by Jim Conrad
(excerpted from the author's contribution to the 2011 book
Freshwater Fish Biotopes in Mexico
by Kai Qvist & Rune Evjeberg of the non-profit organization
"Freshwater Ecotopes", based in Aarhus, Denmark)


If you look at a map of the Yucatán showing surface features such as rivers, lakes and hills, you'll notice that the Peninsula's low-lying, almost flat northern part has no inland rivers at all. What the northern Yucatán does have is many sinkholes, called cenotes, with water in their bottoms.

The Peninsula's limestone bedrock and its cenotes go together, to form a landscape technically called karst topography. Limestone dissolves in acid, and normal rainwater is naturally weakly acid. Limestone is brittle so it tends to develop cracks. Mildly acid rainwater seeps into the cracks, dissolving as it goes, and after a few thousand years you get "solution features" such as caves. If a cave's ceiling collapses leaving a hole in the ground above it, and the collapse zone extends below the water table so that water pools in it, you get a cenote.

An interesting point about the Yucatán's cenotes is that some provide great swimming while others are avoided by local swimmers because they're known to suck you under. The ones famed for good swimming have bottoms formed of rock debris fallen from above, but the ones known for sucking you under have bottoms opening into underground rivers, and the current in those subterranean rivers can be powerful. Here's what's happening with those subterranean rivers:

In The Yucatán, underground water drains inexorably from the southern inland region toward the costs. This is because not only is the southern inland region somewhat higher in elevation -- a peak in northern Calakmul Biosphere Reserve reaches 306m -- but also rainfall in the south is much higher than in the north.

You can see evidence of this hydrological dynamic by taking a half-hour boat ride east of Dzilam de Bravo on the northern coast to a spot maybe 200m offshore and in about 60cm of water where you find an upwelling of freshwater about two meters across. It's a freshwater spring completely surrounded by saltwater. The guide tells you that researchers from the National Geographic Society measured the spring's outflow at 3500 liters per minute. This water originates far away, in the Peninsula's interior.

Of course cenotes have been profoundly important to the history of the Yucatán's Maya people. In this area, for about half of each year, from around November into May, the region endures a severe dry season. Much of the herbaceous layer dies back and many trees lose their leaves. The ground cracks, and farmers make no effort to grow their traditional cornfields, or milpas, until the rains return around May. In the old days, if you didn't have access to cenote water, you were in a mess.


In the southern Yucatán, if you're interested in freshwater habitats, you have to be clear on two local concepts. You know what a lake is, and that in Spanish it's called a laguna. One of the new concepts needed down south is the aguada, which basically is a small lake with shallow water and gently sloped sides. It's formed where the surface limestone simply dips below the water table, with no collapsing of subterranean cavern ceilings involved.

The second new concept needed for understanding freshwater habitats in this area is the petén. Guatemala's northernmost and largest department is called the Petén because so many petenes occur in that area. Petenes also occur in the southern Yucatán. A petén is formed when there's a hole in the limestone and water pressure below pushes water out through the hole, flooding surrounding land, resulting usually in a marsh or shallow, open water.

Often petenes go dry during the dry season, or shrink drastically, then when rains return, the petenes return. But it's been shown that local rainfall has less to do with the petenes returning than with heavy rain in the uplands in the interior, which raises the local water table. Often a petén zone looks like a vast, shallow lake studded with small islands of woody vegetation. This may be because saltwater-intolerant, woody vegetation is taking advantage of freshwater upwelling around the rock hole's mouth.

In fact, all through this southern ecoregion, living things are always having to deal with ever-fluctuating saltwater and freshwater boundaries. If a hurricane blows a surge of saltwater far inland, vast freshwater swamps may temporarily die back. An aguada may be filled with freshwater during the rainy season, but when the dry season comes and its water evaporates it grows ever more salty. Laguna de Bacalar just north of Chetumal in the Yucatán's southeastern corner, the largest lake in that area, is regarded as a freshwater lagoon, but it hosts a mix of freshwater and marine life forms.


Coastal environments frame the Yucatán on the west, north and east. Petenes, mangrove, scrub coastal dunes, and flooded marine grasses are typical. Some coastal zones are incorporated into protected areas such as the Parque Natural del Flamengo Mexicano de Celestún, Parque Natural San Felipe, Parque Natural Río Lagartos and the Reserva de la Biósfera Sian Kaán. However, between these protected areas development is fast destroying important habitat, especially along the coast near Progreso and the "Costa Maya" from Cancún to Tulum, where enormous destruction of mangroves and other critical habitats already has occurred, and is poised to continue.

Over a million acres of mangrove habitat (over 400,000 ha) is considered to exist along the Yucatán's coast, and more than 3% of this ecosystem is being destroyed annually. Mangroves buffer the impact of storms, prevent erosion, and cleanse coastal waters by filtering them.


