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Published byBarry Garrison Modified over 8 years ago
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Why Study Diets??
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The Life of a Diet Sample Background to foraging Why is it important to analyze diets? Collecting diet samples Identifying diet components Quantitative description of diet samples
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Holling’s Disc Equation C.S. “Buzz” Holling Holling, C. S. 1959. The components of predation as revealed by a study of small mammal predation of the European pine sawfly. Canadian Entomologist 91:293–320. Rate of Energy Gained = (λe – s)/(1 +λh) λ = rate of encounter with diet item e = energy gained per encounter s = cost of search per unit time h = average handling time Search Encounter Pursuit Capture Handling
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Predation rates ↑ with ↑ prey densities happens due to 2 effects: 1.Functional response by predator -Type 1 -Type 2 -Type 3 2.Numerical response by predator -Reproduction -Aggregation Holling’s Observations
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Functional Response Type I passive predators
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Functional Response Type II Handling time limited
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Functional Response Type III Learned response
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Functional Response Functional response = same # of predators in area; behavioral change
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Numerical Response ↑ predation due to ↑ predators Two Potential Mechanisms 1. ↑ prey density = ↑ consumption = ↑ predator reproduction = ↑ rate of consumption (↑ reproduction) 2. Attraction of predators to prey aggregations ("aggregational response")
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Numerical Response + + = = Increased Reproduction
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Aggregational Response Numerical Response
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Hollings equation relates diet information to energy and time spent foraging More specific physiological energetic needs can be described using Bioenergetics Numerical Response
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Bioenergetics (Respiration, Digestion, Activity) + (Excretion) + } (Reproduction, Δ Growth) Consumption = From Kitchell et al. 1977
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Why Collect Diets? 1.Fish’s energy: growth and reproduction 2.Aquaculture: assess stock foraging 3.Resource managers: stocking, habitat assessment 4.Environment: indicate change in habitat, population densities
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Environmental Change An example from my research
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Little Rock Lake Whole Lake Manipulation
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Little Rock Lake Results
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Collecting Fish Active sampling techniques (seine, short term gill nets, angling, shocking) Beware of biases -postcapture digestion -regurgitation (stressed fish) -atypical foraging behavior in traps Long term gill net, fyke net, minnow trap
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Collecting Diets Collect diets by: 1. Gastric Lavage 2. Stomach Removal -Remember fish size, population density
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Experimental Strategies 1.Diel patterns (predators and prey) 2.Seasonal patterns (predators and prey) 3.Fish size/gender 4.Digestion rates -slow = over represented (mouse bones) -fast = under represented (earthworms) -correct for these by determining gut passage time for each diet item
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Identifying Diet Items Categorize diet items What is the question you are asking? -More specific taxonomic keying is more information but could be wasted time Broken items: count body parts (# of heads) Sub-sample small diet items
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Enumerating the Diet The “Big 3” 1. Frequency of occurrence 2. % composition by number 3. % composition by weight Diet Indicies
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Frequency of Occurrence Percent of individual diets that contain one or more of a specific diet item Presence/absence indicator - Example: 12/15 walleye diets contain crayfish, frequency of occurrence =.8 = 80% High frequency of occurrence ≠ energetically important, rather selectivity of a group of individuals
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% Composition by Number The number of an individual diet item relative to the total number of items in the diet/diets -Example 1: Brown trout #1: Amphipod = 3 Fantail darter = 1 Amphipod % composition by number = ¾ =.75 = 75%
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% Composition by Number Brown trout #1Brown trout #2Brown trout #3 -Example 2: Sampling event #1: # midge larvae = 3 # total diet items = 11 Midge % composition by number (for this sampling event) = 3/11 =.27 = 27%
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Weight of one type of diet item relative to the total diet weight 1.Wet weight: quicker to obtain 2.Dry weight: more energetically informative Can be calculated similarly to examples shown for % composition by number % Composition by Weight
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Diet Indicies Index of Relative Importance (IRI) IRI = (% number + % weight)(FO) Consistency Overlap Selectivity *all of these are arbitrary units!
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Isopod Diptera Fantail Darter Amphipod
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