Floral diversity of the littoral vegetation of Southeastern Nigeria

An investigation of the floral diversity of the littoral vegetation of Akwa-Ibom State coastline of South eastern Nigeria was carried out using quadrat and transects techniques. The results revealed three (3) vegetation types viz: coastal/shoreline, mangrove and secondary forest vegetation. Although, the most encountered ecosystem was the mangrove yet, coastal vegetation had the highest flora diversity of 100 (47%) as compared to mangrove and secondary forest with 87 (41%) and 27 (12%) species, respectively. A total of 147 taxa belonging to 134 genera and 58 families were recorded from the study area. The dominant family was Fabaceae with 17 species (29.3%), while the dominant genus was Ipomoea with 5 species (3.73%). Of these taxa, 59 occurred in more than one vegetation type while 11 were present in all the three (3) vegetation types. Phanerophytes are the dominant life form covering 36% (53 species) of the encountered species. Similarly, diversity index revealed the highest diversity of species in the coastal ecosystem with Shannon-wiener’s, Simpson’s and Margalef values of 0.99, 4.61 and 21.67, respectively. Correlation matrix and Jaccard similarity coefficient value (0.35) was high between the coastal and mangrove pairs; however, it depicts a minimal significant difference in the species composition within the three ecosystems at 0.05% probability level. Also, PCA scatter plot established less variation between the coastal and mangrove vegetation. Also, 16 economic plants were encountered during the study with uses ranging from edible fruits, spices, vegetables, root crop, and medicine, to palm wine. Factors affecting species distribution a r e introduction of alien species (Nypa fruticans), canalization, dredging, over exploitation of mangrove and pollution. Conservation status of these taxa according to IUCN 2014-2 showed that no endangered species was encountered during the study. However, conservation of this littoral vegetation is highly imperative in other to stabilize the ecosystem dynamics, protect the biodiversity, prevent or reduce potential effects of ecological disasters such as flooding and erosion.


INTRODUCTION
Nigeria's vegetation belts reflect a very close link between vegetation and climate.Vegetation belts are *Corresponding author.E-mail: jolowokudejo@unilag.edu.ng.
Author(s) agree that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License d emarcated on west-to-east zonation pattern characterized by transitional zones from one belt to another hence, resulting into different types o f vegetation including the littoral vegetation that occurs along the shorelines of water bodies.The study area is part of the Niger Delta Region (NDR) which is characterized by rich biodiversity, highly diverse and productive ecosystems, good agricultural land and excellent fisheries.The ecological significance of the region is underlined by some important characteristics (NDES, 1997).The NDR is home to the last remaining intact vestiges of lowland rainforest area in Nigeria which have now been placed under protection as Cross River (Oban and Okwangwo Divisions) and Okomu National Parks.Mangrove ecozone of the region is the most important mangrove ecosystem in Africa and the third largest in the world.
For rural communities that make up much of the NDR, the dependence on biodiversity constitutes the sole means of livelihood.The constant and growing threat to which biodiversity is exposed in the region as a direct consequence of various anthropogenic activities, including oil exploitation, necessitates that constant evaluation of bio-resources status of the region should be undertaken as a tool for constant updating and improvement in conservation strategies.The littoral vegetation is the portion of the aquatic shore that is inhabited by diverse autotrophic plants, that is, areas with water that is shallow enough to admit sufficient light to support rooted vegetation.The littoral zone comprises the zone of fluctuating water levels (that is, the intertidal zone, which is between high/low water marks), wave action at the actual land/water interface and also the deeper inshore areas that support aquatic vegetation.The boundary between vegetation types is sometimes attributable to sharp changes of topography (Kikkawa et al., 1981;Webb and Tracey, 1981;Bowman and Wightman, 1985).The littoral zone may form a narrow or broad fringing wetland, with extensive areas of aquatic plants sorted by their tolerance to different water depths and adaptability (Keddy, 2010).The ecosystem varies from littoral, brackish, freshwater to lowland rainforest ecosystems.Although, natural climate variation and non-climatic factors such as land transformation may also be responsible for some of these trends, human-induced climate and atmospheric changes are the most identified reasons (Hughes, 2000;Parmesan, 1996).In coastal environments, the littoral zone extends from the high water mark, which is rarely inundated, to shoreline areas that are permanently submerged.The plant species of the open coast are referred to as halophytes because they receive large amounts of salt spray (Calder and Taylor, 1968).However, the number of species found in this habitat largely depends on the level of salinity, ability to withstand excessive wave action, fluctuation in water level and tolerance of the species.Depletion of natural resources and the attendant insecurity of rural livelihoods contribute to the social conflicts which plague the Niger Delta Region.
The m a i n objective of this study was to provide a checklist a n d d i v e r s i t y of accurately identified p l a n t species of the south-eastern zone including the creeks, estuaries and shoreline before their habitats are obliterated by raging waves, aggressive incursion of Nypa fruticans and human activities which will assist in future conservation programmes.

