Species Richness Productivity Research Articles

Overyielding in experimental grassland communities – irrespective of species pool or spatial scale

AbstractIn a large integrated biodiversity project (‘The Jena Experiment’ in Germany) we established two experiments, one with a pool of 60 plant species that ranged broadly from dominant to subordinate competitors on large 20 × 20 m and small 3.5 × 3.5 m plots (= main experiment), and one with a pool of nine potentially dominant species on small 3.5 × 3.5 m plots (= dominance experiment). We found identical positive species richness–aboveground productivity relationships in the main experiment at both scales. This result suggests that scaling up, at least over the short term, is appropriate in interpreting the implications of such experiments for larger‐scale patterns. The species richness–productivity relationship was more pronounced in the experiment with dominant species (46.7 and 82.6% yield increase compared to mean monoculture, respectively). Additionally, transgressive overyielding occurred more frequently in the dominance experiment (67.7% of cases) than in the main experiment (23.4% of cases). Additive partitioning and relative yield total analyses showed that both complementarity and selection effects contributed to the positive net biodiversity effect.

Anthropogenic impacts upon plant species richness and net primary productivity in California

AbstractWe assess the importance of anthropogenic land‐use, altered productivity, and species invasions for observed productivity–richness relationships in California. To this end, we model net primary productivity (NPP) c. 1750 AD and at present (1982–1999) and map native and exotic vascular plant richness for 230 subecoregions. NPP has increased up to 105% in semi‐arid areas and decreased up to 48% in coastal urbanized areas. Exotic invasions have increased local species diversity up to 15%. Human activities have reinforced historical gradients in species richness but reduced the spatial heterogeneity of NPP. Structural equation modelling suggests that, prior to European settlement, NPP and richness were primarily controlled by precipitation and other abiotic variables, with NPP mediating richness. Abiotic variables remain the strongest predictors of present NPP and richness, but intermodel comparisons indicate a significant anthropogenic impact upon statewide distributions of NPP and richness. Exotic and native species each positively correlate to NPP after controlling for other variables, which may help explain recent reports of positively associated native and exotic richness.

Invited review: Direct and indirect effects in herbivore - periphyton interactions

In this article we review the indirect interactions prevailing in littoral communities and compare their importance to direct trophic interactions. We focus on the interaction between benthic algae and their herbivorous consumers, i. e. invertebrate grazers and herbivorous fish. In addition to the effects of grazers on periphyton biomass and production, we categorise and describe the types of indirect interactions observed and we present a quantitative meta-analysis to compare the effect magnitude of indirect and direct effects. Our review shows that herbivore-plant interactions comprise considerable complexity beyond consumption. We identify and review three different indirect interactions, which play important roles for periphyton-grazer interactions in benthic food webs. These are keystone predation, trophic cascading and habitat facilitation. Direct negative effects were stronger than indirect positive effect, because grazing leads in almost all studies to a decrease in algal biomass. We found positive effects of grazing on algal nutrient content, biomass-specific productivity and evenness. We found negative overall effects on species richness and area-specific productivity. Moreover, spatial heterogeneity was on average increased by grazing. The magnitudes of direct and indirect effects were often correlated

SPECIES RICHNESS–PRODUCTIVITY PATTERNS DIFFER BETWEEN N-, P-, AND K-LIMITED WETLANDS

We evaluated whether the kind of nutrient limitation (N, P, or K) may affect species richness–productivity patterns and subsequently may explain variation in species richness and in richness of threatened species. We present a data set from previous studies in wetlands in Poland, Belgium, and The Netherlands and examine species richness–productivity patterns for vascular plants in all 150 sites together as well as for N-, P-, and K-limited sites separately. The kind of nutrient limitation was assessed by N:P, N:K, and K:P ratios in the vegetation. Critical values for these ratios were derived from a literature review of fertilization experiments. The kind of nutrient limitation influenced species richness–productivity patterns in our 150 sites through large differences in productivity. P (co)-limitation occurred only at low productivity, K (co)-limitation up to intermediate productivity, and N limitation along the entire productivity gradient. There was a decreasing trend in species richness with increasing productivity for K (co)-limited sites, whereas for both the N-limited sites and P (co)-limited sites a sort of “filled hump-shaped curve” was observed. The species richness–productivity relationship for threatened species was restricted to a much narrower productivity range than that for all species. Richness of threatened species was higher in P (co)-limited sites than in N-limited sites, suggesting that increased P availabilities in wetlands may be particularly important in causing disappearance of threatened species in western Europe. The role of nutrient limitation in species richness–productivity relationships not only reveals mechanisms that may explain variation in species richness and occurrence of threatened species, but it also may be important for nature management practice.

