Oecologia (1993) 95:358-364

Oecologia �9 Springer-Verlag 1993

Plasticity and overcompensation in grass responses to herbivory Richard D. Alward*, Anthony Joern

School of Biological Sciences, University of Nebraska, Lincoln, NE 68588, USA

Received: 12 January 1993 / Accepted: 20 May 1993

Abstract. Several hypotheses predict defoliation-induced increases in individual plant fitness. In this paper we examine three such hypotheses: the Herbivore Optimiza- tion Hypothesis (HOH); the Continuum of Responses Hypothesis (CRH); and the Growth Rate Model (GRM). All three have in common predictions based on responses of defoliated individuals with the objective of explaining community and higher level phenomena. The latter two extend theory by specifying conditions for overcompensatory responses. They differ in whether over- compensation is sensitive to conditions external (CRH) or internal (GRM) to the plant. We tested these hypoth- eses with field experiments in a grassland system in which two native, perennial grass species replace each other along a short topographic/resource gradient. We detect- ed positive, neutral, and negative changes in plant mass in response to partial defoliation. Patterns of responses to the edaphic and competitive environment combina- tions were unique to each species and neither the CRH nor the GRM were able to consistently predict responses in these grasses. Predictions of the HOH were fully sup- ported only by the species naturally limited to lower- resource environments: overcompensation occurred in natural environments and it occurred at herbivory levels these plants experience naturally. Thus, the overcompen- satory response can be important for the maintenance of local plant population distributions. However, new mechanistic theory is needed to account for the trend common to both species: overcompensatory responses to herbivory were greater in the edaphic environment in which each species was naturally most abundant.

Key words: B o u t e l o u a - Grasshopper herbivory - Over- compensation Plant-Herbivore interactions Sandhills prairie

* Present address: Department of Biology, Colorado State Univer- sity, Fort Collins, CO 80523, USA

Correspondence to: R. D. Alward

Defoliation-induced overcompensation in individual plants is predicted by at least three hypotheses. If true, such responses to herbivory could partially explain the persistence of some plant species within natural com- munities. An early, general hypothesis proposed that moderate levels of herbivory could have positive effects on plant fitness (Dyer 1975; McNaughton 1979a). This hypothesis was proposed as a partial explanation for the success of perennial grasses in the face of ubiquitous mammalian grazing in the grassland regions of North America and Africa. Studies on both continents re- peatedly demonstrated greater vegetation yields at moderate levels of grazing, or mowing, than at either lower or higher defoliation levels (Beard 1973; McNaughton 1976, 1979b; Adjei et al. 1980; Dyer et al. 1982; Crawley 1983; Williamson et al. 1989). This "Her- bivore Optimization Hypothesis" (HOH; McNaughton 1979a, Fig. 3.1; Dyer et al. 1982) predicts expected changes in plant fitness over a range of herbivory inten- sities. Increased plant fitness is expected at low to moderate levels of herbivory and decreased fitness is expected at high levels of herbivory.

Predictions of the HOH are explicitly individual- based, although many studies extrapolate responses at the level of the individual plant to explain patterns in plant population demographics and community composition (e.g., McNaughton 1983a, b; Belsky 1987). Failure to consistently recognize the ecological vs. evolutionary and spatial vs. temporal restrictions of the hypothesis has resulted in misdirected discussion, ultimately leading to arguments based on the results of experiments not de- signed to evaluate the hypothesis (reviewed in Belsky 1986, 1987; Brown and Allen 1989). For example, greater vegetation biomass following herbivore exclusion has frequently been found in plots containing more than one individual (Crawley 1988). Such responses confound in- dividual and population or community level responses to herbivory; it is not possible to distinguish the direct from the indirect effects of herbivory in such studies (see Holt 1977; Louda et al. 1990).

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While the HOH stipulates the range of herbivory intensities at which increases in fitness are likely to be observed, it omits consideration of the environmental context in which herbivory occurs. Averaging responses of individuals found over a range of micro-environments may obscure rather than clarify the role of herbivory in natural communities. Two related hypotheses extend the HOH with explicit recognition of some factors within the environment that may alter an individual's qualitative and quantitative response to herbivory. We label these the Continuum of Responses Hypothesis (CRH, Mas- chinski and Whitham 1989) and the Growth Rate Model (GRM, Hilbert et al. 1981). Both hypotheses recognize heterogeneity within the environment as the norm. Both also predict that an individual's response to defoliation is context-sensitive. The CRH and GRM differ in (a) methods of quantifying herbivory, (b) fitness indices em- ployed and, most importantly, (c) environmental con- ditions under which herbivore-induced increases in fit- ness are predicted.

