This research assessed the diversity in species of woody species within the lowland rainforest floristics of Ika region, Nigeria. The research design used was the quasi experimental approach. The Ika region was stratified into 12 sub-areas, while selection of sampling units was made using the technique of random sampling. The sampling units were specified into two different groups as secondary (degraded areas) and primary (mature rainforest within conserved areas) groves. From each sub-area, 2 sampling units were selected randomly, making 24 sampling units. Data collection which was based on species of the woody trees and their populations together with the sampling areas, adopted the quadrat size of 10m x 10m to ensure effectiveness. Standard approaches were followed in data collection; while data generated were analysed using graph, Simpson’s index, and student t-test statistics. Population of the tree species varied in the 2 ecosystems. Tree species such as Milicia excelsa, Swietenia macrophylla, Alstonia boonei, and Aniba rosaeodora are gradually becoming threatened. Tree species diversity varied between the 2 ecosystems, with observed loss of biodiversity. Tree species were more diverse in the primary grove. Simpson’s indices of tree species diversity values for the secondary and primary groves are 0.5783 and 0.8050. With probability, variance and t-values of 0.000, 0.0015 and -14.027 respectively, species diversity between the 2 ecosystems is significant at 0.05 level of confidence. Degradation has negatively impacted on the rainforest and its tree species. Reforestation of the degraded rainforest is recommended for sustainability.