Nelson Ramírez

Beyond species loss: the extinction of ecological interactions in a changing world

Summary 1. The effects of the present biodiversity crisis have been largely focused on the loss of species. However, a missed component of biodiversity loss that often accompanies or even precedes species disappearance is the extinction of ecological interactions. 2. Here, we propose a novel model that (i) relates the diversity of both species and interactions along a gradient of environmental deterioration and (ii) explores how the rate of loss of ecological functions, and consequently of ecosystem services, can be accelerated or restrained depending on how the rate of species loss covaries with the rate of interactions loss. 3. We find that the loss of species and interactions are decoupled, such that ecological interactions are often lost at a higher rate. This implies that the loss of ecological interactions may occur well before species disappearance, affecting species functionality and ecosystems services at a faster rate than species extinctions. We provide a number of empirical case studies illustrating these points. 4. Our approach emphasizes the importance of focusing on species interactions as the major biodiversity component from which the ‘health’ of ecosystems depends.

Comparative pollination biology of Venezuelan columnar cacti and the role of nectar-feeding bats in their sexual reproduction

The floral biology, reproductive system, and visitation behavior of pollinators of four species of columnar cacti, Stenocereus griseus, Pilosocereus moritzianus, Subpilocereus repandus, and Subpilocereus horrispinus, were studied in two arid zones in the north of Venezuela. Our results support the hypothesis that Venezuelan species of columnar cacti have evolved toward specialization on bat pollination. Additional information on the floral biology of a fifth species, Pilosocereus lanuginosus, was also included. All species showed the typical traits that characterize the pollination syndrome of chiropterophily. All species but Pilosocereus moritzianus were obligate outcrossers. Nectar and pollen were restricted to nocturnal floral visitors. Two species of nectar-feeding bats, Leptonycteris curasoae Miller and Glossophaga longirostris Miller, were responsible for practically all the fruit set in these cacti. Frequency of bat visitation per flower per night was highly variable within and between species of cactus, with average frequencies varying between 27 and 78 visits/flower/night. In general terms, the pattern of floral visitation through the night was significantly correlated with the pattern of nectar production and nectar sugar concentration for all species of cactus. Under natural pollination, fruit:flower ratios varied from 0.46 in Subpilocereus repandus to 0.76 in Stenocereus griseus. The efficiency of bat pollination in terms of seed:ovule ratio was high in all species, varying between 0.70 and 0.94.

REPRODUCTIVE PHENOLOGY, LIFE-FORMS, AND HABITATS OF THE VENEZUELAN CENTRAL PLAIN'

Reproductive phenology of 171 plant species belonging to 57 families of angiosperms was studied according to life-forms in four habitat types in a savanna-forest mosaic on the Venezuelan Central Plain. Flowering, unripe fruit, and mature fruit patterns were affected significantly according to life-forms and habitats respectively. Production of flowers, unripe fruits, and mature fruits showed marked seasonality for all habitats except for the forest. Flowering peaked during the rainy season, and fruiting peaked toward the end of the rainy season. The savanna and the disturbed area had similar proportions of species that flowered over the year. The percentage of species with unripe fruits produced throughout the year was more seasonal for the disturbed area than for the other habitats. Mature fruit patterns showed an increase during the late rainy season for the ecotone and savanna. A large number of herbaceous (annual and perennial) and liana species flowered during the wet season, and a smaller fraction flowered during the dry season; and trees, shrubs, and epiphytes increased flowering activity during the dry season. Unripe fruit patterns were similar to those of flowering for all life-forms, however, tree species were less seasonal. Mature fruit production by shrubs peaked in the period of maximum rainfall, while the peak for perennial herbs was in the late rainy season and the peak for annual herbs was during the transition between the rainy season and the dry season. The largest proportion of tree and liana species with ripe fruits occurred during the dry season. Differences among phenological patterns in habitats were caused mainly by life-forms and promote a wider distribution of reproductive events in habitats and overall community in the Venezuelan Central Plain.

