Ecosystem resilience depends on functional redundancy (the number of species contributing similarly to an ecosystem function) and response diversity (how functionally similar species respond differently to disturbance). Here, we explore how land-use change impacts these attributes in plant communities, using data from 18 land-use intensity gradients that represent five biomes and > 2800 species. We identify functional groups using multivariate analysis of plant traits which influence ecosystem processes. Functional redundancy is calculated as the species richness within each group, and response diversity as the multivariate within-group dispersion in response trait space, using traits that influence responses to disturbances. Meta-analysis across all datasets showed that land-use intensification significantly reduced both functional redundancy and response diversity, although specific relationships varied considerably among the different land-use gradients. These results indicate that intensified management of ecosystems for resource extraction can increase their vulnerability to future disturbances.

Land-use intensification reduces functionalredundancy and response diversity in plantcommunities

Large herbivores and carnivores (the megafauna) have been in a state of decline and extinction since the Late Pleistocene, both on land and more recently in the oceans. Much has been written on the timing and causes of these declines, but only recently has scientific attention focused on the consequences of these declines for ecosystem function. Here, we review progress in our understanding of how megafauna affect ecosystem physical and trophic structure, species composition, biogeochemistry, and climate, drawing on special features of PNAS and Ecography that have been published as a result of an international workshop on this topic held in Oxford in 2014. Insights emerging from this work have consequences for our understanding of changes in biosphere function since the Late Pleistocene and of the functioning of contemporary ecosystems, as well as offering a rationale and framework for scientifically informed restoration of megafaunal function where possible and appropriate.

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Megafauna and ecosystem function from the Pleistocene to the Anthropocene

1. Fire is a powerful ecological and evolutionary force that regulates organismal traits, population sizes, species interactions, community composition, carbon and nutrient cycling and ecosystem function. It also presents a rapidly growing societal challenge, due to both increasingly destructive wildfires and fire exclusion in fire-dependent ecosystems. As an ecological process, fire integrates complex feedbacks among biological, social and geophysical processes, requiring coordination across several fields and scales of study. 2. Here, we describe the diversity of ways in which fire operates as a fundamental ecological and evolutionary process on Earth. We explore research priorities in six categories of fire ecology: (a) characteristics of fire regimes, (b) changing fire regimes, (c) fire effects on above-ground ecology, (d) fire effects on below-ground ecology, (e) fire behaviour and (f) fire ecology modelling. 3. We identify three emergent themes: the need to study fire across temporal scales, to assess the mechanisms underlying a variety of ecological feedbacks involving fire and to improve representation of fire in a range of modelling contexts. 4. Synthesis: As fire regimes and our relationships with fire continue to change, prioritizing these research areas will facilitate understanding of the ecological causes and consequences of future fires and rethinking fire management alternatives.

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Fire as a fundamental ecological process: research advances and frontiers

The detrital-based food web of many streams and rivers plays a fundamental role in the cycling and retention of carbon and nutrients. However, we still need to understand which global mechanisms underlie the biogeochemical pathways that control energy transfer from the detrital pool through local food webs into nutrient and energy cycles and storage. Previous attempts to understand the variability in litter breakdown rates have included the search for latitudinal variation patterns and analysis of the influence of different factors. Here we complement those studies by developing a conceptual model to predict litter breakdown dynamics in low order streams. According to the model, litter breakdown rates and the relative role of microbial decomposers and shredder detritivores on this process are hierarchically governed by interactions between climate/hydrology and geology acting upon plant traits, nutrient and leaf availability to decomposers, and metabolism of microbial decomposers and shredders. The model explains variations in leaf litter breakdown rates and shredder abundance across large geographic areas, allowing the formulation of predictions of how anthropogenic pressures may affect litter breakdown rates.

A conceptual model of litter breakdown in low order streams

Most plant species have a range of traits that deter herbivores. However, understanding of how different defences are related to one another is surprisingly weak. Many authors argue that defence traits trade off against one another, while others argue that they form coordinated defence syndromes. We collected a dataset of unprecedented taxonomic and geographic scope (261 species spanning 80 families, from 75 sites across the globe) to investigate relationships among four chemical and six physical defences. Five of the 45 pairwise correlations between defence traits were significant and three of these were tradeoffs. The relationship between species' overall chemical and physical defence levels was marginally nonsignificant (P = 0.08), and remained nonsignificant after accounting for phylogeny, growth form and abundance. Neither categorical principal component analysis (PCA) nor hierarchical cluster analysis supported the idea that species displayed defence syndromes. Our results do not support arguments for tradeoffs or for coordinated defence syndromes. Rather, plants display a range of combinations of defence traits. We suggest this lack of consistent defence syndromes may be adaptive, resulting from selective pressure to deploy a different combination of defences to coexisting species.

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Correlations between physical and chemical defences in plants: tradeoffs, syndromes, or just many different ways to skin a herbivorous cat?

