Wednesday, December 05, 2007

Lines on Maps part 2: México

Continuing southwards from my earlier post, here's my comparable scribble for Mexico:

As before, this is a doodle, the boundaries are very rough and just suggest the general arrangement of these regions. And, unlike the ABA area, where I have actually spent time in most of my mapped regions, I've only actually set foot in a minority (10 of 22) of these Mexican regions. Mexico is an enormous country with complex geography and topography. All these regions interdigitate far, far more than I have suggested. Rainfall, elevation, and latitude determine much of these patterns: deserts to forests, tropical evergreens to subalpine conifers, cold northern winters with frost and snow to the true tropics in the south. One interesting thing to note is that the regions that extend into the US all tend to peter out around the Tropic of Cancer. This line roughly marks the divide between seasonality as rainy/dry (tropical) rather than warm/cold (temperate). Once again, I find it helpful to organize the regions in clusters:

I. Northwestern Mexico.

The far northwest of Mexico is distinct. Ecologically it resembles the southwestern US more than it does most of the rest of Mexico. If you are a vacationing Gringo you might find the terrestrial avifauna here disappointingly familiar beyond a small number of Baja endemics; but if you are working on your Mexican list this is where you will find a lot of good tickies for Yankeebirds you'd be hard pressed to find elsewhere.

1. Southern California.

The southward extension of the grassy/brushy/woodlands familiar to millions.

2. "Sierra Nevada."

See, here's the problem with this name. These mountains are far from the Sierra, but their jeffery pines, incense cedars, and birdlife are far more like what you'd find in Yosemite than in the rest of Mexico. Maybe I should call this region "Montane California" to go with Northern, Interior, and Southern California.

3. Sonora/Mojave Deserts

Another continuation south from the U.S. And just to make this large agglomerated region even more so, I have included the deserts of the Baja peninsula in it. This is a tough call; there is a lot of botanical endemism on the peninsula, but there is even more overlap with the Sonora region. Bird-wise, there's only one endemic here (Gray Thrasher), and it is actually also common farther south towards Cabo. Plus this Sonora-Mojave region already has a pattern of endemic thrashers of limited and discontinuous distribution (Bendire's, LeConte's), so Gray can just be added to that list.

4. Cape Lowlands.

A dry subtropical thornscrub type of environment resembling the much more extensive forests of the Pacific coast of the "mainland," but distinctive enough to earn a split.

5. Cape Highlands.

Another problematic region. Birdwise, it looks mostly like just an impoverished "island" version of the vast Sierra Madre pine-oak forests, with its Northern Pygmy Owls, American Robins, and Yellow-eyed Juncos. But botanically it has many endemics, including some of the dominant canopy tree species. And, other taxonomic treatments that are currently out-of-favor with the AOU would reclassify those birds as Cape Pygmy Owls, San Lucas Robins, and Baird's Juncos. So drink a toast to the splitters and map this as unique region, even if you can't count those tickies officially. This makes the Cabo area into its own biogeographic island, with lowland and montane phases.

II. Pacific slope

Coastal plains, hills, and lower mountains from central Sonora southwards. Many distinctive species are found in these drought-deciduous subtropical and tropical scrubs and forests.

6. Northern Pacific Slope.

Sonora to Nayarit, a frequent destination for birding tourists.

7. South Pacific Slope.

Quite similar in overall structure to the preceeding, but enough changes in species composition to justify a split. The divide seems to happen somewhere in Jalisco where the chachalacas and magpie-jays change species. I've also included the dry scrublands extending well inland up the Río Balsas valley.

8. Central American Dry Lowlands.

Here I've lumped together the dry pacific lowlands from the Soconusco to Guanacaste with the dry interior valleys from Sumidero Canyon to Matagalpa. These dry woods, brushlands, and farmlands from Chiapas to Costa Rica are a mixed bunch, but there's enough similarity overall to take the simple approach and lump them all.

III. Western and Central Mountains.

From cloud forests to alpine meadows, and from Arizona to Central America.

9. Northern Sierra Madre.

The northern portions of the oak-conifer zone of the Sierra Madre from Arizona to Chihuahua, plus the higher mountains of Coahuila, Nuevo Leon, and Big Bend. This includes the "Madrean Sky Islands" of Arizona and New Mexico, the Chisos, Copper Canyon, and Los Cumbres de Monterrey, all well-known birding hotspots. This northern and interior phase of the Sierra Madre forests is set apart by its relatively low diversity and high seasonality. Warblers and hummingbirds leave for the winter, and some species prominent just a bit farther south (e.g. Red Warbler, Mountain Trogon) are hard to find. On the plus side, this is the place to go to get your Arizona Woodpecker or Maroon-fronted Parrot.

