Lecture 8 - Invasive Predation, Spatial Ecology, and Mating Systems

Predation, Invasive Species, and Biodiversity Loss

• Impact of Predators on Survival and Extinction:

  • Predation is a primary driver impacting species survival and potential extinction.

  • Invasive and non-native (exotic) predators have caused devastating impacts on global biodiversity.

  • Invasive species rank alongside climate change and habitat destruction as a leading cause of species decline.

• Island vs. Continental Impacts:

  • Island nations and states (e.g., New Zealand, Australia, Hawaii, Oceania) are particularly susceptible to invasive predators.

  • Case Study: The Stevens Island Wren:

    • This was a flightless wren once widespread in New Zealand, later restricted to a single island.

    • The species was driven to extinction in $1894$.

    • It is believed the entire species was wiped out by a single cat belonging to the lighthouse keeper.

  • Hunt to Extinction vs. Prey Switching:

    • While many predators exhibit "prey switching" (moving to more abundant prey), some (especially cats) will continue to hunt a specific species even when it is in extremely low abundance.

    • This behavior allows them to hunt a species to total extinction despite the presence of alternative prey.

• Global Extinction Patterns by Taxonomic Group:

  • Mammals:

    • Continental areas show higher extinction numbers globally compared to some island populations.

    • This statistic is influenced heavily by Australia, which is both an island and a continent. Since European colonization (roughly $230$ years ago), about $40$ mammal species have gone extinct in Australia.

  • Amphibians, Reptiles, and Birds:

    • These groups have been most heavily affected on islands.

    • In New Zealand, the introduction of rats (via Polynesian boats) led to the rapid decline of birds and reptiles poorly equipped for land-based mammalian predators.

    • The Kakapo: Current survival is restricted to rat-free islands because they are flightless and nest on the ground.

• Predators as Failed Biological Controls:

  • Predators introduced to control pests often become pests themselves due to insufficient ecological data or generalist diets.

  • Examples:

    • Cane Toads: Introduced to Australia to protect the sugar cane industry.

    • Stoats: Introduced to New Zealand to control rodents.

    • Mongoose: Introduced to Hawaii to control rodents.

Human Influence on Predation: Commercial Harvesting

• The Krill Industry:

  • Krill is highly sought after for global health food products.

  • Whales and many fish species rely on krill as a primary food source.

  • Commercial over-harvesting by humans acts as a form of top-down predatory pressure that can destabilize the food web for other predators.

Case Study: Predator Diet Analysis in the Garrywed Landscape (Grampians)

• Background:

  • Conducted by honors student Robin Sinclair in the Garrywed region (Grampians National Park).

  • Focus: Critical Weight Range (CWR) mammals (defined as those between $35\,g$ and $5\,500\,g$ or $5.5\,kg$).

  • CWR mammals are most susceptible to predation by red foxes and feral cats.

  • Variables: Compared fox diets near the park boundary vs. deep within the park, and across a north-to-south gradient (dry/arid north vs. wet south).

• Methodological Challenges and Technology:

  • Human Walked Surveys: Research conducted during COVID-19 lockdowns. Park rangers walked $800\,m$ transects ($200\,m$ on either side of cameras and across the road) looking for fox scats (feces). This was largely unsuccessful as humans are poor at finding scats.

  • Conservation Detector Dogs: A dog was used and was highly successful, finding scats buried under soil or invisible to the human eye due to its powerful sense of smell.

  • Success Rates: Humans found $13$ scats; the detector dog found $183$ scats.

• Dietary Analysis and Sample Size:

  • Scats are washed, broken down, and analyzed for hair, bones, and fragments (e.g., beetle wings).

  • Cumuative Diversity Curve (Brillouin Curve): Used to determine if the diet has been adequately sampled. The curve showed that after approximately $100$ scat samples, no new species were being added to the feeding data.

  • Total sample size collected: $211$ scats ($13$ human, $183$ dog, and others), providing high confidence in the results.

• Fox Diet Findings:

  • Mammals: The dominant food source. Included bandicoots, native rodents, possums, echidnas, and macropods (Swamp wallabies, Red-neck wallabies, Eastern and Western gray kangaroos).

  • Invasive/Other: Livestock (from neighboring properties), hares, feral cats, house mice, black rats, reptiles, invertebrates, plant matter (blackberries), and birds/frogs.

• Conservation Applications of Predator Scat:

  • Foxes can act as "survey tools" to detect rare or cryptic native species.

  • Long-nose Potoroos: Detected in fox scats in areas where they hadn't been seen for years.

  • Brush-tailed Rock Wallabies: Scats contained remains in areas where only one population was known, leading to the discovery of additional populations.

  • Other Examples: Owl pellets at roosts or in caves are similarly used to survey prey populations.

