Field Removal Experiment Research Articles

Plant–soil feedbacks in a diverse grassland: Soil remembers, but not too much

Abstract Plant–soil feedbacks (PSFs) are one of the major drivers of species coexistence in highly dynamic and low‐diversity communities, such as early successional stands or agricultural systems. However, in stable and species‐rich communities, PSFs are much more complex because of numerous species interactions. In this study, we aimed to reveal if PSFs of a dominant species can be detected within a diverse plant community and to what extent they can be influenced by the effects of co‐occurring species. We investigated PSFs between a dominant grass, Festuca rubra, and three other species from a species‐rich mountain grassland, taking advantage of the 18‐year field removal experiment. We tested whether such a long‐term presence/absence of Festuca can shape the feedback of the grassland community and whether these effects prevail in the lower soil layer, where Festuca roots are relatively more abundant, compared to the upper soil layer. We evaluated how experimentally induced soil legacies of Festuca in a pot experiment are modified by subsequently grown plants, both at the level of plant responses and changes in abiotic and biotic soil properties. At the level of the entire community, the soil legacies of the dominant Festuca were not detectable. However, the responses of the plants differed between the soil samples from the upper and lower soil layer. The pot experiment showed that the soil legacies of subsequently growing plants interact, influencing soil properties as well as plant responses to these altered soils. Generally, we found a stronger signal of the most recent conditioning, although the effect of the first conditioning plant was still detectable in many of the measured variables. Plant biomass in the feedback phase was mainly linked to the levels of plant‐available soil nitrogen, although it was also affected by the composition of microbial communities. Synthesis. We showed that plant‐induced soil legacies can be altered by legacies of co‐occurring species, complicating plant–soil feedbacks in diverse communities. Despite the detectable legacy effects on final plant biomass, in the short term, plant growth responds more strongly to the levels of available nutrients. We also highlight the vertical distribution of plant–soil feedbacks.

Seed removal, seed dispersers, and the allocation of tissues in Myrtaceae seeds

AbstractTo avoid seed predation, plants may invest in protective seed tissues. Often related to seed size, allocation in seeds' physical defenses can also be influenced by dispersers. We explore the relationships between seed traits (seed mass and hardness) and seed removal in 22 Myrtaceae species of the Brazilian Atlantic Forest, a dominant and diverse fleshy‐fruited taxon dispersed by birds, rodents, and other mammals. Our goal is to understand how seed traits influence seed removal rates, and whether dispersers can affect tissue allocation in the seed coat. Seeds were exposed to field removal experiments. In the laboratory, total seed mass and seed coat mass were obtained. To evaluate the influence of seed traits on removal, we performed Kruskal–Wallis and Simple Linear Regression tests. We assessed seed coat and seed mass covariation through standardized major‐axis allometric regressions. Harder seeds were larger than softer ones. Seed traits affect removal rates, as tougher and heavier seeds had lower removal. Seed mass significantly predicts seed coat proportion in seven of the 14 species tested. Bird‐dispersed species tend to exhibit lower proportions of seed coat as seed mass increases, whereas rodent‐dispersed species apparently present the opposite trend, with seed coat proportion increasing with seed mass. Such difference may be caused by the contrasting seed predation pressure represented by birds and rodents. Energy allocation for defense, expressed in seed coat proportion, is greater in large seeds, as these are mostly dispersed by rodents whose propensity to cache and disperse seeds is greater for large and well‐protected seeds.

