Why Study Diets??

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

Holling’s Disc Equation

Rate of Energy Gained

= ( λ e – s )/(1 + λ h ) 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 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.

Holling’s Observations

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

Functional Response

Type I

passive predators

Functional Response

Type II

Handling time limited

Functional Response

Type III

Learned response

Functional Response

Functional response = same # of predators in area; behavioral change

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")

Numerical Response

+ = Increased Reproduction + =

Numerical Response

• Aggregational Response

Numerical Response

• Hollings equation relates diet information to energy and time spent foraging • More specific physiological energetic needs can be described using Bioenergetics

Bioenergetics Consumption = (Respiration, Digestion, Activity) + (Excretion) + (Reproduction, Δ Growth) From Kitchell

et al.

1977

Why Collect Diets?

1.

2.

3.

4.

Fish’s energy: growth and reproduction Aquaculture: assess stock foraging Resource managers: stocking, habitat assessment Environment: indicate change in habitat, population densities

Environmental Change

• An example from my research

Little Rock Lake

Whole Lake Manipulation

Little Rock Lake Results

LRR LRT * Year

Collecting Fish

Long term gill net, fyke net, minnow trap Active sampling techniques (seine, short term gill nets, angling, shocking) • Beware of biases -postcapture digestion -regurgitation (stressed fish) -atypical foraging behavior in traps

Collecting Diets • Collect diets by: 1. Gastric Lavage 2. Stomach Removal -Remember fish size, population density

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

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

Enumerating the Diet

• The “Big 3” 1. Frequency of occurrence 2. % composition by number 3. % composition by weight • Diet Indicies

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

% 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%

% Composition by Number

Brown trout #1 Brown trout #2 Brown 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%

% Composition by Weight

• 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

Diet Indicies

• Index of Relative Importance (IRI) IRI = (% number + % weight)(FO) • Consistency • Overlap • Selectivity *all of these are arbitrary units!

Isopod Fantail Darter Amphipod Diptera