Pervasive within-species spatial repulsion among adult tropical trees.

For species to coexist, performance must decline as the density of conspecific individuals increases. Although evidence for such conspecific negative density dependence (CNDD) exists in forests, the within-species spatial repulsion it should produce has rarely been demonstrated in adults. In this study, we show that in comparison to a null model of stochastic birth, death, and limited dispersal, the adults of dozens of tropical forest tree species show strong spatial repulsion, some to surprising distances of approximately 100 meters. We used simulations to show that such strong repulsion can only occur if CNDD considerably exceeds heterospecific negative density dependence-an even stronger condition required for coexistence-and that large-scale repulsion can indeed result from small-scale CNDD. These results demonstrate substantial niche differences between species that may stabilize species diversity.

Intraspecific variability in fluctuating environments: mechanisms of impact on species diversity.

Recent studies have found considerable trait variations within species. The effect of such intraspecific trait variability (ITV) on the stability, coexistence, and diversity of ecological communities received considerable attention and in many models it was shown to impede coexistence and decrease species diversity. Here we present a numerical study of the effect of genetically inherited ITV on species persistence and diversity in a temporally fluctuating environment. Two mechanisms are identified. First, ITV buffers populations against varying environmental conditions (portfolio effect) and reduces variation in abundances. Second, the interplay between ITV and environmental variations tends to increase the mean fitness of diverse populations. The first mechanism promotes persistence and tends to increase species richness, while the second reduces the chance of a rare species population (which is usually homogeneous) to invade, thus decreasing species richness. We show that for large communities the portfolio effect is dominant, leading to ITV promoting species persistence and richness.

Observation resolution critically influences movement-based foraging indices.

Movement-based indices such as moves per minute (MPM) and proportion time moving (PTM) are common methodologies to quantify foraging behavior. Hundreds of studies have reported these indices, many without specifying the temporal resolution of their original data, and others using varying resolutions. This was done despite the likelihood that observation resolution can affect MPM and PTM estimates. Our goal was to empirically determine the sensitivity of these foraging indices to changes in the temporal resolution of the observation. We used a high-speed camera to record movement sequences of 20 Acanthodactylus boskianus lizards. Then, we gradually decreased the resolution of the data and calculated the foraging indices at different temporal resolutions. When considering the range of temporal resolutions that are relevant for field observations with unassisted vision, we found 68% and 48% difference in MPM and PTM estimates, respectively. When using the highest resolution, our estimate of MPM was an order of magnitude higher than all prior reported values for lizards. Our results raise major concerns regarding the use of already published movement-based indices, and enable us to recommend how new foraging data should be collected.

How to use (and not to use) movement-based indices for quantifying foraging behaviour.

Movement-based indices such as moves per minute (MPM) and proportion time moving (PTM) are common methodologies to quantify foraging behaviour. We explore fundamental drawbacks of these indices that question the ways scientists have been using them and propose new solutions.To do so, we combined analytical and simulation models with lizards foraging data at the individual and species levels.We found that the maximal value of MPM is constrained by the minimal durations of moves and stops. As a result, foragers that rarely move and those that rarely stop are bounded to similar low MPM values. This implies that (1) MPM has very little meaning when used alone, (2) MPM and PTM are interdependent, and (3) certain areas in the MPM-PTM plane cannot be occupied. We also found that MPM suffers from inaccuracy and imprecision.We introduced a new bias correction formula for already published MPM data, and a novel index of changes per minute (CPM) that uses the frequency of changes between move and stop bouts. CPM is very similar to MPM, but does not suffer from bias. Finally, we suggested a new foraging plane of average move and average stop durations. We hope that our guidelines of how to use (and not to use) movement-based indices will add rigor to the study of animals' foraging behaviour.

A neutral theory with environmental stochasticity explains static and dynamic properties of ecological communities.

