Geographical and Seasonal Variation in the Richness of Ant-Plant Interactions in México1

The richness and seasonal variation of ant-plant interactions were compared in four habitats in México: lowland tropical dry forest (La Mancha, Veracruz), coastal sand dune matorral (San Benito, Yucatán), semiarid highland vegetation (Zapotitlán, Puebla), and lower montane humid forest (Xalapa, Veracruz). The effects of temperature and precipitation on the seasonal distribution of the number of ant-plant interactions differed among habitats. The general linear models fitted to the ant-plant interaction curves explained 78.8 percent of the variation for La Mancha, 80.1 percent for Zapotitlán, 18 percent for San Benito, and 29.5 percent for Xalapa. Even though rainfall is low in Zapotitlán and San Benito, minimum temperature was the most important factor accounting for the seasonal distribution and low number of interactions. At La Mancha, with milder minimum temperatures and higher water availability, temperature alone did not account for the seasonal distribution and number of interactions, whereas the effect of the precipitation × temperature interaction was highly significant. Xalapa exhibits the lowest temperatures and the highest precipitation, but the role of these factors was only marginal. We suggest that the vegetation at Xalapa, a mixture of tropical and temperate floristic elements, constrains ant-plant interactions due to a limited presence of nectaries. Also, ants are less abundant in cool and relatively aseasonal habitats. The other habitats have tropical floristic elements that are abundant and frequently have nectar-producing structures. We report considerable variation among habitats in the number and seasonal distribution of ant-plant interactions, and suggest that it is due to the effect of variation in environmental parameters, the richness of plants with nectaries in the vegetation, and habitat heterogeneity.     

The role of resource imbalances in the evolutionary ecology of tropical arboreal ants

In numbers and biomass, ants (Hymenoptera, Formicidae) often dominate arthropod faunas of tropical rainforest canopies. Extraordinary ant abundance is due principally to one or a few species able to tap the high productivity of canopy foliage by feeding on plant and homopteran exudates. Prior studies of nitrogen isotopic ratios show that exudate-feeders derive much of their nitrogen (N) by processing large quantities of N-poor, but carbohydrate (CHO)-rich, exudates. CHOs in excess of those that can be coupled with protein for growth and reproduction (postulated as the colony's first priorities) may be directed at little cost and some profit to functions that increase access to limiting protein. High dietary CHO:protein ratios for exudate-feeders appear to subsidize 'high tempo' foraging activity, defence of absolute (level III) territories, and production of N-free alarm/defence exocrine products that enhance ecological dominance in contests with other ants. Among organisms (e.g. plants and Lepidoptera) symbiotic with ants, CHO:protein ratios of ant rewards may control both the identities of ant associates and the quality of ant-rendered services. Dietary ratios of CHO:protein play an important and previously unrecognized role in the ecology and evolution of ants generally. Modifications of worker digestive systems in certain ant subfamilies and genera represent key innovations for handling and processing large volumes of liquid food. The supreme tropical dominants are species released from nest site limitation and able to place their nests in the vicinity of abundant exudate resources. Polydomy appears to be typical of these species and should produce energetic savings by taking colony fragments to the resource.     

Characteristics of floral nectaries and nectar in two species of Crataegus (Rosaceae)

 The structure of floral nectaries of Crataegus coccinea and C. crus-galli was examined using light and scanning electron microscopy. The radial length of the floral nectary, measured from longitudinal sections of flowers, was 30% larger in C. crus-galli than in C. coccinea. For both Crataegus species the glandular tissue thickness was similar – approx. 400 μm. Also, the number of stomata per mm² of nectary surface in C. crus-galli was much higher (by 43%) than for C. coccinea. Stomata were situated in deep hollows. For both taxa the period of nectar secretion was 4 days. The mean quantity of total sugar in nectar per 10 flowers of C. crus-galli and C. coccinea was 3.87 mg and 0.33 mg, respectively. Received August 28, 2002; accepted December 17, 2002 Published online: June 2, 2003     

Floral traits and pollination systems in the Caatinga, a Brazilian tropical dry forest