Study area
The study was carried out along the coastal zone of Akwa-Ibom state on Latitudes 4° 39'N and Longitudes 7° 56'E between the shoreline and the estuaries of the Cross river to Imo river and banks of Qua-Iboe, Imo, Oron and Mbo rivers in Ibeno, Local government area, Eket, Akwa Ibom State, Nigeria (Figure 1), while the details of each points are shown in Table 1.The three (3) ethnic groups in the study area are Ibibio, Annang and Oron.Of these three, the Ibibio remain the majority.The main occupations of the inhabitants of the coastal vegetation include fishing, subsistence farming, water transportation business, trading, and a few are government workers.The Nigerian coastline covers a total length of approximately 859 km and it is divided into four geomorphic plain zones (from east to west) namely the strand coast/estuary, Niger-Delta, Transgressive mahin mud coast and the Barrier beach on the lagoon coast complex (Udo, 1970).It stretches Inland for a distance of about 15 km in Lagos to about 100 km in the Niger Delta and about 25 km east of the Niger Delta.The tidal range increases progressively eastwards to about 3 km at the Calabar estuary.The area is contingent on the movement of the Inter Tropical Discontinuity (ITD) and characterized by very high rainfall (annual total > 4,000mm), temperature values of about 27°C, and relative humidity with mean value of 80.3%.The Niger Delta is one of the major features of the West African coastline which projects into the Gulf of Guinea and is characterized by vast sedimentary basin with a complex river network, and a fragile ecology in which fresh and saline water ecosystems maintain a dynamic equilibrium.The sandy shoreline is backed by extensive mangrove and fresh water swamps.However, mangrove dominates the coast while freshwater swamp forests are dominated with palms and trees (Flyod, 1969); moreso, the Delta contains large natural gas and oil deposits.
The coastline is sandy, heavily incised by numerous creeks, shallow streams and rivers, and drained by a number of rivers including the Cross River, and the Qua Iboe River.Apart from the shoreline and tidal mudflats which are in most areas covered by the invasion of Nypa friutcans, all other areas depict highly disturbed vegetation following persistent and increasing anthropogenic pressure.

Line transects
Twenty line transects were cut across the vegetation formations at two-kilometer-intervals along the coastline, starting from the Cross river estuary and terminating at Imo river estuary for rapid and extensive assessment of ecosystem types.

Quadrats
Twenty quadrats with 5 and 10 m 2 in dimension were established within the ecosystems for intensive and detailed evaluation of various plant species occurrences.

Floristic composition and conservation status
All plant specimens encountered along the transects and within the quadrats were identified to species level either in the field or herbarium, using appropriate Floras, Manuals and Monographs Life form classification was carried out according to Kershaw (1973) and Raunkiaer (1934) and the conservation status of each species was evaluated using the IUCN Redlist v.2014-2.
Life form classification according to Kershaw (1973)

Data Analyses
The frequency and habitat richness were calculated while species indices were carried out with PAST version 2.3 with probability set at the 0.05% level of significance.

Herbarium Specimens
Representative samples of the species were collected, pressed, dried and prepared as herbarium specimens using standard techniques (Radford et al., 1974).The preserved specimens would serve as permanent records of the vegetation and floristic composition of this region.Each specimen would always be available for study or quick reference in connection with future operations in the study area and for future conservation programmes.All representative specimens are deposited in the Herbarium of the University of Lagos, Akoka, Yaba, Lagos, Nigeria (LUH).

