Mark J Gibbons, Room 4.102, BCB Department, UWC Community Ecology BDC331 Mark J Gibbons, Room 4.102, BCB Department, UWC Tel: 021 959 2475. Email: mgibbons@uwc.ac.za Image acknowledgements – http://www.google.com

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

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

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

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

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

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.

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 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: 907-916

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

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: 497-520

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.?

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

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

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.).

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

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: 1094-1105

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

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

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

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

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)

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

THE END Image acknowledgements – http://www.google.com