Mexican Tropical Dry Forest Research Articles

Plant defense, herbivory, and the growth of Cordia alliodora trees and their symbiotic Azteca ant colonies

The effects of herbivory on plant fitness are integrated over a plant's lifetime, mediated by ontogenetic changes in plant defense, tolerance, and herbivore pressure. In symbiotic ant-plant mutualisms, plants provide nesting space and food for ants, and ants defend plants against herbivores. The benefit to the plant of sustaining the growth of symbiotic ant colonies depends on whether defense by the growing ant colony outpaces the plant's growth in defendable area and associated herbivore pressure. These relationships were investigated in the symbiotic mutualism between Cordia alliodora trees and Azteca pittieri ants in a Mexican tropical dry forest. As ant colonies grew, worker production remained constant relative to ant-colony size. As trees grew, leaf production increased relative to tree size. Moreover, larger trees hosted lower densities of ants, suggesting that ant-colony growth did not keep pace with tree growth. On leaves with ants experimentally excluded, herbivory per unit leaf area increased exponentially with tree size, indicating that larger trees experienced higher herbivore pressure per leaf area than smaller trees. Even with ant defense, herbivory increased with tree size. Therefore, although larger trees had larger ant colonies, ant density was lower in larger trees, and the ant colonies did not provide sufficient defense to compensate for the higher herbivore pressure in larger trees. These results suggest that in this system the tree can decrease herbivory by promoting ant-colony growth, i.e., sustaining space and food investment in ants, as long as the tree continues to grow.

The monophyly of Bursera and its impact for divergence times of Burseraceae

AbstractBursera is one of the most diverse and abundant groups of trees and shrubs of the Mexican tropical dry forests. Its interaction with its specialist herbivores in the chrysomelid genus Blepharida, is one of the best–studied coevolutionary systems. Prior studies based on molecular phylogenies concluded that Bursera is a monophyletic genus. Recently, however, other molecular analyses have suggested that the genus might be paraphyletic, with the closely related Commiphora, nested within Bursera. If this is correct, then interpretations of coevolution results would have to be revised. Whether Bursera is or is not monophyletic also has implications for the age of Burseraceae, since previous dates were based on calibrations using Bursera fossils assuming that Bursera was paraphyletic. We performed a phylogenetic analysis of 76 species and varieties of Bursera, 51 species of Commiphora, and 13 outgroups using nuclear DNA data. We also reconstructed a phylogeny of the Burseraceae using 59 members of the family, 9 outgroups and nuclear and chloroplast sequence data. These analyses strongly confirm previous conclusions that this genus is monophyletic. New calculations of the age of Burseraceae date the beginning of its diversification to at least 93 million years ago.

Comparison of Pellet-Group Counting Methods to Estimate Population Density of White-Tailed Deer in a Mexican Tropical Dry Forest

Wildlife species population density estimation is important from both ecological and management perspectives. The pellet-group counting method has been used to evaluate density of the white-tailed deer ( Odocoileus virginianus). This species is an important component of human diet in tropical habitats. The objectives of this study were to: 1) compare density estimates using four methods: counts in circular plots (FSC, Fecal Standing Crop and FAR, Fecal Accumulation Rate), and counts in transects (LT, Line Transect and ST, Strip Transect); 2) simulate the effect of increased sampling effort on density and precision, and; 3) evaluate the effort required to detect changes in population density using LT. From 2006 to 2007, we intensively sampled a 1 km2quadrant in a Mexican tropical dry forest. The results indicate that all four methods produce similar mean population density estimates. However, estimates of precision were dependent on sample size which in turn was associated with the particular counting method used. In descending order of estimate precision, the methods ranked as: LT, ST, FSC, and FAR. To detect population changes of < 20%, we suggest the establishment of 5 to 22 transects (LT) of 390 m during the dry season. To reduce bias in density estimation, it is important to obtain defecation and pellet decomposition rates in the study site.