The CRH is a probabilistic graphical model that predicts a range of possible plant responses to defoliation (Maschinski and Whitham 1989). The model was de- veloped following experimental manipulations with a monocarpic biennial forb (Ipomopsis arizonica). The model predicts that the probability of a particular plant response to defoliation is dependent upon the status of three external variables: (a) the timing of the herbivory event (early to late season); (b) the availability of re- sources (low to high); and (c) the intensity of competition (low to high). Under conditions of low resource-avail- ability and high-intensity competition, herbivory will have consistently detrimental effects on an individual's fitness. Concomitant with increases in resource availability and decreases in competition are decreases in the probability of detrimental effects of herbivory being expressed. Ulti- mately, in high-nutrient, low-competition environments, the timing of herbivory may dramatically alter the plant response observed. Late season herbivory will still proba- bly result in decreased plant fitness, but a single early season herbivory event may result in increased fitness of that individual.

In contrast, the GRM is a mathematical model that predicts quantitative differences in plant relative growth rates dependent upon (a) the proportion of tissue lost and (b) the plant's relative growth rate prior to the herbivory event (Hilbert et al. 1981). Analysis of the model indicates that an increase in relative growth rate following partial defoliation is necessary, but not suf- ficient, for a grazed plant to equal or exceed the net primary productivity (NPP) of an ungrazed plant. Com- pared with heavily grazed individuals, lightly grazed plants require a smaller increase in their growth rates to exceed the NPP of ungrazed plants. Additionally, only plants growing at a rate well below their potential max- imum are likely to exhibit increases in their growth rates sufficient to exceed the NPP of ungrazed plants. Thus, herbivore-induced increases in plant fitness are most probable in lightly-grazed individuals growing well be- low their potential growth rates.

Thus, the CRH and GRM generate two mutually

exclusive predictions of the environmental conditions under which herbivore-induced increases in plant fitness are expected. Following early season defoliation, the CRH predicts that increased plant fitness wilt be most probable in low competition, high resource environ- ments. In contrast, following light defoliation, the GRM predicts that increased plant fitness will be most probable in stressed, slowly growing plants (i.e. : plants in high competition and/or low resource environments). To si- multaneously test these two hypotheses in our experi- ments, we assumed that the relationship between early and late season herbivory is similar to the relationship between low and high tissue loss early in the growing season, for relative impacts on target plant fitness.

Detection of a positive change in an index of fitness (overcompensation) is necessary to support all the above hypotheses. Such responses have been observed in plants growing in field and indoor experiments (Paige and Whitham 1987; Oesterheld and McNaughton 1988; Maschinski and Whitham 1989). Overcompensation alone, however, does not provide sufficient evidence to conclude that herbivory is important in enhancing the success of plant populations, species, or guilds within a community. Additional evidence must include (a) the range of conditions under which overcompensatory re- sponses occur, and (b) the probability that such con- ditions will be encountered in the environment in which the plant is successful. If herbivore-induced increased plant fitness is an important factor explaining the persis- tence of plant populations, then: (a) overcompensatory responses must be detectable in field-growing plants within the environment in which the population is suc- cessful, and (b) the timing and intensity of experimental levels of herbivory under which overcompensation oc- curs must be similar to those that plants are likely to naturally encounter.

In this paper, we report evidence from field experi- ments on the range and likelihood of compensatory re- sponses in two native grasses. We assessed the responses of juvenile Bouteloua gracilis and B. hirsuta individuals to three frequencies of grasshopper herbivory over a range of realistic physical and competitive environments. We used results from these experiments to evaluate the utility of the CRH and the GRM to predict overcom- pensation following defoliation. Furthermore, to evalu- ate the possibility that overcompensatory responses in individuals contribute to the maintenance of local popu- lation distributions (as suggested by the HOH), we deter- mined naturally-occurring frequencies of herbivory across the same range of environments used above.

