BAY DELTA SCIENCE CONFERENCE: The Science of Non-native Species in a Dynamic Delta

The San Francisco Bay-Delta is one of the most invaded estuaries in the world, with non-native species now a large part of the Delta’s ecosystem. The invasion of new non-native species threatens the achievement of the coequal goal of “protecting, restoring, and enhancing the Delta ecosystem.”

Reducing the impact of non-native species is one of the core strategies called for in the Delta Stewardship Council’s Delta Plan. As part of its legislative mandate to provide scientific oversight of programs that support adaptive management, the Delta Independent Science Board (Delta ISB) undertook a review to understand better the scientific needs related to this complex issue.

At the 2021 Bay-Delta Science Conference, Dr. Stephen Brandt, chair of the Delta Independent Science Board, presented the findings and recommendations from the Delta ISB review.

The Delta ISB’s effort on the review of invasive species was two years long and included all of the current members of the Delta Independent Science Board as well as a number of the former members of the board.  

The Delta Independent Science Board has an overall charge of reviewing all the science that is done in the Delta.

Our specific mandate, identified by the Delta Reform Act, is to provide oversight of the scientific research, monitoring, and assessment programs that support adaptive management in the Delta through periodic reviews of those programs,” said Dr. Brandt.  “Although we’ve done a number of reviews of independent reports, our main priority is to review the science by thematic areas. So in each of these themes listed on the slide, we’ve taken a thematic review and looked at the State of the Science overall.  We then looked at particularly what is being done in the Delta and tried to come up with specific recommendations that might improve that scientific effort in the Delta.”

Dr. Brandt noted that throughout this presentation, he would be using the terms ‘non-native species’ and invaders or invasives interchangeably, although the report identifies the differences explicitly in these terms and uses them carefully.

Non-native species are one of the most significant global threats to the integrity of ecosystems, with invasions identified as one of the five fundamental drivers of ecosystem change. The California Bay-Delta has also been recognized as one of the world’s most invaded estuaries. The Delta Reform Act itself discussed reducing the risk from invasive species. Reducing the impact of non-native species is also highlighted as a core strategy in the recent ecosystem amendments to the Delta Plan.

Non-native species directly threaten the achievement of the coequal goal of protecting, restoring, and enhancing the Delta ecosystem,” said Dr. Brandt.  “In fact, the flux of new species into an ecosystem even frustrates our ability to define what the ecosystem is itself.”

The Delta ISB’s review’s overall goal is to understand better the scientific needs related to the complex long-term issue of non-native species and develop a science-based prioritization framework to make decisions and help allocate resources.  The final report includes recommendations that can help agencies prevent and manage the threats and consequences of non-native species in both Delta lands and waters.

The topic of non-native species is very broad. So the Delta ISB focused on the Delta-wide needs that span multiple agency responsibilities, rather than focusing on specific species or specific technologies.  

The review included an extensive literature review, both globally and information related specifically to the Delta.  The Delta ISB hosted two panels of experts that explored the status of science related to non-native species in the Delta.  The Delta ISB held several deliberations, public meetings, and a public review of the report.   The Delta ISB members also participated in several workshops, scientific sessions, and discussions with managers interested in the topic.

Findings of the review

Dr. Brandt said the general findings were not surprising:

  • The science related to invasions and non-native species is extensive and spans over half a century.
  • Non-native species impact almost every ecosystem service and their sustainability. This includes habitat, structure, nutrient and contaminants cycling, water, transportation, drinking water quality, food web dynamics, endangered species, and fisheries.
  • The basic needs and technologies to better prevent, control, and ultimately manage individual non-native species are relatively well known and similar across cross ecosystems. Many of the techniques and approaches that have been used elsewhere in other ecosystems are directly applicable to the Delta. Many of them have been applied in the Delta.
  • Science is needed at each point in the management decision process.

The findings specific to the Delta are:

  • The Delta is a highly modified ecosystem, and that changes its vulnerability to invasions.
  • The global and local forces driving environmental change in the Delta are ongoing, some at an accelerated pace.
  • These changes affect the array of species that could invade the Delta as well as the vulnerability of the Delta. Climate change is particularly important here.
  • The Delta is unique in that the institutional arrangements, responsibilities, scientific collaboration, mechanisms, and funding structures to handle this issue differ from those in other ecosystems.

Addressing non-native species

The approach to handling an individual invasive specie is pretty similar across ecosystems.  First, an array of potential invaders are identified, and a threat assessment is conducted.  The threat assessment asks how likely is this animal to get into the system and survive, and what is the level of impact that would have?

If the potential impact is high, then mechanisms are put in place to prevent that introduction. If those are unsuccessful and the animal enters the ecosystem, immediate attempts are made to eradicate it. If those attempts are unsuccessful, the next step is to try and control the level of the abundance of the animal, and then finally to adapt to the changes that it has made in the ecosystem.  

