Floating invasive plant dominance
- Nutrient loading (regional, well-established): The external nutrient loading into a system decreases success of biological control agents which maintains the regime of floating plant dominance (Coetzee and Hill, 2012).
- Population of floating plants: As plants grow, older leaves decompose and provide nutrients for more plant growth.
- Light (local, well established): As biomass of floating plants decreases light in the water column increases which enables photosynthesis of submerged plants.
Submerged invasive plant dominance
- Sediment stabilization (local, documented): As rooted submerged plants establish and spread, they can stabilize the sediment and decrease sedimentary re-suspension leading to clearer waters and maintaining their growth and this regime (Yarrow et al.,2009).
- Dissolved oxygen levels (local, well established): As biomass of submerged plants increases, levels of dissolved oxygen in the water also increase. This temporarily improves the water quality which in turn facilitates further plant growth.
Shift from floating invasive to submerged invasive plant dominance
- Floods and strong winds (local, observed): Natural shocks such as floods and strong winds can remove mats of floating plants that have dominated systems for years. This leads to a sudden change in opportunities for resource competition, access to light and space.
The main external direct drivers that contribute to the shift include:
- Biological control of floating plants (local, proposed): Biological control of invasive floating plants rapidly remove organisms that were previously dominant. This increases light availability to below-surface biodiversity and leads to a rapid increase of nutrients via the decomposition of the floating plants.
- Nutrient loading post bio-control (local/regional): Intensive and mismanaged anthropogenic activities such as poor disposal of waste water and chemicals can speed up the process by which submerged invasive species can utilize the newly accessible resources gained from the decomposition of the floating plants.
- Seasonal temperature fluctuations (local/regional/global, well established): Changes in temperature can affect the population densities of the biological control agents. Optimum temperatures for the specific BCAs can lead to rapid plant death speeding up the regime shift. Conversely detrimental conditions could lead to the death of the bio-control agents leading to a more resilient floating plant regime.
- Increased human activity: Intensive agricultural practices can lead to increased nutrient loading.
- Aquaria trade: Many submerged invasive aquatic plants, even ones that have been categorized as an offence to own or trade, are readily available throughout South Africa via the aquaria trade (Martin and Coetzee, 2011). The lack of monitoring surrounding this issue has helped the establishment of numerous species in South Africa.
The main external indirect drivers that contribute to the shift include:
Summary of Drivers
|#||Driver (Name)||Type (Direct, Indirect, Internal, Shock)||Scale (local, regional, global)||Uncertainty (speculative, proposed, well-established)|
|1||Biological control of floating plants||Shock/External Direct Driver||Local||Observed/Proposed|
|3||Nutrient loading||External direct||Local/regional||Well established|
|4||Anthropogenic activity||External indirect||Local/regional||Well established|
|5||Aquaria trade||External indirect||Global||Well established|
|6||Seasonal temperature fluctuations||External direct||Regional||Well established|
Floating invasive system to submerged invasive
BCA population: As the population of biological control agents crosses a threshold from steady control to significant collapse of the floating plant population
Damage threshold: The level of damage caused by the BCAs at which point a sudden plant collapse occurs.
Water nutrient threshold: The point at which the level of nutrients that are released from decaying floating plants are available for submerged invasive plants to acquire them.
Shading threshold:Threshold at which light levels in water column are sufficient enough to allow submerged plants to photosynthesize and establish.
Nutrient loading: By reducing levels of eutrophication alongside the control of invasive floating plants we can increase resilience against colonization from submerged plants.
Native flora: Increasing local levels of native vegetation (via seed banks and plant stocking) in systems before the control of the floating plants is underway could increase resilience as there would be less resources available for invasive plants to utilize.
Bio-control agent reserve sites: If the population of floating plants is completely eradicated the population of control agents will also become locally extinct. If the submerged invasive plants were then removed or controlled, and local conditions were not altered, there could be a shift back to regime one and this time there are no agents to control the plant population. Ensuring that a small reserve of the floating invasive plant is maintained will allow the control agents to persist locally and they will be able to continue controlling the floating plant if there are future population increases.
Ecosystem service impacts
This regime shift leads to diminished access to quality freshwater. Livelihoods that are dependent on freshwater biodiversity, such as fishing and eco-tourism are also compromised (Charles and Dukes, 2007). As are all activities that depend on access to freshwater for irrigation and livestock. . Many farmers have lost livestock to drowning as they perceive large mats of floating plants to be solid underfoot (McConnachie et al, 2003). Hydroelectric pumps are damaged, once again limiting water access, and the costs related to repairing these and to the mechanical/chemical control of the submerged plants can be substantial.
Besides impacts of economic activities and livelihoods, this regime shift also directly impacts human well being. Invasive aquatic plants play a key role harboring vectors of diseases such as schistosomiasis (bilharzia) and malaria (Mack and Smith, 2011). Continued mismanagement of invasive species as perpetuated by poor understanding of the systems, can lead to incorrect spending of state funding potentially affecting a wider cross-section of people and communities than those directly affected. Protection against natural disasters are also affected, as floods defenses are compromised by the plants which alter water flow and can increase water levels by raising sedimentation.
Summary of Ecosystem Service impacts on different User Groups
||References (if available)|
|Feed, Fuel and Fibre Crops||-||Yes||Yes||Yes||Yes|
|Wild Food & Products||?|
|Air Quality Regulation||0|
|Soil Erosion Regulation||0|
|Pest & Disease Regulation||-||Yes||Yes||Yes||Yes|
|Protection against Natural Hazards||-||Yes||Yes||Yes||Yes|
|Cognitive & Educational||0|
|Spiritual & Inspirational||-||Yes||Yes|