Ciénegas del Lerma: Mapping the Protected Wetland Landscapes of the Central Mexican Highlands

Ciénegas del Lerma is best known for being the last extensive surviving remnant of the great wetland system that once covered the central Mexican highlands along the upper Lerma River. The reserve is recognized both nationally and internationally for its combination of open-water lakes, floating marshes, and riparian forest that sustain a remarkably diverse community of aquatic plants, fish, amphibians, reptiles, and birds. It is especially valued as a wintering and stopover habitat for migratory waterfowl and as a refuge for several species that are endemic to Mexico or threatened with extinction, including the Lerma axolotl. The site also illustrates how a highland wetland system can continue to coexist with intensive human use, supporting traditional fishing, water-plant harvesting, and grazing within a flood-regulating, biodiversity-rich landscape.

The wetland system now protected as Ciénegas del Lerma was, at the end of the nineteenth century, part of an enormous continuous marshland landscape that covered roughly 27,000 hectares across the upper Lerma basin. Over the course of the twentieth century, agricultural expansion, urban growth, industrial development in the Toluca Valley, and large-scale drainage projects drastically reduced and fragmented this original extent, leaving the three lakes and their associated marshes as the principal surviving fragments.

By the mid-twentieth century, additional areas had been lost, including roughly 10,746 hectares that became permanently inundated through mismanagement and excessive hydrological pressure by 1940. These historical losses framed Ciénegas del Lerma as a rare and increasingly valuable ecological refuge in a landscape that had otherwise lost most of its original wetland character.

In response to this pattern of degradation, the Mexican federal government issued a decree on 27 November 2002 establishing the area as an Área de Protección de Flora y Fauna, recognizing it as an ecological unit that still preserved several interacting ecosystems and a notable biodiversity component. Two years later, on 2 February 2004, Ciénegas del Lerma was designated a Ramsar Wetland of International Importance, placing it on the global register of wetlands considered significant for conservation and embedding it within the international framework of wetland protection.

Subsequent regulatory instruments have shaped how activities inside and around the reserve are managed. Federal norms on endangered native flora and fauna, on water resource availability, on scientific collection, on controlled use of fire, and on training and certification of nature guides all apply to the reserve, formalizing a layered conservation regime that overlays the working agricultural and fishing landscape still present around the lakes.

Ciénegas del Lerma occupies a highland basin position on the central Mexican plateau, with the three protected lakes set within a flat to gently rolling valley floor framed by the surrounding volcanic terrain of the Trans-Mexican Volcanic Belt. The protected area lies within the Lagos y Volcanes de Anáhuac physiographic subprovince, where lakes, swamps, and stream channels interwoven with ranching and farmland define the local topography.

The soils of the area reflect its geological and hydrological history, dominated by alluvial deposits along the watercourses and by three lacustrine soil regions associated with the historical lake beds. The principal soil types are humic histosols, gleysolic humic variants, pellic vertisols, gleyic phaeozems, and mollic andosols, together producing the moisture-retaining substrates that sustain the wetland vegetation.

The hydrology of the reserve is organized around the upper Lerma River and its tributaries, with various drainage channels, irrigation ditches, and smaller streams crossing and encircling the polygons. The three lakes themselves differ in depth and character, with deeper open-water areas exceeding five meters alongside shallower zones dominated by floating plants, emergent vegetation, and deep-rooted aquatic species. This combination of open water, marshland, riparian woodland, and agricultural countryside gives the protected area a strongly mosaic-like landscape in which water defines the spatial structure.

Ecologically, Ciénegas del Lerma is defined by the juxtaposition of open-water lake habitats with floating and emergent aquatic vegetation, extensive marshland communities, and riparian woodlands on the lake margins. The three protected lagoons, Almoloya, Lerma, and San Bartolo, each support distinct submergent and floating plant communities, but together they form a contiguous wetland complex that functions as a single ecological unit.

The vegetation of the reserve is composed of aquatic and subaquatic species as well as moisture-tolerant terrestrial plants. Across the three lakes, records include 226 species of angiosperms, two species of pteridophytes, and at least one algal species, a flora that includes Mexico-endemic taxa such as Cyperus semiochraceus, Sisyrinchium tolucense, Glyceria mexicana, Pennisetum crinitum, Nymphaea gracilis, and Lemna trisulca, as well as culturally important plants whose tubers, leaves, or roots are still harvested by local communities.

At the regional scale, Ciénegas del Lerma acts as a refugium for wetland-dependent ecological processes in a landscape that has otherwise been heavily converted to agriculture and urban land uses. The reserve retains functional examples of native aquatic plant communities, supporting water purification, sediment retention, flood regulation, and recharge of the underlying aquifer, and it provides essential habitat continuity for native fishes, amphibians, reptiles, and birds in an otherwise fragmented highland landscape.

The protected area harbors more than 425 documented species of plants and animals, including 31 taxa classified under some category of risk and a number of exotic or introduced species. Mammals recorded within the reserve include the eastern cottontail, the long-tailed weasel, small shrews, pocket gophers, several bat species, western harvest mice, deer mice, Mexican voles, and hispid cotton rats, together representing a characteristic small-to-medium mammal community of central Mexican wetlands and their margins.