RESULTS
The survey revealed that coastal/littoral/shoreline, mangrove and degraded secondary forest are the dominant vegetation types within the studied area (Table 1) and the highest species diversity was observed along the shoreline vegetation where it covers 47% followed by mangrove that covers 41%, and secondary forest vegetation 31% of the species, respectively.Notably, eleven (11) species were present in all the habitat types (Table 3).Similarly, the diversity index revealed that the coastal ecosystem had the highest Shannon-wiener, Simpson and Margalef values of 0.99, 4.61 and 21.67, respectively as shown in Figure 2 while the highest correlation value was recorded between the coastal and mangrove vegetation (0.35), followed by the value 0.16 for the mangrove and secondary forest vegetation while coastal and secondary forest had the least correlation value of 0.14 at p=0.05% as presented in Table 2. Furthermore, Jaccard similarity index represented with a dendrogram (Figure 4) also confirmed the high correlation between the coastal and mangrove vegetation because they are sisters joining together at the same point on similarity index scale (0.35). Figure 3 presented the principal component analysis (PCA) scatter plot of minimal spanning tree showing less variation between the coastal and mangrove ecosystems.

Data analyses
The frequency and habitat richness were calculated while species indices were carried out with PAST version 2.3 with probability set at the 0.05% level of significance.

Herbarium specimens
Representative samples of the species were collected, pressed, dried and prepared as herbarium specimens using standard techniques (Radford et al., 1974).The preserved specimens would serve as permanent records of the vegetation and floristic composition of this region.Each specimen would always be available for study or quick reference in connection with future operations in the study area and for future conservation programmes.All representative specimens are deposited in the Herbarium of the University of Lagos, Akoka, Yaba, Lagos, Nigeria (LUH).Summarily, a total of 147 taxa belonging to 134 genera and 58 families were encountered and identified during the study (Table 3).The dominant families encountered include Fabaceae and Poaceae with 17 and 16 species, respectively (Figure 5 ) while genus Ipomoea had the highest diversity of five (5) species as shown in Table 3.The dominant life form was Phanerophytes as it covers 36% followed by Chamaephytes (31%), while Geophytes and Cryptophytes cover 17 and 16%, respectively (Figure 6).Moreover, 16 economic species were r e c o r d e d w i t h t h e study area as presented in Table 4.In accordance with the International Union for Conservation of Nature (IUCN) Redlist of threatened species, 15 species fall within the category of least concern (LC), 2 are in Data Deficient (DD) while others are yet to be assessed (NA).

DISCUSSION
The vegetation composition varies because of the transitions from one ecosystem to the other as a result of the abrupt changes in the physical conditions, notably the soil.The similar ecosystems had similar species composition as observed in Imo River, Itak Abasi creek, Iwuo Okpom Opolom creek, Douglas creek, Qua Iboe creek and river sampling points which are of similar mangrove ecosystem dominated by Mangrove species such as Rhizophora racemosa, Avicennia germinans, Nypa fruticans and Dalbergia ecastaphyllum as compared to the shoreline vegetation.However, the three (3) vegetation types encountered during the study are the coastal, mangrove and secondary forest vegetation.

Coastal/shoreline vegetation
This vegetation is the nearest to the Atlantic Ocean and is usually inundated at high tide.It is the most extensive plant formation in the study area extending from the western banks of the Cross River to the shores of Imo River (Plate 1).From the study,100species were identified and the most prominent of these species     in sediment character and nutrients deposition, salinity level which is consequent of the topography of the islands.The coastline and sandy beaches constitute extreme environment for plant life because of the combined effect of factors such as wind, wave, saline water, loose substratum with little water retention capacity which renders the soil very harsh to plant However, some species have to migrate to another ecosystem because they require the terrain that can enhance their growth in order to promote development.Notably, as the distance from the shore increases, the distinct adaptations of plants to tolerate coastal   conditions decrease; although, many plants have adapted and flourished in this harsh coastal environment because of the: 1. Increased thickness of their leaves to protect the plant from dehydration, exposure to sun and salt spray e.g.R. racemosa, Calophyllum inophyllum, etc. 2. Ability to delay germination in response to excessive salt spray, dehydration or other environmentally harsh conditions e.g.Nypa fruticans, Phoenix reclinata 3. Ability to produce large seeds to increase the viability and vigour of seedlings e.g.P. reclinata 4. Dependence on the sea for the dispersal of their seeds e.g Nypa fruticans, Pterocarpus santalinoides 5. Ability to roll the leaves, in response to heat, salt and lack of water e.g.Rhizophora spp., Sphagneticola trilobata 4. Occurrence of hairs on leaves, which helps to avoid heat stress, common in plants found close to the shore as in Diodia rubricosa.5. Wiry stiff leaves and stems which enable the plants to tolerate abrasion by salt-laden winds and sands e.g.Dissotis rotundifolia.

