Beverly J. Rathcke

Competition and Coexistence with A Guild of Herbivorous Insects

Resource exploitation and competition were measured for 13 herbivorous stem—boring insects inhabiting a climax prairie in Illinois. Nine species overlapped > 70% in their exploitation of plant species, stem sizes, and locations within stems, but competition was detected between only two species. In this case the competition resulted from aggression rather than exploitation for limited resources, and exclusion may be delayed or prevented by specific behaviors or refuges. To quantify how closely the guild members were packed upon the available resources, probabilities of coccurrences between species were calculated using resource abundances, population sizes, and overlaps in resource exploitation. The species packing averaged over the entire guild was 30%, but some species had several potential competitors and probabilities of cooccurrences were > 60%. This species structure offers the conditions for diffuse competition, but competition seldom occurred because stems could support several individuals and resources were generally nonlimiting. Although the potential for competition is high and species probably compete occasionally, the high resource overlap and species richness that competition has not been a major organizing force within this guild. When factors known to influence competition are considered, it seems possible that competition has been too infrequent, mild, or nondirectional to cause strong selection for competitive avoidance or displacement. This guild demonstrates that overlap values may not equal competition coefficients and that high overlap may exist because competition is rare. This guild is probably not unique in lacking competitive organization; herbivorous insects may be among the least likely groups to exhibit the patterns predicted by competition theory.

Individual Variation in Nectar Production and Its Effect on Fitness in Kalmia Latifolia

During summer 1986, we assessed the effect of individual variation in per flower nectar production rate on one component of plant reproductive success (percent fruit set) mediated through differential pollinator attractiveness. Thirty-two individual mountain laurels, Kalmia latifolia, were monitored for average 24-h nectar production in individual flowers, average per flower pollinator visitation rate, average floral density, and percent fruit set. Significant positive correlations occurred between (1) average 24-h nectar production by individual flowers and average visitation rate per flower, and (2) average visitation rate per flower and percent fruit set. Pollinators appeared to be differentially attracted to individual shrubs that produce larger quantities of nectar per flower, and the increased visitation rate promoted a higher percentage fruit set because K. latifolia is pollination limited. However, there was no correlation between an individual shrubs 24-h nectar production in 1986 and that same individuals 24-h nectar production in 1985. Thus, while pollinator behavior appears to influence this component of plant reproductive success in Kalmia, any selective advantage to individuals may vary between flowering seasons.

Interactions for Pollination among Coflowering Shrubs

To examine competition for pollination in a natural community, I measured pollinator sharing, pollinator dependence, and limitation of fruit set for four colflowering shrub species in The Great Swamp, Rhode Island. These species included all potential competitors for pollination and have had the long, extensive associations deemed necessary for competitive coevolution. The four shrubs shared the major pollinators (bees), and three of the four species depended upon pollinators for fruit set; but pollination limitation due to competition was detected in only one species. Despite the lack of competition, the shrubs interacted negatively and stronly for bumble bees, which were major pollinators. The species differentially attracted bumble bees, and their relative attractive (competitive) abilities were predicted by an index based on average daily sucrose production per flower x local floral density as: Ilex opaca > Kalmia latifolia > Gaylussacia frondosa > Kalmia angustifolia. Bumble bees rarely visited the less attractive species during flowering overlap in 3 of 4 yr. Competitive avoidance was achieved, not by specialization and divergence as is commonly hypothesized, but by having traits that confer tolerance of rare pollinator visits (e.g., long—lived flowers or self—pollination) or by having diverse pollinator assemblages (small bees and bumble bees). K. latifolia was the only shrub specialized for pollination (by bumble bees), and this specialization increased, rather than decreased, the risk of pollination limitation because K. latifolia was a poorer competitor for bumble bees than Ilex. Pollination limitation was prevented or reduced by the extended longevity of unpolinated flowers. Flowers could remain viable > 2 wk until Ilex ceased flowering and bumble bees switched to K. latifolia K. angustifolia, the poorest competitor, was visited by many species of small bees and could also self—pollinate, so fruit set was maintained even in the absence of bumble bees, Gaylussacia lacked these traits, and it was pollination limited: flowers are short—lived and cannot self—pollinate. For this species small bees were important in maintaining fruit set when bumble bees switched to more attractive species. These traits for competitive avoidance are advantageous when pollination is uncertain for any reason, such as unfavorable weather for pollinator foraging. As a result, their presence cannot be unambiguously attributed to past competition, although they currently are important in reducing competition among these shrubs. Such traits as generalization for resource use of tolerance of sparse resources may be as important as specialization or greater competitive ability for the coexistence of species in competitive communities.

