greater Zandvlei Estuary Nature Reserve
The Rehabilitation of the Salt Marsh area of the Greater Zandvlei Estuary Nature Reserve by removing invassive Waxberries (Morella Cordifolia)
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Greater Zandvlei Estuary Nature Reserve,
City of Cape Town Nature Conservation
This was achieved by doing a preliminary ground dwelling fauna assessment using pitfall traps as well as manually removing M. cordifolia using chainsaws, loppers or slashers. Herbicide was applied to the cut stumps to limit the amount of regrowth of M. cordifolia. The site was also visited regularly to monitor the immediate flora and fauna responses to the removal of M. cordifolia.
It was concluded that the removal of M. cordifolia had positive responses for both the flora and fauna species. The abundance in space, light and nutrients promoted the growth of plant seedlings. The removal of M. cordifolia also resulted in a warmer microclimate that was favourable for the activity of ectotherms. The availability of these ectotherms also supported more consumers in food webs.
Zandvlei has undergone extensive vegetation rehabilitation since the 1950’s and has become the last ecologically functional estuary on the False Bay coastline (Sheasby pers. comm. 2007). It is already known that M. cordifolia existed in the area before the site was extensively rehabilitated seven years ago but M. cordifolia was never observed as an invasive plant in the area during that time (Dorse. pers comm 2008). Photographic evidence indicates that the area was more open and sparsely vegetated in the past. This also indicates that M. cordifolia had a smaller impact on the floral community.
The vegetation types occurring at Zandvlei Nature Reserve, Strandfontein Sewage Works and Rondevlei Nature Reserve (approximately two-three east of Zandvlei) are similar in terms of species composition, density and canopy cover. However, neither Rondevlei nor Strandfontein are as heavily infested with M. cordifolia as the site in Zandvlei. This could suggest that the conditions at Zandvlei are more ideal for M. cordifolia to invade the Salt Marsh plant community and dominate the other plant species.
There are few records regarding the invasion of plant communities by an indigenous plant species. This case study may not only increase the flora and fauna biodiversity of the study area but may also provide the necessary methodology to minimise the damage of these invasive species on plant communities, whether they are alien species or not.
1.2 BACKGROUND INFORMATION ON WAXBERRIES (Morella cordifolia)
1.2.3 Distribution and ecology
SANBI (2005) report that M. cordifolia is wind pollinated and the pollen produced by male plants is favoured by bees. Berries on female plants may remain on the plant for a long period and may form an aerial seed bank (SANBI 2005). The berries on female plants may be eaten by and therefore dispersed by birds (Knevel. 2001).
M. cordifolia is associated with an arbuscular mycorrhizal fungus that grows between the roots and assists the plant in absorbing nutrients from the soil (SANBI. 2005). M. cordifolia’s roots also form root nodules that have nitrogen-fixing bacteria that convert nitrogen in the soil to ammonia (SANBI. 2005). This could have an impact on the nutrient levels in the soil which could in the long term affect the vegetation type M. cordifolia then use this ammonia to make chlorophyll, proteins and enzymes.
1.2.4 Historical uses
According to SANBI (2005), the wax from M. cordifolia berries was also eaten by the Khoi as the wax from the berry is actually a true fat. The bark from the stems and roots of M. cordifolia were used in the tanning of skins (SANBI. 2005). In the early 1900s, M. cordifolia was used for dune stabilization prior to the use of alien species for this purpose.
1.3 THE STUDY AREA
The climate of the Cape Peninsula is predominantly controlled by two systems. During summer months, the Atlantic high-pressure systems (with an anti-cyclone wind flow) are located in the south (Paterson-Jones 1991). Mucina & Rutherford (2006) reported that these high-pressure systems are located near 37° S in summer. These pressure systems forces cold fronts away from the continent and causes strong south easterly winds to blow after relatively calm conditions (Paterson-Jones. 1991). As winter approaches, these pressure systems move north to approximately 32° S (Mucina & Rutherford. 2006) and allow cold fronts to cross the Western Cape and penetrate inland for varying distances (Paterson-Jones 1991). These cold fronts cause strong, north-westerly winds to blow and rain will follow after a couple of days (Cowling & Richardson. 1995).
