Saturday, June 26, 2010

Prehistory of mangroves and humans in Nigeria

Dear Mangrove Action Project
I am the Nigerian Palynologist who informed you about our work on the mangroves in Nigeria some time last year. I am so sorry it took this long for me to send this.


ORIJEMIE AKPO EMUOBOSA, Palynology Laboratory, Department of Archaeology and Anthropology, University of Ibadan, Nigeria

In 2004, a palynological study of an 11m terrestrial core, obtained from Ahanve a village near Badagry in Lagos State, SW Nigeria was carried out by Professor M. A. Sowunmi. One of the aims of the study was to understand the palaeoenvironment of the lowland rain forest (LRF). It was found that the mangrove swamp forest (MSF) was abundant there from the Early Holocene to Mid Holocene (ca 9500-5500 yrs B.P.). However, the MSF declined gradually from the Mid Holocene, and completely disappeared ca 3100yrs B.P. Today, the MSF is not present in the area; the vegetation is a fresh water swamp, dominated by Typha australis. This drastic change in vegetation was unexpected, and thus prompted a further study of the vegetation history of the LRF and MSF in the west coast of Nigeria during the Late Holocene.

More palynological studies of the Lowland Rainforest (LRF) of Southwestern Nigeria during the Late Holocene have been undertaken. The aims of these studies are broadly to reconstruct the palaeoenvironment of the mangroves and ascertain the impact of human interactions with rainforest in this same period (the Late Holocene). Sediment cores with varying depths from three other sites (Ogudu [6m], Otolu [0.45m] and Ikorigho [2m]) along the coasts of SW Nigeria, and one from Ahanve [2m] were obtained. The palynological studies of these cores have been carried out. Furthermore, archaeological excavations were conducted in Ahanve, the same village where the MSF had disappeared some 3100 yrs ago. The aims of this archaeological angle are to ascertain the antiquity of humans there and whether humans actually contributed to the disappearance of the MSF.

The palynological studies have revealed that though environmental and vegetation changes were noted in all the sites, they were comparatively very serious at Ahanve, where a drastic reduction of the LRF and complete disappearance of the MSF occurred some 3100yrs B.P. This change in vegetation has been linked primarily with natural followed by anthropogenic influence. Changes in climate, hydrology, geomorphology and salinity of the mangrove swamp were the initial natural factors that led to the decline and disappearance of the MSF at Ahanve. The dry phase that occurred ca 4500-3000yrs across Africa led to the destruction of the MSF; the Atlantic Ocean was then cut off from Ahanve while fresh water inundated from neighboring rivers and the Badagry creek lowered the salinity of the swamp. All these contributed to the destruction of the MSF there.
These events occurred concomitantly with reductions in rainforest taxa, appearance and increase in secondary forest species, (Elaeis guineensis (oil palm), Alchornea sp), grasses (Poaceae) and weeds (Asteraceae) associated with human habitation. Furthermore, there was a significant increase in microscopic charcoal towards the beginning of the Ahanve core. This indicated the burning of fuel wood by humans. Thus, although the reduction of the MSF at Ahanve was initially caused by change in climate, human activities, presumably felling and burning of forest trees, consequently exacerbated the situation. This led to the total destruction of the MSF.

At the other sites where cores were obtained, the MSF is still present there today. However, the reconstructed palaeoenvironment show that the MSFs in these areas were more extensive in the past. There were also alternating periods of dry and wet. Though these fluctuations in climate affected the MSF especially at dry periods, the MSF recovered when conditions became favorable. However, since the recent past, there have been consistent reductions in the MSF noted both in fossil mangrove pollen and ethnographic records. In addition, there were unprecedented and consistent increases in microscopic charcoal towards the top of all the sediment cores. These charcoal specks are not considered to have resulted from natural fires. Their consistent occurrence and increase even during periods considered to be wet are indications that they were caused by humans. It is presumed that the charcoal specks were derived from the burning of forest trees as domestic fuel.
Therefore, the palynological studies reveal that humans, beginning from some time in the Late Holocene have been having serious impacts on the LRF and MSF in SW Nigeria. Today, the factors having serious effects on mangroves are: felling of the mangrove trees for fuel, conversion of mangrove swamp for other purposes such as building, and dredging of the mangrove swamp. All these human activities alter the ecology of the MSF and destroy mangroves.

Two test pits were excavated at Ahanve. The test pits, named TP1 and TP2, were 210 cm and 110cm deep respectively. The recovered materials includes: Pottery, charcoal, animal and fish bones, rusty nails (iron objects), iron slag, snail and bivalve shells, hearth, palm kernel shells, smoking pipes, snail and bivalve shells and glass beads .
From the archaeological finds, there are indications that the Ahanve people gathered food resources such as the African giant snails (Achatina achatina), fresh water bivalve/clam (Anodonta sp), and possibly engaged in fishing cat fish (Clarias cf. gariepinus). There are also indications that the people hunted. The occurrence of iron slag, hearth, charcoal and charred palm kernel nuts through out the two test pits is an indication of the prehistoric use of fire by humans in Ahanve. But, it is not yet certain, from the archaeological records, if humans contributed to the disappearance of the MSF there ca. 3100yrs B.P.

Unfortunately, dates for these events are not yet available. With the availability of C14 dates, it would be possible to know when these palaeoenvironmental events occurred. Furthermore, a correlation of, and situation of all these sites within a regional (and global) context will be made possible. This will contribute to our understanding of how well to relate with our environment by learning from the past

Best wishes,
Orijemie A Emuobosa

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