RESUMO
The forests of Amazonia are among the most biodiverse plant communities on Earth. Given the immediate threats posed by climate and land-use change, an improved understanding of how this extraordinary biodiversity is spatially organized is urgently required to develop effective conservation strategies. Most Amazonian tree species are extremely rare but a few are common across the region. Indeed, just 227 'hyperdominant' species account for >50% of all individuals >10 cm diameter at 1.3 m in height. Yet, the degree to which the phenomenon of hyperdominance is sensitive to tree size, the extent to which the composition of dominant species changes with size class and how evolutionary history constrains tree hyperdominance, all remain unknown. Here, we use a large floristic dataset to show that, while hyperdominance is a universal phenomenon across forest strata, different species dominate the forest understory, midstory and canopy. We further find that, although species belonging to a range of phylogenetically dispersed lineages have become hyperdominant in small size classes, hyperdominants in large size classes are restricted to a few lineages. Our results demonstrate that it is essential to consider all forest strata to understand regional patterns of dominance and composition in Amazonia. More generally, through the lens of 654 hyperdominant species, we outline a tractable pathway for understanding the functioning of half of Amazonian forests across vertical strata and geographical locations.
Assuntos
Florestas , Árvores , Biodiversidade , Brasil , HumanosRESUMO
Plant secondary metabolites play important ecological and evolutionary roles, most notably in the deterrence of natural enemies. The classical theory explaining the evolution of plant chemical diversity is that new defences arise through a pairwise co-evolutionary arms race between plants and their specialized natural enemies. However, plant species are bombarded by dozens of different herbivore taxa from disparate phylogenetic lineages that span a wide range of feeding strategies and have distinctive physiological constraints that interact differently with particular plant metabolites. How do plant defence chemicals evolve under such multiple and potentially contrasting selective pressures imposed by diverse herbivore communities? To tackle this question, we exhaustively characterized the chemical diversity and insect herbivore fauna from 31 sympatric species of Amazonian Protieae (Burseraceae) trees. Using a combination of phylogenetic, metabolomic and statistical learning tools, we show that secondary metabolites that were associated with repelling herbivores (1) were more frequent across the Protieae phylogeny and (2) were found in average higher abundance than other compounds. Our findings suggest that generalist herbivores can play an important role in shaping plant chemical diversity and support the hypothesis that chemical diversity can also arise from the cumulative outcome of multiple diffuse interactions.
Assuntos
Burseraceae/química , Evolução Molecular , Cadeia Alimentar , Herbivoria , Insetos/fisiologia , Metaboloma , Animais , Burseraceae/classificação , Metabolômica , Modelos Estatísticos , Peru , Filogenia , Árvores/química , Árvores/classificaçãoRESUMO
Arthropods represent most of global biodiversity, with the highest diversity found in tropical rain forests. Nevertheless, we have a very incomplete understanding of how tropical arthropod communities are assembled. We conducted a comprehensive mass sampling of arthropod communities within three major habitat types of lowland Amazonian rain forest, including terra firme clay, white-sand and seasonally flooded forests in Peru and French Guiana. We examined how taxonomic and functional composition (at the family level) differed across these habitat types in the two regions. The overall arthropod community composition exhibited strong turnover among habitats and between regions. In particular, seasonally flooded forest habitats of both regions comprised unique assemblages. Overall, 17·7% (26 of 147) of arthropod families showed significant preferences for a particular habitat type. We present a first reproducible arthropod functional classification among the 147 taxa based on similarity among 21 functional traits describing feeding source, major mouthparts and microhabitats inhabited by each taxon. We identified seven distinct functional groups whose relative abundance contrasted strongly across the three habitats, with sap and leaf feeders showing higher abundances in terra firme clay forest. Our novel arthropod functional classification provides an important complement to link these contrasting patterns of composition to differences in forest functioning across geographical and environmental gradients. This study underlines that both environment and biogeographical processes are responsible for driving arthropod taxonomic composition while environmental filtering is the main driver of the variance in functional composition.
Assuntos
Artrópodes/fisiologia , Biodiversidade , Floresta Úmida , Animais , Artrópodes/classificação , Guiana Francesa , PeruRESUMO
Herbivores are often implicated in the generation of the extraordinarily diverse tropical flora. One hypothesis linking enemies to plant diversification posits that the evolution of novel defenses allows plants to escape their enemies and expand their ranges. When range expansion involves entering a new habitat type, this could accelerate defense evolution if habitats contain different assemblages of herbivores and/or divergent resource availabilities that affect plant defense allocation. We evaluated this hypothesis by investigating two sister habitat specialist ecotypes of Protium subserratum (Burseraceae), a common Amazonian tree that occurs in white-sand and terra firme forests. We collected insect herbivores feeding on the plants, assessed whether growth differences between habitats were genetically based using a reciprocal transplant experiment, and sampled multiple populations of both lineages for defense chemistry. Protium subserratum plants were attacked mainly by chrysomelid beetles and cicadellid hemipterans. Assemblages of insect herbivores were dissimilar between populations of ecotypes from different habitats, as well as from the same habitat 100 km distant. Populations from terra firme habitats grew significantly faster than white-sand populations; they were taller, produced more leaf area, and had more chlorophyll. White-sand populations expressed more dry mass of secondary compounds and accumulated more flavone glycosides and oxidized terpenes, whereas terra firme populations produced a coumaroylquinic acid that was absent from white-sand populations. We interpret these results as strong evidence that herbivores and resource availability select for divergent types and amounts of defense investment in white-sand and terra firme lineages of Protium subserratum, which may contribute to habitat-mediated speciation in these trees.
