2: Trophic cascades & max yield Flashcards
What is a trophic cascade?
indirect interactions between non-adjacent trophic levels
E.g
Predator-prey/prey-resource = direct effect
Predator-resource = indirect effect
How were sea otters discovered to be a keystone species in the US?
- Sea otters hunted to near extinction (for fur)
- (their prey) sea urchin pop. increased
- Kelp forests disappear
- Fish stocks collapsed
Overall loss of biomass and diversity
How did the Rinderpest virus being eradicated increase wildfires in the Serengeti?
- Virus eradicated
- Pop.s of wild large even-toed ungulates (giraffes, buffalo etc.) increase
- Grazing of Serengeti increase
- Decrease in plant biomass = increased wildfires
What are some of the effects of a loss of predators?
→ has - indirect effects on lower trophic levels
Loss of plant diversity
Loss of plant biomass
Loss of plant community structure
Loss of herbivore diversity
… and hard to predict effects on the overall landscape
Water quality, CO2 flux, wildfires, disease epidemics, soil nutrients, invasive species presence
Give 2 examples of misguided attempts to reduce predator pop.s
Sweden’s wolf culling and the UK’s raptor killings
What is the aim of the study on Grasshoppers and Spiders (old field ecosystem)?
To study the ‘ecology of fear’ (impact not of predation itself, but the risk of predation)
→ Is the grasshopper’s changed behaviour (due to fear of predation by the spiders) enough to cause a trophic cascade?
How did the ‘ecology of fear’ study test their hypothesis?
- They glued the spiders mouths together (doesn’t last long) so they couldn’t eat the grasshoppers
- This brings all the aspects of having predators around i.e fear, without actual predation
- So, if the cascade is entirely driven by fear, we should get the same results
How did the ‘ecology of fear’ experiment check they had adequately created risk?
Measured activity levels of glued and non glued spiders = didn’t affect
Also checked if grasshoppers could tell the difference between glued/non-glued by measuring distance moved = no sig. diff
= Appears that an adequate risk treatment has been created
What did the results of the ‘ecology of fear’ study show?
dependent on what spider hunting method dominates = predators and what they do affects the whole ecosystem
Describe the graph for max sustainable yield
logistic growth equation
1. Exponential growth phase
2. Inflection point (where exponents peak before pattern is inverted)
3. negative feedback phase
4. Carrying capacity reached (plateau)
Describe fixed quota harvesting
Constant harvesting rate
Describe how fixed quota harvesting works
Harvest rate always aims to get equilibrium close to K = organisms added to pop. at the same rate as they are harvested
→ due to imperfect data & changing conditions we can only aim for K
→ stable equilibrium (always pushed back to equilibrium)
what are the caveats of max sus yield?
- Ignores pop. structure (age, size, sex ratios)
- Assumes environ does not vary
- In practice impossible to reliably obtain
- Fluctuations in resource abundance can lead to extinction under fixed quota
What is fixed effort harvesting?
Fixed effort and fixed efficiency, pop. size is the only thing that can determine harvest
On a fixed effort harvesting graph, when is equilibrium stable?
When the harvest rate gradient is below the recruitment point
