Miguel Verdú

EARLY EMERGENCE ENHANCES PLANT FITNESS: A PHYLOGENETICALLY CONTROLLED META‐ANALYSIS

The time at which a seedling emerges can determine its future success as a plant. Despite the large number of studies that have examined the effect of emergence time on different components of plant fitness (survival, growth, and/or fecundity), the potential evolutionary response to selection on seedling emergence date is still poorly known. In this study, we review 55 of those studies by a random-effects meta-analysis, considering the phylogenetic relatedness among taxa. We test the following hypotheses: (1) early emergence increases seedling survival, growth, and fecundity, (2) early emergence is more advantageous to large-seeded species than to small-seeded ones, as the former can compensate for the lower number of seeds by increasing seedling survival, (3) perennial plants benefit more than annuals from early emergence, as the iteroparity of the former allows them to risk seedling emergence to the best conditions each year, whereas the semelparity of the latter forces them to spread the risk of emergence over time, and (4) the effect of emergence time may depend upon the experimental conditions (field vs. controlled experiments in a greenhouse or laboratory). Our results show that early emergence differentially affects components of plant fitness, with no effect on seedling survival but large benefits to seedling growth and fecundity. Such effects vary depending upon intrinsic factors like seed size or life-form, and also upon methodology (census time and experimental conditions). Large-seeded species gain from emerging early by growing more during their first growing seasons, although they survive and reproduce similarly to small-seeded species. The survival benefit of early emergence is greater in perennial than in annual species, thus supporting hypothesis 3. The relationship between emergence time and seedling growth appears to be stronger under controlled conditions than in the field, probably as a result of the unlimited nutrient and water resources of the former. In field conditions, in contrast, limited resources probably decelerate the growth of early seedlings, precluding the detection of differences between these and late seedlings.

Modern Quaternary plant lineages promote diversity through facilitation of ancient Tertiary lineages

One of the most important floristic sorting periods to affect modern plant communities occurred during the shift from the wet Tertiary period to the unusually dry Quaternary, when most global deserts developed. During this time, a wave of new plant species emerged, presumably in response to the new climate. Interestingly, most Tertiary species that have been tracked through the fossil record did not disappear but remained relatively abundant despite the development of a much more unfavorable climate for species adapted to moist conditions. Here we find, by integrating paleobotanical, ecological, and phylogenetic analyses, that a large number of ancient Tertiary species in Mediterranean-climate ecosystems appear to have been preserved by the facilitative or “nurse” effects of modern Quaternary species. Our results indicate that these interdependent relationships among plants have played a central role in the preservation of the global biodiversity and provided a mechanism for stabilizing selection and the conservation of ecological traits over evolutionary time scales.

Beyond species loss: the extinction of ecological interactions in a changing world

Summary 1. The effects of the present biodiversity crisis have been largely focused on the loss of species. However, a missed component of biodiversity loss that often accompanies or even precedes species disappearance is the extinction of ecological interactions. 2. Here, we propose a novel model that (i) relates the diversity of both species and interactions along a gradient of environmental deterioration and (ii) explores how the rate of loss of ecological functions, and consequently of ecosystem services, can be accelerated or restrained depending on how the rate of species loss covaries with the rate of interactions loss. 3. We find that the loss of species and interactions are decoupled, such that ecological interactions are often lost at a higher rate. This implies that the loss of ecological interactions may occur well before species disappearance, affecting species functionality and ecosystems services at a faster rate than species extinctions. We provide a number of empirical case studies illustrating these points. 4. Our approach emphasizes the importance of focusing on species interactions as the major biodiversity component from which the ‘health’ of ecosystems depends.

