How do they get their food? Saw earlier in simple way, plants vs meat But is it that simple? What are some of the considerations in an individual actually selecting the food it eats? Chap 5 deals with this
How do they get their food? Basically “rules” in selecting food and where to forage for that food. Initial thing to keep in mind: Foraging animals know what they are doing!!! Why? Their life depends on it! Not just balls bouncing around!
“optimal” foraging theory IF they know what they are doing, then we should see some “optimization” occurring. What are they optimizing? Ratio between costs and benefits. Early theory developed with optimization in mind. Lot of critics…Why?
Arguments against optimizing Optimizing means eventually leading to the BEST way of doing it. Kind of like Evolution’s “survival of the fittest” Should lead to one “optimal” form. Better is: survival of adequate And: Adaptive foraging theory. Semantics? Reflects individual’s constant adjustment to changing conditions.
Ok, Adaptive foraging theory How do they do it? 4 possible areas where they can affect their energy intake vs costs of getting the energy. 1) What they eat: Diet 2) Where they eat: patch choice 3) How long they eat: Patch residence 4) Where do they go next: Patch movement
Foraging rules: diet selection All species have a specific range of food items they use: Why?? Some have very limited diets: specialists Others have wider diet range: generalists Why? “generalists” why do they choose specific foods at specific times? What are the “rules” in selecting a diet
Foraging rules! Diet selection: what factors to take into consideration? Caloric value: obviously Ease of handling: Pecans vs hickory nuts deer vs cow Risk: of being killed… of being injured. Will see later with patch use.
Rules “Generalists” rules Initial models developed from data on feeding response to food density Type II feeding response Forager can only eat so much!
Diet rules Basically, should broaden diet when principle prey 1 drops below a certain level. Considered “optimal” strategy. Here
General predictions Should rank food types relative to profitability Should always include most profitable prey and only expand to less profitable when 1st does not meet needs Thus decision to switch is based on abundance of most profitable not less profitable All or none response: either always accept or never accept them.
Do they hold?? Most controlled experiments seem to produce expected results Great tits (small bird) Reduced use of large worms when reduced in number.
Do they hold? Works the best for herbivores or predators of sessile prey Not so well with predators and mobile prey Also not so well under predation risk. Does not include other needs e.g. minerals, proteins, etc. Not just all Kcal!
Predators and prey Even at zero rabbit abundance, coyotes still had 30% occurrence in their diet!!
Diet summary • Know for sure: • 1) all species have specific diets • 2) some narrow, some broad • 3) even broad diet, have preferred food • Not so sure: • 1) why they have the diet they do! • 2) rules for selecting
Next is patch selection Why “patch” selection? Very few habitats uniform, habitat mosaic is often rule. So… if habitat is patchy, so will food availability. Thus, animals have to select among existing patches: which one to go to. Again, trying to balance gains and costs
Balance • Benefits: How much and quality of food patch supplies: patch quality. • Costs: How much energy needed to get to patch: central place foragers this is biggie • Costs: How much energy needed to harvest food that is there. Again, not just quantity of food resource but how easy it is to get/handle, etc.
Patch selection • Criteria? • 1) Patch quality: resource quantity/quality Similar to diet selection: “should” pick “best” patch to feed in. • What assumes? • - animal’s knowledge of patch locations/quality: Reasonable? • Perfect or imperfect information?
Criteria • Other factors? • - patch depletion rate: as patch is used or has been used, quality goes down. 1) herbivores: renewable/nonrenewable 2) Not so much reduction in # of prey but “catchability” of prey, will deal with later.
Other criteria • - patch predation risk: Is it worth going to high quality patch where you might get killed? So not just patch quality food wise • Introduces “predation risk” into foraging equation as a foraging cost. • Actually transform risk into potential energy loss
Predation risk and patch choice • H = C + P + MOC • Harvest rate (H), what it can get out of the patch. • C = what it needs for metabolism • P =what it could lose (on average) from predation • MOC Missed opportunity costs: energy used for other things instead of eating: grooming, displaying, etc.
What does this tell us? • In patch selection, the patch has to be higher quality than just supplying metabolic needs • This means forager can juggle C and P! • E.g. could go to patch of lower quality IF had lower predation risk! • E.g., could “take the chance” going to high quality/risk patch but for shorter time because of higher harvest rate!
What does this mean? • Much more foraging options than just pure patch quality. • Also affects how long they stay in patch, 3rd area!
Patch residency • Ok, will select a patch based on food quality that gives good balance when it enters the patch. • But patch becomes depleted as it stays in the patch. • When should it leave???
Options • Could stay until all food harvested. • But…. Diminishing returns, longer it harvests, less it gains per time/effort! • Eventually, it would be better (more profitable) if it moved to another patch of now higher quality!!
