invasive species

key concepts: alien, invasive and range-expanding species 

  • alien species →

    • species occurring in a regions due to human activity, either intentional or accidental

    • they have crossed biogeographic barriers that they would not cross naturally

    • examples →

      • escaped aquaculture species

      • species transported on ships

      • introduced fish, mammals, plants, etc

  • invasive species →

    • alien species that successfully establish and spread in the new area

    • the term refers to successful establishment, not merely arrival

  • range expanding species →

    • species moving into new regions naturally, usually due to:

      • climate change environmental shifts

      • habitat changes

  • range expanders and invasive species can have similar ecological impacts, even though range expanders are not introduced by humans

why are range expansions increasing?

  • primary driver: climate change

  • species shift into newly suitable areas because conditions are warming

  • climate change alters:

    • temperature ranges

    • seasonality

    • food availability

    • habitat structure

  • habitat loss and fragmentation:

    • species may move to escape unsuitable or degraded habitat

  • bottom line: organisms shift ranges to track their preferred environments conditions

example: borealisation of the barents sea

this is a major case study showing how warming transforms an arctic ecosystem

physical changes

  • prediceted environmental shifts include:

    • +2-3˚C increase in sea surface

    • retreating sea ice

    • changed circulation, stratification, and ligth environment

    • longer productive seasons

    • increased ocean acidification

biogeochemical changes

  • increase increase in primary production by 30-40%

  • more boreal zooplankton species

  • boreal/sub-arctic fish species move north

  • arctic fish species pushed out

ecological consequences

  • entire food webs are reorganized

  • species adapted to cold conditions become displaced and lose habitat

consequences of borealisation

  • altered food web dynamics

    • new species interaction

    • changes in predator prey relationships

    • competition for resources

  • new winners and losers

    • boreal species may outcompete arctic species

    • some species expand; others decline

  • changing sensitivities to fisheries

    • boreal species may be heavily fished

    • arctic species may become more vulnerable

  • these changes can destabilise the entire ecosystem

key knowledge gaps

  • to manage and predict invasive or expanding species, we must understand:

    • which sub-populations will enter the arctic

    • whether arrival is passive (currents) or active (migration)

    • whether new comers:

      • stay only during certain life stages, or establish permanent, full life cycles

    • whether climate conditions will support spawning, growth, survival

impacts of invasive species

invasive species casues widespread and well-documented ecological effects

  • extinction risk increases

    • alien species have already contributed to: 25% of plant extinction 33% of terrestrial and freshwater animal extinction

    • range expanders, in comparison contributed to <5% and 3% respectively

  • genetic impacts

    • hybirdization

    • introgression

    • loss of native genetic diversity

  • biodiversity loss

    • reduced functional diversity

    • reduced phylogenetic diversity (evolutionary history)

  • food web modifications

    • new trophic pathways

    • altered predation pressure

    • disrupted nutrient flow

  • ecosystem level changes

    • new trophic pathways

    • altered predation pressure

    • disrupted nutrient flow

  • ecosystem level changes

    • productivity

    • nutrient cycling

    • contaminant cycling

    • hydrology

    • disturbance regimes

  • invasive species can reshape entire ecosystems, not just replace a few species

case study: pink salmon

  • introduced by russia in the 1950s

  • life cycle pattern:

    • odd eyar and even year lineages

    • odd year salmon dominate in warmer areas

    • every yyear salmon dominate in cold areas

  • since 2013, massive growth in pink salmon entering european rivers

  • in northern norway, pink salmon outnumber atlantic salmon

why are pink salmon so successful?

  • high abundance

  • 10% straying rate (high dispersal potential)

  • short freshwater residency (lower mortalty risk)

  • short generation time (rapid adaptation)

  • high adaptive potential

    • genetic changes

    • phenological shifts

    • morphological changes

    • higher body size and fecundity

  • they adapt rapidly to new environments and spread aggressively

impacts

  • frehwater: compete with atlantic salmon and sea trout

  • marine: top down predatory effects on pelagic species → food web disruption

predicting their spread

  • pink salmon distribution align with sea surface temperature (SST) habitat suitability maps

  • SST increases → more suitable further north

  • observed reductions in southern norway between 2021-2023 match predicted warming based shifts

  • likely using western norwegian seas as feeding grounds - the same route as atlantic salmon

  • this shows climate warming + biological traits = ongoing range expansion

monitoring actions

  • monitoring tools include:

    • catch statistics

    • traps

    • snorkel surveys

    • environmental DNA 

  • these mathods allow early detection and population tracking

eDNA: metabarcoding and metagenomics

  • eDNA metabarcoding

    • uses PCR with universal primers

    • targets mitochondrial DNA

    • yields massive sequence reads

    • great for presence/absence and species diveristy

  • metagenomics

    • no PCR

    • sequences all DNA in sample

    • captures mitochondrial DNA + nucelar DNA

    • gives broader biological information

  • strenghts

    • non invasive

    • high sensitivity

    • good for detecting rare or ealry stage invasions

    • works across environemnts: water, sediment, soil

  • limitations:

    • primer design challenges

    • species vary in DNA decay rates

    • PCR biases

    • risk of false positives/negatives

    • database dependency

  • these techniques are powerful but require careful interpretation

management actions for pink salmon

  • freshwater removal 

    • large scale removal efforts

      • traps

      • nets

      • fishways

      • harpoons

    • nearly 250,000 pink salmon removed in 2023

    • effective but costly and labour intensive

  • marine removal?

    • not used due to:

      • bycatch of native atlantic salmon sea trout

      • conservation concerns

  • commercial or recreational fishing

    • low commercial value so far

    • recreational inclusion is complicated becasues atlantic salmon populations are already in poor condition

need for better biological knowledge

  • to design better management actions, we need more informatino about:

    • habitat characteristics (temperature, substrate, resources)

    • movement patterns (especially age specific)

    • thermal history

    • health and stress levels

    • reproductive biology

    • trophic interactions

  • these data help predict invasion potential and guide effective mitigation

the papers