Lowering the density: ants associated with the myrmecophyte Tillandsia caput-medusae diminish the establishment of epiphytes.

Ants benefit myrmecophytic plants by two main activities defending them from herbivores and offering nutrients. Ants' territorial defence behaviour also benefits their myrmecophytic plants; in the case of trees, this behaviour includes eliminating structural parasites (epiphytes and lianas). These benefits could also occur with myrmecophytic epiphytes by decreasing the abundance of competing epiphytes. In two subunits of a tropical dry forest in the centre of Mexico, we (i) recorded the diversity of ants associated with the myrmecophyte Tillandsia caput-medusae, and experimentally tested: (ii) the effect of the ants associated with the myrmecophyte in the removal of its seeds and the seeds of other sympatric non-myrmecophyte species of Tillandsia; and (iii) if seed remotion by ants corresponds with epiphyte load in the preferred (Bursera copallifera) and limiting phorophyte species (B. fagaroides, Ipomoea pauciflora and Sapium macrocarpum). In five trees per species, we tied seed batches of T. caput-medusae, T. hubertiana, T. schiedeana and T. recurvata. One seed batch was close, and the other far away from a T. caput-medusae with active ants. Between forest subunits, ant richness was similar, but diversity and evenness differed. Ants diminish seed establishment of all the Tillandsia species; this effect is stronger in the forest subunit with a large ant diversity, maybe because of ant competition. Seed remotion by ants is independent of phorophyte species identity. Although ants can provide benefits to T. caput-medusae, they also could be lowering their abundance.

New host and distribution records for Buprestidae (Coleoptera) from the state of Morelos, Mexico.

New state and host records are provided for 28 species of Buprestidae collected in Morelos, Mexico in the following genera: Acmaeodera Eschscholtz 1829, Actenodes Dejean 1833, Agrilus Curtis 1825, Brachys Dejean 1833, Chrysobothris Eschscholtz 1829, Leiopleura Deyrolle 1864, Lius Deyrolle 1864, Pachyschelus Solier 1833, and Tetragonoschema Thomson 1857.

Spatio-temporal variation of Cerambycidae-host tree interaction networks.

Despite its high ecological importance, the commensal interactions at community level are poorly studied. In tropical dry forests (TDF) there is a great diversity of species adapted to the high seasonality that characterizes them; however, little is known regarding how the spatial and temporal availability of resources generates changes in the pattern of commensal interactions. We experimentally studied changes in the diversity, composition, and pattern of interactions in spatio-temporal associations between the saproxylophagous beetles and their host trees in a TDF in Morelos, Mexico. A total of 65 host tree species were selected, from which 16 wood sections were obtained per species. These sections were exposed in the field to allow oviposition by the cerambycids under four different (spatio-temporal) treatments. We analyzed the network structure and generated indices at species level (i.e., specialization, species strength, and effective partners) and those related to physical characteristics of the wood (hardness and degradation rate) and the cerambycids (body size). In total, 1,323 individuals of 57 species of cerambycids emerged. Our results showed that, independently of the space and time, the network presented a nested and modular structure, with a high specialization degree and a high turnover of cerambycid species and their interactions. In general, we found that the cerambycids are mostly associated with softwood species with a lower decomposition rate of wood, as well as with the most abundant host species. The commensalistic interactions between the cerambycids and their host trees are highly specialized but are not spatio-temporally static. The high turnover in the interactions is caused by the emergence patterns of cerambycids, which seem to restrict their use to certain species. The knowledge of the spatio-temporal variation in Cerambycidae-host tree interactions allows us to predict how environmental and structural changes in the habitat can modify the species ensemble, and therefore its interactions.

Seasonal diversity of Cerambycidae (Coleoptera) is more complex than thought: evidence from a tropical dry forest of Mexico.

Global climate change is expected to affect temperature and precipitation patterns worldwide, which in turn is likely to affect insect phenology, distribution and diversity. To improve our understanding of such processes, it is important to understand how insects may respond to changes in seasonality, and how these affect their activity, patterns of distribution and species richness. The tropical dry forest (TDF) is a highly seasonal ecosystem, for which two seasons are commonly described (rainy and dry) and there is a lack of information on the combined effect of both precipitation and temperature on the insect communities. In order to evaluate the seasonal patterns in the community of Cerambycidae in a TDF, historical climatic variables were obtained, and an annual sampling of the family was carried out, using three collection techniques. We found that the Cerambycidae family showed a more complex response to climate, than simply the rainy and dry season of the year. The relationship between diversity and composition of cerambycids with changes in temperature and precipitation showed four seasonal communities which were synchronized with phenological processes of the TDF. Climate change could reduce biodiversity, causing seasonal patterns to lose complexity, either because the climatic characteristics of a season disappear and/or because the duration of a season expands, these changes will modify the ecological processes of the TDF, since they would generate changes in the flora and fauna associated with the different seasons.