Contrasting herbivory patterns and leaf fluctuating asymmetry inHeliocarpus pallidusbetween different habitat types within a Mexican tropical dry forest

Abstract:Leaf fluctuating asymmetry (FA) is considered an important indicator of environmentally induced stress in plants, but the relationship between herbivory and FA levels is not clear. In this study we compared leaf size and shape, leaf area consumed by herbivorous insects, and FA levels between individuals ofHeliocarpus pallidus(Tiliaceae) from two adjacent and contrasting habitat types (deciduous and riparian) in the Chamela–Cuixmala tropical dry-forest biosphere reserve. Ten individuals ofH. palliduswere collected in each habitat type. Leaf shape was assessed using geometric morphometric techniques. Results indicated statistically significant differences in leaf shape between individuals from the two habitat types. In individuals from the riparian habitat leaf area (mean = 42.3 ± 1.2 cm2), herbivory levels (mean = 25.5% ± 1.8%) and FA levels (mean = 0.38 ± 0.04 cm) were significantly higher than in individuals from the deciduous habitat (17.2 ± 3.5 cm2; 9.6% ± 1.0% and 0.18 ± 0.04 cm, respectively). Within habitats, significant correlations were found between total leaf area and percentage leaf area removed by insects (R2= 0.92 in riparian habitats,R2= 0.9 in deciduous habitats), and between percentage leaf area removed and FA (R2= 0.70 in riparian habitats,R2= 0.79 in deciduous habitats). As has been suggested for other plant species, it is possible that the more favourable conditions in the riparian habitat enhance leaf growth, resulting in higher FA. Also, individuals in this habitat might be characterized by lower chemical defence and/or higher nutritional quality, which would explain the higher herbivory levels.

Nutritional importance of seeds and arthropods to painted spiny pocket mice (Lyomis pictus): the effects of season and forest degradation

Temporal and spatial fluctuations in food abundance may affect the feeding habits of vertebrates in tropical dry forests. We explored the effects of season and forest degradation in dietary patterns of the painted spiny pocket mouse ( Lyomis pictus (Thomas, 1893)) (Heteromyidae) in a Mexican tropical dry forest. We used carbon (13C,12C) and nitrogen (15N,14N) stable isotope analyses to test the hypotheses that (i) L. pictus would increase its use of arthropods during the rainy season when seeds are less available on the forest floor and (ii) that L. pictus would increase its use of arthropods in degraded forest compared with conserved forest. Our hypotheses were wrong because assimilated biomass was derived almost exclusively from seeds in both seasons and the importance of arthropods was marginal in both sites. Examination of food remains in feces and cheek pouches confirmed these trophic patterns. Seed hoarding during the season of high seed availability probably allows L. pictus to subsist on a seed-based diet throughout the year in conserved and disturbed forests. This behavioral trait would enable L. pictus to maintain its specialized feeding habit in environments threatened by habitat degradation.

Importance of the lilac-crowned parrot in pre-dispersal seed predation ofAstronium graveolensin a Mexican tropical dry forest

Abstract:Parrots represent a large biomass of canopy granivores in tropical forests, and may be effective pre-dispersal seed predators. We evaluated the importance of the lilac-crowned parrot (Amazona finschi) as a pre-dispersal seed predator ofAstronium graveolens(Anacardiaceae) in tropical dry forest. Seeds were collected in fruit-traps beneath 22 trees to compare pre-dispersal seed predation by parrots and insects, and determine whether intensity of seed predation was related to fruit-crop size or the aggregation of fruiting conspecifics around focal trees. Ground-level exclosures were established to compare post-dispersal seed predation by vertebrates and insects. The lilac-crowned parrot predated 43% of seeds pre-dispersal, while insects predated only 1.3%. Intensity of pre-dispersal seed predation by parrots was significantly greater in high-fruiting 0.79-ha resource patches, and was not related to fruit abundance of the focal tree. Foraging parrots also discarded immature fruits below the tree, causing a total 56% pre-dispersal loss of seed production, which was greater than post-dispersal removal by vertebrates, mainly rodents (51%) or insects (36%). Our results show that parrots play an important role as pre-dispersal seed predators in tropical dry forests. The reduction of parrot populations in tropical forests may have consequences for seed predation, affecting recruitment patterns of canopy trees.

