Hump-shaped Pattern Research Articles

Movement Strategies of Seed Predators as Determinants of Plant Recruitment Patterns

Plant recruitment in nature exhibits several distinctive patterns ranging from hump shaped to monotonically decreasing with distance from the seed source. We investigate the role of postdispersal seed predation in shaping these patterns, introducing a new mechanistic model that explicitly accounts for the movement strategy used by seed eaters. The model consists of two partial differential equations describing the spatiotemporal dynamics of both seed and predator densities. The movement strategy is defined by how predators move in response to the different cues they can use to search for seeds. These cues may be seed density, seed intake, distance from the plant, density of conspecific foragers, or a mixture of these four. The model is able to reproduce all the basic plant recruitment patterns found in the field. We compare the results to those of the ideal free distribution (IFD) theory and show that hump-shaped plant recruitment patterns cannot be generated by IFD predators and, in general, by foragers that respond exclusively to seed density. These foragers can produce only nonincreasing patterns, the shapes of which are determined by the foragers' navigation capacities. In contrast, hump-shaped patterns can be produced by distance-responsive predators or by foragers that use conspecifics as a cue for seed abundance.

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.

Temporal changes in species diversity and composition in abandoned fields in a trans-Himalayan landscape, Nepal

Secondary succession is an increasing phenomenon due to global changes in agriculture policies and practices. The empirical findings are biased towards the temperate zone. Abandonment of agriculture fields is less frequent in the subtropical and tropical zones where agriculture areas are, in general, expanding. But there are exceptions; a rapid rate of abandonment of agricultural fields have taken place in the arid trans-Himalayan region, due to today’s globalization of economy. We analysed agriculture fields that were abandoned between 1950 and 2003 in a large u-valley in central Nepal (3400 m a.s.l.). The potential forest vegetation is dominated by Pinus wallichina and shrubs of junipers and cotoneaster species. We tested the intermediate richness hypothesis in relation to vegetation cover, soil development and whether old-field succession is convergent or divergent with species data from 242 1 m2 plots in 5 age-classes. The main species compositional turnover expressed by Detrended Correspondence Analyses (DCA) correlated, as expected, with time after abandonment. Fields that were abandoned a long time ago are closer to forest at the periphery of the agricultural landscape. Moisture of the soil significantly increased with age of abandonment, but total vegetation cover and pH were negatively related to age. Beta diversity expressed in DCA SD-units showed an increasing trend with age of abandonment, supporting the divergence pattern in old-field succession. The reason why the succession is not converging may be due to browsing by domestic animals that prevent a closed canopy of pines and juniper to develop. There was a significant hump-shaped pattern in species richness along the temporal gradient, which agrees with the intermediate species-richness hypothesis. There was a rapid increase in species richness in plots close to the villages that were used for haymaking which increased the seed input significantly.

Spatial variability in woody species richness along altitudinal gradient in a lowland-dryland site, Lokapel Turkana, Kenya

The woody species richness patterns in three 2–4 km long transects, approximately 1–3 km apart in a lowland (600–700 m) dryland around Lokapel in Turkana northern Kenya was analyzed in 2003 at 200–500 m intervals using the Point-Centred Quarter (PCQ) method involving 51 observation points. Transect 1 and 2 were set along ephemeral runoff channels locally known as lagga with wet season flow westwards from the Lokapel Hills to the Turkwell River. Transect 3 was a cross-cutting profile dissecting the area initially downhill from the Lokapel Hills and later gently uphill eastwards towards Lokichar. The altitude at each of the 51 observation sites was recorded using a GPS and the woody species identified through local knowledge and taxonomic aids. The results showed that the overall integrated altitudinal gradient for the three transects was approximately 100 m. A total of 43 species of trees and shrubs were identified. The Shannon index showed that Transect 2 had the highest diversity of woody species followed by Transect 1 and Transect 3 while the Sorensen’s index indicated qualitative dissimilarity between all the transects. The results of regression analysis indicated that woody species richness increased linearly with elevation in only one transect but regression analysis of height of woody plants and altitude indicated that only about 20% of the variation in the height of woody plants was accountable by altitude. The spatial analysis of woody species-richness and altitudinal gradient showed a dual peak pattern with the main richness peak in low lying areas below 700 m which was mainly within or close to the riparian floodplain environment of the Turkwell River. A minor richness peak was also identified in higher lying areas around the Lokapel hills. The species richness pattern was similar to the hump-shaped altitudinal species-richness pattern which has been recorded widely around the world but mainly in large-scale studies.