BACKGROUND AND AIMS: Pollination is a critical stage in plant reproduction and thus in the maintenance and evolution of species and communities. The Caatinga is the fourth largest ecosystem in Brazil, but despite its great extent and its importance few studies providing ecological information are available, with a notable lack of work focusing on pollination biology. Here, general data are presented regarding the frequency of pollination systems within Caatinga communities, with the aim of characterizing patterns related to floral attributes in order to make possible comparisons with data for plant communities in other tropical areas, and to test ideas about the utility of syndromes. This paper also intends to provide a reference point for further studies on pollination ecology in this threatened ecosystem. METHODS: The floral traits and the pollination systems of 147 species were analysed in three areas of Caatinga vegetation in northeastern Brazil, and compared with world-wide studies focusing on the same subject. For each species, floral attributes were recorded as form, size, colour, rewards and pollination units. The species were grouped into 12 guilds according to the main pollinator vector. Analyses of the frequencies of the floral traits and pollination systems were undertaken. KEY RESULTS: Nectar and pollen were the most common floral resources and insect pollination was the most frequent, occurring in 69.9 % of the studied species. Of the entomophilous species, 61.7 % were considered to be melittophilous (43.1 % of the total). Vertebrate pollination occurred in 28.1 % of the species (ornithophily in 15.0 % and chiropterophily in 13.1 %), and anemophily was recorded in only 2.0 %. CONCLUSIONS: The results indicated that the pollination systems in Caatinga, despite climatic restrictions, are diversified, with a low percentage of generalist flowers, and similar to other tropical dry and wet forest communities, including those with high rainfall levels.     

The effect of substrate abundance in the vertical stratification of bromeliad epiphytes in a tropical dry forest (Mexico)

Diversity of epiphytes is associated with niche partitioning, through vertical strata and host preferences. However, abundance of substrate offered by hosts differs between vertical strata, misleading if epiphytes prefer a stratum or are randomly distributed. In a tropical dry forest of San Andres de la Cal Morelos, central Mexico, we tested the null hypothesis, that epiphytes follow the abundance of the substrate, rather than showing preference for a particular vertical stratum, and tested whether microclimatic variables, seed germination and seedling survival match with observed epiphyte distribution. Our data show that epiphytes could be randomly distributed inside some host; but in some host species, certain structures presented either a deficit or an excess of all, atmospheric, or tank epiphytes. In the hosts Bursera copallifera and Bursera glabrifolia, distribution of epiphytes was biased towards the upper strata, with a deficit of epiphytes in the lower strata. In Conzattia multiflora, Sapium macrocarpum and Ipomoea pauciflora, epiphyte distribution was biased towards the lower strata. Vertical gradation of light, seed germination and seedling survival did not generally match with epiphyte distribution and did not support the notion that the microclimatic gradient governs the vertical distribution of epiphytes. Our data indicate that vertical distribution of epiphytes in such tropical dry forests is mainly driven by the distribution of the structures, which apparently influence dispersion of the seeds and by the lifespan of branches, which allow the concentration of epiphytes in the stratum that optimizes seed capture and the clonal growth of epiphytes.     

Light microhabitats,growth and photosynthesis of an epiphytic bromeliad in a tropical dry forest

In the tropical dry forest of Dzibilchaltún, Yucatan, Mexico, light microhabitats, tissue acidity changes, chlorophyll fluorescence and growth were investigated for the epiphytic bromeliad Tillandsia brachycaulos Schltdl. Most individuals occurred in sites between 20 and 60% of the height of the tree, and directly on the main trunk or branches closed to the main trunk. During the dry season, individuals received about nine times more photon flux density (PFD) than during the rainy season. Individuals that occurred under 30–59% of the daily ambient PFD showed greater leaf length increase and monthly leaf production, and produced more flowers than individuals in the brighter (>60% of ambient PFD) and the darker (11–29% of ambient PFD) light microhabitats. Well-watered plants acclimated to deep shade (6% of ambient PFD) showed values of tissue acidity similar to plants under higher light treatments. Well-watered plants under 60% of daily ambient PFD showed low values of maximum quantum efficiency at midday during most of the year but showed similar values of tissue acidity than plants under 6 and 30% of ambient PFD. Leaf temperatures of plants in low sites within the canopy and on the forest floor were significantly higher than the air during about 1 h, indicating that those plants could not dissipate the excess of heat. Individuals located in the most shaded microhabitats could also receive less amount of water from rainfall and dew. Therefore, it would be less damaging for this epiphytic bromeliad to occur in the most exposed microhabitats than in the most shaded microhabitats in this tropical dry forest.     