Space–time patterns of co‐variation of biodiversity and primary production in phytoplankton guilds of coastal marine environments

Abstract The relevance of biodiversity to ecosystem processes is a major topic in ecology. Here, we analyse the relationship between biodiversity and productivity of the nano‐ and micro‐phytoplankton guilds in coastal marine ecosystems. The patterns of variation of species richness, diversity and primary productivity (as 14C assimilation) were studied in two marine areas: a eutrophic–mesotrophic area beside the River Po delta (northern Adriatic) and an oligotrophic area around the Salento peninsula (southern Adriatic–Ionian). The study was carried out at 23 sites in the northern area and at 45 sites in the southern area. Sites were arranged on expected spatial and temporal gradients of primary productivity variation, according to distance from the coast, optical depths and seasonal period. 167 taxa were identified in the northern area and 153 taxa in the southern area. In both areas, the taxonomic composition of the nano‐ and micro‐phytoplankton guilds exhibited greater temporal than spatial variation. The latter was much higher in the southern area than in the northern area (average dissimilarity between stations being 70.7±0.8% and 44.7±4.2% respectively). Primary productivity varied in space and time on the gradients considered. Phytoplankton species richness and diversity exhibited significant patterns of variation in space and time; overall, these were inversely related to the primary productivity patterns in the northern area, whereas they were directly related in the southern area. The small individual size and the high turnover rate of phytoplankton are likely to underlie the observed relationships, which emphasized a threshold response to nutrient enrichment in agreement with the ‘paradox of enrichment’. Under resource enrichment conditions, the high turnover of producers leads to hierarchical partitioning of the available resources with an increasing dominance of a few species. Therefore, the relationship observed here seems likely to be explained by the complementarity hypothesis. Copyright © 2003 John Wiley & Sons, Ltd.

EFFECTS OF SEED ADDITIONS AND GRAZING HISTORY ON DIVERSITY AND PRODUCTIVITY OF SUBHUMID GRASSLANDS

Recent studies indicate that plant species richness and primary productivity of grasslands may be limited by seed availability. However, it is not known how widespread this limitation is, whether it is affected by disturbance, or which mechanisms underlie any disturbance effect. We tested for seed limitation and explored the role of litter accumulation in explaining effects of recent grazing history on seedling establishment, species diversity (richness and evenness), and plant productivity in a subhumid grassland ecosystem in Texas, USA. We added seeds and removed litter in a factorial treatment arrangement (seeds added, litter removed, seeds added and litter removed, and control [no seeds added nor litter removed]) within each of five fields: currently grazed, no grazing for 1–2, 2–3, or 6–7 yr, and an unplowed prairie remnant. Seeds from each of 20 native species were added to replicated 1-m2 plots in each field during each of two growing seasons. Adding seeds increased seedling emergence in all fields, but this increase was much greater when litter was removed, especially in ungrazed fields. Counter to our expectations, seedling emergence increased with time since grazing when litter was removed and was greatest in fields not grazed for 1–3 yr when litter was present. As expected, light availability at the soil surface decreased with time since grazing. However, soil water availability to 15 cm depth increased with time since grazing, suggesting that limits on seedling emergence changed from water to light with time since grazing. Aboveground productivity during the second year of the study was significantly reduced following seed additions in the four fields along the grazing gradient, primarily because of a decrease in the productivity of C4 grasses. Species diversity decreased with time since grazing. Seed additions slightly increased species richness but decreased species evenness in fields located along the grazing gradient, resulting in no net effect on diversity. In the more species-rich prairie remnant, seed additions had no effect on aboveground productivity or species richness or evenness. These results suggest that seedling emergence does not relate linearly to time since grazing and that seed availability does not limit productivity and diversity in these subhumid grasslands. Corresponding Editor: D. P. C. Peters.

