Mt. Cameroon
Sao Tome


Additional Resources

Dispatches from the Field
CAS Herpetology Collections
Sao Tome-Principe Country Profile
Gulf of Guinea Islands Biodiversity Network
Consular Information Sheet
Gulf of Guinea Geology I
Gulf of Guinea Geology II

The Gulf of Guinea Islands (sometimes called the Cameroon Line islands) is a group of volcanic islands lying in the Bight of Benin along the west coast of Africa. They consist of Annobon Island (sometimes known as Pagalu), Sao Tome, Principe, and Bioko (formerly known as Fernando Poo). Although technically not an island, the volcanic peak of Mt. Cameroon along the West African coast is a geological and biological part of this island group.

Island Mt. Cameroon Bioko Principe Sao Tome Annobon
Distance from Mainland - 40 km 250 km 300 km 350 km
Elevation 4095 m 3308 m 948 m 2024 m 654 m

There is only a shallow sea bed between Bioko and the mainland, and it is thought that this island was connected to Africa during the most recent ice age (approximately 10,000 years ago) and its lowland fauna and flora should generally reflect that of the mainland. The oldest geological dates, and hence the beginning of biological colonization for Principe, Sao Tome, and Annobon are respectively 31 mya, 14 mya, and 4.8 mya (Lee, et al, 1994).

Map of the Gulf of Guinea Islands off the west coast of Africa. (Annobon is off the map to the lower left).

The theory of island biogeography (MacArthur and Wilson, 1967) indicates that the richness of an island's fauna and flora is proportional to that island's distance from source areas, the island's size, and its elevation. The elevational factor would provide more niches than a similar sized island with simple planar topography. Using this theoretical basis alone, it would be expected that any fauna would become progressively more depauperate from Mt. Cameroon to Bioko to Sao Tome to Principe to Annobon as one proceeds to smaller and lower islands farther from the mainland.

However, it has been demonstrated that islands often develop unique, endemic faunal and floral elements (see Quammen, 1996 for a good summary and examples of this phenomenon). This may be due to the effects of genetic drift upon small populations of colonizing species, or perhaps evolutionary pressures upon colonizing populations which are quite different than presssures of the source area. The longer these genetic factors have had to work on a population in isolation, the more unique that fauna and flora becomes.

Although Sao Tome is a much larger and higher island than Principe, Principe has a geological history more than twice as long as that of Sao Tome. By the time Sao Tome emerged from the ocean, Principe had been available for colonization and evolutionary development for 17 million years. This situation would have created a second, closer source for colonization of Sao Tome.

The faunal and floral elements present on the more distant three islands may have arrived by either of two different routes. 1) The more migratory plants and animals may have arrived recently by island hopping from the nearest island to the coast to ever more distant islands. One would then predict that the plants and animals present on the respective islands would be a progressively more depauperate subset of the nearer islands as the filtering effect of each step reduced the founder diversity.

  View of Luba, town on the west coast of Bioko.

The second alternative is that Principe developed an early fauna and flora through sweepstakes colonization. The few species that were able to colonize this island would have had a small population evolving in isolation, so that evolutionary constraints would have been minimal and genetic drift maximized. Endemic species would have evolved, and some of these species would have populated the next island in sequence to emerge from the ocean floor, as the adjacent island would have been a closer source area than the continent itself. Under this scenario, there would be many endemic species on the islands, especially Principe, and the fauna and flora of Sao Tome would have arrived from both the rich mainland source and a more endemic Principe source.

The only endemic reptile on Bioko, Chamaeleo feae.
Keith Dabney (right) and two assistants collecting insects at an ultraviolet light on Bioko.

In all probability there will be some elements of island hopping on all islands of the group, particularly among the more migratory species. There probably are also differences in composition between various groups of plants and animals. For instance, grasses and insects migrate more easily than oaks and snakes. Elevation and island size must play a role. However, the central question is to what degree did sweepstakes colonization and subsequent isolated evolution contribute to populating the islands? This should be clearly evident by the number and percentage of endemic species.

A second question is the possible existence and composition of temperate fauna and flora so close to the equator. During the last glacial period (approximately 10,000 years ago) the climate of equatorial Africa was much cooler and because so much water was locked up as glaciers, the sea levels were lower, exposing more land surface. As the current warmer interglacial period began, these land areas became progressively more isolated and smaller. The insular fauna and flora were forced to retreat further up the volcanic peaks and appropriate environmental conditions became fewer and smaller in area. To what extent did this temperate fauna and flora survive? To what extent was it modified by changing environmental conditions? The high volcanic peaks of Mt. Cameroon, Bioko, and Sao Tome contain some of the largest areas of this temperate climate in West Africa, and yet the plants and animals are relatively unknown.

