Lecture 23 Eradication Continued

Unintended/Indirect Consequences of Eradication Programs (1) Poisoning of native species (a) Bioaccumulation (b) Direct toxicity (2) Removal of one exotic triggers an invasion or population release of another species. • May have more profound effects on diversity than the eradicated species! (3) May need to reconstruct ecosystems and invest heavily in restoration efforts. • Without restoration, the transformed or degraded ecosystem will not return to something approximating what was there prior to the arrival of the invasive species

Predators • Invasive vertebrate predators are among the most destructive introductions from a conservation standpoint. • Collectively, mammalian carnivores have had a devastating impact on insular fauna worldwide, especially on islands. • Predator removal is challenging and predator-prey relationships must be carefully considered before eradication efforts begin.

Complex effects • Eradication efforts can fundamentally change entire food webs, destabilizing ecosystems if linkages are not carefully considered. • Some food web effects of invasive eradication efforts

Prey release • Invasive prey species increases once a predator is removed. This can have dramatic effects if a predator is an important limit on the invasive population.

Hyperpredation • Predation on native species by introduced predators sustained by high introduced prey. Controlling an invasive species that supports predators may cause a shift in prey selection, transferring mortality onto native species.

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 Unintended Consequences: 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 Australia 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

Unintended Consequences: 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.

Societal Issues Cats § ~600 million cats worldwide, every continent except Antarctica, many oceanic islands § Among introduced species, feral cats rank near the top in their detrimental effect on native species. § In general, most introductions have been deliberate and are especially devastating on islands where unadapted vertebrates are easy meals. § Adaptable, intelligent, kill instinctively even when not hungry

Cat Eradication Cat removal controversial By far the most common pet on earth ◦ - Half of all households in New Zealand own a cat ◦ -20,000,000 stray / feral cats in Australia Killing them is very unpopular Advocacy groups have considerable power and tie up cat culls in the legal system

Easier on Remote Islands Cat eradication on islands ◦ 4 methods (trap, hunt, poison, pathogens) ◦ Disease (FLPV) offers promise for larger areas with difficult terrain ◦ Some cats are immune so it needs to be coupled with other approaches Can be done successfully but very difficult in environments where humans live.

Challenges in 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)