TESTING (O)lpt]blnnlaiI]Foarlg"in(g lTlhlIeaor\y USINGBIRD PREDATION ON GOLDENRODGALLS CHRISTOPHERJ. YAHNKE A ire you the type of personwho would drive gall.The gall of Eurostahas a characteristicspherical shape 10 miles to the nearest BurgerKing instead of eating at (Figure 1). It is distinguishablefrom the gall of another the McDonald's down the block? How many freeway gall-forminginsect, the ellipticalgall moth (Gnorimoshema food billboards will you pass up looking for that perfect gallaesolidaginis),which forms a long, narrowgall (Weber 0U) submarine sandwich before you give up and settle for et al., 2002). The larva will graze on the inner tissue of anything? Too tired to cook and would rather grab a the gall and remainin the centralchamber of the gall for 0- Downloaded from http://online.ucpress.edu/abt/article-pdf/68/8/471/53754/4452045.pdf by guest on 28 September 2021 pizza? Although I don't personally condone fast food 50 weeks, emerging as an adult fly the following spring culture, the gastronomic choices we make are a good to begin the cycle all over again (Weis & Abrahamson, place to start when discussing optimal foraging theory 1998). Gall flies are particularregarding which species of with our students because we, like animals, live in vari- goldenrodthey parasitize.Across the northernstates from able environments and are often forced to make deci- New Englandthrough the GreatPlains, the common gall- sions about where to forage, how long to forage, what formingspecies most likelyto be encounteredin meadows types of food are available,etc. Optimal foraging theory or along roadsidesinclude Canadiangoldenrod (Solidago >4 attempts to explain these behaviors in terms of costs canadensis)and late goldenrod (S. gigantea),whereas the and benefits (Molles, 1999; Ricklefs, 2001). A bird fly- common gall-formingspecies in the Mid-AtlanticStates is ing through a meadow is likely to encounter a variety tall goldenrod(S. altissima)(personal observation, Weis & I_ of different food types. Each potential food item has an Abrahamson,1998). intrinsic value based on its nutrient and energy content, While inside the gall, the larva may be parasitized ,- how much time is requiredto extract the food (handling by the parasitoidwasp Eurytomagigantea or preyed upon time), and how long it takes to locate the food (search by birds or small mammals.Birds such as downy wood- time). Optimal foraging predicts that, given equal han- peckers and black-capped chickadees open galls and dling time, the prey item with a large energy gain will be extract larvae during winter when other food is scarce chosen over the prey item with a small energy gain. Also, (Abrahamson& Weis, 1997; Schlichter,1978), and squir- animals are more likely to select prey with a large energy rels may also use gall fly larvaeas a food source in winter gain when availabilityis high because it decreases search (Shealeret al., 1999). In general,wasps parasitizesmaller time. When search time increases, animals are less selec- galls because the ovipositor of the wasp can reach the tive (Abrahamson& Weis, 1997; Anderson, 1984; Krebs centralcavity of the gall (Weis et al., 1985). Evidencesug- et al., 1977). A number of optimal foragingexperiments gests that in some areas,predatory birds like the downy have been illustrated in The AmericanBiology Teacher woodpecker show a marked preference for larger galls, (Rop, 2001; Wellborn, 2000). Here I present a simple field experiment investigating the goldenrod gall fly and its winter predators, mainly birds. The experiment Figure1. is designed to get students to work together in groups, Gallformation on Canadiangoldenrod (Solidago canadensis). solve problems together in the field, understand the Goldenrodgalls showing disturbance by downy woodpecker design of ecological experiments and hypothesis testing, and learn basic statisticalcalculations. (B)black-capped chickadee (C) and gall fly emergence (D). The goldenrodgall fly (Eurostasolidaginis) is common and widely distributedin North America.A femalegall fly typicallylays a single egg in the terminalbud of a newly emerginggoldenrod stem in spring,and maylay a hundred eggs over the course of her two week adult lifespan(Weis & Abrahamson,1998). After the egg hatches, the larva will burrowits way throughthe bud, down the stem, and induce the plant to producea tumor-likestructure called a CHRISTOPHERJ. YAHNKE is AssistantProfessor and Curatorof Birds and Mammals, Department of Biology and Museum B C ID of NaturalHistory, University of Wisconsin-StevensPoint, AG Stevens Point, WI 54481; e-mail: [email protected]. OPTIMALFORAGING THEORY 471 possibly because a smaller gall is likely to contain larvaof the parasiticwasp (Weis & Table1. Abrahamson,1998). The larvaof the para- Sampledata table that