Ecological interactions are evolutionarily conserved across the entire tree of life

Ecological interactions are crucial to understanding both the ecology and the evolution of organisms. Because the phenotypic traits regulating species interactions are largely a legacy of their ancestors, it is widely assumed that ecological interactions are phylogenetically conserved, with closely related species interacting with similar partners. However, the existing empirical evidence is inadequate to appropriately evaluate the hypothesis of phylogenetic conservatism in ecological interactions, because it is both ecologically and taxonomically biased. In fact, most studies on the evolution of ecological interactions have focused on specialized organisms, such as some parasites or insect herbivores, belonging to a limited subset of the overall tree of life. Here we study the evolution of host use in a large and diverse group of interactions comprising both specialist and generalist acellular, unicellular and multicellular organisms. We show that, as previously found for specialized interactions, generalized interactions can be evolutionarily conserved. Significant phylogenetic conservatism of interaction patterns was equally likely to occur in symbiotic and non-symbiotic interactions, as well as in mutualistic and antagonistic interactions. Host-use differentiation among species was higher in phylogenetically conserved clades, irrespective of their generalization degree and taxonomic position within the tree of life. Our findings strongly suggest a shared pattern in the organization of biological systems through evolutionary time, mediated by marked conservatism of ecological interactions among taxa.

Nucleation processes in a mediterranean bird-dispersed plant

1. The spatial distribution of Pistacia lentiscus, a Mediterranean bird-dispersed plant, in abandoned orchards was found to be strongly linked to the presence of trees or shrubs that act as perches. 2. These perches not only attract seed-disperser birds but also produce favourable microenvironmental conditions for seed germination and seedling establishment. 3. Soil moisture content after a rainfall was always greater beneath perches than not beneath perches. Favourable water potentials for seed germination were maintained for a longer time beneath a perch than elsewhere. 4. After a rainfall, soil was compacted faster where not beneath perches. Seedling radicle penetration into soil was strongly associated with soil compactation.

The Jungle of Methods for Evaluating Phenotypic and Phylogenetic Structure of Communities

The way communities are assembled is an old ecological question currently experiencing renewed interest thanks to the recent advances in molecular biology and phylogenetics. The generality of these new methods has allowed us to understand the structure of communities of organisms from different kingdoms and at different scales. Concomitant with this growing interest, new methods, metrics, terms, and software have appeared that independently solve similar questions, but with different approaches. Here we provide a unifying framework on methods for community structure based on the relationships between four key concepts: phylogeny, phenotype, environment, and co-occurrence. The different approaches are based on different community representations of traits, the phylogenetic relationships of species in the community, or species occurrence along the environmental gradients. We finally provide insights on future directions of this emerging discipline.

The Nested Assembly of Plant Facilitation Networks Prevents Species Extinctions

Facilitation is a positive interaction assembling ecological communities and preserving global biodiversity. Although communities acquire emerging properties when many species interact, most of our knowledge about facilitation is based on studies between pairs of species. To understand how plant facilitation preserves biodiversity in complex ecological communities, we propose to move from the study of pairwise interactions to the network approach. We show that facilitation networks behave as mutualistic networks do, characterized by a nonrandom, nested structure of plant‐plant interactions in which a few generalist nurses facilitate a large number of species while the rest of the nurses facilitate only a subset of them. Consequently, generalist nurses shape a dense and highly connected network. Interestingly, such generalist nurses are the most abundant species in the community, making facilitation‐shaped communities strongly resistant to extinction, as revealed by coextinction simulations. The nested structure of facilitative networks explains why facilitation, by preventing extinction, preserves biodiversity.