In the Yucatán, the farther northwest you go, the drier it becomes and the more scrubby and low the vegetation. Correspondingly, the farther southeast you go, toward Belize, the rainier it gets, and the lusher and taller the forest grows. Low, scrubby "thorn forest" in the northwest, lush forests of a height looking reasonable to a Northerner in the southeast...

Therefore, while geographically the Yucatán definitely is a peninsula, ecologically it's an island -- at least, the northern part is. The northern part is an ecological island surrounded by ocean toward the west, north and east, and toward the south it's separated from the rest of Mexico by a much rainier environment than the north has.

If you're a cactus adapted to northwestern Yucatán's very arid conditions, you'll have no cactus brethren to the south with whom to exchange genetic information. Therefore, you'll be exchanging genetic information only with other cacti living locally, island-like, and Darwin has made clear what happens to species on islands: They evolve adaptations beautifully fit for the local environment but in doing so they change so drastically from their parent stock "on the mainland" that they become genetically isolated -- they become new species.

Therefore, in the Yucatán, if you want to see land-based species found naturally nowhere else on Earth -- narrowly endemic species -- the best place is the Peninsula's arid northwestern corner, its ecological "island."

Endemic plants have been estimated to account for nearly 10% of northern Yucatán's total vegetation. The region contains 10 of the 14 endemic cacti of the Yucatán Peninsula . For the same ecological-island reason the northern Yucatán is considered among the Mexico's richest regions for endemic reptiles and amphibians. It's one of a very few places in Mexico where the Black-beaded Lizard (Heloderma horridum) lives, which is interesting because that species is one of only two venomous lizards known to exist. The northern-Yucatán ecoregion is home to over 290 bird species, of which two are endemic, plus there are about 96 mammal species.

In some cenotes and underground rivers there are endemics, too. There's the Giant Killifish (Fundulus grandissimus), Yucatan Killifish (F. persimilis), Ocellated Killifish (Floridichthys polyommus), the fish known as Cyprinodon suavium, and the uncommon Mexican Blind Brotula (Typhliasina pearsei). Several other species could be classified as near-endemic. Another notable species is the Blind Swamp Eel (Ophisternon infernale).


If you're crossing the Yucatán on Hwy 180 in the north or Hwy 186 in the south you see lots of forest between towns, even around the big city of Mérida. All that greenness can make you think that Nature is getting along relatively well in the Yucatán. Also there's the fact that the vast Sisal plantations of the past, centered around Mérida and at one time bestowing Mérida with the distinction of hosting more millionaires per capita than any other city in the world... now have practically disappeared. Much of that land now is reverting to forest. There's not as much industrial build-up as you see in many other areas, so maybe the groundwater is doing OK, too.

However, there's troubling evidence suggesting otherwise...

Each year more of the Yucatán's scrubby forest is being clearcut, with fallen trunks cut into sections and sold to individual households as firewood. Also an ever-growing percentage of trunks are chainsawed into segments that are heaped into piles, covered with dirt, then very slowly charred with a burn controlled by the size of air holes through the dirt, until charcoal is formed. Charcoal is used in the region's innumerable sidewalk taco stands, small restaurants and in people's homes. In a region where many people are looking for any kind of income, charcoal production seems to have a bright future.

Once a plot is clearcut for charcoal making or some other reason, sometimes the farmer plants a milpa, or traditional cornfield. In a few years weeds and insects move in and the harvest gets so poor that the field is abandoned. In the past, before the forest in that spot would be cleared again for another milpa, years would pass and the forest would regenerate. Soil would regain the greater part of its productive ability. However, now that the Yucatán's population density is much higher, often milpa growers don't wait long enough between clearings for the forest to regenerate and the soil to recuperate. The result is that now throughout the Yucatán soil is very thin to nonexistent, and what's left has little recognizable structure, is low in organic material, and is unable to retain adequate moisture and nutrients. In the old days, with low population density, slash-and-burn was a sustainable technology, but by no means is that the case now. An easy way to see the magnitude of deforestation in the central Yucatan is to set Google Earth for a high-altitude look at the Chichén-Itzá area.

Another concern is the Peninsula's groundwater. Mérida, the capital city with about a million inhabitants, has no sewer system, and I can't find mention of any other Yucatán urban area that does. Theoretically households have septic tanks, but when you see the cities' cracked sidewalks, crumbling buildings and poorly maintained back-streets, you have to wonder what shape an average urban family's septic tank is in. Some of the Yucatán's newer hotels and large stores have their own waste management and treatment systems in place, but a lot don't.

When you think about all the Yucatán's leaking septic tanks, the fact that chicken and turkey farming are among the Yucatán's leading industries, that much pesticide is used in citrus orchards and around people's homes, and that the region's limestone bedrock doesn't filter water that streams through cracks and caverns, it's clear that the groundwater is threatened. Pesticide use is so habitual among average Yucatecans, by the way, that often it's referred to as líquido, the normal Spanish word for "liquid." Of all liquids found in a normal household, they call for líquido when they want to squirt a weed at the house's corner.