Geoscience Reference
In-Depth Information
were able to gain a competitive foothold and continue to evolve, filling similar
niches that placental mammals would fill in Africa, Asia, and the Americas.
The closed canopy forests we see in today's Tropical Rainforest Biome became
widespread around 65-50 mya, in the early Tertiary Period. Recently, a hypothesis
has been put forward that suggests that the similarities we see in some taxonomic
groups among regions are caused by widespread global warming trends in the more
recent geologic past. During times of increased temperature and higher precipita-
tion levels, tropical species were more widely distributed—even at latitudes pole-
ward of the tropics, allowing for periodic interchanges of plants and animals
among the continents. Evidence points to a continuous tropical forest belt during
the Tertiary Period, spreading from Southern Europe through Central Africa into
Madagascar, and through southern Asia and the Far East. This forest was also
present on South America. This new idea increases our understanding of evolu-
tionary relationships and current distributions.
As the continental plates separated, changing climate began to restrict the Trop-
ical Rainforest Biome. When cooling occurred, rainforest species survived only in
those areas that remained tropical in climate. During several epochs, tropical spe-
cies were restricted, ending in their current distribution during the Pleistocene
Epoch. Beginning about 1.8 mya, with the onset of glaciation, the northern lati-
tudes became colder and drier environments with greater seasonality, no longer
favorable for tropical species. More pollen and fossil analyses, along with evolu-
tionary studies, are needed to confirm this hypothesis regarding tropical species ori-
gins and distributions.
The biodiversity of the three main centers of tropical rainforest today is largely
a consequence of evolution along with past climatic events. The Neotropical rain-
forest has the greatest diversity. Its large extent may have allowed more species to
endure through the cooling climate more successfully than their African counter-
parts. Isolation from other continents produced a diverse array of species adapted
specifically to that environment. African rainforests contracted significantly from
the effects of cooling and drier climates, leaving small refugia, which became cen-
ters of endemism still evident today. The Asian-Pacific rainforest was less affected
by the changing climate, although changes in sea level led to wider dispersion of
plants and animals.
Tropical rainforests continuously adapted to changes in climate and geologic
events. Throughout the changing geology and climate of the past, cooler glacial
periods restricted rainforests, possibly confining rainforest species to narrow refu-
ges and small remnants near the Equator. During warming events, the rainforest
expanded poleward beyond the tropical zone. Species in the world's rainforests
evolved and modified, adapting to the changing environment. The paleoecological
record provides evidence of past changes in the distribution of tropical rainforests
and helps in understanding the similarities found among them. Today, geographi-
cally separated rainforests contain their own arrays of species as well as associa-
tions derived from ancestors that had become established over 65 mya. Although
many of the species found in the rainforest today reveal this ancient history, the
