César A. Domínguez

The Role of an Avian Nectar Robber and of Hummingbird Pollinators in the Reproduction of Two Plant Species

1. Flower nectar robbers which extract flower rewards to pollinators but do not pollinate, may reduce the fitness of the plant. In this study we combined field observations and experimental manipulations to assess the role of a primary nectar robber, the bird Diglossa baritula, on flower nectar secretion and reproductive output of two hummingbird-pollinated plant species with contrasting breeding systems. We used the hermaphroditic Salvia mexicana and the androdioecious Fuchsia microphylla. In addition, because the distinction between pollinators and robbers is not so sharp, we compared the pollination efficiencies of D. baritula and of five species of hummingbirds visiting the flowers of the same plant species. 2. Flowers of the two plant species were frequently robbed (92 and 37% of the flowers in Salvia mexicana and Fuchsia microphylla respectively). For both species, field censuses of robbed and unrobbed flowers showed no differences in fruit set. Experimentally induced nectar robbery did not affect the cumulative production of nectar in both plant species. However, in S. mexicana, but not in F. microphylla, intact flowers produced more concentrated nectar. 3. The role of D. baritula and hummingbirds on seed production per flower, was assessed by exposing individual flowers to different schedules of visitation by caged flower visitors. In S. mexicana, the number of seeds produced by flowers visited by the robber only was similar to that of flowers visited by the least efficient pollinator but lower than that of those visited by the other pollinators. 4. In F. microphylla, seed production by flowers visited by the robber only was lower than that of flowers visited by all other pollinators. 5. For both plant species visitation by the robber plus hummingbirds yielded a similar number of seeds as flowers visited by the more effective pollinator. 6. There was no field evidence that nectar robbery by D. baritula damages nectaries or ovules and, since under natural conditions flower visitation by the robber only was very rare, we conclude that for these two plant species D. baritula may be regarded as a commensal or even a low-efficiency pollinator.

The adaptive value of phenotypic floral integration

Floral integration has been deemed an adaptation to increase the benefits of animal pollination, yet no attempts have been made to estimate its adaptive value under natural conditions. Here, the variation in the magnitude and pattern of phenotypic floral integration and the variance-covariance structure of floral traits in four species of Rosaceae were examined. The intensity of natural selection acting on floral phenotypic integration was also estimated and the available evidence regarding the magnitude of floral integration reviewed. The species studied had similar degrees of floral integration, although significant differences were observed in their variance-covariance structure. Selection acted on subsets of floral traits (i.e. selection on intrafloral integration) rather than on the integration of the whole flower. Average integration was 20% and similar to the estimated mean value of flowering plants. The review indicated that flowering plants present lower integration than expected by chance. Numerical simulations suggest that this pattern may result from selection favouring intrafloral integration. Phenotypic integration at the flower level seems to have a low adaptive value among the species surveyed. Moreover, it is proposed that pollinator-mediated selection promotes the evolution of intrafloral integration.

Seed shadows, seed predation and the advantages of dispersal

Given the universal prevalence of plant modifications for dispersal, it is appropiate to ask what advantages do these traits provide. In this paper we propose that ecological phenomena that have selected for these ‘adaptations’ are operational at a local scale and argue that for a proper evaluation of dispersal, it is mandatory to explore the ecological consequences of this phenomenon, besides the dispersal event itself. Here we explore some of the consequences of dispersal, placing special emphasis on the role of post-dispersal seed predation — one of the major presumed advantages of dispersal. In this analysis we include a discussion of i) the hypothesis of escape from predation as a function of distance from propagule source, ii) the effects of several additional factors such as demographic events occurring at subsequent stages of the plant’s life cycle, and iii) the population dynamics of seeds after the dispersal event. An analysis of the possible qualitative and quantitative effects of pre-dispersal predation is also included.

Rainfall and flowering synchrony in a tropical shrub: Variable selection on the flowering time ofErythroxylum havanense

SummaryWe tested the adaptive significance of flowering synchrony by means of a quantitative analysis of selection and by flowering induction experiments with the deciduous shrubErythroxylum havanense. Temporal schedules of flower and fruit production were determined for a local population (in three sites) in a Mexican seasonal forest for 2 years (1987–1988). The consequences of natural variation in flowering time (flowering initiation day) on maternal reproductive success (fecundity) were evaluated. We observed high levels of inter- and intraindividual flowering synchrony in 1987, but not in 1988 and this contrast was related to differences in rainfall patterns between the two years. A significant proportion (15.4%) of the phenotypic variation in flowering initiation day was accounted for by environmental variance. The expression of phenotypic variance of flowering time and, consequently, the opportunity for selection to act, are controlled by annual variation in rainfall. Despite the between-year difference in flowering synchrony, we detected a relatively intense directional selection on flowering initiation day in both years, but selection coefficients were of opposite sign (standardized directional gradients were −0.326 and 0.333 for 1987 and 1988, respectively). For both years there was a significant relationship between individual relative fitness and the number of neighbouring flowering plants in a given day, suggesting positive frequency-dependent selection.

Simulated seed predation reveals a variety of germination responses of neotropical rain forest species

Seed predation, an omnipresent phenomenon in tropical rain forests, is an important determinant of plant recruitment and forest regeneration. Although seed predation destroys large amounts of the seed crop of numerous tropical species, in many cases individual seed damage is only partial. The extent to which partial seed predation affects the recruitment of new individuals in the population depends on the type and magnitude of alteration of the germination behavior of the damaged seeds. We analyzed the germination dynamics of 11 tropical woody species subject to increasing levels of simulated seed predation (0-10% seed mass removal). Germination response to seed damage varied considerably among species but could be grouped into four distinct types: (1) complete inability to germinate under damage ≥1%, (2) no effect on germination dynamics, (3) reduced germination with increasing damage, and (4) reduced final germination but faster germination with increasing damage. We conclude that partial seed predation is often nonlethal and argue that different responses to predation may represent different proximal mechanisms for coping with partial damage, with potential to shape, in the long run, morphological and physiological adaptations in tropical, large-seeded species.

