Complexity of multitrophic interactions in a grassland ecosystem depends on plant species diversity

1.?We studied the theoretical prediction that a loss of plant species richness has a strong impact on community interactions among all trophic levels and tested whether decreased plant species diversity results in a less complex structure and reduced interactions in ecological networks. 2.?Using plant species-specific biomass and arthropod abundance data from experimental grassland plots (Jena Experiment), we constructed multitrophic functional group interaction webs to compare communities based on 4 and 16 plant species. 427 insect and spider species were classified into 13 functional groups. These functional groups represent the nodes of ecological networks. Direct and indirect interactions among them were assessed using partial Mantel tests. Interaction web complexity was quantified using three measures of network structure: connectance, interaction diversity and interaction strength. 3.?Compared with high plant diversity plots, interaction webs based on low plant diversity plots showed reduced complexity in terms of total connectance, interaction diversity and mean interaction strength. Plant diversity effects obviously cascade up the food web and modify interactions across all trophic levels. The strongest effects occurred in interactions between adjacent trophic levels (i.e. predominantly trophic interactions), while significant interactions among plant and carnivore functional groups, as well as horizontal interactions (i.e. interactions between functional groups of the same trophic level), showed rather inconsistent responses and were generally rarer. 4.?Reduced interaction diversity has the potential to decrease and destabilize ecosystem processes. Therefore, we conclude that the loss of basal producer species leads to more simple structured, less and more loosely connected species assemblages, which in turn are very likely to decrease ecosystem functioning, community robustness and tolerance to disturbance. Our results suggest that the functioning of the entire ecological community is critically linked to the diversity of its component plants species.     

Avoiding surprise effects on Surprise Island: alien species control in a multitrophic level perspective

Eradications of invasive alien species have generally benefited biodiversity. However, without sufficient planning, successful eradications can have unexpected and unwanted consequences for native species and ecosystems. In particular, the “surprise effect” is the rapid increase of hitherto unnoticed species following the sudden removal of an invasive alien that was exerting an ecological force on those species (predation, competition or herbivory, for example). The only way to prevent these undesired outcomes is to adapt the control programme following the characterization of the trophic relationships between the invasive alien species and the invaded communities, that is, to view the control with a holistic perspective. Here, we illustrate this point with the study of the role of the ship rat (Rattus rattus), which invaded a tropical pacific atoll, Surprise Island, New Caledonia. We assessed the risk of surprise effects during a pre-eradication phase of several years, and then adapted our eradication strategy accordingly.     

Recurrent habitat disturbance and species diversity in a multiple-competitive species system

To address how species interactions, dispersal and environmental disturbances interplay to affect the spatial distribution and diversity of species, we present a compartment model in which multiple species undergo competitive interaction of Lotka-Volterra type in a patchy environment arranged in a square lattice. Dispersal of species occurs between adjacent patches. Disturbances are periodically imposed on a central part of the environment in a belt-like block or an island-like block of various sizes where each species is killed for a certain time interval and then allowed to recover for the rest of a disturbance cycle. We deal with a case in which the local population dynamics within each patch is analytically determinable and has multiple locally stable equilibrium states in the absence of environmental disturbance. We further assume a trade-off between the reproductive rate of species and its dispersal ability. With these settings, we numerically examine how the spatio-temporal distributions of species are affected by changes in the pattern, size and duration of disturbances. The results demonstrate that: (1) in the undisturbed area, environmental disturbances could generate spatially segregated distributions of species; (2) in the disturbed area, species with higher dispersal abilities quickly invade and preferentially recover their population during the post-disturbance period, being temporarily relieved of competition from other species. These mechanisms collectively lead to increased species diversity in the whole habitat, functioning best when both the size and duration of disturbances are intermediate. In particular, the belt-like disturbance is more effective than the island-like disturbance in sustaining spatial heterogeneity for a wider range of duration of disturbance.     

Effects of anthropogenic habitat disturbance on local pollinator diversity and species turnover across a precipitation gradient

Anthropogenic habitat disturbance can have profound effects on multiple components of forest biotas including pollinator assemblages. We assessed the effect of small-scale disturbance on local richness, abundance, diversity and evenness of insect pollinator fauna; and how habitat disturbance affected species turnover across the landscape and overall diversity along a precipitation gradient in NW Patagonia (Argentina). We evaluated the effect of disturbance on overall pollinator fauna and then separately for bees (i.e. Apoidea) and non-bee pollinators. Locally, disturbed habitats had significantly higher pollinator species richness and abundances than undisturbed habitats for the whole pollinator assemblage, but not for bees or non-bees separately. However, significant differences in species richness between habitats vanished after accounting for differences in abundance between habitat types. At a local scale Shannon–Weaver diversity and evenness did not vary with disturbance. A β diversity index indicated that, across forest types, species turnover was lower between disturbed habitats than between undisturbed habitats. In addition, rarefaction curves showed that disturbed habitats as a whole accumulated fewer species than undisturbed habitats at equivalent sample sizes. We concluded that small patches of disturbed habitat have a negligible effect on local pollinator diversity; however, habitat disturbance reduced β diversity through a homogenization of the pollinator fauna (in particular of bees) across the landscape.     

