Insect herbivory in an experimental agroecosystem: the relativeimportance of habitat area, fragmentation, and the matrix

Kyle J. Haynes and Thomas O. Crist

K. J. Haynes (kjh1087@louisiana.edu), Dept of Biology, Univ. of Louisiana, PO Box 42451, Lafayette, LA 70504, USA. �T. O. Crist, Dept ofZoology, Miami Univ., Oxford, OH 45056, USA.

Habitat area, fragmentation, and the surrounding matrix influence levels of herbivory in various ecosystems, but therelative importance of these effects has rarely been assessed. We compared levels of herbivory and densities of dominantarthropod herbivores (the hemipteran insects Agallia constricta, Empoasca fabae, Therioaphis trifolii, Lygus lineolaris andHalticus bractatus) among experimental plots that varied in the area and fragmentation of clover habitat and thecomposition of the matrix (bare ground or grass) surrounding clover habitat. To assess levels of herbivory, we comparedclover biomass within herbivore exclosures to the biomass accessible to herbivores. The area and fragmentation of cloverhabitat, as well as matrix composition, significantly influenced the collective densities of herbivores, although each speciesexhibited unique responses to habitat structure. Herbivory was strongest in plots with large (64 m2) as compared to small(16 m2) amounts of clover habitat. The difference in clover biomass between the inside and outside of exclosuresincreased significantly with increasing density of Empoasca fabae but was unrelated to the densities of the other herbivores,suggesting that Empoasca fabae was an exceptionally important herbivore in this system. This study supports the view thatherbivore densities and herbivory generally increase with increasing area of plant monocultures, but emphasizes that levelsof herbivory may be driven primarily by one or a few key herbivore species.

There is a large body of literature on the effects of the areaand fragmentation of a focal habitat type and compositionof the surrounding matrix on the distributions andabundances of species (Moilanen and Hanski 1998,Debinski and Holt 2000, Tscharntke et al. 2002, Cronin2003). The effects of these factors on levels of herbivoryhave received a smaller, but still considerable, amount ofattention (Thies et al. 2003, Ostergard and Ehrlen 2005,Valladares et al. 2006). However, few studies have evaluatedthe relative importance of habitat area, fragmentation, andmatrix composition to levels of herbivory (but see Diekotteret al. 2007).

If one or a few herbivore species are responsible for mostof the herbivory in a given community, the feedingspecialization of these key species may determine themanner and strength with which habitat area, fragmenta-tion and matrix composition affect levels of herbivory.Positive, negative, and neutral density responses to habitatarea are frequently observed (Connor et al. 2000, Matter2000, Zaviezo et al. 2006), but species with strongpreferences for a focal habitat type are more likely todisplay positive density�area relationships in the preferredhabitat type than are habitat-generalist species (Hambacket al. 2007). The fragmentation of a focal habitat type isgenerally thought to adversely affect populations but, infact, positive effects of habitat fragmentation may be morelikely for species that use multiple habitat types (Law and

Dickman 1998). Fragmentation of a focal habitat type mayhave positive effects on a habitat generalist by increasing theextent to which complementary resources in the matrixoccur within the organism’s dispersal range (Law andDickman 1998, Fahrig 2003). Finally, whereas habitatspecialists may respond more strongly to the area andfragmentation of a focal habitat type, habitat generalistsmay be more sensitive to the composition of the matrix(Brotons et al. 2003, Steffan-Dewenter 2003). For example,Haynes et al. (2007) found that (adult) densities of thepolyphagous grasshopper Melanoplus femurrubrum in afocal habitat type (clover) were more strongly influencedby the availability of complementary resources in the matrixthan by the area and fragmentation of the focal habitat.

Assessing the relative importance of different attributesof habitat structure and composition to herbivory presentsparticular challenges. The habitat attributes of interest oftencovary in natural landscapes, hindering a researcher’s abilityto isolate their independent effects. For example, Fahrig’s(2003) review revealed that most studies examining theeffects of habitat area and fragmentation on population orcommunity dynamics confounded the effects of these twofactors. Furthermore, plant size or quality may covary withlandscape features (e.g. matrix composition; Haynes andCronin 2004), making it difficult to isolate the effects ofherbivory from the effects of other factors on measures ofplant growth (e.g. biomass, leaf area). One approach to

Oikos 118: 1477�1486, 2009

doi: 10.1111/j.1600-0706.2009.17720.x,

# 2009 The Authors. Journal compilation # 2009 Oikos

Subject Editor: Daniel Gruner. Accepted 6 April 2009

1477

evaluating the relative importance of different habitatattributes to herbivory is to combine observational dataon herbivore densities and plant growth with experimentalmanipulations of both the habitat attributes and herbivoredensities.

In this study, we examine the relative importance ofhabitat area, habitat fragmentation, and matrix compositionin determining levels of herbivory. We created a modelagroecosystem consisting of replicated plots containingsubplots of the forage-crop plant red clover Trifoliumpratense. We manipulated the area and fragmentation ofclover habitat and composition of the matrix (grass or bare-ground) according to a factorial design. Unlike manyprevious experimental studies (Fahrig 2003), our designmanipulated habitat area independently of habitat frag-mentation. In the experimental plots, we recorded thedensities of the five numerically dominant herbivores:Agallia constricta, Empoasca fabae, Therioaphis trifolii, Lyguslineolaris and Halticus bractatus. To quantify levels ofherbivory, we placed small herbivore exclosures in theexperimental plots. We also examined the impacts of eachherbivore species on clover biomass based on regressions ofpaired exclosure-control differences in clover biomass andherbivore abundance. Since clover represents the preferredhabitat for all of the dominant herbivore species (Methods),we expected that densities and herbivory would increasewith the area of clover habitat. We also expected thathabitat-generalist species would have higher densities inclover habitat bordered by the grass matrix because it mayprovide complementary resources. Furthermore, we ex-pected fragmentation to positively affect habitat generalists.Effects of habitat fragmentation on specialist herbivoresmay be positive or negative, because although dispersalmortality and densities of habitat-generalist predators mayincrease with fragmentation, parasitoids often are lesseffective in fragmented landscapes (Kruess and Tscharntke1994, Tscharntke and Brandl 2004).