Study site

We conducted this study at Arapaho Prairie, Arthur County, Nebraska from June 1989 to August 1990, in- clusive. Vegetation-covered, stabilized dunes charac- terize this sandy, low-nutrient, low moisture grassland typical of the southwestern Nebraska upland sandhills grassland. The mixed vegetation includes Ca and C4 representatives from both the eastern tallgrass prairies

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and wes tern shor tg rass pra i r ies , resul t ing in a un ique class i f icat ion as Sandhi l l s Prair ie . The soils o f f lat val ley b o t t o m s genera l ly con t a in h igher levels o f n i t rogen and wa te r and a lower p r o p o r t i o n o f sand t han soils o f the u p p e r dune ridges. Site-specific detai ls o f soil s t ruc tures and d i s t r ibu t ions , vege ta t ion assoc ia t ions a n d loca t ions of exper imen ta l p lo t s have been p rev ious ly r e p o r t e d (Keeler et al. 1980; Barnes et al. 1984). M a n a g e m e n t pract ices include p ro t ec t i on f rom cat t le graz ing and fire since 1976 and a 3 4 year m o w i n g r o t a t i o n o f val ley and lower s lope regions , in i t ia ted in 1982.

W e used two a b u n d a n t and pa la tab le , na t ive C4 shor t - grasses in this s tudy : Bouteloua hirsuta Lag. (ha i ry d r ama) and B. gracilis (H .B .K. ) Lag. ex Grif f i ths (blue g r ama ; nomenc la tu re follows Suther land 1986). A t A r a p - aho Pra i r ie these grasses replace each o ther across a 70 mete r e leva t iona l gradient . Bouteloua hirsuta is mos t a b u n d a n t on the u p p e r steep slopes (" r idges") o f vegeta- t ion-s tab i l i zed sand dunes, whereas B. gracilis is mos t a b u n d a n t in the flat i n t e rdune valleys. Both species are c o m m o n on the mid -g rad i en t s lopes bu t are ra re at the t o p o g r a p h i c ext reme where the o the r is d o m i n a n t (Heinisch 1981).

Methods

We examined the responses of both Bouteloua species to grass- hopper herbivory in field experiments in which we manipulated growth conditions. The variables manipulated included herbivore exposure, neighbor plant density, and growing site along a topo- graphic gradient. Target plants grew in the field continuously over two consecutive growing seasons in 1989 and 1990. They were subjected to identical treatment manipulations in each growing season. We have used similar methods in other experiments (Alward 1992) and so summarize the procedures and treatment manipula- tions here.

Our design employed four treatment combinations of growing site and competitive environments. These were replicated in five blocks of six plants each within each of two sites. The sites were at either extreme of a dune topographic gradient (abbreviated within tables and figures as "V" for valley and "R" for ridge). Detailed descriptions of the spatial and temporal gradients in several edaphic characteristics of sandhills dunes have been reported (Barnes et al. 1984). To place our study in the context of this earlier study, and to quantify differences between sites, we collected soil samples in July and August of each year and analyzed the samples for total Kjeldahl nitrogen and gravimetric soil moisture. Results of analyses of the surface 10 cm of soil are reported since most root growth occurred in this zone (personal observation). Precipitation records for each year were collected by a remote weather station within the valley.

Within each block of six target plants, we randomly assigned half as low-competition and half as high-competition environments. Prior to manipulations, canopy cover on all plots was approximate- ly 50 % (Alward and Joern, unpub, data). We removed vegetation in low-competition plots by application of the general herbicide RomadUp (N-[phosphonomethyl] glycine; diluted according to package instructions) to 0.5 meter diameter plots in June 1989. This ~echnique of neighbor removal eliminated disturbance to initial vegetation structure and soil structure integrity. Since RoundUp has no measurable residual effect in the soil, target seedlings were unaffected. We reapplied RoundUp to plots, in June 1990, by wiping the herbicide on vegetation surrounding target plants. Vol- unteer seedlings emerging within plots over the course of the experi- ment were pulled when detected. We did not manipulate vegetation

in high-competition plots. Low- and high-competition environ- ments are abbreviated as (0) and (+), respectively, within tables and figures. Combinations of site and competition are indicated with the abbreviations V(0), V(+), R(0), and R(+).

