Southwestern Association of Naturalists

The Effect of -Mediated Flower Alteration on Seed Production in Golden-Eye Phlox Author(s): James R. Ott, Jim A. Nelson and Traci Caillouet Source: The Southwestern Naturalist, Vol. 43, No. 4 (Dec., 1998), pp. 430-436 Published by: Southwestern Association of Naturalists Stable URL: http://www.jstor.org/stable/30054079 . Accessed: 16/05/2013 16:16

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This content downloaded from 147.26.169.218 on Thu, 16 May 2013 16:16:49 PM All use subject to JSTOR Terms and Conditions THE SOUTHWESTERNNATURALIST 43(4):430-436 DECEMBER1998

THE EFFECT OF SPIDER-MEDIATED FLOWER ALTERATION ON SEED PRODUCTION IN GOLDEN-EYE PHLOX

JAMESR. OTT,JIM A. NELSON, AND TRACI CAILLOUET

Department of Biology, Southwest Texas State University,San Marcos, TX 78666-4616

ABSTRACT-The crab spider, celer () (Hentz), is a sit-and-wait predator that forages for insects on the flowers of golden-eye phlox, Phlox roemeriana (Scheele) (Polemon- iaceae). often alter flower morphology of this species by tying together two of the flower's five petals to form a "bower" that they then occupy. Access to the flower's corolla tube is partially to completely obstructed by bower construction in 70% of spider-altered flowers. We investigated the effect of bower construction on seed production by comparing the number of matured seeds in unaltered and spider-altered flowers in a natural population of P roemerianaduring the spring of 1996 in Hays Co., Texas. Spider-altered flowers produced significantly fewer seeds per fruit (X = 3.4, n = 135) than did unaltered flowers (X = 5.5, n = 108). This 38% reduction was attributable to a significantly higher incidence of spider-altered flowers failing to produce any seed, 37% versus 9% for unaltered flowers. Because P roemerianais an annual species and at our study site typically produced only a single flower, the observed reduction in seed production due to bower construc- tion resulted in a significant decrease in female lifetime reproductive success for those plants whose flowers were modified. Spiders altered up to ca. 5% of flowers in the population at times during the flowering season. At the observed bower frequencies, the effect of M. celeron P roe- meriana seed production at the population level is minimal.

RESUMEN-La arafia de mar, Misumenops celer (Thomisidae) (Hentz), es una especie rapaz que "se sienta y espera" para forrajear insectos en las flores del flox de ojo aureo (golden-eye phlox), Phlox roemeriana(Schelle) (Polemoniaceae). Las arafias frecuentemente alteran morfologicamente la flor de esta especie, anudando dos de los cinco petalos de la flor, formando un "arco" el cual despues ocupan. El acceso a la corola de la flor estai parcial a completamente obstruido debido a la construcci6n del arco en 70% de las flores alteradas por la arafia. Investigamos el efecto de la construcci6n del arco en la producci6n de semillas, comparando el niimero de semillas maduras en flores alteradas y no alteradas por la arafia en una poblaci6n natural de R roemerianadurante la primavera de 1996 en el Condado de Hays, Texas, E. E. U. U. Las flores alteradas produjeron significativamente menos semillas por fruto (X = 3.4, n = 135) que las flores no alteradas (X = 5.5, n = 108). Esta reducci6n de 38% fue imputable a una incidencia significativamente mas alta de flores alteradas fallando a producir ninguna semilla, 37% contra 9% en las flores no alteradas. Porque P roemerianaes una especie anual y porque en nuestro sitio de estudio tipicamente produjo s6lo una flor, la reducci6n observada en la producci6n de semillas debido a la construcci6n del arco result6 en un decremento significante en el 6xito reproductivo de la vida en las plantas en que las flores fueron modificadas. Las arafias alteraron hasta ca. 5% de las flores en la poblaci6n a veces durante la temporada de las flores. En las frecuencias observadas del arco, el efecto de M. celeren la producci6n de semillas por P roemerianaal nivel de la poblaci6n es minimo.

