A remarkable range disjunction recorded in Metarungia pubinervia ( Acanthaceae )

Abstract

Summary The first occurrence of the genus Metarungia Baden (Acanthaceae) in west Africa is recorded with the discovery of an isolated population of the widespread eastern African taxon M. pubinervia (T. Anderson) Baden in eastern Nigeria. The conservation status of this species is discussed.

Full text

A remarkable range disjunction recorded in Metarungia pubinervia
(Acanthaceae)
Iain Darbyshire
1
, Kaj Vollesen
1
& Hazel M. Chapman
2
Summary. The first occurrence of the genus Metarungia Baden (Acanthaceae) in west Africa is recorded with the
discovery of an isolated population of the widespread eastern African taxon M. pubinervia (T. Anderson) Baden in
eastern Nigeria. The conservation status of this species is discussed.
Key Words. Acanthaceae, Afromontane, Gashaka Gumti, Metarungia pubinervia, Nigeria.
Whilst conducting ecological survey work within the
Gashaka Gumti National Park and surrounding high-
lands of Saurdauna Province, Taraba State, Nigeria in
November 2002, one of the authors (H.M.C.) collect-
ed a spectacular flowering Acanthaceous shrub from
the remote Leinde Fadale Forest at c. 1600 m altitude.
The specimen (H. M. Chapman 24) was subsequently
brought to Kew for identification and was found to be
referable to the genus Metarungia Baden, previously
known only from three species in eastern and
southern Africa (Baden 1981, under the illegitimate
name Macrorungia C. B. Clarke). In view of the large
disjunction in distribution, it was assumed that the
Nigerian material would prove to represent a new
species, although it was noted that the flowering
material closely resembled the widespread east African
taxon M. pubinervia (T. Anderson) Baden, and it
readily keyed to that species in the revision of the
genus (Baden ibid.). Further collections, including
fruiting material, were requested and a second site
visit was made in December 2003, where further
flowering and mature fruiting specimens were collect-
ed (H. M. Chapman 561). More detailed comparison
of this material with M. pubinervia found the Nigerian
gatherings to be inseparable from the east African
material, with all measurements falling well within the
range recorded for M. pubinervia by Baden (ibid.).
Metarungia pubinervia was previously recorded over a
broad distribution in eastern and southeastern Africa,
from montane southern Sudan and southwestern
Ethiopia, through the east African mountain chains
and south to southern Zimbabwe and Mozambique
where it was also recorded from major river catch-
ments. Its westernmost extent was along the mountains
bordering the Albertine Rift in eastern Congo-Kinshasa
and Burundi (see Baden 1981, Fig. 1). The Nigerian
population therefore represents a western range ex-
pansion of approximately 1200 km.
On the western fringes of its east African range,
Metarungia pubinervia is often associated with mid-
altitude (1000 –1200 m alt.) forest typical of the
Congo Basin, with dominant trees including Pterygota,
Newtonia and Pseudospondias, whilst particularly in the
SE part of its range it extends to lower altitudes within
riverine forest. However, through much of its east
African range it is most commonly associated with
various montane and submontane evergreen forest
assemblages up to 2000 m alt. Its distribution there-
fore falls largely within the Afromontane phytocho-
rion, a well-defined archipelago-like floristic region
well represented in the mountain chains of east Africa
and the Cameroon Highlands (White 1978,1981,
1983). Range disjunctions of Afromontane taxa be-
tween these two montane regions are well docu-
mented and rather plentiful; for example Sebsebe
et al. (2006) traced 12 species which occur in the
highlands of Ethiopia and Cameroon but were absent
in the intervening lowlands. Many of these are
common Afromontane trees and shrubs, such as
Agarista salicifolia G. Don (Ericaceae), Bersama abyssinica
Fresen. (Melianthaceae), Nuxia congesta R. Br. ex Fresen.
(Buddlejaceae) and Schefflera abyssinica (Hochst. ex A.
Rich.) Harms (Araliaceae). However, some herbaceous
taxa display a similar trend; for example Sebsebe et al.
Accepted for publication October 2008.
1
Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3AB, UK.
2
School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand.
KEW BULLETIN VOL. 63: 613–615 (2008)
© The Board of Trustees of the Royal Botanic Gardens, Kew, 2008

