Ranids to the rescue

Ranids to the Rescue!
By Jerry Lea & Luca Luiselli
A recent article (Thomas et al., 2004) reports that we are currently
witnessing the sixth major extinction event in Earth’s history, and that
the reasons for this catastrophe are in no small part anthropogenic.
Moreover, we know that amphibians are declining globally as we destroy
and pollute their habitats. Yet, despite this gloomy outlook, there is
good news to report (at least in a few cases!). That is to say, natural
selection always fashions winners and losers, and our studies on
amphibians in southern Nigeria (West Africa) have shown that many anuran
species do in fact benefit from habitat alteration and, although some other
species may lose out, overall species diversity may be maintained or
even increase. The most highly adaptable species appear to belong to the
Ranidae, and one of the reasons for their success is undoubtedly their
ability to utilise multiple habitat types according to prevailing
environmental conditions (Lea et al., 2003; 2004).
Our group has been studying the community composition of species rich
anuran assemblages in West African rainforests (especially in Nigeria)
over the last ten years. We have been particularly interested in the
effects of environmental damage resulting from the activities of the
petrochemical industries (Luiselli & Lea, 2004). Recently, we revisited
areas that have seen at least a tenfold reduction in forest habitat over
the past 20 to 40 years, and we compared current community composition
with that prior to this massive habitat disturbance. In so doing, we
have been able to map out species successions that parallel this
environmental change. We have found that in both lowland and montane forest
habitats the forest specialists (e.g. Ptychadena aequiplicata, Chiromantis
rufescens, Acanthixalus spinosus, Hyperolius sylvaticus, Werneria
mertensi, Woltersdorffina parvipalmata, Cardioglossa spp. etc.) have been
replaced by more generalist species, or those preferring disturbed h!
abitats, and these species now dominate (e.g. Hoplobatrachus
occipitalis, Ptychadena mascareniensis, P. bibroni, P. oxyrhynchus,
Phrynobatrachus latifrons, P. aelleni, P. werneri, Bufo maculatus etc.). In both
environments, species richness has at least been maintained (1960s: 12
spp. from 9 genera; 2002: 12 spp. from 10 genera - Lea et al. 2004), but
in the lowland case species richness has risen quite noticeably (1982:
11 spp. from 8 genera; 2002: 16 spp. from 9 genera - Lea et al. 2003).
This maintenance/ increase in species richness may be as a result of an
increased diversity of microenvironments in the new ‘bushland’ and
secondary forest mosaic compared to the pristine forest (i.e. ‘spatial
heterogeneity hypothesis’: Pianka 1966; Barbault 1991). Nowadays, the most
successful species in both the lowland (riparian) and montane
environments are members of the Ranidae, especially Ptychadena and
Phrynobatrachus spp. It appears that this group of anurans has a wide range of!
adaptations and life-history characteristics (e.g. generalist!
feeding
habits; egg guarding behaviour; within-season clutch partitioning;
large terrestrial component and rapid development of tadpoles; high
fecundity; high mobility and wide habitat tolerances) that enable them to
switch between a variety of habitats depending upon prevailing conditions.
Indeed, the success of the Ranidae is so remarkable that it is
possible to find seven congeners (Ptychadena spp.) within one 5km radius at a
lowland site (see Lea et al. 2003). Given this, we were intrigued to
know how these extremely similar species could persist in such close
proximity. Another facet of our work has therefore been to investigate the
mechanisms that help to maintain species richness within these
communities despite the potential for heavy inter-specific competition. We have
discovered that these mechanisms include a remarkable degree of food
resource partitioning between very similar and closely related ranid
species (Ptychadena), one of which is obviously a generalist feeder and the
other is more of a specialist (Eniang et al. 2003).
Overall, our results show that forest destruction in Nigeria does not
necessarily lead to a reduction in species diversity, but that there
can be a successional change in species composition with generalists
(esp. Ranidae) becoming more species rich at the expense of the forest
specialists. The species that are nowadays the most successful include
those that are highly mobile, have wide habitat tolerances and are
opportunistic (e.g. Ptychadena and Phrynobatrachus); these species cope well
with changing landscapes.
Contact: Jerry Lea, Department of Animal Management, Askham Bryan
College, Askham Bryan, York Y023 3FR, U.K.
jerryleainabraka@yahoo.co.uk
Luca Luiselli, Centre of Environmental Studies ‘Demetra’, via Olona 7,
I-00198 Rome, Italy.
lucamlu@tin.it
References
Barbault, R. (1991) Ecological constraints and community dynamics:
linking community patterns to organismal ecology. The case of tropical
herpetofaunas. Acta Oecol. 12: 139-163.
Eniang, E.A., King, R., Lea, J.M., Capizzi, D. &. Luiselli, L. (2003)
Trophic Niches of Four Sympatric Afrotropical Anurans from Southern
Nigeria: Does Resource Partitioning Play a Role in Structuring the
Community? Rev. Ecol. 58: 321-335.
Lea, J.M., Politano, E. & Luiselli, L. (2003) Changes in the
herpetofauna of a fresh water river in Southern Nigeria, after 20 years of
development. Russ. J. Herpetol. 10(3): 191-198.
Lea, J.M., Luiselli, L & Politano, E. (2004) Are there shifts in
amphibian faunal composition in Nigerian landscapes undergoing long-term
degradation? A case study from a montane environment. Rev. Ecol. in press.
Luiselli, J. & Lea, J. (2004) Pollution: Petrochemicals and Heavy
Metals. In: Heatwole, H. (Ed.) Amphibian Biology (Harvard University Press).
In press.
Pianka, E.R. (1966) Latitudinal gradients in species diversity: a
review of concepts. Am. Nat. 100: 33-46.
Thomas, J.A., Telfer, M.G., Roy, D.B., Preston, C.D., Greenwood,
J.J.D., Asher, J., Fox, R., Clarke, R.T. & Lawton J.H. (2004). Comparative
Losses of British Butterflies, Birds, and Plants and the Global
Extinction Crisis. Science 19 March: 1879-1881.
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Rusty