Plant species richness and community productivity: why the mechanism that promotes coexistence matters

This paper stresses that the mechanism of coexistence is the key to understanding the relationship between species richness and community productivity. Using model plant communities, we explored two general kinds of mechanisms based on resource heterogeneity and recruitment limitation, with and without any trade‐off between reproductive and competitive abilities. We generated different levels of species richness by changing model parameters, in particular the number of species in the regional pool, the degree of recruitment limitation, and the level of heterogeneity. Different diversity–productivity patterns are obtained with different coexistence mechanisms, indicating there is no reason to expect any general relationship between species richness and productivity. We discuss these results in the context of the within‐site and across‐site aspects of the relationship between species richness and productivity. Furthermore, we extend these results to hypothesize the relationship between species richness and productivity for other coexistence mechanisms not explicitly considered here.

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.

Unifying the relationships of species richness to productivity and disturbance.

Although species richness has been hypothesized to be highest at 'intermediate' levels of disturbance, empirical studies have demonstrated that the disturbance-diversity relationship can be either negative or positive depending on productivity On the other hand, hypothesized productivity diversity relationships can be positive, negative or unimodal, as confirmed by empirical studies. However, it has remained unclear under what conditions each pattern is realized, and there is little agreement about the mechanisms that generate these diverse patterns. In this study, I present a model that synthesizes these separately developed hypotheses and shows that the interactive effects of disturbance and productivity on the competitive outcome of multispecies dynamics can result in these diverse relationships of species richness to disturbance and productivity The predicted productivity diversity relationship is unimodal but the productivity level that maximizes species richness increases with increasing disturbance. Similarly, the predicted disturbance diversity relationship is unimodal but the peak moves to higher disturbance levels with increasing productivity Further, these patterns are well explained by the opposite effects of productivity and disturbance on competitive outcome that are suggested by the change in community composition along these two environmental gradients: higher productivity favours superior competitors while higher disturbance levels favour inferior competitors.

THE RELATIONSHIP IN LAKE COMMUNITIES BETWEEN PRIMARY PRODUCTIVITY AND SPECIES RICHNESS

An understanding of the relationship between species richness and productivity is crucial to understanding biodiversity in lakes. We investigated the relationship between the primary productivity of lake ecosystems and the number of species for lacustrine phytoplankton, rotifers, cladocerans, copepods, macrophytes, and fish. Our study includes two parts: (1) a survey of 33 well-studied lakes for which data on six major taxonomic groups were available; and (2) a comparison of the effects of short- and long-term whole-lake nutrient addition on primary productivity and planktonic species richness. In the survey, species richness of all six taxa showed a significant quadratic response to increased annual primary productivity (14C estimate, g C·m−2·yr−1) when lake area is taken into account. However, the richness–productivity relationship for phytoplankton and fish was strongly dependent on lake area. The relationship for phytoplankton, rotifers, cladocerans, copepods, and macrophytes was significantly unimodal. Species richness generally peaked at levels of primary productivity in the range of 30–300 g C·m−2·yr−1. For the average lake size, the highest biodiversity tended to occur in lakes with relatively low primary productivity, such as those found in the Northern Temperate Lakes Long-Term Ecological Research (LTER) site in the upper Midwest (United States) and in the Experimental Lakes Area of Ontario (Canada). Based on short-term (3 yr) and long-term (21–24 yr) experiments, we tested whether individual lakes respond to whole-lake enrichment experiments in the manner suggested by analyses of survey data. Experimental addition of nutrients produced varied and unpredictable responses in species richness, probably due to transient dynamics and time lags. Responses to nutrient addition were taxon and lake specific. Phytoplankton showed a variety of relationships between species richness and pelagic primary productivity (PPR), depending on the history of enrichment and recovery. No significant effect of primary productivity on rotifer richness occurred in any of the experimental lakes, whereas richness of crustacean zooplankton was negatively correlated with primary productivity in both the short- and long-term experiments.

The significance of assemblage‐level thinning for species richness

1 A unimodal relationship between species richness and primary productivity is commonly reported. To explain this pattern, the mechanisms proposed in the many hypotheses are generally complex and almost all are without a strong empirical foundation. Here we evaluate the role of self‐thinning in plant assemblages: assemblage‐level thinning. 2 We developed a simple two‐parameter model of species richness that predicts that plant species richness will be determined by a unimodal relationship between total plant density and above‐ground biomass. This model provides a very narrowly defined set of testable quantitative predictions, and thus is the first falsifiable model of assemblage‐level thinning. We fit this model to the species richness–above‐ground biomass data from 14 empirical studies that are often cited as evidence of a general diversity–productivity relationship. In addition, we compared our model to two other models, one more flexible and one more constrained than our own. 3 We found that our model of species richness explained a substantial and statistically significant portion of the species richness observed in 11 of the 14 empirical studies of species richness–biomass patterns. Therefore, given the conservative nature of our model, and the number of previously published data sets explained by this model, we argue that assemblage‐level thinning not only provides a viable and exceedingly parsimonious explanation, but may also be a widespread phenomenon.

