Earthworm species as pointer for herbaceous flora distribution and health of soil in Onigambari Forest Reserve, Oyo state Nigeria

Amongst various and diverse organisms, earthworms (Annelida: Oligochaeta) are the most important components of soil biota when it comes to soil formation and maintenance of soil structure and fertility. The presence of earthworms modifies the soil quality of the environment due to their burrowing and casting. This affects the activities of other organisms. Thus, they are also termed “ecosystem engineers”. However, while we begin to understand the role of surface casts, it is still unclear to what extent plants utilize subsurface casts. This work is a study of the floral community structure as influenced by surface cast so as to determine the functional groups of earthworms in the natural forest zone of Onigambari Forest Reserve. This reserve is located at latitude 07°25’N and longitude 3°53’E within the low and semi-deciduous forest belt of Nigeria. It is divided into two: natural and plantation forests. Herein, the simple random sampling technique was used to select 10 geo-referenced plots (15m × 15m each). Three 1.0m × 1.0m square quadrats were randomly laid on each plot to give 30 sampling points surveyed for data collection. Data were collected on floral composition, relative importance values (RIV), earthworm composition and diversity, and physicochemical components of the soil. Twenty eight herbaceous plants were enumerated. Andropogon gayanus had the highest relative importance value (RIV) of (20.499), while Acroceras zizanoides and Platostoma africanum had the least RIV (1.1782) amongst other four. Seven earthworm species were enumerated, with Eisenia fetida having the highest RIV (28.571), while Octoclasion cyaneam and Lumbricum rubellus had the least. The species richness of earthworms was high (0.8061) and evenly distributed (0.8405), but with a low diversity (H’ = 1.772). There are two functional groups of earthworm in the natural forest zone, seven were surface dwelling, while only Lumbricus terrestris was sub-surface dwelling. The health of flora and soil of the natural forest zone of Onigambari forest reserve is low. Prevention of outside influence on the forest zone, seeding and deposition of organic waste matter to the soil are strategies that would enhance abundance and diversity of earthworms in the natural forest zone.