EFFECTS OF SMALL RODENTS AND LARGE MAMMALS ON NEOTROPICAL SEEDS

The direct and indirect effects of seed predation by a Neotropical community of small rodents and large mammals were examined in a 1.5-year exclosure experiment in Corcovado National Park, Costa Rica. This park has an intact terrestrial mammalian community, including small rodents and large mammalian seed predators, an essential condition to quantify the dynamics of seed predation. To measure seed predation, three exclosure treatments (1.2 m radius × 1.5 m tall) in two forest types (primary vs. secondary forest) were monitored: (1) fenced exclosures that excluded large mammals, (2) fenced exclosures that excluded both large and small mammals, and (3) open controls. Tethered seeds were added from nine common species of canopy trees and lianas (seven families), and seed removal was measured from February 2001 to July 2002. Small rodents had significant negative effects on four of the nine seed species tested, and the effects of small rodents on seed predation differed significantly from the effects of large mammals and insect/fungal pathogens. Small-rodent seed predation (both rates and total proportion destroyed) did not differ between secondary and primary forest habitats. Throughout the exclosure study, small-rodent populations were marked and recaptured to document their community composition and densities. Small-rodent population fluctuations were observed, and shifts in composition directly affected species-specific and community-wide seed predation. Fewer seeds were destroyed when small-rodent abundance was low. Small-rodent population fluctuations also had an indirect effect, reversing the relative importance of small rodents and large mammals and resulting in large mammals becoming the primary seed predators. This change did not only decrease total seed predation for some species; it had the additional effect of moving from a primarily negative interaction (seed predation from the small rodents) to the varied interactions of seed predation and seed dispersal (scatter-hoarding). This research illustrated that small-rodent community fluctuations are a mechanism responsible for variability in the process of Neotropical seedling regeneration.

Patterns of Individual Variability in Floral Resources

The distribution and presentation of floral rewards to insects has been dem- onstrated to influence pollinator visitation rates, pollinator movements, and consequent plant fitness. However, very little is known about the extant variation in nectar rewards among individuals in a single population. Patterns of nectar production within individual shrubs of Kalmia latifolia L. (Ericaceae) were examined during summer 1985, in a Southern Appalachian heath bald in Giles County, Virginia. Five inflorescences from each of 20 shrubs at two sites in the study area were bagged to exclude insect visitors. Twenty-four hour nectar production was measured using microcapillary extractions in each open flower within each of the bagged inflorescences. For each individual we monitored arithmetic mean reward per flower and variance in reward per flower over the flowering season. There is significant variation among individuals in the mean reward/flower at both sites, but the variance in reward/flower is significant at only one site. The mean reward and the variance in reward are positively correlated among shrubs. If the coefficient of variation is used to uncouple the intrinsic correlation between these two plant variables, then no significant differences emerge among shrubs in levels of relative variability at either site.

Floral longevity and reproductive assurance: seasonal patterns and an experimental test with Kalmia latifolia (Ericaceae)

Floral longevity is assumed to reflect a balance between the benefit of increased pollination success and the cost of flower maintenance. Flowers of Kalmia latifolia (Ericaceae), mountain laurel, have a long duration and can remain viable up to 21 d if unpollinated. I experimentally tested whether this long duration increases pollination success by clipping stigmas to reduce functional floral longevity to 3-4 d. Clipping stigmas decreased fruit set from 65% to only 10%. Flowers with natural life spans were not pollination-limited, demonstrating that long floral duration ensured female reproductive success. The long floral duration of K. latifolia was unique in this site (the Great Swamp, Rhode Island, USA). Coflowering shrubs in summer had a mean floral life span of 3.4 d. Spring-flowering species had significantly longer mean floral durations (7.2 d). These duration differences may reflect seasonal variation in pollinator availability. However, K. latifolia flowers in summer, when its bumble bee pollinators are abundant but it is a poor competitor for bees because its flowers produce little nectar. The long floral duration allows K. latifolia to outlast coflowering competitors and attract sufficient pollinators. I hypothesize that the long floral duration of K. latifolia functions as a mechanism for competitive avoidance and reproductive assurance.

