Distribution of a Fruiting Invasive in the Dwesa-Cwebe Nature Reserve
Background
The introduction of Psidium guajava (guava) into the Dwesa-Cwebe Nature reserve was human induced, and its’ distribution continues to be influenced by anthropogenic land disturbances in addition to the many other unique inhabitants of the reserve. Under the South Africa National Biodiversity Act, the Eastern Cape Park and Tourism Board (ECPTA) is required to implement management plans to control invasive species within its nature reserves. This is in an attempt to minimize the threat to biodiversity alien species pose to the natural and native flora. The Dwesa-Cwebe Nature Reserve, the location of this study site, is recognized by Conservation International for having the highest tree richness of all temperate forests on the planet, for it is part of the Maputaland-Pondoland-Albany biodiversity hotspot. According to Conservation International, this “biodiversity hotspot has the highest tree richness (600 tree species) of all temperate forests on the planet (“Maputaland-Pondoland-Albany”). In order to preserve native species within biodiversity-rich areas, excluding processes that threaten biodiversity and supporting those that maintain or promote biodiversity should be the goal of conservation (Cowling 191).
Invasive alien plant species are an increasing threat to preserving biodiversity due to their sometimes competitive nature, thus hindering or even eliminating native plant species. An outlet for the establishment of non-native species is created when land is disturbed, increasing the availability of soil, sunlight, nutrients and water. Land disturbance is defined as any activity that changes the physical landform, vegetation and hydrology, creates bare soil, or otherwise may cause erosion or sedimentation. The disturbance could be anthropogenic, such the building of a residence, a road, or recreational outdoor courts, which requires the clearing of trees and the removal of soils, or typical in developing countries, “slash and burn” agriculture; or the disturbance may be of natural cause such as fire, flood, and wind. In the case of the Dwesa-Cwebe Nature Reserve, the historically frequent anthropogenic disturbances relating to the establishment of a hotel within the reserve to accommodate tourists, have facilitated the establishment and spread of many highly invasive plant species within the reserve including triffid weed (Chromolaena odorata), lantana (Lantana camara), mauritius thorn (Caesalpinia decapatala), bugweed (Solanum mauritianum), inkberry (Cestrum laevigatum), and common guava (Psidium guajava) (Eastern (1) 3).
The Eastern Cape Parks and Tourism Agency (ECPTA) has undergone a complete analysis of many of the highly invasive species listed, but only preliminary data about P. guajava's distribution has been collected. No invasive shrub removal has been initiated by the Eastern Cape Parks and Tourism to date in the reserve. Psidium guajava is fruit-bearing, facilitating rapid dispersal and is widely tolerant of drought, heat, frost, sea spray, and poor soil composition, suggestive of its potential to become invasive and out-compete native species. (Henderson; Yadava (2)). For invasive plants with abundant dispersers, fruit removal rates are likely to steadily increase with increasing infestation size (Sargent 1289). Moreover, habitats heavily invaded by fleshy-fruited plants, such as the common guava, are likely to receive more seeds than less infested habitats, thereby increasing the number of invasive shrubs. A mutualistic relationship has formed between those who consume the guava’s sweet pulp, and the shrub itself; the fruit provides nutrition to the consumer, and the consumer disperses the seeds (Gosper 554). Research has shown that the number of mutualisms a species maintains and that species invasive potential is positively correlated (Gosper 550). Already, the common guava dominates along roadways and other disturbed patches of terrain on the reserve. It is important that the removal of the guava be marked a priority for removal in order to preserve this marvel of biodiversity in the Eastern Cape. After all, the primary purpose stated in the Dwesa-Cwebe Integrated Management Plan (2009), in regard to invasive species management, is the “conservation of this unique biodiversity” (Eastern (2) 19).
In this analysis, I have mapped the locations of P. guajava shrubs along 7 km of the roadways and trail ways within the Cwebe side of the Dwesa-Cwebe Nature Reserve. This data is baseline data, which will allow the ECPTA to make better informed decisions regarding the control and removal of invasive P. guajava shrubs. The aim of this research was to create a comprehensive map of P. guajava shrub distribution within Cwebe to (a) facilitate the removal and/or control of the shrub by the ECPTA, and to (b) document the patterns of guava distribution in the unique context of Cwebe, which is located in close proximity to rural and impoverished networks of communities, that are denied access to forest resources within the reserve boundaries. To map the species distribution in Cwebe, Trimble GeoXM GPS software was used to locate all guava shrubs along 7 km of roadways. Transects were used to investigate the intrusion of guava in forest interiors, and a detailed literature review was used to explore the primary agents of dispersal (i.e. humans, vervet monkeys, birds, cattle). It is important that one understand the dispersing agents of the common guava seed because of the unique context; high in biodiversity and adjacent to a major river, the Dwesa-Cwebe Nature Reserve is located in close proximity to rural and impoverished networks of communities, which are restricted access to harvesting resources within the reserve. Unlike the communities farther north along the main road into the reserve, it can be argued that these communities have been marginalized by conservation efforts.
Study Site
Located on a twin reserve in the Eastern Cape Providence of South Africa, along the Wild Coast, the study site chosen was both convenient as well as particularly interesting for this study of invasive species. The Dwesa- Cwebe Nature reserve was not established in 1992, although the forests had been demarcated as state forests since 1891 and 1893. The Cwebe reserve is 2200 hectares (ha) and is effectively isolated from the larger Dwesa reserve by the Mbashe River (Eastern (1) 1). Arable potential of the soil in the Dwesa-Cwebe Nature Reserve area is low, with soil types ranging from deep sandy loam to loamy clay soil over eroded shale (Eastern (1) 9). The reserve receives moderate to heavy rainfall, receiving most precipitation in the summer months, mid-October through February. The mean annual rainfall is 965 mm, which is just below the species preferred precipitation of 1000 mm (Samson 1986 in Henderson), and the flora in Cwebe is divided semi-equally between indigenous Coastal Scarp forest and Transkei Coastal Belt grassland (Eastern (1) 9). Elders in the community have said that the area was predominately grassland in the past, rather than equal parts grassland and coastal forest. This shift in grass-forest composition could be the result an increasing fire interval by the change in land use (to a protected area) more than a century ago.
