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ECOLOGICAL IMPACTS OF THE MONTSERRAT VOLCANO: A PICTORIAL ACCOUNT OF ITS EFFECTS ON LAND AND SEA LIFE Dr. Deborah M. Brosnan As biologists, our work in Montserrat has led us to explore not only the marine environment but also the unspoiled terrestrial habitats. These include the coastal mangroves, semi-desert vegetation in the north, and especially the cloud forest currently affected by volcanic impacts. When we first visited Montserrat in late 1994, Chances Peak had some of the finest cloud forest in the Caribbean region (image 1).
Click on this thumbnail to see full size image.
On one of our first visits we hiked the mountain to see the stunning cloud forest at the summit. At that time, Chances Peak cloud forest harbored a high diversity of plant life, including a rich display of tree ferns. and insects, lizards, birds, and bats. On our very first hike, we saw the famed and endemic Montserrat oriole perched on a tree about halfway up the mountain. On that same hike, Heliconia was abundant, and the beautiful black and yellow Heliconius butterflies guided our path through the mid-elevations (image 2).
Click on this thumbnail to see full size image.By January 1996, our helicopter and hiking surveys of the volcano and coastline indicated that vegetation loss from acid rain, gases, heat, and dust on the top of Chances Peak and surrounding area was severe (images 3 - left, and 4 - right).
Click on these thumbnail images to see full size image.
The cloud forest had disappeared. Tree ferns were dead, and the Heliconius butterflies had all but disappeared from the crater area. Vegetation was gradually dying further down the mountain, and during the next few months of 1996 we watched much of the mountainous vegetation in the Gages valley area turn brown and die. This pattern has continued. On the east side, the lush forests of the Tar River Valley were degraded from ash and gases, and finally destroyed by pyroclastic flows (images 5 - left, 6 - center, and 7 - right).
Click on these thumbnail images to see full size image.
One of the factors leading to vegetation death is acid rain, from volcanic sulfur. We are frequently asked about acid rain, and below we have included a brief explanation of the phenomenon and how it affects Montserrat's terrestrial life.
Click on these thumbnail images to see full size image.Volcanoes emit sulfurous gases, most of us know that from the characteristic smell, (image 8, above). In nature, sulfur is released into the atmosphere in three ways, through the formation of sea-spray aerosols, anaerobic respiration by bacteria--e.g. in tidal marshes, and by volcanoes. More recently combustion of fossil fuels has greatly increased the amount of sulfur in the atmosphere, with major consequences for northern forests and lakes. On a global scale the amount sulfur from active volcanoes is minor compared to other sources. On a local scale the impacts from volcanic sulfur emissions have important consequences for plant and animal life. Acid rain is formed when hydrogen sulfide and sulfur dioxide in the atmosphere undergo a complex set of chemical reactions, and eventually combine with water to produce sulfuric acid. It is these acid droplets that are known as acid rain, and they damage plant and animal life. Acid rain affects plants directly by breaking down lipids in the foliage, and by damaging membranes which can lead to plant death. Indirectly acid rain increases leaching of some nutrients and renders other nutrients unavailable for uptake by plants. Vegetation damaged by acid rain shows the characteristic browning that we have observed in Montserrat (image 9).
Click on these thumbnail images to see full size image.
The effect of acid rain on terrestrial and aquatic environments is determined by the acidity of the precipitation and by the geology of the soils and rocks of the area. Different types of soils and rocks have different abilities to neutralize or buffer the acid. Certain types of soils and their associated water ecosystems are particularly sensitive to acid rain because of their low buffering abilities. In general volcanic rocks, and thin topsoils have poor buffering abilities.
Acidity is measured on the pH scale. A pH value of 7.0 is neutral,
and anything less than 7.0 is acidic. (The lower the number the
more concentrated the acid). The pH scale is logarithmic, which
means that the difference between a pH of 6 and a pH of 5 is a
ten-fold difference. The ecosystem of a lake is severely affected
when the pH falls below 5.0. At this level, fish, invertebrates,
and plankton die. In January 1996 we measured the pH of the lake
at the top of Chances Peak (where the mermaid with the golden
comb resides) at 2.0 (i.e. 1,000 times more acidic than a pH of
5.0) (image 10).
Click
on these thumbnail images to see full size image.
Lakes and streams near Farrells measured about 1.5 at the same time (i.e. approximately 5000 time more acidic than a pH of 5.0). We do not know if there are any important biological consequences of these low pH values for the area. However, the streams and lakes away from the crater area are highly unlikely to be affected by acid rain, as they are too distant.
In contrast to our first hikes up Chances Peak, by early 1996 we recorded almost no animal life close to the summit of the volcano, with one notable exception. To our surprise, we found hummingbirds flying within 300m of Chances Peak. The birds that we saw were settling territory disputes. Hummingbirds feed on nectar, and will defend food territories. We cannot imagine what food resources are left to defend, because most of the plant life is dead.