Sex-dependent seasonal feeding activity variations among two species of Nymphalidae (Lepidoptera) in the Mexican tropical dry forest

abstract Tropical dry forest, such as that found in Mexico, is a biome where very few alimentary resources are available during 6 months of the year. Within some families of Lepidoptera such as Nymphalidae, some species are known to survive the dry season as adults using the few decayed pulp fruits available as a food source. We compared the temporal abundance and sex ratio of two model species, one belonging to the subfamily Charaxinae, found world-wide, and one belonging to the subfamily Biblidinae, endemic to the Americas. We used baited traps to measure feeding frequency of both species to asses monthly temporal resource use curves. The butterfly frequency of feeding drops dramatically in the dry season for all species-gender, except for females of the Biblidinae species. There are few differences between the sexes in the relative feeding frequency, assimilated to activity pattern, of Charaxinae between the dry and rainy seasons, while the activity pattern of Biblidinae is very different depending on the sex. These results suggest differential consequences on mate choice competition for both species. Females of the Charaxine species showed a use curve nested on the male curve, while for all other pair-wise comparisons between gender-species, the use curves were significantly different. These differences in the temporal foraging pattern could explain at least in part, the coexistence between both sympatric species in the stressful season.

Does the microarchitecture of Mexican dry forest foliage influence spider distribution

Spider species diversity has been associated with vegetation structure and stratification but there are few studies comparing the spider distribution in different shrubs and trees. In this study we analyzed the species distribution of the spider community of 11 shrub and tree species in two different study sites in a Mexican tropical dry forest. We present results from multivariate analyses that explain their distribution. A classification analysis based on spider abundances separated one shrub, Croton ciliatoglanduliferus, from the rest of the plant species. This was explained by the presence of large numbers of the oxyopid Peucetia viridans (Hentz 1832) on this plant. A second cluster segregated broad-leaved from small-leaved, bipinnate species. This was mainly due to higher spider abundances in the latter type of plants. Four vegetation variables were estimated and their influence on the species distribution was assessed by means of a principal components and regression analysis. With the exception of P. viridans, all spiders were positively associated with number of leaves and number of branchlets per 50 cm branch and negatively with foliage area.

Selective cutting of woody species in a Mexican tropical dry forest: Incompatibility between use and conservation

A major challenge of forest management is to maintain the biodiversity and integrity of the forests while at the same time satisfying human needs through productive activities. While selective extraction of natural resources has less severe consequences on biodiversity and ecosystem function than complete removal of vegetation, such consequences need to be evaluated in detail. “Varas” or stems cut from small trees of tropical dry forests (TDF) in the Pacific Coast of Mexico have been used as plant support stakes in horticultural fields (mainly tomato crops) since the middle of the last century. In this study, we evaluated the effects of selective cutting of plant support stakes on the diversity of woody vegetation of a TDF in northwestern Mexico. Stakes were cut by local harvesters before our assessment of cutting effects. In each of three cutting treatments (T 0 = uncut, T 1 = one cut event, and T 2 = two cut events), we established three sampling plots each consisting of ten 50 m × 2 m parallel transects. All woody plants (stems ≥1.0 cm diameter at 1.3 m height, DBH) were identified and measured on each transect. Species richness ( S) decreased as the number of cuts increased (T 0 = 65 species, T 1 = 50 species, and T 2 = 38 species). The Simpson ( C) and the Shannon ( H′) diversity indices, as well as the rarefaction curves and non-parametric estimates of diversity (Chao1 and ACE) confirmed this tendency of change. Comparison of dominance–diversity curves showed that the woody plant community loses equitability with every additional stake cutting event. The total number of stake providing species did not vary notably across treatments (T 0 = 8 species, T 1 = 9 species, and T 2 = 7 species), but four species reduced their dominance considerably in T 2, while Croton septemnervius, the most used species, increased its abundance and relative basal area with each additional stake harvest, reaching a representation of more than 59% of total number of stems in T 2. The reduction in species diversity, changes in patterns of dominance, and the proliferation of species associated to disturbed sites suggest that current practices of selective cutting require adjustments to make this forest management application more consistent with local conservation of woody plant species diversity and community structure.