Effect of seed removal by ants on the host-epiphyte associations in a tropical dry forest of central Mexico.

Seed depredation is recognized as a determining factor in plant community structure and composition. Ants are primary consumers of seeds influencing abundance of epiphytes on trees. This study was conducted in two subunits of a tropical dry forest established on different soil substrates in San Andrés de la Cal, Tepoztlán, in Morelos, Mexico, and experimentally tested whether seed removal activity is higher in tree species with smaller epiphyte loads compared to those with greater epiphyte loads. Five trees were selected at random from six species of trees with high (preferred hosts) or low (limiting hosts) epiphyte loads. Seed removal differed among hosts and different soil substrates in the forest. On relating seed removal to the abundance of arboreal ants, the most consistent pattern was that lower seed removal was related to lower ant abundance, while high seed removal was associated with intermediate to high ant abundance. Epiphyte seed removal by ants influences epiphyte abundance and can contribute considerably to a failure to establish, since it diminishes the quantity of seeds available for germination and establishment.

Head shape variation in cerambycid saproxylic beetles as a function of host plant selection.

Saproxylic insects depend on deadwood for larval development, and a certain degree of specialization may be involved in their choice of host plants and/or wood in a particular stage of degradation. The plant species chosen for oviposition in turn act as an environmental pressure on the head morphology of larvae and it is expected that head shape plasticity varies directly with the number of woody plant species used for larval development in each insect species. We analyzed head shape variation in saproxylic beetles with respect to host plant species, maximum time of larval emergence and season of the year when insects colonized branches. Generalist species in the use of host plants showed significant variation in head shape and size. Time of emergence and season did not appear to affect head shape, although season was a determinant factor of abundance and possibly head size variation.

Dispersal limitation of Tillandsia species correlates with rain and host structure in a central Mexican tropical dry forest.

Seed dispersal permits the colonization of favorable habitats and generation of new populations, facilitating escape from habitats that are in decline. There is little experimental evidence of the factors that limit epiphyte dispersion towards their hosts. In a tropical dry forest in central Mexico, we monitored the phenology of dispersion of epiphyte species of the genus Tillandsia; we tested experimentally whether precipitation could cause failures in seed dispersal and whether seed capture differs among vertical strata and between host species with high (Bursera copallifera) and low (Conzattia multiflora) epiphyte loads. With the exception of one species that presents late dispersion and low abundance, all of the species disperse prior to the onset of the rainy season. However, early rains immobilize the seeds, affecting up to 24% of the fruits in species with late dispersion. We observed that Tillandsia seeds reach both Bursera and Conzattia hosts, but found that adherence to the host is 4-5 times higher in Bursera. Furthermore, seeds liberated from Bursera travel shorter distances and up to half may remain within the same crown, while the highest seed capture takes place in the upper strata of the trees. We conclude that dispersion of Tillandsia seeds is limited by early rains and by the capture of seeds within the trees where populations concentrate. This pattern of capture also helps to explain the high concentrations of epiphytes in certain hosts, while trees with few epiphytes can be simultaneously considered deficient receivers and efficient exporters of seeds.

Vascular epiphytes and host trees of ant-gardens in an anthropic landscape in southeastern Mexico.

Ant-gardens (AGs) are considered one of the most complex mutualist systems between ants and plants, since interactions involving dispersal, protection, and nutrition occur simultaneously in them; however, little is known about the effects of the transformation of ecosystems on their diversity and interactions. In five environments with different land use within an anthropic landscape in southeastern Mexico, we investigated the diversity and composition of epiphytes and host trees of AGs built by Azteca gnava. A total of 10,871 individuals of 26 epiphytic species, associating with 859 AGs located in 161 host trees, were recorded. The diversity and composition of epiphytes tended to be different between environments; however, Aechmea tillandsioides and Codonanthe uleana were the most important species and considered true AG epiphytes, because they were the most frequent, abundant, and occurred exclusively in AGs. Other important species were the orchids Epidendrum flexuosum, Coryanthes picturata, and Epidendrum pachyrachis, and should also be considered true AG epiphytes, because they occurred almost exclusively in the AGs. The AG abundance in agroforestry plantations was similar or even greater than in riparian vegetation (natural habitat). The AGs were registered in 37 host species but were more frequent in Mangifera indica and Citrus sinensis. We conclude that true epiphytes of A. gnava AGs persist in different environments and host trees, and even these AGs could proliferate in agroforestry plantations of anthropic landscapes.

Is Allelopathic Activity of Ipomoea murucoides Induced by Xylophage Damage?