Understanding the forces shaping ecological communities is crucial to basic science and conservation. Neutral theory has made considerable progress in explaining static properties of communities, like species abundance distributions (SADs), with a simple and generic model, but was criticised for making unrealistic predictions of fundamental dynamic patterns and for being sensitive to interspecific differences in fitness. Here, we show that a generalised neutral theory incorporating environmental stochasticity may resolve these limitations. We apply the theory to real data (the tropical forest of Barro Colorado Island) and demonstrate that it much better explains the properties of short-term population fluctuations and the decay of compositional similarity with time, while retaining the ability to explain SADs. Furthermore, the predictions are considerably more robust to interspecific fitness differences. Our results suggest that this integration of niches and stochasticity may serve as a minimalistic framework explaining fundamental static and dynamic characteristics of ecological communities.

Niche versus neutrality: a dynamical analysis.

Understanding the forces shaping ecological communities is of crucial importance for basic science and conservation. After 50 years in which ecological theory has focused on either stable communities driven by niche-based forces or nonstable "neutral" communities driven by demographic stochasticity, contemporary theories suggest that ecological communities are driven by the simultaneous effects of both types of mechanisms. Here we examine this paradigm using the longest available records for the dynamics of tropical trees and breeding birds. Applying a macroecological approach and fluctuation analysis techniques borrowed from statistical physics, we show that both stabilizing mechanisms and demographic stochasticity fail to play a dominant role in shaping assemblages over time. Rather, community dynamics in these two very different systems is predominantly driven by environmental stochasticity. Clearly, the current melding of niche and neutral theories cannot account for such dynamics. Our results highlight the need for a new theory of community dynamics integrating environmental stochasticity with weak stabilizing forces and suggest that such theory may better describe the dynamics of ecological communities than current neutral theories, deterministic niche-based theories, or recent hybrids.

Temporal fluctuation scaling in populations and communities.

Taylor's law, one of the most widely accepted generalizations in ecology, states that the variance of a population abundance time series scales as a power law of its mean. Here we reexamine this law and the empirical evidence presented in support of it. Specifically, we show that the exponent generally depends on the length of the time series, and its value reflects the combined effect of many underlying mechanisms. Moreover, sampling errors alone, when presented on a double logarithmic scale, are sufficient to produce an apparent power law. This raises questions regarding the usefulness of Taylor's law for understanding ecological processes. As an alternative approach, we focus on short-term fluctuations and derive a generic null model for the variance-to-mean ratio in population time series from a demographic model that incorporates the combined effects of demographic and environmental stochasticity. After comparing the predictions of the proposed null model with the fluctuations observed in empirical data sets, we suggest an alternative expression for fluctuation scaling in population time series. Analyzing population fluctuations as we have proposed here may provide new applied (e.g., estimation of species persistence times) and theoretical (e.g., the neutral theory of biodiversity) insights that can be derived from more generally available short-term monitoring data.

Area-heterogeneity tradeoff and the diversity of ecological communities.

For more than 50 y ecologists have believed that spatial heterogeneity in habitat conditions promotes species richness by increasing opportunities for niche partitioning. However, a recent stochastic model combining the main elements of niche theory and island biogeography theory suggests that environmental heterogeneity has a general unimodal rather than a positive effect on species richness. This result was explained by an inherent tradeoff between environmental heterogeneity and the amount of suitable area available for individual species: for a given area, as heterogeneity increases, the amount of effective area available for individual species decreases, thereby reducing population sizes and increasing the likelihood of stochastic extinctions. Here we provide a comprehensive evaluation of this hypothesis. First we analyze an extensive database of breeding bird distribution in Catalonia and show that patterns of species richness, species abundance, and extinction rates are consistent with the predictions of the area-heterogeneity tradeoff and its proposed mechanisms. We then perform a metaanalysis of heterogeneity-diversity relationships in 54 published datasets and show that empirical data better fit the unimodal pattern predicted by the area-heterogeneity tradeoff than the positive pattern predicted by classic niche theory. Simulations in which species may have variable niche widths along a continuous environmental gradient are consistent with all empirical findings. The area-heterogeneity tradeoff brings a unique perspective to current theories of species diversity and has important implications for biodiversity conservation.