The Delta is home to over 200 non-native species and is among the world’s most invaded estuaries. These species, introduced by human activities, pose a significant threat to the ecosystem. The potential for more non-native species to arrive, coupled with factors like habitat alterations, pollution, and climate change, can make the Delta’s ecosystem more vulnerable to invasion.
Non-native species are recognized as a significant factor driving ecosystem change, as they can disrupt the food web, modify nutrient and contaminant cycles, affect the populations of other species, and alter habitat structures. Such changes significantly impact a wide range of ecosystem services, with native species being particularly affected.
The Delta Stewardship Council’s Delta Plan recognizes the significance of non-native species as a threat to the Delta ecosystem and mandates the restoration of a healthy ecosystem by promoting diverse populations of native species and reducing the risk of harm from invasive species. So the Delta Independent Science Board (Delta ISB) in 2021 assessed the scientific needs in the Delta to address the threat. The Delta ISB periodically conducts reviews as part of their statutory charge in the Delta Reform Act to provide “oversight of the scientific research, monitoring, and assessment programs that support adaptive management of the Delta through periodic reviews.” The findings and recommendations in the report are designed to improve scientific endeavors and priorities with the goal of providing managers with a science-based prioritization framework to make decisions.
What is an invasive specie?
An invasive specie is a non-native specie that causes or is likely to cause harm to the environment, economy, or human health. However, not every non-native species will become a problem; the effects of some non-natives may be benign, while others can have a significant negative impact. So, while every invasive species is non-native, not every non-native species is considered invasive.
Some species can be considered both detrimental and beneficial. For example, striped bass, introduced into the Delta over a century ago, are a popular catch among fishermen and are now managed as a recreational resource in the Delta. However, striped bass may be preying on native fishes, such as salmon. Therefore, whether a species is labeled as invasive can depend on the perception by the public of the benefits and impact of the species and how these are balanced. The management of an invasive specie hinges not only on whether it is feasible but also if it is socially desirable.
How non-native species arrive and become established
For a non-native species to become established in an ecosystem, three key factors are needed:
- Entry Pathways: The species must have a way to enter, such as hitching a ride on goods, escaping captivity, or moving through human-made pathways like canals. Preventing these pathways can help stop invasions.
- Suitable Habitat: The environment must meet the species’ needs for growth and reproduction, often helped by the absence of natural predators. Human changes to the ecosystem can make it more vulnerable to invasions.
- Species Traits: Some species are inherently more adaptable due to traits like rapid reproduction, broad diet, and resilience to predators, allowing them to thrive in new environments.
Ultimately, the success or failure of a species that enters an ecosystem will depend on these characteristics and their match or mismatch to the receiving ecosystem. These relationships are challenging to define quantitatively.
Once it arrives, a non-native species joins the ‘species pool’ of the ecosystem, which is comprised of the types of species present and their abundances at any given time. The ‘species pool’ is a dynamic concept, constantly changing as new species arrive, others disappear, and migratory species pass through. Several forces, such as pollution and climate change, can affect the species pool. As a result, the species pool is constantly in flux, an ever-changing mosaic of ecosystems as species move among locations.
Changes in the species pool can have a variety of consequences. In some cases, non-natives thrive in the new ecosystem and begin to dominate certain habitats or food webs. New species can alter the ecosystem’s biological, hydrological, or physical structure, and changes in aquatic vegetation can alter water flows, temperature, chemistry, and water quality. Sometimes, a new species may have little observed effect until it is too late.
Addressing Non-Native Species in the Delta
While managing invasive species is, in and of itself, challenging, two factors make it especially challenging in the Delta:
First, the Delta is one of Earth’s most heavily modified estuaries. The Delta has been re-engineered to support agriculture and manage water, which has accelerated the successful establishment of non-native species. Many non-native species, such as striped bass, have become “naturalized” members of Delta ecosystems.
Second, climate change, sea-level rise, and the human uses of land and water resources are subject to a complex interplay of forces, many of which are beyond management at the Delta level. The environmental turmoil created by these forces provides new opportunities for non-native species, challenges the capacity of native species to adapt, makes it difficult for scientists to predict ecosystem dynamics, and complicates the management of land and water resources responsibly.
The Delta Plan, a comprehensive management strategy for the Delta ecosystem, identifies reducing the impact of non-native species and protecting native species as a core strategy. The plan outlines specific actions, such as habitat restoration, species monitoring, and public awareness campaigns, to achieve these goals. The Delta also has unique institutional arrangements, responsibilities, scientific collaboration mechanisms, and funding structures to handle non-native species issues.
Several interagency programs have also been formed to prevent, detect, and manage non-native and potentially invasive species:
- The Delta Interagency Invasive Species Coordination Team, organized by the Sacramento-San Joaquin Delta Conservancy, aims to strengthen coordination among agencies to prevent, detect, and manage invasive species.
- The California Invasive Plant Council is a non-profit organization that catalogs invasive plants in California.
- The California Department of Food and Agriculture has the authority to control noxious weeds in California.
