What are Marine Invasive Species?

Marine invasive species, also known as non-native species, non-indigenous species, or alien species, are organisms that have been introduced into ecosystems where they do not historically or naturally occur. These introductions can occur either intentionally, such as through the release of ornamental species or aquaculture, or accidentally as a result of shipping activities or recreational pursuits. Once introduced and established into new environments, they have the potential to cause significant harm to local ecosystems and economies. Here we will provide a comprehensive understanding of marine invasive species, their impacts, and how they are managed.

There are many varying definitions on what constitutes an invasive species in marine waters. However, marine invasive species will typically share one or more of the following characteristics, each of which represent a significant threat to the biodiversity, stability, and overall health of the ecosystems they invade. It is essential to identify and understand these characteristics to develop effective management strategies and minimize the impacts of these species on native ecosystems.

  • Lack of natural predators: In their new environment, marine invasive species may not have any natural predators, which allows their populations to grow unchecked. Without predators to keep their numbers in balance, invasive species can rapidly multiply and dominate their new surroundings.
  • Ability to outcompete native species: Marine invasive species often possess traits that give them a competitive edge over native species, such as a higher reproduction rate, greater tolerance to environmental stress, or more efficient resource utilization. This enables them to outcompete native species for food, habitat, and other resources, leading to a decline in native populations and potentially even the loss of entire native species.
  • Rapid adaptation: In some cases, marine invasive species can quickly adapt to new environmental conditions, which allows them to thrive and spread in their new habitats. This adaptability can make them even more successful than native species, further exacerbating their impacts on local ecosystems.
  • Habitat alteration: Certain marine invasive species can physically alter their new habitats, changing the structure and function of ecosystems. For example, some invasive species may create new habitats by constructing extensive burrows or reef-like structures, while others may release toxins or other chemicals that can harm native species and disrupt ecosystem functioning.

Pathways of Introduction

Understanding and addressing the various pathways through which marine invasive species are introduced is crucial in preventing their spread and minimizing their impact on native ecosystems. By implementing targeted strategies, such as ballast water management, stricter regulations on the aquarium trade, and public education campaigns, we can reduce the risk of introducing and spreading marine invasive species.

Some of the most common pathways of introduction include:

  • Shipping: Invasive species introductions via shipping usually occur vis two main pathways, ballast water release, and ship hull fouling.
    • Ballast Water: Ballast water is used by ships to maintain stability and balance during their voyages. When a ship takes on ballast water in one location and discharges it in another, it can inadvertently transport marine organisms, including invasive species. This has been identified as one of the primary pathways for the introduction of marine invasive species.
    • Hull Fouling: Hull fouling occurs when marine organisms attach themselves to the surfaces of ships, including their hulls, rudders, and propellers. As these ships travel, they can transport these organisms to new locations where they may become invasive.
  • Aquaculture: Aquaculture, the farming of aquatic organisms for food or other products, can inadvertently introduce invasive species through escapes or intentional releases. Species may be imported for farming, and if they escape or are released into the wild, they can potentially establish themselves in new environments and become invasive.
  • Aquarium Trade: The import and export of live aquatic organisms for the aquarium and ornamental fish trade can lead to the introduction of marine invasive species. In some cases, aquarium owners may release unwanted pets into local waterways, not realizing the potential consequences. Additionally, the transport of live organisms can result in accidental releases during transit.
  • Recreational Activities: Recreational activities, such as boating, fishing, and diving, can contribute to the spread of marine invasive species. For example, organisms may hitchhike on boats, fishing gear, or dive equipment, and be introduced to new environments as these items are used in different locations. Anglers may also contribute to the spread by using live bait that is not native to the area.
  • Natural Dispersal and Climate Change: In some cases, marine invasive species may expand their range naturally, driven by factors such as ocean currents or changing environmental conditions. Climate change, in particular, may facilitate the spread of invasive species by altering ocean temperatures and currents, creating more suitable conditions for these species to establish themselves in new areas.

Ecological Impacts

The ecological impacts of marine invasive species are far-reaching and can have significant consequences for the health and stability of invaded ecosystems. A decrease in biodiversity can lead to reduced ecosystem resilience, making it more difficult for ecosystems to recover from disturbances or adapt to environmental changes. Therefore, it is crucial to identify, monitor, and manage marine invasive species to minimize their ecological impacts and preserve the integrity of native ecosystems.