Annual plant community responses to density of small-scale soil disturbances in the Negev Desert of Israel.

We investigated whether plant diversity and productivity in small-scale soil disturbances, which is known to be higher than in undisturbed soil, decreases as the density of the disturbances increases. We studied this in an experiment with soil diggings (15 cm diameter and 15 cm depth) dug at a range of densities, on a north- and a south-facing slope of a watershed in the central Negev Desert of Israel. The diggings were similar to the commonly occurring pits made by porcupines (Hystrix indica) as they forage for below-ground plant parts. We used four levels of digging density, within the naturally occurring range in the region, represented by a rectangular plot with rows of diggings dug at four distances between diggings. The plots were laid out in a blocked design with three replications on both slopes, with each block containing all four levels of digging density. In the spring of 1992, 1994 and 1995 we measured plant density, species richness and plant productivity in the diggings, and in adjacent equal-sized undisturbed control areas ("soil matrix") and on the mounds made by the removed excess soil. Plant density, species richness and productivity of annual plants were higher in the diggings than in the undisturbed matrix, while these responses were very low on the mounds. Plant density, species richness and productivity in the diggings, but not in the matrix or mounds, decreased as digging density increased. This effect varied slightly with location within a watershed and with annual rainfall. The density of seeds captured in the diggings from outside the digging during the 1995 dispersal season decreased with increasing digging density, but only on one of the slopes. At the highest digging density, plant density and species number in the diggings did not decrease down the slope, as expected if interference between diggings in runoff water capture were the cause of the digging density effect. There was a weak decrease in biomass production in 1994-1995 down the slope. We used a simple mathematical model to estimate whether the distribution of rainfall intensities that occurred during the winter of 1994-1995 could result in differences between digging densities in the amount of water captured by the diggings, and whether this could explain the observed effect of digging density. The model showed that there were four events during which less water was captured by the diggings at high digging densities, except in the topmost row of diggings. Soil moisture measurements, however, showed very little difference between diggings at different digging densities. We explain our findings as the result of the interaction between the properties of the disturbance patch with its surroundings, as the diggings capture resources in the form of runoff water, and seeds moved primarily by wind. The additional resources and seeds captured in diggings increase plant density, species richness and productivity relative to the undisturbed matrix. However, the contrast in plant responses between the disturbed patches and undisturbed soil diminishes at higher digging densities. We explain this as interference among diggings at close proximity. As we did not detect a decrease in plant responses down the slopes, we conclude that interference is due to interception of the wind-driven, non-directional flow of seeds. Interception of the down-slope flow of runoff water by upslope diggings is insufficient to affect plant density, determined at the beginning of the season. Later in the season, runoff interception may become important for biomass production.

A Scale-Independent Approach to the Richness vs Biomass Relationship in Ground-Layer Plant Communities

It has been proposed that the much discussed 'hump-backed' relationship between species richness and habitat productivity might be an artifact, arising from the use of an anthropocentric sampling scale. We studied this relationship in 27 Estonian grassland and forest ground-layer plant communities by using scale-independent sampling. Richness was estimated simultaneously from l-m 2 quadrats, and from quadrats inhabited by 500 plant ramets. We also studied richness relative to the size of the actual species pool (relative richness). Modal maximum richness at intermediate standing biomass was observed in both fixed-size and flexible-size quadrats, indicating that the unimodal shape of the studied curve is not merely a sampling artifact. Relative richness varied in a very narrow interval (especially when estimated per 500 ramets). showing that small-scale absolute richness is mostly determined by the size of the local species pool. However, the relationship between relative richness and biomass was slightly parabolic, showing that modal relative richness is expected at intermediate biomass. This could be interpreted as evidence supporting the competition-based explanation of the hump-backed curve, but. more likely, this is an artifact due to possible underestimation of the size of the actual species pool (overestimation of relative richness) in species-rich communities with intermediate productivity. Relative richness is positively correlated with absolute richness in 1-m 2 quadrats, but independent of absolute richness when scale-independent sampling is used. In flexible-size quadrats, richness is predictable as a certain percent of the actual species pool, independent of the density of ramets in the community.