The floral development and floral anatomy ofChrysosplenium alternifolium, an unusal member of the Saxifragaceae

The floral development and anatomy ofChrysosplenium alternifolium were studied with the scanning electron microscope and light microscope to understand the initiation sequence of the floral organs and the morphology of the flower, and to find suitable floral characters to interpret the systematic position of the genus within the Saxifragaceae. The tetramerous flower shows a highly variable initiation sequence. The median sepals and first stamens arise in a paired sequence resembling a dimerous arrangement, but the first sepal and stamen arise on the side opposite to the bract. Transversal sepals and stamens emerge sequentially, as one side often precedes the other; sepals and stamens occasionally arise on common primordia. Initiation of the gynoecium is more constant with two median carpel primordia arising on a sunken floral apex. Several flowers were found to be pentamerous with a 2/5 initiation sequence. Flowers were invariably found to be apetalous without traces of petals in primordial stages; this condition is interpreted as an apomorphy. It is postulated that the development of a broad gynoecial nectary is responsible for the occurrence of an obdiplostemonous androecium. The gynoecium shows a number of anatomical particularities not observed in other Saxifragaceae. The presence and distribution of colleters is discussed.     

Pulses of soil phosphorus availability in a Mexican tropical dry forest: effects of seasonality and level of wetting

Intact cores from the upper soil profile and surface litter were collected at the peak of the dry season and during the rainy period in the tropical deciduous forest of the Chamela region, Jalisco, México, to (1) analyze upper soil phosphorus (P) movement and retention, (2) compare soil P dynamic pools (soluble, bicarbonate, and microbial) in dry and rainy seasons, and (3) determine the response of these P pools to wetting. Unperturbed litter-soil cores were treated in the laboratory with either 10 mm or 30 mm of simulated rain with carrier-free ³²P and compared to a control (no water addition) to determine the fate and retention of added P. ³¹P concentrations and pools in most litter and soil fractions were higher in the dry than in the rainy season. Soluble P was 0.306 g/m² and microbial P was 0.923 g/m² in the dry season (litter plus soil) versus 0.041 (soluble) and 0.526 (microbial) g P/m² in the rainy season. After water addition, rainy-season cores retained 99.9 and 94% of ³²P in the 10- and 30-mm treatments, respectively. Dry-season samples retained 98.9 and 80% of inputs in the same treatments. Retention after wetting occurred mostly in soil (bicarbonate and microbial fractions). Simulated rainfall on rainy-season soils increased P immobilization. On the other hand, simulated rainfall on dry-season soils released P through mineralization. The P release represents between 46 and 99% of the annual litterfall return. Our results suggest that both soluble and microbial P constitute important sources for initiation of plant growth at the onset of the rainy season in tropical dry forest. Received: 23 September 1997 / Accepted: 2 February 1998     

Nitrogen and phosphorus resorption in trees of a Mexican tropical dry forest

Resorption efficiency (RE) and proficiency, foliar nutrient concentrations, and relative soil nutrient availability were determined during 3 consecutive years in tree species growing under contrasting topographic positions (i.e., top vs. bottom and north vs. south aspect) in a tropical dry forest in Mexico. The sites differed in soil nutrient levels, soil water content, and potential radiation interception. Leaf mass per area (g m^–2) increased during the growing season in all species. Soil P availability and mean foliar P concentrations were generally higher at the bottom than at the top site during the 3 years of the study. Leaf N concentrations ranged from 45.4 to 31.4 mg g^–1. Leaf P varied from 2.3 to 1.8 mg g^–1. Mean N and P RE varied among species, occasionally between top and bottom sites, and were higher in the dry than in the wet years of study. Senesced-leaf nutrient concentrations (i.e., a measure of resorption proficiency) varied from 13.7 to 31.2 mg g^–1 (N) and 0.4 to 3.3 mg g^–1 (P) among the different species and were generally indicative of incomplete nutrient resorption. Phosphorus concentrations in senesced leaves were higher at the bottom than at the top site and decreased from the wettest to the the driest year. Soil N and P availability were significantly different in the north- and south-facing slopes, but neither nutrient concentrations of mature and senesced leaves nor RE differed between aspects. Our results suggest that water more than soil nutrient availability controls RE in the Chamela dry forest, while resorption proficiency may be interactively controlled by both nutrient and water availability.     

Dew and its effect on two heliophile understorey species of a tropical dry deciduous forest in Mexico