students use to recordgall measurements. sitic wasp is about one tenth the size of Undisturbedgall Disturbedgall (chickadees) Disturbedgall (woodpeckers) Eurostalarva, and thereforea less reward- ing meal energetically.Also, Shealer et al. Length(mm) Width(mm) Length(mm) Width(mm) Length(mm) Width(mm) (1999) found that larvaof Eurostais signifi- cantly smaller in smaller galls than larger galls. A classic study by Tscharntke(1992) looked at a similarsystem in Germanyand found that the blue tit, a predator of the gall fly Giraudiellainclusa, responded both to gall abundance between habitats and gall clusters within habitats.This observa- tion is consistent with optimal foraging theory as there is a potentially higher energeticbenefit of being in a high quality habitatpatch versus a lower qualitypatch. Downloaded from http://online.ucpress.edu/abt/article-pdf/68/8/471/53754/4452045.pdf by guest on 28 September 2021 Also, search time for the next food item will decrease in patches where resources Plotnumber Totalnumber of galls l are clustered and where there is a higher density of resources. Figure2. Scatterplot of galllength versus width from data collected in the field. Experiment 1: Prey Choice Studentscan draw this by hand or generate it usingsoftware packages like Objective Excel. Test the hypothesis that birds choose largergalls 50 (null hypothesis: birds do not choose galls based on Disturbed Galls *. size). Optimal foraging theory would predict that for- UndisturbedGallsU aging birds would select largergalls than are randomly 40 U availablein the environment. Materials& Methods '30 I suggesthaving students read Weis and Abrahamson (1998) during the week this lab will be conducted to ~20 understand the interactions involved. It will also aid students in thoughtfullyinterpreting the results. 10 Each group of students will need a dial caliper, pencil, and datasheet (Table 1). When you arriveat the study site, demonstrateto the students how to measure 10 20 30 the length and width of a gall using the caliper.Length Gall Width (mm) is more challenging to measure than width, and the imnortant noint to emnhasize is consistencv. I have Results students measureat the point where the gall begins to widen dramatically,rather than where the stem begins to widen The level of statisticalanalysis I requirein class depends (see Figure IA). Also show them examples of undisturbed on the course I am teaching. For my non-majors biology galls and disturbedgalls (Figure 1). Black-cappedchickadees course, I show them how to calculate mean, variance, and make messy, largeirregular holes whereas downy woodpeck- standard deviation using Excel, and how to generate a scat- ers locate the emergence tunnel of the gall-makinglarva ter plot of disturbedversus undisturbedgalls to visualize the (Abrahamson& Weis, 1997). One source of confusion may data (Figure 2). For biology majors and ecology students I be distinguishingfly emergence from avian disturbance.My show them how to run an independent-samplest-test (using rule-of-thumbis that if the hole has a clean border (i.e., it gall width as the test variable and testing chickadees and appears to have been made with a fine drill bit), it is likely woodpeckers separately)using statistical software available emergence(Figure ID). Also, it may be difficultto differentiate to them (in our case SPSS). black-cappedchickadee and downy woodpeckerdisturbance, so I have also pooled these and simply compareddisturbed Experiment 2: Patch Choice versus undisturbedgalls. Have the groups spread out at the study site. Eachgroup is responsiblefor takingmeasurements Objective on all galls found within a 1G.m2plot. Plot size can be varied Test the hypothesis that birds forage more in patches depending on the density of goldenrods at your study site. with higher gall densities (null hypothesis: birds do not 472 THEAMERICAN BIOLOGY TEACHER, VOLUME 68, NO.8, OCTOBER2006 choose patches based on gall densities). Optimal foraging Materials & Methods theory would predict that a greater proportion of galls This experimentdoes not requirethat you measureindi- would be disturbed in goldenrod patches with high gall vidual galls, but can easily be combined with Experiment densities than patches with low gall densities. One because you alreadyhave the data required.Each group Table2. Figure3. Sampledata table that students use to recordclass data for Experiment 2. Scatterplot of killingvalue and galls/mr with PlotNumber NumberofGalls/in NumberDisturbed KillingValue "bestfit" line from the regressionequation Y = a + bX,where Y is the dependentvariable (killing value),X the independentvariable (galls/m2), b is the slopeof the lineand a is theY-intercept. 0.8 Downloaded from http://online.ucpress.edu/abt/article-pdf/68/8/471/53754/4452045.pdf