Tuesday 11th September - Foraging Flashcards
Name the four foraging types and give examples of each
1. Filter feeders
– Animals use specially adapted structures to strain food form water.
– Eat both live and dead material.
– More common in invertebrates.
– But some BIG vertebrates too!
2. Detritivory and scavenging
– Animals that specialise on *dead* material.
– Also ingest live material (microbes).
– Large range of behaviours have evolved that depend on the habitat.
3. Herbivory
– Takes many forms due to different resources available on plants.
- E.g. browsers, miners, seedeaters, pollinators, frugivores, and even “farmers”.
4. Predation
– Animals eating other animals
– can be difficult but the reward is high!
– Evolution of wide range of strategies:
(Trapping, aggressive mimicry, stealth adaptations, adaptation of prey detection)
Investment in time and energy
Successful (optimal) foraging requires balanced investment of which two things?
Time and Energy
S = Search time (to locate food) – shorter if large, widespread, obvious or abundant – longer if small, patchy, cryptic or sparse.
• H = Handling time (to catch or collect and eat food) – shorter if sedentary or grouped (berries, aphids) – longer if well-protected or dangerous (mussels, elephants).
Variation in S and H
• Some foragers live in sight of food that is easy to find but hard to collect (S < H)
– Generalists can switch between many prey and maintain stable numbers
• Others live on food that is patchy and hard to locate but easy to collect when found (S > H)
– Specialist strategy efficient but vulnerable to scarcity.
– True specialization quite rare.
– Seasonal/spatial specialization more common.
Deciding when to give up
G = Giving-up time (before moving to another patch)
– requires a bet on chances of success elsewhere
– G = high if?
– G = low if?
– G = high if? chances perceived as slim and food is evenly distributed
– G = low if? food abundant, easily harvested, but patchy
Variation in S + H energy investment
High? (2 types)
Low? (3 types)
• Searchers
– Frugivorous birds
– pollinators
• Pursuers
– long-distance coursers (wolf, komodo dragon)
• need stamina, cooperation
– short-distance sprinters (cheetah)
• high speed but soon exhausted
Low..
• Ambush (tiger)
– rely on camouflage, surprise
• Traps (web building spiders)
– rely on immobility, fasting ability
• Deceivers (mantis, chameleon)
– rely on perfect imitation, rapid response
The role of food quality
Energy invested in S + H must be less than energy yielded from food
• If S too high?
• If H too high?
• If S too high?
- Search for something else or use a different method
– squirrels and chipmunks reduce S when conditions hard by hoarding winter stores of nuts - but caches can be stolen!
• If H too high?
give up quickly
– cheetahs give up chase after c.200m, must reserve energy for better prospect – general correlation in predator – prey body size. Predators (almost) always larger • high risk and H incurred by small predators attacking large prey (some exceptions!)
Estimating profitability
P.U.V S.Q.D.D?
Equation influenced by characters of potential food items:
• _P_atchy and Unpredictable - means increased profit OR risk of failure to find any, e.g. fruiting trees
- Visibility - if visible, usually unpalatable, e.g. skunk, poison dart frog
• _S_ize - elephant has almost no predators, but huge delicious meal if possible…
• Quality – this depends on the traits that a consumer must maintain – e.g. snails need Ca for shell, spiders need N for silk.
• Defences – makes some food inedible. E.g. millipedes produce everything from phenols to hydrogen cyanide!
• Density - small items profitable for large searchers if very abundant. E.g. whale shark eating krill
Optimal foraging theory (OFT)
OFT assumes correlation between foraging and fitness
- Foraging of individual = ?
- Fitness = ?
- Foraging of individual = success in finding food
- Fitness = lifetime reproductive success
Optimal foraging theory (OFT)
Foraging seen as a series of sequential “decisions”; costs and benefits analysed for each step separately:
1. Food selection
?
2. Searching strategy
?
3. Costs of foraging
?
1. Food selection
a) net profitability b) optimal diet c) nutritive requirements
2. Searching strategy
a) patch dynamics b) marginal value theorem c) search image
3. Costs of foraging
a) energy b) risk of exposure
1a. Food selection: profitability
• Do animals distinguish profitable from unprofitable foods?
– Profitability = net energy gain:
I.e. caloric value minus expenditure on handling + searching
• Crows open whelks by dropping from a height:
- Expense of flight must be exceeded by food gained
– Experiment: minimum drop to break large shell 5m; more drops or higher if shell is smaller.
– total flight height ≈ energy expenditure
– Crows actually choose largest shells and drop from average of 5.23 metres!
1b. Food selection: optimal diet
Optimal diet = ?
– Not merely easy to gather
- Maximises energy gain per unit time spent foraging
– Obviously accept most profitable item whenever encountered, but what to do with 2nd best?
• Depends on abundance of best item, not of 2nd best
– if steak is rare, bread is better than nothing; if steak is freely available, you will bread however abundant.
Optimal diet = when all items profitable
- Tests in captivity confirm that acceptability of less preferred items depends mainly on availability of most preferred
- Tits (Parus major) ate both large and small worms when both scarce; as numbers of both worms increase, specialized on large worms
1c. Food selection: Nutritive requirements
- Moose in N. America have a high-calorie, low-sodium diet of land plants – good for energy intake, but bad for sodium-potassium balance
- Aquatic plants high in sodium but low caloric content.
- Causes moose to have sudden cravings for pond salad, despite low energy value.
2a. Searching strategy: patch dynamics
Choosing a patch
– aggregate where preferred food most common
• Which food supplies can be patchy?
– Flowering trees
– Dung
– Prey dependent on ephemeral water supply
- Describes how to maximise rate of return from patchy resources
- Food will eventually run low in present patch: when should an animal move on?
– When yield for present patch drops below mean for all other patches
– So must pick best patch and keep sampling other patches
• Giving-up time longer if?
• Giving-up time longer if?
– travel cost to next good patch high relative to cost of staying – mean value of all patches declines – value of other patches unknown
2b. Marginal value theorem
• Greater travel times = ?
• Greater travel times also = ?
- The larger the perceived distance among patches, the longer it will take to give up.
- Animal will give up sooner if distances short to escape diminishing returns
• Greater travel times = lower rate of energy gain/time
• Greater travel times also = higher energetic expenditure, thus greater risk of giving up.
Optimal foraging in Blue whales
Foraging dive aborted
• Whales should give up sooner if krill at low density, even if costs are lower due to shallow dive
- Data from tagged whales show they match diving depth and foraging lunges to patches with highest krill densities.
• Whales optimize energetic efficiency as a function of prey depth and density.
References Please read text (Goodenough et al) chapter 12.
Other reading:
- Krebs & Davies 1993. Introduction to Behavioural Ecology 3rd ed. Blackwell Science, Oxford
- Reece et al (2015). Campbell Biology 10th ed. Pearson Australia, Melbourne VIC
- Hazen, E. L., Friedlaender, A. S., & Goldbogen, J. A. (2015). Blue whales (Balaenoptera musculus) optimize foraging efficiency by balancing oxygen use and energy gain as a function of prey density. Science Advances, 1(9)