- The Delta Region Area-wide Aquatic Weed Project is a collaboration among academic and governmental agencies tasked with sustainably managing aquatic weeds in the Delta.
- The Invasive Species Council of California aims to coordinate and strengthen the various organizations that address invasive species across California.
Threat assessment, prevention, and management
Ultimately, the most effective management of non-native species is to prevent their introduction, and so one of the recommendations is to conduct a threat assessment and develop actions to prevent their arrival.
First, science should be used to assess risks and identify species with a high probability of entering the ecosystem and becoming established and the degree of harmful or beneficial impacts, drawing on various available quantitative and qualitative tools. Efforts should then be targeted at the primary pathways for entry; an example would be the efforts to prevent the spread of zebra and quagga mussels.
Once a species has established an initial population, a rapid response to gather more information and eradicate the species is the next potential step. However, eradication requires detecting and responding at the earliest stages of invasion. Few invaders have been eradicated; success has been greatest when invaders have been detected at an early stage and in a small region.
Once a non-native species has gained a foothold in an ecosystem, controls can limit the extent or slow the speed of the spread, reduce the population, or lessen the impacts. Case studies of successful management of many invasive species suggest the problems are manageable, even if species are not eradicated.
There are various control techniques for managing invasive species, such as removal by hand, mechanical harvesting, chemicals, making changes to favor desirable species, biocontrol agents, and integrated pest management. If a non-native species is resistant to control efforts or the efforts fail, management must then shift to adapting to the presence of the new species and altered species pool.
Climate change and non-native species
The introduction of new species is closely linked to big changes in ecosystems, such as how we use resources, climate change, pollution, and habitat changes. In aquatic environments, these changes affect how species spread, the conditions of their new habitats, water flows, salinity, flood patterns, food-web interactions, temperature, and human activities. All these factors influence whether non-native species can enter and settle in new areas and how they might impact those areas, making management very challenging. In addition, when multiple disturbances happen, the ecosystem might transform into something entirely different.
Considering predicted climate change effects in the Delta is the next challenge in forecasting the establishment of new non-native species. Changing habitats will alter the susceptibility of the Delta to different species therefore changing risk assessments and pathways.
Some species will likely become extirpated from the Delta as their temperature limits are exceeded, while other species may invade or encroach as higher temperatures or other disruptions benefit them. Extreme events like floods, droughts, and storms can disturb aquatic and wetland ecosystems and facilitate non-native species invasions.
Restoration and Non-native Species
Keeping non-native species from overtaking restoration projects in the Delta poses a particular challenge. Restoration actions often accompany habitat disturbances, allowing non-natives to become established and resist eradication. Combinations of events and sequences of extreme events may also create opportunities for non-native species to become established at restoration sites. Including such worst-case scenarios in restoration planning is important, as there will be surprises, and decision protocols will be needed throughout implementation and monitoring.
Many researchers assume that restoring diverse vegetation will help ecosystems resist invasion. However, this result is not necessarily true everywhere; some studies have found that diversity hotspots have also been hotspots for invasion.
Restoration projects can be designed as adaptive management experiments, which can reveal the best methods for restoring habitats and managing invaders. Ecological restoration is a long-term process, and adaptive management requires monitoring to determine whether and when adjustments to management practices may be necessary.
Understanding that these ecosystems are not necessarily degraded but rather are different from the prior system is fundamental for management. Indeed, managers should seek to understand and evaluate the potential benefits the ecosystems and the non-native species that comprise them can provide.
Recommendations to Improve Science Capabilities in the Delta
The Delta Independent Science Board’s overall recommendation is to encourage a more ecosystem-level, forward-looking, integrated approach to non-native species science in the Delta, specifically considering climate change. The report stresses the necessity of anticipating and preventing invasive species and highlights the critical role of prioritizing science and enhancing collaboration across different disciplines and agencies.
The Delta ISB outlines seven recommendations to improve science capabilities in the Sacramento-San Joaquin Delta regarding non-native species. These recommendations include developing a comprehensive, Delta-wide food-web model tied to environmental conditions, defining and prioritizing short and long-term science needs, identifying and prioritizing new high-risk species regularly, setting ecosystem-level goals, evaluating threat assessments for non-natives in a changing environment, establishing a multi-agency invasive species coordination plan, and creating a central science source for expertise and information on non-native species in the Delta to aid decision-making and coordination efforts. These recommendations aim to enhance the understanding, prevention, detection, and management of non-native species in the Delta ecosystem, considering the dynamic nature of the environment and the impacts of climate change.
The report is well-researched and a remarkable scientific synthesis of invasive species and their management. The report discusses the impacts of non-natives on ecosystems, the history and status of non-natives in the Delta, prevention, and management of invasive species, and considerations for restoration projects. The appendix provides examples and details of significant non-natives in the Delta, such as bivalves, aquatic plants, wetland vegetation, submerged aquatic vegetation, fish, and mammals. While the focus is on the Delta, the information contained therein is relevant to non-native and invasive species in any ecosystem.
DISB Non native species review
