Soil arthropod composition differs between old-fields dominated by exotic plant species and remnant native grasslands

Secondary succession after agriculture abandonment (old-fields) is mostly dominated by exotic grass species. Non-native plant invasions may alter soil fauna, potentially inducing plant-soil feedbacks. Despite their importance in nutrient cycling and plant-soil interactions, meso and macrofauna received less attention than bacteria or fungi. Here we compared the composition of the soil arthropod community in native remnants and plant exotic-dominated old-fields grasslands in the Inland Pampa, Argentina. We sampled independent remnants and old-field grassland plots within a 100 km² agricultural landscape to test the hypothesis that the abundance of soil arthropod organisms is related to the quality of the plant biomass, whereas the diversity of the soil biota is related to plant species richness, resulting in a different soil biota composition because of differing plant communities. When compared to non-invaded remnant grasslands, soil activity and soil food-web characteristics of the old-fields sites included: 1. Higher total arthropod abundance, particularly of Isopoda, Pseudoescorpionida and Blattaria; 2. Lower abundance of Hymenoptera and Enthomobryomorpha (Collembola); 3. Lower diversity, and evenness, but similar richness of soil organisms orders; 4. Higher soil respiration rates and soil temperature; and 5. Higher total soil N and K⁺content, but lower soil P content. These results illustrate that soil arthropod composition can vary widely within grasslands patches depending on plant species composition. Also, the more diverse plant community of remnant grasslands supports a more diverse soil biota, although soil activity is slower. Our results support the strong linkage between plant community and soil arthropod composition and suggest that changes in soil biota composition might promote plant-soil feedback interactions inducing the persistence of these alternative grassland states in new agricultural human-modified landscapes.     

Species richness and cover along a 60-year chronosequence in old-fields of southeastern Spain

We analyse changes in plant cover and species richness along a 60-year chronosequence in semi-arid Mediterranean old-fields of southeastern Spain. The objectives were: (i) to study patterns of species richness along the abandonment gradient in semi-arid conditions (e.g., to test the “humped-back model” in our system); (ii) to test whether different broad life forms (annuals, forbs, grasses and woody species) showed different patterns along the abandonment gradient, and (iii) to examine to what extent plants with different dispersal strategies dominate at different stages of succession. The explained variance of the regression relating species richness to years since abandonment is improved when considering different life forms. The results suggest that cover and richness of different functional groups show a non-linear unimodal (often positive-skewed) pattern along the gradient (age since abandonment). Maximum total richness is found at young stages of abandonment (<20 years), when most life forms and dispersal strategies coexist. Annuals and perennial forbs reached their maximum richness during the first 10 years of abandonment. About 45% of total woody species richness is reached at this time as a consequence of early colonization of zoochorous shrubs. While the results showed a tendency towards a life-form replacement sequence, the pattern is not so clear when looking at the different dispersal strategies. The results complement previous results in Mediterranean conditions and emphasise the importance of considering different functional types when studying successional patterns. This revised version was published online in June 2006 with corrections to the Cover Date.     

A new index of interactivity in parasite communities.

A new index of interactivity which allows objective evaluation and comparison of interactivity in communities between different host species is presented. The index is derived from the equations for species-accumulation curves generated using non-linear regression (with the Levenberg-Marquardt algorithm) of sample infracommunity richness data. It is advantageous in that it requires only presence/absence data to calculate, is applicable to all parasite taxa (including asexual species), is largely independent of sample size and allows objective comparison of parasite communities while correcting for differences in total richness. Iterative randomisation of infracommunity richness values to generate a mean value for the index avoids spurious results which may be generated by heterogeneity in infracommunity richness and the variation this produces in the non-linear regression results.     

