Arnošt L. Šizling

Between Geometry and Biology: The Problem of Universality of the Species-Area Relationship

The species-area relationship (SAR) is considered to be one of a few generalities in ecology, yet a universal model of its shape and slope has remained elusive. Recently, Harte et al. argued that the slope of the SAR for a given area is driven by a single parameter, the ratio between total number of individuals and number of species (i.e., the mean population size across species at a given scale). We provide a geometric interpretation of this dependence. At the same time, however, we show that this dependence cannot be universal across taxa: if it holds for a taxon composed from two subsets of species and also for one of its subsets, it cannot simultaneously hold for the other subset. Using three data sets, we show that the slope of the SAR considerably varies around the prediction. We estimate the limits of this variation by using geometric considerations, providing a theory based on species spatial turnover at different scales. We argue that the SAR cannot be strictly universal, but its slope at each particular scale varies within the constraints given by species’ spatial turnover at finer spatial scales, and this variation is biologically informative.

Species abundance distribution results from a spatial analogy of central limit theorem

The frequency distribution of species abundances [the species abundance distribution (SAD)] is considered to be a fundamental characteristic of community structure. It is almost invariably strongly right-skewed, with most species being rare. There has been much debate as to its exact properties and the processes from which it results. Here, we contend that an SAD for a study plot must be viewed as spliced from the SADs of many smaller nonoverlapping subplots covering that plot. We show that this splicing, if applied repeatedly to produce subplots of progressively larger size, leads to the observed shape of the SAD for the whole plot regardless of that of the SADs of those subplots. The widely reported shape of an SAD is thus likely to be driven by a spatial parallel of the central limit theorem, a statistically convergent process through which the SAD arises from small to large scales. Exact properties of the SAD are driven by species spatial turnover and the spatial autocorrelation of abundances, and can be predicted using this information. The theory therefore provides a direct link between SADs and the spatial correlation structure of species distributions, and thus between several fundamental descriptors of community structure. Moreover, the statistical process described may lie behind similar frequency distributions observed in many other scientific fields.

Rarity, Commonness, and the Contribution of Individual Species to Species Richness Patterns

Common species have a greater effect on observed geographical patterns of species richness than do rare ones. Here we present a theory of the relationship between individual species occurrence patterns and patterns in species richness, which allows purely geometrical and statistical causes to be distinguished from biological ones. Relationships between species occupancy and the correlation of species occurrence with overall species richness are driven by the frequency distribution of species richness among sites. Moreover, generally positive relationships are promoted by the fact that species occupancy distributions are mostly right skewed. However, biological processes can lead to deviations from the predicted pattern by changing the nestedness of a species’ spatial distribution with regard to the distributions of other species in an assemblage. We have applied our theory to data for European birds at several spatial scales and have identified the species with significantly stronger or weaker correspondence with the overall richness pattern than that predicted by the null model. In sum, whereas the general macroecological pattern of a stronger influence of common species than of rare species on species richness is predicted by mathematical considerations, the theory can reveal biologically important deviations at the level of individual species.

Changes in bird community composition in the Czech Republic from 1982 to 2004: increasing biotic homogenization, impacts of warming climate, but no trend in species richness

