Influence of Species Diversity on the Return of Ecosystem Functions in Replanted Mangroves in Kenya

Abstract

Rates of loss of biodiversity caused by human action are on the increase worldwide. However implications of species loss on the natures' ability to provide ecosystem services and goods are still poorly understood. Despite providing an array of critical services and goods, mangrove ecosystems are under intense threat. In attempts to address the problem, several mangrove restoration initiatives have been formulated in many areas of the world with different objectives. Over the last two decades, experiments manipulating species diversity and measuring ecosystem functions have been conducted mainly using grassland ecosystems and have exhibited positive relationships. More recently, experiments investigating this relationship have emerged in longterm woody species i.e. trees; however these have concentrated on terrestrial forests. This study experimentally manipulated different mangrove species and measured a range of ecosystem functions including sapling survival, above and belowground biomass production and sediment C02 efflux. Enhanced sapling growth was positively correlated with plot height above datum, percentage silt and nitrates and negatively correlated with sediment salinity, ammonium and phosphates. Also high values of above and belowground biomass, root: shoot ratios, mean tree height, leaf area index as well as naturally recruited saplings were observed in mixed plots (particularly with Avicennia marina in the mixture) compared with monospecific plots. Species selection effect, particularly from the fast growing A. marina species was the mechanism behind a range of the observed ecosystem functions. However complementarily effects were observed particularly on aboveground biomass. However sapling survival and sediment CO2 efflux was not influenced by species richness. We conclude that there is variation in the stages of plant development at which species richness effects manifest themselves, in addition the effects of environmental variables has a bearing on the nature and direction of the relationship between species richness and ecosystem function. We anticipate changes in the plots structure over time from A vicennia facilitative effects which are expected to lead to changes in sediment microclimate inducing changes in other species growth and promoting recruitment and development of wildlings.