My recent post, The Whale Trail in the Pandemic, has attracted enormous traffic, by the standards of my site at least. In it I opine that biological control of Acacia cyclops (rooikrans) in De Hoop Nature Reserve is failing because my – admittedly unscientific and subjective – observations of its spread over 19 years of walking The Whale Trail indicate “exponential” infestation. A windsurfing acquaintance of many years ago, who now lives in Switzerland, picked the story up via our LinkedIn connection, and sent it to his friends with whom he had hiked the trail three years ago. One of them knows Prof John Hoffmann of the Plant Conservation Unit at UCT, who is an expert in biological control of aliens, and forwarded it to him. The resultant email response was shared with me again. So I contacted Prof Hoffmann and asked if I could quote his responses in this blog post, because I think it is important that readers of my original post read the expert, scientific perspective as well.
In summary, Prof Hoffmann’s message is optimistic and encouraging: rooikrans is on the backfoot in De Hoop and elsewhere and biological control is working, based on scientists’ data analysis. He points out that although rooikrans seeds prolifically, seed mortality is high in the soil. Fire destroys 90% of the seeds and predation by mice reduces the number of seeds that germinate to a minimum. With the introduction of the gall midge and seed weevil, the numbers of seeds being produced has declined enormously which has tipped the scales against rooikrans.
He notes that my deduction that biological control takes a long time to become effective, is largely correct. The gall midge and seed weevil do not kill the trees. Some external intervention such as fire or mechanical removal is necessary to do that. Based on the fact that rooikrans thickets burn every 15 to 20 years, it will take two fire events, thus 30 to 40 years, before any change becomes obvious. By the time the second fire occurs, there are few seeds left in the soil and more are destroyed by the second fire so that there are insufficient seeds left to replenish the burnt trees. Monitoring progress thus requires sustained, longitudinal studies by scientists over a long period – a period equivalent to the whole research career of an invasion biologist I would think.
Obviously, my opportunistic observations are unscientific and subjective. Scientists have proposed four parameters to categorise biological control outcomes1:
- Density – the number of trees in a defined area;
- Biomass – the amount of plant material in a defined area;
- Area – the extent of coverage by the trees;
- Rate of spread – changes in the amount of time taken for the trees to expand into uninvaded patches or beyond the boundaries of their current distribution.
The four parameters are assessed in three outcome categories following sustained observations by experts, by comparing rooikrans’ status according to the parameters before and after the establishment of the biological agent. It sounds a little complicated and it’s a long-term exercise. There appears to be little published data as present because the studies are ongoing. So we have to trust the experts, who have access to the data, when they tell us that the data is showing significant effects on rooikrans.
I speculate that my observations might roughly speak to Area and, at a stretch, Rate of Spread: the area that I observed is larger than before and the spread seems to have accelerated in recent years. However, I don’t have any empirical data to back this sense up with – and so I am left with some cognitive dissonance.
- V. Cliff Moran, Costas Zachariades, John H. Hoffmann,
Implementing a system in South Africa for categorizing the outcomes of weed biological control,
After CapeNature’s Executive Director: Conservation Operations read my blog posts, Dr Alan Wheeler, Regional Manager (Landscape Manager) for CapeNature’s Southern Landscape, which stretches from the Bot River in the west to the Gouritz River in the east and northwards to about in line with De Doorns, contacted me. I quote his message below, with his permission. Note the reference to insufficient funding for alien eradication and control, an important point that I also picked up in the free ebook, Biological Invasions in South Africa, by Brian van Wilgen et al:
“I asked our Landscape Conservation Intelligence Team (basically our scientific team) to go through your article and to provide information that may be useful feedback for you. Below is what they submitted to me as feedback for you. I hope you find this informative and if you have any questions, please don’t hesitate to contact me.
“We have taken cognisance of the highly favourable reporting on the upgrades to the Whale Trail accommodation – stating that the term “cottage” is more appropriate than “hut” for the overnight facilities and we wish to thank you for this positive feedback and exposure to the public. We have also taken note of the concern raised regarding the levels of the infestation of alien invasive plant species, in particular Rooikrans (Acacia cyclops) within the coastal sections along the Whale Trail and wish to provide a response in this regard.
