Conservation 7- Alien species and introductions

Conservation 7- Alien species and introductions

1. Main threats

   • Change or reduce biodiversity
   • Acts as vectors for new diseases
   • Alter ecosystem processes
   • Disrupt cultural landscapes
   • Reduce value of land and water for human species
   • Estimated 400,000 cumulative introductions globally

2. Mechanisms of introduction in Europe

   • Unintentional transport
   • Intentional release
     • Aesthetic reasons
     • Economic reasons-- maize into Africa
     • ACCIDENT??
   • Unintentional dispersal
   • Tiger mosquito
     • transports dengue, west Nile virus
     • Transported in container ships
     • CLIMATE matching

3. Examples

   • Accidental introduction-- Brown tree snake
     • Native in Australia and Indonesia
     • Transported to Guam on military aircraft
     • No predators, lots of prey
     • 1983--- 6 birds species were extinct, others on the verge
     • Large effects on other taxa-- e.g Geckos and skints
   • Avian malaria in Hawaii
     • Introduced with exotic birds brought by settlers
     • Insect vector in water barrels of ships-- 1800s
     • Native birds had no immune resistance
     • At least 10 species of bird went extinct
   • Rosy wolf snail-- bio control gone wrong
     • Native in SE USA
     • Introduced to Pacific Island to control spread of introduced giant Africa snail
     • Preyed on native tree snails
   • Harlequin Ladybird
     • Biocontrol of aphids
     • Native range in Central and Eastern Africa
     • Released around the world- accidently released in horticultural material
     • Reduction in biodiversity of other aphid-eating insects and non-pest insects
   • Ruddy duck-- Deliberate introduction
     • Ruddy duck introduced in UK
     • Spread to Spain
     • Interbred with white-headed duck-- hybridisation-- genetic extinction
   • Biological control of Opuntia
     • Ornamental catus-- severe pest in Australia
     • Global search for biocontrol agents
     • 150 insects examined
       • 50 tested
         • 12 became established
           • 1 successful

4. Most invasions fail- 10-10 rule

   • About 10% of introduced species become established
   • About 10% of these create problems
   • Some introductions have a higher probability of success
   • High rate of success amoung songbirds in Hawaii

5. The invasion process: biotic resistance

   • Invasion: occurs through a series of consecutive stages
   • Transport from native range
   • Intro to novel range
   • Establishment (survive and repoduce)
   • Spread
   • Biotic resistance from competitors and predators in novel range
     • Repels invaders prior to establishment
     • Regulates spread and impact when established
     • These competitors and predators are completely novel

6. Propagule pressure

   • Propagule size-- absolute number on individuals introduced
   • Propagule number-- number of introduction events
   • Large number of propagules buffers invasives against small pop size
   • Repeated invasion/introduction acts in a similar way
   • Introduced birds in NZ- 41 out of 95 introduced species succeeded

7. Enemy release hypothesis

   • Release from natural enemies allows invaders to become abundant
   • Successful invaders--- free from attack in invaded habitat
   • two aspects
     • Regulatory release
       • In native range-- invader has low defence against natural enemies-- high mortality
       • Abundance is regulated by enemies
       • Loss of enemies after invasion-- increasing survivorship
     • Compensatory release
       • In native range: well defended against enemy
         • Immunity
         • Behaviour
         • Physical
         • Chemical
       • Defences are costly to maintain
       • Compensates for loss of enemies-- resources now allocated for growth, reproduction

8. Enemy of my enemy is my friend hypothesis

   • Negative impact of 'enemies' are carried with invaded
     • American signal crayfishCarried parasite that has exterminated natal crayfish
     • Replacement of native red squirrels with grey enhanced by vulnerability of reds to parapoxvirus introduced with greys

9. Invasion success

   • Muskrat-- introduced from N.America for fur trade
     • Widespread in Europe and Asia
     • Affects vegetation dynamics through grazing
     • Threatens endemic species-- Shellfish, fish and ground nesting birds
     • Found vacant aquatic niche
   • Which species are successful invaders?
     • High reproductive rate
       • Build up large pop size
       • Propagule pressure-- overcome biotic resistance
     • Habitat and/or food generalist-- overcome biotic resistance
     • Good dispersers-- once established can spread more easily
   • Which habitats are vulnerable to invasion?
     • Disturbed areas with empty niches-- little or no competition-- EDGES OF FORESTS
     • Remote islands-- low diversity--- simple communities-- empty niches
     • Islands-- low predation--- easily exploited food resources
   • Critically endangered Juan Fernandez fire crown
     • Found on Juan Fernandez islands
     • Threatened by invasive rats, feral cats and rabbits
     • Removing invasive vertebrates is critical to recovery of island species on brink of extinction-- ETHICAL AND MONEY DILEMMAS!!!