Effects of Paracentrotus lividus (Lamark, 1816) harvesting on benthic assemblages. An experimental approach

AbstractThe effect of harvesting on the diversity and structure of benthic assemblages is nowadays a relevant component of fishery management. Invertebrate harvesting is increasing worldwide and sea urchins are one of the main invertebrate target species. However, current knowledge about the effects of sea urchin harvesting on benthic assemblages is still poor. The aim of this study is to evaluate the influence of Paracentrotus lividus (Lamark, 1816) harvesting on tide pool assemblages from intertidal rocky shores. To achieve this objective, a field removal experiment was done for 10 months. Results of univariate and multivariate analyses showed that P. lividus harvesting had a significant effect on the number of taxa and structure of the benthic assemblage. Assemblages from pools where P. lividus were removed showed significant differences against controls; namely, where P. lividus were eliminated, pools showed higher number of taxa than control pools. This result contrasts with previous studies assessing effects of invertebrate harvesting that generally have found a biodiversity reduction in the exploited habitat. Despite this apparently positive effect of P. lividus harvesting, we should be cautious because effects of P. lividus harvesting on higher trophic levels or during long periods of time remain unexplored.

Two dimensions of demographic differentiation of species in a mountain grassland community: An experimental test

Abstract There is remarkable variation in life histories of coexisting plant species. These “alternative designs” for the given set of environmental conditions are likely to play a role in species niche differences and thus may underlie species coexistence, although there is no clear demonstration of it. Currently available data on within‐community differentiation concern primarily easy and measurable traits that are not fully informative of life history variation. Their relevance for functional differentiation of species and species coexistence is far from clear. Here, we examined differentiation of coexisting species in demographic parameters and how it determines species’ response to neighbours. We determined these parameters for a set of 21 co‐occurring species by fitting a process‐based model to a long‐term (30 years) data series of shoot counts. We examined the functional relevance of these parameters using species responses in a field removal experiment. We further asked with which functional traits they are correlated. Species were differentiated along two largely independent axes: (a) slow vs. fast, separating species according to instantaneous growth rate, competitive response and intraspecific density dependence; and (b) dispersal vs. local dynamics, which separated species with strong dispersal (by seeds or vegetative) from species that tended to stay in the occupied spot. While the slow‐fast axis was associated with commonly used leaf and seed traits, the dispersal axis was best predicted by lateral spreading distance. Each of these two axes predicted different components of species’ responses to neighbour competition: the slow‐fast axis was a good predictor of the short‐term response, whereas dispersal axis was a good predictor of the long‐term response. Synthesis. Demographic differentiation of coexisting species resembles to an important degree demographic differentiation known from large‐scale comparisons. This differentiation is functionally meaningful also at the fine scale; its role in species’ responses to competition implies it is involved in species niche differentiation and coexistence. While seed and leaf traits are important correlates of demographic differentiation, a hitherto underappreciated trait, viz. lateral spreading distance, is an important predictor of the dispersal axis at the fine scale and should be more widely used. A plain language summary is available for this article.

Fraying around the edges: negative effects of the invasive Tradescantia zebrina Hort. ex Bosse (Commelinaceae) on tree regeneration in the Atlantic Forest under different competitive and environmental conditions

Invasive plants modify the structure and functioning of natural environments and threat native plant communities. Invasive species are often favored by human interference such as the creation of artificial forest edges. Field removal experiments may clarify if invasive plants are detrimental to native plant regeneration and how this is related to other local factors. We assessed the joint effect of environment and competition with the invasive Tradescantia zebrina on tree species recruitment in an Atlantic Forest fragment. We carried out the experimental study in the Iguacu National Park, located in southern Brazil, using 30 plots distributed across five invaded sites during 6 months. We counted T. zebrina leaves and recorded the abundance and height of tree recruits over time under contrasting environmental (forest edge vs. forest interior) and removal (all aboveground biomass, only T. zebrina removal, and control) treatments. We analyzed the effects of environment and removal treatment using generalized linear mixed models. The invasive species performed better at the forest edge than in the interior. The higher competitive pressure of T. zebrina led to higher mortality and lower height of tree recruits. Invader removal favored tree recruitment, especially in the forest interior. Our study shows that T. zebrina hampers woody species regeneration in tropical Atlantic Forests, especially at the forest edge.