BREEDING SYSTEMS IN A SECONDARY DECIDUOUS FOREST IN VENEZUELA : THE IMPORTANCE OF LIFE FORM, HABITAT, AND POLLINATION SPECIFICITY

Breeding systems and mating systems of plants in a previously studied secondary deciduous forest were reanalysed in the context of new data. In this analysis, we increased the number of plant species (up to approximately 25% of the plant species in the community), included other life forms (23 annual and perennial species, plus habitat disturbance categories), and considered information about pollinator specificity. The frequencies of species with different sexual systems in a sample of 51 species were 82% hermaphrodite, 14% monoecious, and 4% dioecious. The frequencies of breeding systems in the sample of 49 hermaphroditic and monoecious species were 53% self-incompatible and 47% self-compatible. Self-compatible species included seven partially self-compatible, three self-compatible non-autogamous, and 13 self-compatible autogamous species. None of the species evaluated proved to be agamospermous. Fifty-five percent of the species tested were obligate outbreeders. The proportion of self-incompatible species was higher among trees and shrubs than among annual herbs. The proportion of self-compatible species for perennial herbs and lianas was not different. The association between annual herbs and autogamy was not strong: seven of 13 species were autogamous, five were partially self-compatible, and one was self-incompatible. The main characteristics or factors associated with breeding system were life cycle and successional stage. Short-lived species were mostly self-compatible, and xenogamy tended to be associated with forest and forest-border. In contrast, pollination specificity and life form were not consistently related to breeding system (self-compatibility or self-incompatibility) and mating system (xenogamy or autogamy).

Biologia de polinizacion en una comunidad arbustiva tropical de la Alta Guayana Venezolana

At the community level, 56.2 percent of plant species are pollinated by hymenopterans, 10.9 percent by lepidopterans and 9.6 percent by dipterans, and one plant species (2.7%) by beetles. Bird-pollinated plants represent an important part of plants in the shrubland (12.3%)

Produccion y costo de frutos y semillas entre formas de vida

This study deals with the proportions of seeds, ovules and flowers-fruits aborted, and seeds formed per ovule per inflorescence, related to the weight and number (seed, ovule, flower and fruit) of reproductive strucrutes among five life forms. The study shows that ovule abortion, flower-fruit abortion and seed per ovule per infructescence were significantly higher in trees than in annual and perennial herbs. The correlations between the characteristics of reproductive structures and the abortion levels revealed an increase of ovules aborted with the weight of fruits and seeds, the number of seeds per fruit and flower, fruit weight/flower weight ratio and seed per fruit weight/flower weight ratio from annual herbs to trees (...)

Ecology of pollination in a tropical Venezuelan savanna

Pollination modes ecology of a total of 164 plant species was evaluated according to habitats and plant life forms in the Venezuelan Central Plain. Frequency distribution of nine pollination modes showed that, at the community level bee pollination (38.6%) was dominant. Butterfly (13.9%), fly (12.7%), and wasp (10.8%) pollination were the second most frequent. Moth (6.2%) and wind (10.4%) pollination occurred with similar frequency, and the least common were bird (3.1%), beetle (2.3%) and bat (1.9%) pollination. There was a significant interaction effect indicating that pollination mode was affected by the type of habitat. Bee pollination was the most common pollination mode in all habitats with butterfly, fly and wasp pollination being secondary for forest and forest-savanna transition; and butterfly, wasp, wind and fly pollination being secondary for savanna. Wind, butterfly and fly pollination were found in disturbed areas as secondary pollination modes. Pollination modes were significantly associated and affected by life forms. Bee pollination was dominant in all life forms with wasp, butterfly and fly pollination being the secondary for trees, shrubs, and lianas; and butterfly and wind pollination being the secondary for herbaceous species. The number of pollination modes (richness) among life forms ranged between four and nine for epiphytes and perennial herbs respectively. The highest values of diversity indexes among life forms were found in trees and shrubs. The richness and diversity indices of pollination modes were statistically higher for more structured habitats, forest and forest-savanna transition, than herbaceous habitats, savanna and disturbed areas, which is associated with the highest values of diversity indexes in trees and shrubs. Equitability was higher for forest and disturbed areas than forest-savanna transition and savanna. The results of comparative richness, equitability, diversity indices, and the frequency distribution of pollination modes of 19 samples from tropical and temperate communities indicated that richness of pollination modes may be different between tropical and temperate communities. The proportion of each pollination mode suggests four grouping: (1) rain forests and their strata, (2) grassland savanna, and associated disturbed areas, (3) temperate communities, and (4) the most heterogeneous group, contained mostly neotropical communities, including the four habitats of the Venezuelan Central Plain. The frequency of pollination modes, richness, diversity and equitability of communities, habitats, successional stages, and vegetation strata varies with respect to geography, vegetation structure, and plant species richness.