We have been making regular trips to monitor the Bamboo forest,
home of the Montserrat oriole. To date,
the forest is still in existence, but the plant life is frequently
covered in ash (image 11, below left). The close proximity
of the volcano to the bamboo forest (image 12, below right)
means that a major collapse of the Galways Wall is likely to result
in ash clouds that will severely damage the bamboo forest ecosystem
including the associated animal life including the Montserrat
Oriole.
Click on these thumbnail
images to see full size image.
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How long will it take the cloud forest to recover?
Nature is resilient and natural disturbances are part of the life
cycle of all ecosystems. Today, even in the most impacted zones,
small green shoots continue to sprout, but these do not survive.
Continued volcanic activity and erosion prevent reforestation.
Sustained recovery will not take place until volcanic activity
greatly reduces or stops. Once this happens, a natural recovery
process called succession will begin to take place. Recovery of
the forest will start in two ways, seeds in the seed bank will
start to germinate, and new seeds will blow in from surrounding
areas. The rate of recovery will depend, in part, on the availability
of seeds which in turn depends on the proximity of other forest
species, and the availability of animals (e.g. birds) to disperse
seeds. Presently there is cloud forest in the Center Hills and
this may act as a source of new seeds for South Soufriere Hills.
The recovery process follows a successional pathway, whereby early
species (called pioneers) colonize the soil and make it suitable
for other forest species. Early pioneer species must be able to
tolerate high light intensities and high temperatures (because
there is no longer any forest to provide shade). They must also
be good dispersers, and able to arrive at a new site. Seed dispersal
by bats and birds is very important in the recovery of tropical
forests. Species such as Cecropia are early pioneers, they
are light tolerant, and their seeds are dispersed by a variety
birds and bats-- 76 species of birds feed on Cecropia.
Other pioneer species have seeds that are long-lived in the soil,
and that can survive a wide variety of soil environments. High
light levels, and high temperatures often stimulate the germination
of these seeds. (e.g. Heliocarpus). Heliconia, (the
name means sun-loving) and some palm species will colonize large
disturbed areas. Once these pioneer species have established,
their shade often prevents other members of the same species from
germinating and surviving. Thus new shade tolerant species can
now establish, e.g. the understory palm Crysophilia, and
many arid plants (these are the plants that do well in dim light
and high moisture such as swiss cheese plants Monstera,
(see image 9, above) Philodentron and wild tobacco) and
trees. Thus the forest begins to increase in diversity. Tropical
forests tend to recover faster than their temperate counterparts.
Studies in Puerto Rico showed that some forests recover from hurricane
destruction in about 40 years. However full recovery of a large
landscape that had been completely denuded of forest species i
s likely take considerably longer.
Because of the total loss in vegetation near the crater summit,
erosion has become a severe problem, and with each rain storm
more soil is lost from the mountainsides. The soil and any debris
accumulated along the way, flows into the ghauts and eventually
to the ocean. Vegetation on all sides of the volcano (e.g. north
of Gages Wall) has been lost, and as a result a range of watersheds
are indirectly impacted by the volcano through increased runoff,
e.g. Belham river valley. We have observed plumes of sediment
entering the ocean at Tar River, White River, Gingoes, New Beach,
Fort Ghaut. These plumes are often severe and dramatic. In January
and February, 1996, sediment load was heavy at Whites River, and
each liter of water entering the ocean at contained from 130g
to 150g dry weight of sediment. Water flowed into the ocean at
a rate of 1.5m/sec (image 13).
Click
on these thumbnail images to see full size image.
Generally the sediment settled out of the water column within
300 m from shore. However, on occasions the plume entering Fort
Ghaut extended as far as Bransby Point. The sediment entering
through Belham river flows south over the reefs off Garibaldi
Bluff. The north side of the island had no sediment input. This
is an arid, semi-desert area that is unaffected by the
volcano. Visual analysis of the sediments indicated that sediment
on the reefs and substrate from Garibaldi Bluff to Radio Antilles
was mainly a fine grain sediment of terrigenous origin, and similar
to soil sediment. However, it was not possible to determine the
age of the sediment or how long it had been in the ocean. By contrast
the sediment from the more northern areas (Rendezvous Bay) was
composed of coarse sand grains and showed little terrigenous influence.
VOLCANIC IMPACTS ON THE CORAL REEFS
The volcano is affecting the coral reefs
between the east and southwest of the island. At the most extreme,
we have observed reefs being buried by sediment and ash loads.
On other reefs, we have documented sedimentation (image 14,
below left), coral bleaching, increased disease, and the disintegration
of large sponges. During recent data collection at Garibaldi Bluff
(image 15, photoquadrat sampling, below right), all of
the giant sponges (Xestospongia) were covered in sediment,
some up to 1cm deep.
Click
on these thumbnail images to see full size image.
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UPDATE
We are continuing monitoring the impacts of volcanic activity
on the reefs. In February 1997 we recorded increased degradation
of the reefs in the south of the island. We have, for the first
time, recorded diseased corals at Garibaldi. We will update the
impacts on this page as information continues to become available.
© 2000 Sustainable Ecosystems
Institute
 comments: sei@sei.org |