Reciprocal facilitation between large herbivores and ants in a semi-arid grassland

While positive interactions have been well documented in plant and sessile benthic marine communities, their role in structuring mobile animal communities and underlying mechanisms has been less explored. Using field removal experiments, we demonstrated that a large vertebrate herbivore (cattle; Bos tarurs) and a much smaller invertebrate (ants; Lasius spp.), the two dominant animal taxa in a semi-arid grassland in Northeast China, facilitate each other. Cattle grazing led to higher ant mound abundance compared with ungrazed sites, while the presence of ant mounds increased the foraging of cattle during the peak of the growing season. Mechanistically, these reciprocal positive effects were driven by habitat amelioration and resource (food) enhancement by cattle and ants (respectively). Cattle facilitated ants, probably by decreasing plant litter accumulation by herbivory and trampling, allowing more light to reach the soil surface leading to microclimatic conditions that favour ants. Ants facilitated cattle probably by increasing soil nutrients via bioturbation, increasing food (plant) biomass and quality (nitrogen content) for cattle. Our study demonstrates reciprocal facilitative interactions between two animal species from phylogenetically very distant taxa. Such reciprocal positive interactions may be more common in animal communities than so far assumed, and they should receive more attention to improve our understanding of species coexistence and animal community assembly.

Invasive plants differ from native plants in their impact on native communities

AbstractAimsAlthough the impacts of alien plant invasions on native communities are well documented, the mechanisms that underlie these impacts are poorly resolved. Little is yet known as to why invasive alien plants have greater impacts than native species on their neighbouring species within a community. We investigated two potential mechanisms by which invasive plants may harm neighbouring species: resource competition and novel compounds of the invaders.LocationsField removal experiment in France and Switzerland, and common garden experiment at the University of Fribourg, Switzerland.MethodsWe first conducted a field removal experiment to compare the response of neighbouring communities to experimental biomass removal of two alien invasive vs two dominant native plant species in two sites of each target species (thus eight sites in total). In addition, we conducted a common garden pot experiment to compare the effect of eight triplets of closely related target species (24 species in total), each consisting of an alien invasive species in Europe, a dominant native and a random native species, on the growth of a grass community under the addition or not of activated carbon (AC).Results(1) Field removal experiment: in the control plots, we found that the biomass of native species explained a substantial amount of the variation in biomass of the neighbouring communities, but the invader's biomass did not. Moreover, the neighbouring community recovered from removal of the dominant native species, showing a significant increase in biomass, but did not after removal of the invaders. (2) Common garden pot experiment: we found a larger effect of invasive species than native species on the growth rate of the grass community, but this difference disappeared when AC was added.ConclusionBoth experiments suggest that the impact of some invasive species is not driven by resource competition, but by other mechanisms, most likely by novel weapons or novel plant–soil interactions.

Invasive décor: an association between a native decorator worm and a non-native seaweed can be mutualistic

The likelihood of invasion success increases when non-native species engage in mutualisms with a native or non-native species. Mutualisms formed between native and non- native species have been termed 'novel mutualisms', and research in terrestrial systems has advanced our understanding of the ecological processes involved in their formation and persist- ence. However, documentation of novel mutualisms in marine systems is rare. In Atlantic estuaries of the southeastern USA, the native polychaete worm Diopatra cuprea actively decorates its tube with the non-native red seaweed Gracilaria vermiculophylla. We used field and laboratory exper- iments to test whether the DiopatraGracilaria interaction is mutualistic. We found that Diopatra facilitates Gracilaria by securing the seaweed onto the soft-sediment benthos within a favorable tidal elevation for growth and where hard substrata for attachment are otherwise rare. A combi- nation of laboratory and field experiments also suggests that Gracilaria can enhance the growth of Diopatra by increasing access to epifaunal crustacean prey. However, field removal experiments showed that the benefits of Gracilaria to Diopatra only occurred in some sites and years. We found no evidence that this invader has a significant negative effect on Diopatra, and it appears in some instances to even benefit the worms, which suggests that Gracilaria (and its associated impacts on ecosystems) are likely to remain an important component of southeastern USA estuaries for the foreseeable future.