The Haven hotel, established in the early twentieth century to provide white Transkeian residents the opportunity for a seaside holiday in 1922, is located on the Cwebe reserve, 1 km north of the Mbashe River (Dennison 169). The source of the guava introduction to the Dwesa-Cwebe Nature Reserve, according to Clive Dennison, was a guava fruit brought to and then planted in a garden shortly after the Haven’s establishment (Dennison 180). The fruit has since spread rapidly and is now a major environmental weed. There is a dense thicket of the P. guajava adjacent to the hotel, roughly 100 m2 in size, and located just south of the Haven in the direction of the Mbashe River. This thicket is also located at a forest edge and adjacent to very disturbed turf.
The histories of the former occupants’ actions, in addition to the inhabitants of the reserve today, influence the invasive guava’s distribution on the Cwebe reserve through fragmentation of forests and the disturbance of land to create roadways, trail ways, and additional housing. Roadways within the reserve date back prior to the Haven’s establishment; however, countless anthropogenic disturbances have occurred since 1921. For example, in 1968, there was a shift in management from businessman Gordon Hill to Transkei residents Matthew Dold and Mercia Mitchell (Dennison 171). To better appeal to vacationers, several improvements were made to the property. These improvements included the construction of a new reception office, an oceanfront golf course/ landing strip, and a dozen new guestrooms (Dennison 171). The Haven hotel has a pool and a tennis court, too. Other anthropogenic disturbances near the hotel include three, 50-80 meter-long pathways through the coastal forest from the Haven hotel to the beach, and an area historically used for sand removal. “The badland”, a name coined by current Haven operator, Grant Millar, exists just outside the Haven gate. Harsh in appearance and overturned in surface, the (100m2) patch of grassland was the source of deposits of sand withdrawn for use in local communities and also to maintain the coastal beach near the Haven. Prior to the arrival of present Haven managers, Grant and Nicola Millar, in 2005, masses of debris including glass bottles, kitchen appliances, and porcelain dinnerware, have been thrown haphazardly in the grassland, seen in Figure 2. In August of 2010, a fire was set within the reserve boundaries, and this “badland” burned. This heavily disturbed patch has since become densely populated with the common guava shrub in addition to other invasive and non-native species.
In addition to disturbed patches of land becoming heavily invaded with P. guajava, the pattern of guava being found along roadways also prevailed. In particular, the road from the main gate of the reserve to the Haven hotel was of interest due to the heavy traffic flowing to and from the reserve on this one roadway. There is a constant exchange of people and resources coming and going along this roadway. The hotel employs residents of neighboring communities to care for the hotel and guests, which commute to work on foot. Directly outside of the Cwebe reserve gates, north of the Mbashe River, are several communities including the closest, Cwebe.
The neighboring communities are granted unquestioned access to the reserve, but they are allowed to harvest zero natural resources within its boundaries. In 2004, households per capita income in Cwebe, a community adjacent to the reserve, were between R100 and R199 per month, just under US$1 a day (Timmermans 69). These communities are mostly subsistence farmers and livestock owners, with 84.5% of households deriving income from cropping and 76.7% from livestock care in Cwebe (Timmermans 88). The entire reserve border has been set aside for residential homes and the grazing of livestock. Because the reserve has little to no fencing, and a lack of a “buffer zone” outside of the reserve, these forest edges are likely being harvested for poles and fuel wood, and are inhabited by cattle. Cattle, which will be discussed later, are an important P. guajava distributor.
Methods
Roadways and trail ways are priority survey areas, because disturbed habitats (such as forest edges) experience more frequent fruit removal occurrences (Galetti et al., 200), are favored feeding sites for many frugivorous birds, and are therefore likely to receive proportionately larger amounts of invasive plant seed ((With, 2002) in Gosper, 553). Because there is variance in human, animal, and vehicle traffic along the roadways within the reserve, I surveyed the road most traveled, the road least traveled, and one road moderately traveled. The road stretching from the Haven hotel to the main gate, is the primary route taken by all Haven staff, and is traveled most often. Because those who are employed by the hotel either live at their homes the community or they live at in housing just outside of the gate, they walk this 3 km road to the Haven each morning. In addition, the road is heavily traveled by vehicles, being the primary route to both the Haven as well as the mouth to the Mbashe River. The road from the hotel to the Mbashe R. was also surveyed, because it is traveled semi often by locals and tourists, and less by vehicle. It is also an area most rich in animal inhabitants and seed dispersers (i.e. birds and vervet monkeys). And last, I selected a roadway traveled far less often than the other two, which branches off of the main road north, traveling in a direction opposite of the Mbashe River. Although this road does connect the village of Cwebe to the reserve, Cwebe is a considerable distance away (> 8 km) and staff often reside in the housing provided just outside of the reserve gate. In addition to these roadways, I surveyed the two trail ways cutting through the coastal dune forest from the Haven lawn. Each pathway is about 40 m long, and the two are separated by ~20 m of tree line, which was also surveyed.
Several observations were made at each site where a P. guajava shrub was located in addition to the GPS coordinates using Trimble GeoXM software. These included the estimated height shrubs estimated or relative height (in meters), the presence of fruit, and whether or not the shrub showed signs of harvesting. This data is to be used as a tool by the reserve’s managers informing them of the invasive guava species’ distribution in the problem areas. Although little to no research has been done exploring the successful invasion of P. guajava into forests, Thamsanqa Doctor Empire Mpanza found that guava seeds do not successfully establish in forests (unless those forests are overgrazed by livestock) (Mpanza 111).