Methods

Study system

The focal plant in this study was the herbaceous, perennialred clover Trifolium pratense. Red clover is an importantforage crop worldwide, both in intensely managed cultiva-tion and extensively managed pastures (Guretzky et al.2005), and is commonly grown in clover/grass polycultures(Heath et al. 1973). The dominant herbivores of cloverat our study site were all Hemiptera. They consisted of theleafhoppers Agallia constricta and Empoasca fabae (Cicadel-lidae), the aphid Therioaphis trifolii (Aphididae), the plantbug Lygus lineolaris (Mirididae), and the fleahopper Halticusbractatus (Mirididae). Therioaphis trifolii is only known tofeed on legumes (e.g. alfalfa, clovers; Milne 1998, Nair et al.2003). The remaining herbivore species are known to feedon a wide variety of herbaceous plants (Osborn 1928,Henry 1983, Young 1986, USDA 2002), but were found inmuch higher densities on clover than grass in our system(Haynes unpubl.).

Experimental plots

We conducted the experiment in the summer of 2006 in a2-ha field at Miami Univ. Ecology Research Center nearOxford, OH. Two years earlier (in 2004), we createdreplicated 14�14 m plots, each containing four patches(or subplots) of red clover. The clover subplots varied infragmentation (continuous or isolated by 2 m), habitat area(4 or 16 m2), and matrix type (grass or bare ground)according to a fully factorial design (Fig. 1). Eachcombination of fragmentation, area, and matrix wasoriginally replicated in four separate plots, and arrangedin a completely randomized design. However, we excludedtwo plots from the experiment due to clover mortality inthe spring of 2006, leaving two treatment combinations(large clover area/bare-ground matrix/fragmented and smallclover area/grass matrix/continuous) with three replicateplots. We established the clover subplots and grass matrixby sowing the plots with seeds of red clover Trifoliumpratense and orchard grass Dactylis glomerata. We main-tained bare-ground matrix within experimental plots and inthe 8-m strips between plots by applying herbicide atmonthly intervals. We kept the grass matrix free of forbs byapplying 2,4-D herbicide annually in early summer (June).We mowed the grass monthly to minimize variation in theheight of vegetation among plots because the structuralcomplexity of matrix habitats may influence rates ofmovement into and out of focal patches (Kareiva 1985,Lawrence and Bach 1989). Finally, we regularly weeded allclover habitat.

Herbivory experiment

To quantify the effects of herbivores on clover biomass, weplaced small exclosures in one randomly selected subplotper plot. Within each selected subplot, we placed theexclosure at a stratified random location. An exclosureconsisted of fine-mesh (0.25 mm) polyester ‘no-see-um’netting sewn to fit over a small (0.25w�0.4l�0.5h m)frame constructed of PVC tubing. We pushed the legs ofthe frame �5 cm into the soil. We prevented insects fromentering the exclosures by anchoring the base of the nettingfirmly against the ground using iron rods. To kill anyinsects that may have been inside the exclosures, we sprayed0.15 l of an aqueous solution containing organic pyrethrin

Figure 1. Aerial view of the field containing 36 14�14 mexperimental plots. Photo credits: Beth Dickman.

1478

pesticide through the netting onto the clover inside. Becausethe pesticide was diluted in water (1:500 dilution), wesprayed an equivalent amount of water onto an equivalentarea of clover within the same subplot, which was used as acontrol. We assigned each control area to a stratifiedrandom location. Experiments testing for potential effectsof the exclosures or pesticide on clover are described in thefollowing section.

The exclosures were in place for a 6-week period in 2006while insects were at peak abundance (mid July to lateAugust). After six weeks, we compared clover biomassinside the exclosures to clover biomass at control locations.We clipped the clover biomass at these two locations using arectangular quadrat (0.2�0.35 m) to standardize the areasampled. The plant material was dried at 608C for 48 h andthen weighed.

We evaluated effects of herbivore exclusion on cloverbiomass using a profile ANOVA (Tabachnick and Fidell2000). This procedure, which is comparable to repeatedmeasures ANOVA, allows for the two clover biomassmeasurements (exclosure and control) taken from eachplot to be non-independent. Clover biomass measures takenfrom the same plot could potentially be more similar thanmeasures taken in different plots due to differences inabiotic conditions (e.g. soil quality). We also used theprofile ANOVA to test whether the effect of herbivoreexclusion on clover biomass was dependent on cloverhabitat area, habitat fragmentation or matrix composition.In addition, we tested whether clover biomass at controllocations was influenced by the plot treatments using athree-way ANOVA. The clover biomass data were square-root transformed prior to these analyses to homogenizevariances and improve normality of residuals. We alsoremoved a linear spatial trend in the control-locationbiomass data that we found by performing a regressionagainst the x and y spatial positions of the plots (Crist et al.2006). We did not use detrended data in the profileANOVA, however, because this would preclude directcomparison of differences in biomass inside and outsideof exclosures within each plot.

