et al. 2008) indicate that harvesting, following different regimes similar to the traditional use of this plant, foster biodiversity by allowing more species to be present in harvested plots, particularly rare species. In Hall’s experiment, although cattail was temporarily negatively affected by harvesting, it soon recovered suggesting that cessation of harvesting could lead to over dominance of cattail and to biodiversity loss in the wetlands.
Studies on the effects of fires by Yazmin Escutia (Escutia-Lara et al. 2009) indicate that this disturbance alters wetland plant species distribution and abundance depending on the intensity and frequency of the fires. Fires during the driest days later in the spring and particularly in the afternoon are considerably more intense than fires early in the rainy season or early morning. People choose the season and time of day to create high or low intensity fires depending on the desired effects. Fires to foster resprouting of cattail for handicrafts need to be low in intensity, while fires to foster grass growth for cattle need to be high in intensity.
Figure 3. Burned area near the main spring, this particular fire was started within the wetland system to foster grass expansion.
We have long-term data examining the effects of the intensity of fires on plant communities. Our data indicate that low intensity fires benefit species usually found only in well preserved wetlands in the region, such as the longhair sedge (Carex comosa) (Figure 4), but high intensity or frequent fires can eliminate this species. We also determined that bulrush is not affected by low intensity fires but suffers considerably with a high intensity fire that burns the topsoil and damages the rhizomes. Cattail responds to low and high intensity fires by establishing all along the wetland gradient, invading zones formerly dominated by bulrush. Controlled experimentation indicates that ash produced by burned bulrush is phosphorus rich, and that cattail seedlings respond to high concentrations of this nutrient by doubling their growth rates, a mechanism for cattail expansion.
Cattle browsing has the biggest impact on the wetlands because it eliminates the native plant species, especially on areas dominated by bulrush. Cattle, also consume the leaves of bulrush during the dry season, when this species sets seed and reduce bulrush cover before the wet season which favors invasive plant species, in particular grasses that are also fostered by high intensity fires. Furthermore, by compacting the fragile wetland soil, cattle alter water flow across the wetland. The combined effect of strong fires and cattle browsing alters nutrient dynamics and water flows, reducing the capacity of the wetlands to function as “filters” of agricultural run-off, with the consequent increase of contaminants in spring water.
Figure 4. Effect of fire on longhair sedge (Carex comosa), although the leaves are killed by the fire, low intensity fires do not kill the plant.
In summary, current knowledge strongly suggests that some management practices, in particular those related mostly to traditional uses such as cattail harvesting, foster biodiversity and preserve key ecosystem services. Other practices, such as cattle browsing and high intensity fires, have negative impacts on biodiversity and ecosystem services including water quality improvement. This makes management for conservation and restoration of ecosystem services and biodiversity difficult for two main reasons: first, the accepted paradigm of conservation excludes productive use of resources; and second, wetlands in central Mexico have been used for centuries, making the relationship between use and biodiversity both ecologically and socially complex. It may be the case that some wetlands systems in central Mexico depend on management to maintain biodiversity and also the capacity to provide key ecosystem services. Under these circumstances, it is necessary, although challenging, to establish management strategies that preserve wetlands and cultural practices together. These management strategies may require envisioning nature itself from a different perspective, one that recognizes that for some ecosystems culture is a key component of the complex web of interactions that facilitates their existence.
- Zedler, J.B. 2003. Wetlands at your service: reducing impacts of agriculture at the watershed scale. Frontiers in Ecology and the Environment 1: 65–72.
- Escutia-Lara Y., S. Lara-Cabrera y R. Lindig-Cisneros. 2009. Fuego y dinámica de las hidrófitas emergentes del humedal de la Mintzita, Michoacán, México. Revista Mexicana de Biodiversidad 80: 771-778.
- Dominguez Dominguez O. and G. Perez Ponce de Leon. 2007. Los godeidos, peces endémicos del centro de México. Biodiversitas 75: 12-15.
- Hall, S., J.B. Zedler y R. Lindig-Cisneros. 2008. Does harvesting sustain diversity in Central Mexican Wetlands? Wetlands 28: 776-792.