During dry summers, cloud cover is frequent and fuelled by strong winds (Mucina & Rutherford. 2006). Mucina & Rutherford (2006) report that over 500 mm of water can be precipitated per year without being recorded on standard rain gauges.
Mucina & Rutherford (2006) also report that relative humidity is highest along the coast in summer but high values are also reached inland during winter. In terms of wind speed, the southerly gradient winds are reinforced by the sea breeze over False Bay and raises maximum wind velocities in the early afternoon (Mucina & Rutherford. 2006). Rutherford & Westfall (1994) add that lightning frequency and hail are rare in the western parts of the biome, where the two study sites are located.
1.3.2 Vegetation Type
The study site is located within the Fynbos biome, a biome renowned for its floristic diversity. The fynbos biome is characterised by the co dominance of evergreen, sclerophyllous plants that do not exceed three metres in height (Rutherford & Westfall. 1994). According to Cowling & Richardson (1995), the driving forces of this biome are the summer droughts, lack of soil nutrients, recurring fire and the wind. These forces have an effect on how the fynbos plants appear and how they live. Within the fynbos biome, many different vegetation types exist depending on the geology, soil and other environmental conditions occurring at a particular area.
The vegetation type of the study site is classified as Cape Flats Dune strandveld. Cowling et al (2003) describe strandveld as a dense to open shrubland of medium height (0.5 – 1.5 m) and a biomass of 3500 – 8250 g m-2. Cowling et al (2003) add that the vegetation in this vegetation type is mostly sclerophyllis, deciduous and evergreen plants with shrub, grass and restiod growth forms. Mucina & Rutherford (2006) add that the shrub structure of this vegetation type is extremely low especially when it is closer to the seashore due to stunting from salt spray, high winds and extreme conditions.
188.8.131.52 Distribution of Cape Flats Dune Strandveld
184.108.40.206 Landscape Features
220.127.116.11 Geology and soils
18.104.22.168 Important taxa
22.214.171.124 Threats to this vegetation type
1.4 RESEARCH OBJECTIVES
1. Determine what small ground-dwelling fauna species are currently using the area by using pitfall traps. These data will be used to compare changes in fauna diversity before and after rehabilitation.
2. Remove the majority of the invasive M. cordifolia and other invasive plant species from the area and appropriate control measures such as herbicide to prevent the re-growth of these species.
3. Record all plant species currently occurring in the Salt Marsh area to form baseline data to monitor the changes in floral composition after the removal of M. cordifolia.
4. Determine what fauna species is using the area after the rehabilitation by using pitfall traps and visual observations.
5. Monitor the site for any re-growth of invasive plant species.
6. Recommend indigenous plant species that are compatible with the area.
7. Use the data from this to draw up a management plan for the site.
MATERIALS AND METHODS
The pitfall traps were arranged approximately two meters away from each other and in a circular fashion. The pitfall traps were arranged at the edges of M. cordifolia stands in order to monitor the ground-dwelling fauna species currently living among M. cordifolia. The pitfall traps were checked twice a day for a period of ten days.
Removing M. cordifolia and alien invasive species:
Rooikrans (Acacia cyclops) was also removed using the same method described above.
Monitoring flora and fauna:
The study area was also monitored for re-growth of M. cordifolia on a regular basis. If the plant had re-sprouted, the plant would undergo the same removal procedures described above. The study area was also monitored for the re-growth or seedlings of alien invasive plant species.
Several observations were noted in field notes as well as communications with invasive plant specialists. Pitfall traps were also placed in the same designated places after the M. cordifolia stands had been removed. These pitfall traps would aid in monitoring changes in fauna composition after the stands were removed.
Table 1 Assessment of ground-dwelling fauna in the Salt Marsh site
Removing M. cordifolia and alien invasive plant species:
All flora species noted in the study site after the removal of M. cordifolia is attached as Appendix 1.
Fauna related responses:
In terms of avian responses to the removal of M. cordifolia, twice the number of bird species was observed in the study area. A large proportion of these bird species were successfully hunting in the study area and were using different levels of the plant community.