(ibid.) noted Moraea schimperi (Hochst.) Pic. Serm.
(Iridaceae)andScadoxus multiflorus (Martyn) Raf.
(Amaryllidaceae). Within the Acanthaceae this distribu-
tion has previously been noted in, for example,
Dicliptera laxata C. B. Clarke and Mimulopsis solmsii
Schweinf., both of which are widespread in the east
African mountains and in the Cameroon Highlands.
Two points are however worthy of note with regard to
the Metarungia disjunction. The first is the apparent
lack of evolutionary divergence (at least based upon
morphological evidence) of the highly isolated
Nigerian population. This is unusual in the Acanthaceae
in which regional differences are often evident within
taxa, even on a considerably smaller scale than the
current disjunction. More remarkable still is the
apparent absence of this species from suitable habitats
in the intervening area, most notably in neighbouring
forests of Chappal Waddi and the remainder of the
Cameroon Highlands (as defined by Cheek et al. 2004)
where significant areas of evergreen forest at a similar
altitude to Leinde Fadale (c. 1600 –1700 m) remain
extant. With large, showy red flowers and often a
colonial ecology, populations of this species are unlikely
to be overlooked. As evidenced by the Nigerian
population, this species appears to have a regular
annual flowering period during the drier season
(November –December), a period in which much
collecting has been carried out in the adjacent
mountains of Cameroon over recent decades. There-
fore, although it may yet be discovered in the more
remote parts of that country, it is almost certainly
absent from the well-known mountain regions (Mt
Cameroon, the Kupe-Bakossi-Mwanenguba complex
and the Bamenda Highlands). In contrast, the majority
of Afromontane taxa displaying a similar range disjunc-
tion are widely recorded and locally common in both
eastern Africa and the Cameroon Highlands, often with
scattered satellite populations in the intervening Congo
Basin or Angola (White 1981). The species with
perhaps the closest distribution pattern to that of
Metarungia pubinervia is the Afromontane tree Apodytes
dimidiata E. Mey. ex Arn. (Icacinaceae), which is
widespread and common in montane east Africa but
is known in the Cameroon Highlands from only five
records, three of which are from Nigeria; it was first
recorded in the well-collected Bamenda Highlands of
Cameroon as recently as 2004 (Cheek 2004).
Not only does this represent a new species record
for west Africa but also a new generic record.
However, Metarungia is morphologically very close to
the genus Anisotes Nees, which is recorded in west
Africa with two species in Cameroon and one species
in Guinea-Conakry. The only significant diagnostic
character for the two genera lies in the dehiscence of
the capsule, the placenta of Metarungia rising elasti-
cally at dehiscence, that of Anisotes remaining attached
to the wall of the capsule (Baden 1981). This
character has arisen independently in several unrelat-
ed groups within the Acanthaceae and has previously
been found to be of insufficient consistency to
maintain the separation of Peristrophe Nees from
Dicliptera Juss. (Darbyshire & Vollesen 2007)and
Rungia Nees from Justicia L. (Vollesen in prep.). It is
therefore likely that future analysis of Metarungia will
result in its reduction to synonymy within Anisotes,an
approach already adopted by Heine (1966).
In both Anisotes and Metarungia, the large, brightly
coloured corollas are almost certainly bird-pollinated,
although direct pollination studies are yet to be
conducted on these groups. It is envisaged that future
observations of the Nigerian population of M. pubiner-
via will further enlighten our understanding of this
pollination mechanism.
Metarungia pubinervia (T. Anderson)Baden (1984:638).
DISTRIBUTION. Montane regions of eastern and south-
eastern Africa (Ethiopia, Uganda, Kenya, Burundi,
eastern Congo-Kinshasa, Tanzania, Mozambique,
Malawi, Zambia and Zimbabwe); eastern Nigeria.
NIGERIA. Saurdauna Province: Gashaka Gumti National
Park, Leinde Fadale forest, fl. 7 Nov. 2002, H. M.
Chapman 24 (K!, UCI); idem,fl. & fr. 29 Dec. 2003, H.
M. Chapman 561 (BR!, CAS!, K!, UCI, WAG!).
CONSERVATION STATUS. This species is widely distributed
in eastern and southeastern Africa where it can be
locally common and it is represented by numerous
collections in the Kew herbarium. Therefore, although
habitat destruction and degradation continue to occur
in parts of its range, it cannot be considered globally
threatened and is assessed as of Least Concern (LC)
under IUCN (2001) criteria. On a national level, the
single site in Nigeria, Leinde Fadale, is a small montane
forest patch of approximately 8 km
2
, lying just beyond
the boundary of the Gashaka Gumti National Park. It is
locally abundant here in one small area at c. 1600 –
1670 m alt., forming an almost monotypic understorey.
This forest is rather remote and is currently
undisturbed, but as it lies outside the boundary of the
formally protected National Park, it is vulnerable to
future agricultural encroachment. The presence of this
nationally scarce species could be used to support the
possible future extension of the boundary of the park
to incorporate this important forest remnant. If the
high diversity and high numbers of endemic and/or
scarce taxa recorded in the adjacent montane regions
of Cameroon are indicative, it is quite likely that the
discovery of Metarungia pubinervia here will be followed
by further equally interesting botanical finds during the
on-going ecological research programme in the
Gashaka Gumti region.
614 KEW BULLETIN VOL. 63(4)
© The Board of Trustees of the Royal Botanic Gardens, Kew, 2008

Acknowledgements
Research in the Gashaka Gumti National Park was
supported by Flora and Fauna International, in
collaboration with the University of Canterbury and
the North of England Zoological Society. We thank
Martin Cheek for valuable input, and thank two
anonymous reviewers for comments on an earlier
draft of this manuscript.
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615REMARKABLE RANGE DISJUNCTION IN METARUNGIA PUBINERVIA
© The Board of Trustees of the Royal Botanic Gardens, Kew, 2008