Options • But when should it do it??? • “leaving rules”: when should an animal leave a patch? • Early idea: Charnov 1976 proposed the “Marginal value theorem” as a leaving rule
Marginal value theorem • Consider: 10 patches of different value. • Can calculate average value of these ten patches (V1 +V2 + V3….+V10/10) • Some will be above average • Some below average. • Assume (based on previous discussion) animal will first enter an above average patch.
Marginal value theorem • As it forages, value of patch approaches overall average. • MVT states that when the value of that patch reaches average of all patches, animal should leave • Why? Because there are now other patches out there that are more valuable! • Goes to one of these, harvest rate/time would be higher than if it stays!
Sound pretty simple! • Is there support for this idea? • Some, lots of studies looking at ”quitting harvest rates” and in general animals do leave BEFORE resource depleted, so there is some type of leaving rule • Whether it is the MVT becomes debatable. • Especially if we add predation risk!!
Predation risk and patch residence time • How does the threat of being eaten affects how long you stay in a patch?? • Again, not just food value of patch but probability of being killed
Patch safety • In “safe” patch, animal can afford to stay longer,= lower quitting harvest rate. • In “dangerous” patch, longer you stay the more likely you will be killed! • Again, a balance between foraging and safety.
Measuring predation risk • We know how we can measure food resources, saw earlier. • How do we measure predation risk?? • Obviously important in understanding/predicting patch selection and residency time.
Giving up densities • As mentioned, safer areas should have lower quitting harvest rates: can measure but difficult. Need to observe animals and measure how much they are harvesting just before they leave. • Brown 1988 demonstrated you can use not the harvest rate when they leave but what they leave behind, Giving up densities (GUDS)
Guds • What?? • Given any resource, animal will not eat all of it. Will leave some, gets to point not worth eating rest: • What it leaves is its GUD • More important Brown demonstrated that how much it leaves is related to predation risk!
How do you measure GUDs? • Artificial feed trays/boxes: • Mix in standard amount of seed/food • Inedible substrate
They come • Animals come and search for the seeds • Give up depending on how dangerous it is!
So what do you get?? • Make comparisons between patch types. • Identifies safe vs risky habitat for species
Patch selection and residence • All this helps to adjust the patch selection and residence time rules. • With these types of studies can adjust for predation risk. • Leads to better understanding of how animals use the habitat they live in. • Will see more later.
Patch movement rules • Ok… have idea on why they might select patch and how long the might stay, but… • Once you leave a patch, where do you go? • This leads to 4th concern, patch movement rules.
Patch movement here • What are factors to consider? • Quality obviously • Predation risk, obviously, • Travel time is another one: if travel is energetically costly, distance figures in • Travel risk: if you have to pass through dangerous area, becomes important in your decisions
Dangerous travel, example • Sheep often have to travel through wooded areas to get from one open area to the next.
Summary • Have now seen that foragers need to consider the 4 aspects of foraging. • Have seen that patch quality is dependent on food value and predation risk • Result is a pattern of patch use by an individual
Patch use to habitat use • Though we talk about patch use relative to foraging, patches represent habitats • So… foraging represents or reflects a large part of the habitat use patterns of a species. • Habitat use: spatial landscape use patterns of an individual related to the types of dominate vegetation in use and non-use areas.
What more is important? • What other considerations relative to “patch use” (habitat use) are there? • Move away from getting energy to the MOC’s
Missed Opportunity Costs • From a foraging basis, these are all those things that “get in the way” of eating! • Obviously important, more important at the appropriate time than eating!! • What are they?
MOCs • One big one is resting/sleeping! • Although ruminants can combine resting and eating (re-eating), don’t take in new food. • Few can eat and sleep at the same time! • A large portion of the 24 hours is devoted to these different stages of rest (doing nothing with eyes open – sound sleeping
Rest and sleep • Not concerned with reasons for nor physiology behind them but how do they affect habitat use. • Where to rest/sleep? • Often not same areas where you eat. • Comfortable and/or safe (predation again!)
Resting and Sleeping • Comfort: cool if it is hot, warm if it is cold • So varies with season/weather • Most species will have preferred warm/cold day resting or “loafing” sites. • Nothing like a sunny rock on a cool day! • When most likely to rest? After meals of course!
Resting/sleeping • When most likely to sleep? • For many, night-time (a lot of wildlife species are diurnal). Most birds, squirrels, primates, etc. • For them, finding a sleeping spot that affords protection from night chill.
When do you sleep? • For many, sleep during the day! Nocturnal or crepuscular. • Cre..what! Most active around sunrise/sunset • For these, daytime sleeping sites often are to alleviate daytime heat.