Birds are the most diverse vertebrate group, with around 118 species recorded, and the reserve is notable for its role as a wintering area for migratory waterfowl and other aquatic birds attracted by the temperate conditions and the relatively mild microclimate generated by the open water. Protected and threatened species in the area include the least grebe, American bittern, Mexican duck, Swainson's hawk, white-tailed hawk, swallow-tailed kite, peregrine falcon, yellow rail, king rail, Virginia rail, great horned owl, song sparrow, Savannah sparrow, and the endemic black-polled yellowthroat.

Reptiles of the reserve include the southern garter snake, the Mexican black-bellied water snake, the Mexican bull snake, the Trans-Mexican rattlesnake, the mesquite lizard, the mountain horned lizard, and the Mexican alligator lizard, occupying a range of aquatic, marsh, and upland habitats around the lakes. The amphibian fauna is small but ecologically significant, featuring salamanders such as the granular salamander and the critically important Lerma axolotl, together with tree frogs such as the ridged tree frog and the Moctezuma frog, several of which are classified as species at risk.

Native freshwater fishes have historically formed part of the cultural and economic identity of the wetland, and the reserve continues to sustain populations of native species alongside introduced carp, goldfish, and grass carp. The combination of endemic and threatened amphibians, restricted-range reptiles, a substantial migratory bird community, and a mix of native and introduced fishes gives Ciénegas del Lerma a distinctive and nationally important wildlife profile shaped by its position at the intersection of highland basin and major migratory flyway.

Ciénegas del Lerma is recognized as a conservation priority because it represents one of the last intact functioning wetland complexes of the Central Mexican highlands, an ecosystem type that has been almost entirely lost elsewhere in the region through drainage, urban expansion, and pollution. Federally designated as a Flora and Fauna Protection Area in 2002 and inscribed as a Ramsar site in 2004, the reserve carries both national and international legal recognition of its ecological value.

The conservation significance of the area rests on several reinforcing factors. It is a refugium for endemic species such as the Lerma axolotl, it anchors a migratory bird network across central Mexico, and it continues to provide regulating ecosystem services including flood control, water-quality maintenance, and aquifer recharge for the densely populated Toluca Valley. These functions tie the protection of the wetland directly to the hydrological and climatic stability of one of Mexico's most important highland basins.

At the same time, conservation here is inseparable from active management of human pressures. Over-extraction of groundwater to supply the Toluca Valley and Mexico City, urban and industrial land-use change, untreated wastewater discharges crossing the polygons via drainage channels during rain events, and shifts in agricultural practices all affect the lakes. The site's conservation regime therefore combines federal protection status, Ramsar obligations, and a set of national norms controlling endangered species protection, water availability accounting, scientific collection, controlled use of fire, and guide certification, in an effort to keep traditional livelihoods compatible with the long-term functioning of the wetland.

Ciénegas del Lerma is woven into the cultural and economic life of the communities of the upper Lerma basin, where the lakes have long supported subsistence and small-scale commercial activities. Rainfed agriculture for household consumption and limited sale, extensive grazing of cattle, sheep, and horses, harvesting of aquatic plants such as water potato, watercress, soft-stem bulrush, and round and palm bulrush, and small-scale fishing for native species such as the tiro, amarillo, and tambulla fish coexist within and around the protected area.

Hunting of migratory waterfowl, particularly ducks and teal, has historically been part of the seasonal rhythm of the wetlands, alongside aquaculture practices using both native species and introduced Asian carp and crayfish. These activities give the reserve a strong cultural dimension, embedding it within a still-living rural wetland tradition rather than separating it from the surrounding working landscape.

The wetland also carries a broader symbolic and ecological significance, as the surviving fragment of the great marshland that once characterized the Toluca Valley and the central Mexican plateau. Its recognition as a Ramsar site and its continuous presence in the regional identity of Toluca and the surrounding municipalities underline its role not only as a natural protected area but also as a cultural landscape shared by farmers, fishers, hunters, and urban residents who depend, directly or indirectly, on the environmental services the wetland provides.

Ciénegas del Lerma stands out as the principal surviving wetland of the upper Lerma basin and one of the last extensive highland marshes of central Mexico, with three named lake polygons each preserving distinct aquatic plant communities. Its combination of Ramsar status, flora and fauna protection designation, and ongoing traditional rural use makes it a notable example of layered conservation governance applied in a working landscape.

The reserve's particular strengths include its importance for migratory aquatic birds, its refuge role for the Lerma axolotl and other threatened amphibians, its mosaic of open water, floating marsh, and riparian woodland habitats, and its continuing role as a flood-regulating and water-quality-supporting element of the Toluca Valley landscape.

The climate of the Toluca Valley around Ciénegas del Lerma is temperate subhumid with summer rains, so the wetland's character shifts seasonally between a drier cool season and a warmer, water-rich rainy season. Winter, particularly December and January, brings frosts and the lowest temperatures of the year, but it is also the period when many migratory waterfowl are present at the lakes, making it the most rewarding season for observing wintering birdlife on the open water.

Spring and early summer, especially April and May, are the warmest and driest months, with the highest temperatures and the lowest atmospheric humidity, conditions in which water levels tend to be lower and aquatic vegetation is most visible. The rainy season that follows recharges the lakes and surrounding marshes and brings the wetland to its fullest hydrological expression, with the highest abundance of aquatic plants and the most active breeding conditions for resident wildlife.