Effects of male size and female dispersion on male mate-locating success in Nephila clavipes

The reproductive success of sexually reproducing organisms depends on their capacity to locate potential partners. Research on the mating systems of web-building spiders, like other terrestrial invertebrates, has focused around male courtship and the signalling behaviours that facilitate copulation. In contrast, both mate searching and mate location behaviours are largely ignored and rarely quantified in field studies. We conducted a series of field removal experiments to explore the effects of male body size and female dispersion on mate locating behaviour of the golden orb-weaving spider (Nephila clavipes). The number of males that arrived on females’ webs was influenced by travel distance, male body size, male body weight and female dispersal. More males returned to webs that were closer to the release point; smaller males experienced a greater weight loss; and larger numbers of males arrived at webs where males had been previously present. Female dispersion influenced the size but not the number of arriving males: males were no more likely to arrive at solitary than aggregations of webs, but males arriving at solitary webs were typically larger than those arriving at aggregations. We discuss the implications of various male traits that could act as selective pressures on male mate location and mate searching behaviours.

Intraspecific competition between shrubs in a semi-arid savanna

Tree-on-tree competitive interactions may be more important in affecting the distribution of the tree components of savannas than inter-specific competition with grasses. The presence of intraspecific competition is expected to negatively affect inter-tree spacing, individual size distributions and plant physiology as well as survival/mortality. In this field removal experiment on Acacia mellifera, one of South Africa’s most common encroachers on nutrient-poor soils, the growth, water relations and mortalities of shrubs where all neighbouring woody competitors were removed (target) were monitored three times during each of three growing seasons. After 3 years, the nitrogen and carbon isotopic ratios of the study plants were analysed. Target shrubs benefitted from removal of neighbours, resulting in greater growth, less water stress, a relatively small degree of canopy dieback and reduced reliance on N2 fixation. Target shrubs grew by 25 ± 4% in height relative to 7 ± 4% for controls, with the targets suffering a maximum of <15% canopy dieback compared to up to 60% in the controls. Severe environmental stress is known to affect neighbour interactions among shrubs and competition may constrain shrub sizes and avoid density-dependent mortality. In contrast, release from competition in our study may have allowed greater growth of target plants, increasing their total evapo-transpirational leaf surface areas and leaving them vulnerable to drought and water stress. Intratree competition on shallow nutrient-poor soils in savannas may thus aid the persistence of bush encroachment by regulating the sizes of individual shrubs below the threshold of drought vulnerability.

Why do some species in arid lands increase under grazing? Mechanisms that favour increased abundance of Maireana pyramidata in overgrazed chenopod shrublands of South Australia

AbstractWhile the abundance of some plant species decreases under high grazing intensity, others become more abundant. Release from competition by decreaser species contributes to this pattern in mesic systems, but this may not be the case in xeric systems where competition may be less intense. Here we examine three mechanisms that may be involved: (i) increased recruitment and growth because of soil changes produced by grazing, for example, increased soil nutrient availability through dung accumulation; (ii) increased recruitment favoured by the breaking up of the lichen crust; and (iii) reduced competition because of the decline of decreaser species. We used field and glasshouse experiments to determine the possible contribution of these mechanisms to the increase of the chenopod Maireana pyramidata around a watering point in a chenopod shrubland of South Australia. There was no evidence of nutrient accumulation close to the watering point, and while seedlings of M. pyramidata responded to nutrient addition, their growth was the same in soil collected from areas with different grazing intensity. While a broken lichen crust increased the emergence of both M. pyramidata and the decreaser Atriplex vesicaria, the effect was larger for the former. We found no competition between seedlings of the two species or between juveniles of A. vesicaria and seedlings of M. pyramidata in glasshouse experiments. Adult plants of both A. vesicaria and M. pyramidata produced similar growth reduction in seedlings of M. pyramidata. Furthermore, a field removal experiment failed to detect any competitive effect of A. vesicaria on M. pyramidata. Our data indicate that the disintegration of the soil crust by grazer activities can be a major factor controlling floristic changes in overgrazed rangelands. These results imply that factors that control establishment may be more important than competition in shaping shrub population dynamics in these systems. Ground surface itself can affect establishment opportunities, and this should be taken into account in management and restoration efforts in arid lands.