Testing for local species saturation with nonindependent regional species pools

Identifying the factors controlling local community structure is a central problem in ecology. Ecologists frequently use regression to test for a nonlinear saturating relationship between local community richness and regional species pool richness, suggesting that species interactions limit the number of locally coexisting species. However, communities in different regions are not independent if regions share species. We present a Monte Carlo test for whether an observed local-regional richness relationship is significantly different from that expected when regions are nonindependent and species interactions do not limit community membership. We illustrate this test with data from experimental microcosm communities. A conventional F -test suggests a significant saturating relationship between realized community richness and species pool richness. However, the Monte Carlo test fails to reject the null hypothesis that species interactions do not affect community richness. Strong species interactions do not necessarily set an absolute upper limit to the number of locally coexisting species.     

Effect of species richness and relative abundance on the shape of the species accumulation curve

Abstract We explain how species accumulation curves are influenced by species richness (total number of species), relative abundance and diversity using computer‐generated simulations. Species richness defines the boundary of the horizontal asymptote value for a species accumulation curve, and the shape of the curve is influenced by both relative abundance and diversity. Simulations with a high proportion of rare species and a few abundant species have a species accumulation curve with a low ‘shoulder’ (inflection point on the ordinate axis) and a long upward slope to the asymptote. Simulations with a high proportion of relatively abundant species have a steeply rising initial slope to the species accumulation curve and plateau early. Diversity (as measured by Simpson's and Shannon–Weaver indices) for simulations is positively correlated with the initial slope of the species accumulation curve. Species accumulation curves cross when one simulation has a high proportion of both rare and abundant species compared with another that has a more even distribution of abundance among species.     

Assessing the diversity pattern of cryophilous plant species in high elevation habitats

This study aimed to better document the diversity and distribution patterns of vascular cryophilous species across major habitat types in a high-elevation Mediterranean system in central Italy. The research addressed the following questions: (a) whether different habitats support similar levels of biodiversity in terms of total vascular plants richness and cryophilous species richness, and (b) how each habitat contributes to the total cryophilous species diversity. A random stratified sampling approach based on a habitat map was applied to construct rarefaction curves for overall cryophilous species richness and habitat type-specific cryophilous richness. Rarefaction curves were also constructed for all-species and exclusive species. To determine whether the targeted species represented a constant proportion of all species, the ratio between the rarefaction curves of the cryophilous species and all species was also calculated. The results highlight the importance of the different habitat types in overall and cryophilous species conservation because these different habitat types had progressively higher richness values. At the regional scale, steep slopes had the highest species diversity, the greatest exclusive species richness and a steep rarefaction curve. The diversity pattern of cryophilous taxa was not related to the general pattern of total species richness, with these species being more common in three habitat types with extreme environmental conditions: ridges, cliffs, and screes. For the establishment of successful biodiversity conservation programs, it is imperative to include species-poor habitats containing a high proportion of cryophilous species, which are considered to be threatened by climate warming.     

Nested spatial patterns in seed bank and vegetation of Mediterranean old-fields

A nested sampling design was used to describe the spatial patterns for the species richness and composition in the seed bank and vegetation of three Mediterranean old-fields (1, 7, and 15 yr after the last ploughing). Three scales were examined hierarchically: sampling units within plots of 0.25 m² for the vegetation and of 0.05 m² for the seed bank, 100 m² plots within fields, and fields of 1000 m². In spite of the strong spatial variation among sampling units, species richness and composition of both seed bank and vegetation showed hierarchically structured patterns of heterogeneity, while each old-field was a homogeneous entity. These spatial patterns tended to be partially masked when the data were aggregated at the scale of the plot. Such results stress the use of a nested sampling design for studying variation in species richness and taxonomic composition in both vegetation and seed bank. This design, in combination with CCA, also showed that the vegetation showed a coarser grain than the seed bank, probably in relation to seed clumping.     