African American and White Differences in the Impacts of Monetary Policy on the Duration of Unemployment

An emerging literature documents monetary policy’s impacts on labor market outcomes. For example, Willem Thorbecke (200 ) and Seth B. Carpenter and William M. Rodgers III (2004) find that recursive vector autoregressions (VAR) and autoregressive distributed lag (ADL) models indicate that the response of the employmentpopulation-ratio, unemployment rate, and labor force participation rate to increases in the federal funds rate follows the hump-shaped pattern in which the peak impact occurs several quarters after the initial shock (Carl E. Walsh 2003). An interesting dynamic occurs. The drop in aggregate demand reduces the number of available jobs. This reduction lengthens job search, but not to the extent that job seekers leave the labor force. An extension of this work is to identify the type of unemployment that contractionary monetary policy generates. Is the unemployment short or long term? To answer this question, Rodgers (2007) uses published Bureau of Labor Statistics (BLS) data on unemployment duration and type of unemployment to estimate models to assess whether job loss is concentrated among shortor long-term spells, or whether a ripple

Life-Cycle Portfolio Choice with a Realistic Retirement Age Distribution

This paper numerically solves the optimal life-cycle portfolio choice when the model is calibrated to match the empirical retirement age distribution: people tend to retire only starting from their 50s. The model shows that financial incentives for keeping investors in labor force and low leisure preference for young investors endogenously restrict investors from retiring early. Thus, this paper suggests a novel effect of the early retirement option on portfolio choice. As opposed to results from earlier models, the optimal portfolio share of stock does not increase monotonically prior to retirement. Wealthy investors might find it optimal to reduce the stock share in their early stage of life in order to decrease the possibility of having insufficient wealth for retirement when they are older. Thus, the model predicts either an increasing or a hump-shaped pattern for life-cycle stock holding, consistent with empirical observations.

Impact of two different flood pulses on planktonic communities of the largest floodplain lakes of the Daugava River (Latvia)

An impact of two different flood pulses on phyto- and zooplankton communities of the two largest floodplain lakes of the Daugava River were studied in the spring and summer of 2005. Samples of phyto- and zooplankton were taken at weekly and biweekly intervals. At the end of March, a medium size pulse of spring flood was observed. At the beginning of May, it was followed by an unusually high pulse of flush floods caused by heavy rainstorms in the local drainage area. An overall increase of biomass and the number of taxa of planktonic communities during the filling and drainage phases of the spring floods was stated. The pulse of the flush floods resulted in a lower total biomass and higher species diversity, and can be regarded as a disturbance event. The high species diversity represented by a hump-shaped pattern caused by an intermediate disturbance that was measured by the rate of water level change during the floods according to Intermediate Disturbance Hypothesis (IDH). This study reflected both linear and a slight hump-shaped relationships between the rate of water level change and the number of phytoplankton and zooplankton taxa.

The Sales Effect of Word of Mouth: A Model for Creative Goods and Estimates for Novels

Weekly sales of creative goods - like music records, movies or books - usually peak shortly after release and then decline quickly. In many cases, however, they follow a hump-shaped pattern where sales increase for some time. A popular explanation for this phenomenon is word of mouth among a population of heterogeneous buyers, but previous studies typically assume buyer homogeneity or neglect word of mouth altogether. In this paper, I study a model of new-product diffusion with heterogeneous buyers that allows for a quantification of the sales effect of word of mouth. The model includes Christmas sales as a special case. All parameters have an intuitive interpretation. Simulation results suggest that the parameters are estimable for data that are not too volatile and that cover a sufficiently large part of a title's life cycle. I estimate the model for four exemplary novels using scanner data on weekly sales.

The sales effect of word of mouth: a model for creative goods and estimates for novels

Weekly sales of creative goods—like music records, movies, or books—usually peak shortly after release and then decline quickly. In many cases, however, they follow a hump-shaped pattern where sales increase for some time. A popular explanation for this phenomenon is word of mouth among a population of heterogeneous buyers, but previous studies typically assume buyer homogeneity or neglect word of mouth altogether. In this paper, I study a model of new-product diffusion with heterogeneous buyers that allows for a quantification of the sales effect of word of mouth. The model includes Christmas sales as a special case. All parameters have an intuitive interpretation. Simulation results suggest that the parameters are estimable for data that are not too volatile and that cover a sufficiently large part of a title’s life cycle. I estimate the model for four exemplary novels using scanner data on weekly sales.

