Mangroves in peril: unprecedented degradation rates of peri-urban mangroves in Kenya

[摘要] Marine ecosystems are experiencing unprecedented degradation rates higherthan any other ecosystem on the planet, which in some instances are up to4 times those of rainforests. Mangrove ecosystems have especially beenimpacted by compounded anthropogenic pressures leading to significant coverreductions of between 35 and 50% (equivalent to 1–2% loss pa) forthe last half century. The main objective of this study was to test thehypothesis that peri-urban mangroves suffering from compounded and intensepressures may be experiencing higher degradation rates than the global mean(and/or national mean for Kenya) using Mombasa mangroves (comprising Tudorand Mwache creeks) as a case study. Stratified sampling was used to samplealong 22 and 10 belt transects in Mwache and Tudor respectively, set tocapture stand heterogeneity in terms of species composition and structure inaddition to perceived human pressure gradients using proximity to humanhabitations as a proxy. We acquired SPOT (HRV/ HRVIR/ HRS) images of April1994, May 2000 and January 2009 and a vector mangrove map of 1992 at a scaleof 1:50 000 for cover change and species composition analysis. Resultsfrom image classification of the 2009 image had 80.23% overall accuracyand Cohen's kappa of 0.77, thus proving satisfactory for use in this context.Structural data indicate that complexity index (CI) which captures standstructural development was higher in Mwache at 1.80 compared to Tudor at1.71. From cover change data, Tudor lost 86.9% of the forest between1992 and 2009, compared to Mwache at 45.4%, representing very highhitherto undocumented degradation rates of 5.1 and 2.7% pa,respectively. These unprecedentedly high degradation rates, which far exceednot only the national mean (for Kenya of 0.7% pa) but the global meanas well, strongly suggest that these mangroves are highly threatened due tocompounded pressures. Strengthening of governance regimes through enforcementand compliance to halt illegal wood extraction, improvement of land-usepractices upstream to reduce soil erosion, restoration in areas where naturalregeneration has been impaired, provision of alternative energysources/building materials and a complete moratorium on wood extractionespecially in Tudor Creek to allow recovery are some of the suggestedmanagement interventions.

[学科分类] 地球化学与岩石