1. Mark J Gibbons, Room 4.102, BCB Department, UWC
Community Ecology
BDC331
Mark J Gibbons, Room 4.102, BCB Department, UWC
Tel:
Image acknowledgements –

2. Types of Predators
True predators – eat whole organisms; prey individuals suffer immediately from the actions of an individual predator; eat more than one prey item during their life
Herbivores – eat bits of organisms; prey individuals do not usually suffer immediately from the actions of an individual predator; eat more than one prey item during their life

3. Parasites – eat bits of organisms; prey individuals do not usually suffer immediately from the actions of an individual predator; intimately associated with a single prey item during their life
Parasitoids – “eat” whole organisms; prey individuals suffer “immediately” from the actions of an individual predator; intimately associated with a single prey item during their life

4. The effects of predation on prey populations
Since the effects of predation on an individual prey item are eventually deleterious, it might be supposed that predators are bad news for prey populations
BUT……….
Predation may occur at a demographically unimportant stage of the prey life
For example – if plant recruitment is not influenced by the number of seeds produced then a seed-predator is unlikely to have any effect on recruitment
Predators remove individuals from population that make, or are likely to make, no contribution towards reproduction – the old, the sick or the very young

5. Link between predation and prey populations not easy
Predation serves to reduce prey population numbers, thereby reducing the effects of intra-specific competition
The impacts of predation may be limited by compensatory reactions amongst survivors because of less competition
For example…………….
Massive shoot of wood pigeons in autumn has no impact on numbers breeding the following year because numbers determined by available food resources over winter
Murton et al (1966) J Applied Ecology 11: 61-81
Link between predation and prey populations not easy

6. Numerical responses of predator limited by generation time
If predator populations are unable to respond, numerically, to the prey when they are abundant, they will have a limited effect on the prey populations. Mismatch
Generation Time = 30 days
Biomass 10
Time (days)
Diatom Bloom in Southern Benguela

7. The effects of consumption on consumers
An increase in the amount of food consumed leads to increased growth and reproduction – subject to intraspecific competition
All individuals require a certain amount of food to maintain themselves – a threshold amount. Only if the threshold is exceeded can the excess be diverted to growth and reproduction, i.e. low consumption rates rather than leading to small benefits to the consumer simply alter the rate at which starvation occurs. This also means prey populations have a refuge at very low abundances.
BUT….
At the other extreme, growth and reproduction cannot continue ad infinitum for an individual – it becomes satiated, and consumption rate reaches a plateau whereafter an increase in prey density is not reflected by any change to consumption. This also means that the effect of the predators on prey populations (per capita) is less at high abundances than at low abundances.

8. TYPE I Functional Response
Rigler (1961) Canadian J Zoology 39:
Constant volume of space per unit time
Plateau reached because food caught not eaten
TYPE II Functional response – most common
Feeding Rate determined by Search and Handling Time
Bergman et al (2000) Functional Ecology 14: 61-69
Thompson (1975) J Animal Ecology 44:

9. TYPE III Functional Response – S-shaped
Holling (1959) Canadian Entomologist 91:
Switching…….

10. Food quality also important
Eaten - Available
Living - Dead
Animals eat most nutritious food available and those that die during the dry season do so not only because of the shortage of food but also the shortage of quality food – they are in a poorer condition than those that survive
Sinclair (1975) J Animal Ecology 1974:

11. Diet widths and composition
Types
Monophagous – eat one prey type. E.g.?
Polyphagous – eat many prey types. E.g.?
Oligophagous – eat several prey types. E.g.?

12. Evolution of Diet Width…
Phylogenetic constraints to diet width….

13. Over time, generalists increasingly become specialists due to co-evolution
Predator
Prey Sp 1
Camouflage
Prey Sp 2
Prey Sp 3
Speed
Fossorial habit
More Specialist
Predator
Prey Sp 2
Prey Sp 1
Problems with intra-specific competition if too specialised: also stochastic changes in environment

14. Evolution of Diet Width…
Caroll (1872) Through The Looking Glass and What Alice Found There, MacMillan
Alice finds herself hand-in-hand with the Red Queen, running faster and faster but without getting anywhere.  The Red Queen explains, "Now, here, you see, it takes all the running you can do, to keep in the same place.  If you want to get somewhere else, you must run at least twice as fast as that."
Red Queen Hypothesis, ongoing adaptation may be the evolutionary equivalent of this scene, requiring each species to evolve continuously just to keep in the same place among other evolving species (parasites, competitors, predators, etc.).

15. Food Preferences
Oligo- and polyphagous predators are not indiscriminate – they show preferences
Preferences based on maximizing energy returns
Davies (1977) J Animal Ecology 46: 37-57
Two Types –
Ranked preferences – based on maximizing returns
Balanced preferences – based on ensuring a balanced intake

16. Switching Preferences maybe fixed
Murdoch & Stewart-Oaten (1975) Advances in Ecological Research 9: 1-131
Preferences may vary with prey availability
Murdoch et al (1975) Ecology 56:

17. When tubificids and fruit-flies offered in equal numbers
Switching occurs in the following situations:
Different types of prey found in different microhabitats
Consumers develop a search image towards a common prey
Increased probability in pursuing a common prey
Increased probability of catching a common prey
Increased efficiency in handling a common prey
Often – observed population “preferences” are due to an increase in the number of specialist individuals – not due to an overall change in diet amongst all individuals
When tubificids and fruit-flies offered in equal numbers

18. E = Average energy content of current diet
Optimal Foraging Theory – Behavioural Ecology
Attempts to look at rules that govern the behaviour of individuals when foraging: animals maximize energetic returns
e.g.
Should a predator expand its diet to include additional prey items?
Ei ≥ E
(s + h) hi
E = Average energy content of current diet
s = Average search time of current diet
hi = handling time of the next most profitable prey item
Ei = Energy content of the next most profitable prey item
h = Average handling time of current diet

19. Predicts that:
Individuals with short handling times will be generalists
Individuals with long handling times will be specialists
Other things being equal, an individual in an unproductive or very variable environment will have a broader diet than one in a predictable and productive environment
Predators should ignore insufficiently profitable prey, irrespective of their abundance

20. Foraging in a patchy environment
Food is patchily distributed – and so as a consequence are predators. Predator density in high density food patches is greater than that in low density food patches

21. Two types of behaviour underlie aggregation by predators
1 - Location of patches
2 - Responses of consumers in patches
Change in search pattern after encountering prey
Change in rate of patch abandonment (profit margins)

22. Consumers aggregate in patches when the expected rate of food intake exceeds background levels i.e. profitable
Immediately aggregated predators start feeding – the profitability of the patch declines.
The rate at which the profitability declines will depend on the number of consumers BUT…………
As predator density increases, the predators will also spend an increasing amount of time interacting with each other – and less time foraging – impacts on patch profitability. Known as pseudo-interference

23. THE END
Image acknowledgements –