We measured the densities of the herbivore species in theclover habitat of each experimental plot in mid July and lateAugust. For each plot, we collected insects from the subplotcontaining the herbivore exclosure using a D-vac insectvacuum. Within each of these subplots, we placed the0.08 m2 sampling head of the D-vac over the clover at fourstratified random locations. The suction samples weretransferred to plastic bags and placed in a freezer until allinsects were dead. We then counted the insects using astereoscopic dissecting microscope. To evaluate the relation-ships between the densities of individual herbivore speciesand the level of herbivore damage to clover, we performedmultiple regression analysis using the densities of eachherbivore species as predictor variables and the difference inclover biomass between the exclosure and control location(exclosure biomass � control biomass) as the responsevariable. In addition, we examined the collective effects ofthe herbivores on clover biomass by performing a linearregression with total herbivore density as the predictorvariable. We also evaluated the effects of the area andfragmentation of clover habitat and matrix composition onthe densities of the five herbivore species using MANOVA.

Following significant results from a MANOVA, we eval-uated the effects of habitat area, fragmentation, and matrixcomposition on the individual herbivore species usingunivariate ANOVA (Quinn and Keough 2002). Weanalyzed the July and August data separately because theeffects of herbivores on clover biomass could potentially shiftover time. To protect against inflated type I error due toconducting two non-independent tests (on July and Augustdensities), we set the significance level for MANOVA andmultiple regression analyses to a�0.025 (0.05/2). Tominimize heterogeneity of variance and to normalizeresiduals, the densities of each insect species were square-root transformed prior to the analyses. As with the cloverbiomass data, we removed linear spatial trends in thedensities of three herbivore species (Halticus bractatus inJuly and August, Empoasca fabae in July, and Therioaphistrifolii in July) prior to fitting the MANOVA and ANOVAmodels.

Insect dispersal or spatially correlated environmentalconditions (e.g. soil quality) could potentially have lead tonon-independence of the insect density or clover biomassmeasurements taken from neighboring plots. We tested forspatial autocorrelation of the residuals from the ANOVAand regression models (Ver Hoef and Cressie 1993) usingthe nonparametric spatial correlation function (Bjørnstadand Falck 2001), which revealed that none of the residualsexhibited significant spatial autocorrelation at any lag dis-tance between plots.

Exclosure and pesticide effects on clover

Based on a previous field study (Cronin 2007) using insectcages similar to ours, we did not expect the exclosures toaffect clover biomass. Nonetheless, we conducted a separatebut concurrent experiment to examine exclosure effects onplant biomass. We built ‘leaky’ exclosures, which allowedinsects to access the clover inside. These ‘leaky’ exclosureswere identical to the exclosures used in the herbivoryexperiment except that there were two large rectangularholes in the netting on opposite sides of the exclosure. Theholes were 10 cm wide and extended from the soil to aheight of 20 cm. We placed ten ‘leaky exclosures’ in a smallsecondary field (�0.2 ha) of red clover 1 km from theexperimental plots. Exclosures and paired control locationswere arrayed along a linear transect. Exclosure-control pairswere spaced 2 m apart. We placed each exclosure 1 m tothe left or right of the transect (at random) and located thepaired control on the opposite side (also 1 m from thetransect). After six weeks, we sampled clover biomass ateach location using methods described previously. Wetested whether clover biomass differed between the exclo-sures and controls using a paired t-test. Prior to thisanalysis, the clover biomass data were ln-transformed.

We also experimentally tested whether clover sprayedwith the aqueous pesticide solution differed from cloversprayed with water, as a check on the direct effects ofpyrethrins on clover growth. We established 20 herbivoreexclosures in the secondary clover field. We sprayed half ofthe exclosures with the pesticide solution and other halfwith an equivalent amount of water. The treatments wereassigned to the exclosures at random and clover biomass was

1479

measured after six weeks. We assessed whether cloverbiomass differed between exclosures sprayed with pesticidesolution or water with ANOVA. Clover biomass was ln-transformed prior to this analysis.

Results

Herbivory experiment

We found a significant interactive effect of clover habitatarea and matrix composition on clover biomass at controllocations (Table 1). In plots with 64 m2 of clover habitat,biomass was relatively constant across the combinations offragmentation and matrix composition (Fig. 2). On theother hand, clover biomass in plots with 16 m2 of habitatwas substantially higher in plots with bare-ground matrixthan in plots with grass matrix.

Clover biomass was significantly higher inside theexperimental exclosures compared to control locationswith no exclosure (Table 2, Fig. 3). In addition, the effectof the exclosures on clover biomass differed according toclover habitat area. In plots with 16 m2 of clover habitat,clover biomass did not differ strongly between exclosuresand controls. In plots with 64 m2 of clover habitat,however, clover biomass was 1.4 times higher inside theexclosures. Thus herbivores had strong effects on cloverbiomass in larger patches of clover. Herbivory effects onclover biomass were unrelated to matrix composition andhabitat fragmentation.

Based on a multiple regression analysis, the differences inbiomass between the exclosures and controls (exclosurebiomass � control biomass) were not significantly related tothe July densities of any of the herbivore species (Table 3).However, there was a positive relationship between thedifference in biomass and the density of Empoasca fabae inAugust (Table 3, Fig. 4). The August densities of all otherinsect species were unrelated to the difference in biomassbetween exclosures and controls. For the August data, aregression model including all of the herbivore speciesexplained 26.6% of the variation in the biomass differentialamong plots (based on the r2 value). Despite the apparenteffects of Empoasca fabae on clover biomass, we foundno relationship between total herbivore density and thebiomass differential in July (biomass differential��1.121�(density)0.5�64.823, n�30, p�0.585, r2�0.011) or August (biomass differential�10.102�(den-sity)0.5 � 43.323, n�30, p�0.287, r2�0.04).

