Community Ecology BCB331

1. Community Ecology BCB331 Mark J Gibbons, Room Z108, BCB Department, UWC Tel: 021 959 2475. Email: mgibbons@uwc.ac.za Image acknowledgements – http://www.google.com

2. 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 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

3. 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 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

4. 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 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……….

5. 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 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 Murton et al (1966) J Applied Ecology 11: 61-81 Link between predation and prey populations not easy

6. BUT…. 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. The effects of consumption on consumers An increase in the amount of food consumed leads to increased growth and reproduction – subject to intraspecific competition 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.

7. TYPE II Functional response – most common Feeding Rate determined by Search and Handling Time TYPE I Functional Response Rigler (1961) Canadian J Zoology 39: 857-868 Constant volume of space per unit time Plateau reached because food caught not eaten Bergman et al (2000) Functional Ecology 14: 61-69 Thompson (1975) J Animal Ecology 44: 907-916

8. TYPE III Functional Response – S-shaped Holling (1959) Canadian Entomologist 91: 385-398 Switching…….

9. Generation Time = 30 days 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 Biomass 10 Time (days) Diatom Bloom in Southern Benguela

10. Living - Dead Eaten - Available Food quality also important 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: 497-520

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. Camouflage Speed Fossorial habit More Specialist Predator Prey Sp 1 Prey Sp 2 Over time, generalists increasingly become specialists due to co-evolution Generalist Predator Prey Sp 1 Prey Sp 2 Prey Sp 3 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. Preferences based on maximizing energy returns Food Preferences Oligo- and polyphagous predators are not indiscriminate – they show preferences 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. Preferences maybe fixed Preferences may vary with prey availability Switching Murdoch & Stewart-Oaten (1975) Advances in Ecological Research 9: 1-131 Murdoch et al (1975) Ecology 56: 1094-1105

17. 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 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

18. Ei≥ E hi (s + h) E = Average energy content of current diet s = Average search time of current diet h = Average handling time of current diet Ei= Energy content of the next most profitable prey item hi= handling time of the next most profitable prey item 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?

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. 1 - Location of patches 2 - Responses of consumers in patches Change in search pattern after encountering prey Two types of behaviour underlie aggregation by predators 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 – http://www.google.com