Herbivory activates the synthesis of allelochemicals that can mediate plant-plant interactions. There is an inverse relationship between the activity of xylophages and the abundance of epiphytes on Ipomoea murucoides. Xylophagy may modify the branch chemical constitution, which also affects the liberation of allelochemicals with defense and allelopathic properties. We evaluated the bark chemical content and the effect of extracts from branches subjected to treatments of exclusion, mechanical damage and the presence/absence of epiphytes, on the seed germination of the epiphyte Tillandsia recurvata. Principal component analysis showed that branches without any treatment separate from branches subjected to treatments; damaged and excluded branches had similar chemical content but we found no evidence to relate intentional damage with allelopathy; however 1-hexadecanol, a defense volatile compound correlated positively with principal component (PC) 1. The chemical constitution of branches subject to exclusion plus damage or plus epiphytes was similar among them. PC2 indicated that palmitic acid (allelopathic compound) and squalene, a triterpene that attracts herbivore enemies, correlated positively with the inhibition of seed germination of T. recurvata. Inhibition of seed germination of T. recurvata was mainly correlated with the increment of palmitic acid and this compound reached higher concentrations in excluded branches treatments. Then, it is likely that the allelopathic response of I. murucoides would increase to the damage (shade, load) that may be caused by a high load of epiphytes than to damage caused by the xylophages.

Estratificación vertical y preferencia de hospedero de las epífitas vasculares de un bosque nublado de Chiapas, México

Vertical stratification and host preference by vascular epiphytes in a Chiapas, Mexico, cloud forest. The high diversity of vascular epiphytes in neotropical montane forest has been explained as the result of vertical stratification of the forest and specific relationships between epiphytes and their hosts trees at local scales. In a lower montane cloud forest, we studied the vertical stratification and host preferences of vascular epiphytes in a 0.0625 ha plot where 41 trees ≥ of 10 cm DBH were sampled during 12 months in 2001 and 2002. We found 43 epiphyte species growing on 15 tree genera. We tested for vertical strata and host preferences using 19 epiphyte taxa. We found strong evidence that epiphytes divided the canopy, but those epiphyte species dispersed by animals were generalists with respect to hosts and vertical strata. Wind dispersed epiphytes were vertically stratified, with a higher richness in the lowest tree strata. On average the epiphytes preferred 3.5 host species, suggesting low host preference. Two host species, Ardisia and Quercus, were avoided by the majority of the epiphyte species. Our results show that epiphytes divided the canopy horizontally and were stratified vertically, suggesting that host identity could be important determining the abundance of colonizing sites for epiphytes. Rev. Biol. Trop. 56 (4): 2069-2086. Epub 2008 December 12.

En escala local, la alta diversidad de epífitas vasculares en bosques montanos neotropicales ha sido explicada por que logran estratificar verticalmente al dosel y tienen relaciones específicas con sus hospederos. En un bosque de montaña estudiamos la estratificación vertical y las preferencias de hospedero de las epífitas vasculares en un cuadrante de 0.0625 ha en donde muestreamos 41 árboles ≥10 DAP, durante 12 meses en los años 2001 y 2002. Encontramos 43 especies de epífitas en 15 géneros de árboles. Probamos preferencia de estratos verticales o de hospedero con 19 táxones epífitos. Encontramos evidencia de que las epífitas se dividen el dosel; aunque, las epífitas dispersadas por animales tendieron a ser generalistas. Entre las epífitas dispersadas por el viento reconocimos una clara estratificación vertical, concentrando especies en la parte baja de los árboles. En promedio las especies de epífitas prefirieron 3.5 especies de hospederos, sugiriendo una baja preferencia. Dos especies de árboles resultaron ser malos hospederos, Ardisia y Quercus. Nuestros resultados muestran que las epífitas parten el dosel horizontalmente, se estratifican verticalmente y sugieren que la identidad de los hospederos es importante por que puede determinar la abundancia de sitios colonizables para muchas epífitas.

[Vertical stratification and host preference by vascular epiphytes in a Chiapas, Mexico, cloud forest]. / Estratificación vertical y preferencia de hospedero de las epifitas vasculares de un bosque nublado de Chiapas, México.

The high diversity of vascular epiphytes in neotropical montane forest has been explained as the result of vertical stratification of the forest and specific relationships between epiphytes and their hosts trees at local scales. In a lower montane cloud forest, we studied the vertical stratification and host preferences of vascular epiphytes in a 0.0625 ha plot where 41 trees > or = of 10 cm DBH were sampled during 12 months in 2001 and 2002. We found 43 epiphyte species growing on 15 tree genera. We tested for vertical strata and host preferences using 19 epiphyte taxa. We found strong evidence that epiphytes divided the canopy, but those epiphyte species dispersed by animals were generalists with respect to hosts and vertical strata. Wind dispersed epiphytes were vertically stratified, with a higher richness in the lowest tree strata. On average the epiphytes preferred 3.5 host species, suggesting low host preference. Two host species, Ardisia and Quercus, were avoided by the majority of the epiphyte species. Our results show that epiphytes divided the canopy horizontally and were stratified vertically, suggesting that host identity could be important determining the abundance of colonizing sites for epiphytes.

