Ecological
studies on Aerangis biloba(Lindl.) Schltr. (Orchidaceae) in NACGRAB field gene bank, southwestern
Nigeria
Temitope Israel Borokini1 & Adedotun Onoyinka Afolayan 2
1,2 National Centre for Genetic Resources and Biotechnology
(NACGRAB), P.M.B 5382, Moor Plantation, Ibadan, Oyo State 23402, Nigeria
Email: 1 tbisrael@gmail.com (corresponding
author), 2 onoyinka@yahoo.com
Date of publication (online): 26 September 2012
Date of
publication (print): 26 September 2012
ISSN
0974-7907 (online) | 0974-7893 (print)
Editor:Pankaj Kumar
Manuscript
details:
Ms # o2728
Received
15 March 2011
Final
received 27 February 2012
Finally
accepted 05 September 2012
Citation: Borokini, T.I. & A.O. Afolayan(2012). Ecological studies on Aerangis biloba (Lindl.) Schltr. (Orchidaceae) in NACGRAB field gene bank, southwestern
Nigeria. Journal of Threatened Taxa 4(11): 3075–3081.
Copyright:© Temitope Israel Borokini& Adedotun Onoyinka Afolayan 2012. Creative Commons
Attribution 3.0 Unported License. JoTT allows unrestricted use of this article in any medium
for non-profit purposes, reproduction and distribution by providing adequate
credit to the authors and the source of publication.
Abstract: Aerangis biloba (Lindl.) Schltr.
belongs to the family Orchidaceae, which is listed as
threatened under the Appendix II of the CITES. This study was
conducted to estimate the population and distribution of Aerangis biloba in the field by National Centrefor Genetic Resources and Biotechnology (NACGRAB), Ibadan, southwestern
Nigeria. Twenty
five sample plots of 50m by 50m each along line transects were randomly
chosen and studies were carried out on them. The results were correlated with the
factors that affect the distribution and survival of A. biloba. A total of 129
specimens of A.biloba were observed within the study
area, giving a population density of 5.16 orchids per m2. This indicates that the species is
abundant in the sampled area, as a result of good population of the selected phorophytes. The analysis of the data indicated a high
correlation between and among the plots (P=0.0003). A. biloba was observed only on nine trees among over 160 trees present in
the sampling area, suggesting that these nine trees could act as phorophyte for A. biloba of which, Irvingia gabonensis and Irvingia wombulu harbored the highest number of orchids. The paper gives a synopsis of the
economic and ecological significance of orchids. It also recommends controlled
harvesting, reduced deforestation and establishment of orchid gardens as some
of the effective ways of enhancing orchid conservation in Nigeria.
Keywords: Aerangis biloba, epiphytes, field gene bank, NACGRAB, phorophytes.
For
figures, images, tables -- click here
The family Orchidaceae comprises
over 850 genera and an estimated 25,000 species representing about 10% of the
world’s flowering plants and the largest family in species number (Roberts
& Dixon 2008). The African
continent harbours around 2,400 orchid species
(Madison 1977). Of the 25,000 known
orchid species, more than 70% are thought to live as epiphytes in tree canopies
(Gravendeel et al. 2004). An epiphyte is considered to be a plant
living on another plant or sometimes on other objects above ground surface and
growing either partly or entirely in the air, into suspended soils, or in woody
debris (Richards 1996; Moffett 2000). Barkman (1958) coined the term ‘phorophyte’ for the plants that support epiphytes. Orchidaceaecontains 60% of all epiphytic species and 10 times as many epiphyte species as
any other family of vascular plants (Kres 1986).
The genus Aerangis has many
species in Africa and Madagascar with strikingly pretty flowers. Most African epiphytic orchids were
classified in the genus Angraecum in the mid
18th century and Lindley described this species as the ‘two-lobed Angurek’ - Angraecum bilobum (Stewart 1975). The name was chosen to describe the leaf
apex, which is often conspicuously bilobed. Although, Reichenbachhad established the genus Aerangis in 1865, it
was not until 1914 that Schlechter transferred this
species into the genus Aerangis (Stewart
1975).
