Testing the Janzen-Connell model for species diversity in a West African montane forest.
Thesis DisciplinePlant Ecology
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
A major question in ecology is 'why are tropical forests so species diverse?' One hypothesis to explain tropical species diversity is the Janzen-Connell hypothesis. This model assumes high levels of host-specific seed and seedling predation and / or pathogen attack when seedlings occur at high density near to the parent tree; seedlings are more likey to survive and reach maturity the further they are away from parents / conspecific adults. Theoretically this should lead to a random distribution of each species in the forest, which in turn will lead to high species diversity. Here I test the Janzen-Connell hypothesis for the first time in a submontane dry forest in Nigeria, West Africa. Specifically I tested whether or not a) leaf herbivory decreases and b) seedling survival increases with distance from parent / conspecific adult trees. These two components were tested separately on naturally occurring seedlings and on experimentally planted seedlings. I also tested whether or not conspecific adult trees showed clumped distributions by testing if conspecific nearest neighbours were observed more often than would be expected by chance alone. Naturally occurring seedlings of three species, Pouteria altissima, Newtonia buchananii and Isolona pleurocarpa showed significantly greater survival at distances away from parent / conspecific adult trees. Two out of a total of three species (Entandrophragma angolense, Deinbollia pinnata and Sterculia pinnata) of experimentally planted seedlings showed increased survival at distances away from conspecific adult trees, but this trend was non-significant. Leaf herbivory decreased with distance from parent / conspecific adult trees for four out of a total of six study species, but all relationships of leaf herbivory with distance for these six species were non-significant. Of two individual species, Anthonotha noldeae and Carapa procera, and two species groups tested for clumping, all had a greater number of conspecific nearest neighbours than would be expected to occur by chance alone, and this was significant for the two species groups. The decreased survival of seedlings under parent / conspecific adult trees is likely to maintain tree species diversity in West African submontane forests as predicted by the Janzen-Connell model. The role of host specific seedling herbivores in reducing recruitment under parent / conspecific adult trees requires further investigation. Although conspecific adults showed some degree of clumping no conclusion was reached as to whether this was evidence for or against the Janzen-Connell model.