We raised target plants from seed in the spring of 1989, and in late June transplanted 60 seven-week old seedlings of each species into plots, individually prepared as above. We enclosed seedlings in cylindrical hardware cloth cages and manipulated herbivory levels by introducing a grasshopper (Ageneotettix deorum Scudder) into each cage for zero, one, or two 24 h periods in early July. In a concurrent study, we found that a single herbivory event resulted in a mean loss in above-ground tissue of 23 % (range: 5-56 %) for B. gracilis seedlings and 36% (1-100%) for B. hirsuta seedlings and a second herbivory event resulted in a mean total loss of 61% (19-100%) and 76% (19-100%) for B. gracilis and B. hirsuta, respec- tively (Alward and Joern, unpub, data). We did not quantify proportional tissue loss in second-year juveniles but did verify its occurrence. Because these plants were often four times as large as first year plants, the proportion of tissue lost was likely much less than in seedlings. In mid-August 1990, we harvested entire plants (shoots and roots to a depth of 20 cm) and obtained dry masses of shoots, roots and inflorescences.

We analyzed each species' response to herbivory for each of the four unique site and competition combinations using analysis of variance for randomized complete block designs (Steel and Torrie 1980) with PC SAS 6.04 (1987, General Linear Models Procedure). Inflorescence mass data included frequent zero values; natural log-transformed data (/n(y + 1)) were used in the analyses, but we present back-transformed means in the figures. We exploited the two degrees-of-freedom available from the three herbivory treat- ments by evaluating orthogonal contrasts to investigate linear and quadratic patterns of response. Significant quadratic trends indicate curvilinear responses in plant biomass to increasing herhivory. The F-values reported within Table 2 are from these ANOVAS and contrasts.

To assess site-specific natural frequencies of herbivory at Arapa- ho Prairie, we transplanted an additional 90 B. gracilis seedlings and 48 B. hirsuta seedlings, without cages, during 1990. We manipulated growing site and neighbor density as above but paid special atten- tion to nearest neighbor identity in the unmanipulated neighbor- hood. For B. hirsuta we arranged seedlings in four blocks of six plants within each of the two sites in early July. We planted 24 seedlings in high-density environments and within 5 cm of an in situ congener (congener identity was site-specific: B. hirsuta on the ridge and B. gracilis in the valley). We planted the remaining 24 seedlings in low-density environments, as described for caged plants.

For B. gracilis we arranged seedlings in five blocks of nine plants within the two sites in late June. We planted 30 seedlings in low- density environments and 30 seedlings in high-density environments and within 5 cm of an in situ congener as described for B. hirsuta. In addition, we planted an extra 30 seedlings in high-density en- vironments but within 5 cm of Sporobolus cryptandrus (Tom) A. Gray (sand dropseed), a C4 midgrass abundant within all blocks in both the valley and ridge. Seedling locations were marked with short, painted wooden skewers. We recorded the initial number of leaves on all seedlings. Over the season, we inspected seedlings and recorded total leaf number and number of leaves showing herbivore damage at least weekly, but occasionally daily, until harvest on 1 September 1990. This method allowed a conservative estimate of the natural frequency of herbivory and the proportion of leaves damaged at each herbivory event. ANOVA for split-plot design (PC SAS 1987; Proc GLM) permitted us to evaluate the effects of growing sites and neighbors on naturally occurring rates of herbiv- ory.

Results

Resul t s o f the July and A u g u s t edaph ic surveys were cons is ten t wi th the pa t t e rns descr ibed by Barnes et al.

Table 1. Differences in soil nitrogen and water [Mean (SE)] in upper 10 cm of soil from ridge and valley sites at Arapaho Prairie from samples collected on 8 July and 8 August 1989 and the effects of RoundUp application on soil resources in the valley only. Months in which there were significant topographic site differences are noted

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with an asterisk (Student's t; P=0.05; n=12-15). One month following herbicide application there were no significant differences in abundance of these soil resources in valley sites as indicated by the similarity of the August measurements (Student's t; P=0.05; n= 10)

Resource Month Topographic site comparisons

Ridge

Herbicide effects (valley only)

Valley Untreated Treated

Soil nitrogen July 37.5 (2.5) * (mg N/kg soil) Aug 55.1 (10.5)

Soil water July 1.65 (0.30) * (%) Aug 4.41 (0.25) *

63.5 (4.8) 66.3 (2.1) 61.4 (2.6) 66.6 (4.4) 65.0 (2.4) 62.6 (2.1)

2.19 (0.13) 1.64 (0.04) 1.66 (0.04) 7.03 (0.25) 7.55 (0.10) 7.57 (0.15)

