Unmanipulated Plots Research Articles

Globally consistent response of plant microbiome diversity across hosts and continents to soil nutrients and herbivores

All multicellular organisms host a diverse microbiome composed of microbial pathogens, mutualists, and commensals, and changes in microbiome diversity or composition can alter host fitness and function. Nonetheless, we lack a general understanding of the drivers of microbiome diversity, in part because it is regulated by concurrent processes spanning scales from global to local. Global-scale environmental gradients can determine variation in microbiome diversity among sites, however an individual host’s microbiome also may reflect its local micro-environment. We fill this knowledge gap by experimentally manipulating two potential mediators of plant microbiome diversity (soil nutrient supply and herbivore density) at 23 grassland sites spanning global-scale gradients in soil nutrients, climate, and plant biomass. Here we show that leaf-scale microbiome diversity in unmanipulated plots depended on the total microbiome diversity at each site, which was highest at sites with high soil nutrients and plant biomass. We also found that experimentally adding soil nutrients and excluding herbivores produced concordant results across sites, increasing microbiome diversity by increasing plant biomass, which created a shaded microclimate. This demonstration of consistent responses of microbiome diversity across a wide range of host species and environmental conditions suggests the possibility of a general, predictive understanding of microbiome diversity.

Assessing the potential of invertebrate natural enemies of insect pests inhabiting Miscanthus x giganteus shelterbelts in pasture

ABSTRACT Miscanthus x giganteus (Miscanthus) grass shelterbelts can deliver multiple ecosystem services on New Zealand commercial dairy farms. However, there has been little investigation into how these shelterbelts contribute to insect pest management. Here, on a Canterbury dairy farm, we investigated what generalist surface-dwelling invertebrate natural enemies of insect pests inhabit three separate Miscanthus shelterbelts compared to three unmanipulated field margin plots. The potential contribution of these natural enemy assemblages to future biological control was also investigated. To measure this, live moth egg baits with associated pitfall trapping were deployed in autumn, winter and spring of 2015. Miscanthus and the field margin plots were found to have similar potential natural enemy richness but differed in their community composition. The potential predation rate of pests in Miscanthus was 85% higher than in the field margin. Infrared video and Sanger sequencing confirmed that the harvestman Phalangium opilio and the slug Deroceras reticulatum consumed the egg baits in Miscanthus. Conversely, no bait-consuming invertebrates were identified in the field margin. These preliminary results indicate potential natural enemies inhabit Miscanthus and may consume insect pest eggs on the soil-surface. However, to achieve anyinsect pest suppression, further habitat manipulation would be required.

Heterotrophic and rhizospheric respiration in coniferous forest soils along a latitudinal gradient

Northern forest soils are a major carbon (C) reservoir of global importance. To estimate how the C balance in these soils will change, the roles of tree roots and soil microbes in C balance should first be decoupled. This study determined how the activity of heterotrophs and tree roots together with root-associated microbes in the rhizosphere varies in coniferous forest soils in boreal, hemiboreal, and temperate climates along a latitudinal gradient using a trenching approach. We created experimental plots without living tree roots, measured soil respiration (CO2 efflux) from these and from unmanipulated plots using the chamber technique, and partitioned the efflux into root-rhizosphere (RR) and heterotrophic (RH) respiration. The share of RR in ecosystem gross primary production (GPP) decreased from north to south in the Scots pine (Pinus sylvestris L.) and the Norway spruce (Picea abies (L.) Karst.) forests, with the exception of a mixed site, where the share of RR in GPP varied strongly between the years. RR per ground area and per root biomass were mainly independent of climate within the gradient. RH per ground area increased from north to south with temperature, while RH per soil C did not change with temperature. Soil moisture did not significantly affect the respiration components in the northernmost site, whereas soil moisture was positively connected with RH and negatively with RR in other Scots pine sites and positively connected with RR in pure Norway spruce stands. The dynamic ecosystem model LPJ-GUESS was able to capture the seasonal dynamics of RH and RR at the sites, but overall accuracy varied markedly between the sites, as the model underestimated RH in the southern site and RR elsewhere. Our study provides knowledge about the nature of soil respiration components. The valuable insights can be used in more accurate land-ecosystem modelling of forest ecosystems.

