The ecological tradeoffs of invasive rhizophora mangle on the hawaiian islands

Abstract

My research reevaluates historical nonnative mangrove eradication and habitat restoration goals on in Hawaii. As a part of this reevaluation, I establish a framework for weighing modern ecological services with negative ecological and socioeconomic costs. The purpose of my framework is to support land managers by providing updated assessments that accurately represent the contribution nonnative mangroves provide regarding Hawaii's current climate mitigation goals. This framework focuses on invasive Rhizophora mangle (red mangrove) stands as they are the most abundant type of mangrove on the islands. Recently published Rhizophora mangle carbon sequestration and soil accretion rates on the Hawaiian island of Molokai now allow us to estimate their ability to offset current greenhouse gas emissions and sea level rise rates. These offsets can then be weighed against the traditional view that all Rhizophora mangle must be removed. I estimate 4 km2 of ??. mangle currently sequesters 0.3% of Hawaii's current greenhouse gas reduction target and if removed would set this goal back by 8%. I also find that Rhizophora mangle vertically accretes sediment faster than current seal level rise and coastline erosion rates predicted for Hawaii, making this ecosystem service is a valuable asset in mitigating the effects of climate change. Historically, Rhizophora mangle removal is justified by a reduction in water quality and endemic bird habitat in both anthropogenic features like harbors, canals, and fishponds, and also sensitive coastline features like anchialine pools. By weighing the ecological impacts against services, I recommend strategic mangrove removal in sensitive areas and areas that hinder commerce. Mangrove removal efforts on the islands are currently active and on-going, financially supported by state and federal funding in conjunction with local conservation groups. I examine reported financial costs of previous mangrove removal projects and discuss the feasibility of island-wide eradication. Based on the cost of prior removal projects, I estimate the cost of full eradication above $41 million not including future monitoring and maintenance. With new climate mitigation goals being added to already burdened state and federal conservation and land management budgets, I promote a more pragmatic view of nonnative species by recommending strategic Rhizophora mangle management in lieu of full state-wide eradication.