Assuntos
Evolução Biológica , Burseraceae/fisiologia , Ecossistema , Insetos/fisiologia , Animais , Brasil , Burseraceae/química , Burseraceae/genética , Herbivoria , Densidade Demográfica , Solo , ÁrvoresRESUMO
Tropical forests include a diversity of habitats, which has led to specialization in plants. Near Iquitos, in the Peruvian Amazon, nutrient-rich clay forests surround nutrient-poor white-sand forests, each harboring a unique composition of habitat specialist trees. We tested the hypothesis that the combination of impoverished soils and herbivory creates strong natural selection for plant defenses in white-sand forest, while rapid growth is favored in clay forests. Recently, we reported evidence from a reciprocal-transplant experiment that manipulated the presence of herbivores and involved 20 species from six genera, including phylogenetically independent pairs of closely related white-sand and clay specialists. When protected from herbivores, clay specialists exhibited faster growth rates than white-sand specialists in both habitats. But, when unprotected, white-sand specialists outperformed clay specialists in white-sand habitat, and clay specialists outperformed white-sand specialists in clay habitat. Here we test further the hypothesis that the growth defense trade-off contributes to habitat specialization by comparing patterns of growth, herbivory, and defensive traits in these same six genera of white-sand and clay specialists. While the probability of herbivore attack did not differ between the two habitats, an artificial defoliation experiment showed that the impact of herbivory on plant mortality was significantly greater in white-sand forests. We quantified the amount of terpenes, phenolics, leaf toughness, and available foliar protein for the plants in the experiment. Different genera invested in different defensive strategies, and we found strong evidence for phylogenetic constraint in defense type. Overall, however, we found significantly higher total defense investment for white-sand specialists, relative to their clay specialist congeners. Furthermore, herbivore resistance consistently exhibited a significant trade-off against growth rate in each of the six phylogenetically independent species-pairs. These results confirm theoretical predictions that a trade-off exists between growth rate and defense investment, causing white-sand and clay specialists to evolve divergent strategies. We propose that the growth-defense trade-off is universal and provides an important mechanism by which herbivores govern plant distribution patterns across resource gradients.
Assuntos
Ecossistema , Meio Ambiente , Desenvolvimento Vegetal , Árvores , Animais , Cromatografia Gasosa-Espectrometria de Massas , Geografia , Insetos/classificação , Peru , Fenóis/análise , Folhas de Planta/fisiologia , Fenômenos Fisiológicos Vegetais , Proteínas de Plantas/análise , Plantas/classificação , Solo , Terpenos/análiseRESUMO
Environmental heterogeneity in the tropics is thought to lead to specialization in plants and thereby contribute to the diversity of the tropical flora. We examine this idea with data on the habitat specificity of 35 western Amazonian species from the genera Protium, Crepidospermum, and Tetragastris in the monophyletic tribe Protieae (Burseraceae) mapped on a molecular-based phylogeny. We surveyed three edaphic habitats that occur throughout terra firme Amazonia: white-sand, clay, and terrace soils in eight forests across more than 2000 km in the western Amazon. Twenty-six of the 35 species were found to be associated with only one of three soil types, and no species was associated with all three habitats; this pattern of edaphic specialization was consistent across the entire region. Habitat association mapped onto the phylogenetic tree shows association with terrace soils to be the probable ancestral state in the group, with subsequent speciation events onto clay and white-sand soils. The repeated gain of clay association within the clade likely coincides with the emergence of large areas of clay soils in the Miocene deposited during the Andean uplift. Character optimizations revealed that soil association was not phylogenetically clustered for white-sand and clay specialists, suggesting repeated independent evolution of soil specificity is common within the Protieae. This phylogenetic analysis also showed that multiple cases of putative sister taxa with parapatric distributions differ in their edaphic associations, suggesting that edaphic heterogeneity was an important driver of speciation in the Protieae in the Amazon basin.
Assuntos
Burseraceae/genética , Meio Ambiente , Filogenia , Solo/análise , Sequência de Bases , Teorema de Bayes , Burseraceae/fisiologia , DNA Espaçador Ribossômico/genética , Equador , Geografia , Modelos Genéticos , Dados de Sequência Molecular , Peru , Análise de Sequência de DNA , Especificidade da EspécieRESUMO
In an edaphically heterogeneous area in the Peruvian Amazon, clay soils and nutrient-poor white sands each harbor distinctive plant communities. To determine whether a trade-off between growth and antiherbivore defense enforces habitat specialization on these two soil types, we conducted a reciprocal transplant study of seedlings of 20 species from six genera of phylogenetically independent pairs of edaphic specialist trees and manipulated the presence of herbivores. Clay specialist species grew significantly faster than white-sand specialists in both soil types when protected from herbivores. However, when unprotected, white-sand specialists dominated in white-sand forests and clay specialists dominated in clay forests. Therefore, habitat specialization in this system results from an interaction of herbivore pressure with soil type.