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Department of Ecology, Ethology, and Evolution, University of Illinois at Urbana-Champaign, Shelford Vivarium, 606 E. Healey St., Champaign, Illinois 618202
Received for publication June 5, 1998. Accepted for publication October 15, 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Castilleja linariaefolia • competition • density dependence • Ipomopsis aggregata • Polemoniaceae • pollination
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Kunin, 1993
). Understanding the balance between intra- and interspecific competition is important because the mechanisms by which these processes decrease pollination and reproductive success may differ. Intraspecific competition occurs through changes in pollinator visitation (e.g., Rathcke, 1983
). At high plant density, pollinators can become limiting, reducing pollen deposition and subsequent reproductive success (Waser, 1978a
, 1983
; Rathcke, 1983
). Interspecific competition may also be mediated by changes in pollinator visitation, but interspecific pollen transfer is a more common mechanism (Waser, 1983
). Interspecific pollen transfer occurs when pollinators deposit both heterospecific and conspecific pollen on flowers (Waser, 1978a
, 1983
; Rathcke, 1983
). If this heterospecific pollen interferes with conspecific pollen deposition or fertilization success (e.g., Waser and Fugate, 1986
; Galen and Gregory, 1989
) or conspecific pollen is lost on other species' flowers (Campbell and Motten, 1985
; Feinsinger, Busby, and Tiebout, 1988
; Feinsinger and Tiebout, 1991
; Murcia and Feinsinger, 1996
), then subsequent reproductive success may decrease.
Because intraspecific competition occurs through changes in pollinator visitation, but interspecific competition can occur through both pollinator visitation and interspecific pollen transfer, they may have different consequences for a plant species' ecology and evolution. For example, if competition occurs by changes in pollinator visitation, then traits that make plants more attractive to pollinators, such as nectar production, flower number, or flower color, should be selected for. If competition occurs by interspecific pollen transfer, then traits that mechanically minimize the receipt of heterospecific pollen and/or maximize the receipt of conspecific pollen, such as the orientation of floral parts, should also be selected for (e.g., Brown and Kodric-Brown, 1979
; Campbell and Motten, 1985
). The evolution of the overall plant floral display may therefore depend on the relative strength of inter- and intraspecific competition. The relative importance of pollinator visitation and interspecific pollen transfer as mechanisms of competition may also influence the genetic structure of plant populations (Campbell, 1985a
) and plant species' spatial patterns (Pleasants, 1980
).
I used Ipomopsis aggregata (Polemoniaceae) and Castilleja linariaefolia (Scrophulariaceae) as a model system to assess the relative importance of intra- and interspecific competition for pollination. Several lines of evidence suggest that C. linariaefolia competes with I. aggregata for pollination. Flowers of both species are red and pollinated almost exclusively by Broad-tailed (Selasphorus platycercus) and Rufous (Selasphorus rufus) Hummingbirds at my study sites (personal observation). When I. aggregata and C. linariaefolia co-occur, their flowering phenologies overlap extensively, they are spatially intermingled, and interspecific movements by pollinators are frequent (personal observation). In a hand-pollination study, C. linariaefolia pollen, when deposited first, reduced subsequent conspecific pollen deposition and seed set of I. aggregata (C. M. Caruso and M. Alfaro, unpublished data). The probability of intraspecific competition among I. aggregata plants for pollination is less certain. Yet because hummingbird foraging and territoriality are strongly density dependent (e.g., Kodric-Brown and Brown, 1978
), pollinators could become a limited resource for I. aggregata as conspecific density increases.
