Non-native invasive species pose a significant threat to aquatic ecosystems and can disrupt the use of invaded systems. For example, alien plants often outcompete indigenous flora and form monocultures that cannot be used by native fauna, which require a diverse habitat to thrive (Dibble et al., 1996; Jeppesen et al., 1998; Madsen, 2014). In addition to reducing the richness of the species, invasive species can modify ecosystem services. Aggressive growth of alien species can change soil texture and deplete substrate nutrient reservoirs, resulting in insufficient resource availability for native flora (Madsen, 2014). Exotic trees may produce dense canopies that reduce the amount of light available for photosynthesis by understory plants. A similar situation occurs in aquatic ecosystems; rapid growth and expansion by floating [e.g., waterhyacinth (Eichhornia crassipes)], floating-leaved [e.g., crested floatingheart (Nymphoides cristata)], and canopy-forming submersed [e.g., hydrilla (Hydrilla verticillata)] introduced species can interfere with the air–water interface, thereby blocking light, reducing oxygen in the water column, and degrading the habitat for native flora and fauna (Madsen, 2014).
Dense growth of exotic plants can interfere with anthropocentric interests. Terrestrial weeds can reduce crop yields by diverting water, nutrients, and other resources from cultivated plants and cause power outages and flooding (Bridges, 1994; Duryea and Kampf, 2017). Excessive growth of terrestrial invaders can exacerbate wildfires (Brooks et al., 2004). Fast-growing weedy trees in Florida and other hurricane-prone regions may be ill-suited for heavy rains and high winds; ultimately, they may become projectiles, damage power lines, or crush residential and commercial properties (Duryea and Kampf, 2017).
Aquatic ecosystems are not immune to the hazards posed by invasive species. In fact, some argue that the problems associated with the introduction of exotic plants are more pronounced in these unique ecosystems. Crop production is somewhat uncommon in aquatic areas, but yields of aquatic food crops can be greatly reduced when invasive species are present. Weed pressure is considered a major limiting factor for rice (Oryza sativa) production, with losses that range from 10 to 100% in the absence of weed management efforts (Rao et al., 2007). Similar to terrestrial weeds, aquatic invaders pose risks to human health by creating a drowning hazard and serving as a habitat for undesirable insects [e.g., mosquitoes (Anopheles sp., Aedes sp. Culex sp., Mansonia sp., and others)] that vector human disease (Cuda, 2014). Aquatic weeds can impact human uses of waterbodies by disrupting hydropower operations, clogging irrigation intakes, blocking access to recreational resources, interfering with the use of watercraft, and reducing aesthetics (and property values) (Holm et al., 1969). Overgrowth of aquatic plants can also inhibit water movement and negatively impact flood control operations, thereby increasing the risk of catastrophic flooding of communities, farms, roadways, and other anthropocentric infrastructure during heavy rainfall events (Grant, 1962).
It is clear that invasive plants cause a wide variety of problems, but how are these species able to produce such large and troublesome populations? Exotic species can become weedy and exhibit aggressive growth in an invaded region for a number of reasons. Abiotic factors such as temperature, rainfall, and nutrient availability may be more amenable in the invaded range compared to the native habitat of the species, thus fostering growth at levels not seen in the home territory of the invader. An important biotic factor that can result in explosive growth of a non-native species is the lack of specialist biological stressors in the introduced range. This phenomenon is often referred to as the “enemy release hypothesis,” and it posits that phytopathogenic organisms (primarily fungi and bacteria) and phytophagous animals (e.g., insects) that have co-evolved with the species in question prevent aggressive growth of the plant in its native range. When a plant is introduced to a new ecosystem where these stressors are not present, they are “released” from the pressure imposed by these organisms and can experience unchecked growth in the invaded range (Keane and Crawley, 2002). This hypothesis forms the basis of classical biological control, which uses the introduction of co-evolved host-specific insects or pathogens as a means to manage invasive species. This strategy has been used with varying levels of success to reduce populations of a variety of noxious weeds, including waterhyacinth (Tipping et al., 2014), alligatorweed [Alternanthera philoxeriodes (Buckingham, 1996)], and air potato [Dioscorides bulbifera (Center et al., 2015)]. However, indigenous herbivores that are present in the invaded range are often polyphagous and feed indiscriminately on both native and exotic plants. In fact, Macel et al. (2017) suggested that exotic plants may be more susceptible to herbivory by phytophagous generalists in newly invaded regions compared to indigenous plants. Therefore, the release from herbivory is incomplete and invasion success is likely influenced by a combination of abiotic and biotic factors.
By definition, invasive species are introduced and not native to the ecosystem in which they become problematic (U.S. Department of Agriculture, 1999). However, it is becoming evident to resource managers that excessive growth of indigenous plants can cause similar ecosystem disruptions. The reasons for formerly well-behaved plants “going rogue” and overtaking formerly diverse areas are unclear, but several factors may contribute to these new growth patterns. For example, warmer average temperatures may facilitate range expansion by allowing plants to move into areas that have become newly hospitable (Upson et al., 2016). Extirpation or reduction of natural enemies may occur in response to changing environmental conditions, disease, predation, and intentional or inadvertent pest control operations. An important factor that seems to drive weedy behavior in native aquatic plants is what I term “competition release,” or targeted management of introduced invasive species. Similar to the enemy release hypothesis, competition release frees native plants from competition with exotic species and allows them access to formerly unavailable resources such as light and nutrients. If the management techniques used do not include physical removal of the undesired plant material, then additional nutrient pulses are likely to occur as the dying plants decompose (Grimshaw, 2002; Jewell, 1971), which can further subsidize excessive growth by native plants.
The most intuitive way to address invasive behavior in native aquatic plants is to discuss them in the context of introduced exotic weeds with similar growth habits. The characteristics of representative floating, floating-leaved, submersed, and littoral (shoreline and shallow water) invaders are outlined and “weedy” native plants that are analogous to these exotic species are described in this work.
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