Mangrove Tree Specificity and Conservation Implications of the Arboreal Crab Parasesarma leptosoma at Mngazana, a Mangrove Estuary in the Eastern Cape, South Africa (original) (raw)

Abstract

The arboreal crab Parasesarma leptosoma has been recently discovered at Mngazana, a southerly mangrove system in southern Africa, where crab tree preferences were studied using an indirect (browse leaf damage) and a direct (tree traps) method. The extent of crab induced leaf damage was compared for three mangrove species at two sites, one next to a tidal creek and one away from the creek. Using ANOVA, significant differences were found between tree species (P < 0.001) at different distances from the creek (_P_ < 0.022). Crabs were found to occur on _Rhizophora mucronata_ and _Brugueira gymnorrhiza_, but not on _Avicennia marina_. This reflected a gradient in browsing, from well-browsed _R. mucronata_ (100% near the creek and 25.7% away from the creek), to medium browsing of _B. gymnorrhiza_ (51.5% near and 0% away) and no browsing on _A. marina_ (near or away). These differences could be explained in terms of palatability, as both _R. mucronata_ and _B. gymnorrhiza_ are salt excluders, while _A. marina_ secretes salt from its leaves. Leaf consumption levels averaged between 1.73% and 2.6% of leaf area for _R. mucronata_ and 0–1.76% for _B. gymnorrhiza_. For both _R. mucronata_ and _B. gymnorrhiza_ there was a significant correlation between the number of crabs caught directly and the amount of browse leaf damage (_P_ < 0.01). Crab number was also significantly correlated with tree circumference for _R. mucronata_ (_r_ 2 = 0.67) and _B. gymnorrhiza_ (_r_ 2 = 0.76, _P_ < 0.05), with crabs more prevalent on the former tree species and no crabs trapped on _A. marina_ (91.7%, 38.3% and 0% catches, respectively), thus reflecting the results obtained by the indirect method. Total Nitrogen and Phosphate were measured for both sediment and leaves of the three mangrove species at the two sites. Leaf comparisons showed significant differences (_P_ < 0.01) for both Total Nitrogen and Phosphate with _R. mucronata_ having the highest values, followed by _A. marina_ and lastly _B. gymnorrhiza_. Total Nitrogen was significantly higher for both _B. gymnorrhiza_ and _R. mucronata_ compared with _A. marina_, while leaf phosphate was significantly lower for _B. gymnorrhiza_ when compared with both _R. mucronata_ and _A. marina_. No significant differences were found for leaf nutrients between sites, with the exception of _A. marina_ and _R. mucronata_ Total Nitrogen, which was significantly higher at the near creek sites (_P_ < 0.05). Sediment analysis showed no significant differences (_P_ > 0.05) in either nutrients or median particle size. Thus, R. mucronata, especially near the creek, had higher nutrient value and was probably more palatable and could explain observed differences in crab distribution. Very little browse damage was encountered in saplings below 10 cm. Most poles chopped by the local communities are R. mucronata in the 15–20 cm category, which coincides with peak crab frequencies in the 15–25 cm size classes for R. mucronata and B. gymnorrhiza, so that this selective harvesting is affecting this crab population maximally. Predictions were made as to the effect of crab loss, tree replacement rate and alternatives to chopping, which would boost community socio-economic levels and reduce the anthropogenic pressure on this biodiverse southerly mangrove system.

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Acknowledgements

This research was funded by a National Research Foundation (NRF) Natural Resources, Utilisation, Ecology and Management grant (GUN number 2050973). The assistance of Tshawe Makanandana, Mzo Nkaitshana and Sintu Hola in the field and for collecting additional soil and leaf samples for analysis is greatly appreciated. This paper is for Thandi my post-graduate student, friend and colleague who died tragically in October 2003.

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1. University of Transkei, P. Bag X1, Unitra, 5117, Umtata, East Cape, South Africa
   W. D. Emmerson & T. T. Ndenze
2. Bowbarrie, Fasque, Fettercairn, Aberdeenshire, AB30 1ER, Scotland, UK
   W. D. Emmerson

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1. W. D. Emmerson
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2. T. T. Ndenze
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Emmerson, W. ., Ndenze, T.T. Mangrove Tree Specificity and Conservation Implications of the Arboreal Crab Parasesarma leptosoma at Mngazana, a Mangrove Estuary in the Eastern Cape, South Africa.Wetlands Ecol Manage 15, 13–25 (2007). https://doi.org/10.1007/s11273-006-9008-4

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• Received: 03 November 2004
• Accepted: 23 March 2006
• Published: 20 July 2006
• Issue Date: February 2007
• DOI: https://doi.org/10.1007/s11273-006-9008-4

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