CEC Research | https://doi.org/10.21973/N3R36F Fall 2017 1/6

Foragingstrategiesandresourcepartitioningofdesertharvesterants

AliBrunschwiler1,AmeliaMaurer2,JohnParsons3,SageRadecki2,

JoeScullin4,SelenaVengco2

1UniversityofCalifornia,Davis,2UniversityofCalifornia,SantaCruz,3UniversityofCalifornia,SantaBarbara,4UniversityofCalifornia,SanDiego

ABSTRACT

Resource limitations force desert species to forage efficiently and partitionresources. Ant species have different foraging strategies, which may act as amethod of niche partitioning. We aimed to measure differences in foragingbehavior between colonies of Messor and Pogonomyrmex. Using anexperimental treatment of seeds, we measured collection rate and resourcepreferencebetweenhigh and lowdensity resources between the two species.We found significant differences in multiple aspects of foraging behavior,includingoverallabundanceofseedscollected,proportionofdifferentresourcescollected, and the effect of time on resource preference. These differencessuggest a mechanism for resource partitioning between Messor andPogonomyrmex.Additionally,theroleofthesetwospeciesinseeddispersalmaybeinfluencedbytheirforagingstrategies.

INTRODUCTION

Deserts are among the mostabiotically stressful ecosystems due tolow rates of precipitation and extremetemperatures. As a result, naturalselectionhasfavoredspeciesthatadaptto these extreme conditions byoptimizing energy efficiency (Davidson1977).Optimal foraging theorypredictsthat animalswillmaximize the amountof energy gained when foraging whileminimizing the amount of time andenergy lost. Additionally, species withsimilar niche requirements can reduceenergy losses associated with

competition by partitioning resources(Hutchinson 1978, Begon et. al 1986,Ricklefs1990). Sedentary ants form colonies in fixed

locationsandrarelymove,limitingtheirforaging abilities to a small area justaround their nest. Since limitingresources may impose strong selectivepressures in deserts, foraging behaviorand behavioral cohesion of a colonymayreflecttheirefforttomaximizethedifference between resources gainedand energy lost while foraging(Bernstein 1975). In areas of higherabundance, sedentary ants have beenobserved utilizing a solitary foraging

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strategywherescouts leave thenest ina certain direction until food is found.Once the food is returned to the nest,theantwill returndirectly to theplacewhere it last found food and beginsearching for more food. This limitscompetition between foragers of thesame colony, and maximizes the totalsurfaceareacoveredbyasinglecolonywhileforaging.Inareaswhereresourcesare scarce, sedentary ants are typicallyobserved utilizing a group foragingstrategy where scouts forage indifferent directions and recruit moreforagers once a food source is found.Thismaximizestheirgatheringpotentialin areas of scarce resources by puttingthe colony’s efforts towards highdensityfoodresources.Two common ants found in arid

regions of the southwestern UnitedStates are Messor sp. andPogonomyrmex sp. (both hereafterreferred to by Messor andPogonomyrmex, respectively). Bothgenera are harvester ants althoughMessor tends to utilize the groupforaging strategy while Pogonomyrmexgenerally uses the solitary foragingstrategy (Rissing and Pollock 1984). Inorder to quantify the differencesbetweenthesetwoforagingtechniqueswe examined ant responses to highdensity (clumped) food sources vs. lowdensity(dispersed)foodsources.Inourstudy,weexaminedhowthese

two ant genera utilized high vs. lowdensity resources. By laying out equalquantities of clumped and dispersedseeds, we were able to compare seedintakeandproportionofeach resourcetypecollectedbetweenthetwospeciesovertime.IfMessorandPogonomyrmex

can coexist they must be able toresource partition through foragingstrategy. Our research allowed us toassess the resource dispersionpreference of both Messor andPogonomyrmex.

