Long-Term Changes in Mangrove Landscape of the Niger River Delta, Nigeria

Abstract

The Niger Delta mangrove is the third largest in the world and the largest in Africa. Since the 1960s oil and gas exploration has become an important economic activity, resulting in significant alteration of the landscape via pollution, urbanization and invasion. Landsat images of six different years (1984, 1986, 2000, 2003, 2005 and 2007) were used to determine land cover across 3, 700 km². Landscape was segregated between areas with oil and gas exploration and those without. Two forest types were identified namely mangrove and mixed, which were further decomposed into high (Mang 1) and low (Mang 2) density mangroves and palms. A total of 145 landscape square samples, each 6.76 km² were randomly selected in each map and statistically analyzed and modeled. The results showed that the kappa coefficient for the six years were all >0.9, (i.e., 0.93, 0.07, 0.94, 0.93, 0.90 and 0.94) indicating high classification accuracy. Also great change in mangrove landscape occurred in the last decade. Locations with increased oil and gas activities had significantly decreased amount of mangrove and palm forests. Also mixed forests increased over time and had a significant negative relationship with mangrove, Mang 1 and Mang 2. Even though the total area of mangrove forest did not change significantly (p>0.05), the total biomass of mangrove decreased (p<0.01). Nypa palm abundances increased over time, yet, it is negatively affected by the exploration. Increase in mixed forest and urban region has negatively affected the mangrove forest in the Niger’s delta landscape. High density mangrove forest withstood better the impacts of oil and gas exploration compared to mixed forest, but low density mangrove forest was the opposite. This suggests complex landscape level effects among different forest types.