Some Aspects of the Ecology of a Potentially Invasive Species in Martinique: The Case of Dichrostachys cinerea

Biological invasions are the second most significant cause of biodiversity loss. They lead to the introduction of exogenous plant species which today threaten autochthonous and/or endemic species. Indeed, many plant species have disappeared. Other highly endangered species are currently in danger of extinction. Human activities, the overexploitation of resources, climate change but also biological invasions have advanced these extinctions. These phenomena have led to the progressive deterioration of the environments and a decrease in the diversity of the landscapes (a considerable depletion of the plant ecosystem) thus contributing to a loss of biodiversity. Natural (cyclones, etc.) or anthropogenic (pollution, etc.) perturbations create openings in the ground cover allowing more competitive species to install themselves. Introduced plant species install themselves in the open areas caused by these perturbations. They overcome a succession of barriers (physical, geographic, environmental) before they can become invasive. When conditions are favourable to their installation, they develop there to the detriment of the native populations. Insular environments (islands) are more sensitive to the phenomena of biological invasions. In Martinique, forests have gradually degraded and are losing their ecosystem resilience. This favours the installation and development of invasive species. Dichrostachys cinerea is an introduced species which is native to Africa and which has become naturalised on the island of Martinique. It colonises extensively perturbed areas, fallow land, abandoned farm plots located in the south of the island. A floristic analysis of the forests of the communes of southern Martinique (Sainte Anne, Marin, Vauclin) was carried out in order to establish the ecological profile of this shrub.


Introduction
Invasive alien species are the second leading cause of biodiversity loss (Vitousek et al., 1997). Man has long contributed to the influx of plant species outside their natural distribution range when travelling to the New World or conquering territories. Some of these introduced species have adapted to their new environment. During their development, they have gradually expanded their regenerations to the detriment of the autochthonous and/or indigenous plant species of the given territory. In addition to their rapid growth, invasive alien plant species possess great ecological plasticity (Jakobs et al., 2004;Richards et al., 2006;Terral and Ater, 2016). This gives them the ability to expand their ecological niches in different environmental conditions.
In addition, these taxa tend to alter the characteristics of the environment (the chemical composition of the soil, etc.) or the specific interactions with indigenous plant communities, thereby reducing local biodiversity.
Insular ecosystems are very sensitive to these invasion phenomena. Their geographic isolation and limited surface area are two features which favour the installation of invasive alien species (Vitousek and Walker, 1989). Martinique contains approximately 3200 plant species (Howard, 1988;Fournet, 2002) of which 1536 are indigenous or autochthonous species (48%), 236 are naturalised (about 7%), 846 are cultivated species (26%), 180 are in the process of naturalising (about 6%) and the remaining 405 species are likely to have disappeared or were not found.
The first studies of invasive alien species in Martinique were led by Professor Philippe Joseph (Joseph, 1999). They enabled him to establish the ecological profile and the potential for invasion of these species.
These potentially invasive species develop in perturbed regions of xeric, mesophilous or hygrophilous environments (especially Funtumia elastica) and expand their populations to form dense monospecific stands.
Dichrostachys cinerea (L.) Wight & Arn is an invasive bush or shrub (Fournet, 2002), introduced in the 19 th century in the French West Indies. The species proliferates in deciduous xeric forests, fields, roadsides, agricultural areas and other perturbed areas. It causes significant agricultural losses and managing it requires resources which are extremely costly.
This paper aims to outline the main life history traits of Dichrostachys cinerea in the xeric formations of Martinique and to establish its ecological profile.

1.1
Study site Located in the centre of the island arc of the Lesser Antilles, the island of Martinique has a total surface area of 1,128 km 2 . The humid tropical climate is marked by two seasons: a dry season or carême (Lent) that corresponds to the drought period from February to April, and a wet (or rainy) season that runs from May to November. Its highest peak, Mount Pelée, rises to 1,397 m. The numerous reliefs (mountainous massifs) create climatic variations which initiate a bioclimatic staging. This staging gives rise to several types of vegetation ranging from dry to hyper-humid. The minimum and maximum annual temperatures of the study areas were 23°C and 28°C respectively with annual precipitation not exceeding 1,500 mm, which corresponds to a dry bioclimate.  Several areas showing plant formations at different dynamic and ecological stages were inventoried by the BIORECA team. Among those inventoried, we chose to present fourteen which were at more or less advanced stages of development, marked by the greater or lesser presence of the species Dichrostachys cinerea. With the help of data from the National Geographic Institute (IGN in the French acronym), we were able to extract maps revealing the different soil types of Martinique. These revealed the presence of two types of soils in the study area: vertisols and brown soils containing halloysites. A characteristic of these soils is that they develop in dry areas (with rainfall of less than 1500 mm).

