Fire and Invasive species: The case of Acacias in Madagascar
Silvija Budaviciute
Jesús J. Contreras Pascual
Adrián A. Monge Monge
Slash and burn in Madagascar
Madagascar is the fourth biggest island of the world with a size around two and a half times
England. It´s located in the southeast coast of Africa in the Indic ocean (WWF 2007).
Madagascar has a wide conditions variety. The east and the north parts have tropical climate. The
western and south are dominated for a dry climate. The orography also changes around the country,
since the low altitude of the coast to the high plateau where is located the capital, Antananarivo. It
makes that the island has a high ecosystem variety. Tropical rainforest, tropical dry forest, thorn
forest, shrubs land and desert, of high and low land, and also mangroves in the coast (WWF 2007).
Madagascar is really an ecological important point. Its different ecosystem conditions and the
isolation of Africa´s continent make that Madagascar has the 5% of the word biodiversity, 80% of
this organism are indigenous and many of these are endemics. In Madagascar we can found
mammals like lemurs or fossa, reptile as chameleons or tortoises, five endemics bird species, the
coelacanth that is a Lazarus taxon. Lazarus taxon is the term used form the species classified as
extinct who reappear whit the same genetic code. Many varieties of palms and baobab are
endemics of Madagascar (Irwin et al. in press).
Figure 1. Madagascar’s geography makes it diverse
in terms of ecosystems and its insular environment
is rich in endemic species
The slash and burn practice, which is call tavy for the
Madagascar´s people, is one of the major causes of wildfires, deforestation and biodiversity loss of
the island. The practice implies the cut and burn of primary or secondary forest vegetation to later
cultivate one cycle of rice and another of a tuber crop (Kull 2002). After cultivation, land is
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allowed to rest under a fallow period; current fallow time is now between 5 and 7 times shorter than
30 years ago (between 3 and 5 years in length). The process repeats several times until productivity
decreases (Styger et al. 2007). This process induces landscape changes from indigenous forest to
either grassland or sometimes to acacia´s forest.
There are several historical reasons for the continuous use of slash and burn in Madagascar (Kull
2002):
 Madagascar´s exponential population growth between 1900 and 2000 (this growth continues
around 3% per year).
 Madagascar is a poor country and many of its citizens depend directly of the primary
production.
 During colonial times, tavy was a form of rebellion against French authority, which
considered that practice was destructive and difficult to administrate (for tax collection
proposes).
Today, tavy is still consider by many as a traditional production practice and as a new form of
protest against a new “conservation colonialism”.
Acacias in Madagascar
Australian acacias have been widely introduced in Africa; only in Madagascar, 39 species have in
formally introduced in the last 150 years. Today, five acacia species are consider as invasive in
Madagascar, but many others are already considered as naturalized and have the potential of
becoming invasive under the right circumstances (Kull et al. 20007a).
From all the invasive acacia species, A. dealbata and A. mearnsii are consider the most aggressive
and currently dominate more than 300 000 ha in the central highlands of Madagascar (Kull et al.
20007b). This species were originally introduce along roads and railway lines in order to secure fuel
supply, reduce erosion and provide shade. A. dealbata became popular in the late 1800 as a
substitute for eucalyptus plantations in high areas along railway lines for charcoal production; in a
way, the expansion of A. dealbata is strongly correlated to the expansion of the railway to the south
and to the east of the capital (Kull et al. 20007a).
After been introduced in the early 1900, A. mearnsii was heavily planted around 1950 as to expand
the vegetable tannins industry in Madagascar. This industrial endeavor failed and many plantations
were abandoned and expanded naturally. Later both acacias were planted in different areas to cover
barren hills, many of which were older tavy fields that were too degraded to recover (Kull et al.
20007a).
Nowadays, the continuous use of slash and burning in edge forest areas and the degradation of
endemic spiny forest are creating suitable environments for the expansion of both acacia species,
particularly A. dealbata in the highlands (Kull et al. 20007b, Styger et al. 2007). The control of
these species becomes very difficult as they are highly appreciated by poor rural people due to their
multiple uses: fuelwood, minor construction, charcoal production, green manure, medicinal
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properties and others. Local authorities are also unwilling to deal with the problem as they see these
acacias as free, fast-growing trees that do well in poor soils and even beneficial for areas under tavy
due to the nitrogen fixation (Kull et al. 20007b).
