Predator-prey interactions

Transcription

1 Predator-prey interactions Dik Heg The major types of organism-organism interactions 1. Competition - Interspecific competition (competition between different species) - Intraspecific competition (competition within the same species) 2. Predation - Interspecific predation (predator-prey interactions) - Intraspecific predation (cannibalism, infanticide) 3. Cooperation - Interspecific cooperation (mutualism, symbiosis) - Intraspecific cooperation (kin selection, reciprocal altruism) 4. Parasitism - Interspecific parasitism (host-parasite interactions, e.g. ectoparasites, endoparasites, viruses, pathogens) - Intraspecific parasitism (within-species brood parasitism, e.g. egg dumping, sneaking) I. Overview II. III. Predator-prey population dynamics I. : stages of predation and the antipredator defences 1. Encounter rarity, hiding, activity budget 2. Detection immobility, crypsis, confusion, sensory limits 3. Identification masquerade, confusion, Mullerian mimicry, Batesian mimicry, honest warning colorations 4. Approach (attack) 5. Subjugation (prevent escape) 6. Consumption Overview 1

2 Prey selection Example apostatic selection Predators might select prey based upon: - Species - Size - Morph etc. etc. Apostatic selection: predator preys on more common type of prey Reasons for apostatic selection 1. Formation of search image It takes time to learn to detect and handle prey: known prey preferred 2. Optimal search rate hypothesis Optimal search speeds for prey 1 and 2 differ: first search for more abundant prey 1, until 1 < 2, than switch search rate strategy to 2 3. Search intensity hypothesis ( stare duration ) Prey species 2 might be more difficult to detect Search rate increase: trade-off between increase in encounter frequency and a reduction in detection probability 2

3 Cryptic prey Cryptic prey: colour, morphology and/or chemical profile resembles a random sample of the background as perceived by the predator (at the time and place at which the prey is most vulnerable to predation) Argentine horned frog Sandgrouse chicks Prominent moth Leaflitter frog Aposematic prey Aposematic colouration (or sounds, odours etc.): conspicuous colouration combined with noxiousness (incl. unpalatability) Predator training and hence protection more efficient when: 1. Higher density of the signal 2. Higher densities of the aposematic prey 3. Fewer or more similar aposematic colour patterns around 4. Clumped- or higher local densities of prey 3

4 Aposematic prey How can natural selection favour a defence that operates during or after the subjugation and consumption phases of predation? 1. Kin selection (Fisher 1930) 2. Synergistic selection (Guilford 1985, Maynard-Smith 1989) 3. Combination with a common post-attack defence (Wicklund & Jarvi 1982) Mimicry Mimicry: two species resemble each other (in morphology and often also in behaviour). Batesian mimicry Müllerian mimicry Species 1 Species 2 Batesian: one palatable species resembles unpalatable species Müllerian: two unpalatable species resemble each other Mimicry ring: species complex of groups of Batesian and Müllerian mimics Example: pairs of Batesian mimics Unpalatable swallowtail model species Palatable female races of Papilio memnon Example: pairs of Müllerian mimics from Ecuador and Northern Peru Heliconius melpomene H. erato 4

5 Example: Müllerian mimics of Heliconius 1. H. cydno morphs have increased survival when they match locally abundant co-model species when transferred at low densities (a, c & d). 2. H. cydno morphs do not differ in survival when many H. cydno morphs are transferred leading to high densities of both models (b). Populations A: `white` Müllerian mimics Populations B: `yellow` Müllerian mimics H. cydno Co-models Control (normal transfer, A to A or B to B) Experiment (reciprocal transfer, A to B or B to A) Kapan (2001). Nature 409: Kapan (2001). Nature 409: II. The aim of optimal foraging theory is to predict the foraging strategy to be expected under specified conditions Major assumptions: 1. Natural selection on foraging in the past = present 2. High net rate of energy intake = high fitness 3. Experimental setup = close to natural environment The diet-switch model (McArthur & Pianka 1966) Specialist: only pursues profitable prey, but has to search more. Generalist: also eats more and less profitable prey, has to search less. E = energy content h = handling time prey s h s h s = search time prey i th prey = the next most-profitable prey type Optimal strategy: predator should also pursue i th prey when: E i / h i > E / (s + h) 5