 A series of horizontal and vertical measurements of dew deposition and dew duration were carried out in a tropical dry deciduous forest in western Mexico (19° 30′ N, 105° 03′ W). The effect of dew on transpiration in heliophile species was also investigated. The amount of dew was very variable with no temporal or spatial pattern. The amount of dew measured at two horizontal transects (maximum and minimum) (at heights of 0.20 m and 1.30 m above ground level) was from 0.014 to 0.203 mm and from 0.013 to 0.061 mm in the middle and at the end of the dry season, respectively. Dew deposition at different vertical levels (0.50–12.5 m height) ranged from 0.04 to 0.36 mm. The duration of dew formation ranged between 60 and 129 min after sunrise above the canopy (a height of 11 m), and between 259 and 290 min after sunrise at a height of 2 m. Daily transpiration rates were 883 and 632 g m^–2 d^–1 in Coccoloba liebmannii and 538 and 864 g m^–2 d^–1 in Jacquinia pungens in January and April, respectively. Transpiration was restricted from sunrise to early afternoon in April, as a result of the pronounced midday closure of stomata in both species. The reduction of transpiration by dew ranged from 13.2 to 50.1 g m^–2 d^–1 and from 4.5 to 77.7 g m^–2 d^–1 for C. liebmannii and J. pungens, respectively. Dew can play an important role in enhancing the survival of heliophile species in the dry season by reducing transpiration rates during the morning. Received: 30 April 1998 / Accepted: 14 December 1998     

Changes in the spatial variation of soil properties following shifting cultivation in a Mexican tropical dry forest

The role of secondary vegetation in restoring soil fertility during shifting cultivation in the tropics is well known. Yet the effect of secondary succession on the spatial patterns of soil properties has received little attention. To determine whether changes in the plant community as a result of shifting cultivation affect the scale of spatial dependence for biologically important soil nutrients, we sampled three dry tropical forest stands in Campeche, Mexico. These stands represented a gradient of cultivation history: one mature forest stand, a forest fallow that had undergone one cultivation-fallow cycle, and a forest fallow that had undergone two cultivation-fallow cycles. We used an analysis of semivariance to quantify the scale and magnitude of spatial dependence for organic matter content (OM), phosphorus (P), potassium (K), and aluminum (Al) in each stand. The scale of spatial dependence varied with cultivation history, but the degree of spatial dependence did not differ among stands. In the mature forest P and K were autocorrelated over distances >7.5 m. In the forest fallows 48–88% of the variation in soil P and K was autocorrelated over distances up to 1.1–5.1 m. In contrast, the range of autocorrelation for Al (∼2.5 m) did not differ among stands. We conclude that shifting cultivation changes the range of autocorrelation for biologically important soil nutrients at a scale that may influence plant growth. The finer scaled pattern of soil nutrients in forest fallows is likely to persist with continued shifting cultivation, since fallows are cleared every 3–15 years.     

The occurrence and significance of amino acids in floral nectar

Approximately 1 500 angiosperm species, in previous papers, have been sampled for the assessment of the amino acids (a. a.) in their nectar. We reaffirm that the findings provide statistically significant data linking differences in the concentration with pollinator type. Flowers that are pollinated by animals that have alternative sources of protein-building a. a.’s show lower a. a. concentration than those that are not. There is a tendency for woody plant nectar a. a.’s to be less concentrated than those of herbaceous plants, but there can be “phylogenetic constraints” which may reduce the correlations of a. a. concentration with pollinator type and with life form. The individual a. a.’s form complements which are qualitatively extremely constant within species. Proline is a normal constituent of many nectars and does not necessarily indicate contamination of the nectar by pollen. Criticism of our findings byGottsberger & al. (1984) is answered by reference to our previous publications and those of other workers, and to the presentation of data from California native species, not published previously. All previous postulates are borne out by these new data with the exception of positive correlations of a. a. concentration with “primitive” and “advanced” floral characteristics taken one at a time, which appear to be inconsistent and are affected strongly by the nature of the family in which they occur. Summary data are provided for families and genera which indicate that high or low a. a. concentration can typify certain families and genera of both relatively “primitive” and relatively “advanced” nature. Needs for future research on an ecosystem basis are quoted.     

The timing of nectar secretion in staminal and staminodial glands in Lauraceae

The flowers of many Lauraceae have two kinds of glandular organ: paired glands at the base of the filaments of the third androecial whorl, and staminodes with a glandular head, corresponding to a fourth, sterile androecial whorl. So far, it is unknown why there are two different kinds of organ with apparently the same function. Observations now show that the staminal and the staminodial glands secrete nectar at different times in the heterodichogamous flowering cycle, and are therefore essential for the pollination of bisexual Lauraceae flowers.     

Induced production of extrafloral nectar in intact lima bean plants in response to volatiles from spider mite-infested conspecific plants as a possible indirect defense against spider mites

We found that intact lima bean plants increased the secretion of extrafloral nectar (EFN) after exposure to Tetranychus urticae-induced plant volatiles. Predatory mites, Phytoseiulus persimilis, dispersed more slowly from an exposed intact plant than from a control plant (plant exposed to volatiles from intact conspecific). The predators also dispersed more slowly from those plants that were provided with extra EFN than from untreated plants. We further show that EFN was a potential alternative food source for P. persimilis. From these results, we concluded that increased EFN was involved in the slow dispersal of P. persimilis from the plants exposed to herbivore-induced plant volatiles. Our data suggest that the increase of EFN in an HIPV-exposed intact plant could be an induced indirect defense against spider mites.     