AbstractRecent major environmental changes could lead to homogenization in the composition of plant and animal communities, with generalist species replacing more specialized species, as well as to the increased domination of species adapted to warmer climatic conditions. Using common bird monitoring data, we have tested whether these patterns can be observed in the long-term changes in the structure and species richness of bird communities in the Czech Republic. We focused on a comparison of two time periods (1982–1990 and 1991–2004) that differ in land use (high agricultural intensity in the former, and a drop in agricultural intensity accompanied by forest expansion in the latter). We found that bird communities became less specialized and that this decline in specialization did not change after 1990. In contrast, taxonomic homogenization increased during the first time period but declined at the beginning of the second one. Moreover, the community composition developed towards a dominance of species breeding in warmer climates, and this change coincided with an increase in spring temperatures. Therefore, it would appear that both functional and taxonomic homogenization took place in the 1980s but the latter did not continue in subsequent decades. Species richness of local bird communities did not show any trend over time. We suggest that climate warming might be an important driving force of changes in the bird community composition in the Czech Republic, but the role of land-use changes is less clear, although it is probable that habitat specialists probably did not benefit from lower intensity of agricultural activities and increased forest cover.ZusammenfassungÄnderungen in der Zusammensetzung von Vogelgemeinschaften in der Tschechischen Republik zwischen 1982 und 2004: zunehmende biotische Vereinheitlichung, Einfluss von Klimaerwärmung, aber kein Trend im Artenreichtum Aktuelle große Veränderungen in der Umwelt könnten zu einer Vereinheitlichung der Zusammensetzung von Tier- und Pflanzengemeinschaften führen, wobei Generalisten die spezialisierteren Arten ersetzen und Arten dominieren, die an wärmere Klimate angepasst sind. Anhand normaler Vogel-Monitoring Daten testen wir in dieser Studie, ob sich diese Muster in langfristigen Veränderungen in der Struktur und dem Artenreichtum der Vogelgemeinschaften in der Tschechischen Republik beobachten lassen. Wir konzentrieren uns auf einen Vergleich zwischen zwei Zeiträumen (1982–1990 und 1991–2004), die sich in der Landnutzung unterscheiden (intensive Landwirtschaft im früheren und ein Absinken der Intensität der Landnutzung begleitet von einer Ausbreitung von Waldgebieten im letzteren). Wir fanden, dass die Vogelgemeinschaften weniger spezialisiert wurden, und diese Abnahme der Spezialisierung änderte sich nicht nach 1990. Dagegen stieg die taxonomische Vereinheitlichung während des ersten Zeitraums an, nahm aber am Beginn des zweiten wieder ab. Darüber hinaus entwickelte sich die Zusammensetzung der Gemeinschaft hin zu einer Dominanz von Arten, die ihre Brutgebiete in wärmeren Klimaten haben, und diese Änderung fiel zusammen mit einem Ansteigen der Frühlingstemperaturen. Daher scheint es, als hätten sowohl die funktionale als auch die taxonomische Vereinheitlichung in den 1980er Jahren stattgefunden, aber die erstere schritt später nicht fort. Der Artenreichtum lokaler Vogelgemeinschaften zeigte keinerlei Trend über die Zeit. Wir stellen zur Diskussion, dass die Klimaerwärmung eine treibende Kraft sein könnte für Änderungen in der Zusammenstellung der Vogelgemeinschaft in der Tschechischen Republik, jedoch ist die Rolle der Landnutzung weniger klar. Arten, die auf ein bestimmtes Habitat spezialisiert sind, konnten wahrscheinlich nicht profitieren von der weniger intensiven Landnutzung und der Zunahme der Wälder.

Invariance in species-abundance distributions

Many attempts to explain the species-abundance distribution (SAD) assume that it has a universal functional form which applies to most assemblages. However, if such a form does exist, then it has to be invariant under changes in the area of the study plot (the addition of neighboring areas or subdivision of the original area) and changes in taxonomic composition (the addition of sister taxa or subdivision to subtaxa). We developed a theory for such an area-and-taxon invariant SAD and derived a formula for such a distribution. Both the log-normal and our area-and-taxon invariant distribution fitted data well. However, the log-normal distributions of two adjoined sub-assemblages cannot be composed into a log-normal distribution for the resulting assemblage, and the SAD composed from two log-normal distributions fits the SAD for the assemblage poorly in comparison to the area-and-taxon invariant distribution. Observed abundance patterns therefore reveal area-and-taxon invariant properties absent in log-normal distributions, suggesting that multiplicative models generating log-normal-like SADs (including the power-fraction model) cannot be universally valid, as they necessarily apply only to particular scales and taxa.

Artifactions in the Log-Transformation of Species Abundance Distributions

One of the most frequently studied pattern in ecology is the Species Abundance Distribution (SAD) that represents the frequency distribution of species abundances in an assemblage. Two main approaches to displaying such information have been employed: histograms constructed using exponentially increasing bin widths as pioneered by Preston (1948), and plots of ranked species abundances. While both techniques have been extensively used in the investigation of community ecology hypotheses, the Preston-style species-abundance histogram has become central to current debates concerning appropriate characterization of the SAD and the processes generating it. Here we point out an important issue in the Preston approach that has profound implications to this debate: by employing bins of exponentially increasing size, the resultant histogram may display a hump-shaped pattern that is not congruent with the shape of the untransformed distribution. Moreover, any distribution constructed from log-transformed abundances will necessarily reveal at least one internal mode, even when the non-transformed probability density function is strictly decreasing. We warn against misinterpretation of such transformed datasets, and suggest that rank-abundance plots, which are equivalent to the cumulative distribution functions extensively used in other branches of science, represent a more informative approach as they allow for better discrimination between a number of probability distributions. Ecologists should be aware that logarithmic transformation often generates a log-normal-like shape, and are encouraged to use rank abundance curves to visualize and analyze species-abundance patterns.

