- Title
- Top-down control by grapsoid crabs in South African estuarine salt marshes
- Creator
- Smith, Leigh-Ann
- Subject
- Salt marsh ecology – South Africa
- Subject
- Crabs --South Africa
- Date Issued
- 2023-04
- Date
- 2023-04
- Type
- Master's theses
- Type
- text
- Identifier
- http://hdl.handle.net/10948/61223
- Identifier
- vital:69807
- Description
- Salt marshes are one of the most productive ecosystems globally. These ‘blue carbon’ habitats are in many respects resilient to climate change effects, able to track sea level rise and act as carbon sinks. Since salt marshes are economically and ecologically essential habitats (for example providing foraging grounds for invertebrates, fish, birds and humans), understanding the processes that control their structure is crucial to best maintain ecosystem services provided by these habitats. For decades the importance of top-down forces, such as grazing, has not been recognised as being important in controlling salt marsh plant productivity. Rather, bottom-up forces, such as the availability of oxygen, was thought to be the main constraint to primary production. Therefore, a better understanding of salt marsh ecosystem dynamics and whether grazer control exists is needed. Although grazer top-down control has been demonstrated in salt marshes in locations such as the United States, this has not been investigated in South Africa. This is a crucial knowledge gap locally considering the importance of salt marsh ecosystems. The aim of this study was therefore to determine what role consumers play in structuring South Africa’s estuarine salt marsh vegetation, thereby also contextualising this process internationally. In order to achieve this, the abundance of grapsoid crabs was estimated and related to key salt marsh features, such as Spartina maritima stem height and stem density, which was quantified from observational surveys. Sampling took place at the beginning and end of the early-spring and late-summer growth period (September 2021 and April 2022) in the Knysna, Kromme and Swartkops estuaries. If a correlation was observed between crab density and salt marsh features, it was important to determine whether they consume the salt marsh vegetation or if the effect was due to their burrowing activities. Hence, the diet of grapsoid crabs was investigated using stable isotope and gut content analysis. To examine the importance of crab grazing in controlling the growth of salt marsh vegetation, crab abundance was manipulated in situ by means of exclusion cages. This experiment ran for five months in the Kromme Estuary, from November 2021 to March 2022 over the salt marsh growing season. An ex situ feeding experiment was also conducted on 30 crab specimens collected from the Kromme Estuary, during May 2022. These crabs were starved for 48 hours and thereafter fed different components of Spartina (growing and dead blades, and roots and rhizomes). The food consumed was quantified as the change in plant length and dry weight after 48 hours, which gave a maximum grazing rate for the different food sources. It was found that crabs are having a minimal impact on the salt marsh vegetation structure. Spartina maritima does form an important component of the diet of grapsoid crabs, however, this is not their only source of food, and possibly not their preferred source of food. They are however contributing to sediment properties through their burrowing activities. The iii ex situ feeding experiment showed that grapsoid crabs can consume all vegetative components of Spartina, with a significantly greater amount of roots and rhizomes consumed. There was no consistent effect in terms of the exclusion of grazers, indicating that other mitigating factors could be reducing the impacts of grapsoid exclusion in situ. Together, these findings contribute knowledge that can be used to inform conservation by quantifying what the salt marsh risks losing in the absence or overabundance of crabs, as has occurred elsewhere globally. These results influence how salt marshes are managed, and highlight how not recognising the importance of consumers, such as crabs, could misalign management objectives.
- Description
- Thesis (MSc) -- Faculty of Science, School of Environmental Sciences, 2022
- Format
- computer
- Format
- online resource
- Format
- application/pdf
- Format
- 1 online resource (154 pages)
- Format
- Publisher
- Nelson Mandela University
- Publisher
- Faculty of Science
- Language
- English
- Rights
- Nelson Mandela University
- Rights
- All Rights Reserved
- Rights
- Open Access
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