Sources and Sinks of Diversification and Conservation Priorities for the Mexican Tropical Dry Forest

Elucidating the geographical history of diversification is critical for inferring where future diversification may occur and thus could be a valuable aid in determining conservation priorities. However, it has been difficult to recognize areas with a higher likelihood of promoting diversification. We reconstructed centres of origin of lineages and identified areas in the Mexican tropical dry forest that have been important centres of diversification (sources) and areas where species are maintained but where diversification is less likely to occur (diversity sinks). We used a molecular phylogeny of the genus Bursera, a dominant member of the forest, along with information on current species distributions. Results indicate that vast areas of the forest have historically functioned as diversity sinks, generating few or no extant Bursera lineages. Only a few areas have functioned as major engines of diversification. Long-term preservation of biodiversity may be promoted by incorporation of such knowledge in decision-making.

Nodulation dynamics and nodule activity in leguminous tree species of a Mexican tropical dry forest

Biological N fixation by symbiotic micro-organisms plays a key role in making atmospheric N largely available to other organisms and nodule formation by rhizobia in roots of many legume species represents a very important pathway (Postgate 1998). Estimates of the contribution of symbiotic fixation to total N content in soils range from 44 to 200 Tg y−1(Bøckman 1997).

Interactions Among Three Trophic Levels and Diversity of Parasitoids: A Case of Top-Down Processes in Mexican Tropical Dry Forest

The objective of this study was to analyze the relationship between plant hosts, galling insects, and their parasitoids in a tropical dry forest at Chamela-Cuixmala Biosphere Reserve in western Mexico. In 120 transects of 30 by 5 m (60 in deciduous forest and 60 in riparian habitats), 29 galling insects species were found and represented in the following order: Diptera (Cecidomyiidae, which induced the greatest abundance of galls with 22 species; 76%), Homoptera (Psylloidea, 6.9%; Psyllidae, 6.9%; Triozidae, 3.4%), Hymenoptera (Tanaostigmatidae, 3.4%; which were rare), and one unidentified morphospecies (3.4%). In all cases, there was a great specificity between galling insect species and their host plant species; one galling insect species was associated with one specific plant species. In contrast, there was no specificity between parasitoid species and their host galling insect species. Only 11 species of parasitoids were associated with 29 galling insect species represented in the following families: Torymidae (18.2%), Eurytomidae (18.2%), Eulophidae (18.2%), Eupelmidae (9.1%), Pteromalidae (9.1%), family Braconidae (9.1%), Platygastridae (9.1%), and one unidentified (9.1%). Most parasitoid species parasitized several gall species (Torymus sp.: 51.1%, Eurytoma sp.: 49.7%, Torymoides sp.: 46.9%). Therefore, the effects of variation in plant defenses do not extend to the third trophic level, because a few species of parasitoids can determine the community structure and composition of galling insect species in tropical plants, and instead, top-down processes seem to be regulating trophic interactions of galling insect species in tropical gall communities.