What does a technology shock do? A VAR analysis with model-based sign restrictions

This paper estimates the effects of technology shocks in VAR models of the U.S., identified by imposing restrictions on the sign of impulse responses. These restrictions are consistent with the implications of a popular class of DSGE models, with both real and nominal frictions, and with sufficiently wide ranges for their parameters. This identification strategy thus substitutes theoretically motivated restrictions for the atheoretical assumptions on the time-series properties of the data that are key to long-run restrictions. Stochastic technology improvements persistently increase real wages, consumption, investment and output in the data; hours worked are very likely to increase, displaying a hump-shaped pattern. Contrary to most of the related VAR evidence, results are not sensitive to a number of specification assumptions, including those on the stationarity properties of variables.

Platinum-group element geochemistry of mantle eclogites: a reconnaissance study of xenoliths from the Orapa kimberlite, Botswana

Eclogite xenoliths recovered from kimberlites and alkali basalts are clinoproxene and garnet rich rocks formed at high pressures within the mantle. They are broadly 'basaltic' in composition and their genesis remains controversial. Early models suggested that they represent the metamorphic end product of (frozen) magma that never made it to shallow depths and instead crystallised at mantle pressures. However, most recent models favour an origin where basalts and gabbros generated at shallow pressures are subducted back into the mantle, converted into eclogite and that subduction is the mechanism by which eclogites become incorporated into the ancient subcontinental lithospheric mantle. The platinum-group elements (PGE) and Au are powerful tracers of mantle processes and their limited mobility during metamorphism and alteration allows them to be used as tools to see through later metamorphic events to the original formation mechanism. The present study presents new geochemical data, including the first complete PGE spectra, for a reconnaissance suite of nine eclogite xenoliths (comprising both group 1 and group 2 types) from the Orapa mine in Botswana. While the bulk chemical data presented here inevitably contain an element of contamination from the kimberlite, three out of the four group 2 eclogites show pronounced enrichment in Nb and Nb/Ta fractionation relative to average mantle that cannot be ascribed to interactions with the kimberlite. None of the group 1 eclogites show this feature. Both groups of eclogites show fractionated patterns, with enrichment in the palladium-group PGE (PPGE) over the iridium-group PGE (IPGE). This is similar to many basalts, and both groups overlap with one another on chondrite normalised plots. Some group 1 eclogites show evidence (hump-shaped patterns) for loss of Pd and Au that maybe related to metamorphism or interactions with the kimberlite. Total PGE (and particularly IPGE) concentrations of eclogites are very low compared with mantle peridoties from similar depths and it is clear that the IPGE enrichment observed in the PGE signatures of on-craton kimberlites probably comes predominantly from assimilated peridotite, even in cases such as the Roberts Victor kimberlite where eclogites dominate the xenolith suite.

Dispersal frequency affects local biomass production by controlling local diversity

Dispersal is a major factor regulating the number of coexisting species, but the relationship between species diversity and ecosystem processes has mainly been analysed for communities closed to dispersal. We experimentally investigated how initial local diversity and dispersal frequency affect local diversity and biomass production in open benthic microalgal metacommunities. Final local species richness and local biomass production were strongly influenced by dispersal frequency but not by initial local diversity. Both final local richness and final local biomass showed a hump-shaped pattern with increasing dispersal frequency, with a maximum at intermediate dispersal frequencies. Consequently, final local biomass increased linearly with increasing final richness. We conclude that the general relationship between richness and ecosystem functioning remains valid in open systems, but the maintenance of ecosystem processes significantly depends on the effects of dispersal on species richness and local interactions.

Sample size and the detection of a hump-shaped relationship between biomass and species richness in Mediterranean wetlands

Abstract Questions: What is the observed relationship between biomass and species richness across both spatial and temporal scales in communities of submerged annual macrophytes? Does the number of plots sampled affect detection of hump-shaped pattern? Location: Donana National Park, southwestern Spain. Methods: A total of 102 plots were sampled during four hydrological cycles. In each hydrological cycle, the plots were distributed randomly along an environmental flooding gradient in three contrasted microhabitats located in the transition zone just below the upper marsh. In each plot (0.5 m × 0.5 m), plant density and above- and below-ground biomass of submerged vegetation were measured. The hump-shaped model was tested by using a generalized linear model (GLM). A bootstrap procedure was used to test the effect of the number of plots on the ability to detect hump-shaped patterns. Result: The area exhibited low species density with a range of 1 - 9 species and low values of biomass with a range of 0.2–87....