Spatial Ecology and Animal Movement

• Definitions:

  • Spatial Ecology: The study of where animals are, what they are doing, and how movement affects survival.

  • Home Range: The area individuals regularly move through to acquire resources (food, shelter, mates) for survival and reproduction. It is maintained over a significant portion of an animal's lifespan.

  • Activity Range: An area used for a short duration (days or weeks), relative to the animal's life.

  • Territory: A home range that is actively defended.

• Case Study: The Bobuck (Mountain Brushtail Possum):

  • Physical Traits: Weight between $2\,kg$ and $4\,kg$ (larger than Brushtail possums). Lifespan up to $17\,years$. One offspring per year.

  • Habitat: Tall, wet forests (e.g., Strathbogie Ranges, Northeast Victoria) and some coastal areas.

  • Diet: Specialized, primarily Silverwattle and fungi.

  • Nesting: Relies on tree hollows.

• Technology in Movement Ecology (Blake Allen's Study):

  • Devices: Used "iGachu's" (GPS loggers) and accelerometers.

  • Accelerometers: Measure high-resolution activity (Overall Dynamic Body Acceleration) in three directions (up/down, side-to-side, forwards/backwards).

  • Cost Innovation: Commercial units cost thousands; Blake Allen developed custom units for roughly $150$.

  • Activity vs. Distance: The relationship between distance traveled (x-axis) and activity levels (y-axis) is weak, with an $r^2 < 0.1$.

  • Inference: Animals may move quickly to a resource (like an orchard) but remain there with low activity (feeding/mating) or high localized activity with low distance.

Energy Expenditure and Nutrition

• Doubly Labeled Water (DLW):

  • Uses uncommon isotopes: Hydrogen ($^2\text{H}$) and Oxygen ($^{18}\text{O}$). The oxygen has two additional neutrons.

  • Method: Inject the animal, release it, recapture it, and measure the metabolism of the isotopes as a proxy for kilojoules expended.

  • Cons: Expensive, invasive, stressful for the animal, and only provides a single data point per individual.

• Accelerometer as a Proxy:

  • Calibrating accelerometer data with DLW data showed a very strong relationship.

  • Accelerometers provide a non-invasive way to measure energy expenditure without recapture.

Resource Distribution and Mating Systems

• Research by Jen (Strathbogie Ranges):

  • Compared Forest vs. Roadside populations of Bobucks to see how resources influence social systems.

  • Forest Habitat: $36\,ha$ block (part of $150\,ha$). Selectively logged. Fewer large trees. Hollow-bearing trees concentrated at one end (upslope). Silverwattle concentrated at the other end (downslope).

  • Roadside Habitat: Linear strips of $26\,ha$ total. Large remnant trees (many hollows). Resources (hollows and Silverwattle) uniformly spread.

• Findings on Social Systems:

  • Forest (Social Monogamy):

    • Large home ranges for females ($3\times$ larger than roadside).

    • High overlap ($80\%+$) between a male and a single female.

    • Pair bonds are often lifelong (unless one dies).

    • Low resource density/poor distribution makes it hard for a male to overlap multiple females.

  • Roadside (Polygyny):

    • Small female home ranges due to uniform, high-quality resources (hollow-food proximity).

    • Males overlap home ranges of multiple (often $3$) females.

    • Genetics confirmed males in roadsides sired more offspring ($2-3$ per year) compared to forest males (typically $1$ per year).

• Experimental Manipulation - Nest Boxes:

  • Jen added $48$ nest boxes to foraging areas in the forest to test if shelter was the limiting resource.

  • Results: Nest box use increased over weeks/months. Animals preferred boxes close to food resources.

  • Thermal Properties: Natural hollows in gum trees provide better insulation than plywood nest boxes. Chain-saw hollows (arborist-carved) are often preferred over boxes.

Human Impact on Movement and Behavior

• Global Analysis (Driscoll and Doherty):

  • Human presence (walking, recreation) often has a larger impact on animal movement than habitat fragmentation itself.

  • Animals increase movement (up to $70\%+$) to avoid humans or decrease it to stay cryptic.

  • A $70\%$ increase in home range creates a massive energetic cost.

  • COVID-19 Observation: During lockdowns, wildlife (Coyotes, Kangaroos, Goats) entered urban areas because human presence vanished (fear of humans was the barrier).

Questions & Discussion

• Predators of Bobucks: Red foxes and Powerful Owls are the main threats. Juvenile Bobucks are particularly vulnerable to Powerful Owls. High activity/movement increases the likelihood of detection by these predators.

• National Park Management: Trails are designated to limit human impact. Walking off-path or with dogs can cause birds and other animals to avoid those areas entirely.

• Human Presence and Pests: In some cases, human presence might deter cryptic pests like deer, which could indirectly benefit plant communities.

• Administrative Note: Oral presentation groups will be released next week. Literature reviews should be ongoing.