Analytical evidence for scale-invariance in the shape of species abundance distributions

The distribution of species abundances within an ecological community provides a window into ecological processes and has important applications in conservation biology as an indicator of disturbance. Previous work indicates that species abundance distributions might be independent of the scales at which they are measured which has implications for data interpretation. Here we formulate an analytically tractable model for the species abundance distribution at different scales and discuss the biological relevance of its assumptions. Our model shows that as scale increases, the shape of the species abundance distribution converges to a particular shape given uniquely by the Jaccard index of spatial species turnover and by a parameter for the spatial correlation of abundances. Our model indicates that the shape of the species abundance distribution is taxon specific but does not depend on sample area, provided this area is large. We conclude that the species abundance distribution may indeed serve as an indicator of disturbances affecting species spatial turnover and that the assumption of conservation of energy in ecosystems, which is part of the Maximum Entropy approach, should be re-evaluated.

An alternative theoretical approach to escape decision-making: the role of visual cues.

Escape enables prey to avoid an approaching predator. The escape decision-making process has traditionally been interpreted using theoretical models that consider ultimate explanations based on the cost/benefit paradigm. Ultimate approaches, however, suffer from inseparable extra-assumptions due to an inability to accurately parameterize the models variables and their interactive relationships. In this study, we propose a mathematical model that uses intensity of predator-mediated visual stimuli as a basic cue for the escape response. We consider looming stimuli (i.e. expanding retinal image of the moving predator) as a cue to flight initiation distance (FID; distance at which escape begins) of incubating Mallards (Anas platyrhynchos). We then examine the relationship between FID, vegetation cover and directness of predator trajectory, and fit the resultant model to experimental data. As predicted by the model, vegetation concealment and directness of predator trajectory interact, with FID decreasing with increased concealment during a direct approach toward prey, but not during a tangential approach. Thus, we show that a simple proximate expectation, which involves only visual processing of a moving predator, may explain interactive effects of environmental and predator-induced variables on an escape response. We assume that our proximate approach, which offers a plausible and parsimonious explanation for variation in FID, may serve as an evolutionary background for traditional, ultimate explanations and should be incorporated into interpretation of escape behavior.

Scaling Biodiversity: Scaling species richness and distribution: uniting the species–area and species–energy relationships

Introduction Two macroecological patterns of species richness are sufficiently common and occur across such a wide range of taxa and geographic realms that they can be regarded as universal. The first is an increase in the number of species with the area sampled, the species–area relationship (hereafter SAR). The other is the relationship between species richness and the availability of energy that can be turned into biomass – the species–energy relationship (hereafter SER). Both patterns have a long history of exploration (e.g. Arrhenius, 1921; Gleason, 1922; Preston, 1960; Wright, 1983; Williamson, 1988; Currie, 1991; Rosenzweig, 1995; Waide et al., 1999; Gaston, 2000; Hawkins et al., 2003). However, attempts to interpret them within one unifying framework, or at least to relate them to each other, have been surprisingly rare. The most notable exception has been Wright’s (1983) attempt to derive both patterns from the assumed relationship between total energy availability (defined as the product of available area and energy input per unit area) and population size. According to this theory, both area and energy positively affect species’ population abundances, which decreases probabilities of population extinction, and thus increases the total number of species that can coexist on a site. Then, species richness should increase with increasing area or increasing energy in the same way. Although this theory can be valid in island situations where the total number of species is determined by the rate of extinctions which are not balanced by immigration events (MacArthur & Wilson, 1967), the situation on the mainland is more complicated. The local occurrence of a species is given not only by the viability of this population itself, but by the broader spatial context (Rosenzweig,

Invasiveness Does Not Predict Impact: Response of Native Land Snail Communities to Plant Invasions in Riparian Habitats

Studies of plant invasions rarely address impacts on molluscs. By comparing pairs of invaded and corresponding uninvaded plots in 96 sites in floodplain forests, we examined effects of four invasive alien plants (Impatiens glandulifera, Fallopia japonica, F. sachalinensis, and F.×bohemica) in the Czech Republic on communities of land snails. The richness and abundance of living land snail species were recorded separately for all species, rare species listed on the national Red List, and small species with shell size below 5 mm. The significant impacts ranged from 16–48% reduction in snail species numbers, and 29–90% reduction in abundance. Small species were especially prone to reduction in species richness by all four invasive plant taxa. Rare snails were also negatively impacted by all plant invaders, both in terms of species richness or abundance. Overall, the impacts on snails were invader-specific, differing among plant taxa. The strong effect of I. glandulifera could be related to the post-invasion decrease in abundance of tall nitrophilous native plant species that are a nutrient-rich food source for snails in riparian habitats. Fallopia sachalinensis had the strongest negative impact of the three knotweeds, which reflects differences in their canopy structure, microhabitat humidity and litter decomposition. The ranking of Fallopia taxa according to the strength of impacts on snail communities differs from ranking by their invasiveness, known from previous studies. This indicates that invasiveness does not simply translate to impacts of invasion and needs to be borne in mind by conservation and management authorities.