In July, the densities of herbivore species were collec-tively influenced by all three main effects, clover area, cloverfragmentation, and matrix composition (Appendix 1).Three of the five herbivore species displayed a trendtowards higher density in the plots with greater cloverarea (Halticus bractatus, Lygus lineolaris, Empoasca fabae ;Fig. 5), though this trend was significant only for Halticusbractatus (Appendix 2). Densities of this species were 3.2times greater in plots with 64 compared to 16 m2 of cloverhabitat. Conversely, Therioaphis trifolii density was 3.5times higher in the latter plots. Matrix compositionsignificantly affected the densities of both Therioaphistrifolii and Agallia constricta (Appendix 2). Therioaphistrifolii densities were 5.8 times higher in clover habitatembedded within the bare-ground matrix. Agallia constrictaexhibited the opposite response; densities were twice as highin clover habitat within the grass matrix. Finally, fragmen-tation did not significantly affect the July densities of anysingle herbivore species. In August, herbivore densities werenot collectively influenced by any of the experimentalfactors (Appendix 1). With regard to the effect of cloverhabitat area, however, all of the herbivore species individu-ally exhibited the same trends that were observed in July.

Table 1. ANOVA results on the effects of habitat area, habitatfragmentation and matrix composition on clover biomass in controllocations. Significant results at the a�0.05 level are shown in bold.

Source DF F p

Area 1 0.054 0.819Matrix 1 3.026 0.096Fragmentation 1 1.068 0.313Area�Matrix 1 9.544 0.005Area�Fragmentation 1 0.094 0.763Matrix�Fragmentation 1 0.090 0.767Area�Matrix�Fragmentation 1 0.439 0.514Error 22

Table 2. Results from a profile ANOVA on effects of herbivoreexclosure, habitat area, matrix composition, and habitat fragmenta-tion on clover biomass. Significant results at the a�0.05 level areshown in bold.

Source DF F p

Exclosure 1 7.178 0.014Exclosure�Area 1 5.256 0.032Exclosure�Matrix 1 1.036 0.320Exclosure�Fragmentation 1 0.028 0.868Exclosure�Area�Matrix 1 1.862 0.186Exclosure�Area�Fragmentation 1 0.512 0.482Exclosure�Matrix�Fragmentation 1 0.828 0.373Exclosure�Area�Matrix�Fragmentation 1 0.168 0.686Error 22

0

4

8

12

16

20 Bare, continuousBare, fragmentedGrass, continuousGrass, fragmented

Large Small

Sq.

roo

t (cl

over

bio

mas

s [g

m-2

])

Habitat area

1, 2

1, 2

1

2

Figure 2. Effects of habitat area, habitat fragmentation, andmatrix composition on clover biomass in control locations(means�1 SE). Means labeled by different numbers respresentsignificant differences at the a�0.05 level (Tukey’s HSD testsevaluated differences among all area and matrix combinations).

1480

Exclosure and pesticide effects on clover

Our experimental test for enclosure effects on cloverbiomass revealed no difference between controls and the‘leaky’ exclosures (0.00890.427 g m�2, mean differenceof ln(g m�2)995% CI, t9�0.466, p�0.652), whichallowed insects to move freely in or out of the exclosures. Inaddition, clover biomass did not differ between exclosuressprayed with pesticide solution (82.498.4 g m�2, mean91 SE) and exclosures sprayed with water (79.696.1 g m�2;F1,18�0.003, p�0.955).

Discussion

Our study shows that herbivore damage to red clover wasinfluenced by the area of clover habitat. We found asubstantial difference in clover biomass between exclosuresand open controls in plots with 64 m2 of clover habitat, butno such difference in biomass in plots with 16 m2 of cloverhabitat. Because we found no exclosure effects on cloverbiomass, we attribute differences in clover biomass between

exclosures and controls to differences in the intensity ofherbivory. Thus, herbivory was much stronger in plots withgreater clover area. Similar findings of insect herbivoryincreasing with increasing patch area have been reported inother systems (e.g. understory herbs, forest fragments;Ostergard and Ehrlen 2005, Valladares et al. 2006). Unlikeclover area, the fragmentation of clover habitat andcomposition of the matrix had no effects on herbivory.

Habitat-structure effects on herbivore densities andherbivory

The strong effect of habitat area on herbivory may be partlyexplained by the collective effects of habitat area onherbivore densities. Densities of three of the five herbivorespecies (Halticus bractatus, Lygus lineolaris and Empoascafabae) increased with increasing clover area, whereas twoherbivore species (Therioaphis trifolii and Agallia constricta)exhibited negative responses to increasing clover area (theseresponses were statistically significant only for Halticus

Bio

mas

s di

ffere

ntia

l (g

m-2

)

-70

0

70

140

210

Large Small

Bare, continuous

Bare, fragmented

Grass, continuous

Grass, fragmented

Habitat area

Figure 3. Differences in clover biomass between herbivoreexclosures and control locations (biomass inside exclosures �control biomass, means�1 SE). Results from a profile ANOVA(Table 2) showed that the effect of herbivore exclusion on biomasswas greater in plots with 64 m2 of clover habitat than in plots with16 m2 of clover habitat.

Table 3. Results from multiple regression analyses examining the relationships between the densities of each herbivore species in July andAugust and the clover biomass differential (biomass inside exclosure � biomass outside exclosure). Densities were square-root transformed.Significant results at the a�0.05 level are shown in bold.

Herbivore species July August

Coefficient t p Coefficient t p

Agallia constricta 3.990 �0.400 0.693 5.490 �1.153 0.735Empoasca fabae �2.434 0.303 0.764 39.112 2.634 0.015Therioaphis trifolii �0.843 �0.119 0.906 �10.791 �1.148 0.260Lygus lineolaris 10.841 0.507 0.617 8.444 0.468 0.644Halticus bractatus 14.733 1.124 0.272 5.545 0.679 0.504

July: r2�0.101.August: r2�0.266.