A second sampling technique was employed, this time to determine the range of the species invasion, if any at all, into the forest’s interior. Although little to no research has been done exploring the successful invasion of P. guajava into forests, Thamsanqa Doctor Empire Mpanza found that guava seeds do not successfully establish in forests (Mpanza 111). In order to investigate this phenomenon, twelve transects, each 10 m in length, were spaced 500 m apart, perpendicular to the road. Transects alternated sides of the road, from left to right. A distance of 500 m were chosen by dividing the total distance (in kilometers) of roadway to be sampled, by the desired number of transects to be explored, twelve. [6 km/ 0.5 km= 12 transects.] A vehicle was used to space the transects apart. A meter tape was run perpendicular from the roadway to the 10 m mark and then placed on the ground. Perpendicular to that tape, parallel to the road, another meter tape was run 2 m across, forming a “T” with the 10 meter tape. The 2 m wide tape was used to reference the buffer zone of 1 m, from each side of the tape for the total transect distance of 10 m. The entire transect was 20 square meters.
Results
In total, more than 5,142 shrubs were recorded. Along all 12 transects observed, P. guajava shrubs were found along 2. The other 10 lacked guava both along the transects in addition to in the surrounding area. The two transects which yielded P. guajava shrubs, (1 and 8) were both located in open, or semi-open areas. Transect number 1, was located 500 m from the Haven hotel, and within 20 square meter area, 14 guava shrubs were found. The tallest shrub was 3 m tall and the shortest was 1.7 m. The shrub located the farthest distance from the road was 6.3 meters inward. Along transect number 8, located 3500 m from the Haven, one guava shrub was noted in the transect area. However, the location of this transect was not within the forest, but rather in an open grassland area, which can be observed by referencing map (c) in Figure 4. A possible source of error may have been the quantitative, equi-distance spacing of the transects. As expected, guava was restricted to roadways and was not found in adjacent forests. Perhaps it would have been more informing if transects were located only in sections along roadways where guava was already present. Psidium guajava shrubs tend to be concentrated near a parent plant. Therefore, where there is one large (>2 m) shrub, there are many (~3) within just one or two meters from that more mature plant. If there are no shrubs in an area, there will not be any shrubs several meters into the forest.
Discussion
The abundance of guava along roadways frequented by people and animals suggests the importance of dispersal in its dominated distribution, but also highlights the potentially critical role of the fruit as a diet supplement for both humans and wildlife. Preferring riverine forests, vervet monkeys (Chlorocebus), semi-terrestrial and semi-arboreal, are particularly fond of guava, and are one of the many consumers of the fleshy fruit residing in the reserve forests (Cawthon). Because Cwebe is bordered by the Mbashe River and the area is forested, vervet monkeys are abundant. Not to mention, the monkeys have no natural predators within the reserve. Although the nutritional requirements of the species Chlorocebus have been under-researched, vervet monkeys are known in raiding crops, and are particularly fond of guava (Cawthon). These inhabitants are an important disperser of the guava seeds, for they have to potential to disperse the seeds not only once through consumption, but twice. Once a seed is passed from the fruit to the ground, the seed becomes a candidate for secondary dispersal. There are many circumstances that may lead to a secondary dispersal of seeds including ripe fruit falling onto the ground or birds discarding seeds rather than ingesting them. Guava seeds are considerably large (>0.7 cm) and are therefore more likely to be discarded by birds rather than gulped, so the occurrence of guava seeds being on the ground and then transferred on the bottom of an animal’s foot is another possible mode of dispersal.
Greater than 290 bird species have been recorded on the reserve, and this number has likely increased (http://www.wildcoast.co.za/dwesa). Bird-plant mutualisms occur when dispersers shift their foraging patterns to match the location of the invasive plant species (Gosper 550). Although not much pattern shifting occurs, because the guava shrubs are located in areas when the landscape has already been disturbed, and birds favor disturbed habitats, these mutualisms are prevalent; the diverse collection of bird species within the reserve likely has a considerable impact on the dispersal of the seeds. Researchers in the Karoo of South Africa concluded in their study on the dispersal of non-native prickly pears (Opuntia ficus-indica) that the invasive flora density was substantially greater (800 times greater) under telegraph and transmission poles (Dean 305). This applies directly to this study, for the prickly pear is also a fruity invasive species. Telegraph lines within the Cwebe Nature Reserve trace the roadway precisely, as is common in most forested areas. Therefore, the birds within the reserve, if gulping the guava seeds, are a primary dispersing agent of the invasive shrub, and the shrubs predominance along roadways could be due to the birds’ preferences of fragmented habitats and perching locations.
Cattle are both a primary and a secondary disperser of common guava seeds in the nature reserve. The Global Invasive Species Database lists cattle as dispersers of P. guajava, but only in the Galapagos Islands; however, the cattle in the vicinity of the Dwesa-Cwebe Nature Reserve are definite dispersers of the shrub’s seeds. The consumption of guava seeds by cattle has been documented in T. D. E. Mpanza’s Master’s thesis on the impacts of livestock on rehabilitating post-mining forest in Zululand in South Africa. Cattle are attracted to the Cwebe reserve grasslands because they are less grazed, and access to the reserve is open due to the lack of fencing. With 76.7% of the community of Cwebe deriving some source of income from livestock goods and services (Timmermans 74), cattle are abundant. Not to mention, the communal land directly outside of the reserve, seen in Figure 3, has been set aside for grazing. The Cwebe side of the twin Dwesa-Cwebe Nature Reserve has historically been ground used by the communities for emergency grazing, in times of drought (Timmermans 161). The cattle often travel to the coast of the reserve along the same route as those who commute to work at the Haven each day, along the main road, likely snacking on the guava as they pass through. More research is needed to determine the extent of livestock’s role in the dispersal of P. guajava.
The community members in addition to the owners and operators of the Haven hotel are additional dispersing agents for the invasive shrub. The fruiting shrub makes delicious jams and jellies, and the shrubs’ removal would likely have dynamic impacts on both social as well as ecological systems. It is important to consider the livelihoods of the neighboring communities; the role of the nutritious guava fruit in the lives of the local Xhosa people and their nutrition restricted diets needs to be discussed. The main diseases afflicting the people of Dwesa/Cwebe were water and nutrition related diseases (i.e. scabies, diarrhea, kwashiorkor, stunting, tuberculosis) (Timmermans 79). The guava fruit is an important food crop as well as medicinal plant. The fruit pulp is very nutritious, for it is rich in vitamin C, dietary fiber, protein, calcium, and potassium(Yadava (1)). In South Africa, the leaf is used to treat a plethora of ailments, predominantly diarrhea and diabetes (Gutiérrez 4; Oh 411). The nearest health clinics from the community of Cwebe are located in Nkanya and Manganyela, both are more than a three hour walk by foot, and the second required a taxi (Timmermans 59). The communities in the near vicinity of the Cwebe Nature Reserve are mostly relying on indigenous knowledge and folk medicine; therefore, the removal of the P. guajava shrub could have detrimental effects on both human health in addition to nutrition. The mutualistic relationships that have formed between the non-native P. guajava shrub and its’ consumers (i.e. humans and animals) allow one another to remain viable, as seen in Figure 4.