A study of the notes attached to the herbarium specimens of Aerangis biloba at
Kew revealed that most have been collected in dense forest in the Ivory Coast
and on isolated trees in the savannah areas of northern Nigeria. They are found in thickets, forest edges
and small patches of woodlands as well as high forests. Many have been observed growing among
plantation crops and frequently on village trees. Citrus, mango, orange, palm and coffee
trees have been recorded as actual hosts for this epiphyte (Stewart 1975). Most of the plants collected in flower
have been found in July and August, which is towards the end of the main rainy
season in West Africa. This
indicates that the plants require plenty of water prior to blooming and have a
specific and predictable blooming season that corresponds to seasonal rainfall
and coordination of the life-cycle of its
pollinators. It thrives when
planted in a slatted basket with large pieces of bark (Stewart 1975).
In terms of their ecological significance, orchids are a part
of the autotrophic web of organisms, absorbing energy and converting it to
food, part of which is transferred along a complex ecological food web when living in association
with other living organisms. The
flowers are highly attractive to nectar-feeding insects and birds. Furthermore, epiphytic orchids
contribute to the complexity, structure and function of the canopy, and are
important components in terms of both biomass and species diversity; they
interact with vast numbers of canopy invertebrates, as well as many vertebrate
species (Nadkarni 1984; Ellwood et al. 2002; Wolf
& Flamenco 2003; Gravendeel et al. 2004; Monteiro et al. 2009).
As a result of the threat from over-collecting all orchids
have been placed on Appendix II or higher of the Convention on International
Trade in Endangered Species (CITES). Despite this protection, the ongoing general decrease in the number and
size of orchid populations is mainly due to the loss and degradation of their
habitats (Koopowitz et al. 2003). Because orchids are susceptible to human
activities, they have been proposed as indicators of levels of human
disturbance and overall ecosystem health (Turner et al; 1994; Benzing 1998).
In Nigeria, very little attention has been given to the study
of orchids. Worse still, there are
no accurate surveys on the diversity, population and conservation status of
orchids. It has been reported that
the orchids Habenaria nigerica,
and Diaphananthe dorotheae are exclusive endemics for Nigeria; Segerback(1983) described 104 species of orchids for Nigeria, but Jayeola(1991) encountered over 400 species for the country indicating a richer
diversity of orchid species than was previously thought. Since very few people are involved in
orchid cultivation in Nigeria, most of the orchid flowers harvested for
ornamental purpose comes from the wild. With increasing anthropogenic
disturbances, epiphyte species richness declines (Hietz1999; Kromer & Gradstein 2003) and community
composition changes (Werner et al. 2005). Therefore, this study was conducted to assess the population of Aerangis biloba in
the field gene bank of National Centre for Genetic Resources and Biotechnology
(NACGRAB), Ibadan, southwestern Nigeria with the view of studying the effects
of the ecological factors on the population and distribution of this orchid
within the gene bank. This study is
part of the proposed extensive study on the orchid survey in Nigeria.
Materials
and Methods
Study
Area
The field gene bank of National Centre for Genetic Resources
and Biotechnology (NACGRAB) was established in 1987, as an ex situ collection
site for indigenous recalcitrant plants in Nigeria. It is about 12ha wide, lying on the
latitude 7022’ North of the equator and longitude 3050’
East of the Greenwich Meridian. The
Field Gene bank of NACGRAB contains a large variety of plants with diverse growth
forms. According to Borokini et al. (2010), 361 plants have been identified in
the field gene bank, comprising 160 trees, 119 herbs, 55 shrubs and 27
vines. Some of the plants
established on the field gene bank are in plantations, while others are scattered
within the field. The climate of
the study area is characterized by high temperature and a bimodal rainfall
pattern. The annual total and mean
rainfall was 1,435.8 and 99.87 mm respectively for year 2008. Rainfall for the area spans from
February to December, with peak in September (289.9mm); no rainfall was
recorded for November. The mean annual minimum and maximum temperatures were
24.570C and 32.10C respectively for 2008, while the
relative humidity was lowest at 53% in January and highest at 92% in
August. Agrometeorologicalrecords from 1979 to 2008 show insignificant difference in the data taken for
rainfall, temperature and relative humidity. Image 1 shows the location of the study
area within Nigeria.