Variation in pollinator availability, flower vores, and seed predators (Gertsch, 1979; herbivory, and subsequent seed predation can Wise, 1993). The net effect of the presence each contribute to variation in fitness within and activities of flower-dwelling spiders on populations and to the dynamics of plant pop- plant fitness likely depends on whether the spi- ulations (Harper, 1977; Crawley, 1983; Denno der is present during anthesis, duration of its and McClure, 1983; Crawley, 1989). Spiders of stay, its relative efficiency as a predator of in- a number of different families commonly use sect visitors, and spider density. For example, angiosperm flowers as hunting sites and thus the presence of the green lynx spider, Peucetia interact with insect pollinators, flower herbi- viridans, decreased seed set per inflorescence

This content downloaded from 147.26.169.218 on Thu, 16 May 2013 16:16:49 PM All use subject to JSTOR Terms and Conditions December 1998 Ott et al.-Flower alterationin golden-eye phlox 431 in Haplopappus venetus by one-third (presum- corolla ca. 1.5 cm long. Anthers of the five stamens ably by reducing pollinators) but increased are located ca. 0.75 cm below the orifice, and the number of seeds matured by 17% (presumably three-lobed stigma of a single gynoecium is located ca. 1.2 cm below the orifice. The three-celled by reducing seed predators-Louda, 1982). ovary contains up to 18 ovules. Seeds mature within a Conversely, thomisid and salticid did cap- spiders sule which dehisces at not affect insect visitation or seed maturity.Primary pollinators production of R roemerianaare that umbel in Daucus carota et lepidopterans specialize in per (Wilkinson al., tongue-tip pollination, as well as bumblebees and or 1991) Asclepias syriaca (Morse, 1986a) be- honeybees (Grant, 1959; Grant and Grant, 1965). cause spiders did not continuously occupy um- Crab spiders are commonly observed foraging for bels and spider density was low. With the ex- insect visitors on flowers and are well-documented ception of those studies; however, potential predators of lepidopterans, bumblebees, and hon- contribution of flower-dwelling spiders to vari- eybees (Gertsch, 1979; Morse, 1981, 1986a). Thom- ation in plant fitness via their interaction with isids are visuallyoriented sit-and-waitpredators that on stealth rather than construction of a flower-visiting insects is largely unknown. rely silken web to capture prey (Gertsch, 1979). They are rela- Here we examine the effect of the crab spi- tively well studied with respect to patch choice der, celer (Araneae: Thomisidae), Misumenops and Fritz, on success of (Morse 1982; Morse, 1988, 1993), foraging reproductive golden-eye phlox, behavior (Fritz and Morse, 1985; Morse, 1986b, 1988; Phlox roemeriana Phlox (Polemoniaceae). roemer- Schmalhofer, 1996), reproductive success (Morse iana typically produces only a single flower, spi- and Fritz, 1987), survival strategies (Vogelei and ders hunt for insect prey on these flowers (Mu- Greissl, 1989; Morse, 1992), and prey capture niappan and Chada, 1970), and spiders alter (Morse, 1979, 1981; Kareiva et al., 1989, Schmalho- flower morphology. Misumenops celeralters flow- fer, 1996). The crab spider, M. celer, commonly is found on flowers er morphology of P roemerianaby tying togeth- of P roemerianain central Texas. its silken line, M. celerhas er the flower's petals to form conspicuous Using drag developed a method of webbing together two of the flower "bowers." This interaction provides an excel- petals in P roemerianato form a bower.The function of this lent opportunity to investigate the effect of a bower presently is unknown [but see Schmalhofer flower-dwelling spider on fitness because plant (1996) for possible thermal benefits and hunting ef- effects on individual flowers translate into ef- ficiency enhancement]. fects on fitness. The ac- plant bower-building This study was conducted in a population of P tivities of M. celer may indirectly decrease re- roemerianalocated on Southwest Texas State Univer- productive success of individual P roemeriana sity's Freeman Ranch in Hays Co., Texas. The habi- flowers (plants) by decreasing the probability tat, grazed pastures, is typical for the species in this and success of pollinator visitation. Alternative- region. To estimate the effect of spider alteration of ly, bowers may directly reduce the number of floral morphology on seed production, we randomly selected numbers of effective pollinator visits per flower if bowers approximately equal plants with unaltered flowers and altered flowers on each of increase predation efficiency of hunting spi- three dates. These dates coincided with flow- ders (Schmalhofer, 1996). Here we peak compare Both altered and unaltered flowers success of and un- ering period. reproductive spider-altered were individuallymarked with thread to avoid con- altered flowers to estimate the effect of bower fusion in plants that produced multiple flowers. In construction on individual we plant fitness; total, 108 unaltered and 135 spider-altered flowers also examine the of frequency bower construc- were marked. The two types of flowers were sampled tion in a natural population to assess the po- within a 200 by 200 m area. tential impact of spider occupancy on plant Spider-altered flowers were partitioned into three population dynamics. subcategories defined at the outset of the study based on degree to which access to the corolla tube METHODS--Phloxroemeriana is a common herba- appeared to be obstructed by the bower. Categories ceous annual found throughout the EdwardsPlateau were 1) corolla obstructed (two opposing petals tied of central Texas. This species flowers from February tightly together such that the corolla tube was not to Maydepending on yearlyand local environmental visible); 2) corolla partially obstructed (two oppos- conditions. Inflorescence typically consists of a sin- ing or non-opposing petals tied together such that gle violet flower with a distinct golden eye in the petals only partially obscured the interior of the co- center of the corolla. Flowers have five sepals that rolla tube); and 3) corolla unobstructed (the spider are fused to form a tubular calyx and four or five tied two petals, usually non-opposing petals, corolla petals that are fused at the base to form a tubular tube was still in plain view and access to the corolla