METHODS

2.1StudyArea

We conducted our study at theSweeney Granite Mountains DesertResearchCenterintheMojaveDesertofCalifornia (34.7813° N, 115.6538° W).The reserve hosts a wide number ofgranivorous ant species, and iscomposed of mixed cactus and yuccascrub habitats. We observed antcolonies located at Norris Camp andCottonwood Basin, locations that weresimilar inhabitatandabioticconditionsincluding temperature and vegetativefeatures.Oursamplesiteswereatleast5 m apart to avoid inter-colonyinteractions, and we only surveyedactively foraging colonies. Weconsideredacolonytobeactiveifthereweremorethan10antsoutsidearadiusof0.5mfromthecolony.Wesurveyed12 Pogonomyrmex and 11 Messorcolonies,23intotal.

2.2SamplingDesign

Weconductedourfieldsamplingoverthecourseof fivedays,November4–8,2017. At each colony, we baited antswith 8 g of bulgur seeds. Our twotreatments included seeds set out athigh (clumped) and low (dispersed)densities. For the high densitytreatments, we set out four 1.0 gramclumps of seeds 4 cm in diameter and0.5metersfromthecolonyentranceat

CEC Research | https://doi.org/10.21973/N3R36F Fall 2017 3/6

eachofthefourcardinaldirections.Forthe lowdensity (dispersed) treatments,we sprinkled 4.0 g of seeds evenly

Figure1.Experimental layout.Largeredcirclesrepresent 1.0 g clumped seed treatments andbluedottedlinerepresents4.0gofseedsevenlydispersed0.5mfromcolonyentrance.

around the colony entrance at a 0.5mradius (Figure 1). In order todifferentiate between clumped anddispersed seeds in the field, we dyedseeds in different treatments differentcolors using food coloring (WatkinsTMAssorted Food Coloring). One color ofseedswasusedforthefourhighdensityclumps, and a different color for thedispersed arcs. To avoid potentiallyconfounding effects of different colorsofdyeonexperimental treatments,werandomized colors of clumped anddispersedseedseachsamplingday. At each colony we observed ant

activityfor60minutes,beginningwhenall seedswere set out.We divided theobservation period into five minuteintervals to observe changes in seedcollection over time. We recorded thecolor of every bulgur seed carried intothecolonyinordertodistinguishwhichtreatmenttheantcollecteditfrom.

2.3StatisticalAnalysis

We excluded the last fifteenminutesof our observation period, because weobserved that the number of seedscollected per interval began to declineafter 45 minutes, indicating that seedresources may have been exhausted.Thus, seeds may have been collectedbased on availability rather thanpreference. To test for differences in handling

time between species we used a two-tailed t-test.We defined handling timeasthetimebetweenanantfirstpickingup and first returning a seed to thecolony.Wedidnotdistinguishbetweenclumped and dispersed resources forhandling time. Inorder todetermine ifthereweredifferencesinthenumberoftotalseedscollectedbyspeciesperfiveminute time interval, we performed atwo-tailedt-test.Atwo-tailedt-testwasalso used to test for differences in theproportion of clumped resources thateach species collected over the wholetimeperiod.WeusedaANCOVAtestingthe effects of ant genus on proportionof clumped seeds taken with timeintervalasacovariate.

RESULTS

We found no significant difference inhandlingtimebetweenspecies(n=207,p = 0.467). On average, Messorcollected significantly more seeds perfive minute time interval thanPogonomyrmex (n = 207, p < 0.0001).Messor collected over twice as manyseeds per interval thanPogonomyrmex(Figure 2). Additionally, 31.3% of theseeds thatMessor collectedwere fromclumped resources, whereasPogonomyrmex collected 21.6% fromclumpedresources(n=207,p<0.0134;

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Figure 4). The proportion of clumpedseeds taken by ants increasedsignificantly over time (n = 207, p <0.003;Figure4).Additionally,therewas

Figure 2. Total seeds per time interval byspecies.Mean number of seeds collected perinterval±sebetweenantspecies.

