The role of zooplankton predator-prey interactions in structuring plankton communities

This group of papers derives from a session, organized heldto recognize the fundamental contributions made by Gustav-AdolfPaffenhöfer to the field of zooplankton behavioural ecology.The session was organized by his colleagues, Maria Grazia Mazzocchi,Marie Bundy and Done Deibel, at the American Society of Limnologyand Oceanography Aquatic Sciences meeting in Salt Lake City,20–25 February     

The role of indirect interactions in structuring tropical insect communities

Apparent competition is a form of indirect interaction among species that can potentially structure biological communities. In insect communities, parasitoid-mediated apparent competition has been proposed as a particularly important structuring force. We argue that short-term apparent competition may be less important in structuring insect communities in tropical regions, compared with temperate regions. This prediction arises because, compared with temperate insects, tropical insects that share natural enemies are more likely to be isolated in both space and time.     

Evaluating the role of fire disturbance in structuring small reptile communities in temperate forests

Fire is an integral disturbance shaping forest community dynamics over large scales. However, understanding the relationship between fire induced habitat disturbance and biodiversity remain equivocal. Ecological theories including the intermediate disturbance hypothesis (IDH) and the habitat accommodation model (HAM) offer predictive frameworks that could explain faunal responses to fire disturbances. We used an 80 year post-fire chronosequence to investigate small reptile community responses to fires in temperate forests across 74 sites. First, we evaluated if changes in species richness, abundance and evenness post-fire followed trends of prior predictions, including the IDH. Second, using competing models of fine scale habitat elements we evaluated the specific ways which fire influenced small reptiles. Third, we evaluated support for the HAM by examining compositional changes of reptile community post-fire. Relative abundance was positively correlated to age post-fire while richness and evenness showed no associations. The abundance trend was as expected based on the prior prediction of sustained population increase post-disturbance, but the trend for richness contradicted the prediction of highest diversity at intermediate levels of disturbance (according to IDH). Abundance changes were driven mainly by changes in overstorey, ground layer, and shelter, while richness and evenness did not associate with any vegetation parameter. Community composition was not strongly correlated to age since fire, thus support for the HAM was weak. Overall, in this ecosystem, frequent fire disturbances can be detrimental to small reptiles. Future studies utilizing approaches based on species traits could enhance our understanding of biodiversity patterns post-disturbance.     

The role of the floodplain gradient in structuring of testate amoebae communities in the Ilych River

Forty-two testate amoebae taxa were identified in alluvial soils of floodplain islands in the Ilych River. Among the pedo- and eurybionts, there were aquatic rhizopods. Along the floodplain transect (willow meadow ?? deciduous forest ?? coniferous forest), the testate amoebae community changed directly. There are spatially homogeneous (low beta-diversity) testacean communities but species rich on the local level (high alpha-diversity) within forests. Within willows and meadows, communities are characterized by low alpha-diversity and high heterogeneity that leads to high gamma-diversity.     

Uncertain biotic and abiotic interactions in benthic communities

We analyze marine benthic communities at different sites in Skagerrak with the purpose of understanding the role of exogenous and endogenous factors in explaining the species' temporal dynamics. The previous finding that the dynamics of these species communities are mainly driven and synchronized by environmental (temperature) forcing was only weakly supported when analyzing single-species dynamics at five sites where four of the species were present every year. There was no consistent pattern in how the temperature affected the realized per capita growth rate, either across species at a given site, or among sites for a given species. Furthermore, there was no net-interaction from the community on a given species strong enough to give rise to second-order dynamics. However, when implementing a Multi Dimensional Scaling (MDS) analysis and incorporating all sampling sites and species -we found that the different communities clustered in relation to depth, hence, communities at the same depth were more "similar" than communities at different depth. Revealing the underlying interactions shaping these marine benthic communities is a challenge that calls for an array of various and complementary approaches.     

The role of reproductive plant traits and biotic interactions in the dynamics of semi-arid plant communities