Parallel changes in genetic diversity and species diversity following a natural disturbance

We examined the spatial and temporal variation of species diversity and genetic diversity in a metacommunity comprising 16 species of freshwater gastropods. We monitored species abundance at five localities of the Ain river floodplain in southeastern France, over a period of four years. Using 190 AFLP loci, we monitored the genetic diversity of Radix balthica , one of the most abundant gastropod species of the metacommunity, twice during that period. An exceptionally intense drought occurred during the last two years and differentially affected the study sites. This allowed us to test the effect of natural disturbances on changes in both genetic and species diversity. Overall, local (alpha) diversity declined as reflected by lower values of gene diversity H _S and evenness. In parallel, the among-sites (beta) diversity increased at both the genetic ( F _ST) and species ( F _STC) levels. These results suggest that disturbances can lead to similar changes in genetic and community structure through the combined effects of selective and neutral processes.     

Effects of a natural flood disturbance on species richness and beta diversity of stream benthic diatom communities

Natural hydrological disturbances in streams may reduce biomass and species richness and change community composition within streams. Disturbances can also affect beta diversity among streams if their effects are species specific or vary across sites. We investigated the effect of a natural flood on species richness, community composition and among-streams beta diversity of benthic diatoms (total community and three functional groups: low profile, high profile and motile) of seven streams in New Zealand. Sampling occurred shortly before, 10 days after and 40 days after the flood. Species richness of the total diatom community did not change after the flood. The high-profile group was the only one whose species richness declined after the flood, whereas species richness of the low-profile group increased. Community composition changed after the flood, mostly as a result of species replacement rather than richness differences over time. Finally, among-streams beta diversity did not change after the flood, suggesting that variation in species composition of benthic diatoms among streams may be maintained in the face of flood disturbances.     

Effects of fire disturbance on ant abundance and diversity: a global meta-analysis

We conducted a meta-analysis of the effects of fire on the abundance and alpha diversity of ants based upon data published over the past 70 years. Overall, fire reduced ant diversity by 18 %, but had no effect on ant abundance. However, there was significant variation in the effect of fire on ant diversity amongst different vegetation types. Fire significantly decreased ant diversity in forests—especially in tropical forests—whereas in deserts, grasslands, and savannas it did not. Similarly, fire had a strong negative mean effect on ant diversity in sites where it is uncommon, but did not significantly affect diversity where it is a recurrent phenomenon. There is evidence that, in forests, wildfires have a stronger negative effect on ant diversity than does prescribed burning. In addition, we found marginally significant differences in the effect of fire on the abundance and diversity of forest ants among studies that sampled ants at different times post-fire, or that sampled ants from different soil strata. In contrast, fire did not significantly affect the abundance or diversity of savanna ants, and this was true even after we took into account the geographic location of the study, the ant community sampled, the time since fire, and the fire regime. Overall, the results of our study indicate that habitat type is an important predictor of ant community responses to fire. However, even within a given habitat, reported effects were quite variable among the studies reviewed, evidencing the idiosyncratic nature of fire effects on ants.     

Chemical ecology of host-plant selection by herbivorous arthropods: a multitrophic perspective

Most herbivorous arthropods are specialists that feed on one or a few related plant species. To understand why this is so, both mechanistic and functional studies have been carried out, predominantly restricted to bitrophic aspects. Host-selection behaviour of herbivorous arthropods has been intensively studied and this has provided ample evidence for the role of secondary plant chemicals as source of information in behavioural decisions of herbivores. Many evolutionary studies have regarded co-evolution between plants and herbivores to explain the diversity of secondary plant chemicals and host specialisation of herbivores. However, many cases remain unexplained where herbivores select host plants that are suboptimal in terms of fitness returns. A stimulating paper by Bernays and Graham [(1988) Ecology 69, 886-892)] has initiated a discussion on the need of a multitrophic perspective to understand the evolution of host-plant specialisation by herbivorous arthropods. However, this has hardly resulted in ecological studies on host-selection behaviour that take a multitrophic perspective. Yet, evidence is accumulating that constitutive and induced infochemicals from natural enemies and competitors can affect herbivore behaviour. These cues may constitute important information on fitness prospects, just as plant cues can do. In this paper I selectively review how information from organisms at different trophic levels varies in space and time and how herbivores can integratively exploit this information during host selection. In doing so, research areas are identified that are likely to provide important new insights to explain several of the questions in herbivore host selection that remain unanswered so far. These research areas are at the interface of evolutionary ecology, behavioural ecology and chemical ecology.     