Slugs as Generalist Herbivores: Tests of Three Hypotheses on Plant Choices

Three hypotheses explaining plant choices by generalist herbivores were tested by mea- suring leaf acceptabilities to three slug species, each of which lives in a different successional stage of vegetation. The acceptabilities of 61 plant species were determined in laboratory feeding trials by offering individual slugs leaf discs of a test and a control plant (lettuce) and recording consumption. The plant-defense hypothesis, that plants predictable to herbivores should have greater defense com- mitments and be less acceptable than unpredictable plants, was contradicted when predictability was judged by successional status. The acceptabilities of climax forest plants and early-successional plants were not significantly different for any of the three slug species. This was also true within woody species and within herbs. However, the plant-defense hypothesis was supported when predictability was measured as seasonal leaf persistence. As seasonal leaf persistence of forest herbs increased, acceptability declined. As alternative explanation based on growth rate is discussed. Alternatively, feeding choices were hypothesized to be determined by adaptations of these generalist herbivores to plants available in their local habitats (the herbivore-adaptation hypothesis). Because slugs are generally restricted to feeding on herbs, woody species were predicted to be less acceptable than herbs and this was strongly supported. Tree leaves were seldom eaten by any of the three slug species. In addition, each slug preferred a few herbaceous species unique to its specific habitat, sug? gesting local adaptation, but the actual mechanism remains to be determined. If both plants and slugs are coevolving, plant choices would depend upon where we examine the coevolutionary spiral, but they should be unique for each slug species as it coevolves with local plants (the coevolution hypothesis). In these tests, feeding responses ofthe three slug species were significantly positively correlated and unique responses were not evident. These hypotheses depend upon the probabilities of plant-herbivore encounters, which can be difficult to determine. For these slug species stem heights of only 10 cm acted as effective feeding barriers. Plants were vulnerable during emergence, but they can escape slugs if they emerge early in the spring. Because such subtle factors may determine encounter rates, predictions of feeding choices and plant defenses may be difficult to make. Specific adaptations may not be evident from these tests of general patterns. Field observations suggest that these introduced slug species are exerting significant selective pressure on the morphologies, phenologies, and defenses of native plants.

HURRICANE CAUSES RESOURCE AND POLLINATION LIMITATION OF FRUIT SET IN A BIRD‐POLLINATED SHRUB

Hurricanes can strip leaves from plants and provide other stresses that can reduce resource availability for subsequent reproduction. In addition, hurricanes commonly reduce populations of bird pollinators. I measured both resource and pollination limitation of fruit set for Bahama Swamp-bush (Pavonia bahamensis) on San Salvador Island in the Bahamas before and after two seasons of hurricanes. Before the hurricanes in the winter flowering season in 1994–1995, fruit set of P. bahamensis was 100% for most shrubs. After a mild hurricane, in 1995–1996, fruit set was 48%. After the severe Hurricane Lili (Category 2) in 1996–1997, mean fruit set was only 11%. In both years after the hurricanes, lower fruit set reflected resource limitation. In 1996–1997 after Hurricane Lili, fruit set was severely reduced an additional 74% by pollination limitation. Pollination limitation was caused by declines of the two bird pollinators, Bananaquits and Bahama Woodstars. In 1994–1995, both species were frequent visitors to fl...