Habitat loss and possible effects on local species richness in a species-poor system: a case study of southern Baltic Sea macrofauna

The effects of habitat loss on local species richness depend on the characteristics of the endangered system (including its total species pool and the distribution of species among the habitats). The present study focuses on the species-poor southern Baltic marine benthic biota. Macrobenthic samples were collected in three habitats: (1) soft bottom covered with vegetation; (2) stony reefs; (3) unvegetated sands. Fourty one percent of 54 observed macrozoobenthic species were habitat specific, while 30% occurred in all three habitats. There were no significant differences in total species richness among the three habitats. The accumulation curves plotted for subsets of data with selected habitats excluded lay below the curve plotted for the whole dataset, but only in one case the 95% confidence intervals of the subset curve did not overlap with those plotted for the whole dataset. The exclusion of samples from selected habitats produced a species richness drop ranging from 9 to 13%. The present study showed that habitat loss in a species-poor area with a relatively large ratio of generalist species cannot produce local species richness declines similar to those predicted for diverse marine systems. However, it must be emphasized that in species-poor systems, the loss of ecological function accompanying habitat loss could be disproportionally higher than that predicted based on decreases in species richness, as some functions are performed by a single species.     

Effects of repeated burning on species richness in a Florida pine savanna: A test of the intermediate disturbance hypothesis

Abstract. We studied the effect of burning frequency on the density and species richness of understory flowering stems in a Florida sandhill. Flowering stems were censused weekly for 54 weeks in six sites that had been burned one to six times in the previous 16 years. We concurrently measured overstory characteristics such as species composition, density and basal area. We used maximum likelihood and Akaike's Information Criterion to compare linear, quadratic, saturating, and null models of community response to repeating burning. We did not find a relationship between species richness, diversity or flowering stem density and fire frequency. Tree density was related to fire frequency and may represent an indirect pathway for fire effects on understory characteristics. While we found no support for the Intermediate Disturbance Hypothesis, an analysis of our experimental design indicated that we had low statistical power. We develop the hypothesis that a saturating model of response to fire best describes understory species richness in our system. We test this hypothesis using the most extensive published fire data set we are aware of and find support for a saturating model.     

Mosses in Ohio wetlands respond to indices of disturbance and vascular plant integrity

We examined the relationships between an index of wetland habitat quality and disturbance (ORAM score) and an index of vascular plant integrity (VIBI-FQ score) with moss species richness and a moss quality assessment index (MQAI) in 45 wetlands in three vegetation types in Ohio, USA. Species richness of mosses and MQAI were positively associated with ORAM and VIBI-FQ scores. VIBI-FQ score was a better predictor of both moss species richness and MQAI than was either ORAM score or vegetation type. This result was consistent with the strict microhabitat requirements for many moss species, which may be better assessed by VIBI-FQ than ORAM. Probability curves as a function of VIBI-FQ score were then generated for presence of groups of moss species having the same degree of fidelity to substrate and plant communities relative to other species in the moss flora (coefficients of conservatism, CCs). Species having an intermediate- or high degree of fidelity to substrate and plant communities (i.e., species with CC ≥ 5) had a 50% probability of presence (P₅₀) and 90% probability of presence (P₉₀) in wetlands with intermediate- and high VIBI-FQ scores, respectively. Although moss species richness, probability of presence of species based on CC, and MQAI may reflect wetland habitat quality, the 95% confidence intervals around P₅₀ and P₉₀ values may be too wide for regulatory use. Moss species richness, MQAI, and presence of groups of mosses may be more useful for evaluating moss habitat quality in wetlands than a set of “indicator species.”     

Effects of species evenness can be derived from species richness – ecosystem functioning relationships

Although species richness effects on ecosystem functioning have been studied thoroughly in countless experiments, the effects of the other side of diversity – species evenness – remain less identified, especially at high species richness. Due to the large number of different model ecosystems that need to be created, the explanatory power of the experimental approach for evenness is indeed limited. We show here that experimental studies on the influence of species richness on ecosystem functions contain hidden information on the influence of species evenness. Both the effects of maximum and minimum evenness, and of a key set of intermediate evenness levels, can be derived from species richness – ecosystem function curves, and that for every richness level, by using communities with low species richness as the equivalent of highly uneven communities with higher richness. We show that evenness effects on ecosystem functioning have the same direction as richness effects, however with increasing effect sizes at higher richness levels. We validated our technique for a wide range of ecosystem functions and applied it to the species richness – community biomass data from an existing biodiversity experiment. Our approach could provide a fast and easy alternative to resource‐intensive experiments in which evenness is experimentally varied, as we can build on the elaborate existing literature on species richness to assess its effects.     