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means to be made while controlling the experi- mentwise error rate at alpha (Day and Quinn, 1989). To gain insight into causes of differences among means, two additional analyses were performed. First, a 4 by 2 test of independence of the frequency of unsuccessful flowers (flowers that produced zero seeds) and flower condition was performed using the G-statistic. This procedure was followed by a si- multaneous test procedure (Sokal and Rohlf, 1969) to determine which flower conditions differed from one another in frequency of unsuccessful flowers. Second, a one-way ANOVA of seed production per fruit among the four flower conditions was per- formed using only data on seed production from successful flowers. All analyses were performed using SAS (SAS Institute Inc., 1985).

RESULTs-Alteration of flowers by spiders significantly reduced seed production per fruit. Two-way ANOVA followed by comparison of means demonstrated significant differences in seed production among the four flower con- = ditions (F3,237 4.87, P < 0.01) and showed that each of the three classes of spider-altered flowers fewer seed FIG. 1-Phlox roemeriana flowers that have had produced significantly per their petals tied together by the silken drag line of fruit than did unaltered flowers (Ryan's Q, P Misumenops celer.The corolla tube is completely ob- < 0.05; Table 1). On average, bower construc- scured on the flower to the left and unobscured on tion reduced number of seed per fruit by 38% the flower to the right. (X = 3.4, n = 135) compared with control flowers (X = 5.5, n = 108). The degree to which behavior of M. celer altered tube to be see Pres- petal-tying appeared unobstructed; Fig. 1). access to the corolla of P roemerianavaried con- ence or absence of spiders on altered flowers was recorded at the time each flower was marked. Plants siderably; 46% of flowers were categorized as as with marked flowers were later collected, prior to totally obstructed, 24% partially obstructed, fruit dehiscence and seed dispersal, and the fate of and 30% as unobstructed. Mean seed produc- each flower was determined. Flowers were scored as tion per fruit, however, did not differ signifi- successful if one or more seeds were produced per cantly among the three classes of corolla ob- fruit and unsuccessful if no seeds were produced. struction (Ryan's Q, P > 0.05). Thus, presence Number of seeds fruit successful produced per by of a bower, but not degree of obstruction of flowers was counted. the corolla, was associated with reduction in To estimate frequency of spider-altered flowers in seed production per flower. No date (F2,231 = the population throughout the flowering season of 1.70, P > nor date flower condition R roemeriana,two 20 by 20 m grids were established 0.19) by = P > was de- just after initiation of flowering, and number of al- interaction (F6,231 0.93, 0.47) tered and unaltered flowers was recorded daily in tected; hence, means presented in Table 1 rep- each grid between 3 April and the termination (1 resent flower condition effects pooled across May, 1996) of the flowering season. the three marking dates. Statistical Analysis-Effect of spider alteration on A 4 by 2 test of independence of the fre- seed fruit was a two- production per analyzed using quency of failed flowers and flower condition way mixed-model ANOVA with flower condition showed that flower condition classes differed (four levels-unaltered and three levels of corolla significantly in incidence of failed flowers obstruction) and flowering date (three levels) as zero = fixed and random effects, respectively. Following (flowers producing seeds) (G 20.24, df = ANOVA, means of the four flower conditions were 3, P < 0.001). Simultaneous comparison of compared using Ryan's Q at alpha = 0.05. This test percentages showed that all three classes of spi- statistic allows all possible comparisons among der-altered flowers failed to set seed more fre-