Some descriptive aspects of the Dichrostachys cinerea (Mimosaceae)
Dichrostachys cinerea or Acacia Saint-Domingueor marabout in some parts of the world, especially Africa -is a shrub or bush of the Mimosaceae family, Saman family, of the Inga. It was introduced in the Lesser Antilles in the 19 th century after having colonised certain islands of the Greater Antilles (Cuba). It is native to southern and tropical Africa.
The fruits of this mimosaceae were introduced from the Greater Antilles when cattle were imported for the production of meat.
Today, the species has a large geographical distribution worldwide. Large populations of the species are found in the tropics. It is an aggressive invasive plant with the ability to grow in sunny open areas as well as in severely degraded areas, especially in abandoned agricultural plots, fallow land in dry bioclimate. It is found mostly in poor soils which have been degraded by agriculture along the coast of Martinique. In Martinique, it is found in low-lying regions (0 -200 m) although it is also found in tropical forests at up to 2000 m altitude. It is spread mainly by seeds which can be easily dispersed by animals, although it is propagated by suckers (GISD) and by barochory (large bank of seeds viable in the soil). This species tends to form dense and impenetrable monospecific thickets in degraded xeric formations. The bark of the young twigs is green and quite densely pubescent, while that of adult twigs is brown with many whitish lenticels. Its bipinnate leaves are composed of 5 to 10 pairs of pinnae with leaflets of 10 to 30 pairs of pinnae. The fragrant flowers are characterised by a bicolour inflorescence in racemes: pink or white in the upper part and yellow in the lower part. The upper flowers are sterile with protruding staminodes. The staminodes are pink when they open and then change gradually to white in maturity. The lower flowers are hermaphroditic with a pistil and 10 yellow stamens. The fruits are dark brown pods and twisted in the form of clusters which contain pale brown biconvex seeds. The pods give off a strong aroma which attracts animals and this facilitates the dispersal of the seeds.
This species has many uses. In agroforestry, it is used to improve and rehabilitate soils, to stabilise sand dunes and to fight against erosion. It also has medicinal uses; indeed the bark, roots and leaves are used to treat headaches, toothaches and stings and also to treat snake bites, syphilis and gonorrhoea.

Method
The method proposed by Professor Philippe Joseph is based on floristic inventories. During these inventories, we demarcate a study area called a transect which is divided into quadrats. The surface area of the transect varies between 500 and 1000 m² depending on the plant formations. We look at the descriptors: scientific name, total height, first branch height, trunk diameter (measured at 1.33 m from the ground according to international standards: diametric class). The ecological and structural parameters taken into account in this study are the following: -The absolute frequency f a , that is to say the number of times a given species was observed in the quadrats or in the stations. It characterises the distribution of the species within each quadrat; -The relative frequency f r , ratio of the absolute frequency to the total number of quadrats in a transect or of stations; -The density, corresponding to the number of individuals in the survey area; -The index of distribution I d = f r * d (density, defined as the ratio between the number of individuals of the species considered and the basal area S t ). It allows us to ascertain the modalities of occupation of the space by the species population in the station; -The index of dominance (ID) enables us to determine species dominance with regards to each other, and is obtained by the following relation: ID = Id * S t (basal area); -The CFA: the comparison between the stations was done by means of a confirmatory factor analysis (CFA) using the software XLSTAT; -The AHC: the comparison of the hierarchical links between individuals and groups of individuals was carried out using ascending hierarchical classification.

Results
The results are presented in the form of tables which show the main ecological and structural parameters used to characterise each of the stations with regards to each other.   Given the number of individuals and their distribution within the station, Pisonia fragrans is therefore the most competitive species in this station. Station C3

1.4
Average dominance Station V1 Dichrostachys cinerea is slightly more distributed in the station (eleven individuals surveyed) ( Table 4.3). Its population is marked by individuals of larger sections up to 10 centimetres (Figure 4.5).    It is a seasonal tropical evergreen formation of lower horizon and xeric facies at pre-forest to bush stage. The predominant species are -in order of ecological significance -Croton bixoides, Pisonia fragrans and Erythroxylum havanense. In this station, the floristic composition is characterised for the most part by individuals with very small measurements for sections (2.5 cm on average) and height, ranging between 1 and 8 metres.
Dichrostachys cinerea is more predominant (71%) in this station (Table 4.5). Its individuals present relatively large sections measuring 2.5 or even 15 centimetres.

Absolute frequency (fa) = presence of the species in the different quadrats; Relative frequency (fr) = Absolute frequency / by the number of quadrats; Density = number of individuals / survey area; Index of distribution (Id) = Relative frequency * Density; Index of dominance (ID) = Index of distribution * basal area
This seasonal tropical evergreen floristic unit of lower horizon and xeric facies at the pre-forest to bush stage is dominated by a large population of Croton bixoides (120 individuals per species excluding regenerations and dead trees), most of whose individuals have sections measuring 2.5 centimetres (i.e. 64.7% and 35.3% have diameters of 5 cm) (

Absolute frequency (fa) = presence of the species in the different quadrats; Relative frequency (fr) = Absolute frequency / by the number of quadrats; Density = number of individuals / survey area; Index of distribution (Id) = Relative frequency * Density; Index of dominance (ID) = Index of distribution * basal area
The plant stand of this station is composed of individuals with very low height (less than 8 m) and section (2.5 or even 5 cm) measurements, with the exception of a few units (3) of the dominant species of the station, Acacia sp, which present a large basal area measurement (Table 4.