What Does a Technology Shock Do? A VAR Analysis with Model-Based Sign Restrictions

This Paper estimates the effects of technology shocks in VAR models of the United States, Japan and Germany, identified imposing restrictions on the sign of impulse responses. These restrictions are motivated with priors on the parameters of a class of DSGE models with both real and nominal frictions. Estimated technology shocks lead to substantial and persistent increases in labor productivity, real wages, consumption, investment and output. In contrast with most results in the VAR literature, hours worked are much more likely to increase, displaying a hump-shaped pattern. These results are shown to stem primarily from the identification strategy proposed in the Paper, which substitutes theoretical restrictions for the atheoretical assumptions on the time series properties of the data, that are the hallmark of long-run restrictions.

The Mid‐Domain Effect and Diversity Gradients: Is There Anything to Learn?

The mid-domain effect (MDE) has been proposed as a null model for diversity gradients and an explanation for observed patterns. Here we respond to a recent defense of the concept, explaining that it cannot represent a viable model in either real or null worlds. First, the MDE misrepresents the nature of species ranges. There is also an internal logical inconsistency underlying the MDE because the range size frequency distribution, necessary to generate a hump-shaped pattern under randomization, cannot exist in the absence of environmental gradients and is generated by the ecological and historical processes that the MDE claims to exclude.

Diversity and productivity of plant communities across the Inland Northwest, USA

No definitive explanation for the form of the relationship between species diversity and ecosystem productivity exists nor is there agreement on the mechanisms linking diversity and productivity across scales. Here, we examine changes in the form of the diversity-productivity relationship within and across the plant communities at three observational scales: plots, alliances, and physiognomic vegetation types (PVTs). Vascular plant richness data are from 4,760 20 m2 vegetation field plots. Productivity estimates in grams carbon per square meter are from annual net primary productivity (ANPP) models. Analyses with generalized linear models confirm scale dependence in the species diversity-productivity relationship. At the plot focus, the observed diversity-productivity relationship was weak. When plot data were aggregated to a focus of vegetation alliances, a hump-shaped relationship was observed. Species turnover among plots cannot explain the observed hump-shaped relationship at the alliance focus because we used mean plot richness across plots as our index of species richness for alliances and PVTs. The sorting of alliances along the productivity gradient appears to follow regional patterns of moisture availability, with alliances that occupy dry environments occurring within the increasing phase of the hump-shaped pattern, alliances that occupy mesic to hydric environments occurring near the top or in the decreasing phase of the curve, and alliances that occupy the wettest environments having the fewest species and the highest ANPP. This pattern is consistent with the intermediate productivity theory but appears to be inconsistent with the predictions of water-energy theory.

SCALE DEPENDENCE IN THE SPECIES-RICHNESS–PRODUCTIVITY RELATIONSHIP: THE ROLE OF SPECIES TURNOVER

Recent research in aquatic systems suggests that productivity-richness re- lationships change with spatial scale and that species turnover (i.e., spatial and temporal variation in species composition) plays an important role in generating this scale depen- dence. The generality of such scale dependence and the effects of variation in temporal scale remain unknown. We examined the extent to which the richness-productivity rela- tionship in terrestrial plant communities depends on spatial or temporal scale and evaluated how spatial and temporal turnover (i.e., species turnover in space and time) generates scale dependence in these relationships using data from two Long-Term Ecological Research (LTER) sites (Jornada and Konza). We found a weak hump-shaped relationship (Jornada) and no relationship (Konza) between richness and productivity at the smallest focal scale (1 m 2 at Jornada and 50 m 2 at Konza) at each site, but strong hump-shaped relationships at the largest focal scale (49 m 2 at Jornada and 200 m 2 at Konza) for each site. Relationships between spatial turnover and productivity at each site mirrored the productivity-richness relationships that emerged at the larger spatial scale (i.e., a significant hump-shaped pattern). In contrast, temporal turnover was unrelated to productivity, and hence increasing temporal scale did not appreciably change the form of the productivity-richness relationship. Our study suggests that the way in which productivity-richness relationships change with spatial or temporal scale depends on the form and strength of the underlying relationship between species turnover and productivity. Moreover, we contend that a dominant effect of increasing productivity is the generation of dissimilarity in species composition among localities that comprise a region, rather than increasing the number of species that occur within local communities. Thus, understanding the mechanisms that cause species turnover to vary with productivity is critical to understanding scale dependence in richness-productivity rela- tionships.