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TABLE1-Number of seeds per fruit produced by unaltered and spider-altered flowers and percent failed flowers per flower condition in a population of Phlox roemerianain central Texas. Mean seeds per fruit was calculated on the basis of both successful and unsuccessful flowers; mean seeds per successful fruit was calculated for successful flowers only (flowers that produced 21 seed per fruit). Percent failed flowers is the percent that did not mature fruit (0 seeds produced). Means and percentages marked with the same do not differ at = 0.05. superscript significantly ea

Mean seeds per Percent Mean seeds successful fruit failed Flower condition per fruit p SE n u SE n flowers

Unaltered (control) 5.5 + 0.31z 108 6.1 c 0.29z 98 9.3z Spider altered Unobstructed 3.7 + 0.49Y 41 5.1 g 0.46z 30 26.8Y Partially obstructed 3.3 a 0.52Y 32 4.4 0.-53z 24 25.OY Obstructed 3.2 + 0.42Y 62 5.1 i 0.43z 39 37.1Y

quently than did control flowers (mean of 31% ber of seed produced per successful flower versus 9%, respectively; Table 1). Frequency of (F3,185= 1.09, P > 0.50). Thus, reduction in flower failure, however, did not differ signifi- number of seeds produced per fruit when bow- cantly among unobstructed, partially obstruct- ers were present was due to a greater proba- ed, and totally obstructed flowers (G = 1.95, df bility of failure to set seed rather than a gen- = 3, P > 0.05). eral reduction in number of seeds produced After removing failed flowers from the data per flower when bowers were present. set, a one-way ANOVA showed that the four Seventy percent of spider-altered flowers flower conditions did not differ in mean num- were observed to have resident spiders on them at the time of marking. Frequency of spi- der-altered flowers in was low 300- study grids throughout the entire flowering season of P. roemeriana.The maximum observed frequency in grid 1 was and spiders did not col- onize flowers in-4.5%, grid 2 (Fig. 2). (%) 200- DiscussioN-Spider alteration of flowers re- spidersduced average seed production per flower by flowers by 38% compared with controls. Because P roe- of meriana is an annual species and, at our study 10 site, typically produced a single flower, this re- duction in seed production represents a Number100- occupied marked decrease in female lifetime plant fit- ness for those plants whose flowers were al- 5 Flowerstered by spiders. We did not attempt to mea- sure effect of altered floral morphology on male reproductive success. Our estimates of ef- fect of bowers on seed are conser- 0- 0 production 90 100 110 120 130 vative and may considerably underestimate ef- Julian date fects on female plant fitness because flowers may have been before arrival. FIG. 2-Flowering phenology of Phlox roemeriana pollinated spider The observed reduction in mean seed (closed circles, grid 1: closed diamonds, grid 2) and pro- duction in the frequency of spider-altered flowers (open circles, spider-altered flowers could result grid 1) in the Freeman Ranch population during the from a higher incidence of complete failure to spring of 1996. set seed, reduction in number of seeds pro-