Some of the key ways in which marine invasive species can disrupt ecosystems are:

  • Outcompeting native species: Marine invasive species can compete with native species for essential resources such as food and habitat. Due to their competitive advantages, such as higher reproduction rates or greater adaptability, invasive species can outcompete native species, leading to declines in native populations and, in extreme cases, local extinctions.
  • Altering food webs: The introduction of marine invasive species can cause disruptions in the intricate balance of food webs within an ecosystem. Invasive species may become new predators, preying on native species, or they may compete with native species for the same food sources. These disruptions can lead to cascading effects throughout food webs, impacting multiple species and trophic levels.
  • Changing the structure and function of ecosystems: Some marine invasive species can physically alter their environment, either directly or indirectly. For example, invasive species such as the zebra mussel can filter large volumes of water, removing plankton and affecting water clarity, which in turn can influence the distribution and abundance of other species. Other invasive species, like the European green crab, can damage habitats by digging burrows or uprooting seagrasses, leading to increased erosion and habitat loss for native species.
  • Hybridization and genetic pollution: In some cases, marine invasive species can interbreed with closely related native species, resulting in hybrid offspring. This process, known as hybridization, can lead to a loss of genetic diversity within native populations and potentially threaten the survival of native species.
  • Disease transmission: Marine invasive species can introduce new diseases or parasites to native species, which may not have evolved defenses against these novel threats. The spread of disease can have devastating effects on native populations, leading to declines in abundance or even extinctions.

Economic Impacts

The economic impacts of marine invasive species can be considerable, affecting a wide range of industries and sectors. It is crucial to implement effective prevention and management strategies to minimize these impacts and protect the economic vitality of affected communities and industries.

These economic consequences can manifest in several ways, including:

  • Damage to fisheries: Invasive species can negatively affect commercial and recreational fisheries by reducing the abundance of target species, either through direct predation or by outcompeting them for resources. This decline in target species can lead to reduced catch rates, decreased profits for fishers, and potential job losses within the industry.
  • Impacts on aquaculture: Aquaculture operations can be affected by invasive species that prey on farmed organisms, compete for resources, or introduce diseases and parasites. These impacts can lead to reduced production, increased costs for prevention and mitigation measures, and even the collapse of some aquaculture businesses.
  • Harm to tourism: The presence of invasive species can negatively impact marine tourism by damaging the aesthetic value of marine ecosystems, reducing the abundance of iconic native species, or posing direct risks to human health and safety. This can result in decreased visitor numbers and revenue for tourism-dependent businesses and communities.
  • Infrastructure damage: Some marine invasive species can cause physical damage to infrastructure, such as ports, marinas, and water supply systems. For example, invasive species like zebra mussels can attach themselves to underwater structures, leading to corrosion and fouling, which require costly maintenance and repair efforts.
  • Costs of monitoring, control, and eradication: Governments and communities often bear the financial burden of managing marine invasive species through monitoring programs, control measures, and eradication efforts. These costs can be substantial, diverting resources away from other important environmental or social initiatives.
  • Loss of ecosystem services: Marine invasive species can disrupt the provision of essential ecosystem services, such as water purification, coastal protection, and carbon sequestration. The loss of these services can have far-reaching economic consequences, affecting industries and communities that rely on healthy marine ecosystems for their livelihoods and well-being.

Human Health Impacts

Marine invasive species can pose various risks to human health, both directly and indirectly. These risks can be broadly categorized as follows:

