沙漠研究 19-1, 217- 219 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

The Importance of Vegetables Crops in the Oases of Gabes
Rim TWITI*1, Mansour HADDAD1, Ali FERCHICHI1
Abstract: The prospecting and evaluation of the plant biodiversity on the coastal oasis ofGabes were conducted through an investigation covering 13 oases to determine the species
richness, composition based on indexes of diversity of Shannon (H ') and equitability. The
inventory of species present in the oasis of Gabes has proved that the carrot, turnip, onion arethe most cultivated species. The other species occupy small areas. Some local cultivars ofonion, carrot and turnip have been distinguished.
Keywords: Biodiversity, Oases, Vegetable crops

1. Introduction
The oases of south-east of Tunisia extend along the coasts of gulf of Gabes which give them amaritime climate. Several studies have targeted to study cropping systems practiced in this oasis divided
into three distinct stages of cultures: Palm trees, which form the upper floor and symbolise the oasis
landscape, a panorama of fruit trees forming the intermediate floor (pomegranate, olives, several varietiesof grapes, fig, apricot, plum, apple, pear, citrus) and other herbaceous crops that occupy the lower groundfloor formed by various vegetable crops, fodder and industrial ones (henna and tobacco). In spite of theirimportance, the vegetables species cropped in the underlying floor were very partially studied. The aim of
this study is to estimate the biodiversity of vegetables species grown in these oases.
2. Materials and Methods
The inventory of species was done through a questionnaire which takes into account the objectives ofthe study, namely the characterization of farms, the inventory of species distribution and the abundance ofspecies and cultivated varieties. The survey was prepared in collaboration with leaders members in local
NGOs and the official services of agricultural promotion. The study area had covered 5 oases: Gabes city,Gabes south, Gabes West, Gannouch and Metouia. The sampling size for each one is proportional to its
size with a total number of farmers counting 366.
2.1. Evaluation of species diversity
To appreciate the diversity of species in each oasis, the number of different species cropped wascounted. For that several indexes and parameters were used to assess the biodiversity within eachecosystem. The level of species diversity is quantified by specific richness (S), Shannon diversity andequitability index.
2.1.1. The species richness (S)This variable is essential to quantify the biological diversity of an ecosystem: It consist on counting
different species found in the exploitations visited and conducted on permanent grassland (Nicklès, 1839
cited by Pervanchonm, 2004). According to Blondel (1995), this method gives a rapid view to express thebiodiversity.
2.1.2. Index of species diversity
The index of Shannon-Weaver (Shannon and Weaver, 1949) takes into account the number of species
present and their relative abundance and allows us to obtain additional information. The diversity index
Shannon H '(Shannon, 1948) is used in ecology as a measure of species diversity (Margalef, 1958 cited byFrontier, 1983). It allows us to quantify the diversity by combining two components: the number ofspecies and their distribution on land. It is calculated by this formulas

H '= -Σp (log2 pi), With:i=1 i
1Institute of Arid Regions, Arid and oasis cultivation Laboratory Km 22, 4119 ElFjé Mednine Tunisia* E-mail: rym_twiti@yahoo.com, Fax number: 75 228 088

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pi = (area of exploitation / total area studied): S the total number of species present;
H ' is equal to 0 where the population is formed by one specie only and log2 S if all species are present
with an equivalent abundance (This index is usually ranged between 4.5 and 5 for the most diversified
plant population).
2.1.3. Index of equitability (R)The equitability index is also used to appreciate the level of fair distribution of the diversity (Ricottaand Avena, 2003). This index can measure the fair distribution of species compared to a theoretical equal
distribution for all species (Barbault, 1992). It is given by the following formula:
R= Η′ / Η max
With: H max = log2 S.
R = 0 (one species dominates) or 1 (all species have the same abundance).
3. Results and Discussion
3.1. Estimating the diversity of the 3rd floor of the cropping system
The importance of various speculations in the oases studied and their distribution are illustrated by theFigure 1. It shows that the palm cultivation remains especially in the oasis of Gabes city with an area
covering nearly 51%, the tree cultivation is mainly practiced in Gabes south covering 29 % of the area andvegetable crops are especially developed in the oasis of Ghannouch where they occupied 53 % of the area.
3.2. The species richness
Our survey revealed the existence of many species in the oases visited. Indeed, species richness S at
this stage culture is 21.
3.3. Specific composition
The exploration has shown that each oasis is specialized at the production of certain species (Table 1).
This table shows that there is a great diversification of vegetable crops in the oasis of Ghannouch. To alesser degree, vegetable crops are important in the oasis of Gabes city. The oasis of Metouia is specialized
at the cultivation of coret (Corchorus olitorius), Gabes north east is specialized at leek (Allium Porrum)
and Gabes south in forage crops especially (Medicago sativa).
3.4. Index diversity of Shannon
The richness and the specific composition of flora are not enough to measure the diversity of speciesand to estimate the degree of diversification. The Shannon index is 0.84. This value is closer to 0 thanlog2s (4.39) which reflected a very significant loss of biodiversity in the ecosystem. The index Shannon
diversity was also calculated within each oasis. The results are presented in Table 2. The oasis of Metouia
is the poorest in terms of biodiversity (H '= 0.017). Even if the Ghannouch oasis’s index is relatively high,its value did not indicate really a great biodiversity.
60% of areas (ares)

50403020100

Vegetable cropshoutGS

ityorthGN

Gcia
touMe

houcannGh

Palm cutivation

Fruit tree

Fig. 1. Distribution of different areas of speculation in the oasis studied.

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Table 1. The most practiced species for each oasis.

G.city- Parsley,- Blett,- Fennel,- Spinach,
Coriander

G.north- LeekG.south- Alfalfa,- Oats- Henna,-Tobacco.

Ghannouch- Onions- Carrot,- Turnip,- Garlic,- Squash,- Cucumber,- Cabbage,- Chilli,- Tomato,- Lettuce- Watermelon,- Melon,- Beet- Celery

Metouia- CoretTable 2. Index of diversity of Shannon for each oasis.

oasis
H'G.city
0.052

G.north
0.0342

G.south
0.0774Ghannouch
0.2111

Metouia
0.017

Table 3. Index of equitability for each oasis.

oasis
R

G.city0.0118G.north
0.0078G.south
0.0176Ghannouch
0.048

Metouia
0.00393.5. Index of equitability
The equitability of vegetables in the 5 oases studied is 0.19, this value closer to 0 than 1, indicates adeterioration of the biodiversity of the lower floor. The index of equitability calculated for each oasis isillustrated in Table 3. All values are also closer to 0 than 1. Metouia has a very low index (0.0039), reflecting
the scarcity of biodiversity in this oasis, while Ghannouch shows a highest index of equitability highest.
4. Conclusions
Nowadays life transformation in the oases of the Southeast of Tunisia had led to an increase in theabundance of agricultural activity at the expense of industrialization and the development of otherattractive economic sectors. This work was carried out to study the biodiversity of vegetable crops in theoasis of Gabes. The different indexes studied (species richness Shannon and equitability) had reflected aloss of diversity of vegetable crops in the oasis of Gabes. The richness of flora biodiversity in the oasis ofGhannouch can be explained by water availability. The plots are irrigated in an interval of 10 to 15 days
(water tower) which is the shortest tour through contribution to others oasis which promotes the practice ofvegetable crops. The poorest oasis in terms of plant diversity is the oasis of Metouia. Indeed this oasis
suffers most from the problem of hydromorphy, neglection and immigration. The hydromorphy causes soilsalinity, the degradation of soil quality and their inability to crops production.
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