I assessed the relative importance of intra- and interspecific competition for pollination by simultaneously manipulating the density of the focal species (I. aggregata) and the presence of C. linariaefolia in a field experiment. If changes in conspecific density have a greater effect on I. aggregata's pollination and reproductive success than the presence of a co-occurring species, then I can conclude that intraspecific competition for pollination is stronger than interspecific competition. If the presence of a co-occurring species has a greater effect on I. aggregata's pollination and reproductive success than conspecific density, then I can conclude that interspecific competition for pollination is stronger than intraspecific competition.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Grant and Wilken, 1986
). The plants used in this study were located in xeric subalpine meadows near the Rocky Mountain Biological Laboratory (RMBL) in west-central Colorado (elevation 2900 m). Ipomopsis aggregata (Waser and Price, 1989
) and C. linariaefolia (Cronquist et al., 1984
) are long-lived perennials. Ipomopsis aggregata is monocarpic (Waser and Price, 1989
), self-incompatible (Waser and Price, 1983
), and its reproduction is pollen limited at RMBL (Hainsworth, Wolf, and Mercier, 1985
; Campbell, 1991
; Campbell and Halama, 1993
). Castilleja linariaefolia plants are polycarpic (Carpenter, 1988
) and self-incompatible (Carpenter, 1983
). Reproduction of C. linariaefolia growing in the Sierra Nevada is not pollen limited (Carpenter, 1988
). Both species flower from late June until August at RMBL (Waser, 1978a
; C. M. Caruso, personal observation). Ipomopsis aggregata flowers produce 0–5 µL of nectar/24 h (Mitchell, 1993
). Measurements of 24-h nectar production are not available for C. linariaefolia, but milligrams of sugar secreted per flower per hour varies from 0.002 to 0.204 for plants growing in the Sierra Nevada (Carpenter, 1983
).
Experimental design
A split-plot design (e.g., Underwood, 1997
) was used, with the presence of heterospecifics (present vs. absent) as the between-plot factor and conspecific density (high vs. low) as the within-plot factor. I conducted the experiment in a meadow where there were no other hummingbird-pollinated species. This meadow was divided into 18 plots, each 10 m wide and extending out 
25 m from an escarpment. The distance between neighboring plots varied from 0 to 30 m. All plots contained I. aggregata and C. linariaefolia, but C. linariaefolia inflorescences were removed from half of the plots to create the heterospecifics-absent treatment level. Within each plot, I randomly chose one group of 12 I. aggregata plants growing within a 1-m2 area to be a high conspecific density experimental unit. I also randomly chose a single I. aggregata plant per plot and removed all conspecific inflorescences within a 5-m radius to create a low conspecific density experimental unit. This radius was chosen because <1% of flights by I. aggregata's pollinators are >5 m (Waser, 1982
). In summary, the experiment contained 18 plots, with two experimental units nested within each of these plots. The distance between high and low conspecific density experimental units within a plot varied from 5 to 25 m.
No manipulations were done to create the heterospecifics-present and high conspecific density treatment levels, which means that treatment effects cannot be unambiguously separated from the possible effects of correlated variables. The effects of several confounding variables can, however, be discounted.
Pre-existing differences in the floral neighborhood
The density of I. aggregata might be systematically lower within plots in the heterospecifics-present treatment, and this difference in the floral neighborhood could influence plant pollination and subsequent reproductive success (e.g., Feinsinger, Tiebout, and Young, 1991
). But the number of I. aggregata in the neighborhood (5-m radius) of the high conspecific density experimental units was not significantly different between plots where heterospecifics were present and absent (separate variance t test: t = 1.810; df = 9.8; P = 0.101). If areas where high conspecific density experimental units were placed contained many I. aggregata because they were favorable sites for growth, then density of C. linariaefolia may also be greater in the neighborhood of these units. But the number of C. linariaefolia in the neighborhood (5-m radius) of high vs. low conspecific density experimental units was not significantly different (separate variance t test: t = 0.548; df = 11.6; P = 0.594).
Pre-existing differences in plant size
If I. aggregata in high conspecific density experimental units experience strong resource competition, then plants in these units may be smaller and produce fewer flowers. If pollinators are responding to the density of flowers rather than plants, manipulating the density of plants will only be effective if plant and flower densities are positively related. Using number of buds as a proxy for number of flowers, I found that flower number per plant did not differ between high and low conspecific density experimental units (F1,16 = 2.584; P = 0.128), indicating that density-dependent resource competition did not influence inflorescence size. The high-density experimental units contained 7.5 times more total flowers than the low-density experimental units (F1,16 = 128.653; P = 0.000), indicating that plant and flower densities were positively related.