Interactions Among Three Trophic Levels and Diversity of Parasitoids: A Case of Top-Down Processes in Mexican Tropical Dry Forest

The objective of this study was to analyze the relationship between plant hosts, galling insects, and their parasitoids in a tropical dry forest at Chamela-Cuixmala Biosphere Reserve in western Mexico. In 120 transects of 30 by 5 m (60 in deciduous forest and 60 in riparian habitats), 29 galling insects species were found and represented in the following order: Diptera (Cecidomyiidae, which induced the greatest abundance of galls with 22 species; 76%), Homoptera (Psylloidea, 6.9%; Psyllidae, 6.9%; Triozidae, 3.4%), Hymenoptera (Tanaostigmatidae, 3.4%; which were rare), and one unidentified morphospecies (3.4%). In all cases, there was a great specificity between galling insect species and their host plant species; one galling insect species was associated with one specific plant species. In contrast, there was no specificity between parasitoid species and their host galling insect species. Only 11 species of parasitoids were associated with 29 galling insect species represented in the following families: Torymidae (18.2%), Eurytomidae (18.2%), Eulophidae (18.2%), Eupelmidae (9.1%), Pteromalidae (9.1%), family Braconidae (9.1%), Platygastridae (9.1%), and one unidentified (9.1%). Most parasitoid species parasitized several gall species (Torymus sp.: 51.1%, Eurytoma sp.: 49.7%, Torymoides sp.: 46.9%). Therefore, the effects of variation in plant defenses do not extend to the third trophic level, because a few species of parasitoids can determine the community structure and composition of galling insect species in tropical plants, and instead, top-down processes seem to be regulating trophic interactions of galling insect species in tropical gall communities.

Carrying Capacity and Potential Production of Ungulates for Human Use in a Mexican Tropical Dry Forest

ABSTRACTData are provided on the carrying capacity and potential production for sustainable human use of white‐tailed deer (Odocoileus virginianus) and collared peccary (Pecari tajacu) in a protected tropical dry forest at Chamela on the Pacific coast of Mexico. In this paper, the carrying capacity was defined as the equilibrium density plus the number of animals removed by predators. The equilibrium point was estimated from the density dependent relationship between the finite population growth rate and the current density according to a logistic model. Annual density was estimated using the line transect method. Carrying capacity estimates were 16.5 to 17.2 deer/km2 and 9.3–9.5 peccaries/km2, representing a combined biomass of 841–874 kg/km2. A potential production for human use of 2.1 deer/km2 and 4.4 peccaries/km2 was estimated employing the model of Robinson and Redford (1991). The data suggest that, in the protected tropical dry forest of Chamela, the density and biomass of wild ungulates can maintain a similar or greater density and biomass than other Neotropical forests. To obtain an accurate estimation of the maximum sustainable yield (MSY), it is necessary to consider predation. From a management point of view, it is important to consider that carrying capacity varies as a function of the rainfall pattern.

The impact of herbivore–plant coevolution on plant community structure

Coevolutionary theory proposes that the diversity of chemical structures found in plants is, in large part, the result of selection by herbivores. Because herbivores often feed on chemically similar plants, they should impose selective pressures on plants to diverge chemically or bias community assembly toward chemical divergence. Using a coevolved interaction between a group of chrysomelid beetles and their host plants, I tested whether coexisting plants of the Mexican tropical dry forest tend to be chemically more dissimilar than random. Results show that some of the communities are chemically overdispersed and that overdispersion is related to the tightness of the interaction between plants and herbivores and the spatial scale at which communities are measured. As coevolutionary specialization increases and spatial scale decreases, communities tend to be more chemically dissimilar. At fairly local scales and where herbivores have tight, one-to-one interactions with plants, communities have a strong pattern of chemical disparity.

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.

Preliminary Evidence of the Importance of ENSO in Modifying Food Availability for White‐tailed Deer in a Mexican Tropical Dry Forest1

ABSTRACTThe influence of El Niño/Southern Oscillation (ENSO) on rainfall and its possible effect on availability of food for white‐tailed deer (Odocoileus virginianus) in a tropical dry forest in the Pacific coast of Mexico was studied. From 1977 to 2003 there were three significant El Niño and La Niña events. During El Niño years rainfall decreased during the wet season ( June to October) and increased during the dry season (November to May), with the opposite effect during La Niña years. Plant diversity was monitored in permanent plots during the wet and dry seasons of 1989–1993. The results provide evidence that ENSO events affect deer food availability, particularly in the dry season.

