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The
vast tracks of lowland forest harbour a stunning number of terrestrial
species. Many scientists have been puzzled in the past by the apparent
contradictory monotony of the landscape and high species richness, and
this has invoked some intriguing theories about the origin of Amazonian
biodiversity. The
monotony of the lowlands, that contain about 90% of the Amazon forests,
is only superficial. Subsurface sediments and soils show unexpected heterogeneity,
which has been shown to be an important factor in distribution of plant-groups
(and related insects), and thus biodiversity. The relationship between
edaphic heterogeneity and biodiversity in lowland Amazonia is addressed
at the University of Turku Biodiversity Centre “Amazon Biodiversity Programm”
that collaborates in the Climazonia project. Isolation of small pockets
of edaphic similar areas has promoted habitat segregation and may have
resulted in allopatric evolution. Rivers also form allopatric barriers,
and animals and plants have been shown to diverge in areas cut through
by rivers. Stream avulsion and capture, a common feature in Western Amazonia,
must have added an extra dimension to habitat segregation and allopatry. Considering
Western Amazonia, we know that landscapes capable of supporting rainforests
only emerged c. 8 Myr ago. The high terrestrial species richness there
is thus of a relative recent age. We also know that the successive stage
s have produced very different types of deposits, each in its own way susceptible
to the strong tropical weathering trends. Neotectonic uplift in Western
Amazonia has exposed these deposits, and together with progressive weathering
and fluvial denudation resulted into a large-scale edaphic mosaic. The
dynamics of edaphic mosaics are thought to have played a major role in
the diversification of Amazonian biota, at least for Western Amazonia.
The landscape of Amazonia is almost entirely characterised as lowlands covered
with thick tropical forest, that is cut through by large rivers. Only the
rims of Amazonia are mountainous: the Andes in the west, the Guyana Shield
in the north, the Brazilian Shield in the south and some inselbergs in
the northwest.
Species
distribution patterns and diversity have been explained as a result of
repeated fragmentation of the now almost continuous forest into forest
refugia during glacial times. However, molecular dates of divergence of
several groups indicate that speciation of terrestrial taxa is mostly of
a Tertiary age, and the scanty palynological data do show shrinkage of
the forest area during glacial times, but also make it plausible that the
core forest would have remained intact. In the last decade, geological
evidence in favour of arid conditions in ice-age Amazonia have been overturned.
Other
historical hypotheses explaining species distributions and biodiversity
have been put forward, including the existence of a huge Quaternary Lake
in Amazonia (“Lago Amazonas”) as well as a possible role of tectonic structures
(“arches”) deep down in the subsurface.
The
aquatic ecosystems also harbour a stunning number of taxa. The Amazon harbours
most species of fresh water taxa in the world. Again, the question is if
fluvial landscapes were the act of the scene for speciation, or that this
happened in other aquatic landscapes in the geological past.
Our
research underlines severe changes in lowland Amazonia during the past
30 Myr. During that period the Amazon region experienced roughly three
phases of landscape evolution:
(2)
From c. 25 until c. 8 Myr ago, the western fringes of lowland Amazonia,
the Andean foreland basins, drowned. The lakes gradually expanded eastward,
and during the Middle Miocene (c. 14-10 Myrs ago) they occupied most parts
of western Amazonia
(3)
About 8 Myr ago tectonic uplift of western Amazonia caused the breakthrough
of the modern Amazon River and terminated the lake phase.
The
aquatic fauna’s that developed in the huge lakes of Miocene Western Amazonia
did add to the signature of the modern Amazonian aquatic biota. Lake Pebas,
with its restricted marine connections, is thought to have played a major
role in the adaptation of marine biota (e.g. manatees, dolphins, stingrays
and other fish-groups) to freshwater ecosystems. Lake Pebas also was the
scene for extensive radiation’s of invertebrate faunas, but their presence
in modern Amazonian ecosystems is remarkably scant.
Frank Wesselingh