Square root (no. m-2)

Bio

mas

s di

ffere

ntia

l (re

sidu

als)

0 1 2 3 4 5 6 7-400

-300

-200

-100

0

100

200

300

Figure 4. Relationship between the density of Empoasca fabae andthe effect of herbivores on clover biomass. The effect of herbivoreson clover biomass was measured as the difference in clover biomassbetween herbivore exclosures and control locations (biomass insideexclosure � control biomass). To indicate the effect of Empoascafabae alone on clover biomass, the residuals from a modelincluding all of the herbivores except Empoasca fabae wereregressed against the density of Empoasca fabae (biomass differ-ential [residuals]�38.26�(Empoasca fabae density)0.5 � 114.21,n�30, r2�0.219).

1481

bractatus and Therioaphis trifolii). Positive density�arearelationships in herbivorous insects are traditionally thoughtto result from increased rates of immigration into and (or)decreased rates of emigration out of large host-plant patchesin order to maximize feeding efficiency (the resourceconcentration hypothesis; Root 1973). Given that all ofthe herbivore species appeared to strongly prefer the cloverhabitat over the grass matrix (all exhibited much higherdensities in clover; Haynes unpubl.), this hypothesis mayexplain the positive responses of Halticus bractatus, Lyguslineolaris and Empoasca fabae to increasing clover area. Evenamong habitat-specialist species, however, negative density�area relationships, like we report for Therioaphis trifolii andAgallia constricta, are not atypical. In a meta-analysis,Hamback and Englund (2005) found that various aphid,leafhopper and planthopper species exhibited density�arearelationships ranging from slightly negative to slightlypositive. Negative density�area relationships, they showed,

are likely for species exhibiting area-dependent immigrationand perimeter-dependent emigration (Hamback and Eng-lund 2005). An alternative explanation for the negativeresponses of Therioaphis trifolii and Agallia constricta toincreasing clover area is that predation pressure on thesespecies was higher in plots with large clover subplots.Reductions in habitat area are often expected to havestronger adverse effects on predators than on their prey(Holt et al. 1999, Tscharntke and Brandl 2004). One majorconsequence of this effect is that reducing the area of habitatcould potentially lead to outbreaks of herbivores and plantdestruction (With et al. 2002). If predation was responsiblefor the negative density�area relationships of Therioaphistrifolii and Agallia constricta, this effect was not sufficient tohave a cascading influence on clover biomass.

For herbivores that forage in multiple habitat types,densities may often be elevated in patches of the focalhabitat type that border matrix habitat types that provide

0500

1000150020002500

Therioaphis trifolii

05

10152025

Den

sity

(no

. m-2

)

Lygus lineolaris

01020304050

Halticus bractatus

Agallia constricta

0

40

80

120

July August

Empoasca fabae

0

10

20

30

0

10

20

30

01020304050

01020304050

02468

10

100

020406080

Large Small Large SmallHabitat area Habitat area

Bare, continuous

Bare, fragmented

Grass, continuous

Grass, fragmented

Large > Small(p = 0.001)

Small > Large(p = 0.005)

Bare > Grass(p= 0.002)

Grass > Bare(p < 0.001)

Figure 5. Effects of habitat area, habitat fragmentation, and matrix composition on the densities of Agallia constricta, Empoasca fabae,Therioaphis trifolii, Lygus lineolaris and Halticus bractatus (means�1 SE). After finding that the densities of the herbivore species werecollectively influenced by the experimental factors in July, but not August (based on MANOVAs), the July data was further analyzed byperforming separate ANOVAs for each herbivore species (Quinn and Keough 2002). Significant results at the a�0.05 level are listed inthe figure.

1482

supplementary or complementary resources (Dunning et al.1992, Haynes et al. 2007). Consistent with this hypothesis,we found that Agallia constricta, a polyphagous leafhopperwhich feeds on both forbs and grasses, exhibited higherdensities in clover habitat embedded within grass matrixcompared to bare-ground matrix. The more-specialized,legume-feeding aphid, Therioaphis trifolii, showed theopposite response to matrix composition; densities werehigher in clover within bare-ground matrix. The higherdensities of aphids in clover habitat embedded within bare-ground matrix may be the result of a reluctance to moveover bare ground. The planthopper Prokelisia crocea, amonophagous herbivore of the grass Spartina pectinata,displays a similar distributional pattern. The planthopperreaches higher densities in host-plant patches bordered bymudflats than in patches bordered by non-host grassesbecause the rate of emigration from patches in mudflats ismuch lower (Haynes and Cronin 2003, Cronin 2007,Reeve et al. 2008). Despite the effects of matrix composi-tion on herbivore densities in our system, matrix composi-tion did not influence levels of herbivory. The lack of effectsof matrix composition on herbivory may be explained bythe fact that neither Agallia constricta nor Therioaphis trifoliiappeared to strongly affect clover biomass.

Like matrix composition, habitat fragmentation did notaffect levels of herbivory. The collective densities ofherbivore species were influenced by habitat fragmentation,but no single species was strongly affected by this factor.The lack of clear fragmentation effects may be partiallyexplained by the fact that the scale of fragmentation in thisstudy was likely small relative to the dispersal abilities ofthree of the five herbivore species. The leafhoppers Agalliaconstricta and Empoasca fabae and the plant bug Lyguslineolaris are sufficiently mobile that the fragmentation ofclover habitat probably did little to improve access toresources in the matrix (Fleischer et al. 1988, Hoffman andHogg 1992, B. Schroeder pers. comm.). For Halticusbractatus, however, it is conceivable that fragmentationincreased the ability to forage both in clover and the matrixsince a large percentage of adult females are wingless (Day1991). Similarly, most Theroaphis trifolii adults werewingless in July and August (92% and 100% of capturedadults). Thus, rates of movement of this habitat-specialistherbivore among clover subplots were likely substantiallyreduced by the fragmentation of clover habitat.