Mangrove vegetation
Mangrove swamp forests are fully developed in the estuaries of the Cross River, Qua Iboe River, Imo River, Douglas, Itak Abasi and Iwuo-Okpom Opolom Creeks and on the banksof Oron and Mbo Rivers (Plate 2).Mangroves also occur either as large stretches or pockets of vegetation throughout the study area.Mangroves are the characteristic littoral plant formations of sheltered tropical and subtropical coastlines.Saenger et al.(1983) have divided the mangroves into two broad groups;exclusive species(only found in mangrove) and non-exclusive species (commonly associated with mangrove).Most of the dominant and important species are Rhizophora spp.Conocarpus erectus,Nypa fruticans,A.germinans.Mangrove vegetation is not generally rich in plant species because of the complexities and rigours of the habitat.These plants are generally referred to as halophytes, adapted to anaerobic conditions of both salt and fresh water in muddy and dark coloured soil with high organic content through the development of stilt roots and pneumatophores which project above the mud and water in order to absorb oxygen.Moreso, mangroves have the ability to grow where no other tree can, thereby making significant contributions that benefit the environment.They protect the coast from erosion, buffer adjacent marine ecosystems (often coral reefs) from terrestrial inputs, and form the primary habitat for a high diversity of fish and invertebrates, some of which are commercially important (Rützler and Feller 1996;Mumby et al., 2004;Layman et al., 2004).Their coverage of coastal shorelines and wetlands provides many diverse species of birds, mammals, crustacean, and fish a unique, irreplaceable habitat.Mangroves preserve water quality and reduce pollution by filtering suspended material and assimilating dissolved nutrients.The presence of some non-mangrove plant species in this zone shows that the habitat has been altered by human activities.Mangrove is the preferred wood for several purposes including fish drying and the construction of temporary shelters used by fishermen.Human interference which involves cutting down of the mangrove provides the foothold for Nypa invasion which subsequently gains advantage and then displaces the mangrove.Large areas within the study sites, e.g.Ibeno, Ikot Abasi and Imo River estuary, have been decimated by the aggression of this invasive species.Nypa palm has a superficial fibrous root system which lacks the holding ability to maintain coastline integrity.

