dispersal, plants quickly colonize (plate 12- 11). As the
colonizers stabilize the sandbar, it builds, forming a
point bar and eventually supporting a sandbar scrub
community that replaces the original colonizers. But
by then more sand has been added, so the colonizers
persist as the bar grows ever larger. It may, depending
upon the dynamics of the river, become sufficiently large
to form a substantial river island. The dynamic nature of
tropical rivers is evident in the ever- changing pattern of
point bars and riverine island distribution.
Oxbows
Where the flow dynamics of the river become unstable
(typically during the high water period), the river may
cut a new channel, effectively isolating a meander
and creating what is called an oxbow lake, a habitat of
essentially standing water. Oxbows are common in rivers
subject to a variable flood cycle and they provide yet
another kind of riverine habitat, where water stagnates
rather than flows rapidly. Such still water supports vast
growth of water hyacinth, as well as the giant Victoria
Water- lilies (Victoria amazonica; chapter 10). It is here
that the peculiar Hoatzin can be found (see “Hoatzin:
Bizarre Bird of the Riverbanks,” below).
Floating Meadows
Entire islands of floating grasses can be encountered
along the Amazon and within Amazonian lakes.
Along the main rivers, some of these grassy islands
occasionally reach a size at which they can be a hazard
to navigation. Two grasses, Paspalum repens and
Echinochloa polystachya, are abundant components
of the floating meadows and together make up about
80– 90% of all the floating grass species of Amazonia.
Paspalum is adapted to float, forming dense, floating
mats for the four- to five- month rainy season, when
the river is high. The plants grow and spread asexually
during this time, but also flower and make seeds, so
that during dry season multitudes of seeds fall on the
newly exposed ground, to quickly germinate. Thus
Paspalum is adapted to be both a floating and terrestrial
plant. Terrestrial Paspalum has a distinctly different
morphology from the aquatic form, even though they
are the same species. Unlike Paspalum, Echinochloa
has no floating morph, but remains rooted throughout
the flood cycle. This species is most common in lakes.
Moriche Specialists: Birds and Moriche Palms
Stands of Moriche Palm are prime feeding areas for
various large macaw species whose powerful bills are
sufficiently strong to crack the hard palm nuts that
occur in dense clusters below the fronds. Other bird
species have become Moriche specialists, rarely if ever
found away from Moriche stands. These species provide
examples of an important component of Neotropical
species diversity, the tendency toward extreme
habitat specialization. The Moriche Oriole (Icterus
chrysocephalus), a small, striking black oriole with bright
yellow crown, shoulders, rump, and thighs, feeds and
nests within Moriche fronds. The Sulphury Flycatcher
(Tyrannopsis sulphurea) is a gray- headed, yellow- bellied
bird, easily confused with the widespread Tropical
Kingbird (Tyrannus melancholicus). Both species can
be difficult to see well in the dense palm fronds. More
obvious, the Fork- tailed Palm- Swift (Tachornis squamata)
is a common aerial feeder, streaking through the skies
in the vicinity of Moriche stands in search of insect prey.
This pale swift with a deeply forked tail builds its nest on
the underside of a dead Moriche Palm leaf. Finally, the
Point- tailed Palmcreeper (Berlepschia rikeri), a member of
the Furnariidae, or ovenbird family, is entirely confined
to Moriche Palm stands in Amazonia, and is nowhere
really common. It is difficult to spot among the dense,
fanlike palm fronds, but its presence can be known from
its song, a loud series of ringing notes. The bird is bright
cinnamon on the wings and tail and streaked boldly with
black and white on its head and breast. View it really,
really well, and you’ll see its bright red eyes.Plate 12- 12. Moriche Palm swamp. Photo by John Kricher.212 chapter 12 cruising the rivers to the sea