Data collection
Data were collected between 15 July and 11 August 1995. Three individuals within each high conspecific density experimental unit were randomly designated as focal plants, and all data for each unit were from these individuals. Each low conspecific density experimental unit contained only one individual, making it the focal plant. Because I. aggregata is self-incompatible (Waser and Price, 1983
), any self-pollen deposited on flowers cannot fertilize ovules. To prevent deposition of self-pollen, all flowers on the focal plants were emasculated before anther dehiscence. Emasculation can influence the dynamics of pollen transfer (Price and Waser, 1982
; Waser and Price, 1982
) and subsequent seed set (de Jong et al., 1992
; Waser and Price, 1991
), but outcross pollen deposition cannot be quantified without emasculating flowers. Because I emasculated all of the plants for which data were collected, relative comparisons between treatments should be valid.
To measure pollen deposition on stigmas, I randomly chose one or two flowers per plant seven times over the course of the experiment. These flowers were marked, collected 3 d later, stained with basic fuschin (Kearns and Inouye, 1993
), and the number of con- and heterospecific pollen grains counted. Most heterospecific pollen was from C. linariaefolia. By collecting flowers that can live 3–5 d (Campbell and Halama, 1993
) after only 3 d, I may have underestimated the magnitude of, but not relative differences in, pollen deposition. I observed each experimental plant or plants for 80 min to assess pollinator visitation rates, but there were too few visits (six foraging bouts to 48 flowers) for the data to be analyzed. I collected ten fruits from focal plants in each experimental unit to estimate seed set per fruit. Because the growing season was delayed by late snowmelt in 1995, the experiment had to be terminated before total fruit set per plant could be assessed.
Data analysis
I analyzed the data using a split-plot ANOVA, with presence of heterospecifics as the between-plot factor and conspecific density as the within-plot factor. In this design, the presence of the heterospecifics factor was tested over the plot (presence of heterospecifics) term, while the conspecific density factor and the interaction between the two factors were tested over the conspecific density x plot (presence of heterospecifics) term (Wilkinson, 1997
). I ran separate analyses for each response variable: the amount of conspecific pollen deposited per flower, the amount of heterospecific pollen deposited per flower, the ratio of heterospecific to total pollen deposited per flower, and seed set per fruit. Sample size for the analysis of seed set per fruit was reduced because fruits could not be collected from some experimental units. Each response variable was tested for the assumption of normality using Lilliefors' test and homogeneity of variance using Levene's test (Wilkinson, 1997
). The amount of heterospecific pollen deposited per flower and the ratio of heterospecific to total pollen deposited per flower were log transformed to meet these assumptions.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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25% less I. aggregata pollen per flower than those in plots where heterospecifics were removed (Fig. 1A). But the amount of I. aggregata pollen deposited per flower was not influenced by changes in conspecific density (F1,16 = 0.007; P = 0.933; Fig. 1B). There was no significant interaction between presence of heterospecifics and conspecific density (F1,16 = 3.263; P = 0.090; Fig. 2A). Flowers received a maximum of 198 conspecific pollen grains.
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Ratio of heterospecific to total pollen deposited
The ratio of heterospecific to total pollen deposited was influenced by the presence of heterospecifics (F1,14 = 5.531; P = 0.034). The proportion that was heterospecific pollen was twice as large for plants in plots where heterospecifics were present than where they were absent (Fig. 1E). Conspecific density also influenced the proportion of the pollen load that was heterospecific pollen (F1,14 = 6.872; P = 0.020). The proportion that was heterospecific pollen was three times as great in areas where conspecific density was lowered as in areas where density was high (Fig. 1F). There was no significant interaction between the presence of heterospecifics and conspecific density (F1,14 = 0.000; P = 0.998; Fig. 2C).