Plant reproductive biology of herbaceous monocots in a Venezuelan tropical cloud forest

The reproductive biology of 29 herbaceous understory monocot species was studied in a tropical cloud forest of northern coastal Venezuela (Henri Pittier National Park). Of the 23 plant species, 6 (26.1%) were self-incompatible and 17 (73.9%) self-compatible, the latter consisting of 12 (52.2%) fully self-compatible and 5 (21.7%) partially self-compatible species. An analysis of sexual systems and temporal variation of sex expression showed that of 29 monocot species, 14 (48.3%) were hermaphrodite and adichogamous, 14 (48.3%) monoecious and dichogamous, and one (3.4%) dioecious. The combination of monoecy and dichogamy prevented autogamy in 11 (91.7%) of the 12 self-compatible and monoecious species; one (8.3%) was autogamous. Of all species, 24 (82.8%) were obligate outbreeders, 2 (6.9%) spontaneously autogamous, and 3 (10.3%) partially autogamous. Ovule and flower abortion were not significantly different between the different groups. Pollen-ovule ratio was significantly different between monoecious and hermaphrodite species. The number of pollen grains and ovules per flower and the pollen-ovule ratio were variable among various pollination types. The average pollen-ovule ratio was higher in beetle-, dipteran-, and wind-pollinated species than in bee-, bird-, and bat-pollinated species. The first three pollination types were restricted to monoecious species, and the second three groups were restricted to hermaphrodite species. Of 33 plant species investigated, 12 (36.4%) displayed vegetative reproduction. The reproductive features of the understory monocot species are discussed in the context of life form, devices of cross-pollination, and geological history of the tropical cloud forest.

Reproductive biology in a tropical shrubland of Venezuelan Guayana

Plant sexual systems were evaluated in a tropical shrubland community of Venezuelan Guayana. Of 89 plant species, 73.0% were hermaphrodite, 12.4% monoecious, 10.1 % submonoecious, and 4.5 % dioecious. Temporal variation of sexual expression showed that hermaphrodite species were mostly adichogamous and monoecious, submonoecious species dichogamous (protandrous andprotogynous). Herkogamy was an important mechanism promoting cross-pollination. Of 38 hermaphrodite species, 39.5 % were herkogamous. Plant sexual systems were not associated with life form. Most species were pollinated mostly by large and medium-size bees. Pollinator specificity and pollen load transportation, were higher for monoecious species than for hermaphrodite, submonoecious and dioecious ones. Both pollinator specificity and pollen transportation were not significantly different among representatives of the latter three sexual systems. Plant sexual systems were significantly associated with dispersal syndromes; dioecious species had bird-dispersal syndromes, the other sexual systems mostly anemochory, epizoochory and endozoochory; but many hermaphrodites were anemochorous.

Produccion y Costo de Frutos y Semillas Relacionados a las Caracteristicas de las lnflorescencias1

The levels of seed set and aborted seeds, ovules, and flowers and fruits were determined in 232 species of plants in relation to inflorescence type, inflorescence position, synchronization of anthesis within the inflorescence and the pollination unit, and the combination of these variables. The percentage of variance between the levels of aborted ovules, aborted flowers and fruits, and seed set was significantly greater for inflorescence type than the other variables. The percentage of variance explained by different groups of variables (grouped by duster analysis) in the abortion levels and seed set does not differ from that of the individual variables. The lowest levels of aborted seeds occur in spadices, as well as in axillary and terminal/axillary inflorescences. The lowest levels of aborted ovules and the highest levels of seed set occurred in terminal, capituliform inflorescences. The lowest levels of flower-fruit abortion were found in solitary flowers and in terminal, capituliform inflorescences, and in synchronous anthesis. There was a general tendency in this sample for low levels of abortion and high seed set to be associated with the low cost of fruits and individual seeds in inflorescence units. These results are interpreted on the basis of the biomass assigned to flowers, fruits, and seeds, as well as particular characteristics of the qualitative characters.