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Invasive floating to invasive submerged plant dominance

Main Contributors:

Emily Strange

Other Contributors:

Prof. Julie Coetzee, Julie Coetzee

Summary

Man-made dams and lakes can be highly vulnerable to colonization from floating non-native invasive plants. These plants can form dense mats restricting light to the water column, damaging hydroelectric equipment as well as reducing water quality and biodiversity. The implementation of classical biological control (CBC) programs has been a widely beneficial tool in the control of floating invasive plants. CBC uses host specific natural enemies (bio-control agents) of the invasive plant to control plant populations in invaded ranges. The overall aim is to induce a regime shift into a functioning system with high native biodiversity and freshwater access. However, we propose that whilst the bio-control agents do lead to a dramatic reduction in the biomass and health of the floating plant, it can also act as a catalyst inducing a shift into a second degraded stable regime. This second regime is dominated by submerged invasive plants. The establishment of bio-control agents on the floating plants can lead to a rapid plant population crash and the release of the nutrients they locked up. The increase in space, light and nutrients enables rapid growth of invasive submerged plants. These species can alter water flow, turbidity, sedimentation and can degrade water quality and biodiversity, restrict access to freshwater and damage hydro-electrical equipment.

Drivers

Key direct drivers

  • External inputs (eg fertilizers)
  • Species introduction or removal
  • Environmental shocks (eg floods)

Land use

  • Urban
  • Small-scale subsistence crop cultivation
  • Large-scale commercial crop cultivation
  • Intensive livestock production (eg feedlots)
  • Extensive livestock production (rangelands)
  • Conservation
  • Tourism

Impacts

Ecosystem type

  • Freshwater lakes & rivers

Key Ecosystem Processes

  • Nutrient cycling
  • Water cycling

Biodiversity

  • Biodiversity

Provisioning services

  • Freshwater
  • Livestock
  • Fisheries
  • Hydropower

Regulating services

  • Water purification
  • Natural hazard regulation

Cultural services

  • Recreation
  • Aesthetic values
  • Knowledge and educational values

Human Well-being

  • Food and nutrition
  • Health (eg toxins, disease)
  • Livelihoods and economic activity
  • Cultural, aesthetic and recreational values

Key Attributes

Typical spatial scale

  • Local/landscape

Typical time scale

  • Years

Reversibility

  • Unknown

Evidence

  • Models
  • Contemporary observations
  • Experiments

Confidence: Existence of RS

  • Speculative – Regime shift has been proposed, but little evidence as yet

Confidence: Mechanism underlying RS

  • Well established – Wide agreement on the underlying mechanism

Alternate regimes

Invasive floating plant dominance

This regime is characterized by dense, mono-specific mats of free-floating invasive plants that cover vast areas of the water’s surface. The water below lacks adequate light for many species to survive and biodiversity is restricted. These mats can completely conceal the water-body beneath and as they are free floating they can be displaced by flash floods and strong winds, often damaging hydroelectric equipment and causing a sudden change in ecosystem structure and function.

Invasive submerged plant dominance

Dense monocultures of rooted submerged invasive plants dominate the system, often not seen until they reach the water’s surface by which point they are usually problematic. Native vegetation is out competed, and whilst initially these plants can push up levels of dissolved oxygen and improve water quality they rapidly grow to a state where only the plants near the waters’ surface are healthy and just below the water is turbid and hypoxic. Water flow is also limited and pumping mechanisms can become clogged and damaged, increasing flood risk. These plants often reproduce via fragmentation and can easily be spread through river systems by wildlife, people, boats and often by the machines used to mechanically clear the plants from invaded areas. 

Drivers and causes of the regime shift

Shift from floating invasive to submerged invasive plant dominance

The main causes of the regime shift, we believe, are the biological control of floating invasive plants in freshwater systems. Nutrients that were previously locked within the floating invasive plants are now available for submerged invasive plants to acquire. Another major factor is the intrinsic nature of these systems, they are man-made and historically not a common feature of South African topography meaning there are fewer native species to readily occupy them. As the floating plants are controlled there may not be a good stock of native submerged plants to utilize the resources and instead resilience is very low in this unstable state. Poorly regulated human activities such as waste water treatment and unregulated agricultural endeavors have also led to problematically high nutrient loading in many of these systems.