Table 2. Results Of ANOVA of herbivore exposure (HERB) upon plant biomass and orthogonal contrasts (values of F-statistic) to evaluate the significance of quadratic (Q) and linear (L) trends in juvenile total mass and shoot, root and inflorescence mass in re- sponse to increasing duration of exposure to herbivory. "Environ- ment" column lists symbols for the site and competition combina-

tions: valley (V) and ridge (R) sites with (+) and without (0) competitors. Column entitled "S.V." is the sources of variation from the ANOVAs. Significance of F-statistics indicated with sym- bols:?P_<0.1; *P_<0.05; **P_<0.01. Significance levels for shoot, root, and inflorescence responses have been Bonferroni-adjusted

Environment S.V. Bouteloua gracilis

Total Shoot Root Inflor

Bouteloua hirsuta

Total Shoot Root Inflor

V(0) HERB: Q: L:

V(+) HERB: Q: L:

R(0) HERB Q: L:

R(+) HERB Q: L:

4.29* 4.20 2.75 3.82 7.64** 7.62* 4.40? 5.19t 0.93 0.78 1.09 2.46

5.16. 5.29 3.85 n/a 0.38 0.14 1.28 n/a

10.25"* 10.59" 6.98 + n/a

0.35 0.08 3.85 4.15 0.01 0.01 1.02 0.03 0.67 0.15 6.137 8.11"

5.53* 8.95 t 2.80 n/a 2.53 t 1.34 3.76 n/a 8.54** 16.56"* 1.85 n/a

0.13 0.23 0.01 0.50 0.02 0.03 0.00 0.64 0.23 0.41 0.02 0.24

1.30 2.59 0.30 0.91 2.46 t 5.14 0.06 1.57 0.68 0.68 0.60 0.71

3.89 3.46 1.50 0.44 7.78* 6.92? 2.84 0.63 0.50 0.56 0.00 0.48

0.37 0.97 0.50 n/a 0.29 1.57 0.63 n/a 0.53 0.58 0.26 n/a

(1984); nitrogen and moisture levels in the key growing zone of ridge soils were consistently lower than valley soils (Table 1). In this study, early August rains were responsible for significantly higher soil moisture mea- surements in August. Neither soil nitrogen nor soil mois- ture were significantly affected by RoundUp treatments (Table 1).

The removal of neighboring plants accomplished by the herbicide applications did, however, significantly af- fect the growth of target individuals. Within a site, plants without neighbors were, on average, 3.5 times greater in mass than plants with neighbors. Plants with neighbors generally failed to initiate inflorescences. However, Bouteloua hirsuta individuals with neighbors in valley sites produced, on average, 2.7 inflorescences.

Grasshopper herbivores regularly had significant ef- fects on B. #racilis total mass (Table 2, Fig. 1 a). The trends of plant response to increasing exposure to her- bivores were sensitive to the environment in which the plant was found. In valley, low-competition environ- ments, a significant quadratic response to herbivory was observed. However, in both the valley and the ridge, under high-competition conditions, a negative linear re- sponse to herbivory was observed. Trends of response in

shoot and inflorescence tissues generally were similar to trends of whole plant response whereas root mass was less frequently affected by herbivory (Table 2, Fig. 2 a, b, c). The notable exception was in ridge, low- competition environments. In this case, although whole plant mass was not significantly affected by herbivory, root mass declined linearly and inflorescence mass in- creased linearly in response to increased exposure to grasshopper herbivory (Table 2, Fig. 2 b, c).

Bouteloua hirsuta was seldom significantly affected by the levels of grasshopper herbivory used in this study (Table 2, Figs. 1 b and 2 d, e, f). However, in valley, high-competition environments a significant whole- plant quadratic response was detected. In ridge, low-competition environments significant quadratic re- sponses in shoot tissues resulted in a significant whole plant quadratic response. Bouteloua hirsuta inflorescence mass was not significantly affected by these herbivory treatments.