Specific trends in guava distribution within the Cwebe Nature Reserve can be noted. These include the shrubs’ presence along roadways, forest edges, and in disturbed patches of turf. The lack of P. guajava in forest interiors indicates an interaction between the availability of light and successful seed germination. And finally, the combination and abundance of guava seed dispersing agents are unique to this study, and can be useful in comparing shrub distribution in other areas of the world where agent composition differs.
Further Research
While collecting GPS coordinates, I took notice to multiple locations where large and fruitful guava shrubs were distributed in a perfect circle around a previously cut guava shrub. The P. guajava shrub removal effort by Grant Millar near the hotel, including the cutting and then painting of chemicals onto the trunk base, proved to be unsuccessful. Innovative and experimental removal techniques need to be researched, criticized, and made available for public use. Also, because the invasive guava dominates in disturbed patches and fragmented landscapes, a more intensive survey which can relate P. guajava shrub distribution to the severity or type of land disturbance would be beneficial for the literature. Given that Dwesa-Cwebe is a critical case-study for exploring post-Apartheid conservation and land management issues, the removal of the shrubs likely would have complex socio-ecological implications. The complex relationship of the invasive guava shrubs in a mosaic of socio-ecological systems should be evaluated prior to the removal of the species.
The introduction of Psidium guajava (guava) into the Dwesa-Cwebe Nature reserve was human induced, and its’ distribution continues to be influenced by anthropogenic land disturbances in addition to the many other unique inhabitants of the reserve. Under the South Africa National Biodiversity Act, the Eastern Cape Park and Tourism Board (ECPTA) is required to implement management plans to control invasive species within its nature reserves. This is in an attempt to minimize the threat to biodiversity alien species pose to the natural and native flora. The Dwesa-Cwebe Nature Reserve, the location of this study site, is recognized by Conservation International for having the highest tree richness of all temperate forests on the planet, for it is part of the Maputaland-Pondoland-Albany biodiversity hotspot. According to Conservation International, this “biodiversity hotspot has the highest tree richness (600 tree species) of all temperate forests on the planet (“Maputaland-Pondoland-Albany”). In order to preserve native species within biodiversity-rich areas, excluding processes that threaten biodiversity and supporting those that maintain or promote biodiversity should be the goal of conservation (Cowling 191).
Invasive alien plant species are an increasing threat to preserving biodiversity due to their sometimes competitive nature, thus hindering or even eliminating native plant species. An outlet for the establishment of non-native species is created when land is disturbed, increasing the availability of soil, sunlight, nutrients and water. Land disturbance is defined as any activity that changes the physical landform, vegetation and hydrology, creates bare soil, or otherwise may cause erosion or sedimentation. The disturbance could be anthropogenic, such the building of a residence, a road, or recreational outdoor courts, which requires the clearing of trees and the removal of soils, or typical in developing countries, “slash and burn” agriculture; or the disturbance may be of natural cause such as fire, flood, and wind. In the case of the Dwesa-Cwebe Nature Reserve, the historically frequent anthropogenic disturbances relating to the establishment of a hotel within the reserve to accommodate tourists, have facilitated the establishment and spread of many highly invasive plant species within the reserve including triffid weed (Chromolaena odorata), lantana (Lantana camara), mauritius thorn (Caesalpinia decapatala), bugweed (Solanum mauritianum), inkberry (Cestrum laevigatum), and common guava (Psidium guajava) (Eastern (1) 3).
The Eastern Cape Parks and Tourism Agency (ECPTA) has undergone a complete analysis of many of the highly invasive species listed, but only preliminary data about P. guajava's distribution has been collected. No invasive shrub removal has been initiated by the Eastern Cape Parks and Tourism to date in the reserve. Psidium guajava is fruit-bearing, facilitating rapid dispersal and is widely tolerant of drought, heat, frost, sea spray, and poor soil composition, suggestive of its potential to become invasive and out-compete native species. (Henderson; Yadava (2)). For invasive plants with abundant dispersers, fruit removal rates are likely to steadily increase with increasing infestation size (Sargent 1289). Moreover, habitats heavily invaded by fleshy-fruited plants, such as the common guava, are likely to receive more seeds than less infested habitats, thereby increasing the number of invasive shrubs. A mutualistic relationship has formed between those who consume the guava’s sweet pulp, and the shrub itself; the fruit provides nutrition to the consumer, and the consumer disperses the seeds (Gosper 554). Research has shown that the number of mutualisms a species maintains and that species invasive potential is positively correlated (Gosper 550). Already, the common guava dominates along roadways and other disturbed patches of terrain on the reserve. It is important that the removal of the guava be marked a priority for removal in order to preserve this marvel of biodiversity in the Eastern Cape. After all, the primary purpose stated in the Dwesa-Cwebe Integrated Management Plan (2009), in regard to invasive species management, is the “conservation of this unique biodiversity” (Eastern (2) 19).
In this analysis, I have mapped the locations of P. guajava shrubs along 7 km of the roadways and trail ways within the Cwebe side of the Dwesa-Cwebe Nature Reserve. This data is baseline data, which will allow the ECPTA to make better informed decisions regarding the control and removal of invasive P. guajava shrubs. The aim of this research was to create a comprehensive map of P. guajava shrub distribution within Cwebe to (a) facilitate the removal and/or control of the shrub by the ECPTA, and to (b) document the patterns of guava distribution in the unique context of Cwebe, which is located in close proximity to rural and impoverished networks of communities, that are denied access to forest resources within the reserve boundaries. To map the species distribution in Cwebe, Trimble GeoXM GPS software was used to locate all guava shrubs along 7 km of roadways. Transects were used to investigate the intrusion of guava in forest interiors, and a detailed literature review was used to explore the primary agents of dispersal (i.e. humans, vervet monkeys, birds, cattle). It is important that one understand the dispersing agents of the common guava seed because of the unique context; high in biodiversity and adjacent to a major river, the Dwesa-Cwebe Nature Reserve is located in close proximity to rural and impoverished networks of communities, which are restricted access to harvesting resources within the reserve. Unlike the communities farther north along the main road into the reserve, it can be argued that these communities have been marginalized by conservation efforts.
