Jozani biodiversity inventory report 2002 - Coastal Forests of Kenya ...

are scarce and big trees are rare. Increased livestock number cause negative impact on fodder
species (Plate 29). With heavy grazing the habitat may change for such an extent that some
species may not be able to survive and these will be lost from the ecosystem, causing a
change in biodiversity. This threat is made worse by the free grazing system practiced in most
of the villages surrounding the Jozani forest and sometimes livestock encroach into forest
(See Appendix V).
3. Effect of alien species
This is one of the major threats to native biological diversity (IUCN, 1990). Invasive species
are found in all taxonomic groups including introduced viruses, fungi, algae, mosses, ferns,
higher plants, invertebrates, amphibians, reptiles, birds and mammals. The impacts of
invasive species are immense, insidious, and usually irreversible. Hundreds of extinctions,
especially on islands, have been caused by alien species (IUCN, 1990). Article 8 (h) of the
Convention on Biological Diversity (CBD) of which Tanzania is a Party states that "each
Contracting Party shall, as far as possible and as appropriate, prevent the introduction of,
control or eradicate those alien species which threaten ecosystem, habitats or species".
The decision to use only alien species in plantation forests may have a negative effect on the
local biodiversity through destruction of the habitat prior and after planting. Such introduced
exotic species may suppress local species which may disappear from the area (Plate 31a and
b). For example Maesopsis eminii tree (Rhamnaceae) was introduced in East Usambara from
western Tanzania in the 1960s. Since then the tree has been spreading fast and it is predicted
that in 200 years it will cover 50% of the area originally under natural forest (Binggeli, 1989).
In all forest plantations plant species diversity was low. Some are comprised of pure stands of
planted trees with only a handful of local species. This may be due to low competitive ability
by the local species suggesting that in the long run the area may have most of local species
completely out-competed. In plantation forestry indigenous species are considered as weeds
so that during tending operations they have to be weeded out.
A number of alien species have been introduced to Zanzibar including the Javan civet
(Viverricula indica), House rat (Rattus rattus) and Common rat (Rattus norvegicus) (Moreau
and Pakenham, 1941). These species are common in the proposed national park. Their effect
remains unknown but some like the house rat are pests and potential vectors for diseases such
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