6 The diet-switch model (McArthur & Pianka 1966) The constrained diet model (Belovsky ) Some general predictions from the model: 1. Predators with short handling times should be generalists. 2. Predators should be more generalistic in an unproductive environment (low density = large search time). 3. Predators should ignore insufficiently profitable i th prey irrespective of their density (formula is s i independent). Example using linear programming with herbivore moose Alces alces: Consumption Energy value Sodium (gram) per gram per gram Aquatic plants A 3.8 kj gram Terrestrial plants T 4.25 kj gram How much A and T can and should the moose eat to survive? Maintenance requirement: kj < 3.8A T (minimum energy needed for survival per day) Sodium requirement: 2.57 gram < 0.003A (Only aquatic plants contain sodium, gram needed per day) Digestive limitation: gram > 20A T Constraints, limitations, currencies to be optimized are very often individual (phenotypic/genotypic), population and/or species specific! Predator present Predator (gram of wet food per day which can be maximally processed, aquatic plants contain 5x more water) Number of fish No predator present largemouth bass Micropterus salmoides Prey % Vegetation cover where prey is feeding bluegill sunfish Lepomis macrochirus 6

7 The functional response Functional response (Solomon 1949): the relationship between individual s consumption rate (P) and local food density (N) Three types of functional responses (Holling 1959): The numerical response Numerical or aggregative response: the relationship between the local consumer density and local food density. Type 1 Type 2 Type 3 No handling time At large N: predator Switching, search image, Max P = maximum throughput only handling prey increased (handling) efficiency Interference: functional response and numerical response interact This will lead to animals distributing themselves over the habitats or patches until fitness: Patch use: the marginal value theorem Given: prey distributed in patches (which might differ in quality) prey within patches are depleted by foragers forager pays travel cost to reach other patch Question: at what point should a forager leave a patch? Equal: Ideal free distribution Unequal: Ideal despotic distribution (dominants occupy best patches or have less negative effects from interference) 7

8 Patch use: the marginal value theorem III. Predator-prey population dynamics The basic dynamics of predator-prey and plant-herbivore systems: a tendency towards cycles. Why?? Fig. 10.1c Begon p.370 The Lotka-Volterra Model Description of the changes in abundance of predators and prey: P = number of predators (or consumers) N = number of prey (or biomass) Without predators, exponential increase in N per time step t: dn = rn dt As predator or prey densities increase, more predators encounter prey (PN) and prey will be consumed with attack rate a : dn = rn a PN dt The Lotka-Volterra Model Predator numbers are assumed to decline exponentially through starvation in the absence of prey, q is the mortality rate: dp = -qp dt This is counteracted by predator birth, which depends on predator consumption a PN and the efficiency f of converting this food into predator offspring: dp = fa PN - qp dt 8

9 The Lotka-Volterra Model The Lotka-Volterra Model Predator abundance (P) Prey abundance (N) Despite the underlying tendencies, predator-prey cycles are not always to be expected! Parasitoid present Example: effects of piscivore predator on group-living in the Lake Tanganyika cichlid Neolamprologus pulcher host Plodia interpunctella Piscivore: Lepidiolamprologus elongatus No parasitoid present parasitoid Venturia canescens Group-living fish: Neolamprologus pulcher 9

10 Lake Tanganyika 10

11 Cages (2x2m) Piscivore (control, medium, large) Survival (a) breeder males breeder females large helpers medium helpers small helpers 1.0 large helpers medium helpers survival survival small helpers (b) control medium large predator treatment number of adults Reproduction Feeding rate (a) 40 number of offspring fry juveniles n = Large helpers Medium helpers Feeding rate (bites/minute) (b) control medium large control medium large predator treatment Mean distance from shelter (cm) 11