Study Site
Located on a twin reserve in the Eastern Cape Providence of South Africa, along the Wild Coast, the study site chosen was both convenient as well as particularly interesting for this study of invasive species. The Dwesa- Cwebe Nature reserve was not established in 1992, although the forests had been demarcated as state forests since 1891 and 1893. The Cwebe reserve is 2200 hectares (ha) and is effectively isolated from the larger Dwesa reserve by the Mbashe River (Eastern (1) 1). Arable potential of the soil in the Dwesa-Cwebe Nature Reserve area is low, with soil types ranging from deep sandy loam to loamy clay soil over eroded shale (Eastern (1) 9). The reserve receives moderate to heavy rainfall, receiving most precipitation in the summer months, mid-October through February. The mean annual rainfall is 965 mm, which is just below the species preferred precipitation of 1000 mm (Samson 1986 in Henderson), and the flora in Cwebe is divided semi-equally between indigenous Coastal Scarp forest and Transkei Coastal Belt grassland (Eastern (1) 9). Elders in the community have said that the area was predominately grassland in the past, rather than equal parts grassland and coastal forest. This shift in grass-forest composition could be the result an increasing fire interval by the change in land use (to a protected area) more than a century ago.
The Haven hotel, established in the early twentieth century to provide white Transkeian residents the opportunity for a seaside holiday in 1922, is located on the Cwebe reserve, 1 km north of the Mbashe River (Dennison 169). The source of the guava introduction to the Dwesa-Cwebe Nature Reserve, according to Clive Dennison, was a guava fruit brought to and then planted in a garden shortly after the Haven’s establishment (Dennison 180). The fruit has since spread rapidly and is now a major environmental weed. There is a dense thicket of the P. guajava adjacent to the hotel, roughly 100 m2 in size, and located just south of the Haven in the direction of the Mbashe River. This thicket is also located at a forest edge and adjacent to very disturbed turf.
The histories of the former occupants’ actions, in addition to the inhabitants of the reserve today, influence the invasive guava’s distribution on the Cwebe reserve through fragmentation of forests and the disturbance of land to create roadways, trail ways, and additional housing. Roadways within the reserve date back prior to the Haven’s establishment; however, countless anthropogenic disturbances have occurred since 1921. For example, in 1968, there was a shift in management from businessman Gordon Hill to Transkei residents Matthew Dold and Mercia Mitchell (Dennison 171). To better appeal to vacationers, several improvements were made to the property. These improvements included the construction of a new reception office, an oceanfront golf course/ landing strip, and a dozen new guestrooms (Dennison 171). The Haven hotel has a pool and a tennis court, too. Other anthropogenic disturbances near the hotel include three, 50-80 meter-long pathways through the coastal forest from the Haven hotel to the beach, and an area historically used for sand removal. “The badland”, a name coined by current Haven operator, Grant Millar, exists just outside the Haven gate. Harsh in appearance and overturned in surface, the (100m2) patch of grassland was the source of deposits of sand withdrawn for use in local communities and also to maintain the coastal beach near the Haven. Prior to the arrival of present Haven managers, Grant and Nicola Millar, in 2005, masses of debris including glass bottles, kitchen appliances, and porcelain dinnerware, have been thrown haphazardly in the grassland, seen in Figure 2. In August of 2010, a fire was set within the reserve boundaries, and this “badland” burned. This heavily disturbed patch has since become densely populated with the common guava shrub in addition to other invasive and non-native species.
In addition to disturbed patches of land becoming heavily invaded with P. guajava, the pattern of guava being found along roadways also prevailed. In particular, the road from the main gate of the reserve to the Haven hotel was of interest due to the heavy traffic flowing to and from the reserve on this one roadway. There is a constant exchange of people and resources coming and going along this roadway. The hotel employs residents of neighboring communities to care for the hotel and guests, which commute to work on foot. Directly outside of the Cwebe reserve gates, north of the Mbashe River, are several communities including the closest, Cwebe.
The neighboring communities are granted unquestioned access to the reserve, but they are allowed to harvest zero natural resources within its boundaries. In 2004, households per capita income in Cwebe, a community adjacent to the reserve, were between R100 and R199 per month, just under US$1 a day (Timmermans 69). These communities are mostly subsistence farmers and livestock owners, with 84.5% of households deriving income from cropping and 76.7% from livestock care in Cwebe (Timmermans 88). The entire reserve border has been set aside for residential homes and the grazing of livestock. Because the reserve has little to no fencing, and a lack of a “buffer zone” outside of the reserve, these forest edges are likely being harvested for poles and fuel wood, and are inhabited by cattle. Cattle, which will be discussed later, are an important P. guajava distributor.
Methods
Roadways and trail ways are priority survey areas, because disturbed habitats (such as forest edges) experience more frequent fruit removal occurrences (Galetti et al., 200), are favored feeding sites for many frugivorous birds, and are therefore likely to receive proportionately larger amounts of invasive plant seed ((With, 2002) in Gosper, 553). Because there is variance in human, animal, and vehicle traffic along the roadways within the reserve, I surveyed the road most traveled, the road least traveled, and one road moderately traveled. The road stretching from the Haven hotel to the main gate, is the primary route taken by all Haven staff, and is traveled most often. Because those who are employed by the hotel either live at their homes the community or they live at in housing just outside of the gate, they walk this 3 km road to the Haven each morning. In addition, the road is heavily traveled by vehicles, being the primary route to both the Haven as well as the mouth to the Mbashe River. The road from the hotel to the Mbashe R. was also surveyed, because it is traveled semi often by locals and tourists, and less by vehicle. It is also an area most rich in animal inhabitants and seed dispersers (i.e. birds and vervet monkeys). And last, I selected a roadway traveled far less often than the other two, which branches off of the main road north, traveling in a direction opposite of the Mbashe River. Although this road does connect the village of Cwebe to the reserve, Cwebe is a considerable distance away (> 8 km) and staff often reside in the housing provided just outside of the reserve gate. In addition to these roadways, I surveyed the two trail ways cutting through the coastal dune forest from the Haven lawn. Each pathway is about 40 m long, and the two are separated by ~20 m of tree line, which was also surveyed.