Seed set per fruit
Mean seed set per fruit varied from 9.616 to 11.609 among treatment groups (Fig. 1G, H). But neither the presence of heterospecifics (F1,7 = 0.470; P = 0.515; Fig. 1G) nor conspecific density (F1,7 = 0.051; P = 0.829; Fig. 1H) influenced seed set per fruit. The interaction between presence of heterospecifics and conspecific density was not significant (F1,7 = 3.863; P = 0.090; Fig. 2D).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Feinsinger, Tiebout, and Young, 1991
). The pollination of I. aggregata, like that of many other plant species, is clearly influenced by the presence of competitors for pollinator service. The presence of C. linariaefolia may also influence components of male fitness such as the amount of I. aggregata pollen exported. Although I did not measure any components of male fitness, results from other studies (Campbell, 1985a
) suggest that the amount of pollen exported should also decrease in the presence of a competitor species.
The lack of any effect of changes in conspecific plant density on the pollination of I. aggregata (Fig. 1B) is more difficult to explain. Plant pollination has frequently been reported to increase (Kunin, 1997
, and references therein) and occasionally to decrease (Zimmerman, 1980
) with the density of conspecifics. But rarely (Aizen, 1997
) is there no effect of changes in conspecific density on at least one component of pollination, particularly when such a large density manipulation (from 12 plants/m2 to 0.05 plants/m2) is done in a self-incompatible species. Because all I. aggregata plants within a 5-m radius of the focal plant were removed to create the low conspecific density units, these plants experienced isolation from conspecifics in addition to low conspecific density. In contrast to my results, this isolation should have magnified the effect of the density manipulations, enhancing the chance of observing intraspecific competition for pollination.
Alternatively, the lack of an effect of conspecific density on pollination of I. aggregata may be a function of the shape of the relationship between plant density and pollination. If pollinators are a limiting resource and their foraging is density dependent, then the relationship between plant density and pollination may be modeled as a quadratic function (Rathcke, 1983
). Pollination will be low at low plant density because there are too few plants to attract pollinators. As the density of plants increases, they should be more attractive to pollinators and pollination should increase until there are too many plants for pollinators to adequately service, at which point pollination should again decrease (Rathcke, 1983
). If my low and high conspecific density experimental units fell at the extremes of this quadratic function, then I might not expect to detect any changes in pollination.
The lack of an effect of changes in conspecific density on pollination of I. aggregata may also be caused by the effect of my treatments on the mean distance pollinators move between flowers. The distance a pollinator moves between flowers is inversely related to plant density; as plant density increases, the mean distance moved by pollinators decreases (e.g., Handel, 1983
). Pollinators may have been able to move the 5 m between the plants in my low-density treatment and other conspecifics without incurring large energy costs. If this is the case, then the amount of I. aggregata pollen deposited may not differ between the high- and low-density treatments. Lastly, the manipulation of conspecific density in my experiment may also have influenced an unmeasured component of pollination such as pollinator visitation rate. Further studies that manipulate conspecific density without isolating plants, include more levels of the conspecific plant density treatment, and measure all components of pollination are needed to definitively determine whether pollination of I. aggregata is density independent.
Despite interspecific competition for pollination, the component of I. aggregata's reproductive success measured in this experiment (seed set per fruit) was not significantly reduced when C. linariaefolia was present. Dose–response curves relating I. aggregata pollen deposition and seed set per fruit (Kohn and Waser, 1985
; Waser and Fugate, 1986
) indicate that an increase in deposition within the range of pollen loads observed in this experiment (Fig. 1A) will increase seed set. But although I. aggregata in plots with C. linariaefolia set almost two fewer seeds per fruit than plants in plots without heterospecifics (Fig. 1G), the difference was not significant. This weak effect of changes in I. aggregata pollen deposition on seed set per fruit could occur if reproduction was not pollen limited. Although I did not determine whether my plants were pollen limited, 1995 should have been a year of pollen limitation as hummingbirds were scarce because of an abnormally late snowmelt (personal observation). Several studies have found that fruit and seed set of I. aggregata are pollen limited at RMBL (Hainsworth, Wolf, and Mercier, 1985
; Campbell, 1991
; Campbell and Halama, 1993
). Reduced I. aggregata pollen deposition in the presence of C. linariaefolia might also have influenced a component of reproductive success besides seed set per fruit. For example, fruit set is more commonly pollen limited than seed set per fruit, and fruits that reach maturity may not differ in seed set if plants abort inadequately pollinated fruits before aborting individual ovules within fruits (Burd, 1994
). It is also possible that the nonsignificant reduction in seed set per fruit in the heterospecifics-present treatment could be significant if scaled up to the whole-plant level by multiplying by fruit set per plant.