A review on the elevational patterns of plant species diversity

Patterns of biodiversity along environmental gradients is one of the basic issues in the biodiversity studies. The elevational gradient incorporates multiple resource gradients which vary continuously in different fashions. As patterns along environmental gradients are significantly scale-dependent, we reviewed plant diversity patterns along the elevational gradient in different scales: α diversity, β diversity and γ diversity, as well the factors which maintain these patterns. At the local scale, there is no fixed elevational pattern for herb species richness. For woody plants, the monotonic decrease of species richness as elevation increases is thought to be a general pattern, yet other patterns exist. The decrease of β diversity with increasing elevation has been observed in different ecosystems. Species richness in different elevational bands exhibits two different patterns, monotonic decrease and hump-shaped pattern along the elevational gradient.

Does the Fretwell–Oksanen model apply to invertebrates?

Despite the myriads of herbivores in terrestrial ecosystems, the world is green (Hairston et al. 1960). Nevertheless, all these animals consume plant material and they should therefore have some influence on plant individuals, populations and communites. Hence, in order to understand community and ecosystem processes, we need a quantitative understanding of the impact of herbivores on plant communities and of its variability across ecosystems. Many ecologists will agree, that invertebrates have a lower impact on plant communities than vertebrates (Crawley 1989). However, exclusion experiments revealed that invertebrates influence biomass, species richness, competition regimes and nutrient cycling in plant communities (Bach 1994, Brown 1994, Brown and Gange 1989, Carson and Root 1999, Fraser and Grime 1997, Gibson et al. 1990). Furthermore, the results of these experiments showed considerable variability of the herbivore effect across plant communities (Hendrix et al. 1988, Fraser and Grime 1997). One explanation for this variability is the so-called Fretwell–Oksanen model. Fretwell (1977, 1987), Oksanen (1990) and Oksanen et al. (1981) suggested that the relative importance of top-down and bottom-up forces within communities changes with productivity. A low primary productivity should not be sufficient to sustain appreciable populations of herbivores (bottomup control of herbivores), whereas at high levels of productivity predators control herbivores (top-down control of herbivores). Hence, on the community level herbivore impact should show a hump at intermediate levels of productivity. Oksanen et al. (1981) and Oksanen (1990) emphasised that their model applies only to vertebrates. Invertebrates should have limited possibilities to respond on primary productivity, due to their low mobility, narrow dietary niche breadth and low costs of dormancy. Furthermore, invertebrate carnivores exploit also detritus-feeding prey and need little energy to run their physiological machinery throughout the year. Nevertheless, the Fretwell–Oksanen model has been applied to invertebrate grazing systems. The results of some recent studies provided some support for a hump-shaped pattern along nutrient gradients (Fraser 1998, Fraser and Grime 1997). Therefore, the objective of our study was to approach the following question: Do the available experimental results suggest a general hump-shaped relationship between productivity and the impact of invertebrate phytophages on plant communities?

What Is the Observed Relationship between Species Richness and Productivity?

Understanding the relationship between species richness and productivity is fundamental to the management and preservation of biodiversity. Yet despite years of study and intense theoretical interest, this relationship remains controversial. Here, we present the results of a literature survey in which we examined the relationship between species richness and productivity in 171 published studies. We extracted the raw data from published tables and graphs and subjected these data to a standardized analysis, using ordinary least-squares (OLS) regression and generalized linear-model (GLIM) regression to test for significant positive, negative, or curvilinear relationships between productivity and species diversity. If the relationship was curvilinear, we tested whether the maximum (or minimum) of the curve occurred within the range of productivity values observed (i.e., was there evidence of a hump?). A meta-analysis conducted on the distribution of standardized quadratic regression coefficients showed that the average quadratic coefficient was negative (i.e., the average species richness–productivity relationship was curvilinear and decelerating), and that the distribution of standardized quadratic regression coefficients was significantly heterogeneous (i.e., the studies did not sample the same underlying species richness–productivity relationship). Looking more closely at the patterns of productivity–diversity relationships, we found that, for vascular plants at geographical scales smaller than continents, hump-shaped relationships occurred most frequently (41–45% of all studies). A positive relationship between productivity and species richness was the next most common pattern, and positive and hump-shaped relationships co-dominated at the continental scale. For animals, positive, negative, and hump-shaped patterns were common at most geographical scales, and no one pattern predominated. For both plants and animals, hump-shaped curves were relatively more common in studies that crossed community boundaries compared to studies conducted within a community type, and plant studies that crossed community types tended to span a greater range of productivity compared to studies within community types. Sample size and plot size did not affect the probability of finding a particular productivity–diversity relationship (e.g., positive, hump-shaped, etc.). However, hump-shaped curves were especially common (65%) in studies of plant diversity that used plant biomass as a measure of productivity, and in studies conducted in aquatic systems.