Community assembly time and the relationship between local and regional species richness

Many previous studies have assumed that a linear relationship between local and regional species richness indicates that communities are limited by regional processes, while a saturating relationship suggests that species interactions restrict local richness. We show theoretically that the relationship between local and regional richness changes in a consistent fashion with assembly time in interacting communities. Communities show saturation in their early assembly stages because only a subset of the regional pool may colonize a locality. At intermediate assembly times, communities will appear unsaturated until significant competitive exclusion occurs. Finally, when communities reach equilibrium, we found saturation as a result of resource competition resulting in the dominance of a limited number of species. We show that habitat size and species fecundity are important in determining the time needed for the community to reach equilibrium and thus affect the relationship between local and regional species richness. Our results suggest the number of coexisting species is a function of local and regional processes whose relative influences might vary over time and that research using the relationship between local and regional species richness to infer mechanisms limiting species richness must have knowledge of the assembly time of the community.     

Species richness patterns in the understorey of Pyrenean Pinus sylvestris forest

Abstract. Species richness was studied in the understorey of natural Pinus sylvestris forest in the eastern Pyrenees. Understorey plant species were grouped in three structural groups as woody species, herbs and mosses. The response curves of total species richness and species richness of each structural group were fitted against environmental and stand-structural parameters, using Generalized Linear Models. The results suggested that, to predict species richness, environmental parameters were more important than tree-canopy structural parameters, in particular incoming radiation and soil nutrient concentration. The species richness response curve was often humped in relation to soil nutrient concentration. Different patterns of species richness were found for each structural group.     

On the estimation of species richness based on the accumulation of previously unrecorded species

Estimation of species richness of local communities has become an important topic in community ecology and monitoring. Investigators can seldom enumerate all the species present in the area of interest during sampling sessions. If the location of interest is sampled repeatedly within a short time period, the number of new species recorded is typically largest in the initial sample and decreases as sampling proceeds, but new species may be detected if sampling sessions are added. The question is how to estimate the total number of species. The data collected by sampling the area of interest repeatedly can be used to build species accumulation curves: the cumulative number of species recorded as a function of the number of sampling sessions (which we refer to as “species accumulation data”). A classic approach used to compute total species richness is to fit curves to the data on species accumulation with sampling effort. This approach does not rest on direct estimation of the probability of detecting species during sampling sessions and has no underlying basis regarding the sampling process that gave rise to the data. Here we recommend a probabilistic, nonparametric estimator for species richness for use with species accumulation data. We use estimators of population size that were developed for capture‐recapture data, but that can be used to estimate the size of species assemblages using species accumulation data. Models of detection probability account for the underlying sampling process. They permit variation in detection probability among species. We illustrate this approach using data from the North American Breeding Bird Survey (BBS). We describe other situations where species accumulation data are collected under different designs (e.g., over longer periods of time, or over spatial replicates) and that lend themselves to of use capture‐recapture models for estimating the size of the community of interest. We discuss the assumptions and interpretations corresponding to each situation.     