The dynamics of semi-arid plant communities are determined by the interplay between competition and facilitation among plants. The sign and strength of these biotic interactions depend on plant traits. However, the relationships between plant traits and biotic interactions, and the consequences for plant communities are still poorly understood. Our objective here was to investigate, with a modelling approach, the role of plant reproductive traits on biotic interactions, and the consequences for processes such as plant succession and invasion. The dynamics of two plant types were modelled with a spatially-explicit integrodifferential model: (1) a plant with seed dispersal (colonizer of bare soil) and (2) a plant with local vegetative propagation (local competitor). Both plant types were involved in facilitation due to a local positive feedback between vegetation biomass and soil water availability, which promoted establishment and growth. Plants in the system also competed for limited water. The efficiency in water acquisition (dependent on reproductive and growth plant traits) determined which plant type dominated the community at the steady state. Facilitative interactions between plant types also played an important role in the community dynamics, promoting establishment in the driest conditions and recovery from low biomass. Plants with vegetative propagation took advantage of the ability of seed dispersers to establish on bare soil from a low initial biomass. Seed dispersers were good invaders, maintained high biomass at intermediate and high rainfall and showed a high ability in taking profit from the positive feedback originated by plants with vegetative propagation under the driest conditions. However, seed dispersers lost competitiveness with an increasing investment in fecundity. All together, our results showed that reproductive plant traits can affect the balance between facilitative and competitive interactions. Understanding this effect of plant traits on biotic interactions provides insights in processes such as plant succession and shrub encroachment.     

The role of specialist parasites in structuring host communities

Natural enemies can be a powerful force when structuring natural communities, and in facilitating or preventing species coexistence depending on the nature of the trophic interaction. In particular, “keystone” predators can promote species coexistence, provided they preferentially attack the competitively dominant species. However, it is not clear whether parasites can play a similar structuring role; parasites typically form chronic associations with their victims, reducing their fitness (i.e., fecundity) rather than survival, and allowing infected hosts to remain viable competitors within the community. Therefore the density-dependent suppression of the host is likely to be more subtle than that due to predation. Using a series of simple population-dynamic models we show that specialist parasites can facilitate species coexistence, although possibly less so than predators. These results contrast with those typically found with models of generalist parasites, which can reduce the likelihood of species coexistence through apparent competition. In addition, we show that the likelihood of parasite-facilitated species coexistence depends greatly on the specific type of parasite. In particular, macroparasites (e.g., parasitic helminths) may be less likely to facilitate species coexistence than microparasites (e.g., viruses or bacteria) due to their typically highly aggregated distribution amongst their hosts. Furthermore, species coexistence is more likely if the parasite is relatively benign to its host. Parasitism by apparently “harmless” specialist parasites may provide an important but overlooked factor in the maintenance of species diversity, facilitating species invasions into new communities and the emergence of novel infectious diseases.     

The varying role of population abundance in structuring indices of biotic homogenization

Aim An important component of human‐induced global change is the decrease or increase in community distinctiveness (taxonomic homogenization or differentiation, respectively) that follows the loss of native species and gain of non‐native species. We use simulation approaches to assess the extent to which conclusions about the outcome of the homogenization process depend on whether or not abundance data are incorporated. Location Data were produced through computer simulation. Methods The frequency with which occurrence‐based similarity indices and abundance‐based similarity indices give different views of changes in community similarity, and the conditions under which such differences occurred were assessed using both deterministic and stochastic modelling approaches to simulate species assemblage states. Results Occurrence‐based and abundance‐based indices were positively correlated across the set of simulations for both the deterministic and stochastic models. However, in both situations approximately one quarter (25%) of models resulted in contrasting outcomes for the two approaches of calculating changes in compositional similarity; that is, one data type showed a positive value (homogenization), whereas the other showed a negative value (differentiation). Main conclusions In the majority of cases, species abundances will not change drastically enough after perturbation to produce large differences between homogenization scores measured using occurrence versus abundance information. However, in cases where these changes are large, it is important to recognize that the choice of metric to analyse homogenization trends will influence the qualitative and quantitative conclusions drawn. Studies of real assemblages are therefore necessary to evaluate the role of species abundance in defining the magnitude and direction of changes in community composition across space, and the implications of these changes for native biodiversity.     

The biotic resistance role of fish predation in fouling communities

Predation may often influence native species dynamics and so may be important for the control of introduced species as well. Here, we examine how predation can regulate fouling communities on artificial substrates in the coast of Brazil. Specifically, we tested whether predators limit colonization and establishment of introduced species. A predation experiment using plastic plates as experimental replicates (predator excluded and not excluded) was carried out in Ceará, Bahia, São Paulo and Santa Catarina, between 3°S and 27°S. Ninety-eight species colonized the plates, 14 of which were introduced. While species richness was similar among the treatments (except in Santa Catarina), community structure varied by treatment and introduced species were less abundant in the predation treatment at all sites. Also, predation was selective and controlled introduced, poorly defended, species, specifically ascidians. Thus, biotic resistance driven by predation was not ubiquitous but apparently targets one group of major economic and environmental impact. As a consequence, introduced barnacles were released from competition with dominant ascidians and thus the ecosystem service (biotic resistance) performed by fish alone was unable to completely avoid or control the introduction of non-native species. Control measures must rely on various approaches, but if we want fish to contribute in this control, it is very important to conserve the local native assembly of predators in order to reap their benefits in this ecosystem service.     