10. Sierra Madre Montane.

The well-named heart of the Mexican montane with all its distinctive biota. Much of the pine/oak biota is widespread from Sonora to Chiapas (on both sides of the Balsas Basin), so I have pooled them all in one great region. Below the pine zone you will variously enter semiarid plains, pacific thorn forest, or tropical cloud forest, depending on where you are. But within and above the pine zone there's a remarkable consistency in the species lists.

11. Interior Montane.

Away from the Pacific crest, the sierras and volcanos rising from the plateaus of central Mexico have a somewhat different aspect to their biota. I've drawn this as one big blob but it is really a collection of many smaller blobs on the highlands of Durango, Aguascalientes, Guanajuato, Hidalgo, México, DF, Morelos, Tlaxcala, and Puebla. This includes the pine zone up through the firs to the alpine summits (birdwise, Mexico has no alpine specialists; volcanic eruptions may have a little to to with this...).

12. Pacific Cloud Forests.

On the Pacific slope of the Sierra Madre del Sur from Guerrero southwards a belt of cloud forest occurs above the dry pacific forests and seaward of the oak/pine zone. Many species occur in this belt that are scarce elsewhere in Mexico. I think this probably can be pooled with cloud forests farther south in Central America on both the Atlantic and Pacific slopes; but I've never been there myself and birding reports from these areas are spotty.

IV. Central Plateau.

The Altiplanice and other high plains and valleys from New Mexico and Texas to Oaxaca. Diverse deserts, grasslands, and shrublands that are wintering grounds for vast numbers of waterfowl and grassland birds from the north; home as well to the world's largest city.

13. Chihuahuan Desert.

Continuation from the U.S border to around the Tropic of Cancer, most of Chihuahua and Coahuila

14. Central Plateau.

Differs from the Chihuahuan portion by milder winters and fewer wintering grassland birds from the great plains. Still sees a large influx of waterfowl.

15. Southern Plateau.

Distinct in its species lists, this area includes interior Oaxaca and parts of surrounding states. It grades into the pacific lowlands down the Río Balsas basin.

V. Atlantic Slope.

Mountain slopes and plains of the east.

16. Tamaulipas Brushlands.

Distinctive brushlands and riparian woods over south Texas, most of Tamaulipas and much of Nuevo Leon.

17. Gulf Lowland.

A belt of diverse lowland forests from southern Tamaulipas to Tabasco, this is the closest approach of tropical moist lowland forests to the U.S. border.

18. Sierra Madre Oriental.

Separating the Altiplanice from the coastal lowlands is this band of mountains that supports a rich cloud forest. It spans the borders of San Luis Potosí, Tamaulipas, Veracruz, Hidalgo, Querétaro, and Oaxaca. El Cielo Biosphere Reserve in southern Tamaulipas is a well-known birding hotspot at the northern end of this region; this is the closest approach of cloud forest to the U.S. border.

19. Yucatan.

Covering most of Campeche, Yucatán, and Quintana Roo, this is a drier, more seasonal tropical lowland. Endemic species combine with outlier populations of widespread species of the dry tropics to make this a region distinct from its neighbors.

20. Cozumel.

Considering how small and flat it is and how close it is to the mainland, the island of Cozumel has an amazingly distinct biota and large number of endemic species.

21. Caribbean Lowland.

Extending southwards over much of Central America, in Mexico this region is seen by birding tourists most often around Palenque in Chiapas. It has higher diversity and a more fully "tropical" gestalt than the Gulf Lowland. A must-see region if you are working on your Mexico list, of course.

22. Central American Atlantic Cloud Forests.

Found on the mountain slopes of northern Chiapas, this is the western extension of an extensive belt of cloud forests along the wet slopes from Guatemala to Nicaragua. I suspect this can be pooled with the "Pacific Cloud Forest" above to make one great big "Southern" or "Central American" cloud forest region (which would of course be confusing with the two additional quite distinct but equally "Central American" cloud forest zones of the Talamanca and of Darién in Costa Rica and Panama... "Western Central American Cloud Forest???" Names, names).


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