  • Physical harm: Certain marine invasive species can cause direct harm to humans through stings, bites, or other physical interactions. For example, the warty sea squirt (Styela clava), an invasive tunicate found in some California harbors, can cause skin irritation if handled improperly. Although not venomous like the lionfish, it serves as a reminder that caution should be exercised when encountering unfamiliar marine organisms.
  • Toxin production: Some marine invasive species produce toxins that can accumulate in the tissues of fish and shellfish, leading to potential health risks when these organisms are consumed by humans. For example, the invasive Asian clam (Potamocorbula amurensis) in the San Francisco Bay can accumulate high levels of selenium, which is toxic when ingested in large quantities. This can have negative consequences for both wildlife and humans consuming the contaminated clams.
  • Disease transmission: Marine invasive species can act as vectors for the transmission of diseases to humans, either directly or indirectly. For example, the invasive Japanese seaweed (Sargassum horneri), found along the Southern California coast, can provide suitable habitat for mosquitoes that are potential disease vectors, such as the Aedes aegypti mosquito, which can transmit diseases like dengue fever and Zika virus.
  • Impacts on water quality: The presence of certain marine invasive species can negatively affect water quality, with potential implications for human health. For instance, invasive species like the European green crab (Carcinus maenas) can increase the turbidity of water by disturbing sediment, potentially affecting the balance of nutrients, and promoting the growth of harmful bacteria or algae.
  • Impacts on mental health and well-being: The presence and consequences of marine invasive species invasions can also have indirect effects on human health and well-being, as the degradation of marine ecosystems can lead to feelings of loss, anxiety, and decreased enjoyment of natural areas. This can be particularly pronounced in communities that have strong cultural or economic ties to the marine environment, such as coastal fishing communities.

Management and Prevention Strategies

By implementing a combination of early detection and rapid response, prevention and control measures, and eradication and control efforts, we can effectively manage marine invasive species and minimize their impacts on ecosystems, economies, and human health.

  • Early Detection and Rapid Response: Efficient monitoring and reporting systems are crucial in detecting invasive species early and responding rapidly to prevent their establishment and spread. Early detection involves regular monitoring and surveillance efforts, as well as public awareness and education campaigns to encourage community participation in monitoring and reporting programs. Once an invasive species is detected, rapid response measures, guided by a well-developed and coordinated management plan, must be implemented to prevent its establishment, and spread. Collaboration and coordination among different agencies, organizations, and stakeholders are vital for the success of early detection and rapid response efforts.
  • Prevention and Control Measures: Preventing the introduction of invasive species is the most effective way to protect ecosystems. Several strategies can be employed to reduce the risk of introduction, including:
    • Ballast water management: Implementing regulations and guidelines for the treatment and exchange of ballast water in ships can help prevent the unintentional transport of invasive species across different ecosystems.
    • Aquarium trade regulations: Establishing strict regulations on the import, sale, and release of non-native aquatic species in the aquarium trade can help prevent the intentional or accidental introduction of invasive species into local ecosystems.
    • Public awareness campaigns: Raising public awareness about the risks and impacts of marine invasive species can encourage responsible behavior and promote the adoption of prevention measures, such as cleaning recreational equipment or not releasing unwanted aquarium species into the wild.
  • Eradication and Control Efforts: Once an invasive species is established, control and eradication measures can be employed to minimize their impacts. Various methods can be used, depending on the specific species and the local environmental conditions:
    • Physical removal: Manual or mechanical removal of invasive species can be an effective control method for some species, particularly when populations are small and localized.
    • Biological control: Introducing natural predators or parasites that specifically target invasive species can help control their populations, although this method requires careful evaluation to avoid unintended ecological consequences.
    • Chemical control: The use of chemical treatments, such as pesticides or herbicides, can be an effective means of controlling invasive species populations, but their application must be carefully managed to minimize potential harm to non-target species and the environment.

Hotspots of Marine Invasions in California

California's extensive coastline and diverse marine ecosystems make it particularly vulnerable to marine invasive species. Certain areas along the coast, referred to as hotspots, are at a higher risk of invasion due to factors such as increased shipping activity, aquaculture operations, and recreational use. To protect California's marine ecosystems and minimize the impacts of invasive species, it is essential to focus on prevention, early detection, and rapid response measures in these hotspots. By implementing targeted management strategies in these areas, California can help safeguard its unique and valuable coastal environments from the damaging effects of marine invasive species.

