Marco D. Visser

Functional traits as predictors of vital rates across the life cycle of tropical trees

Summary The ‘functional traits’ of species have been heralded as promising predictors for species’ demographic rates and life history. Multiple studies have linked plant species’ demographic rates to commonly measured traits. However, predictive power is usually low – raising questions about the practical usefulness of traits – and analyses have been limited to size-independent univariate approaches restricted to a particular life stage. Here we directly evaluated the predictive power of multiple traits simultaneously across the entire life cycle of 136 tropical tree species from central Panama. Using a model-averaging approach, we related wood density, seed mass, leaf mass per area and adult stature (maximum diameter) to onset of reproduction, seed production, seedling establishment, and growth and survival at seedling, sapling and adult stages. Three of the four traits analysed here (wood density, seed mass and adult stature) typically explained 20–60% of interspecific variation at a given vital rate and life stage. There were strong shifts in the importance of different traits throughout the life cycle of trees, with seed mass and adult stature being most important early in life, and wood density becoming most important after establishment. Every trait had opposing effects on different vital rates or at different life stages; for example, seed mass was associated with higher seedling establishment and lower initial survival, wood density with higher survival and lower growth, and adult stature with decreased juvenile but increased adult growth and survival. Forest dynamics are driven by the combined effects of all demographic processes across the full life cycle. Application of a multitrait and full-life cycle approach revealed the full role of key traits, and illuminated how trait effects on demography change through the life cycle. The effects of traits on one life stage or vital rate were sometimes offset by opposing effects at another stage, revealing the danger of drawing broad conclusions about functional trait–demography relationships from analysis of a single life stage or vital rate. Robust ecological and evolutionary conclusions about the roles of functional traits rely on an understanding of the relationships of traits to vital rates across all life stages.

Negative density dependence of seed dispersal and seedling recruitment in a Neotropical palm

Negative density dependence (NDD) of recruitment is pervasive in tropical tree species. We tested the hypotheses that seed dispersal is NDD, due to intraspecific competition for dispersers, and that this contributes to NDD of recruitment. We compared dispersal in the palm Attalea butyracea across a wide range of population density on Barro Colorado Island in Panama and assessed its consequences for seed distributions. We found that frugivore visitation, seed removal and dispersal distance all declined with population density of A. butyracea, demonstrating NDD of seed dispersal due to competition for dispersers. Furthermore, as population density increased, the distances of seeds from the nearest adult decreased, conspecific seed crowding increased and seedling recruitment success decreased, all patterns expected under poorer dispersal. Unexpectedly, however, our analyses showed that NDD of dispersal did not contribute substantially to these changes in the quality of the seed distribution; patterns with population density were dominated by effects due solely to increasing adult and seed density.

Tri‐trophic interactions affect density dependence of seed fate in a tropical forest palm

Natural enemies, especially host-specific enemies, are hypothesised to facilitate the coexistence of plant species by disproportionately inflicting more damage at increasing host abundance. However, few studies have assessed such Janzen-Connell mechanisms on a scale relevant for coexistence and no study has evaluated potential top-down influences on the specialized pests. We quantified seed predation by specialist invertebrates and generalist vertebrates, as well as larval predation on these invertebrates, for the Neotropical palm Attalea butyracea across ten 4-ha plots spanning 20-fold variation in palm density. As palm density increased, seed attack by bruchid beetles increased, whereas seed predation by rodents held constant. But because rodent predation on bruchid larvae increased disproportionately with increasing palm density, bruchid emergence rates and total seed predation by rodents and bruchids combined were both density-independent. Our results demonstrate that top-down effects can limit the potential of host-specific insects to induce negative-density dependence in plant populations.

Speeding Up Ecological and Evolutionary Computations in R; Essentials of High Performance Computing for Biologists

Computation has become a critical component of research in biology. A risk has emerged that computational and programming challenges may limit research scope, depth, and quality. We review various solutions to common computational efficiency problems in ecological and evolutionary research. Our review pulls together material that is currently scattered across many sources and emphasizes those techniques that are especially effective for typical ecological and environmental problems. We demonstrate how straightforward it can be to write efficient code and implement techniques such as profiling or parallel computing. We supply a newly developed R package (aprof) that helps to identify computational bottlenecks in R code and determine whether optimization can be effective. Our review is complemented by a practical set of examples and detailed Supporting Information material (S1–S3 Texts) that demonstrate large improvements in computational speed (ranging from 10.5 times to 14,000 times faster). By improving computational efficiency, biologists can feasibly solve more complex tasks, ask more ambitious questions, and include more sophisticated analyses in their research.

Tree species vary widely in their tolerance for liana infestation: a case study of differential host response to generalist parasites

This study was supported by the Netherlands Organization for Scientific Research (NWO-ALW 801-01-009; M.D.V.), the Smithsonian Tropical Research Institute (M.D.V.) and the HSBC Climate Partnership (H.C.M.). The datasets were collected with funding from the National Science Foundation (DEB 0425651 & 0948585 to S.P.H.; 0453445 to H.C.M.; 0453665 to S.J.W.; 0613666, 0845071, 1019436 & 1558093 to S.A.S., and 1242622 & 1464389 to L.S.C.), the Smithsonian Tropical Research Institute, the Centre for Tropical Forest Science, the John D. and Catherine T. MacArthur Foundation, the Mellon Foundation and the Small World Institute Fund.

Surviving in a Cosexual World: A Cost-Benefit Analysis of Dioecy in Tropical Trees

Dioecy has a demographic disadvantage compared with hermaphroditism: only about half of reproductive adults produce seeds. Dioecious species must therefore have fitness advantages to compensate for this cost through increased survival, growth, and/or reproduction. We used a full life cycle approach to quantify the demographic costs and benefits associated with dioecy while controlling for demographic differences between dioecious and hermaphroditic species related to other functional traits. The advantage of this novel approach is that we can focus on the effect of breeding system across a diverse tree community. We built a composite integral projection model for hermaphroditic and dioecious tree populations from Barro Colorado Island, Panama, using long-term demographic and newly collected reproductive data. Integration of all costs and benefits showed that compensation was realized through increased seed production, resulting in no net costs of dioecy. Compensation was also facilitated by the low contribution of reproduction to population growth. Estimated positive effects of dioecy on tree growth and survival were small and insignificant for population growth rates. Our model revealed that, for long-lived organisms, the cost of having males is smaller than generally expected. Hence, little compensation is required for dioecious species to maintain population growth rates similar to those of hermaphroditic species.

trackdem: Automated particle tracking to obtain population counts and size distributions from videos in r

The aim of trackdem is to obtain unbiased, automated estimates of population densities and body size distributions, using video material or image sequences as input. It is meant to assist in evolutionary and ecological studies, which often rely on accurate estimates of population size, population structure and/or individual behaviour. The package trackdem includes a set of functions to convert a short video into an image sequence, background detection, particle identification and linking, and the training of an artifical neural network for noise filtering. This vignette provides a step-by-step introduction on the usage of all functions to analyse image sequences of moving organisms. No movie files are required as all functions are illustrated by simulated image sequences.

A host–parasite model explains variation in liana infestation among co‐occurring tree species

The datasets were collected with funding from the National Science Foundation (DEB 0453445; 0453665; 0613666, 0845071, 1019436 & 1558093), Netherlands Organization for Scientific Research (NWO-ALW 801-01-009), the Smithsonian Tropical Research Institute, the Centre for Tropical Forest Science, the John D. and Catherine T. MacArthur Foundation, the Mellon Foundation and the Small World Institute Fund.