Mating systems are in¯uenced by several ecological factors, including plant density, number of owers per plant, and pollinator movements. In this study, we investigated the simultaneous eects of these three factors on the mating system of a self-compatible Brazilian shrub species: Helicteres brevispira St. Hil. Outcrossing rate is directly correlated with plant density. Changes in the number of owers per plant aect outcrossing rate through their eect on the density of¯owers. Variation in foraging behaviour of hummingbird pollinators is a consequence of the interaction between plant density and number of¯owers per plant. Territorialist pollinators are common in high density areas but visit few¯owers on each plant, thereby promoting outcrossing. In areas of low plant density, trapliners and rare territorialists visit several¯owers per plant, thus increasing sel®ng. Our results indicate that outcrossing rate is a dynamic parameter, with the extent of variation depending on a number of ecological factors. In successional species such as those in the genus Helicteres, demographic changes may be accompanied by alterations in mating system parameters, with concomitant eects on the genetic structure of populations.Keywords:¯ower density, Helicteres, mating system, outcrossing rate, plant density, pollinator behaviour. IntroductionMating systems in plants can be in¯uenced by several ecological factors. Wright (1946) demonstrated theoretically that inbreeding is inversely correlated with plant density and gene dispersal, if mating within neighbourhoods is random. As density increases, the eective number of reproducing individuals inside the neighbourhood areas also increases, as does the outcrossing rate. This is particularly true in plant species pollinated by animals, because pollinators tend to¯y to the closest neighbour and most of the pollen is exchanged among neighbouring plants (Levin, 1986; Parra et al., 1993; Boshier et al., 1995; Stacy et al., 1996).Several authors have suggested that high concentration of rewards in space may enhance pollinator activity between plants and thus pollen¯ow (e.g. Ausgspurger, 1980; Wyatt, 1980; Gerber, 1985). Others have suggested that this concentration may restrict pollinator movement and gene¯ow between plants and within populations (e.g. Linhart, 1973;Feinsinger, 1978;Schmitt, 1983; Stacy et al., 1996). Species with dierent spatial patterns of¯oral rewards can also attract guilds of pollinators with distinct foraging strategies. For example, Linhart (1973) andFeinsinger (1978) found that species that produce few¯owers per plant attract trapliner hummingbirds (which visit sequentially the same plants over a wide geographical area), whereas species that produce many¯owers per plant are pollinated by territorial hummingbirds (which visit and defend owering plants that are within their feeding territories). These authors have suggested that traplining and foraging over long distances should promote outcrossing, but that territorial foraging should result in inbreeding (see also B...

-(Changes in the floristic and structure of a dry deciduous forest during a period of 4-years (1994-1998), southeastern Brazil). Changes in the structure and composition of a fragment of dry deciduous forest in southeastern Brazil were described for 4-year period (1994)(1995)(1996)(1997)(1998). This fragment was quite disturbed by human activities up to 1993, but it has been protected since 1994. A survey of woody plants (≥ 3.2 cm dbh) was conducted in 26 transects of 50 x 6 m (0.78 ha) in 1994. Four years later (1998), all the transects were revaluated In 1994, a total of 114 tree species from 46 families were identified. Species richness was almost identical in 1998. The values of the Shannon index (3.72) and the equability (0.79) did not differ significantly during the studied period (t test, p > 0.1). The species richness were greater than Neotropical dry forests due probably to the building sucessional phase of this fragment. Although tree density had declined by about 10.2%, total basal area had a small change, increasing 1.5%. Those alterations were reflected in the size class distribution, which had significant differences between 1994 and 1998 (Chi-square test, p < 0.01). Although overall forest composition showed little change, several species showed significant imbalance between recruitment and mortality rates. The decline of pioneer species (Chi-square test, p < 0.001) and the growth in total basal area of the canopy species caused the changes observed during the four years. The preservation of the studied fragment has been important for its recovery in the last years.RESUMO -(Mudanças na florística e estrutura de uma floresta decídua durante um período de quatro anos (1994)(1995)(1996)(1997)(1998), na região do Triângulo Mineiro, MG). As alterações na estrutura e na composição florística de um fragmento de floresta decídua foram descritas para um período de quatro anos (1994)(1995)(1996)(1997)(1998), na região do Triângulo Mineiro, MG. Este fragmento, denominado Mata da Zilda, foi bastante perturbado por atividades humanas até 1993, sendo protegido a partir de 1994. Logo após a sua proteção, realizou-se um inventário das plantas lenhosas (CAP ≥ 10 cm) em 26 transectos de 50 x 6 m (0,78 ha). Quatro anos depois (1998), todos os transectos foram reavaliados e medidos os novos ingressos. Em 1994, foram identificadas 114 espécies, representando 46 famílias. A riqueza de espécies foi idêntica em 1998. Também, os valores do índice de Shannon (3,72) e a equabilidade (0,79) não diferiram significativamente durante os quatro anos (teste t, p > 0,1). A riqueza de espécies na Mata da Zilda foi superior à observada nas florestas secas Neotropicais, devido provavelmente à fase de sucessão em que se encontra. Embora a densidade tenha diminuído cerca de 10,2%, durante os quatro anos, a área basal total mudou pouco, aumentando apenas 1,5%. Essas alterações refletiram-se na distribuição em classes de tamanho, que apresentou diferenças significativas entre os dois períodos avaliados (teste Qui-quadrado, p < 0...