Data
collection and analysis
This survey was carried out between 1 and 5 September
2010. It involves an enumeration of
the orchids within 50x50 m plots laid along line transects within the field
gene bank at an interval of 25m. Twenty five sample plots were laid; making a total of
sampled area to be 6.25ha that is 52.08% of the total field gene bank
area. Furthermore, the phorophytes on which the orchids were found were noted, the
population of the phorophytes and the height of the
orchids on the phorophytes was also documented. Photographs of A. bilobaon the phorophytes were taken. The results were subjected to
statistical analysis to determine the population density, analysis of variance
(ANOVA) and test of significance using PAST Statistical software (Hammer et al.
2001). The population density
within each plot was calculated as the number of occurrence divided by the area
of the plot in sq.m.
Results
A total of 129 Aerangis biloba were identified in the 25 plots sampled within
the field gene bank of NACGRAB, Ibadan, Nigeria; giving a population density of
5.16 (Table 1). This result
indicates that the species is abundant in the field gene bank. Table 1 shows the occurrences of the
orchid in each of the sampled plot and the trees identified in each of the
plots. Furthermore, the phorophytes for the epiphytic orchid include Irvingia gabonensis,Citrus spp, Spondias mombin, Irvingia wombulu, Tecoma stans, Azadirachta indica,Psidium guajava,Albizia odoratissimaand Mangifera indica. Table 2 shows the population of each of
the identified phorophytes for this orchid species
within the sampled area of the field gene bank; while
images 2–3 are the photographs that illustrate the orchids growing on thephorophytes within the field genebank. The epiphytes were attached to the phorophytes either on the main trunk or the branches. The orchid population on each of the phorophyte ranges from one to seven per plant. I. gabonensis had up to seven orchids per phorophyte, and many of the I. gabonensisstands have A. biloba growing on them. It was also noted that the height of the
epiphytes on the phorophytes range from 0.7 to 2.9 m.
The statistical analysis showed that the variation
of the occurrence of the orchids within and among the plots are highly
significant (P=0.0003) indicating that the occurrence of the epiphytic orchid
is relatively dependent on the availability and abundance of its specific phorophyte. The
standard deviation was 6.89 (Table 3) and the population density within each
plot ranged from 0 to 0.0336 (Table 1), which further shows the interdependency
of epiphytic orchids on their corresponding phorophytes.
Discussion
Aerangis bloba is
relatively abundant in the field gene bank due to high population of the
preferred phorophytes, which provided medium of
establishment. The pollination of
the flowers by native moths and spread of the seeds by air currents from tree
to tree further promoted the establishment of the orchid in this area. Only nine of the 160 trees in the field
gene bank were identified as phorophytes for A. biloba. This indicates that A. biloba, like
many other epiphytic orchids, are only able to germinate and survive to
reproductive age on certain phorophytes. The occurrence of orchids in a plot
increases with increase in the phorophyte population
within the plot. This indicates
that the occurrence of the orchids is dependent on the phorophytes,
therefore, the presence, population and distribution of the phorophytesdetermines the distribution and the frequency of the orchids within the
plots. Likewise, the survival and
conservation of the orchids is highly dependent on specific tree species, which
act as phorophytes (hosts) for the orchids. This study has, therefore, revealed a
significant number of phorophytes for A. biloba. The
abundance and distribution of these phorophytes will
help in the conservation of this orchid species.