How the regime shift works

Shift from floating invasive to submerged invasive plant dominance

The invasive floating plant regime occurs in man-made, impounded freshwater systems, and has been frequently observed in places such as South Africa. The invasive plants have often been intentionally introduced via the aquarium and ornamental trade, and un-intentionally via ‘hitchhiking’ on other species and aquatic machinery. In many cases external nutrient loading facilitates their growth and minimizes the impact of biological control agents therefore perpetuating the regime. The floating plants quickly form mats that block light to the water column, which can reduce the presence of other aquatic plants that might compete for resources, again maintaining their dominance. Eventually they can alter the whole ecosystem structure and function diminishing the presence of and access to quality freshwater.

Once invasive floating plants are dominant biological control agents may be introduced to control the floating plants, with the hope of inducing a regime shift to a clear water system with healthy biodiversity and good quality water. However, the regime shift we believe is happening in many systems is quite different. As the floating plants decompose dues to the effects of control agents the system receives a sudden influx of freely available nutrients, and at the same time light levels within the water column are restored. As nutrients, light and space become available submerged invasive plants are able to utilize the resources and establish.

The submerged plants’ ability to photosynthesize temporarily increases the levels of dissolved oxygen in the water column thus improving water quality (short-term) and allowing the submerged plants to become dominant. After this period of improvement, the water quality deteriorates as the plant biomass crosses a threshold after which it begins to block out light, reduce biodiversity and alter sediment stability. External nutrient loading further facilitates their growth and helps to sustain the new invasive submerged plant regime. 

Impacts on ecosystem services and human well-being

Shift from floating invasive to submerged invasive plant dominance

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 plants as perpetuated by poor understanding of the systems, leads 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.

Management options

Potentially, once the dominating invasive submerged plants are controlled (manually, physically or biologically) there could be a shift back to the floating plant dominance if the environmental conditions that facilitated the floating plants have not changed or been managed and if no ‘reserve’ of the floating plant has been left to support a population of it’s bio-control agents.

By reducing levels of eutrophication alongside the control of invasive floating plants we can increase resilience against colonization from submerged plants. We also propose that 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.

Key References

  1. Charles, H., Dukes J.S. 2007. Impacts of invasive species on ecosystem services. Biological Invasions; Ecological Studies (193)
  2. Coetzee et al., 2011 Prospects for the biological control of submerged macrophytes in South Africa. African Entomology : Biological control of invasive alien plants in South Africa (1999 - 2010) : Special Issue 2
  3. Coetzee, J., Hill M.P. 2012. The role of eutrophication in the biological control of water hyacinth, Eichhornia crassipes, in South Africa. BioControl 57:247-261
  4. Hill, M.P., 2002. The impact and control of alien aquatic vegetation in South African aquatic ecosystems. African Journal of Aquatic Science 28 (1): 19-24
  5. Mack, RN., and Smith, MC.,2011 Invasive plants as catylsts for the spread of human parasites. NeoBiota 9: 13-29
  6. Martin, GD., and Coetzee, JA. Pet stores, aquarists and the internet trade as modes of introduction and spread of invasive macrophytes in South Africa. Water SA [online]. 2011, vol.37, n.3, pp. 371-380. ISSN 0378-4738
  7. McConnachie, J., de Wit,MP., Hill, M.P.,Byrne, M.J., Economic evaluation of the successful biological control of Azolla filiculoides in South Africa, Biological Control, 28 (1) ISSN 1049-9644
  8. Moorhouse and Mcdonald, 2015. Are invasives worse in freshwater than terrestrial ecosystems? Wiley Periodicals
  9. Yarrow,. M., Marin, V.H., Finlayson, M., Tironi, A., Delgado L.E. and Fishcher F. 2009. The ecology of Egeria densa Planchon (Liliopsida: Alismatales): A wetland ecosystem engineer? Revista Chilena de Historia Natural 82: 299-313

Citation

Emily Strange, Prof. Julie Coetzee, Julie Coetzee. Invasive floating to invasive submerged plant dominance. In: Regime Shifts Database, www.regimeshifts.org. Last revised 2017-01-25 07:37:11 GMT.
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