Mod 4 Lec 32 - Predation and Conservation

Learning Outcomes

• Understand basic species interactions at different trophic levels.
• Understand the impact of introduced predators on mammal conservation in Australia.
• Understand how changes to predator-prey interactions can lead to conservation challenges, such as driving prey to extinction.
• Understand the theory behind meso-predator release and its relevance to wildlife conservation and management.

Trophic Levels

• Apex predators have no natural predators and are at the top of the food chain.

Predation

• Predator: An individual eats all or part of another live individual (focus on carnivory – animal kills and consumes another animal).
• Usually involves interactions between trophic levels where one species negatively affects the other.
• Implications for conservation and wildlife management:
  • Predators can cause significant decline of conservation-dependent prey species, especially if the predator is introduced.
  • Particularly important on small islands.
  • Predators may control overabundant prey.
  • The role of human harvesting in conjunction with or instead of predators.

Predators as Regulators

• Predation can regulate prey densities, keeping prey at low densities.
  • Example: Caribou populations and wolf predation.
    • Areas of constant predation: 0.003-0.2/km^2
    • Areas with no predation: >2/km^2
• Predation can remove malnourished animals, influencing intraspecific competition in prey species.

Destabilizing Effect of Predation

• Under certain conditions, predation can destabilize a prey population and push it to extinction:
  • When there is no prey switching.
  • When there is no refuge for prey at low densities.
  • When predators have an alternative prey species to maintain their population.

Destabilizing Effect of Predation - Canada

• Boreal caribou are declining in Canada.
• Moose recently moved into the area and are now the primary prey of wolves, sustaining the population when caribou are at low densities.
• Caribou suffer predation rates which reduce calving success to 6.9%.
• Caribou adult mortality is 29% (mostly wolf predation).
• Caribou population is declining, with predation rate increasing as density decreases.

Destabilizing Effect of Predation - Canada

• Moose and white-tailed deer populations have increased in western North America due to climate change and forestry.
• This has also increased predators that are causing the decline and extirpation of woodland caribou.

Destabilizing Effect of Predation - Alaska

• Apex predator vs. apex predator interactions.
• Cross-system cascades: terrestrial and marine (separate and intertwined).
• System shift from deer to sea otters (marine apex predator) slowly re-colonizing the area.
• Wolves naturally colonized the island in 2013. In 2015, wolf scat = 98% deer, and by 2018, 0% deer. Presence of sea otter increasing.
• Cross-system shift in prey and supplementary food source increased wolf population, decimating deer populations.

Prey Evasion Strategies

• Migrate outside of predators' range:
  • Predators can’t move the same distances as migrating herds.
• Spatial clumping:
  • Group size should increase with increasing predator densities.
  • Balance between minimizing predation risk and increasing intraspecific competition.
  • Parturient females often leave the herd and become solitary, relying on the fact that predators will focus on areas of highest prey density.
• Synchronizing reproduction to “swamp” predators:
  • Synchronized over and above normal seasonal influences.

Invasion of Exotic Carnivores

• Cats introduced with European settlement now occupy all regions of Australia since 1900.
• Red foxes introduced for hunting in the mid-1870s; widespread but not as expansive range as cats.

Invasion of Exotic Carnivores

• Introduced rabbits formed an important part of the diet of both cats and foxes, probably facilitating their spread.
• Potential impacts of cats and foxes on endemic species were not recognized in early years:
  • No species known to have declined or gone extinct in Britain because of cats or foxes.

Invasion of Exotic Carnivores - Early Observations

• Early observations of declines coinciding with the spread of the fox:
  • NSW records of bounties paid for wildlife harvested:
    • Rat-kangaroos (bettongs and potoroos) were harvested at significant rates in the 1890’s:
    • 1892-1895: >200,000 p.a.
• Strong inverse relationship between rat-kangaroo bounties and fox bounties
  • Fox spread through NSW between 1900-1910
  • 1906-12: ~80,000 rat-kangaroos harvested p.a.
  • 1916-20: <500 rat-kangaroos harvested p.a.
• Rat-kangaroo populations typically collapsed within <15 years of the fox arriving.