Effects of productivity and disturbance on species richness: a neutral model

Productivity and disturbance are major determinants of species diversity, and results from theoretical models predict that species richness should peak at intermediate levels of both factors. Such "unimodal" responses have been documented in many field and laboratory studies and have usually been attributed to differences among species in competitive ability and/or trade-offs between competitive ability and tolerance to disturbance. Here we show that most documented patterns of disturbance-richness and productivity-richness relationships, as well as the observed interactions between the two factors, can be explained by a simple neutral model where all species are ecologically identical and lack trade-offs in species characteristics. This finding suggests that current neutral theories can be extended to explain patterns of species responses to productivity and disturbance.     

Impact of pocket gopher disturbance on plant species diversity in a shortgrass prairie community

Summary We examined the impact of pocket gopher disturbances on the dynamics of a shortgrass prairie community. Through their burrowing activity, pocket gophers (Thomomys bottae) cast up mounds of soil which both kill existing vegetation and create sites for colonization by competitively-inferior plant species. Three major patterns emerge from these disturbances: First, we show that 10 of the most common herbaceous perennial dicots benefit from pocket gopher disturbance; that is, a greater proportion of seedlings are found in the open space created by pocket gopher disturbance than would be expected based on the availability of disturbed habitat. Additionally, these seedlings exhibited higher growth rates than adjacent seedlings of the same species growing in undisturbed habitat. Second, we tested two predictions of the Intermediate Disturbance Hypothesis and found that species diversity was greatest for plots characterized by disturbances of intermediate age. However, we did not detect significant differences in diversity between plots characterized by intermediate and high levels of disturbance, indicating that many species are adapted to or at least tolerant of high levels of disturbance. Third, we noted that the abundance of grasses decreased with increasing disturbance, while the abundance of dicots increased with increasing disturbance.     

昆虫与植物的协同进化:寄主植物-铃夜蛾-寄生蜂相互作用

近数10年内,Ehrlich和Raven于1964年提出的协同进化理论及Jermy于1976年提出的顺序进化理论极大地促进了对昆虫与植物相互作用的研究。文章首先简要介绍有关理论,对植食性昆虫与植物关系研究的若干核心问题进行评述。主要问题包括(1)植食性昆虫如何选择寄主植物?(2)植物次生物质是否作为植物防御昆虫取食的重要屏障?(3)昆虫能否适应植物的化学防御?(4)植食性昆虫寄主范围是否是从广到专演化的?随之,作者结合对铃夜蛾Helicoverpa系统研究取得的结果,对上述问题做了进一步的论证和阐述。最后,在继承协同进化、顺序进化等理论精髓的基础上,根据当今三营养级相互作用领域的研究新进展,提出一个新的假说,即多营养级协同进化假说。该假说肯定植物次生物质在植物防御和昆虫识别寄主植物上的重要作用,同时把其他营养级并列放入交互作用的系统,特别强调第三营养级在昆虫与植物关系演化过程中的参与和寄主转移与昆虫食性专化和广化的联系。     

Experimental disturbance and the maintenance of species diversity in a stream community

Summary In order to test the role of disturbance and the effects of disturbance frequency on stream communities, an experiment was conducted in New Hope Creek, North Carolina, USA. Patches of cobbles were tumbled 0, 1 or 2 times in a 6 week span. These tumbling disturbances lasted only 30 seconds. The recovery of the macroinvertebrates was monitored.Most taxa showed major reductions in population density immediately following the disturbance. The percent reduction of a given taxon in disturbed vs. control patches ranged from 21.4–95%. Recovery to near normal population levels was achieved in about four weeks. A second disturbance caused similar population reductions as the first one, and delayed the recovery.The macroinvertebrate community in cobbles was demonstrated to be resilient in that populations quickly regained their predisturbance densities. Rare taxa did not selectively colonize disturbed patches. The implications of these findings for the intermediate disturbance hypothesis and the structure of stream communities is discussed. Disturbance is a major determinant of lotic community structure and species diversity.     