Diversity and genetic structure of the Mexican endemic epiphyte Tillandsia achyrostachys E. Morr. ex Baker var. achyrostachys (Bromeliaceae).

BACKGROUND AND AIMS: The monoecious, bird-pollinated epiphytic Tillandsia achyrostachys E. Morr. ex Baker var. achyrostachys is an endemic bromeliad of the tropical dry forests of Mexico with clonal growth. In the Sierra de Huautla Natural Reserve this species shows a host preference for Bursera copallifera (Sessé & Moc ex. DC) Bullock. As a result of deforestation in the study area, B. copallifera has become a rare tree species in the remaining forest patches. This human-induced disturbance has directly affected the population densities of T. achyrostachys. In this study the genetic consequences of habitat fragmentation were assessed by comparing the genetic diversity, gene flow and genetic differentiation in six populations of T. achyrostachys in the Sierra de Huautla Natural Reserve, Mexico. METHODS: Allozyme electrophoresis of sixteen loci (eleven polymorphic and five monomorphic) were used. The data were analysed with standard statistical approximations for obtaining diversity, genetic structure and gene flow. KEY RESULTS: Genetic diversity and allelic richness were: HE = 0.21 +/- 0.02, A = 1.86 +/- 0.08, respectively. F-statistics revealed a deficiency of heterozygous plants in all populations (Fit = 0.65 +/- 0.02 and Fis = 0.43 +/- 0.06). Significant genetic differentiation between populations was detected (Fst = 0.39 +/- 0.07). Average gene flow between pairs of populations was relatively low and had high variation (Nm = 0.46 +/- 0.21), which denotes a pattern of isolation by distance. The genetic structure of populations of T. achyrostachys suggests that habitat fragmentation has reduced allelic richness and genetic diversity, and increased significant genetic differentiation (by approx. 40 %) between populations. CONCLUSIONS: The F-statistic values (>0) and the level of gene flow found suggest that habitat fragmentation has broken up the former population structure. In this context, it is proposed that the host trees of T. achyrostachys should be considered as a conservation priority, since they represent the limiting factor to bromeliad population growth and connectivity.

Effects of floral display and plant abundance on fruit production of Ryncholaelia glauca (Orchidaceae)

Flowering plant density can increase number of visits and fruit set in multi-flowering plants, however this aspect has not been studied on few flower species. We studied the effects of individual floral display and plant density on the fruit production of the epiphytic, moth-pollinated orchid, Ryncholaelia glauca, in an oak forest of Chavarrillo, Veracruz, Mexico. Species is non-autogamous, and produced one flower per flowering shoot each flowering season. We hypothesized that orchids with more flowering shoots and those on trees with clumps of conspecific should develop more fruits than isolated ones. R. glauca population flowers synchronouly, and individual flowers last up to 18 days, with flowers closing rapidly after pollination. Individuals produced few flowers per year, although some plants developed flowers in both seasons and fewer of them developed fruits both years. There was no relationship between flower number per orchid, or per host tree, with the number of fruits developed per plant. Host trees with flowering and fruiting orchids were randomly dispersed and the pattern of distribution of flowering and fruiting plants was not related. Apparently, pollinators visit the flowers randomly, with no evidence of density dependence. The fruit set of R. glauca was as low as fruit set of multi-flowered orchids moth pollinated, suggesting that fruit set on moth pollinated orchids could be independent of the number of flowers displayed.

Effects of floral display and plant abundance on fruit production of Ryncholaelia glauca (Orchidaceae).

Flowering plant density can increase number of visits and fruit set in multi-flowering plants, however this aspect has not been studied on few flower species. We studied the effects of individual floral display and plant density on the fruit production of the epiphytic, moth-pollinated orchid, Ryncholaelia glauca, in an oak forest of Chavarrillo, Veracruz, Mexico. Species is non-autogamous, and produced one flower per flowering shoot each flowering season. We hypothesized that orchids with more flowering shoots and those on trees with clumps of conspecific should develop more fruits than isolated ones. R. glauca population flowers synchronouly, and individual flowers last up to 18 days, with flowers closing rapidly after pollination. Individuals produced few flowers per year, although some plants developed flowers in both seasons and fewer of them developed fruits both years. There was no relationship between flower number per orchid, or per host tree, with the number of fruits developed per plant. Host trees with flowering and fruiting orchids were randomly dispersed and the pattern of distribution of flowering and fruiting plants was not related. Apparently, pollinators visit the flowers randomly, with no evidence of density dependence. The fruit set of R. glauca was as low as fruit set of multi-flowered orchids moth pollinated, suggesting that fruit set on moth pollinated orchids could be independent of the number of flowers displayed.