Resource limitation may also account for why I. aggregata did not set significantly more seeds per fruit in the absence of C. linariaefolia. Reproduction in I. aggregata can also be resource limited, and pollen and resource limitation can interact to influence reproductive success (Campbell and Halama, 1993
). If reproduction of I. aggregata was not greater in the absence of C. linariaefolia because of resource limitation, then I. aggregata receiving supplemental nutrients and growing in the absence of C. linariaefolia should produce more seeds per fruit that those growing with heterospecifics. Seed set of unfertilized I. aggregata growing in the presence and absence of heterospecifics should not differ.
I could not determine the mechanism by which C. linariaefolia competed with I. aggregata for pollination in this experiment, but one possibility can be eliminated. A hand-pollination experiment indicated that deposition of C. linariaefolia pollen (range = 32–513 grains; mean = 189.37 grains) can reduce subsequent conspecific pollen deposition onto I. aggregata flowers (C. M. Caruso and M. Alfaro, unpublished data). Based on these results, I conclude that the amount of heterospecific pollen deposited on conspecific stigmas in the present experiment (range = 0–98 grains; mean = 7.38 grains) was too small to be the primary cause of competition for pollination. Interspecific competition must therefore occur through changes in pollinator visitation or loss of I. aggregata pollen on C. linariaefolia flowers. The spatial scale of my manipulations suggests that the latter explanation is more likely. The amount of pollen lost on heterospecifics will depend on the sequence of species visited by a pollinator prior to visiting a focal plant. But the visitation rate of pollinators may be determined by the distribution of floral resources over a much larger area than was manipulated in this study (e.g., Thomson, 1981
).
Although there was no intraspecific competition among I. aggregata plants for pollination (Fig. 1B), my results indicated that changes in the density of I. aggregata could influence the strength of interspecific competition with C. linariaefolia. When I. aggregata density was increased, the absolute (Fig. 1D) and relative (Fig. 1F) amount of heterospecific pollen deposited decreased by 70 and 56%, respectively. This decrease in heterospecific pollen deposition was presumably caused by pollinators moving among conspecifics and carrying predominately conspecific pollen loads when visiting the high conspecific density experimental units. Such an inverse relationship between the degree of aggregation of conspecifics and the amount of interspecific pollen transfer has been suggested by models (Waser, 1978b
; Campbell, 1986
), but has not been previously experimentally demonstrated. The amount of C. linariaefolia pollen deposited in this experiment was too small to depress the pollination and reproduction of I. aggregata, but C. linariaefolia pollen deposition on I. aggregata flowers can be substantially higher in natural populations (C. M. Caruso, unpublished data). Therefore, my results suggest that an increase in the aggregation of conspecifics could ameliorate one of the negative effects of interspecific pollen transfer in natural populations of I. aggregata.
My assessment that interspecific competition for pollination was stronger than intraspecific competition may be used to predict changes in the evolution and ecology of I. aggregata. For example, interspecific competition between I. aggregata and C. linariaefolia in this experiment was caused by loss of conspecific pollen on heterospecific flowers and/or changes in pollinator visitation. Thus, when C. linariaefolia is present, there should be selection via female fitness on traits of I. aggregata that increase plant attractiveness to pollinators and those that mechanically maximize conspecific pollen deposition. Phenotypic selection via female fitness of I. aggregata is currently being measured in populations with and without C. linariaefolia to test this prediction.
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FOOTNOTES |
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2 Alternate address: The Rocky Mountain Biological Laboratory, P.O. Box 519, Crested Butte, Colorado 81224.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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