Detrimental effects of introduced honeybees (Apis mellifera L.) on native plant pollination have been predicted based on their observed deterrence or expulsion of native pollinators or their depletion of floral resources. Here, a case is reported in which floral visitation by the introduced honeybee affects male fitness and probably fruit and seed production of a tropical tree without affecting the resource sought by the native pollinator. Clusia arrudae is a dioecious species and is pollinated by individuals of Eufriesea nigrohirta that visit its flowers to collect resin. Male flowers, however, are also visited by individuals of A. mellifera, which remove ca 99 percent of their pollen grains. When E. nigrohirta leaves flowers previously visited by A. mellifera, they carry on their bodies less than 0.1 percent of the pollen grains carried by bees leaving flowers not visited by the honeybee. This may explain why the frequency of A. mellifera at male flowers is negatively correlated with the number of seeds produced by female flowers. This novel situation may affect other plants offering alternative rewards for pollinators, such as resins, oils, and aromatic compounds. RESUMO A polinização de espécies nativas de plantas pode ser prejudicada em decorrȩncia das visitas da abelha introduzida Apis mellifera. Isto tern sido previsto com base em registros de situaço̧es em que esta abelha impede o acesso de polinizadores nativos ou os expulsa das flores e também pelo fato dela exaurir os recursos florais. Clusia arrudaeé polinizada por Eufriesea nigrohirta que busca resina em suas flores. Apis mellifera visita as flores masculinas de Clusia arrudae e remove cerca de 99 percento dos grãos do pólen da flor visitada. Quando E. nigrohirta visita flores já visitadas por A. mellifera, ela carrega em seu corpo menos de 0.1 percent dos grãos de pólen carregados por abelhas que visitam flores ainda não visitadas por A. mellifera. Isto pode explicar porque a freqüȩncia de A. mellifera em flores masculinas é negativa‐mente correlacionada com a quantidade de sememes produzidas por flores femininas. Esta é uma situação que ainda não havia sido registrada e que pode afetar outras plantas que oferecem recursos alternatives para o polinizador, como resinas, óleos, e compostos aromáticos.

Helicteres brevispira is a pioneer species of the tropical riparian forest whose populations appear to cycle through episodes of extinction and recolonization. Therefore, genetic consequences of founding events may strongly a ect the genetic structure of its populations. An analysis of F-statistics showed that the studied population of H. brevispira is genetically substructured with the highest values of F ST found in areas of high plant densities. Spatial autocorrelation analysis showed that genetic patches have diameters of 3±6 m. Although pollinator movements are usually between plants which are 3±6 m apart, longer¯ights occur and the neighbourhood area is estimated to have a diameter of 15 m. This suggests that genetic patches are smaller than the neighbourhood area. Seed dispersal is limited, mostly less than 2 m from the mother plants. Thus, short seed dispersal, seed dormancy and founder e ects in the seed bank may be the most important determinants of genetic structure in populations of H. brevispira. Factors such as drift and inbreeding may also increase the level of substructure in this population, but the equilibrium model of isolation by distance does not ®t our data.

Pollinators provide an essential service to natural ecosystems and agriculture. In tomatoes flowers, anthers are poricidal, pollen may drop from their pore when flowers are shaken by the wind. However, bees that vibrate these anthers increase pollen load on the stigma and in fruit production. The present study aimed to identify the pollinator richness of tomato flowers and investigate their morphological and functional traits related to the plant-pollinator interaction in plantations of Central Brazil. The time of anthesis, flower duration, and the number and viability of pollen grains and ovules were recorded. Floral visitors were observed and collected. Flower buds opened around 6h30 and closed around 18h00. They reopened on the following day at the same time in the morning, lasting on average 48 hours. The highest pollen availability occurred during the first hours of anthesis. Afterwards, the number of pollen grains declined, especially between 10h00 to 12h00, which is consistent with the pollinator visitation pattern. Forty bee species were found in the tomato fields, 30 of which were considered pollinators. We found that during the flowering period, plants offered an enormous amount of pollen to their visitors. These may explain the high richness and amount of bees that visit the tomato flowers in the study areas. The period of pollen availability and depletion throughout the day overlapped with the bees foraging period, suggesting that bees are highly effective in removing pollen grains from anthers. Many of these grains probably land on the stigma of the same flower, leading to self-pollination and subsequent fruit development. Native bees (Exomalopsis spp.) are effective pollinators of tomato flowers and are likely to contribute to increasing crop productivity. On the other hand, here tomato flowers offer large amounts of pollen resource to a high richness and amount of bees, showing a strong plant-pollinator interaction in the study agroecosystem.Keywords: agroecosystem, buzz pollination, Exomalopsis, poricidal anther, Solanum lycopersicum L. Alta riqueza de espécies de polinizadores nativos em plantações Brasileiras de tomateiro ResumoPolinizadores fornecer um serviço essencial para os ecossistemas naturais e para agricultura.