Metagenome of the Mediterranean deep chlorophyll maximum studied by direct and fosmid library 454 pyrosequencing

The deep chlorophyll maximum (DCM) is a zone of maximal photosynthetic activity, generally located toward the base of the photic zone in lakes and oceans. In the tropical waters, this is a permanent feature, but in the Mediterranean and other temperate waters, the DCM is a seasonal phenomenon. The metagenome from a single sample of a mature Mediterranean DCM community has been 454 pyrosequenced both directly and after cloning in fosmids. This study is the first to be carried out at this sequencing depth (ca. 600 Mb combining direct and fosmid sequencing) at any DCM. Our results indicate a microbial community massively dominated by the high-light-adapted Prochlorococcus marinus subsp. pastoris, Synechococcus sp., and the heterotroph Candidatus Pelagibacter. The sequences retrieved were remarkably similar to the existing genome of P. marinus subsp. pastoris with a nucleotide identity over 98%. Besides, we found a large number of cyanophages that could prey on this microbe, although sequence conservation was much lower. The high abundance of phage sequences in the cellular size fraction indicated a remarkably high proportion of cells suffering phage lytic attack. In addition, several fosmids clearly belonging to Group II Euryarchaeota were retrieved and recruited many fragments from the total direct DNA sequencing suggesting that this group might be quite abundant in this habitat. The comparison between the direct and fosmids sequencing revealed a bias in the fosmid libraries against low-GC DNA and specifically against the two most dominant members of the community, Candidatus Pelagibacter and P. marinus subsp. pastoris, thus unexpectedly providing a feasible method to obtain large genomic fragments from other less prevalent members of this community.

FIRE REDUCES MORPHOSPACE OCCUPATION IN PLANT COMMUNITIES

The two main assembly processes claimed to structure plant communities are habitat filtering and competitive interactions. The set of species growing in fire-prone communities has been filtered in such a way that species without fire-persistence traits have not successfully entered the community. Because plant traits are evolutionarily conserved and fire traits are correlated with other plant traits, communities under high fire frequency should not include all possible trait combinations, and thus the morphospace occupation by species in these communities should be lower than expected by chance (underoccupied). In contrast, communities under low fire frequency would lack the filtering factor, and thus their underoccupation of the morphospace is not expected. We test this prediction by comparing the morphospace occupation by species in communities located in the western Mediterranean Basin, five of them subject to high fire frequency (HiFi) and four to low fire frequency (LowFi). We first compile a set of morphological and functional traits for the species growing on the nine sites, then we compute the morphospace occupation of each site as a convex hull volume, and finally, to assert that our results are not a product of a random branching pattern of evolution, we simulate our traits under a null model of neutral evolution and compare the morphospace occupation of the simulated traits with the results from the empirical data. The results suggest that, as predicted, there is a clear differential morphospace occupation between communities under different fire regimes in such a way that the morphospace is underoccupied in HiFi communities only. The simulation of a neutral evolutionary model does not replicate the observed pattern of differential morphospace occupation, and thus it should be attributed to assembly processes. In conclusion, our results suggest that fire is a strong community assembling process, filtering the species that have fire-persistent traits and thus assembling phenotypically and phylogenetically clustered communities with vacant zones in the morphospace.

The network structure of plant–arbuscular mycorrhizal fungi

Ecological network theory predicts that in mutualistic systems specialists tend to interact with a subset of species with which generalists interact (i.e. nestedness). Approaching plant-arbuscular mycorrhizal fungi (AMF) association using network analyses will allow the generality of this pattern to be expanded to the ubiquitous plant-AMF mutualism. Based on certain plant-AMF specificity recently suggested, networks are expected to be nested as a result of their mutualistic nature, and modular, with certain species interacting more tightly than others. Network analyses were used to test for nestedness and modularity and to compare the different contribution of plant and AMF to the overall nestedness. Plant-AMF networks share general network properties with other mutualisms. Plant species with few AMFs in their roots tend to associate with those AMFs recorded in most plant species. AMFs present in a few plant species occur in plant species sheltering most AMF (i.e. nestedness). This plant-AMF network presents weakly interlinked subsets of species, strongly connected internally (i.e. modularity). Both plants and AMF show a nested structure, although AMFs have lower nestedness than plants. The plant-AMF interaction pattern is interpreted in the context of how plant-AMF associations can be underlying mechanisms shaping plant community assemblages.