Richness of Primary Producers and Consumer Abundance Mediate Epiphyte Loads in a Tropical Seagrass System

Consumer communities play an important role in maintaining ecosystem structure and function. In seagrass systems, algal regulation by mesograzers provides a critical maintenance function which promotes seagrass productivity. Consumer communities also represent a key link in trophic energy transfer and buffer negative effects to seagrasses associated with eutrophication. Such interactions are well documented in the literature regarding temperate systems, however, it is not clear if the same relationships exist in tropical systems. This study aimed to identify if the invertebrate communities within a tropical, multispecies seagrass meadow moderated epiphyte abundance under natural conditions by comparing algal abundance across two sites at Green Island, Australia. At each site, paired plots were established where invertebrate assemblages were perturbed via insecticide manipulation and compared to unmanipulated plots. An 89% increase in epiphyte abundance was seen after six weeks of experimental invertebrate reductions within the system. Using generalised linear mixed-effect models and path analysis, we found that the abundance of invertebrates was negatively correlated with epiphyte load on seagrass leaves. Habitat species richness was seen to be positively correlated with invertebrate abundance. These findings mirrored those of temperate systems, suggesting this mechanism operates similarly across latitudinal gradients.

Altered leaf litter quality exacerbates the negative impact of climate change on decomposition

Abstract Leaf litter decomposition is a key component of global biogeochemical cycles that influence soil carbon storage, nutrient availability and plant productivity. Ongoing climate change will lead to warmer and drier conditions in many dryland regions, potentially affecting litter decomposition and nutrient dynamics. Climate change effects can be direct and/or indirect, for example, through changes in litter quality, yet their relative importance on litter decomposition remains unclear. We conducted a manipulative study in a semi‐arid shrubland to assess the effects of leaf litter quality, forecasted climate change, that is, +2.5°C warming (W), 30% rainfall reduction (RR) as well as their interaction (W + RR) to elucidate their relative effects on litter decomposition. Climatic effects alone reduced decomposition of a homogeneous Control leaf litter collected from Helianthemum squamatum shrubs growing in unmanipulated plots by 23.4%, 18.1% and 29.8% in the W, RR and W + RR treatments respectively. Leaf litter quality was lower in shrubs that had been growing in warmed plots (W and W + RR), as they had lower nutrient concentrations (P, Fe) and higher C:N and C:P ratios than leaf litter produced under ambient (Control) conditions. Lignin concentration was significantly lower in litter from W + RR plots, yet when both climate and litter quality were considered simultaneously, decomposition rates were 32.0%, 26.3% and 39.9% lower in W, RR and W + RR plots compared to Controls. In addition, we found greater microbial N immobilization in leaf litter incubated within warmed (W and W + RR) than within non‐warmed plots (Control and RR). Structural equation modelling showed that higher litter moisture and microbial biomass contents stimulated decomposition. Simulated climate change (W, RR and W + RR) reduced decomposition indirectly by negatively affecting litter moisture contents and litter microbial biomass. Microbial nitrogen immobilization was stimulated by the lower quality (i.e. high C:N ratios) of the leaf litter collected in shrubs from warmed plots (W and W + RR). Synthesis. Our findings indicate that forecasted climate change conditions slow down C and N cycling in a dryland ecosystem, an effect that is further exacerbated by climate change‐induced reductions in litter quality and related reductions in bacterial and fungal biomass in litter.

Alternative state? Experimentally induced Fucus canopy persists 38 yr in an Ascophyllum‐dominated community

