Tectonic Changes And The Rise Of The Andes

For the last 20 million years the Andes have been rising as a result of the subduction of several oceanic plates beneath the South American Plate. The uplift transformed a rather flat continent into one with strong physical separation of lowlands and a host of new habitats ranging from humid foothills to ice-covered summits. The rise of the Andes had no less radical an effect on the biogeography of the continent. Drainages of great rivers were reversed (Damuth and Kumar, 1975; Hoorn et al., 1995), and the related orogeny in Central America provided, first, stepping stones and, ultimately, a landbridge connecting a Gondwanan to a Laurasian flora and fauna (Terborgh, 1992; Webb and Rancy, 1996). The great American faunal interchange in which successive waves of taxa moved north and south and then underwent adaptive radiation began as early as 16 million years ago and reached its peak (Webb, 1997) following the closure of the Isthmus of Panama, a progressive process in which the final phase took place between 5 and 4 million years ago.

The arrival of such animals as monkeys, sloths, elephantids, camelids, rats, and cats left a lasting impression on these systems. Many entered unoccupied niches, while others may have gone into direct competition with marsupial counterparts, or the indigenous array of flightless predatory birds. The net result was rather lop-sided with relatively few genera moving into North America, though Glyptodont, a re-radiation of sloth species, possum, armadillos, and porcupines were clear exceptions. While only the latter three have surviving representatives in North America, >50% of mammal genera in South America are derived from Laurasian immigrants (Terborgh, 1992).

This pattern obeys the basic biogeographic rule that the flora of larger source areas will outcompete those of smaller source areas. Consequently, lowland rainforest taxa from South America surged up into Central America, and became the dominant vegetation of the lowland tropics. Contrastingly, Laurasian elements swept south along mountain chains occupying the climatically temperate zone of Central and South American mountains.

Many modern genera were extant and clearly recognizable in the pollen of Miocene sediments (23 to 6 million years ago) (Jaramillo and Dilcher, 2002). During this time the Andes were rising, attaining about half their modern height reaching c. 2,500-3,000 m about 10 million years ago (Hoorn et al., 1995). Thus, these were forest-shrouded systems. Additionally, low passes—such as the Guayaquil gap and the Maracaibo area—maintained lowland connectivity from the Pacific to the interior of the continent until the mid- to Late Miocene (Hoorn et al., 1995). Only in later stages of uplift did the Andes rise above elevations supporting diverse montane forests (i.e., above 3,300-3,500m).

Montane-dwelling migrants into this setting from North America had to island-hop—either literally, or from hilltop to hilltop—including passage across a broad lowland plain in central Panama. This gap without highlands over 1,000 m was at least 130 km and may have acted as a severe filter to large seeded species, such as Quercus. Indeed, Quercus diversity in western Panama is about 13 species, whereas only one species is present in eastern Panama (Gentry, 1985).

The southward migrations of Laurasian taxa such as Annonaceae, Hedyosmum,

Salix and Rumex are inferred rather than observed, but the arrival of Myrica, Alnus, and Quercus are apparent in the paleoecological records from the high plain of Bogota (Hooghiemstra, 1984, 1989; Van der Hammen, 1985; Van der Hammen et al., 1992; Van't Veer and Hooghiemstra, 2000). Myrica arrived in the Middle Pliocene, Alnus first occurs in the Colombian pollen record about 1 million years ago. Quercus first occurs about 478 kyr bp (Van't Veer and Hooghiemstra, 2000) but probably only attained its modern dominance between 1,000 m and 2,500 m elevation about 200 kyr bp (Hooghiemstra et al., 2002). Since the first arrival of these species Alnus has spread as far south as Chile, whereas the southernmost distribution of Quercus coincides with the Colombian-Ecuadorian border (Gentry 1993). Alnus, a pioneer species, thrives in disturbed settings, whereas Quercus humboldtii is a dominant of mature Andean forests. The arrival of Quercus in Colombia clearly impacted previously established taxa such as Hedyosmum, Vallea, and Weinmannia (Hooghiemstra, 1984)—species that are still the common components of upper Andean forest in Peru and Ecuador.

Progressive cooling during the Quaternary led the upper limit of diverse forest to move downslope ranging between 3,600m and 2,800 m during warm periods and probably as low as 1,900 m during peak glacial conditions. The consequent expansion of montane grasslands, through a combination of uplift and cooling, provided habitat for newly arriving holarctic species that enriched Puna and Paramo floras.

The new arrivals to forest and grassland settings created novel communities. The concept of no-analog communities is usually used in reference to suggest that communities of the past differed from those of the present (e.g., Overpeck et al., 1985), but ecologically a no-analog community is most significant if it is the formation of a novel community compared with those that preceded it. The sequential arrival of Myrica, Alnus, and Quercus established such novel communities. Furthermore, the faunal interchange between the Americas altered predator-prey relationships, seed-dispersal and plant recruitment (Janzen and Martin, 1982; Wille et al., 2002). While many of the megaherbivores died out in the terminal Pleistocene, the surviving camelids exert a significant grazing influence on montane grasslands. Such a basic observation is an important reminder that the loss of some of the other megafauna could have had a substantial impact on the openness of all Neotropical settings (Janzen and Martin, 1982). Thus, during the Quaternary there has been a major re-shuffling of community composition, both in plants and animals, that resulted from plate tectonics. Glacial activity added additional layers of migrational change.

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