While some previous studies reported
little or no phorophytespecificity among some epiphytes (Johansson 1974; Sanford
1974; Todzia1986; Ackerman et al. 1989; Zimmerman & Olmsted 1992; Ackerman et al. 1996);other authors reported phorophytespecificity among some vascular epiphytes (Went 1940; Frei 1973; ter Steege & Cornelissen 1989; Merwin et
al. 2003). Although
epiphytes are a significant component of overall plant diversity, relatively
little is known about the specificity of the association between epiphytes and
their phorophytes. This association is of particular
interest since epiphytes are confined to a patchy and discontinuous
distribution of suitable substrate. The specificity of the orchid-phorophyterelationship may have direct bearing on the abundance of epiphytic taxa,
particularly as increasing rates of anthropogenic-related habitat disturbance
exacerbate the patchiness of suitable substrate (Trapnell& Hamrick 2006). Therefore,
there is the need to study further the ecological associations between orchids
and phorophytes and study the selective nature of
epiphytic orchids for phorophytes, with the aim of
assessing their conservation status.
Orchids are dependent on their host trees for survival, and
hence, deforestation could likely increase the chances of orchids being
threatened in their habitats. Riofrio et al. (2007) noted that deforestation practices
pose a major threat for the survival of orchids, as they are greatly dependent
on the environmental conditions of the forests that sustain them and the host
trees on which they grow. In the
light of this, Riofrio et al. (2007) advocated for
studies on the patterns of spatial distribution and colonization of secondary
succession forests regenerated after deforestation. It is therefore, recommended that
deforestation should be discouraged, as they do not only cause a lot of
environmental damage to the natural ecosystem, but also increase the risk of
extinction of orchids that depend on them. It is also interesting to note that as some orchids are considered as
threatened, some new species are being discovered such as Vanda longitepala, which was discovered in northern Burma
(Roberts et al. 2008). However,
such new discoveries would not be possible if the habitats (the forests) have
been degraded through deforestation and land use changes.
As all orchids are considered threatened and extinction of
orchids reported in some countries; it should be noted that extinction of
orchids have extensive detrimental effects on ecosystem health. The field gene bank has a uniform climatic
condition, elevation and altitude most suitable for the colonization of Aerangis biloba. It is essential that continuous
management and conservation of the phorophytes in the
field gene bank should be undertaken to ensure the survival and sustenance of
the epiphytic orchids and their complex ecological relationship with other
biotic components of the community. Especially since the selective nature of A. bilobafor a specific type of phorophyte might be the major
factor responsible for their distribution within the gene bank.
From literature studies, it could be noted that
over-collection of wild orchids for sale and deforestation are the greatest
risk factors for the survival of wild orchid populations (Koopowitzet al. 2003). Therefore,
cultivation of orchids should be encouraged, in order to reduce the pressure on
the wild collections and at the same time, be a source of income generation and
self-employment for interested people, especially the youth, since there is a
ready market for orchids and the capital involved is relatively small.
It is also strongly recommended that research attention be
given to Nigerian orchids, to conduct a comprehensive survey on their natural
distribution, population, the phorophytes of the
epiphytic orchids, samples taken and cultures created of their mycorrhizae symbionts and their
conservation status.
Conclusion
and Recommendation
Aerangis biloba, like many other
orchids, is an important component of a complex ecosystem, and members of its
populations as producers of nectar, are fundamental to some complex food
webs. This study has revealed that
the abundance and conservation of epiphytic orchids, such as A. biloba is dependent on the abundance and population of
the phorophytes and the natural presence of the mycorrhizae associated with the phorophytes. Therefore, felling of the phorophytes for this orchid will reduce the population of
the orchid and threaten its survival.
Research attention needs to be focused on the study of many
of the other highly diverse Nigerian orchid flora for their phorophytespecificity, effects of altitudinal gradients, mycorrhizalassociations and habitat preferences, among other environmental factors, in
order to preserve, understand and enjoy these unique Nigerian orchid species.
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