AbstractExperimental tests of the hypothesis that ecological communities can exist in “multiple stable states” are rare, and some argue, impossible, because of the unlikelihood that any system will meet the necessary criteria. These are that alternative states (1) are in the same location, (2) experience the same environment, (3) persist for multiple generations, (4) resist repeated perturbations, and (5) result from a pulse manipulation. In 1974, we initiated an experiment testing the ability of Ascophyllum nodosum‐dominated rocky intertidal communities to recover from complete canopy removal. Manipulations were monitored frequently for 5 yr after clearance and resurveyed again after a 35‐yr hiatus. After clearance, Ascophyllum was replaced immediately by another fucoid alga, Fucus spp, which continued to dominate the space through 1979 despite regular annual recruitment by Ascophyllum. Observations in 2009 revealed that Fucus spp. still dominated the cleared plots. Surveys in 2011 and 2013 demonstrated that Fucus persisted in experimental plots and Ascophyllum persisted in adjacent, unmanipulated plots. All criteria for testing an alternative state were met. Fucus persisted through multiple generations of both fucoids, a steady annual rain of recruits of both species, a high frequency of storm‐driven perturbations, and it resulted from a pulse manipulation. Likely mechanisms include poor Ascophyllum recruitment directly under the Fucus spp. canopy despite abundant recruitment in adjacent areas, self‐maintenance by Fucus spp. through high recruitment and fast growth, and recruitment facilitation of Fucus by the barnacle Semibalanus balanoides. Several lines of evidence indicate that other possible mechanisms including indirect facilitation of fucoids through predation on competitors (mussels), and positive or negative effects of littorine grazing, are unlikely. Although prior results in Maine suggested that the mussel Mytilus edulis was an alternative stable state, the new results suggest that Fucus spp. was the alternative state to Ascophyllum. During the nearly four decades of this experiment, a number of important species including fucoids, littorine grazers, and mussels all declined in abundance, most likely due to climate change. The presumed impact of climate change makes prediction of the long‐term response of this system difficult, but it already differs dramatically from its structure in the 1970s.

Manipulation of basking sites for endangered eastern massasauga rattlesnakes

Generating open-canopy basking sites via manipulation of vegetative cover has been proposed as a conservation strategy for snakes and other reptiles. We assessed how endangered eastern massasauga rattlesnakes (Sistrurus catenatus) responded to 2 types of manipulations at a wetland site in New York, USA: 1) cutting shrubs to create ≤100-m2 plots within known gestation areas in 2008, 2011, and 2012, and 2) cutting tree and shrub cover in the adjacent forest to create a 4-ha forest clearing in 2011. Based on systematic visual surveys from June to August in 2011 and 2012, we estimated snake presence among manipulated and unmanipulated plots in response to plot treatment date. We observed massasaugas selecting newly manipulated plots more often than unmanipulated plots within known gestation areas in both 2011 and 2012, but we detected no increase in massasauga activity within the adjacent forest clearing in either year. The effectiveness of vegetation cutting appeared to decline after 3 years because of vegetation re-growth. We suggest that cutting shrubs to ≤0.25 m in height can benefit this population of rare snakes, which faces limited availability of basking sites because of vegetative succession. © 2016 The Wildlife Society.

Contrasting effects of warming and increased snowfall on Arctic tundra plant phenology over the past two decades.

Recent changes in climate have led to significant shifts in phenology, with many studies demonstrating advanced phenology in response to warming temperatures. The rate of temperature change is especially high in the Arctic, but this is also where we have relatively little data on phenological changes and the processes driving these changes. In order to understand how Arctic plant species are likely to respond to future changes in climate, we monitored flowering phenology in response to both experimental and ambient warming for four widespread species in two habitat types over 21years. We additionally used long-term environmental records to disentangle the effects of temperature increase and changes in snowmelt date on phenological patterns. While flowering occurred earlier in response to experimental warming, plants in unmanipulated plots showed no change or a delay in flowering over the 21-year period, despite more than 1°C of ambient warming during that time. This counterintuitive result was likely due to significantly delayed snowmelt over the study period (0.05-0.2days/yr) due to increased winter snowfall. The timing of snowmelt was a strong driver of flowering phenology for all species - especially for early-flowering species - while spring temperature was significantly related to flowering time only for later-flowering species. Despite significantly delayed flowering phenology, the timing of seed maturation showed no significant change over time, suggesting that warmer temperatures may promote more rapid seed development. The results of this study highlight the importance of understanding the specific environmental cues that drive species' phenological responses as well as the complex interactions between temperature and precipitation when forecasting phenology over the coming decades. As demonstrated here, the effects of altered snowmelt patterns can counter the effects of warmer temperatures, even to the point of generating phenological responses opposite to those predicted by warming alone.