Competition between clover and matrix vegetation

Our results also showed that clover biomass was influencedby an interactive effect of clover habitat area and composi-tion of the matrix. Clover biomass was highest in plots withless clover area and bare-ground matrix. Given that wedid not see interactive effects of area and matrix onherbivore densities, this effect was probably not a result ofherbivory. It is more likely that the observed differences inclover biomass resulted because of competition betweenclover plants and grass in the matrix, particularly alongclover-matrix edges. In plots with bare-ground matrix,clover plants near the edges of subplots may have exhibitedincreased growth due to reduced competition for water ornutrients. Because clover plants in small (4 m2) clover

subplots were closer to clover-matrix edges, on average, thanin large clover subplots (16 m2), the lack of competitors atthe clover�bare-ground boundary would likely be morebeneficial to clover in small clover subplots. A review byHaynes and Cronin (2004) found that matrix compositionoften influences the quality (e.g. plant size, tissue nitrogencontent) of focal patches of vegetation by altering competi-tion between plants in the patch and the matrix or byaltering nutrient subsidies across the patch edge.

Species-specific impacts of herbivores

The density of only one herbivore species, Empoasca fabae,was significantly related to the difference in clover biomassbetween exclosures and controls. The biomass differential(exclosure biomass � control biomass) increased withincreasing density of Empoasca fabae in August, explainingapproximately one fifth of the variation in the biomassdifferential among plots (Fig. 3). This greater importance ofEmpoasca fabae is likely due to the feeding habits of thisspecies. Unlike many other hemipterans which are sheathfeeders (stylet tips are sealed into a plant’s vascular cell usinga sheath composed of hardened saliva), Empoasca spp. feedby rupturing plant cells with a unique form of styletmovement, secreting saliva, and then sipping the contentsof damaged cells (reviewed by Backus et al. 2005).Consequently, Empoasca fabae and its congeners areespecially likely to cause hopperburn, a type of damagethat results in tip-wilting, leaf yellowing, and plant stunting(reduced growth). Stunting caused by Empoasca fabae is amajor cause of reduced yield in legume forage crops inNorth America (Backus et al. 2005).

Empoasca fabae was likely the most damaging herbivorein our clover agroecosystem, but spatial variation in levels ofherbivory can not be understood solely on the basis of thespatial distribution of this species. Although our experimentrevealed that the effects of herbivores on clover biomasswere much stronger in plots with more clover area, densitiesof Empoasca fabae were only slightly higher in these plots.One possible explanation for the disproportionately highreduction in clover biomass in plots with large cloversubplots is that increased clover habitat area had positiveeffects on the densities of three of the five dominantherbivores (Empoasca fabae, Lygus lineolaris and Halticusbractatus) in both July and August. Densities of Halticusbractatus, for example, were 1.9 and 3.2 times higher inlarge than in small clover subplots in July and August,respectively. Furthermore, although the multivariate re-sponse of all five herbivore species in August to habitat areawas not significant, the consistently positive effects of cloverhabitat area on the densities of Empoasca fabae, Lyguslineolaris and Halticus bractatus likely explains the impor-tance of habitat area in influencing levels of herbivory.

Conclusions

Our findings support the view that the area of a focalhabitat (e.g. crop) is an important factor influencingherbivore densities and herbivore damage (Root 1973)as well as the concept that the practice of planting in largemonocultures may ensure continued high losses of crop

1483

productivity to herbivores (Russell 1989, Daily 1997,Altieri 1999, Landis et al. 2000, Tscharntke et al. 2005).Given the small size of our experimental plots, however,how well our conclusions apply to agricultural landscapes isan important question. Monotonically increasing or decreas-ing density-area relationships have frequently been reportedfor herbivorous insects (MacGarvin 1982, Nowicki et al.2007, but see Hamback and Englund 2005), suggestingthat herbivore density�area relationships and levels ofherbivory may remain more or less constant across a widerange of spatial scales. In contrast, matrix composition mayoften be more important in influencing herbivory withinsmall experimental fields than in larger agricultural fields.Cross-edge foraging of matrix-dwelling or habitat-generalistherbivores and natural enemies into agricultural fieldsresults in greater impacts on community structure nearthe field margin than in the field interior (Clough et al.2005, Rand et al. 2006). Therefore, herbivore impacts inlarge fields, with their low edge-to-area ratios, may be lessdependent on composition of the surrounding matrix.Finally, given that colonization rates typically decreasewith increasing distance between source and recipienthabitats (Hanski 1999), habitat fragmentation may bemore important to community structure and herbivory atlarger scales.

By documenting considerable asymmetry in the impactsof different herbivore species on plant biomass, this studysuggests that the effects of landscape features on herbivorymay differ among systems supporting different suites ofherbivores. The relative importance of habitat area, frag-mentation, and matrix composition to herbivory in a givensystem could depend on how the distributions of a few keyherbivore species, rather than overall herbivore densities, areinfluenced by these factors. This notion suggests that therelative importance of different landscape attributes toherbivory may be difficult to predict without a detailedunderstanding of the ecological impacts of the species in agiven system (Rosenlew and Roslin 2008).

Acknowledgements � Special thanks to Candace Crist for sewing theherbivore exclosures and to Tim Diekotter for assisting with pilotstudies. Laura Douglas, Sam Evans, Rodney Kolb, Brad Schroederand Dave Stasek provided valuable help in the field. DanielGruner provided useful comments on earlier drafts of thismanuscript. Funding for this project was provided by MiamiUniv. and a Miami Univ. Postdoctoral Research Scholarship.

References

Altieri, M. A. 1999. The ecological role of biodiversity inagroecosystems. � Agric. Ecosyst. Environ. 74: 19�31.

Backus, E. A. et al. 2005. Mechanisms of hopperburn: an overviewof insect taxonomy, behavior and physiology. � Annu. Rev.Entomol. 50: 125�151.

Brotons, L. et al. 2003. Are fragments islands? Landscape contextand density-area relationships in boreal forest birds. � Am.Nat. 162: 343�357.