Naturally occurring herbivory occurs at different fre- quencies for transplanted seedlings of these two species (Fig. 3). On average, B. #racilis seedlings were grazed 4.3 times over the 67 days of observations, whereas B. hir- suta seedlings were grazed an average of 1.5 times over

362

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HERBIVORY EXPOSURE (days) Fig. 2a-f. Juvenile responses to herbivory (means, SE) in Bouteloua 9racilis a aboveground mass; b belowground mass; e inflorescence mass; and Bouteloua hirsuta d aboveground mass; e belowground mass; f inflorescence mass. Symbols for site and competition com- binations as in Fig. 1. Bonferroni-adjusted statistically significant linear and quadratic response patterns are reported in Table 2. Note different scales for dry mass

53 days of observations. Individuals of B. 9racilis in the valley were grazed more frequently than individuals on the ridge but neighbor density and species composition did not affect grazing frequencies. In contrast, in- dividuals of B. hirsuta in open sites in the valley were grazed significantly more frequently than individuals on the ridge or individuals next to a congener.

Bouteloua graeilis individuals were grazed from 1 to 9 times, all were grazed at least once, and only 15 of the 90 seedlings were grazed fewer than 3 times. Of these 15 seldom grazed individuals, all were severely defoliated and only 4 individuals, all on the ridge, had any visible green leaves after day 18. Bouteloua hirsuta individuals were grazed from 0 to 5 times, 85% were grazed at least once, and only 9 of the 48 seedlings were grazed more frequently than twice. Of these 9 frequently grazed in- dividuals, 7 were in open valley sites. The modal fre- quency of herbivory on B. hirsuta seedlings in open ridge sites equaled 1.

These natural rates of herbivory data were collected in 1990 when mean grasshopper densities (combining grass and forb feeders) were 1.5 m -2 on the ridge and 1.75 m-2 in the valley (Joern, unpublished data). Grass- hopper densities on the ridge were within a standard deviation of the twelve-year mean for Arapaho Prairie while grasshopper densities in the valley were below the twelve-year mean (ridge: 2.1 m -2 (sd 1.1); valley: 4.8 m -z (sd 2.8); Joern 1982, unpublished data).

Discussion

Both Bouteloua gracilis and B. hirsuta exhibited plasticity in response to grasshopper herbivory. A continuum of responses in shoot, root, inflorescence, and total plant mass was observed as the frequency of herbivory in- creased. Patterns of response ranged from negative linear relationships between herbivory duration and plant mass, through neutral relationships, to significant qua- dratic relationships. In some cases, total plant mass re- sponse was the result of similar patterns of response in shoot, root, and inflorescence mass (e.g., both species in V(0) sites; Table 2). In other cases, only shoot mass was significantly affected by herbivory treatments (e.g., B. hirsuta in R(0) sites and B. graeilis in R(+ ) sites; Table 2). Since shoots contributed from 51% to 78% to total plant mass, a statistically significant shoot response re- sulted in a similarly significant total plant response, re- gardless of the responses of roots and inflorescences.

The specific response observed was related to the environmental conditions in which the plant was grow- ing. Environment-specific responses were, to a large ex- tent, predictable from the Continuum of Responses Hypothesis or the Growth Rate Model, but the utility of these hypotheses was species-specific, as described below. The responses of B. gracilis were better predicted by the CRH. The responses of B. hirsuta were, arguably, better

O9

Z LU > LU >- n- O > s163 cc LU T

7

6 5

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Open Bouteloua Sporobolus

B. hirsuta b 7

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NEIGHBOR Fig. 3a, b. Frequencies of natural herbivory (means, SE) on seed- lings of a Bouteloua 9raeilis and b Bouteloua hirsuta in valley and ridge sites, with and without nearby competitors. Neighbor treat- ments include: OPEN (no living neighbors within 25 cm) and undisturbed ambient, site-specific neighbor densities with an in situ Bouteloua spp. or Sporobolus eryptandrus adult within 5 cm of the target seedling

predicted by the GRM, but significant negative effects of herbivory were seldom observed.

The continuum of responses hypothesis

If overcompensatory responses in these grama grasses occur at Arapaho Prairie, the CRH (Maschinski and Whitham 1989) predicts that such responses would be most probable in the resource-rich valley plots from which competitors have been removed (V(0) plots). In- deed, this was the only context in which significant qua- dratic responses, indicative of overcompensation, were observed in B. 9racilis (Table 2, Figs. 1 a and 2 a, b, c). In the three other environmental contexts the effects of herbivory on B. 9racilis were, generally, neutral or nega- tive (Table 2). This pattern was also consistent with the CRH.