Competitive effects of the exotic Bombus terrestris on native bumble bees revealed by a field removal experiment

AbstractA commercialized bumble bee pollinator (Bombus terrestris) introduced from Europe has colonized in Japan and potentially competes with native bumble bees for food and nest sites. To examine the competitive impacts, a field removal experiment was conducted in an area in northern Japan where B. terrestris has become feral. In 2005 and 2006, totals of 1,511 and 2,978 B. terrestris bees, respectively, were killed in six removal sites. In those 2 years and the pre‐removal year (2004), the bee abundance and worker body size were measured in the six removal sites and seven control sites, and effects of the removal on the measurements were examined using statistical models. In 2005 only, the removal decreased the number of B. terrestris queens and increased that of two native species, B. ardens and B. hypocrita, the tongue length of which overlaps that of B. terrestris. The removal in 2005 affected the worker body size of neither B. terrestris nor any native species. These results show the competitive impacts of exotic B. terrestris on the queen abundance of the native species that are likely to share floral resources with B. terrestris.

HEMIPARASITES GENERATE ENVIRONMENTAL HETEROGENEITY AND ENHANCE SPECIES COEXISTENCE IN SALT MARSHES

Tidal inundation and salinity are considered to be controlling factors in salt marsh species distributions. Parasitic plants may also influence community organization as parasite-host interactions may play a functional role in stress amelioration due to physiological mechanisms for salinity tolerance and resource acquisition. Endangered root hemiparasites (Cordylanthus maritimus ssp. palustris and Cordylanthus mollis ssp. mollis) occupy unique habitat within fragmented northern California tidal wetlands. My objective was to examine the effects of these root hemiparasites on soil salinity, aeration, and community composition. I compared experimentally established bare patches, shaded and unshaded, and parasite removal patches to controls with hemiparasites across intertidal elevation gradients. Plant community composition, soil salinity, and redox potential were measured as response variables. In this field removal experiment, I demonstrated that parasite-host associations can enhance the amelioration of physical stress conditions in the salt marsh exceeding the passive role of shading by vegetation. Consumer-driven reduction of physical stress resulted in increased plant species richness, and the effect was most pronounced with elevated salinity and hypoxia stress. Although previous studies have demonstrated that removal of dominant plant biomass by herbivores can increase physical stress in salt marshes, this is one of the first examples of a positive indirect effect of a consumer on community diversity through physical stress relief. Greater understanding of biological interactions coupled with abiotic factors may improve rare plant conservation and salt marsh restoration success.

INTERGENERIC SALAMANDER INTERACTIONS ACROSS AN ECOTONE

Although many field experiments have examined interactions among terrestrial salamanders or among semi-aquatic salamanders, virtually no research has examined potential interactions among salamander species across ecotones. Past research has indicated that numerous factors may influence salamander distributions, including competition, predation, and abiotic factors. We conducted a field removal experiment to investigate potential interactions among salamanders across the steam-woodland ecotone and to determine the effects of such interactions on community structure. We used removals of a terrestrial salamander, Plethodon cinereus, and/or several species of semi-aquatic salamanders of the genus Desmognathus to investigate potential interactions between these genera and the semi-aquatic salamander Eurycea cirrigera. Although removals did not significantly decrease numbers of P. cinereus or Desmognathus on plots, distribution of E. cirrigera shifted unexpectedly closer to streams in response to removals of P...