Species richness and canopy productivity of Australian plant communities

The species richness (number of vascular plants per hectare) of Australian plant communities (containing a mosaic of gap, regeneration, maturation and senescent phases) is correlated with the annual biomass productivity of the overstorey canopy. The annual production of leaves and stem in the canopy of the plant community is shown to be limited by the requirements of photosynthesis (particularly light and the availability of water) and the length of the growing season. The species richness of Australian plant communities is the product of the blance between the dominance of the overstorey and the response of the understorey to the shading of the overstorey. For all climatic regions and zones the species richness of the overstorey of the plant community is shown to be exponentially related to the annual shoot growth of the overstorey canopy, until the latitudinal or altitudinal tree line is reached. With latitudinal increase outside the tropics, overstorey canopies of forest communities absorb increasingly more of the incident solar radiation. markedly reducing the species richness of the understorey strata. In contrast, in these latitudes the overstorey of plant communities with widely spaced trees or tall shrubs will absorb far less solar radiation, thus enabling the species richness of the understorey to be maintained.

Australian Rock-Mammals: A Phenomenon of the Seasonally Dry Tropics

Mammal species specializing on rocky habitats constitute 11 percent of Australia's marsupial and rodent species. The majority of rock-dwelling species is limited to the seasonally dry tropics. These habitats experience a single wet season followed by a long dry season. Runoff from rocky escarpments, water absorption by large rock formations, and the presence of aquifers in rock result in greater water availability close to rock escarpments. Escarpment vegetation, therefore, is higher in species richness and plant productivity than the surrounding forested habitaat. These species-rich habitats provide a greater variety of potential foods for hebivores. The impact of large rock formations on surrounding vegetation is less marked in regions with a more evenly distributed or very low rainfall and/or a temperate climate. The impact of rock escarpments on vegetation in the seasonally dry tropics, rather than the existence of more opportunities for speciation via genetic isolation, is likely to have been instrumental in the evolution of the often regionally endemic rock faunas of Australia's dry tropics. THE EXISTENCE OF GEOGRAPHIC AND PAST CLIMATIC BARRIERS to gene flow is often emphasized in studies of speciation and zoogeographic differentiation (e.g., Keast et al. 1959; Kikkawa and Pearse 1969; Horton 1972, 1973; Keast 1981). Although speciation is unlikely to occur without geographically induced genetic isolation, the ecological framework within which natural selection occurs is likely to be equally crucial in determining whether genetic divergence occurs. The northwestern portion of Australia (the Timorian zoogeographic subregion; Kikkawa & Pearse 1969), has a highly endemic fauna. The region's fauna is usually assumed to have been the product of genetic isolation caused by climatic barriers to the eastern and southern regions of Australia. A large component of the Timorian subregion's endemic fauna is composed of species specializing on rock habitats. These habitats are composed of rock escarpments or isolated residual masses that can be more than 100 m above the surrounding country (e.g., Fig. 1). In western Arnhemland (Fig. 2A) alone there are two species of rock rat (Zyzomys woodwardi, Z. argurus), one species of rock ringtail possum (Pseudocheirus dahli), the rock pigeon (Petrophassa rufipennis), the black wallaroo (Macropus bernardus), a rock wallaby (Petrogale brachyotis), the nabarlek (Peradorcas concinna, a minature rock wallaby), together with species of gecko, skink, and frog, and a species each of python, honeyeater, fruit pigeon, grass wren, and shrike thrush. All these species are restricted to rock habitats. If genetic isolation alone had been sufficient to produce these rock specialists, then rock-dwelling species should be equally abundant in other parts of Australia. If, however, the rock-dwelling fauna is unusually larger than in other parts of Australia, then some ecological factor in addition to genetic isolation is required to explain the richness of rock faunas in Australia's dry tropics. The pattern of distribution of rock-dwelling mammal species across Australia is presented, and climatological correlates with species richness established. Vegetation of a rock escarpment in western Arnhemland is analyzed to determine the impact of climatic variables on rock habitats in the dry tropics. These results are compared with data from other climatic regions. The role of ecological factors in the evolution of endemic rock-faunas is assessed.