Fire is an important ecological factor that structures savannas, such as the cerrado, by selecting plant species and altering soil nutrient content. In Emas National Park, central Brazil, we compared soils under three different fire regimes and their relationship to the cerrado species they support. We collected 25 soil and vegetation samples at each site. We found differences in soil characteristics (p < 0.05), with fertility and fire frequency positively related: in the annually burned site we found higher values of organic matter, nitrogen, and clay, whereas in the protected site we detected lower values of pH and higher values of aluminum. We also observed differences in plant community structure, with distinct floristic compositions in each site. Floristic composition was more related to sand proportion (intra-set correlation = 0.834). Different fire frequencies increase environmental heterogeneity and beta diversity in the Brazilian cerrado.

Soil–vegetation relationships in cerrados under different fire frequencies

Plants have traits against herbivory that may occur together and increase defense efficiency. We tested whether there are defense syndromes in a cerrado community and, if so, whether there is a phylogenetic signal in them. We measured nine defense traits from a woodland cerrado community in southeastern Brazil. We tested the correlation between all pairs of traits and grouped the species into defense syndromes according to their traits. Most pairwise correlations of traits were complementary. Plants with lower specific leaf area also presented tougher leaves, with low nitrogen, more trichomes, and tannins. We found five syndromes: two with low defenses and high nutritional quality, two with high defenses and low nutritional quality, and one with traits compensating each other. There were two predominant strategies against herbivory in cerrado: “tolerance” and “low nutritional quality” syndromes. Phylogeny did not determine the suite of traits species presented. We argue that herbivory exerts significant selection pressure on these plant defense traits.

Defense syndromes against herbivory in a cerrado plant community

The reproductive phenology of plants is expected to be influenced by climatic factors and by the phylogenetic history of the species. In savannas, the peaks of flowering and fruiting are associated with climate seasonality. However, there is still a controversy about the reproductive phenology of plants in riparian forests, a vegetation type that does not experience a severe water shortage. We tested whether the reproductive peaks in riparian forests are different from those of the savannas. We also tested if the number of species in flower is correlated with rainfall and day length. We sought evidence of phylogenetic signals in the reproductive periods of the species. Most of species in savannas and riparian forests flowered and fruited in the wet season. The number of flowering species was positively correlated with monthly rainfall and day length. However, we did not find phylogenetic signals in the reproductive periods. The phenological pattern of riparian forests was similar to that of savannas. At the community level, the reproductive periods of plants may not be under phylogenetic constraints, as observed in forests under nonseasonal climate. Sun-related variables seem to be the main cues for the reproductive phenology of plants in savannas and riparian forests.

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Reproductive phenology of Brazilian savannas and riparian forests: environmental and phylogenetic issues

Differences in plant species richness and composition are associated with soil properties and disturbances such as fire, which can therefore be key determinants of species occurrence in savanna plant communities. We measured species richness, using nine plant functional traits and abundance to calculate three functional diversity indices. We then used model selection analyses to select the best model for predicting functional diversity and richness based on soil variables at sites with three different fire frequencies. We also calculated the community-weighted mean of each trait and used ordination to examine how traits changed across fire frequencies. We found higher species richness and functional dispersion at sites that were more fertile and where fire was frequent, and the opposite at such sites where fire was infrequent. However, soil properties influenced functional evenness and divergence only where fire was infrequent, with higher values where soils were poorer. Fire can change functional traits directly by hindering development of plants and indirectly by altering competition. Different fire frequencies lead to different plant-soil relationships, which can affect the functioning of tropical savanna communities. Functional diversity components and functional identity of the communities are both affected by fire frequency and soil conditions.

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Influence of fire history and soil properties on plant species richness and functional diversity in a neotropical savanna

O fogo e um importante agente evolutivo que pode causar alteracoes floristicas, filogeneticas e funcionais nas comunidades vegetais de cerrado, alterando a composicao do solo e modificando as interacoes interespecificas. Aqui discutimos os efeitos do fogo sobre a vegetacao de cerrado e levantamos sugestoes para o seu manejo em unidades de conservacao. Com especial enfase para trabalhos realizados no Parque Nacional das Emas, na fisionomia de campo cerrado, compilamos os seguintes resultados: em maiores frequencias de fogo (queimadas anuais ou bienais) ocorre agrupamento fenotipico, diminuicao da competicao, diminuicao da biomassa vegetal e enriquecimento dos solos; em menor frequencia (sem queimadas ha doze anos), ha maior competicao entre as especies e grande acumulo de biomassa seca. Alem disso, diferentes regimes de fogo suportam diferentes composicoes floristicas, com grupos de especies exclusivos em cada regime, tanto de especies herbaceo-subarbustivas quanto de arbustivo-arboreas. Portanto, sugerimos que seja mantido um mosaico com diferentes regimes de fogo e que se evitem areas de cerrado sem queimadas por muitos anos. Palavras-chave : biomassa vegetal, diversidade filogenetica, diversidade funcional, manejo, solo.

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Danilo Muniz da Silva

Papers

Soil–vegetation relationships in cerrados under different fire frequencies

Defense syndromes against herbivory in a cerrado plant community

Reproductive phenology of Brazilian savannas and riparian forests: environmental and phylogenetic issues

Influence of fire history and soil properties on plant species richness and functional diversity in a neotropical savanna

Os efeitos dos regimes de fogo sobre a vegetação de Cerrado no Parque Nacional das Emas, GO: considerações para a conservação da diversidade