Although B. hirsuta was also plastic in response to herbivory, the patterns of responses observed were poor- ly predicted by the CRH (Table 2, Figs. 1 b and 2 d, e, f). That herbivory had no significant effect on plant mass in V(0) plots was not inconsistent with the CRH; however, the mean individual responses in the other three environ- ments were not as predicted. Neutral effects were ob- served in R(+ ) plots where negative effects were expect- ed. Significant quadratic responses in total and shoot mass, indicative of overcompensation, were observed in

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R(0) and V(+) plots, environments in which the CRH predicts neutral to negative responses to herbivory.

The 9rowth rate model

Analysis of the GRM indicates that plants most likely to overcompensate following herbivory are those which are stressed and growing slowly, compared to their potential rates (Hilbert et al. 1981, p. 17). Thus, according to the GRM, if overcompensatory responses occur at Arapaho Prairie, they are most probable in ridge and/or high- competition environments: A comparison of the model with the results of these experiments indicate that the GRM poorly predicts the patterns of responses observed in B. gracilis (Table 2, Figs. 1 a and 2 a, b, c). Herbivory had negative or neutral effects on vegetative tissues of slowly growing plants (V(+), R(0), and R(+) plots), and, in contrast with predictions, only the fastest growing B. gracilis, in the least stressful environment (i.e. : V(0) plots), exhibited overcompensation to herbivory.

The GRM was more successful in predicting the re- sponses of the ridge species, B. hirsuta, to herbivory. Rapidly growing B. hirsuta in the valley at low- competition levels (V(0) plots) did not overcompensate (Table 2). More slowly growing individuals (in V(+ ) and R(0) plots) did overcompensate, although the slowest growing plants (R(+)) did not.

Natural rates of the frequency of herbivory on seed- lings of these grasses were consistently different between species and occasionally among sites (Fig. 3). Bouteloua 9racilis seedlings were grazed more than twice as fre- quently as B. hirsuta seedlings. Mean frequencies of her- bivory experienced by B. gracilis far exceed the single event that promotes overcompensation, even in these years of low grasshopper abundance. Additionally, the three seedlings naturally grazed only once during the observation period were used only once by default: these seedlings were completely defoliated and did not survive to be grazed again. In contrast, B. hirsuta seedlings were, on average, grazed by grasshoppers once during the observation period (Fig. 3). The mean natural frequen- cies of herbivory experienced by B. hirsuta in low- competition, ridge plots coincided with the experimental conditions that promoted overcompensatory responses. The Herbivore Optimization Hypothesis was supported by these responses in B. hirsuta. Plant fitness did increase following defoliation, within sites that this species is currently most abundant, at the low herbivory levels that most indviduals of B. hirsuta experienced naturally.

Unfortunately, these experiments indicate that presently available models which specify the conditions that promote overcompensation cannot be generalized to all species, and possibly not even to all populations with- in a species (see also Bergelson and Crawley 1992). Al- though individuals of both these grasses exhibited over- compensatory responses in both vegetative and re- productive tissues, neither resource abundance nor rela- tive growth rates consistently predicted the occurrence of this response. However, one pattern emerged. Overcom- pensatory responses were observed under the condition

364

of reduced competi t ive pressure within sites in which each species is natural ly mos t a b u n d a n t : B. 9racilis in the valley and B. hirsuta on the ridge. Such results suggest that theory must account for (a) spatial gradients in herbivory intensity, and var ia t ion in p lant species' (b) life his tory strategies, (c) sensitivity to defoliation, and (d) sensitivity to the envi ronmenta l contexts in which herbiv- ory occurs.

Acknowledgements. Valuable assistance in the field, greenhouse, and workshop from M. Zeisset, G. Drohman, and R. Randall, respec- tively, is greatly appreciated. Comments from S. Behmer, F.S. Chapin, III, S.M. Louda, J. Maschinski, S. Mole, J. Stubben- dieck, Y. Yang, and an anonymous reviewer greatly improved this manuscript. Soil nitrogen analyses were conducted by the University of Nebraska Soils Testing Lab. Arapaho Prairie is owned by The Nature Conservancy and is managed, through a lease agreement, by Cedar Point Biological Station (School of Biological Sciences, University of Nebraska). Funding was provided in part by a USDA Competitive Grant to AJ (USDA/NRIC 89-37153-4467) and re- search awards to RDA from the School of Biological Sciences, University of Nebraska.

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