Salinity gradient and nutrients as major structuring factors of the phytoplankton communities in salt marshes

This study investigated the major environmental factors structuring, for a year, phytoplankton assemblages in the Sfax saltern (Tunisia): salinity and nutrients. A STATICO analysis based on 11 environmental variables and the abundances of 64 phytoplanktonic species was conducted. STATICO is used to analyze the stable part of the relationships between the environment and species, and then to determine how these relationships change over time. The analysis confirmed that the salinity gradient had a considerable influence on the composition of the phytoplanktonic communities. Bacillariophyceae and Dinophyceae dominated in the least salty ponds, whereas Chlorophyceae and Cyanophyceae dominated in the saltiest ponds, in accordance with the halotolerance level estimated for each species by calculating the optimum salinity and salt tolerance. Nitrogen (N) and phosphorus (P) ions could have a secondary influence on the phytoplankton distribution and its dynamics. Dinophyceae seem to be favored by high ammonium (NH₄ ⁺) concentrations, whereas diatoms seem to be favored by high orthophosphates (PO₄ ^3?) and nitrates (NO₃ ^?) values. The Chlorophyceae Dunalliela salina thrived in the saltiest ponds when the NO₃ ^? concentrations increased.     

The role of a spiny plant refuge in structuring grazed shortgrass steppe plant communities

This study evaluates the hypothesis that biological grazing refuges have an important role in plant-grazer interactions of grasslands with a long history of grazing. We assessed the hypothesis that clumps of the spiny cactus Opuntia polyacantha provide biological refuges from cattle grazing, affecting cover and seedhead production of associated vascular plants in the shortgrass steppe of the North America. The study was based on sampling inside and outside Opuntia clumps in eight long-term moderately grazed pastures established 60 yr ago and their respective ungrazed controls. Opuntia clumps provided a refuge for seedhead production of the dominant grass ( Bouteloua gracilis ) and for cover and seedhead production of many plant groups. Clumps were also a refuge for species sensitive to grazing (species that decrease with grazing) and barrel-cacti, but not for species preferred by cattle (species with greater proportion in the diet than in the field), exotics or weeds. Our results suggest that these effects were mainly through changes in the microenvironmental conditions resulting from protection effects, even though all potential microenvironmental effects could not be measured. Cacti promoted some negative effects on other plant groups, probably due to the space occupied by cladodes inside cactus clumps. The refuge effects observed at the group level did not translate into strong community level effects. Species diversity ( H' ) was greater in cactus clumps due to lower dominance rather than greater richness. The presence of Opuntia clumps increased landscape-scale diversity. This ecological role of Opuntia clumps as refuge from cattle grazing should be taken into consideration in management practices aimed at cactus eradication in order to increase forage availability for livestock. We discuss the potential role of plant community productivity and grazing history with regard to the importance of natural refuges in structuring grassland communities.     

Organism–quartz interactions in structuring benthic communities: towards a marine bio-mineralogy?

Abiotic and biotic factors determine the distribution and structure of marine benthic communities but the processes invoked to explain such patterns are often inadequate. We demonstrate that the presence of quartz grains in sediments inhibits animal colonization on marine soft-bottoms. We suggest that this phenomenon may be related to the oxidant properties of the quartz crystal surface derived by the generation of silicon-based radicals due to water movement. The interaction between organisms and the crystallographic structure of substrata might play a neglected but important role in structuring coastal benthic communities.     

Consensus genetic structuring and typological value of markers using multiple co-inertia analysis

Working with weakly congruent markers means that consensus genetic structuring of populations requires methods explicitly devoted to this purpose. The method, which is presented here, belongs to the multivariate analyses. This method consists of different steps. First, single-marker analyses were performed using a version of principal component analysis, which is designed for allelic frequencies (%PCA). Drawing confidence ellipses around the population positions enhances %PCA plots. Second, a multiple co-inertia analysis (MCOA) was performed, which reveals the common features of single-marker analyses, builds a reference structure and makes it possible to compare single-marker structures with this reference through graphical tools. Finally, a typological value is provided for each marker. The typological value measures the efficiency of a marker to structure populations in the same way as other markers. In this study, we evaluate the interest and the efficiency of this method applied to a European and African bovine microsatellite data set. The typological value differs among markers, indicating that some markers are more efficient in displaying a consensus typology than others. Moreover, efficient markers in one collection of populations do not remain efficient in others. The number of markers used in a study is not a sufficient criterion to judge its reliability. "Quantity is not quality".     