Response of fire ants (Formicidae: Solenopsis invicta and S.gerninata) to artificial nectars with amino acids

Abstract.

• 1 The role that amino acids in extrafloral nectars play in attracting ants to plants was investigated. Workers from laboratory colonies of Solenopsis invicta Buren and S.geminafa (F). (Formicidae) fed from artificial nectaries containing mimics of the extrafloral nectar of Passiflora menispermifoh and P.caerulea; P.menispermifoh nectar contains higher levels of amino acids (1347.3 pdml) than does the nectar of P.currulea (125.2 μm /ml).
• 2 When sugar-only and sugar—amino acid nectar mimics were presented simultaneously, more S.invicta workers were counted at sugar—amino acid nectar mimics than at sugar-only nectars. S.geminatu did not discriminate between the two nectars.
• 3 When the two Pamiflora L. nectar mimics were presented simultaneously, S.invicta and S.geminata workers were more abundant at the nectaries containing high levels of amino acids (P.menispermifolia HBK mimic) than at the nectaries containing low levels of amino acids (P.cuerulea L. mimic).
• 4 The behaviour shown by S.invicta and S.gerninata suggests that plants with high levels of amino acids in their extrafloral nectars attract more ant protectors and might suffer less herbivory than plants producing nectars with low levels of amino acids. If so, ants may favour, over evolutionary time, plants that produce nectars with high levels of amino acids.
• 5 Day-to-day variability in ant behaviour was considerable even among laboratory colonies maintained on the same diet in similar environmental conditions. This variability will reduce the selective impact that ants have on plants and may help to explain why most ant-plant interactions are facultative.

     

The role of tropical dry forest as a long-term barrier to dispersal: a comparative phylogeographical analysis of dry forest tolerant and intolerant frogs

We used a comparative phylogeographical approach to investigate the origins of the disjunct wet forest biota of the Golfo Dulce region along the Pacific slope of Costa Rica. This region is isolated by Pacific dry forests north and south and isolated from Caribbean wet forests by mountains. We studied three sympatric lowland frog species in the Craugastor fitzingeri species group that prefer wet forest but differ in their response to dry habitats. In dry forest, C. fitzingeri can survive along streams while C. crassidigitus and C. talamancae are entirely absent. We collected samples from across the ranges of all three species, and obtained mitochondrial DNA sequence data from the COI and cytochrome b genes. We observed significant phylogeographical structure in C. crassidigitus and C. talamancae, but much less in C. fitzingeri, demonstrating that mountain barriers and dry forest habitat have reduced mitochondrial gene flow in the strictly wet-forest species. Additionally, we discovered that the Golfo Dulce and Central Panama populations of C. crassidigitus appear to have diverged in the Pliocene or earlier, suggesting that the dry forest separating these populations is old. Our phylogenetic analysis of 12 of approximately 16 species of the C. fitzingeri species group suggests that the three lowland species are each other's closest relatives. Because of this shared phylogenetic history, we attribute the striking differences in phylogeographical structure to the different ecologies of the frogs. In summary, we find that what appear to be minor differences in the natural history of these three closely related species may profoundly impact the potential for dispersal, range size, and cladogenesis.     

The relative importance of positive and negative interactions for pollinator attraction in a plant community

Plant–pollinator interactions provide ideal frameworks for studying interactions in plant communities. Despite the large potential influence of such interactions on plant community structure, biodiversity and evolutionary processes, we know surprisingly little about the relative importance of positive and negative interactions among plant species for pollinator attraction. Therefore, we explored the relationships between conspecific and heterospecific floral densities and the flower visitation rates of nine plant species mainly visited by bumble bees, and six plant species mainly visited by flies, in a temperate grassland, through stepwise multiple regressions. Significant relationships were interpreted as interactions for pollinator attraction. Our results revealed that positive intra- and interspecific interactions for pollinator attraction were far more frequent than negative ones. Seventeen interspecific interactions were revealed of which 14 were significantly positive, whereas three of four significant intraspecific interactions were positive. Seven species experienced only positive interactions and two species experienced only negative interactions. The results presented here indicate that negative interactions are not necessarily the dominant ecological interaction for pollination among plants within a community, and the study represents a straightforward approach to study intra- and interspecific interactions among multiple species within a community. We discuss which mechanisms may drive the positive interactions for pollinator attraction and whether this may result in facilitative effects on reproductive success. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.