Bjørnstad, O. N. and Falck, W. 2001. Nonparametric spatialcovariance functions: estimation and testing. � Environ. Ecol.Stat. 8: 53�70.

Clough, Y. et al. 2005. Spider diversity in cereal fields: comparingfactors at local, landscape and regional scales. � J. Biogeogr.32: 2007�2014.

Connor, E. F. et al. 2000. Individuals�area relationships: therelationship between animal population density and area.� Ecology 81: 734�748.

Crist, T. O. et al. 2006. Spatial variation in insect community andspecies responses to habitat loss and plant communitycomposition. � Oecologia 147: 510�521.

Cronin, J. T. 2003. Matrix heterogeneity and host�parasitoidinteractions in space. � Ecology 84: 1506�1516.

Cronin, J. T. 2007. From population sources to sieves: the matrixalters host�parasitoid source�sink structure. � Ecology 88:2966�2976.

Daily, G. C. et al. 1997. Ecosystem services: benefits supplied tohuman societies by natural ecosystems. Iss. Ecol. no. 2. � Ecol.Soc. Am.

Day, W. H. 1991. The peculiar sex ratio and dimorphism of thegarden fleahopper, Halticus bractatus (Hemiptera, Miridae).� Entomol. News 102: 113�117.

Debinski, D. M. and Holt, R. D. 2000. A survey and overview ofhabitat fragmentation experiments. � Conserv. Biol. 14: 342�355.

Diekotter, T. et al. 2007. Direct and indirect effects of habitat areaand matrix composition on species interactions among flower-visiting insects. � Oikos 116: 1588�1598.

Dunning, J. B. et al. 1992. Ecological processes that affectpopulations in complex landscapes. � Oikos 65: 169�175.

Fahrig, L. 2003. Effects of habitat fragmentation on biodiversity.� Annu. Rev. Ecol. Evol. Syst. 34: 487�515.

Fleischer, S. J. et al. 1988. Dispersal of Lygus lineolaris (Hetero-ptera, Miridae) adults through cotton following nursery hostdestruction. � Environ. Entomol. 17: 533�541.

Guretzky, J. A. et al. 2005. Species diversity and functionalcomposition of pastures that vary in landscape position andgrazing management. � Crop Sci. 45: 282�289.

Hamback, P. A. and Englund, G. 2005. Patch area, populationdensity and the scaling of migration rates: the resourceconcentration hypothesis revisited. � Ecol. Lett. 8: 1057�1065.

Hamback, P. A. et al. 2007. Habitat specialization, body size, andfamily identity explain lepidopteran density�area relationshipsin a cross-continental comparison. � Proc. Natl Acad. Sci.USA 104: 8368�8373.

Hanski, I. 1999. Metapopulation ecology. � Oxford Univ. Press.Haynes, K. J. and Cronin, J. T. 2003. Matrix composition affects

the spatial ecology of a prairie planthopper. � Ecology 84:2856�2866.

Haynes, K. J. and Cronin, J. T. 2004. Confounding of patchquality and matrix effects in herbivore movement studies.� Landscape Ecol. 19: 119�124.

Haynes, K. J. et al. 2007. Resource complementation and theresponse of an insect herbivore to habitat area and fragmenta-tion. � Oecologia 153: 511�520.

Heath, M. E. et al. 1973. Forages: the science of grasslandagriculture (3rd ed.). � Iowa State Univ. Press.

Henry, T. J. 1983. The garden fleahopper genus Halticus(Hemiptera, Miridae) � resurrection of an old name and keyto species of the western hemisphere. � Proc. Entomol. Soc.Wash. 85: 607�611.

Hoffman, G. D. and Hogg, D. B. 1992. Effect of alfalfa waterstress on potato leafhopper (Homoptera, Cicadellidae) plantpreference and oviposition rate. � Ann. Entomol. Soc. Am. 85:506�516.

Holt, R. D. et al. 1999. Trophic rank and the species�arearelationship. � Ecology 80: 1495�1504.

Kareiva, P. 1985. Finding and losing host plants by Phyllotreta:patch size and surrounding habitat. � Ecology 66: 1809�1816.

1484

Kruess, A. and Tscharntke, T. 1994. Habitat fragmentation,species loss, and biological control. � Science 264: 1581�1584.

Landis, D. A. et al. 2000. Habitat management to conserve naturalenemies of arthropod pests in agriculture. � Annu. Rev.Entomol. 45: 175�201.

Law, B. S. and Dickman, C. R. 1998. The use of habitat mosaicsby terrestrial vertebrate fauna: implications for conservationand management. � Biodiv. Conserv. 7: 323�333.

Lawrence, W. S. and Bach, C. E. 1989. Chrysomelid beetlemovements in relation to host-plant size and surrounding non-host vegetatation. � Ecology 70: 1679�1690.

Matter, S. F. 2000. The importance of the relationship betweenpopulation density and habitat area. � Oikos 89: 613�619.

MacGarvin, M. 1982. Species�area relationships of insects on hostplants: herbivores on rosebay willowherb. � J. Anim. Ecol. 51:207�223.

Milne, W. M. 1998. Suitability of clovers (Trifolium species andcultivars) as hosts of spotted clover aphid, a biotype ofTherioaphis trifolii (Monell) (Hemiptera: Aphididae). � Aust.J. Exp. Agric. 38: 241�245.

Moilanen, A. and Hanski, I. 1998. Metapopulation dynamics:effects of habitat quality and landscape structure. � Ecology79: 2503�2515.

Nair, R. M. et al. 2003. Evaluating pasture legumes for resistanceto aphids. � Aust. J. Exp. Agric. 43: 1345�1349.

Nowicki, P. 2007. From metapopulation theory to conservationrecommendations: lessons from spatial occurrence and abun-dance patterns of Maculinea butterflies. � Biol. Conserv. 140:119�129.

