Abundance of macrofauna in dense seagrass is due to habitat preference,not predation

Summary Two main hypotheses compete to explain why prey abundance decreases when seagrass density is reduced. One proposes that predators are more successful amongst seagrass of lower density; the other invokes habitat choice by prey. We reduced the density of seagrass in the presence, and in the absence, of predators in a field experiment to discriminate between these hypotheses. When seagrass was manipulated abundances of all six prey species decreased simultaneously both in the presence and in the absence of predators. We conclude that correlations of prey abundance and shoot density within a seagrass bed are proximately due to habitat preference of dense seagrass by prey. We report another experiment which supports this conclusion and shows that habitat preference is exercised at the earliest opportunity. However, the habitat preferences may have been selected by predation pressure.     

Germination biology of selected central Australian plants

Abstract Risk spreading of germination may be particularly common in environments with unpredictable climates. Germinability, propensity to germinate at different temperatures and germination speed were classified for seeds of 105 species from the central Australian arid zone, and related to plant growth form, perenniality, seed size and seed dispersal mode. Almost all species had at least some seeds which were dormant, consistent with the idea that risk spreading is important in arid zones. Dispersal mode and plant perenniality were not found to be associated with germinability. Seeds of most species germinated rapidly relative to what has been recorded from higher-rainfall environments, as might be expected in an environment where wet soils are usually temporary. Faster germination tended to be associated with low germinability, suggesting a spectrum of strategies from species that risk a small number of their seeds in many rainfall events, to those that germinate only in large rainfall events but then risk large numbers of seeds.     

Is there a latitudinal gradient in seed production?

We compiled data from seed rain studies at 33 sites from around the world to determine whether the greater mean seed mass of tropical plants is associated with production of fewer seeds per square meter of ground. We found no significant linear relationship between latitude and annual seed rain density, but found some evidence for a mid‐latitude peak in seed rain density (quadratic relationship, p=0.018; R²=0.23). Combining seed rain data with seed mass data suggests that vegetation at the equator produces between 19 and 128 times more total mass of seed per year than does vegetation at 60°. This gradient in seed production would far outweigh the doubling in net primary productivity (NPP) over the same range of latitudes. Thus, our (admittedly small) dataset suggests that tropical vegetation allocates a much greater proportion of NPP to reproduction. This raises two important questions for the future: 1) why might tropical vegetation commit more energy to seed production than vegetation further from the equator? 2) What aspect of plant growth might receive proportionally less energy in tropical ecosystems?     

A roadmap to plant functional island biogeography

Island biogeography is the study of the spatio-temporal distribution of species, communities, assemblages or ecosystems on islands and other isolated habitats. Island diversity is structured by five classes of process: dispersal, establishment, biotic interactions, extinction and evolution. Classical approaches in island biogeography focused on species richness as the deterministic outcome of these processes. This has proved fruitful, but species traits can potentially offer new biological insights into the processes by which island life assembles and why some species perform better at colonising and persisting on islands. Functional traits refer to morphological and phenological characteristics of an organism or species that can be linked to its ecological strategy and that scale up from individual plants to properties of communities and ecosystems. A baseline hypothesis is for traits and ecological strategies of island species to show similar patterns as a matched mainland environment. However, strong dispersal, environmental and biotic-interaction filters as well as stochasticity associated with insularity modify this baseline. Clades that do colonise often embark on distinct ecological and evolutionary pathways, some because of distinctive evolutionary forces on islands, and some because of the opportunities offered by freedom from competitors or herbivores or the absence of mutualists. Functional traits are expected to be shaped by these processes. Here, we review and discuss the potential for integrating functional traits into island biogeography. While we focus on plants, the general considerations and concepts may be extended to other groups of organisms. We evaluate how functional traits on islands relate to core principles of species dispersal, establishment, extinction, reproduction, biotic interactions, evolution and conservation. We formulate existing knowledge as 33 working hypotheses. Some of these are grounded on firm empirical evidence, others provide opportunities for future research. We organise our hypotheses under five overarching sections. Section A focuses on plant functional traits enabling species dispersal to islands. Section B discusses how traits help to predict species establishment, successional trajectories and natural extinctions on islands. Section C reviews how traits indicate species biotic interactions and reproduction strategies and which traits promote intra-island dispersal. Section D discusses how evolution on islands leads to predictable changes in trait values and which traits are most susceptible to change. Section E debates how functional ecology can be used to study multiple drivers of global change on islands and to formulate effective conservation measures. Islands have a justified reputation as research models. They illuminate the forces operating within mainland communities by showing what happens when those forces are released or changed. We believe that the lens of functional ecology can shed more light on these forces than research approaches that do not consider functional differences among species.