Reference is made to the newly erected signage along the Whale Trail, which includes signs regarding the biological control agents which have been released to control the alien invasive species present. One of these is for the Acacia seed weevil (Melanterius servulus) which was introduced to control Rooikrans, for which the adults and the larvae feed on the seeds. As correctly stated, the weevils do not kill the plant but reduce the seed bank. One of the reasons for the success of Rooikrans as an invasive species is the massive number of seeds produced and the long period for which the seeds remain viable, and which are stimulated to germinate by fire (although it is not essential for germination). The weevils are therefore able to address the immense seed bank that develops in the soil, particularly where there are dense infestations. De Hoop is one of the study sites for monitoring of the long term impact of the biological control agents on the target species undertaken by the Plant Protection Research Institute (PPRI) within the Agricultural Research Council (ARC) and includes a monitoring site where Melanterius servulus has been released on Rooikrans. The monitoring takes places twice a year both before and after the seeds are dropped from the trees (“seed rain”) and there is a significant reduction in the number of seeds in the seedbank comparing post-seed rain to pre-seed rain. The study has indicated that there is a steady decline in the seed bank over the 12 year period taking into account annual fluctuations, however there is still a considerable seed bank remaining . We do wish to note that biological control does not ever result in the total eradication of the target species as the agent will die out once the food source dies out and instead reduces the population to a manageable level. It is however also a method of controlling aliens which requires significantly less resources than other methods.
It is important to note that although the signage focuses on the biological control agents, this is not the only component of the management of the alien invasive species within De Hoop Nature Reserve, as an integrated approach is required in order to address the problem. The primary method of control of the alien invasive species is through mechanical and chemical control, mainly through chopping down of aliens and application of herbicide to ensure that the plant dies and does not resprout. It should be noted that Rooikrans is one the few alien invasive tree species prevalent within the Fynbos Biome which does not require herbicide, provided the plants are cut down at the base. This is undertaken by teams of workers and is funded by the Natural Resource Management (NRM) project within the Department of Environment, Forestry and Fisheries and is a major job creation project. The funding allocation is split between all CapeNature nature reserves as well as the rest of the country and currently the allocation for De Hoop is not sufficient to adequately address the levels of alien infestation across De Hoop. There are teams working at De Hoop and we are currently focusing on the eastern sector around Noetsie and Elandspad.
Reference was also made to the concessions given to woodcutters. This is a separate project to the alien clearing project, whereby commercial wood harvesters will harvest Rooikrans trees of sufficient size to provide firewood. The wood harvesting contracts are not located adjacent to the Whale Trail and would therefore not have been observed while on the hike. The wood harvesters are required to provide 2000 pieces a month to CapeNature which is used for the Whale trail and the remainder is sold commercially. This project does provide for local economic development as well. The nature reserve is divided into compartments to manage the alien invasive plants and the strategies for each compartment would differ e.g. a compartment which is allocated to biological control would not be put out for clearing by a team in order to provide for a reserve for the population of the biological control agent. There are compartments in the vicinity of Noetsie and Vaalkrans which are allocated to biological control of Rooikrans. The biological control and woodcutters should therefore be viewed as supplementary approaches
In summary, we do appreciate your concern for the threat presented by the alien invasive species to De Hoop Nature Reserve and its unique and precious biodiversity and acknowledge that the population levels of alien invasive species do need to be brought under control. This has been identified as a top priority management intervention to be addressed in the protected area management plan for De Hoop Nature Reserve and we are endeavouring to reduce the alien infestations through an integrated approach and attempting to identify other avenues of sourcing funding to supplement the existing initiatives. It should be noted that currently external funding sources are focusing on mountain catchment areas which are vital for increasing the water released from catchments to supply water to the Western Cape, which is a high socio-economic priority and De Hoop would not fall into this category”.”
Hi Ralph. We are a bunch of friends, who like you, have seen, over the past 20 years, how the aliens of De Hoop has been ignored and that it, has now reached a critical level. Do you have any suggestions as to how we can tackle this problem. It is obvious that Cape Nature is not capable. Surely those of us who love De Hoop, can do something together, to save such an amazing place. Regards Petro