This content downloaded from 147.26.169.218 on Thu, 16 May 2013 16:16:49 PM All use subject to JSTOR Terms and Conditions 434 The SouthwesternNaturalist vol. 43, no. 4 duced per flower, or both. Our analyses tality early in the life history contributes greatly showed that a significantly higher proportion to overall mortality (Schmidt and Levin, 1985; of altered flowers failed to set seed, suggesting Schwaegerle and Bazzaz, 1987) and that repro- that alteration of flowers often precluded visi- ductive value is maximized at onset of flower- tation by pollinators rather than decreasing ing (Leverich and Levin, 1979). If demography successful pollination when flowers were visit- of P roemerianais similar, each plant that has ed. survived to reproductive age is very valuable Reduced seed production in flowers attend- with respect to its expected contribution to fu- ed by spiders parallels, in part, data obtained ture population growth. Given the magnitude by Louda (1982), who examined the net effect of the reduction in seed production due to of attendance at flowers by the spider Peucetia bower construction that we observed at the in- on seed in viridans production Haplopappus ve- dividual flower level, we can ask whether spider netus. The P venetus viridans-H. study system alteration of flowers affects seed production at differs from this in that hunted system spiders the population level in P. roemeriana.Whether on inflorescences containing many flowers individual level effects could translate into de- rather than flowers and then remained single tectable decreases in seed production at the on inflorescences fruit maturation. In through population level is expected to depend on the both attendance at flowers re- systems spider frequency of spider-altered flowers in the pop- duced mean seed production by over 30%, and ulation. We can use our data on individual this reduction was attributable to a marked re- plant effects to estimate the effect of spider duction in of flowers that set seed. proportion alteration of flowers on seed production in P In the P venetus however, viridans-H. system, roemerianaat the population level as a function the net effect of attendance spider ultimately of spider frequency. If we assume that access was because the reduction in seed positive pro- to the corolla is totally obstructed in all spider- duction flower head attributable to the per altered flowers, use 28% as our estimate of of was the presence spiders outweighed by spi- flower failure rate due to alteration, [the dif- ders' role in seed loss to seed reducing pred- ference in failure rates of unaltered flowers ators. In the only other study of this sort of (9%) and totally obstructed flowers (37%)], which we are aware, Wilkinson et al. (1991) and use 4.5% as our estimate of the found no difference in fruit frequency production per of flowers maximum umbel between umbels with and without spider-altered (the ob- spi- served then the number of suc- ders in a of Daucus carota. frequency), population cessful v1 seed construction of bowers M. celer plants (producing per flower) Although by in the is to decrease in flowers of P roemerianaresulted in reduced population predicted by only 1.3% compared with a spider-free popu- seed production via an increase in the proba- lation. At the observed bower frequency, the bility of failure to set seed, we found no sup- effect of M. celeron P roemerianaseed produc- for the hypothesis that increasing corolla port tion at the level is minimal. obstruction (as perceived by the observer) de- population The extent to which of M. celer, creased either mean seed production or inci- frequencies and hence the effect on P roemeriana, dence of flowers failing to set seed (Table 1). spiders' and is unknown. Thus the simple presence of a bower was suf- vary temporally spatially Based on our of the and cli- ficient to interfere with pollination success. Di- knowledge species matic fluctuations in Texas, our estimate of rect manipulation of the system will be re- spi- der be low in relation to the quired to tease apart separate and interactive frequency may in fall 1995 effects of bowers and presence of spiders on long-term average. Beginning early and cen- seed set. P roemerianaflowers vary considerably continuing throughout spring 1996, tral Texas suffered a of historic in size, color, and in pattern of petal attach- drought pro- ment. Whether spiders choose flowers ran- portions. Abundance of many plant species domly and whether some variants naturally usually present prior to the onset of blooming produce more seed than others is an open in P roemerianawas severely reduced. Because question. M. celer is dependent on other plant species Demographic studies of a related annual prior to onset of flowering of P roemeriana,it species of Phlox, P drummondii, show that mor- is likely that recruitment onto P roemerianawas