Several observations were made at each site where a P. guajava shrub was located in addition to the GPS coordinates using Trimble GeoXM software. These included the estimated height shrubs estimated or relative height (in meters), the presence of fruit, and whether or not the shrub showed signs of harvesting. This data is to be used as a tool by the reserve’s managers informing them of the invasive guava species’ distribution in the problem areas. Although little to no research has been done exploring the successful invasion of P. guajava into forests, Thamsanqa Doctor Empire Mpanza found that guava seeds do not successfully establish in forests (unless those forests are overgrazed by livestock) (Mpanza 111).
A second sampling technique was employed, this time to determine the range of the species invasion, if any at all, into the forest’s interior. Although little to no research has been done exploring the successful invasion of P. guajava into forests, Thamsanqa Doctor Empire Mpanza found that guava seeds do not successfully establish in forests (Mpanza 111). In order to investigate this phenomenon, twelve transects, each 10 m in length, were spaced 500 m apart, perpendicular to the road. Transects alternated sides of the road, from left to right. A distance of 500 m were chosen by dividing the total distance (in kilometers) of roadway to be sampled, by the desired number of transects to be explored, twelve. [6 km/ 0.5 km= 12 transects.] A vehicle was used to space the transects apart. A meter tape was run perpendicular from the roadway to the 10 m mark and then placed on the ground. Perpendicular to that tape, parallel to the road, another meter tape was run 2 m across, forming a “T” with the 10 meter tape. The 2 m wide tape was used to reference the buffer zone of 1 m, from each side of the tape for the total transect distance of 10 m. The entire transect was 20 square meters.
Results
In total, more than 5,142 shrubs were recorded. Along all 12 transects observed, P. guajava shrubs were found along 2. The other 10 lacked guava both along the transects in addition to in the surrounding area. The two transects which yielded P. guajava shrubs, (1 and 8) were both located in open, or semi-open areas. Transect number 1, was located 500 m from the Haven hotel, and within 20 square meter area, 14 guava shrubs were found. The tallest shrub was 3 m tall and the shortest was 1.7 m. The shrub located the farthest distance from the road was 6.3 meters inward. Along transect number 8, located 3500 m from the Haven, one guava shrub was noted in the transect area. However, the location of this transect was not within the forest, but rather in an open grassland area, which can be observed by referencing map (c) in Figure 4. A possible source of error may have been the quantitative, equi-distance spacing of the transects. As expected, guava was restricted to roadways and was not found in adjacent forests. Perhaps it would have been more informing if transects were located only in sections along roadways where guava was already present. Psidium guajava shrubs tend to be concentrated near a parent plant. Therefore, where there is one large (>2 m) shrub, there are many (~3) within just one or two meters from that more mature plant. If there are no shrubs in an area, there will not be any shrubs several meters into the forest.
Discussion
The abundance of guava along roadways frequented by people and animals suggests the importance of dispersal in its dominated distribution, but also highlights the potentially critical role of the fruit as a diet supplement for both humans and wildlife. Preferring riverine forests, vervet monkeys (Chlorocebus), semi-terrestrial and semi-arboreal, are particularly fond of guava, and are one of the many consumers of the fleshy fruit residing in the reserve forests (Cawthon). Because Cwebe is bordered by the Mbashe River and the area is forested, vervet monkeys are abundant. Not to mention, the monkeys have no natural predators within the reserve. Although the nutritional requirements of the species Chlorocebus have been under-researched, vervet monkeys are known in raiding crops, and are particularly fond of guava (Cawthon). These inhabitants are an important disperser of the guava seeds, for they have to potential to disperse the seeds not only once through consumption, but twice. Once a seed is passed from the fruit to the ground, the seed becomes a candidate for secondary dispersal. There are many circumstances that may lead to a secondary dispersal of seeds including ripe fruit falling onto the ground or birds discarding seeds rather than ingesting them. Guava seeds are considerably large (>0.7 cm) and are therefore more likely to be discarded by birds rather than gulped, so the occurrence of guava seeds being on the ground and then transferred on the bottom of an animal’s foot is another possible mode of dispersal.
Greater than 290 bird species have been recorded on the reserve, and this number has likely increased (http://www.wildcoast.co.za/dwesa). Bird-plant mutualisms occur when dispersers shift their foraging patterns to match the location of the invasive plant species (Gosper 550). Although not much pattern shifting occurs, because the guava shrubs are located in areas when the landscape has already been disturbed, and birds favor disturbed habitats, these mutualisms are prevalent; the diverse collection of bird species within the reserve likely has a considerable impact on the dispersal of the seeds. Researchers in the Karoo of South Africa concluded in their study on the dispersal of non-native prickly pears (Opuntia ficus-indica) that the invasive flora density was substantially greater (800 times greater) under telegraph and transmission poles (Dean 305). This applies directly to this study, for the prickly pear is also a fruity invasive species. Telegraph lines within the Cwebe Nature Reserve trace the roadway precisely, as is common in most forested areas. Therefore, the birds within the reserve, if gulping the guava seeds, are a primary dispersing agent of the invasive shrub, and the shrubs predominance along roadways could be due to the birds’ preferences of fragmented habitats and perching locations.