Community assessment techniques and the implications for rarefaction and extrapolation with Hill numbers

Diversity estimates play a key role in ecological assessments. Species richness and abundance are commonly used to generate complex diversity indices that are dependent on the quality of these estimates. As such, there is a long‐standing interest in the development of monitoring techniques, their ability to adequately assess species diversity, and the implications for generated indices. To determine the ability of substratum community assessment methods to capture species diversity, we evaluated four methods: photo quadrat, point intercept, random subsampling, and full quadrat assessments. Species density, abundance, richness, Shannon diversity, and Simpson diversity were then calculated for each method. We then conducted a method validation at a subset of locations to serve as an indication for how well each method captured the totality of the diversity present. Density, richness, Shannon diversity, and Simpson diversity estimates varied between methods, despite assessments occurring at the same locations, with photo quadrats detecting the lowest estimates and full quadrat assessments the highest. Abundance estimates were consistent among methods. Sample‐based rarefaction and extrapolation curves indicated that differences between Hill numbers (richness, Shannon diversity, and Simpson diversity) were significant in the majority of cases, and coverage‐based rarefaction and extrapolation curves confirmed that these dissimilarities were due to differences between the methods, not the sample completeness. Method validation highlighted the inability of the tested methods to capture the totality of the diversity present, while further supporting the notion of extrapolating abundances. Our results highlight the need for consistency across research methods, the advantages of utilizing multiple diversity indices, and potential concerns and considerations when comparing data from multiple sources.     

Is vascular plant species diversity a predictor of bryophyte species diversity in Mediterranean forests?

This study aimed to (i) investigate the congruence among the species composition and diversity of bryophytes and vascular plants in forests; (ii) test if site prioritization for conservation aims by the maximization of the pooled number of vascular plant species is effective to maximize the pooled number of bryophyte species. The study was performed in six forests in Tuscany, Italy. Four-hundred and twenty vascular plant species (61 of which were woody) and 128 bryophyte species were recorded in 109 plots. Despite the good predictive value of the compositional patterns of both woody plants and total vascular with respect to the compositional pattern of bryophytes, the species richness of the latter was only marginally related to the species richness of the former two. Bryophyte rare species were not spatially related to rare plant species and neither coincided with the sites of highest plant species richness. The species accumulation curves of bryophytes behaved differently with respect to those of woody plants or total vascular plants. Reserve selection analysis based on the maximization of the pooled species richness of either woody plants or total vascular plants were not effective in maximizing the pooled species richness of bryophytes. This study indicates that species diversity of vascular plants is not likely to be a good indicator of the bryophyte species diversity in Mediterranean forests.     

Patterns of Species Richness at Varying Scales in Western Kenya: Planning for Agroecosystem Diversification

Agroforestry tree domestication research is geared at promoting diversification of on-farm tree species composition. A survey was conducted in western Kenya with the objective of exploring possibilities for diversification for a particular agroecosystem, involving a complete tree census, tree measurement and collection of ethnobotanical information in 201 small-scale farms. Various approaches to landscape diversification were explored, including random distribution of trees to increase alpha richness and species richness at higher scales in the landscape, and random distribution of species composition over villages to increase the average richness of villages. The results showed that random distribution would result in increments of average species richness in the landscape, without requiring increments of total and average abundance. A new, fast and exact method of calculating site-based species accumulation curves was presented. The method yielded results that were extremely close to classical algorithms using 10,000 randomisations. Four use-groups (beverage, fodder, charcoal and soil fertility enhancement) were identified as use-groups with alpha richness smaller than one species, but only beverage and fodder had lowest richness at all scales (fruit and construction wood joined the four use-groups of lowest average species richness at higher scales in the landscape). The novel approaches used in this study could be used in future biodiversity studies on species accumulation patterns, or on spatial distribution patterns of species richness in a landscape.     