The expansion of acacia species in central Madagascar is likely to continue as long as local people
consider them more beneficial than harmful. This is a clear example of a collision between
international objective (biodiversity conservation) and local objectives (sustenance of livelihoods);
any solution to the invasive problem would have to consider the needs and dependency on rural
communities to the new acacia forests.
Impacts on local biodiversity
Biological invasions cause important immediate threats to biodiversity. As with many invasive
species Acacias are not exception. Indeed, Acacias are considered as very serious invasive species
(Kull et al. 2007a).
There are several reasons to raise concerns on Acacia impacts on biodiversity. They are habitat
transformers. After they establish in the area, they grow in dense clumps and thus reduce native
vegetation cover. By changing cover density they modify microclimate conditions. They also
compete with local biodiversity for space, light, water and nutrients. Native plants usually loose the
competition, as they are more sensitive to changing conditions. As native biota is exchange with
Acacia’s ecosystem functions and structure is changed too. Thus the whole chain of change
reactions initiate in ecosystems that might be or might not be reversed.
Despite all these recognized impact the effects of invasive species on biodiversity are little studied
in Madagascar. We are able to find only the couple of studies on Acacia’s effects on arthropod
communities in South Africa. It is possible that references are available in other languages for
Madagascar, but we were looking only at English references. Therefore below we will present two
case studies from South Africa on Acacia dealbata and A. mearnsii effects on invertebrate
communities.
A case studies on Acacia dealbata and A. mearnsii effects on invertebrate communities, South
Africa
Invertebrates are a functionally important group and disturbances can have a significant effect on
species diversity, abundance, biomass and species dominance. The study by Coetzee et al. (2007)
showed that Acacia dealbata invasions increase rarity and vulnerability of native invertebrates in
South Africa. The study shows (figure 2) that invaded grassland sites had higher Coleopteran
morphospecies richness and abundance values compare to invaded sides. The evenness of
assemblages also confirmed differences between invaded and not invaded sites. In particular
important difference to notice is between invaded and not invaded habitats concerning the
occurrence of rare species. The figure 2 indicates that in invaded sites rare Coleoptera species were
not present.
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Figure 2. Species accumulation corve for grassland and invaded sites (from Coetzee et al. 2007).
Authors suggest that A. mearnsii might have similar effect on arthropod diversity. Thus we looked
at the other study done by Samways et al. (1996) on A. mearnsii effect on several invertebrate taxa.
These study verified the results by Coetzee et al. (2007). The study showed that taxa that occurred
in areas with exotic vegetation were mostly Blattodea and Brachinus sp. and there were not
restricted to exotic vegetation only. In addition, several species were restricted to indigenous
vegetation and were not found in exotic vegetation sites. Therefore, results verified poorest
invertebrate richness and diversity under A. mearnsii.
Both Acacias are a concern because they cause the loss of unique species of Coleoptera and other
arthropods. They also increase abundance of common species. Species that should not be present in
grasslands or other habitats with native vegetation move there. Such changes in arthropod diversity
composition, richness and abundance may affect dynamics and diversity of insectivorous birds and
other taxa.
Conclusions
Tavy is not a sustainable system any more as rotations have become very short. Forest under heavy
tavy management could become grasslands or acacia forests in less than 30 years
Younger farmers understand the disadvantages of tavy, but older ones are difficult to convince due
to its traditional and political connotations.
Studies on the effect of invasive acacias on local biodiversity are not available for Madagascar (at
least not in English) and these studies are needed. Research from main land Africa indicates that
Acacias invasions have a negative impact on native insect communities particularly by favouring
the expansion of common insect species (e.g. cockroaches).
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A main concern is that the ecological impacts of several acacia invasions, particularly in
Madagascar, are currently under-evaluated, due to their utility to farmers and for soil rehabilitation.
Local authorities are also unwilling to act in order to avoid public unrest in rural areas.
References
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