Cattle are both a primary and a secondary disperser of common guava seeds in the nature reserve. The Global Invasive Species Database lists cattle as dispersers of P. guajava, but only in the Galapagos Islands; however, the cattle in the vicinity of the Dwesa-Cwebe Nature Reserve are definite dispersers of the shrub’s seeds. The consumption of guava seeds by cattle has been documented in T. D. E. Mpanza’s Master’s thesis on the impacts of livestock on rehabilitating post-mining forest in Zululand in South Africa. Cattle are attracted to the Cwebe reserve grasslands because they are less grazed, and access to the reserve is open due to the lack of fencing. With 76.7% of the community of Cwebe deriving some source of income from livestock goods and services (Timmermans 74), cattle are abundant. Not to mention, the communal land directly outside of the reserve, seen in Figure 3, has been set aside for grazing. The Cwebe side of the twin Dwesa-Cwebe Nature Reserve has historically been ground used by the communities for emergency grazing, in times of drought (Timmermans 161). The cattle often travel to the coast of the reserve along the same route as those who commute to work at the Haven each day, along the main road, likely snacking on the guava as they pass through. More research is needed to determine the extent of livestock’s role in the dispersal of P. guajava.
The community members in addition to the owners and operators of the Haven hotel are additional dispersing agents for the invasive shrub. The fruiting shrub makes delicious jams and jellies, and the shrubs’ removal would likely have dynamic impacts on both social as well as ecological systems. It is important to consider the livelihoods of the neighboring communities; the role of the nutritious guava fruit in the lives of the local Xhosa people and their nutrition restricted diets needs to be discussed. The main diseases afflicting the people of Dwesa/Cwebe were water and nutrition related diseases (i.e. scabies, diarrhea, kwashiorkor, stunting, tuberculosis) (Timmermans 79). The guava fruit is an important food crop as well as medicinal plant. The fruit pulp is very nutritious, for it is rich in vitamin C, dietary fiber, protein, calcium, and potassium(Yadava (1)). In South Africa, the leaf is used to treat a plethora of ailments, predominantly diarrhea and diabetes (Gutiérrez 4; Oh 411). The nearest health clinics from the community of Cwebe are located in Nkanya and Manganyela, both are more than a three hour walk by foot, and the second required a taxi (Timmermans 59). The communities in the near vicinity of the Cwebe Nature Reserve are mostly relying on indigenous knowledge and folk medicine; therefore, the removal of the P. guajava shrub could have detrimental effects on both human health in addition to nutrition. The mutualistic relationships that have formed between the non-native P. guajava shrub and its’ consumers (i.e. humans and animals) allow one another to remain viable, as seen in Figure 4.
Specific trends in guava distribution within the Cwebe Nature Reserve can be noted. These include the shrubs’ presence along roadways, forest edges, and in disturbed patches of turf. The lack of P. guajava in forest interiors indicates an interaction between the availability of light and successful seed germination. And finally, the combination and abundance of guava seed dispersing agents are unique to this study, and can be useful in comparing shrub distribution in other areas of the world where agent composition differs.
Further Research
While collecting GPS coordinates, I took notice to multiple locations where large and fruitful guava shrubs were distributed in a perfect circle around a previously cut guava shrub. The P. guajava shrub removal effort by Grant Millar near the hotel, including the cutting and then painting of chemicals onto the trunk base, proved to be unsuccessful. Innovative and experimental removal techniques need to be researched, criticized, and made available for public use. Also, because the invasive guava dominates in disturbed patches and fragmented landscapes, a more intensive survey which can relate P. guajava shrub distribution to the severity or type of land disturbance would be beneficial for the literature. Given that Dwesa-Cwebe is a critical case-study for exploring post-Apartheid conservation and land management issues, the removal of the shrubs likely would have complex socio-ecological implications. The complex relationship of the invasive guava shrubs in a mosaic of socio-ecological systems should be evaluated prior to the removal of the species.
Figure 1: A diagram displaying how land disturbance can lead to a decrease in native species populations.
Figure 2. Image showing a disturbed patch of land near the Haven Hotel.
Figure 3: A betterment map, displaying the intended land use type for Cwebe and Ntubeni, just outside of the Cwebe Nature Reserve (Timmermans 40).
Figure 4.1. Overview of the central portion of the Cwebe Nature reserve where guava was sampled.
Figure 4.2. Guava shrubs in the historic garden. Largest shrubs were also observed here, validating the hypothesis that the source of introduction was in this garden.
Figure 4.3. Guava shrubs in at the intersection of two roadways. A highly disturbed area.
Figure 5: A diagram displaying the dynamic relationship between humans and animals and the guava.
Figure 6. Image of the common guava (Psidium guajava).
REFERENCES
[1] Cawthon Lang KA. 2006 January 3. Primate Factsheets: Vervet (Chlorocebus) Taxonomy, Morphology, & Ecology . <http://pin.primate.wisc.edu/factsheets/entry/vervet>. Accessed 2012 July 21.
[2] Cowling, R.M (07/01/2003). "A conservation plan for a global biodiversity hotspot—the Cape Floristic Region, South Africa". Biological conservation (0006-3207), 112 (1-2), p. 191.
[3] Dean, W.R.J. & Milton, S.J. (2000) Directed dispersal of Opuntia species in the Karoo, South Africa: are crows the responsible agents? Journal of Arid Environments, 15, 305–311.
[4] Dennison, Clive. A History of the Wild Coast. KwaZulu-Natal: Brevitas, 2010. Print.
[5] Eastern Cape Parks and Tourism Agency. Integrated Reserve Management Plan: Dwesa-Cwebe Nature Reserve. South Africa, June 2009. Print.
[6] Eastern Cape Parks and Tourism Agency. Invasive Alien Plants—Strategic Management Plan for Dwesa Cwebe Nature Reserve. South Africa, 2009. Print.
[7] Gosper, C. R., Stansbury, C. D. and Vivian-Smith, G. (2005), Seed dispersal of fleshy-fruited invasive plants by birds: contributing factors and management options. Diversity and Distributions, 11: 549–558. Print.
[8] Gutiérrez, Rosa Martha Pérez (01/01/2008). "Psidium guajava: A review of its traditional uses, phytochemistry and pharmacology". Journal of ethnopharmacology(0378-8741), 117(1), p.1. Print.