  • San Francisco Bay: The San Francisco Bay is one of the most well-known hotspots for marine invasions in California. Its busy ports and marinas, combined with its diverse and productive ecosystems, make it an ideal environment for invasive species to thrive. The bay is home to a plethora of non-native species, including the European green crab (Carcinus maenas), the Asian clam (Potamocorbula amurensis), and the invasive cordgrass (Spartina spp.).
  • Southern California Bight: Stretching from Point Conception to the U.S.-Mexico border, the Southern California Bight is a hotspot for marine invasions due to its dense population centers, extensive shipping activity, and numerous marinas and harbors. Invasive species, such as the Japanese seaweed (Sargassum horneri) and the red algae (Grateloupia turuturu), have significantly impacted native ecosystems, leading to concerns about the long-term health of these coastal environments.
  • Channel Islands: The Channel Islands, located off the coast of Southern California, are also at risk of marine invasions. Their proximity to mainland harbors and the popularity of recreational boating increase the likelihood of invasive species introductions. Notable invasive species in the area include the Mediterranean fanworm (Sabella spallanzanii) and the clubbed tunicate (Styela clava).
  • Ports and Harbors: California's numerous ports and harbors, such as the Port of Los Angeles, the Port of Long Beach, and the Port of Oakland, serve as entry points for invasive species that hitchhike on the hulls of ships, in ballast water, or in shipping containers. These areas, with their high levels of shipping activity and regular exchange of goods and organisms, are particularly susceptible to new marine invasions.

Climate Change and California Marine Invasive Species

Climate change plays a significant role in the spread and establishment of marine invasive species in California by altering various environmental factors that can provide more suitable conditions for their establishment and spread. It is important that we consider the effects of climate change when developing and implementing management strategies for invasive species, as well as the need for ongoing research and monitoring to better understand and predict these complex interactions.

The impacts of climate change on marine invasive species in California can be summarized as follows:

  • Changes in water temperature: As ocean temperatures rise due to climate change, the range of some marine invasive species may expand, allowing them to establish themselves in new areas, including California's coastal waters. For example, warmer waters might allow tropical or subtropical invasive species to thrive in regions that were previously too cold for them. This could lead to an increase in the number of invasive species along the California coast and potentially exacerbate their ecological impacts.
  • Altered ocean currents: Climate change can also affect ocean currents, which play a critical role in the dispersal of marine organisms, including invasive species. Changes in current patterns may transport non-native species to new areas where they can establish themselves, including California's coastal ecosystems. Additionally, altered currents can modify the distribution of larvae and other life stages of invasive species, potentially increasing their chances of successful colonization.
  • Changes in species distribution: As native species shift their ranges in response to climate change, they may leave ecological niches vacant, creating opportunities for invasive species to establish themselves. In California, this could lead to new invasive species taking advantage of the changing conditions and outcompeting native species for resources, ultimately altering the structure and function of local marine ecosystems.
  • Changes in coastal habitats: Climate change can also impact coastal habitats, such as wetlands and estuaries, which can influence the establishment and spread of invasive species. For example, sea-level rise may lead to the loss or alteration of coastal habitats, providing new opportunities for invasive species to colonize these areas. Additionally, increased storm frequency and intensity could lead to the introduction or spread of non-native species through the disturbance of native habitats and the transport of invasive species by stormwater runoff or debris.
  • Changes in species interactions: Climate change can alter the interactions between native and invasive species by impacting factors such as predation, competition, and parasitism. For instance, changes in water temperature may affect the metabolic rates, growth rates, and reproductive success of both native and invasive species, potentially altering the balance of these ecological interactions and allowing invasive species to gain an advantage over their native counterparts.

Well-Known Marine Invasive Species in California

California's coastal and estuarine environments are highly diverse and productive ecosystems, making them attractive to marine invasive species. Factors such as increased shipping, aquaculture, and recreational activities contribute to the risk of invasion.

Here are some examples of several well-known marine invasive species that have impacted California:

  • European Green Crab: The European green crab (Carcinus maenas) is native to Europe and North Africa and has invaded California's coastal waters. This highly adaptable crab is a voracious predator that feeds on native shellfish, such as clams and mussels, causing significant ecological and economic impacts. The green crab can also damage eelgrass habitats, which provide critical nursery grounds for many fish species.
  • Asian Kelp: The Asian kelp, also known as Wakame or Undaria pinnatifida, is a large brown seaweed native to the cold temperate coastal areas of Japan, Korea, and China. It has become invasive in California, where it can rapidly grow and outcompete native kelp species for light and nutrients, ultimately altering the structure and function of kelp forest ecosystems.
  • Japanese Bubble Snail: The Japanese bubble snail (Haminoea japonica) is a small marine gastropod native to the Northwest Pacific, including Japan, Korea, and China. It has been introduced to California, where it can impact native species by preying on bivalve eggs and larvae and compete with native snails for resources. This invasive species can alter the balance of marine ecosystems and negatively affect native populations of mollusks.
  • Inland Silverside: The inland silverside (Menidia audens) is a small fish native to the Atlantic and Gulf coasts of the United States. It was intentionally introduced to California as a form of gnat and algae control, as it feeds on larvae. However, the inland silverside has had negative effects on native fish species, such as the Delta smelt, by preying on their eggs and larvae, and competing for the same food sources, including zooplankton.
  • Clubbed Tunicate: The clubbed tunicate (Styela clava) is a solitary, sessile marine invertebrate native to the Northwest Pacific. It has become invasive in California, where it can rapidly reproduce and form dense aggregations on submerged structures such as docks, pilings, and boat hulls. The clubbed tunicate can outcompete native species for space and resources, which can lead to a decline in native tunicate populations and have negative impacts on local biodiversity.
  • Watersipora spp: The Watersipora genus of bryozoans includes several invasive species that have become established in California, such as Watersipora subtorquata and Watersipora arcuata. These encrusting bryozoans can form dense colonies on hard substrates, including artificial structures like ship hulls, marina infrastructure, and even native kelp blades. Watersipora species can outcompete native fouling organisms for space and contribute to the alteration of local marine communities.

Importance of Invasive Species Research

Ongoing research is crucial in understanding the biology, ecology, and potential impacts of marine invasive species. By emphasizing the need for ongoing research in the following areas, we can better understand the complex dynamics of marine invasive species and develop more effective and adaptive management strategies to protect our marine ecosystems and the valuable resources they provide.

The following areas highlight the importance of research in addressing marine invasive species:

  • Understanding species biology and ecology: Gaining insights into the life history, reproductive strategies, dispersal mechanisms, and environmental tolerances of invasive species can help predict their potential for establishment, spread, and impacts on native ecosystems. This information can inform targeted prevention, early detection, and control efforts, ultimately improving the effectiveness of management strategies.
  • Assessing ecosystem impacts: Research focused on understanding the ecological consequences of invasive species can help identify priority areas for management and conservation efforts. This may involve studying the direct impacts of invasive species on native species and habitats, as well as their indirect effects on ecosystem processes, such as nutrient cycling, food web dynamics, and habitat structure.
  • Evaluating the effects of climate change: Investigating how climate change influences the dynamics of marine invasive species is essential for anticipating and mitigating future invasions. This could include studying the effects of changing water temperatures, ocean currents, and coastal habitats on species distributions, as well as the potential for novel interactions between native and invasive species under altered environmental conditions.
  • Developing novel control methods: Research into innovative control and eradication techniques can help improve the management of established invasive species populations. This may involve exploring the use of advanced technologies, such as genetic modification or biocontrol agents, as well as refining existing methods, such as mechanical removal or chemical treatments, to minimize potential harm to non-target species and the environment.
  • Evaluating management effectiveness: Monitoring the outcomes of management actions and conducting research to assess their effectiveness can help inform adaptive management approaches and optimize the allocation of resources. This could include evaluating the success of prevention measures, such as ballast water management or aquarium trade regulations, as well as the efficacy of control and eradication efforts in reducing the impacts of invasive species on native ecosystems.
  • Enhancing collaboration and information sharing: Promoting interdisciplinary research and fostering collaborations among scientists, managers, policymakers, and stakeholders can help bridge gaps in knowledge and ensure that management strategies are informed by the best available science. Establishing networks and platforms for sharing research findings, data, and best practices can also facilitate the exchange of information and expertise across regions and sectors, ultimately improving our collective ability to address marine invasive species challenges.


Shawn McBride, Aquatic Bioassessment Lab Supervisor
Email: Shawn.McBride@wildlife.ca.gov
Phone: (916) 358-2861

Anthony Rietl, Senior Environmental Scientist, Lead
Phone: (916) 341-6958

Sharon Shiba, Senior Environmental Scientist

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