9).
Dichrostachys cinerea is widely distributed in this station (80%) but presents young individuals which are small in size (Figures 4.15 and 4.16).   (Figure 4.5). In addition to these individuals, Acacia sp also presents high diametric distributions (up to 20 cm), but its small population means it is not part of the species of the dominant cortege. The plant stand of the station is marked by individuals with low measurements for diameter (between 2.5 and 5 cm) and height, not exceeding 8 metres (Figure 4.6).
Dichrostachys cinerea is distributed to a much greater extent in this station (80%) and presents a larger population with small diameters and heights (Figures 4.5 and 4.6).          Analysing our data, we can see that Dichrostachys cinerea presents a number of individuals (excluding dead trees and regeneration) and the highest indices of distribution. Dichrostachys cinerea is therefore the most competitive species of these two stations. Station M3 having reached their level of optimal morphogenetic development.
In these stations, the opening of the canopy has enabled the installation and development of the most competitive species, Dichrostachys cinerea. In these plant communities, the species has managed to form monospecific thickets.

1.6
Dominance ratio for all stations Dichrostachys cinerea differs from the other species in its high number of individuals and high level of density. For all the stations, it is the most competitive species with regard to the environmental factors.

Figure 0.29 : Abundance of Dichrostachys cinerea in all stations
To complete our observation, we plotted the number of individuals excluding regenerations and dead trees in the primary axis and the index of distribution in the secondary axis according to the stations we inventoried. The data indicate variations in the abundance of Dichrostachys cinerea depending on the different stations. Dichrostachys cinerea has great capacity to install in several biocenotic conditions. It is a plastic species.

Discussion
The study areas comprise 114 plant species divided into 49 families and 97 genera. Dichrostachys cinerea was found in all stations we inventoried. We identified 915 mature individuals (excluding seedlings and dead trees) ( Table 4.16). The individuals we counted have diameters measuring between 2.5 and15 cm. Depending on the dynamic development stage of the stations (sylvatic secondary, presylvatic or bush), we observed variations in the dominance of the species. In some stations, the species forms monospecific stands.

Secondary sylvatic formation
This CFA (Confirmatory Factor Analysis) allowed us to identify the essential relationships existing between species and stations. The inertia generated by the first axis is 26.95% and that of the second axis is 16.67%. These two axes express a maximum of inertia of 43.62% ( Figure 5.1). The analysis was conducted in the stations with the same mesological conditions. Axis 1 distinguishes changes in the floristic composition between the different stations. Axis 2 highlights a dynamic and ecological gradient of the plant formations of each of the stations.
This CFA disclosed plant formations in three dynamic stages. Stations V1, V2, V3, V4, V5, V6 and V9 have a similar floristic composition because they present values which are highly homogeneous.
The development of woody heliophilous species (Pisonia fragrans, Zanthoxylum monophyllum) leads to a gradual closure of the stratum. They correspond to plant communities located between pre-forest to bush stages. Stations M1, M2, M3, V4, V8 and V7 represent the most degraded stations. They relate to bush strata marked mainly by individuals of low biomasses punctuated by some mature trees of significant sections. They belong to bush formations. Stations C1 and C3 have a similar floristic composition: they are made up of old pioneer ecounits (Citharexylum spinosum and Haematoxylon campechianum) and give way to post-pioneer ecounits (Bursera simaruba and Lonchocarpus punctatus). They belong to secondary sylvatic formations.

Figure 0.2 : Hierarchy of partitions obtained by AHC
The results of the AHC (Ascending Hierarchical Classification) confirm those obtained by projection of the individuals in factorial designs based on the CFA. The AHC shows a more crude analysis than with the CFA. However, this method (the AHC) shows that stations V7 and V8 have a very similar floristic composition. Indeed, in these two stations, Dichrostachys cinerea is the dominant species and the other species are very marginal. Stations V9, V4, V3 and V5 are fairly homogeneous. In a more refined/affine manner, stations V3 and V5 are the most similar.

Conclusion
Invasive plants are the direct cause of the regression, instability, or disappearance of autochthonous species or even species communities. In addition, they lead to the homogenisation of fauna and ecosystems and are also the source of emerging diseases. Martinique is a small island system characterised by highly anthropogenic vegetation.
The communes studied (Marin and Vauclin) are both areas with very high and medium ecosystem vulnerabilities. Dichrostachys cinerea is a threat to the development of autochthonous and/or indigenous species.
In addition to the fact that it produces its own regenerations, it continues to expand progressively in the bush, presylvatic and secondary sylvatic stages.
In conclusion, Dichrostachys cinerea possesses the profile and ecological traits of a naturalised plant in the study area as it has invaded all areas unoccupied by autochthonous species. Moreover, its manner of inserting itself in different biocenotic conditions makes it possible to describe the plasticity of the species with regard to the environments.