Mount Cameroon

Mount Cameroon is the highest point in West Africa, reaching 4095 m height. It has the largest area of temperate plants and animals in this region. As such, it also provides a source area and refugium for other, smaller temperate areas, such as the higher peaks on Bioko (3308 m) and Sao Tome (2024 m). Baseline data for spiders and ants of West Africa have been developed through the collections of Dr. Charles E. Griswold (Jan.-Feb. 1992) and Dr. Brian L. Fisher (April 2000).

Mt. Cameroon from the air.


Mt. Cameroon Photo Album


Since it is known that Bioko was in contact with the mainland during relatively recent periods of lowered sea level, this island should serve as a benchmark of which species are available to provide a source for island hopping of fauna and flora from the mainland lowlands and Mt. Cameroon. This is the largest and highest island, and is presumed to have the greatest number of niches for animal and plant survival. For this reason, a group of California Academy of Sciences researchers from the Herpetology and Entomology Departments travelled to Bioko Island from 14 September to 21 October, 1998. They spent six weeks collecting in as many habitats and using as many trapping techniques as possible. Participants included: Dr. Robert C. Drewes (Herpetology & team leader), Jens Vindum (Herpetology), Lindsay Henwood (Herpetology), Darryl Ubick (Entomology) and Keith Dabney (Entomology). The resulting collections, along with other published records, serve as baseline data from which to judge the fauna and flora of the more distant islands.

Tent site pitched at about 2000 m between a road and steep mountain slope. Darryl Ubick seen attempting to prevent flow of water from entering the tent.

One of the more colorful caterpillars seen on Bioko

Keith Dabney (in blue hat) and Darryl Ubick (with backpack) using umbrellas to collect spiders beaten from vegetation on Bioko.

Report on the Spiders (Araneae), Acrocerid flies (Diptera), and Ground Beetles (Carabidae) of Bioko


Principe is the oldest of the Gulf of Guinea Islands not near the mainland. Some rock samples have tested to be 31 million years of age. Although not as high as Bioko and Sao Tome, the highest peak is 948 m altitude, so that some remnants of a previously more widespread cooler fauna and flora may remain.

Sao Tome

Center of the capital city, Sao Tome.
A rural home near Bomboaim built onto a steep hillside.


From 25 March 2001 to 22 May 2001 a second, larger multidisciplinary team will travel to Sao Tome and Principe Islands to better document the fauna and flora of those two islands. Participants include: Dr. Robert C. Drewes (team leader - Herpetology), Dr. Tomio Iwamoto (Ichthyology), Dr. Charles D. Griswold (Entomology), Dr. Sarah Spaulding (Invertebrate Zoology), Dr. Norman D. Penny (Entomology), Dr. Doug Long (Mammalogy and Ornithology), Jens Vindum (Herpetology), Dong Lin (Photographer), Joel Ledford (Entomology), Ricka E. Stoelting (Herpetology) and Fabio Penny (Computer Services). Transects will be made up the central mountain to explore as many elevational niches as possible. Fresh water fish collections will be made using seine nets and small electro-shock probes. Bats will be studied with mist nets. Snakes, frogs, and lizards will be collected by hand collecting and use of long grasping poles. Insects, spiders, and other arthropods will be collected using pitfall traps, flight intercept traps, light traps, Molasses traps, detritus sifters, dusting spider webs with corn starch, and beating arthropods from the vegetation onto inverted umbrellas. Freshwater diatoms will be scraped from as many moist micro-habitats and streams as can be determined.

Collections from this second expedition will be compared with the results of the collections from Bioko to determine which species they have in common and which species came from other source areas. Results may help us better understand which species are long distant colonizers and how evolution has affected small, isolated populations of these species. We hope to find remnants of a previously more widespread temperate fauna at the higher elevations.


One of many freshwater streams on Sao Tome.
Secondary peaks below the principal peak of Sao Tome (to the right).


A volcanic dike on Sao Tome which created this distinctive landform when the softer surrounding rock was worn away.


Lee, D-C., A.N. Halliday, J.G. Fitton, G. Poli. 1994. Isotopic variations with distance and time in the volcanic islands of the Cameroon line: evidence for a mantle plume origin. Earth and Planetary Science Letters 123:119-138.

MacArthur, R.H. and E.O. Wilson. 1967. The Theory of Island Biogeography. Princeton University Press, Princeton, New Jersey.

Quammen, D. 1996. The Song of the Dodo. Island Biogeography in an Age of Entinctions. Touchstone Books. New York, New York. 702 pp.


Web site created by Norman D. Penny and Fabio R. Penny.

Last updated 10 April 2001

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