Impact of Fox Management

• Positive response of regional fauna to extensive fox baiting (Kinnear et al. 1988):
  • Black-flanked rock wallabies persisted in refuges of WA.
  • By 1979-82, all populations declining or stable.
  • A poison baiting program was initiated in 1982.
  • Resulted in a rapid increase in wallaby populations, with unbaited sites continuing a decline in population
    • By 1990, one of the unbaited site populations had disappeared completely.

Impact of Cat Management

• Cats are generalist feeders, preferring small prey items.
• Feral cats are often significantly larger than domestic cats
• In many arid regions, native mammals survived for long periods in the presence of cats.
• On larger islands (e.g., Tasmania and Flinders), cats have coexisted with native species for many decades.
• Early losses pre-fox introduction were of species within the weight-range of cat diet.
• Cats have been implicated in the failure of reintroduction attempts
• Lack of effective, broad-scale cat control methods has limited capacity to get experimental evidence of the impacts of cats.

Susceptibility to Predation - Critical Weight Range (CWR) mammals:

• Extinctions and declines of mammals in Australia almost entirely confined to a particular group of mammals
  • Non-flying mammals with a mean adult body weight between 35g – 5500g
• No clear pattern of susceptibility within the CWR
• CWR species generally secure on offshore islands where foxes are absent.
• Species that dwell in rock-pile habitats have not fared as poorly as others.
• Species in mesic areas have fared better than arid species.
• Other species outside the CWR can become endangered or extinct under certain conditions.

Conservation Next Steps

• Carnivores can have harmful or beneficial effects on other species.
• Often difficult to foresee the flow-on effects from predator management, due to the complexity of the systems.
• But, it is important that we acknowledge the potential for flow-on effects
  • Phenomenon known as “mesopredator release”.

Mesopredator Release - WA

• Fox control in Shark Bay, WA leads to a three-fold increase in cats, resulting in an 80% decline in native small vertebrates.

Mesopredator Release - WA

• Dingo control in the Tanami Desert leads to a rapid invasion by foxes and local extinction of rufous-hare wallabies.

Top-Order Predators

• Top-order predators are important in maintaining ecosystem function
  • Limit populations of prey
  • Can limit populations of subordinate predators
  • Modulate diversity
• Removal can have a profound impact on diversity at lower trophic levels
  • Herbivores can become overabundant (Yellowstone example)
  • Subordinate predators may increase if unchecked, potentially decimating prey populations
• Depends on the system and the species found within, with unique behaviours and niches, and the strength of the interspecific interactions

Mesopredator Release

• Apex predators have no natural predators and are at the top of the food chain.

Apex Predators as Management Tools

• Bald eagles reintroduced onto Santa Cruz Island to deter golden eagles from preying on endangered foxes.

Fenced Reserves

• Wildlife sanctuaries in Australia have focused extensively on the use of predator-proof reserves
  • Offshore islands without introduced predators
  • Predator-proof fenced reserves
• Considerable success in conserving faunal communities
  • Expensive to establish and maintain
  • Need for ongoing intensive management
• Important short-term strategy

Intertwined with Other Conservation Management – Kruger National Park, South Africa

• Artificial waterpoints were installed in the 1930s to increase water during drought and provide opportune focal points for wildlife viewing.
• Very little thought was given to the ongoing impacts of increased water availability, as well as where in the park they should be constructed.

Intertwined with Other Conservation Management – Kruger National Park, South Africa

• Increased elephants caused a habitat shift in wooded areas from the destruction of vegetation, opening the habitat for large herds of grazing herbivores.
• Increased prey resulted in increased predators.
• Increased risk for slower prey species who were used to thick woodland habitat (like roan antelope).