[9] Henderson, S., 2006. Psidium guajava (shrub, tree). Global Invasive Species Database, Web. 10 April 2012. <http://www.issg.org/pdf/publications/worst_100/english_100_worst.pdf>
[10]Lang KA. 2006 January 3. Primate Factsheets: Vervet (Chlorocebus) Taxonomy, Morphology, & Ecology. Web. 30 March 2012.
[11] “Maputaland-Pondoland-Albany.” The Hotspots. Conservation International, Web. 20 April 2012. <http://www.conservation.org/where/priority_areas/hotspots/africa/Maputaland-Pondoland-Albany/Pages/default.aspx>
[12] Morton, J. 1987. Guava. p. 356–363. In: Fruits of warm climates. Julia F. Morton, Miami, FL. Print.
[13] Mpanza, T.D.E., 2008. Impacts of livestock on rehabilitating post-mining dune forest in Zululand. MSc Thesis. University of Zululand, Empangeni. South Africa. Print.
[14] Oh, Won Keun (01/01/2005). "Antidiabetic effects of extracts from Psidium guajava". Journal of ethnopharmacology(0378-8741), 96(3), p.411.
[15] Psidium guajava (Linn.) Joseph, B. and R.M. Priya, 2011. Phytochemical and biopharmaceutical aspects of Psidium guajava (L.) essential oil: A review. Res. J. Med. Plant, 5: 432-442.
[16] Sargent, Sarah. (Aug. 1990). “Neighborhood Effects on Fruit Removal by Birds: A Field Experiment with Viburnum Dentatum (Caprifoliaceae)” Sarah Sargent Ecology, Vol. 71, No. 4, pp. 1289-1298.
[17] Timmermans, H. Rural Livelihoods at Dwesa-Cwebe: Poverty, Development, and Natural Resource Use on the Wild Coast, South Africa. M.Sc. Thesis (Grahamstown: Rhodes University, 2004). Print.
[18] Yadava, U.L. 1996. Guava production in Georgia under cold-protection structure. p. 451-457. In: J. Janick (ed.), Progress in new crops. ASHS Press, Arlington, VA.
[19] Yadava, UL. (09/01/1996). "Guava (Psidium guajava L): An exotic tree fruit with potential in the southeastern United States". HortScience (0018-5345), 31(5), p.789. Print.
[1] Cawthon Lang KA. 2006 January 3. Primate Factsheets: Vervet (Chlorocebus) Taxonomy, Morphology, & Ecology . <http://pin.primate.wisc.edu/factsheets/entry/vervet>. Accessed 2012 July 21.
[2] Cowling, R.M (07/01/2003). "A conservation plan for a global biodiversity hotspot—the Cape Floristic Region, South Africa". Biological conservation (0006-3207), 112 (1-2), p. 191.
[3] Dean, W.R.J. & Milton, S.J. (2000) Directed dispersal of Opuntia species in the Karoo, South Africa: are crows the responsible agents? Journal of Arid Environments, 15, 305–311.
[4] Dennison, Clive. A History of the Wild Coast. KwaZulu-Natal: Brevitas, 2010. Print.
[5] Eastern Cape Parks and Tourism Agency. Integrated Reserve Management Plan: Dwesa-Cwebe Nature Reserve. South Africa, June 2009. Print.
[6] Eastern Cape Parks and Tourism Agency. Invasive Alien Plants—Strategic Management Plan for Dwesa Cwebe Nature Reserve. South Africa, 2009. Print.
[7] Gosper, C. R., Stansbury, C. D. and Vivian-Smith, G. (2005), Seed dispersal of fleshy-fruited invasive plants by birds: contributing factors and management options. Diversity and Distributions, 11: 549–558. Print.
[8] Gutiérrez, Rosa Martha Pérez (01/01/2008). "Psidium guajava: A review of its traditional uses, phytochemistry and pharmacology". Journal of ethnopharmacology(0378-8741), 117(1), p.1. Print.
[9] Henderson, S., 2006. Psidium guajava (shrub, tree). Global Invasive Species Database, Web. 10 April 2012. <http://www.issg.org/pdf/publications/worst_100/english_100_worst.pdf>
[10]Lang KA. 2006 January 3. Primate Factsheets: Vervet (Chlorocebus) Taxonomy, Morphology, & Ecology. Web. 30 March 2012.
[11] “Maputaland-Pondoland-Albany.” The Hotspots. Conservation International, Web. 20 April 2012. <http://www.conservation.org/where/priority_areas/hotspots/africa/Maputaland-Pondoland-Albany/Pages/default.aspx>
[12] Morton, J. 1987. Guava. p. 356–363. In: Fruits of warm climates. Julia F. Morton, Miami, FL. Print.
[13] Mpanza, T.D.E., 2008. Impacts of livestock on rehabilitating post-mining dune forest in Zululand. MSc Thesis. University of Zululand, Empangeni. South Africa. Print.
[14] Oh, Won Keun (01/01/2005). "Antidiabetic effects of extracts from Psidium guajava". Journal of ethnopharmacology(0378-8741), 96(3), p.411.
[15] Psidium guajava (Linn.) Joseph, B. and R.M. Priya, 2011. Phytochemical and biopharmaceutical aspects of Psidium guajava (L.) essential oil: A review. Res. J. Med. Plant, 5: 432-442.
[16] Sargent, Sarah. (Aug. 1990). “Neighborhood Effects on Fruit Removal by Birds: A Field Experiment with Viburnum Dentatum (Caprifoliaceae)” Sarah Sargent Ecology, Vol. 71, No. 4, pp. 1289-1298.
[17] Timmermans, H. Rural Livelihoods at Dwesa-Cwebe: Poverty, Development, and Natural Resource Use on the Wild Coast, South Africa. M.Sc. Thesis (Grahamstown: Rhodes University, 2004). Print.
[18] Yadava, U.L. 1996. Guava production in Georgia under cold-protection structure. p. 451-457. In: J. Janick (ed.), Progress in new crops. ASHS Press, Arlington, VA.
[19] Yadava, UL. (09/01/1996). "Guava (Psidium guajava L): An exotic tree fruit with potential in the southeastern United States". HortScience (0018-5345), 31(5), p.789. Print.