Osborn, H. 1928. The leafhoppers of Ohio. � Ohio Biol. Surv.Bull. 14.

.Ostergard, H. and Ehrlen, J. 2005. Among population variationin specialist and generalist seed predation � the importance ofhost plant distribution, alternative hosts and environmentalvariation. � Oikos 111: 39�46.

Quinn, G. P. and Keough, M. J. 2002. Experimental design anddata analysis for biologists. � Cambridge Univ. Press.

Rand, T. A. et al. 2006. Spillover edge effects: the dispersal ofagriculturally subsidized insect natural enemies into adjacentnatural habitats. � Ecol. Lett. 9: 603�614.

Reeve, J. D. et al. 2008. Diffusion models for animals in complexlandscapes: incorporating heterogeneity among substrates,individuals and edge behaviours. � J. Anim. Ecol. 77: 898�904.

Root, R. B. 1973. Organization of plant�arthropod association insimple and diverse habitats � fauna of collards (Brassicaoleracea). � Ecol. Monogr. 43: 95�120.

Rosenlew, H. and Roslin, T. 2008. Habitat fragmentation and thefunctional efficiency of temperate dung beetles. � Oikos 117:1659�1666.

Russell, E. P. 1989. Enemies hypothesis: a review of the effect ofvegetational diversity on predatory insects and parasitoids.� Environ. Entomol. 18: 590�599.

Steffan-Dewenter, I. 2003. Importance of habitat area andlandscape context for species richness of bees and wasps infragmented orchard meadows. � Conserv. Biol. 17: 1036�1044.

Tabachnick, B. G. and Fidell, L. S. 2000. Using multivariatestatistics. � Allyn and Bacon.

Thies, C. et al. 2003. Effects of landscape context on herbivoryand parasitism at different spatial scales. � Oikos 101: 18�25.

Tscharntke, T. and Brandl, R. 2004. Plant�insect interactions infragmented landscapes. � Annu. Rev. Entomol. 49: 405�430.

Tscharntke, T. et al. 2002. Contribution of small habitatfragments to conservation of insect communities of grass-land�cropland landscapes. � Ecol. Appl. 12: 354�363.

Tscharntke, T. et al. 2005. Landscape perspectives on agriculturalintensification and biodiversity�ecosystem service manage-ment. � Ecol. Lett. 8: 857�874.

USDA (US Dept Agric.) 2002. Crop profiles for potatoes inMaryland. (/<www.cfmx1.ent.ncsu.edu/cropprofiles/wservices/cpdisplay.cfm?filename�Mdpotato/>).

Valladares, G. et al. 2006. Habitat fragmentation effects ontrophic processes of insect�plant food webs. � Conserv. Biol.20: 212�217.

Ver Hoef, J. M. and Cressie, N. 1993. Spatial statistics: analysis ofexperiments. � In: Scheiner, S. M. and Gurevitch, J. (eds),Design and analysis of ecological experiments. Chapman andHall, pp. 319�341.

With, K. A. et al. 2002. Threshold effects of landscape structureon biological control in agroecosystems. � Ecol. Appl. 12: 52�65.

Young, O. P. 1986. Host plants of the tarnished plant bug, Lyguslineolaris (Heteroptera, Miridae). � Ann. Entomol. Soc. Am.79: 747�762.

Zaviezo, T. et al. 2006. Effects of habitat loss, habitat fragmenta-tion, and isolation on the density, species richness, anddistribution of ladybeetles in manipulated alfalfa landscapes.� Ecol. Entomol. 31: 646�656.

1485

Results from MANOVAs on the effects of habitat area, matrix composition, and habitat fragmentation on the collective densities of the five herbivore species. Densities were square-root transformed.Significant results at the a�0.025 level are shown in boldface.

Source July August

Wilk’s l DF F p Wilk’s l DF F p

Area 0.351 5, 18 6.657 0.001 0.539 5, 18 3.084 0.035Matrix 0.318 5, 18 7.724 B0.001 0.696 5, 18 1.571 0.218Fragmentation 0.500 5, 18 3.593 0.020 0.649 5, 18 1.946 0.136Area�Matrix 0.787 5, 18 0.974 0.460 0.693 5, 18 1.598 0.211Area�Fragmentation 0.799 5, 18 0.903 0.501 0.874 5, 18 0.519 0.759Matrix�Fragmentation 0.815 5, 18 0.818 0.553 0.744 5, 18 1.238 0.332Area�Matrix�Fragmentation 0.642 5, 18 2.004 0.127 0.801 5, 18 0.896 0.505

Results from univariate ANOVAs examining effects of habitat area (A), matrix composition (M), and habitat fragmentation (F) on the densities of each herbivore species in July. Densities were square-roottransformed. Significant results at the a�0.025 level are shown in boldface.

Source DF Agallia constricta Empoasca fabae Therioaphis trifolii Lygus lineolaris Halticus bractatus

F p F p F p F p F p

A 1, 22 0.424 0.522 0.362 0.643 9.486 0.005 2.736 0.112 13.143 0.001M 1, 22 24.393 B0.001 0.864 0.363 12.941 0.002 0.035 0.854 0.021 0.886F 1, 22 3.457 0.076 0.274 0.606 0.847 0.367 2.490 0.129 0.224 0.641A�M 1, 22 0.057 0.813 1.391 0.251 2.651 0.118 0.377 0.546 0.734 0.401A�F 1, 22 2.232 0.149 3.171 0.089 0.435 0.516 0.914 0.350 0.615 0.441M�F 1, 22 2.145 0.157 1.504 0.233 0.026 0.873 0.824 0.374 0.045 0.835A�M�F 1, 22 0.015 0.903 0.010 0.922 0.642 0.432 4.287 0.050 3.726 0.067

Appendix 1

Appendix 2

14

86