Figure 3. Proportion of clumped resources byant species. Mean proportion of clumpedresources.

Figure4. Proportionof clumped resourcespertime intervalper species.Black line representsMessor,redlinerepresentsPogonomyrmex.a marginally significant interactionbetweentheeffectsofspeciesandtimeon proportion clumped seeds collected(n = 207, p < 0.056 ; Figure 4). Overtime, the proportion of clumped seedstakenbyMessor increasedbya4%perinterval, while the proportion taken byPogonomyrmex did not increase overtime.

DISCUSSION

We determined that Messor andPogonomyrmex exhibit differences inmany aspects of their foragingstrategies, including harvesting rate,resource preference, and shifts inforagingbehaviorovertime.Despitethesimilarity of individual efficiencybetween these two species, Messorcoloniesasawholewereabletocollectseeds at twice the rate ofPogonomyrmex colonies. This suggeststhat differentiation between the twospecies is occurring at the level of thecolony,notattheleveloftheindividualforager. Because foraging efficiencydoes not differ between individuals ofMessorandPogonomyrmex,colonysizeand activity levels may be driving the

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differences in collection rates weobserved.Furthermore,Messorcoloniesfocused

a higher proportion of their foragingeffort on clumped seeds thanPogonomyrmex colonies. This suggeststhatMessor isbetteratexploitinghigh-density resources thanPogonomyrmex,perhaps by use of a group-foragingstrategy.Additionally,Messor exhibiteda greater increase in the proportion ofclumped seeds over time thanPogonomyrmex. Messor’s preferenceforhigh-density resourcesmay indicatethat they shift their energy away fromcollecting the dispersed seeds as theydiscovermoreclumpedseedsovertime.Meanwhile, Pogonomyrmex does nothave as strong of a preference forclumped seeds and therefore does notchange its foraging patterns as itdiscovers new seed clumps. Theincreaseinproportionofclumpedseedscollectedmayalsobeattributabletothehigher encounter rate of dispersedseeds in the beginning of theexperiment. As dispersed seeds werecollected, a higher proportion ofclumpedseedsbecameavailable,whichcontributed to the increase in clumpedseedscollectedlateron. Foraging strategy differentiation has

been suggested as a mechanism forspecies coexistence (Laughlin andWerner 1980) and thus, the foragingdifferences observed in our study maybe mechanisms for resourcepartitioning. Our results suggest thatthe coexistence of Messor andPogonomyrmex may be facilitated bydifferences in foraging strategies indifferentenvironments,asseeddensitycan bemuch higher under shrubs than

inopenareas (NelsonandChew1977).Messormaybespecializedtoforaginginareas of higher seed density, whilePogonomyrmexmay be better adaptedtoopenareaswithlowseeddensitydueto their lower preference for clumpedseeds.ThisissupportedbythefactthatMessor has a higher elevation rangethan Pogonomyrmex and is thereforebetteradaptedtohabitatswithahigherdensityofresources. The differences in seed resource use

between Messor and Pogonomyrmexalso have implications for desert seedbanks and plant survival. Ant foragingsignificantly depletes the abundance ofseeds stored in the soil (Crist andMacMahon 1992). Due to its higherpreference for clumped resources,Messor may have a larger role in thedepletion of high-density seedresources. Ant foraging can also havepositive impacts on plant survival bydispersing seeds to a more favorablemicrohabitat (Hanzawa et al. 1987,SlingsleyandBond1985)andfacilitatinggermination (Culver and Beattie 1978).Messor and Pogonomyrmex may differin their influencesonplant speciesduetotheirdifferentforagingbehaviorsandtheseeffectsmayhavecommunitylevelconsequences.Moreresearchwouldbeneeded to determine the how foragingimpacts differ between Messor andPogonomyrmexdependingontheplantspecies.

ACKNOWLEDGEMENTS

This work was performed at theUniversity of California’s SweeneyGranite Mountains Desert ResearchCenter,doi:10.21973/N3S942.

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