Although an individual approach requires science and monitoring at each stage of the process, there are common things that are applied across species.  This could include looking at pathways and multiple entry points, and developing monitoring programs that search for an array of species, Dr. Brandt said.

We suggest that we move beyond this and begin to look at an ecosystem-level approach to dealing with non-native species,” he said.  “We’re introducing the concept of species pool dynamics, where new species come in through immigration and introductions, but some species also leave. That species pool is also driven to a large extent by some of the major drivers that affect the ecosystems: climate change, resource use pollution, land-use change, and habitat alterations.”

That species pool then is changing; it’s dynamic,” he continued.  “The consequences of what’s in that pool are changes in food webs, changes in native species, effects on most ecosystem services or habitat structure itself, and even effects on water flow and quality.”


The report makes several recommendations, including:

Improve scientific capabilities and understanding in the Delta

The primary scientific need is to develop a comprehensive, spatially explicit food web model for the Delta tied to environmental driving forces and conditions.

Any species that enters the ecosystem does, in one way or another, affect the food web,” said Dr. Brandt.  “If we understand the food web, it can improve our mechanistic understanding of non-native species currently in the system, help us predict the potential impact of a new invader, and also help us to get a more quantitative threat of an invasive species under changing environmental conditions.

Conducting focused workshops and syntheses can help prioritize science in different science programs, including workshops on pathways, technologies such as satellites, or species groups such as emergent vegetation.  He acknowledged several workshops have been held to help identify what is needed in the Delta; the Delta ISB endorses continuing and expanding these workshops.

Prioritize Current Management Actions: Individual Species

Regarding individual species approaches, the Delta ISB recommends that a prioritized list of species be developed that pose the most significant immediate and long-term threats to the Delta.  This list should include the likely invaders, the likely pathways of introduction, as well as a more quantitative evaluation of the expected ecosystem and economic impacts.

We recognize that some of these lists already exist at a statewide level, but a focus specifically on what’s most threatening to the Delta would help managers prioritize their monitoring programs and approaches, and set expectations of what might happen down the road,” said Dr. Brandt.

Shift Focus to an Ecosystem Level

The shift to an ecosystem-level focus is highly recommended. Any new species that is established is going to change the ecosystem in some way.  So management needs to adapt to the continually changing ecosystems, and better forecasting of changes can help set expectations. 

Taking that ecosystem perspective and realizing that things are dynamic, one can ask the question, what are the ecosystem goals for the Delta in the context of changing drivers in a changing species pool?” said Dr. Brandt.  “We also recognize that ecosystem restoration programs need to formally implement non-native species management research into ecosystem restoration programs, perhaps even using ecosystem restoration in an experimental way of looking at non-native species.”

Consider ongoing and future changes of drivers in the Delta

The Delta ISB recommends the ongoing and future changes of the Delta be considered in light of ever-changing conditions.

Ongoing threats for invasive species should be evaluated considering changing drivers, particularly climate, that might change the pathways to the Delta, the types of invaders that might be able to invade the system, the susceptibility of the Delta to invasion, and the actual impacts that might happen,” said Dr. Brandt.

Overall recommendation …

The Delta ISB’s overall recommendation is to encourage a broader, more forward-looking integrated approach to non-native species science in the Delta to inform management goals.  

By broader, we mean expanding the multiple species,” said Dr. Brandt.  “By forward-looking, we mean developing predictions and scenarios and forecasting in the context of ongoing and projected changes in drivers, and by integrated means coordinating efforts across disciplines and agencies.”

Non-native species are a fundamental part of the ecosystem and a driver of ecosystem change. New invaders will likely disrupt all essential services to Delta stakeholders, and that a coordinated approach to both the science and management is needed to address this growing problem.”

The first step would be to form a non-native species task force or non-native species center to expand communication and coordination and perhaps provide a source of expertise. The Delta ISB suggests this could be done under the auspices of the Delta Interagency Invasive Species Coordination Team, which is already making significant progress in this area.

The Delta ISB also thinks that developing a comprehensive multi-agency invasive species coordination plan is important. The plan should outline the responsibilities and authorities across monitoring, rapid response, control, and scientific expertise.


To conclude, recognize that science can help assess the risk and consequences to help prioritize management actions, and perhaps provide a proactive understanding,” said Dr. Brandt.  “Monitoring is also critical.”

Dr. Brandt suggests the comprehensive plan could be tested by asking the question, how would we deal with a new invasion by a ‘nutrina’ (a fictitious specie)?  What would happen if Nutrina entered and became abundant in the ecosystem? How do we deal with this species? Who is responsible for it? How do we deal with the consequences of it?

By having improved science that would help look at the array of potential invaders and its consequences, and have a food web modeling program, we might be able to reduce the impact of these surprises,” he said.

DISB Non native species review
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