Mate guarding, competition and variation in size in male orb-web spiders, Metellina segmentata: a field experiment

In the common orb-web spider Metellina segmentata, males are more powerfully built with longer legs, although females are heavier because of their egg load. Males guard females before attempting to mate, and there is considerable male–male competition because of the male-biased operational sex ratio. We used a field removal experiment to examine (1) seasonal changes in the average morphology of guarding males and (2) whether there is a pool of small males that is excluded from the webs of females. Morphological measures were subjected to a principal components analysis and changes in PC scores were examined for seasonal effects and the effects of previous removal of males. The size of guarding males (PC1) increased over the season, suggesting that smaller males were increasingly excluded from webs, but the condition of guarding males (PC2) decreased, indicating that energy reserves are depleted because the males gain little access to food during the reproductive season. When guarding males were removed, smaller males were able to take up residence. Our results show that large males have a clear advantage in monopolizing females. We discuss the manner in which selection acts to maintain large male size in this spider.Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Can interaction coefficients be determined from census data? Testing two estimation methods with Negev Desert rodents

Two approaches based on regression models are proposed to estimate competition from census data. The “static” approach is based on censuses of population sizes among species at one point in time over many sites. The “dynamic” approach relies on a time series of species abundance data to examine whether per capita changes in one species are associated with the abundance of other species. We estimated competition interactions in a Negev rodent community consisting of 10 species using both approaches, basing on 8 years (16 half‐year periods) of observations. The static approach revealed significant competitive interactions in four of 45 pairs of species, whereas the dynamic approach did so in the same four plus two more pairs. For each species pair, both approaches revealed significant negative interactions in only 1–4 of 16 seasons. The static approach provided nearly symmetric estimations of competition, whereas estimations of dynamic approach were asymmetric. Moreover, estimations of the two approaches did not coincide in time. Cases of negative interactions recorded by the static approach were more frequent at peak and increase phases of population density dynamics, whereas those recorded by the dynamic approach were more frequent at peak and decline phases. Results of field removal experiments with Mus musculus and Gerbillus dasyurus supported predictions of dynamic but not static approaches. We hypothesized that in harsh and fluctuating desert environments that disrupt equilibrium, the dynamic approach indicates true (exploitation) competition, whereas the static approach reflects negative interspecific spatial association (interference).

Estimating Competition Coefficients from Census Data: A Re-Examination of the Regression Technique

In this paper we assess the regression technique used to estimate competition coefficients (α) from census data for two species of rodents. We compare the results from this technique to those calculated from data collected in a field removal experiment conducted in coastal heathland in southeastern Australia. We examined the relationship between the competition coefficient and the population variance. Our results and analyses show that α depends heavily on the population variance, increasing the likelihood of producing a statistical artefact in some cases: they also indicate the need for a reappraisal of the conclusion that common species exert weak competitive effects while rare species exert strong effects. To overcome this artificial dependence we propose a procedure of normal standard-ization of species census data before calculating regression parameters. Our results show disagreements between the estimated α using regression and the observed α calculated from the removal experiment, when the census data were not standardized. However, after the standardization procedure was applied to the census data, each estimated α matched the observed value well and the results were consistent with previously published results. This supports the validity of a standardized regression technique. We also discuss the inconsistency existing among different methods of factoring out habitat effects and recommend the use of competition coefficients calculated from the Residual PCA method as the best estimate for α from our data. The results presented are stimulating and encouraging because they resurrect the regression technique and inspire future refinement and application.

The evolution of competing species of terrestrial salamanders: niche partitioning or interference?

One of the central assumptions of evolutionary ecology is that interspecific competition is a potent evolutionary force acting on coexisting species. There are few animal species that provide an opportunity for an experimental analysis of the evolutionary consequences of the phenomenon. We have taken advantage of the fact that two species of terrestrial salamander,Plethodon glutinosus andP. jordani, have different altitudinal distributions on two mountain ranges in North Carolina. Field removal experiments showed that interspecfic competition was much stronger in the Great Smoky Mountains than in the Balsam Mountains, and transplant experiments between the two mountain ranges showed that neither species from the Balsam Mountains had a measurable effect on its congener in the Smokies, although both species from the Smokies had strong negative effects on the Balsam congeners. Other experiments were conducted on the behavioral and ecological changes that have (or have not) evolved in the two areas. Our studies show that increased interspecific interference was the major evolutionary response of these largePlethodon species to interspecific competition, and that partitioning of food or microhabitat was not involved.