Seed dispersal and defecation patterns of Cebus capucinus and Alouatta palliata : consequences for seed dispersal effectiveness

Primates are primary seed dispersers for many tropical tree species. Different species of primates vary considerably in ranging and feeding behaviour, seed processing, and in seed defecation patterns. Here we compare the role of two arboreal primate species, howlers (Alouatta palliata), and white-faced monkeys (Cebus capucinus) as seed dispersers in a tropical dry forest in Costa Rica. We found that Cebus produce smaller defecations, spend shorter times feeding per tree, have longer seed dispersal distances, and produce a more scattered pattern of seed deposition in the forest than Alouatta. In addition, Cebus moved more frequently between trees, and consumed fruits of more species than Alouatta. We examined the consequences of the contrasting defecation patterns produced by Cebus and Alouatta on the early seed fate of Acacia collinsii. We found that quantity, but not the identity (Cebus vs. Alouatta) of faecal material affected post-dispersal activity. Seeds in scattered faeces, sufficiently apart from each other (the common defecation pattern of white-faced monkeys), had higher short-term survival than seeds in clumped patterns of faeces (the pattern associated with Alouatta).

The evolution of distyly from tristyly in populations of Oxalis alpina (Oxalidaceae) in the Sky Islands of the Sonoran Desert

The evolution of distyly from tristyly was investigated in populations of Oxalis alpina at high elevations throughout the Sky Islands of the Sonoran Desert. Incompatibility systems in tristylous populations, where self-incompatible short-, mid-, and long-styled morphs occur in populations, vary from those typical of tristylous species in which each morph is equally capable of fertilizing ovules of the other two morphs, to breeding systems in which incompatibility relationships are asymmetric. In these populations, selection against the allele controlling expression of the mid-styled morph is likely. The degree of modification of incompatibility in the short- and long-styled morphs in 10 populations was strongly associated with fewer mid-styled morphs, supporting models predicting the effect of these modifications of incompatibility on frequency of the mid-styled morph. Self-compatibility of the mid-styled morph may be important for maintaining the frequency of this morph, depending on the level of self-pollination, self-fertilization, and the extent of inbreeding depression. Modifications of incompatibility in tristylous populations and the distribution of distylous populations of O. alpina in the Sky Island region have similar geographic components, indicating the potential importance of historical factors in the evolution of distyly from tristyly.

Flower morphometry of Rhizophora mangle (Rhizophoraceae): geographical variation in Mexican populations

We explored the patterns of intra- and interpopulation variation in flower morphology of the red mangrove, Rhizophora mangle. Twelve populations in Mexico were studied: five from the Gulf of Mexico and the Caribbean Sea, and seven from the Pacific Coast. Six metric floral attributes were measured from a sample of 1370 flowers. Significant differences among populations were found for all six attributes. Because floral attributes were all correlated, scores derived from principal factor analysis were used to describe the variation in flower morphology. Two factors explained essentially all of the variance in flower morphology. Corolla and calyx size had a strong effect on factor 1, while gynoecium size had the higher effect on factor 2. Nested analyses of variance on the scores from both factors revealed significant differences among coasts, among populations within coasts, and among plants within populations. Nonetheless, this variation cannot be explained as a result of clinal variation, as indicated by a series of regression analyses. Cluster analysis (UPGMA) showed that a population from the Pacific coast was clustered together with those of the Atlantic, and the arrangement of populations within each coast showed no evident geographical pattern. We propose that frequent events of extinction and recolonization by a few individuals, followed by selfing, may produce differentiation among populations of red mangrove.

Are reward polymorphisms subject to frequency‐ and density‐dependent selection? Evidence from a monoecious species pollinated by deceit

Abstract Most deceit‐pollinated species involve floral dimorphisms characterized by the presence of rewarding male flowers and nonrewarding female flowers. It has been proposed that this polymorphism establishes the conditions for the action of frequency dependent selection (FDS). The tendency of foraging animals to aggregate in areas of high resource density suggests that pollination efficiency and fruit production may be positively influenced by flower density (density dependent selection, DDS). In this paper we offer a graphical model describing the effects of FDS and DDS on a monoecious species pollinated by deceit. We test the FDS and DDS assumptions and the predictions of the model using field observations and experimental populations of Begonia gracilis in which population sex ratio and flower density were controlled. We found a marked effect of both FDS and DDS on pollinator visitation, fruit‐set, and on the probability of female flowers to setting fruits. We conclude that these two types of selection have had a strong influence on the evolution of deceit‐pollinated species.

On the function of floral nectar in Croton suberosus (Euphorbiaceae)

In this paper we propose that the role of floral nectar in Croton suberosus is the attraction of herbivore pr,dators rather than reward for pollinators. The flowers of this monoecious plant species present several features associated with anemophily and wind is the prime pollen vector. Furthermore, in the field, levels of herbivore damage to the leaves are extremely low, even though laboratory tests indicate that the foliage is readily acceptable. Experimental herbivore addition to plants of C. suberosus shows that Polistes instabilis wasps act as defenders while they forage in search of floral nectar. In the deflowered plants treatment, the rate of activity of wasps on the plants was much lower than in intact controls.