Flood disturbance and riparian species diversity on the Colorado River Delta

We investigated the influence of channel migration and expansion on riparian plant species diversity along the lower Colorado River near the United States–Mexico border. Using repeat aerial photography in a GIS we identified and classed areas of low, moderate, and high disturbance frequency caused by channel expansion and migration. Replicate vegetation plots (12m×12m) were sampled in each of the three disturbance classes. One-way ANOVA was used to test for differences in species richness, species diversity (using the Shannon–Weiner Index) and overall percent ground cover of plants between the three disturbance classes. Regardless of disturbance class, plots were dominated by trees or shrubs, especially the non-native Tamarix ramosissima, as well as Pluchea sericea, Baccharis salicifolia and Salix goodingii. Clearly woody species constitute the great bulk of overall species richness, percent ground cover, and species diversity (H) in each disturbance group. No overall statistically significant differences were revealed among the disturbance groups for values of species richness, percent ground cover, or the Shannon–Wiener Index, though paired contrasts of means revealed that total percent ground cover on low disturbance plots was significantly higher than on moderately disturbed plots. Spatial and temporal variability in riparian diversity in the study area appears to hinge on factors other than disturbance frequency such as salt or drought stress. Alternately, our results could be interpreted as suggesting that in the presence of intensive flow regulation, disturbance plays a secondary role to ecological stresses, similar to that demonstrated by others. Intentional flood pulses are advocated as a restorative management strategy for improving plant productivity, management of exotic species (particularly T. ramosissima), and restoration of overall biodiversity.     

Predation and disturbance interact to shape prey species diversity

Though predation, productivity (nutrient richness), spatial heterogeneity, and disturbance regimes are known to influence species diversity, interactions between these factors remain largely unknown. Predation has been shown to interact with productivity and with spatial heterogeneity, but few experimental studies have focused on how predation and disturbance interact to influence prey diversity. We used theory and experiments to investigate how these factors influence diversification of Pseudomonas fluorescens by manipulating both predation (presence or absence of Bdellovibrio bacteriovorus) and disturbance (frequency and intensity of disturbance). Our results show that in a homogeneous environment, predation is essential to promote prey species diversity. However, in most but not all treatments, elevated diversity was transitory, implying that the effect of predation on diversity was strongly influenced by disturbance. Both our experimental and theoretical results suggest that disturbance interacts with predation by modifying the interplay of resource and apparent competition among prey.     

Intermediate disturbance in the ecology of phytoplankton and the maintenance of species diversity: a synthesis

This paper concludes a collection of contributions presented at the 8th Workshop of the International Association of Phytoplankton Taxonomy and Ecology. It derives a consensus as to the virtues and strengths of J. H. Connell's Intermediate Disturbance Hypothesis (IDH), its applicability to phytoplankton ecology and its theoretical and practical weaknesses. The view is expressed that the IDH is too useful a concept to reject and that, as a word model, it provides a powerful link between diversity and disturbance. The more robust investigations that are necessary to consolidate the tenancy of IDH need to concentrate upon the separation and quantification of the stimulus- and response-components of disturbance.     

Patterns of plant species diversity during succession under different disturbance regimes

Summary I suggest that between-community variations in diversity patterns during succession in plant communities are due to the effects of selection on life history strategies under different disturbance regimes. Natural disturbances to plant communities are simultaneously a source of mortality for some individuals and a source of establishment sites for others. The plant community consists of a mosaic of disturbance patches (gaps) of different environmental conditions. The composition of the mosaic is described by the size-frequency distribution of the gaps and is dependent on the rates and scales of disturbance. The life-history strategies of plant species dependent on some form of disturbance for establishment of propagules should reflect this size-frequency distribution of disturbance patches. An extension of island biogeographic theory to encompass relative habitat area predicts that a community should be most rich in species adapted to growth and establishment in the spatially most common patch types. Changes in species diversity during succession following large scale disturbance reflect the prevalent life history patterns under historically common disturbance regimes. Communities in which the greatest patch area is in large-scale clearings (e.g. following fire) are most diverse in species establishing seedlings in xeric, high light conditions. Species diversity decreases during succession. Communities in which such large patches are rare are characterized by a large number of species that reach the canopy through small gaps and realtively few which regenerate in the large clearings. Diversity increases during succession following a large scale disturbance.Evidence from communities characterized by different disturbance regimes is summarized from the literature. This hypothesis provides an evolutionary mechanism with which to examine the changes in plant community structure during succession. Diversity peaks occurring at intermediate levels of disturbance as discussed by Connell and Huston are interpreted in this context.     

Gap formation and species diversity in Japanese beech forests: a test of the intermediate disturbance hypothesis on a geographic scale

To evaluate whether the intermediate-disturbance hypothesis applies on regional scales, the relationship between the species diversity and gap formation regime of beech forests was examined. The mean gap size and the variation of gap sizes showed no correlation with species diversity. The mean windstorm interval varied widely, but geographical trends, such as latitudinal gradient, were not observed. However, locations that sustained an intermediate frequency of disturbance had the highest species diversity. Although a latitudinal gradient of disturbance was not apparent, the intermediate-disturbance hypothesis was partly supported on a geographic scale. The most predictable model for species diversity was a multiple regression model composed of two factors, the windstorm interval and the cumulative temperature of the growing season. The fact that the temperature was of greater importance than the disturbance interval indicates that the most important factor in predicting forest species diversity is the amount of available energy on a geographic scale.