This content downloaded from 147.26.169.218 on Thu, 16 May 2013 16:16:49 PM All use subject to JSTOR Terms and Conditions December 1998 Ott et al.-Flower alterationin golden-eye phlox 435 significantly reduced during the period of this dation by salticid spiders and ichneumonid study. wasps (Morse, 1989, 1992), shelter (camou- The function of petal-tying behavior in M. flage) provided by bowers could serve to re- celer and other thomisid spiders is just begin- duce predation. ning to be explored (Schmalhofer, 1996). Crab To determine whether construction of bow- spiders routinely lay down silken drag lines ers on P roemerianarepresents an adaptive nov- (Gertsch, 1979) as they move about their en- el use of the drag line directed toward prey vironment. These drag lines have previously capture, shelter, or defense from natural ene- been considered unlikely to play a role in prey mies, data on when and under what conditions capture (Morse, 1989). However, three genera flowers are tied are needed as well as data on of thomisid spiders, , Misumenops, foraging success of spiders on flowers with and and , have been observed to use without bowers. drag lines in a novel manner to tie together flower petals to form bowers in a variety of The authors thank M. Eubanks,D. Morse, and V. plant species. Whether use of drag lines to con- Schmalhoferfor helpful comments on a draft of the struct bowers functions as an adaptation, or ex- manuscriptand E. Silverfine for editorial assistance. D. A. Dean identification for aptation (Gould and Vrba, 1982), directed at provided Misumenops celer.We thank the SouthwestTexas State prey capture is unknown. Morse University enhancing Freeman Ranch for the field site for this (1981) that bowers formed by Mis- providing suggested research,which was a Research umena vatia in flowers of rose, Rosa car- supported by Faculty pasture Enhancement Grant from SWT to J. Ott and a olina, served as shelters. How- only overnight StrandtmannField Biology Awardto T. Caillouet. ever, because the ability of spiders to remain active during daylight hours depends on the LITERATURECITED temperature of their environment (Reichert and it is that construc- Tracy, 1975), possible CRAWLEY,M. 1983. the of an- tion of bowers enhance J. Herbivory: dynamics may indirectly spider imal-plant interactions. University of California success a crab hunting by ameliorating spider's Press, Berkeley. thermal microenvironment and thus extend CRAWLEY,M. J. 1989. Insect herbivores and plant the spider's activity time. Schmalhofer (1996) population dynamics. Annual Review of Ento- found body temperatures of crab spiders, Mis- mology 34:531-564. umenoidesformosipes, occupying bowers on the DAY,R. W., ANDG. P. QUINN. 1989. Comparisons of composite Bidens aristosa were significantly less treatments after an analysisof variance in ecolo- 59:433-463. than those of spiders occupying an exposed gy. Ecological Monographs R. AND M. S. MCCLURE.1983. Variable position on the upper surface of the inflores- DENNO, F., cence. Measurements of killed plants and herbivores in natural and managed freshly spiders Academic New York. revealed that the thermal benefit of a bower systems. Press, FRITZ,R. S., ANDD. H. MORSE.1985. increased with radiant and Reproductive increasing intensity success and foraging of the crab spider Misumena wind decreasing speed. vatia. Oecologia 65:194-200. flowers Alternatively, tied-up might directly GERTSCH,W. J. 1979. American spiders. Second ed. enhance the probability of prey capture by al- Van Nostrand Reinhold Company,New York. lowing spiders to position themselves closer to GOULD,S. J., ANDE. S. VRBA.1982. Exaptation-a pollinators. For , distance be- missing term in the science of form. Paleobiology tween predator and pollinator determined 1:4-15. whether a visitor was attacked (Morse, 1986a). GRANT,V. 1959. Natural history of the Phlox family, Vol. 1. Martinus The Flower alteration might also make it more dif- Systematicbotany. Nijhoff, The Netherlands. ficult for pollinators to work the flower effec- Hague, GRANT,V., ANDK. A. GRANT.1965. Flower pollination tively, thereby increasing handling time and in the Phlox family. Columbia University Press, risk of predation. For Misumena vatia hunting New York. on flowers of the insect Asclepias syriaca, longer HARPER,J. L. 1977. The population biology of plants. visitors remained on the umbel, the greater Academic Press, New York. was their chance of approaching a spider close- KAREIVA,P., D. H. MORSE, AND J. ECCLESTON. 1989. ly and of being attacked (Morse, 1986a). Fi- Stochastic prey arrivals and crab spider giving-up nally, because crab spiders are subject to pre- times: simulations of spider performance using

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