In terms of amphibian responses to the removal of M. cordifolia, a larger number of frog species was noted for the area as well as an increase in the population of common frogs in the study area.
A larger number of insect species were also noted in the area after the removal of M. cordifolia and they were also had longer periods of activity.
In terms of mammal responses to the removal of M. cordifolia, more tracks were noted in the area by Cape grysbok (Raphicerus melanotis). Sightings of small rodents were also reported in the study area.
All animal species and their occurrence in the study site are attached as Appendix 2.
Ectotherms (such as insects, reptiles and amphibians) in particular were noted to rapidly change in terms of species composition and populations. Their longer periods of activity were possibly the result into the change in the microclimate. Dense stands of M. cordifolia cause colder microclimates because of the immense shade throughout the day and do not favour ectotherms. Once M. cordifolia was removed, the sun could heat up more open sand and the warmer microclimate was more suitable for these ectotherms. The removal of M. cordifolia also provided these ectotherms with more room to hunt and breed successfully.
The increase in insect species and periods of activity correspond with periods of activity of other ectotherms and insect feeding birds. This indicates that insects form the basis of most food webs in the study area. The increase in the number of insect species would mean that there is more food available to other animal species.
From field observations, bird species also hunted more successfully in the study site. The removal of M. cordifolia provided more space for the birds to manoeuvre around while hunting and more space for birds that make their nests on the ground. The removal of M. cordifolia also opened up different levels in the canopy of the plants. This provided more niches for different bird species to occupy and therefore increased the diversity of bird species in the area.
Since more tracks were noted in the area by R. melanotis, it suggests that the study area had become more accessible now that the dense stands of M. cordifolia had been removed.
In terms of flora responses to the removal of M. cordifolia, the facts that so many seedlings from different plant species were present after the removal of the invasive species indicate that conditions were not ideal for these seedlings to grow in previous conditions. Previously, these seedlings could not compete with M. cordifolia for resources such as nutrients, space and sunlight. However, once M. cordifolia was removed, these resources became available to the seedlings and helped them to grow.
From the results of the field observations, there was a significant increase in not only the number of species but the number of individuals per species as well. This indicates that dense stands of M. cordifolia have a negative effect on the diversity of ground-dwelling fauna species. The dense stands of M. cordifolia affect the microclimate of these ground-dwelling fauna species and limit their activity. Once M. cordifolia was removed, the ambient temperature of the microclimate was higher because the sun had more surface area to heat up. The warmer temperature was more favourable to the ground-dwelling fauna species because they were mostly ectotherms. The warmer microclimate also promoted higher species diversity of insects as well as longer periods of activity by insects. The availability of these insects supported many more food webs within the study site.
The floral diversity of the study site also had a positive response to the removal of M. cordifolia. Seedlings of various plant species started to grow in response to the availability of space, sunlight and nutrients. By removing the invasive M. cordifolia, the different levels of the plant community were restored and this opened new niches to fauna species, particularly bird species. Like any invasive plant, M. cordifolia has to be monitored on a regular basis for regrowth in order to successfully rehabilitate the study site. The study site has been disturbed by removing M. cordifolia and is more prone to invasion by alien plant species such as Acacia saligna (Port Jackson) and Acacia cyclops (rooikrans). However, through careful monitoring and applying the appropriate eradication methodology, the Salt Marsh area of the Greater Zandvlei Nature Reserve can be successfully rehabilitated.
Sheasby (pers comm, 2008) recommends that Euphorbia marlothianii should also be put back into the area from Rondevlei. Other annuals and bulbs typically occurring Cape Flats dune strandveld can also be introduced in the area to increase plant diversity. The species listed above are all species that are well adjusted to coastal sands and would survive the environmental conditions at the study site as well as keep the veld open.
The author would also recommend that pitfall traps be placed in the area during similar climate conditions (late summer-early autumn) next year for a statistically valid measure of biodiversity changes of ground dwelling fauna over a longer period of time.
APPENDIX 1: PLANT SPECIES LIST PRIOR TO THE REMOVAL OF M. cordifolia
APPENDIX 2: FAUNA SPECIES LIST AND OCCURRENCE WITHIN THE SALT MARSH AREA
X = PRESENT