Tree Diversity, Environmental Heterogeneity, and Productivity in a Mexican Tropical Dry Forest1

ABSTRACTSpecies diversity–environmental heterogeneity (D–EH) and species diversity–productivity (D–P) relationships have seldom been analyzed simultaneously even though such analyses could help to understand the processes underlying contrasts in species diversity among sites. Here we analyzed both relationships at a local scale for a highly diverse tropical dry forest of Mexico. We posed the following questions: (1) are environmental heterogeneity and productivity related?; (2) what are the shapes of D–EH and D–P relationships?; (3) what are individual, and interactive, contributions of these two variables to the observed variance in species diversity?; and (4) are patterns affected by sample size, or by partitioning into average local diversity and spatial species turnover? All trees (diameter at breast height ≥5 cm) within twenty‐six 0.2‐ha transects were censused; four environmental variables associated with water availability were combined into an environmental heterogeneity index; aboveground standing biomass was used as a productivity estimator. Simple and multiple linear and nonlinear regression models were run. Environmental heterogeneity and productivity were not correlated. We found consistently positive log‐linear D–EH and D–P relationships. Productivity explained a larger fraction of among‐transect variance in species diversity than did environmental heterogeneity. No effects of sample size were found. Different components of diversity varied in sensitivity to environmental heterogeneity and productivity. Our results suggest that species' differentiation along water availability gradients and species exclusion at the lowest productivity (driest) sites occur simultaneously, independently, and in a scale‐dependent fashion on the tree community of this forest.

The potential misapplication of rapid plant diversity assessment in tropical conservation

The rapid assessment of tropical plant biodiversity has become an important tool for the quantitative investigation of regional scale (between location) biogeographical patterns. However, the analysis presented here of local scale (within location) variation in a Mexican tropical dry forest suggests that a significant part of regional scale variation in this forest type may be an artefact of the undersampling of sites within locations. Such undersampling is common but is shown to have potentially serious implications for regional conservation assessment in tropical dry forest. It is argued that several sites within each location need to be sampled before the plant diversity at those locations can be meaningfully compared.

Abundance and Leaf Damage Caused by Gall‐Inducing Insects in a Mexican Tropical Dry Forest1

ABSTRACTWe evaluated the temporal and spatial patterns of abundance and the amount of damage caused by gall‐inducing insects (GII) in deciduous and riparian habitats in a seasonal tropical dry forest in Mexico. Plants occurring in these habitats differ in their phenology and moisture availability. Deciduous habitats are seasonal and xeric, while riparian habitats are aseasonal and mesic. We found 37 GII species and each one was associated with a specific plant species. In total, 19 species (51.3%) were present in deciduous habitats, 13 species (35.2%) in riparian habitats, and only 5 species (13.5%) occurred in both. Abundance and leaf damage by GII were greater in deciduous than in riparian habitats during the wet season. For each GII species that occurred in both habitats, host plant species supported greater abundance and leaf damage by GII in deciduous habitats during the wet season. These results indicate a greater association of GII species with host plants in deciduous than in riparian habitats during the wet season. In riparian habitats, 11 plant species (61.1%) had greater density of GII in the dry than in the wet season. Similarly, leaf damage by GII was significantly greater in the dry than in the wet season in riparian habitats for 12 plant species (66.7%). Dry forest plants of riparian habitats presented two peaks of leaf‐flushing: GII colonized leaves produced in the first peak at the beginning of the wet season, and accumulated or recolonized leaves in the second peak at the beginning of the dry season. The levels of leaf damage by GII detected in this study in the rainy season were considerably higher than those obtained for folivorous insects in other neotropical forests, suggesting that this GII guild might have an important impact on their host plant species in this tropical community.