Predation and Conservation Notes
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 – one species negatively affects the other
Has implications for conservation and wildlife management:
Predators can cause significant decline of conservation-dependent prey species (esp. if predator is introduced)
Particularly important on small islands
Predators may control overabundant prey
Role of human harvesting in conjunction with or instead of predators
Predators as Regulators of Prey Species and Populations
Predation can regulate prey densities, keeping prey at low densities
Caribou populations and wolf predation
Areas of constant predation:
Areas with no predation: >2/km^2
Predation can remove malnourished animals, influencing intraspecific competition in prey species
Destabilising 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
Destabilising Effect of Predation - Canada
Boreal caribou are declining in Canada
Moose recently moved into 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
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
Destabilising Effect of Predation - Alaska
What about apex predator vs. apex predator?
Cross-system cascades: terrestrial and marine (separate and intertwined)
System only deer à sea otters (marine apex predator) slowly re-colonising area
Wolves naturally colonised 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 -> decimated deer populations
Prey Evading Predators
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 minimising predation risk and increasing intraspecific competition
Parturient females often leave herd and become solitary, relying on the fact that predators will focus on areas of highest prey density
Synchronising reproduction to “swamp” predators
Synchronised 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
Introduced rabbits formed an important part of the diet of both, probably facilitating their spread
Potential impacts of cats and foxes on endemic species not recognised 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 spread of 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
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 complexity of the systems
But, important that we do acknowledge potential for flow-on effects
Phenomenon known as “mesopredator release”
Mesopredator Release
WA Fox control in Shark Bay, WA à three-fold increase in cats à 80% decline in native small vertebrates
Dingo control in Tanami Desert à rapid invasion by foxes à 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 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 into the ongoing impacts of increased water availability, as well as where in the park they should be constructed
Increased elephants caused a habitat shift in wooded areas from 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)