Diversity of benthic macrofauna and physical parameters of sediments in natural mangroves and in afforested mangroves three decades after compensatory planting

Study Number:

112

Author:

Jassim A. Al‑Khayat, Maryam A. Abdulla, Juha M. Alatalo

Abstract:

Mangroves in the Arabian Gulf are under pressure from land use change, leading to compensatory planting at sites without natural mangroves. In this study, we examine the potential long-term success of this form of afforestation of mangroves as a conservation measure supporting biodiversity. We compared the community of benthic macrofauna (in total 13,522 individuals from 75 species) and physical parameters at 15 sampling points in five afforested mangrove communities and 19 stations in four natural mangrove communities in Qatar. Three decades after afforestation, there were no significant differences between the benthic macrofauna in natural and afforested mangroves as regards total abundance, species richness, Shannon’s diversity, effective number of species, or evenness. Only species richness of Crustacea was significantly higher in natural mangroves. Total organic carbon (TOC), sand, and clay content did not differ. This contradicts findings at the same locations 10 years after afforestation, when benthic macrofauna diversity, TOC, and clay content were lower at afforested sites than at natural mangrove sites. Our results suggest that compensatory planting of mangroves in areas without previous natural occurrence can potentially compensate for loss of biodiversity due to coastal development and that physical parameters may become more similar in afforested mangroves and natural mangroves over time. Furthermore, the increase in TOC in sediments in afforested mangroves over time can help mitigation of climate change. Importantly, the results highlight the need for long-term studies and perspectives when evaluating conservation practices.

Main Results and Conclusions:

• After afforestation, the created mangrove ecosystem closely resembled a naturally occuring mangrove forest.
  • “Three decades after afforestation, there were no significant differences between the benthic macrofauna in natural and afforested mangroves as regards total abundance, species richness, Shannon’s diversity, effective number of species, or evenness.” (1)                
  •  “Moreover, there was no difference in total carbon in the sediments between afforested and natural mangroves, supporting our second hypothesis.” (5)    
  • “Thus, our results are in line with findings in a previous study in Vietnam, where benthic species richness in mangroves increased with time since initial afforestation over the period from 2004 to 2013 (Zvonareva et al. 2015).” (6)    
  • “While the effect of tidal zone on biodiversity was not the focus of our study, we found that except for crustaceans, which showed higher abundance at the upper tidal level, there were no significant differences in biodiversity or abundance between upper and middle tidal levels.” (6)   
• Climate change, specifically rising sea levels, is a huge threat to existing mangrove communities, and all those that dwell in them.
  • “The Middle East contains an estimated 3377 km2 of mangroves, up to 96% of which are at risk of being lost due to sea level rise (Blankespoor et al. 2014; Ward et al. 2016).” (1)                    
  • “Restoring and increasing coastal vegetation also has the potential to mitigate climate change (Duarte et al. 2013). Rehabilitation of existing mangroves has been conducted worldwide (Feng et al. 2014; Li et al. 2015; Nusantara et al. 2015; Zvonareva et al. 2015; Liu et al. 2016; Zabbey and Tanee 2016).” (1)    
  • “…afforestation with mangroves could contribute to some degree to mitigating climate change, as the sediments can function as a carbon sink for a long period. In contrast, clearing of mangroves causes large losses of carbon to the atmosphere (Grellier et al. 2017).” (7) 
• Benthic biodiversity plays a huge role in the success of mangrove afforestation.
  • “The functionality and success of restored mangroves vary (Kairo et al. 2001; Bosire et al. 2008; Primavera and Esteban 2008), but plantations have been shown to have positive effects on ecological functions even at polluted sites (Leung and Cheung 2017).” (2)            
  • “Much of the diversity in mangrove forests consists of benthic fauna that live on or in the bottom sediments (Saravanakumar et al. 2007; George et al. 2010; Zhou et al. 2015; Chen et al. 2017). These benthic macrofauna provide many ecological functions and play an important role in connecting primary producers and top carnivores or predators, helping in the decomposition of organic matter and nutrient cycling through trophic activities, enhancing sediment porosity, and creating pathways for oxygen, nutrients, water, and other elements to pass into the sediment (Kon et al. 2011; Pravinkumar et al. 2013).” (2) 
• The only difference seen between natural and afforested mangrove communities is the abundance of Crustacea.
  • “For Crustacea, there was a significant effect of mangrove type on species richness (p = 0.001), which was lower in afforested compared with natural mangroves (Figs. 4, 5, Table S2). Position had a significant effect on Crustacea abundance, which was significantly lower in the middle tidal level…”     
  • “Prolonged inundation of mangroves in Brunei has been shown to decrease crab abundance, with crab populations then being able to recover after the inundation returned to more normal timing (Choy and Booth 1994). This may help to explain the distribution of crustaceans in our study, which were found in highest abundance in the upper level tidal mangrove, i.e., that with the shortest period of daily inundation.” (6) 
• Planting mangroves in areas where there were not previously mangroves (afforestation) has been successful.
  • “Afforested mangroves had significantly (p = 0.016) higher salinity and lower pH (0.032) than natural mangroves (Table 3). However, no significant difference was found in TOC (p = 0.543), sand content (p = 0.274), or clay con- tent (p = 0.274) between natural and afforested mangroves (Tables 3, 4, Table S3).” (4).    
  • “However, our analysis of natural and afforested mangroves in the present study revealed that, given enough time, compensatory planting of mangroves following land use change at sites without previous natural mangroves may be successful in terms of conserving biodiversity in the extreme environments of the Arabian Gulf.” (6)                 
  • “Thus, our results are in line with findings in a previous study in Vietnam, where benthic species richness in mangroves increased with time since initial afforestation over the period from 2004 to 2013 (Zvonareva et al. 2015).” (6)
  • “…our data suggest that compensatory planting of mangroves in areas without previous natural occurrence may be successful in the longer term in supporting benthic biodiversity and in contributing to mitigation of climate change by increasing storage of total carbon in sediments over a long time.” (8)  
• There are some unknown aspects of afforestation that are emerging as these projects continue in length.
  • “This contradicts findings in a previous study in Gujarat that sites positioned farthest from and closest to the seashore had higher macrofauna density than middle-distance sites (Saravanakumar et al. 2007).” (6)  
  • “These studies highlight the danger of drawing conclusions on the effects of conservation measures based on short-term responses. Lack of consistency between short-term and longer-term responses has also been reported for other plant communities (Alatalo et al. 2014). Restored and afforested mangroves need time to mature (Alongi 2009; Salmo et al. 2017), and it is unlikely that benthic communities will reach the same levels of biodiversity until this has happened (Nagelkerken et al. 2008).” (7)

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