Effects of floral restrictiveness and stigma size on heterospecific pollen receipt in a prairie community

Plant species vary greatly in the degree to which floral morphology restricts access to the flower interior. Restrictiveness of flower corollas may influence heterospecific pollen receipt, but the impact of floral morphology on heterospecific pollen transfer has received little attention. We characterized patterns of pollinator visitation and quantities of conspecific and heterospecific pollen receipt for 29 species with a range of floral morphologies in a prairie community dominated by the introduced plant Euphorbia esula (leafy spurge) which has an unrestrictive morphology. Pollinator overlap was significantly greater between Euphorbia and other unrestrictive flowers than restrictive flowers. Compared to flowers with restrictive morphologies, unrestrictive flowers received significantly more Euphorbia pollen, more heterospecific pollen from other sources, and a greater diversity of pollen species, but not more conspecific pollen. However, stigmatic surface area was significantly larger for flowers with unrestrictive morphologies, and the density of Euphorbia and other heterospecific pollen per stigmatic area did not significantly differ between flower types. These findings suggest that the smaller stigma size in restrictive flowers partly accounts for their decreased heterospecific pollen receipt, but that restrictiveness also allows species to increase the purity of pollen loads they receive. Given that restrictive flowers receive fewer heterospecific pollen grains but at a higher density, the effect of restrictiveness on fecundity depends on whether absolute quantity or density of heterospecific pollen affects fecundity more. Our results also indicate that abundant neighbors are not necessarily important heterospecific pollen sources since Euphorbia pollen was rarely abundant on heterospecifics.

Pollination and Predation Limit Fruit Set in a Shrub, Bourreria succulents (Boraginaceae), after Hurricanes on San Salvador Island, Bahamas1

Hurricanes have been assumed to reduce the reproduction of plants, either directly by leaf stripping and stress or indirectly by reducing pollinators. I examined the pollination and fruit set of a common shrub, Bourreria succulenta, after hurricanes on San Salvador island, Bahamas. Contrary to the assumption of resource limitation, B. succulenta showed unusually prolific flowering after Hurricane Lili stripped leaves from most of the plants in October 1996. I predicted that the abundant flowering would saturate pollinators and that fruit set would be pollination-limited. Fruit set was strongly pollination-limited by 71 percent. Butterflies are probably the major pollinators and were present at the site, but they rarely visited B. surculenta flowers even though flowers were brimming with nectar. Nectarivorous birds (Bananaquits and Bahama Woodstars) visit B. surrulenta flowers, but their populations were decimated by Hurricane Lili and they rarely visited flowers during this time. Fruit set was also severely predation-limited; a moth caterpillar (Gelechiidae) was extremely abundant and ate buds, flowers, and fruits, causing a further 68 percent reduction in fruit set. Together, pollination limitation and predation limitation reduced fruit set to only 7 percent or less. Predation was also intense in 1999 after Hurricane Floyd and resulted in 11 percent fruit set or less. Whether or not hurricanes were the cause of limited pollinators or abundant predators, the resulting low fruit set could have population effects because hurricanes can provide opportunities for the recruitment of new plants. These results emphasize that understanding plant-animal interactions may be necessary for predicting the effects of hurricanes on plant reproductive success, which may affect subsequent recruitment. Species on small islands like San Salvador (1 50 km2) with relatively few species may be especially vulnerable to environmental disturbances such as hurricanes. Ky words: Bahamas; Bourreria; Jower prehtion; j u i t predation; j u i t set; hurricane effects; island pollination; plantanimal interactions; pollination limitation; scrublands. UNDERSTANDING THE EFFECTS OF HURRICANES ON PLANTS is a topic that has received much recent attention. Studies have documented that hurricanes can strip plants of their leaves and provide other stresses, such as high rainfall, high winds, or salt spray, which destroy flower and fruit resources for some time afterwards (Reilly 1991, Walker et al. 1992, Boose et al. 1994, Zimmerman et af. 1994, Bronstein & Hossert-McKey 1995, Grant et al. 1997, Pascarella 1998a). These stresses might also be expected to decrease the ability of plants to produce flowers or fruit after the hurricane; however, subsequent flower production after hurricanes was not significantly reduced in an orchid (Ackerman & Moya 1996), a fig species (Bronstein & HossertMcKey 1995), or Pavonia baharnensis on San Salvador, Bahamas (Rathcke 2000a,b), and even increased in a shrub (Pascarella 1998b). In addition, fruit set did not appear to be resource-limited in two of these species, the orchid and fig (Bronstein & Hossert-McKey 1995, Ackerman & Moya I Received 15 August 1999; revision accepted 22 March 2000. E-mail: brathcke@umich.edu 1996), although resource limitation significantly reduced fruit set for I? bahamensis (Rathcke 1998,