Lecture 21 Eradication Continued
• Eradication of some groups may be technically possible but sociologically
difficult. Most of these species are charismatic or utilitarian species of birds,
mammals, and fish.
• Perhaps the most controversial eradication programs are those aimed at
feral cats
• Pets, people have decided they should be allowed to roam free
• Adaptable, intelligent, kill instinctively even when not hungry
•
• Cat eradication on islands
• 4 methods (trap, hunt, poison, pathogens)
• Disease (FLPV) offers the most promise for larger areas with difficult
terrain
• Some cats are immune
Complex effects
• Some food web effects of invasive eradication efforts
• Prey release (removal of a predator), prey increase unchecked
• Hyperpredation (predation on native species by introduced
predators sustained by high introduced prey). For rare native species,
even small amounts of additional predation can drive them to
extinction.
• Mesopredator release (removal of a higher predator resets food web
and elevates a middle predator to a greater role).
•
• Unintended Consequences: Prey Release
• Macquarie Island, Australia: myxoma virus introduced to kill rabbit
populations that reached 130,000 by 1978
• Successful, rabbits dropped to <20,000 by mid-1980’s
• Cats, introduced in 1800’s, switched to native birds as rabbits
declined, decimating breeding colonies
• In 1985, cat eradication began, in 2000, last cat was killed
• However, the virus by itself was insufficient to maintain rabbit
population at low levels and the population rebounded to more than
150,000
• Cat eradication had the unintended consequence of prey release
Hyperpredation
• High populations of an exotic prey sustain relatively high densities of
exotic predators. Some spillover of predation onto native species.
Even if it occurs infrequently, the additional predation pressure on
native species of conservation concern can be important
• Examples, cats and foxes in Australi are sustained by high rodent .
rabbit numbers but relatively low levels of hyperpredation on
endangered marsupials is really important.
• In New Zealand, rats sustain cats which occasionally eat the large
flightless kakapo parrot
Mesopredator Release
• In many ecosystems, apex predators have been greatly reduced or
eliminated.
• This often allows smaller predators, the so-called mesopredators, to reach
much higher densities
• True for both exotic (e.g., cats, foxes, stoats, mongoose) and native predators.
• These mesopredators assume a different role and can have significant impact
on the species below as they are no longer held in check by an apex predator.
• Eradication of an introduced apex predator can ramify through the trophic
web as can the introduction of a mesopredator into an environment where it
becomes the apex.
The difficulties of eradication
• Scale
• Islands, restricted areas
• Cheat grass – dominates an area the size of California!!
• Complex indirect effects
• Removal may imperil natives through food web changes, especially if
other invasives are present.
• May need to eradicate more than one species similtaneously
• Plants / invertebrates - difficult to eradicate
• Detection, effective techniques, resting propagules
• Post-eradication susceptibility to other invasive species
• Long term changes to ecosystem structure or function (salinity,
nitrogen, water table, etc.)
• Politics
• Inaction, cost, ‘cute’, cultural, economic (useful, hunting, fishing)