Differential hypogeous sporocarp production from Nothofagus dombeyi and N. pumilio forests in southern Argentina

Mycorrhizal fungi that form hypogeous sporocarps are an important component of the temperate forest soil community. In many regions, such as the Nothofagus forest in the Patagonian Andes, this group of fungi has been poorly studied. Here we examined the spring and autumn community composition of "sequestrate fungi", based on sporocarp production in pure forests of Nothofagus dombeyi (evergreen) and N. pumilio (deciduous). We investigated the possible relationships between these communities and environmental factors over 2 y. The rarefaction curves and the minimal richness estimates converged at nearly the same level for each forest type, and the asymptotes suggested that the sampling effort was sufficient to capture most of the hypogeous sporocarp richness in these forest stands. In total 27 species were recovered. Basidiomycota, Ascomycota and Glomeromycota respectively accounted for nine, two and one genera. Species richness of hypogeous sporocarps varied in relation to forest type but not to season (fall and spring), whereas sporocarp biomass varied according to an interaction between season and forest type. Species richness and sporocarp biomass were positively correlated with rainfall and negatively correlated with altitude. In addition sporocarp species richness was positively related to number of trees per transect. We found that two different forest stands, each dominated by different species of Nothofagus, exhibited different hypogeous sporocarp communities.     

Bird species numbers in an archipelago of reeds at Lake Velence, Hungary

1 Bird species numbers were studied on 109 reed islands at Lake Velence, Hungary, in the 1993 and 1994 breeding seasons. The aim was to describe and account for the abundance and distribution patterns of the bird species. 2 It was expected that an exponential model would fit the calculated species–area curves. However, for the 1993 data, both the power function (LogS ~ LogArea) and the exponential (S ~ LogArea) models did so, while the power function, exponential and linear (S ~ A) models fitted the curves for the 1994 data. 3 The results showed that the pattern was not random: a collection of small islands held more species than a few large islands with the same total area. 4 The relative species richness of small islands is a result of the preference of most common passerine bird species for the edges of reed islands. Most individuals were found in the first 5 m of the reedbed, and no edge avoidance was detected on a local spatial scale. Large, rarer species (e.g. Great White Egret), however, were found to be dependent on large reed islands. 5 Comparison of results with two other studies on bird communities of reed islands revealed that the type of landscape matrix (e.g. deep water, shallow water or agricultural lands) among reed patches significantly influences bird communities. Deep water was dominated by grebes and coot, shallow water by reed‐nesting passerines, and farmed areas by reed‐ and bush‐nesting passerines.     

Assessing biodiversity with species accumulation curves; inventories of small reptiles by pit‐trapping in Western Australia

Abstract We examined 11 non‐linear regression models to determine which of them best fitted curvilinear species accumulation curves based on pit‐trapping data for reptiles in a range of heterogeneous and homogenous sites in mesic, semi‐arid and arid regions of Western Australia. A well‐defined plateau in a species accumulation curve is required for any of the models accurately to estimate species richness. Two different measures of effort (pit‐trapping days and number of individuals caught) were used to determine if the measure of effort influenced the choice of the best model(s). We used species accumulation curves to predict species richness, determined the trapping effort required to catch a nominated percentage (e.g. 95%) of the predicted number of species in an area, and examined the relationship between species accumulation curves with diversity and rarity. Species richness, diversity and the proportion of rare species in a community influenced the shape of species accumulation curves. The Beta‐P model provided the best overall fit (highest r²) for heterogeneous and homogeneous sites. For heterogeneous sites, Hill, Rational, Clench, Exponential and Weibull models were the next best. For homogeneous habitats, Hill, Weibull and Chapman–Richards were the next best models. There was very little difference between Beta‐P and Hill models in fitting the data to accumulation curves, although the Hill model generally over‐estimated species richness. Most models worked equally well for both measures of trapping effort. Because the number of individuals caught was influenced by both pit‐trapping effort and the abundance of individuals, both measures of effort must be considered if species accumulation curves are to be used as a planning tool. Trapping effort to catch a nominated percentage of the total predicted species in homogeneous and heterogeneous habitats varied among sites, but even for only 75% of the predicted number of species it was generally much higher than the typical effort currently being used for terrestrial vertebrate fauna surveys in Australia. It was not possible to provide a general indication of the effort required to predict species richness for a site, or to capture a nominated proportion of species at a site, because species accumulation curves are heavily influenced by the characteristics of particular sites.