Lecture 7: Invasive Species Flashcards
What is an invasive species?
- Dispersal is a natural process. (Species don’t stay in one place, due to climate & environment change).
- Anthropogenic change has drastically modified natural dispersal patterns & increased its speed.
Terminology
Native Species: occurs within its natural range (past or present) & dispersal potential (i.e. within the range it could occupy without introduction by humans).
Alien Species (aka **non-native): occurs outside of its natural range or dispersal potential.
Invasive Alien Species (aka invasive species): an alien species that becomes established in natural or semi natural ecosystems or habitat, is an agent of change , & threatens native biological diversity (e.g. Japanese knotweed, grey squirrel has displaced our native red squirrel, pheasants released for shooting).
- An increasing problem (as people move around the world more and faster).
- A global problem. Higher rates in some countries (e.g. Australia, New Zealand and USA) due to deliberate introductions by European settlers.
- Islands are quite vulnerable (fauna adapted to not have certain predators in environment).
How does invasion happen?
Of 1517 recorded invasive species reviewed by Turbelin et al (2017)
- 39% were introduced intentionally. E.g. starlings in North America, Cane toad in Australia to try control pests on sugar cane but ate native organisms (& they toxic so if native predators ate them they died), Giant African Land snail.
- 26% were introduced unintentionally. E.g. rats in irelands, Zebra Mussels by ships in the larval stage, Giant salvinia (aquatic plant).
- 22% were spread by both intentional & unintentional introductions.
- 13% had no information available.
What makes an invasive species?
3 stages:
- Must survive dispersal to a new area and undergo dispersal.
Once species arrive, there 2 ‘filters’ that determine whether it will establish. - Abiotic environment - are environmental conditions suitable for the species?
- Biotic environment - can it compete with local species & defend from predators, parasites & pathogens?
If get through these filters, they can become invasive.
Stage 1: How do they disperse?
- Some more able to survive human-mediated dispersal.
- Maritime transportation: attach to boat hulls (e.g. molluscs), survive in ballast water (water pumped into boat to maintain safe operating conditions) (e.g. seeds & larvae), live in close association with humans (e.g. rats, cats, livestock).
- (Creating new transport routes) Shipping canals can allow dispersal between previously disconnected places e.g. Suez Canal (between Red Sea and Mediterranean, species disperse through canal either on ships or natural dispersal).
- Over-land transportation: species involved in pet or horticultural (cultivation of plants in gardens or greenhouses) trade.
- Stowaways in cargo (e.g. food shipments) can occur via any transport route.
Stage 2: How do they establish? - Abiotic factors
- Most cases, species die before they establish new area.
- Climate & resources must be suitable.
- Chances of establishment increases with: Number of introductions & number of individuals introduced.
- Large stochastic (phenomenon that involves random element or unpredictable outcomes) element involved e.g. the first 31 purposeful attempts to introduce red deer (Cervus elaphus) to New Zealand failed. On 32nd attempt, population exploded & now is a major pest.
Stage 3: How do they establish? - Biotic factors
4 hypotheses proposed:
- Empty niche hypothesis (alien species exploit niche not filled by native species (e.g. plant with deeper roots, capture nutrients from deeper soil).
- Novel weapons hypothesis (some species outcompete native species by producing some kind of weapon), e.g. plant expressing allelopathic chemicals that kill resident plants. Ni et al (2012) found allelopathic chemicals have contributed to successful establishments of 25 out of 33 of the most noxious invasive plants in China.
- Enemies release hypothesis (when species gets transported might have a disease/pathogens, but most of the time they don’t get transported with them, so loose part of a natural control of that species). Torchin et al (2003) found species had half the number of parasite species infecting them in invasive ranges compared to native ranges.
- Novel environment hypothesis (invasions more likely successful if species is associated with some kind of human modified habitat) humans create novel habitats that well suited for alien species e.g. roadsides and rats, cockroaches, houseflies.
Impacts on biodiversity
- Murphy & Romanuk (2014) found that: a) invasive alien species decrease native species richness by an average 24%, b) they ranked biodiversity impacts of invasives as 2nd to land-use change.
- Invasive mammalian predators (e.g. cats & mice) caused extinction of 87 birds, 45 mammals, 10 reptiles.
Impacts on biodiversity
- Introduction of alien species might not cause an over-all decline in biodiversity in an area.
- Because introductions happen faster than extinctions.
- BUT do cause homogenisation. Endemic species lost & replaced with globally common ones.
Ecosystems-level impacts
- E.g. invasive plants speed up rates of nutrient cycling (nitrogen accumulation & decomposition) & increase primary production.
- Nutrient rich ecosystems tend to have low biodiversity as some species can dominate.
Economic impacts
- E.g. emerald ash borer (Agrilus planipennis) was accidentally introduced to USA and Canada in the 1990’s. Killed 53 million ash trees (Fraxinus spp) causing $11 billion of damage.
- High income regions e.g. North America & Europe experience highest absolute cost from invasive species by affecting agriculture, forestry. Lower income countries spend more of GDP.
Alien species aren’t always bad
- Most relatively benign. E.g. Aesculapian snake - native to mainland Europe & accidentally released in UK.
- Can have positive impacts e.g. non-native flowering plants introduced to gardens are good for pollinators.
- Some species used for food e.g. potatoes, tomato’s.
- Or recreation.
- Biological pest control - 16% of introductions of non-native enemies result in complete control of target pest.
- E.g. the cassava mealy bug, accidentally introduced to Africa in 1970s, insecticide control was costly & environmentally damaging, now controlled by a specialist parasitic wasp, originally discovered in mealybug’s native range.
There can be benefits of alien species
- Arguments over which species native. e.g. not known what was there in past.
- e.g. Pool frog thought to be introduced to UK from Europe - not conserved in UK, presumed extinct in UK in 1995, after conservationists realised they were probs native. Reintroduction programmes now underway.
Management and control
- Start with horizon scanning (figure out what might happen).
- Risk assessment : estimate probability of introduction, establishment, spread. Estimate potential impact, species disruption modelling. = Risk summary
- Risk management : prevention, eradication, containment, long-term management. = Management options
- Use Risk summary & Management options to prioritise what to do by decision makers (e.g. governments etc).
- Management plan
Management and control: Risk assessment
Risk assessment is a formal process for determining probability that event will occur & consequences or impact if it does occur. (Ones that likely to occur & have severe impact need to be focused on).
- Identify threats.
- Decide how might be affected and how.
- Analyse the risks, & decide on precautions.
- Record findings & generate summary.
- Review and monitor. (Risk assessments need to be done continually as risks might change over time)