Degraded secondary forest vegetation
The forest encompasses three vegetation types: the swamp forest, tropical rain forest and secondary regrowth.swamp forest is found along the coastal and deltaic regions of Nigeria as observed within the study area.It harbors a w i d e variety of tree species such as R. racemosa, A. germinans, Sterculia tragacantha and Laguncularia racemosa.Structurally, secondary forest can be grouped into three: tree layers, shrub layer and the undergrowth that are of great economic values.The forests are supposed to be dominated by trees, shrubs, woody climbers (lianas) and abundance of epiphytes.The rate of forest degradation is on the high side because of intense anthropogenic factors such as indiscriminate bush burning, conversion of forest land for other uses e.g.Oil palms plantation, introduction of alien species e.g.Nypa fruticans, farming and replacement of forest species with fruits trees and arable crops and pollution, etc.All these determine the vegetation structure/floristic composition of this vegetation.
Generally, species distribution takes the form of a mosaic of local populations of greater or less degree of isolation, and therefore it is to be expected that most will show a degree of micro-geographical variation, with correlatable differences.
Avicennia and Rhizophora are som e of the dominant plants s pec ies of mangrove communities in the Niger Delta genera with several closely related species, this is similar to the report of Ellison (1991) in the study of pacific paleogeography of Rhizophora mangle L. Moreso, plants develop adaptive features to survive the ecosystem conditions.Nypa fruticans is an invasive species (Plate 3) that has posed significant threat to the mangrove ecosystem.Invasion of N. fruticans in the mangrove swamps was observed to be consequent on human activities; N. fruticans i s a s t r o n g c o m p e t i t o r and lack utilization by the local people.The impacts include the reduction in primary and secondary productivity, disruption of food chain and erosion ofriverbanks.The biological diversity in the mangrove can be enhanced if Nypa palm is replaced with red and white mangroves in the mangrove ecosystem.Fruits and seeds of some non-coastal species are dispersed by water and were deposited along the shoreline as a result of tidal movement.These species include:Pentaclethra macrophylla,Dioclea reflexa and Pterocarpus santalinoides.The coastal vegetation, which is the most extensive and the nearest to the Atlantic Ocean had the highest floral diversity than the other vegetation types/ecosystems.The links between forest structure and environmental conditions have been studied by McKee ( 1993) and Fromard et al. (1998) who suggested that the wide variation in tree heig ht and Plate 3. Young Acrostichum aureum and dense thickets of Nypa fruticans which pose serious threat to the health of the mangrove ecosystem.
productivity are in response to temporal and spatial variation of environmental factors across the intertidal zone, such as soil salinity, soil waterlogging and nutrient availability (Koch, 1997;Feller et al., 2002Feller et al., , 2003)).There is a significant difference in the species composition of three ecosystems at probability level of 0.05 even though they are positively correlated.This is as a result of some species adapting to the changing environment e.g. A.
cordifolia, A. germinans, Chromolaena odorata, Emilia coccinea, Dalbergia ecastaphyllum, N. fruticans, Pennisetum sp., Ipomoea pes-caprae and Ipomoea cairica were recorded across the three dominant ecosystems within the study area while others are found in one or two of the vegetation types.The coastal and mangrove vegetation are similar because there are no clear margins between the habitats, they are both halophytic, mangrove protects the coastal area and are usually parallel to the coastline hence leading to occurrence of same species as compared to the secondary forest which are majorly fallow and farm land.The economic species were mostly found around the communities in some parts of the study area, w h e r e t h e y a r e u s e d a s food and medicine.Jessica and Dianna (2004)

Conclusion
The study has provided the checklist of the floral diversity and the various vegetation types across the littoral zones of eastern part of Nigeria.The c h a n g e i n vegetation structure was attributed to the fierce action of strong waves and active coastal erosion which are quite evident in several locations; fishing activities, overexploitation of plant resources, invasion of alien species e.g.N. fruticans, and replacement of original plant communities by tree crops, such as coconut, oil palms, mango, plantain and vegetables.Dredging, pollution and canalization remain constant industrial activities which promote mangrove clearing constitute a threat to the well-being of the ecosystems.Species conservation in these ecosystems is highly imperative in order to maintain the ecosystem dynamics, and the benefits that can be derived from these natural resources in perpetuity.Conservation efforts are often hampered by dearth of basic botanical information such as taxonomic identification and nomenclature of plant taxa.Therefore, information provided in this study is expected to fill such gaps.

Figure 1 .
Figure 1.Map of Akwa Ibom State Coastal region showing the study area.

Figure 2 .
Figure 2. Species indices across the studied area.

Figure 5 .
Figure 5. Family distribution across the studied area.

Figure 6 .
Figure 6.Life form distribution across the study area.
stated that the introduction and spread of non-native species has become a global ecological and conservation crisis as invasive organisms are increasingly altering terrestrial and aquatic communities worldwide.The loss of biodiversity and species extinction are, likewise, major ongoing crises.Native species decline often occur simultaneously and in the same place as invasion by non-native species, leading many conservationists and researchers to believe that invasions and extinctions are closely linked.The identified anthropogenic factors are farming, dredging, drilling and laying of oil pipes, improper waste disposal, introduction of alien species and over-exploitation of the resources e.g.R. racemosa are used as fuelwood.As the world's biodiversity faces incessant threats f r o m environmental and human induced factors, it is imperative to know the conservation status of the flora and fauna in any ecosystem.In accordance with the International Union for Conservation of Nature (IUCN) Red list of threatened species, 15 species fall within the category of Least Concern (LC), 2 are in Data Deficient (DD) while others are yet to be assessed (NA).

Table 1 .
Details of the sampling points within the studied area. S

Table 2 .
Cross correlation tabulation for the encountered ecosystem.

Table 3 .
Checklist of the flora diversity encountered across the studied area.

Table 4 .
Checklist of the encountered economic plants.