The role of geologic grazing refuges in structuring Mediterranean grassland plant communities

We tested the hypothesis that small rock-enclosed geologic refuges have an important role in maintaining grazing-sensitive species in grassland with a long history of intense grazing. The study was carried out in Mediterranean grassland in a basalt landscape in northern Israel. Community composition was compared in 63 sites between samples of 1 m² quadrats in two microhabitats: (1) rock-enclosed, presumed “refuges” and (2) rock-adjacent, “near refuges” but just outside them, accessible to cattle grazing. Median refuge area was 2.75 m², median rock height and diameter around refuges were 1.10 m and 1.50 m. Median height of residual dry herbage was 1.41 m in refuges, compared to 0.38 m outside, indicating the difference in grazing intensity. Species richness at three scales (quadrat, site, all sites) was significantly greater in the near-refuge than in the refuge habitat. In the latter, many annuals were excluded by dominance of tall perennials. Twelve species (of 103) had significantly higher cover in refuges, including tall perennial grasses, tall annuals, climbers, and a shrub. A total of 53 species with a strong significant negative response to refuges were mostly small and medium height annuals. The intermediate group of 38 species with weak or non-significant responses to refuges included, among others, dominant tall grasses that were abundant both in refuges and just outside them. The latter, as well as most refuge-positive species had shown a positive response to protection in exclosures. The results support the hypothesis that small rock-enclosed habitats—more so than artificial exclosures—are effective grazing refuges for rare, grazing-susceptible species. The contribution of refuges to species richness at the landscape scale is much greater than their proportion of the area. Dispersion from refuges maintains small populations of rare species near refuges and can initiate expansion into the landscape when grazing pressure is lowered.     

Evaluating the multiple mechanisms underlying herbivore-algal interactions in streams

Stream algal responses to herbivory were investigated under different environmental conditions. Snail densities and nutrient concentrations were manipulated in experimental enclosures to document the influence of nutrient availability on the magnitude of algal responses to herbivory. Periphyton mats in other enclosures were subjected to physical disruption by artificial means to evaluate the influence of disturbance on algal abundance. The impact of herbivory on algal abundance decreased substantially with increase in water column nutrient concentrations. This result was explained by findings that: (1) algal accumulation was constrained by nutrient availability under ambient water quality conditions. (2) accumulation of most algal populations was stimulated by nutrient enrichment only under grazed conditions. Thus, snail grazing simultaneously exerted a negative impact on algal abundance, by removal and consumption of a portion of the periphyton mat, and a stimulative effect, by increasing the availability of nutrients to remaining cells. Algal responses to artificial disturbance indicated that stimulative effects of herbivory were caused by the physical disruption of thr assemblage rather than by other processes (e. g., nutrient regeneration). However, consumptive losses far outweighed stimulative effects on algal abundance under ambient nutrient conditions. The magnitude of these two antagonistic effects was comparable under enriched conditions because grazing had only slight effects on algal abundance. Thus, the importance of different mechanisms of algal-herbivore interactions is strongly influenced by ambient environmental conditions, a finding that has important implications for predicting the outcome of herbivore-algal interactions in ecosystems with pronounced temporal and spatial variation in biotic and abiotic conditions.     

Evaluating the role of regional and local processes in structuring a larval trematode metacommunity of Helisoma trivolvis

Metacommunity theory has advanced our understanding of how local and regional processes affect the structure of ecological communities. While parasites have largely been omitted from metacommunity research, parasite communities can provide the large sample sizes and discrete boundaries often required for evaluating metacommunity patterns. Here, we used assemblages of flatworm parasites that infect freshwater snails (Helisoma trivolvis) to evaluate three questions: 1) what factors affect individual host infections within ponds? 2) Is the parasite metacommunity structured among ponds? And 3) what is the relative role of local versus regional processes in determining metacommunity structure and species richness among ponds? We examined 10 821 snails from 96 sites in five park complexes in the San Francisco Bay area, California, and found 953 infections from six parasite groups. At the within‐pond level, infection status of host snails correlated positively with individual snail size and pond infection prevalence for all six parasite groups. Using an ordination method to test for metacommunity structure, we found that the parasite metacommunity was organized in a non‐random pattern with species responding individually along an environmental gradient. Based on a model selection approach involving local and regional predictors, parasite species richness and metacommunity structure correlated with both local abiotic (pH and total dissolved nitrogen) and biotic (non‐host mollusk density, and H. trivolvis biomass) factors, with little support for regional predictors. Overall, this trematode metacommunity most closely followed the predictions from the species sorting or mass effects metacommunity paradigm, in which community diversity is filtered by local site characteristics.