Exploring Restoration Methods for the Olympia OysterOstrea luridaCarpenter, 1864: Effects of Shell Bed Thickness and Shell Deployment Methods on Shell Cover, Oyster Recruitment, and Oyster Density

Oysters provide habitat, sediment stabilization, and improved water quality, and are important foundation species in many estuarine ecosystems. Worldwide oyster population declines have been dramatic and efforts to restore declining populations and the services they provide are ongoing. Several commonly used oyster restoration techniques were examined to determine which would be the most successful for restoring the Olympia oyster Ostrea lurida in Newport Bay, CA. Replicate (n = 5) 2×2 m shell beds were constructed of two initial shell planting thicknesses (bed thicknesses of 4 versus 12 cm) and two methods of deployment (bagged versus loose shell). Shell cover, oyster spatfall (settlement), oyster recruitment, and adult oyster densities were analyzed over 2 y; 12-cm-thick oyster beds maintained higher shell cover, experienced less sedimentation, and received greater numbers of oyster recruits than 4-cm-thick beds. There was no significant effect of shell deployment method on shell cover, recruitment, or adult density; however, spatfallwas greater on loose shell beds comparedwith bagged shell beds in the final year of the study. Overall, augmenting mudflat habitat with oyster shell significantly increased adult O. lurida oyster density compared with unmanipulated plots and increased oyster density relative to the average density of oysters measured elsewhere in Newport Bay. Collectively, the data suggest that building thicker shell beds might increase the longevity of a constructed shell bed, and therefore, this approach is recommended for future restoration activities in southern California. This study highlights the advantages of augmenting habitat in a manner that provides vertical relief from sedimentation.

The influence of prolonged flooding on the growth of Spartina alterniflora in Cape Cod (Massachusetts, USA)

The tolerance of Spartina alterniflora L. to prolonged flooding is a subject of considerable debate. Understanding how this species responds to extended periods of continuous flooding in the field is crucial for making decisions about its management where it is exotic and in predicting losses due to sea level rise where it is native. In this study, plots of S. alterniflora within poorly-drained areas of four salt marshes in Cape Cod National Seashore (CCNS; Massachusetts, USA) were lowered to create conditions of continuous flooding over the duration of one growing season. These were compared to control plots where Spartina was removed and put back in place at the same elevation and un-manipulated plots. The density of plants in the lowered treatment was significantly reduced by 86%, whereas the manipulated and un-manipulated controls declined only marginally by 28% and 11%, respectively. Lowering also reduced the extent of height growth, although not significantly compared to manipulated controls. Porewater sulfide concentrations were initially over fourfold higher in lowered plots than the other treatments, and in the range toxic to S. alterniflora growth, whereas there was no effect on salinity. The results suggest that prolonged flooding in its natural setting over the course of several months may be enough to eliminate S. alterniflora. Thus, when areas of S. alterniflora in marshes become continuously flooded as a result of sea level rise, they may undergo substantial decline. Moreover, in controlling S. alterniflora as an exotic species hydrologic manipulation may be a useful management tool.

Disturbance effects of kelp thalli on structure and diversity of a coastal Arctic marine soft-bottom assemblage

The effects of biotic disturbances, like seaweed whiplash, on the diversity of benthic communities are well documented for temperate coastal systems, yet missing for Arctic benthos. In Arctic soft-bottom habitats, kelp thalli occur either continuously (e.g. trapped by sediment) or sporadically (by drifting on the sediment) after detachment from rocky shores. To explore whether a kelp thallus can disturb the structure and diversity of a coastal Arctic soft-bottom assemblage, we continuously fixed a single thallus of the kelp Saccharina latissima to or sporadically (i.e. biweekly) moved it on the sediment and compared treatment effects to unmanipulated plots (=controls). On 6 September 2013 (i.e. after 73 days of manipulation), one sediment core was taken from each of the 30 plots (n = 10), from which the number of individuals of each of the 45 encountered animal taxa was recorded. The continuous presence of an experimentally fixed kelp thallus significantly reduced the number of individuals on average by 27 %. This disturbance effect was even stronger, on average 49 %, where a kelp thallus was biweekly moved on the sediment. Likewise, taxon richness was lowered by an average of 19 and 36 % where a S. latissima thallus was continuously or sporadically present, respectively. While the composition of taxa was also significantly different among all treatment groups, evenness and biomass were unaffected by kelp treatments. We conclude that the presence and already movements of a single kelp thallus can promote small-scale patchiness in near-shore soft-bottom assemblage structure and diversity and exemplify a significant connection between rocky and sedimentary coastal habitats.

Effects of a simulated hurricane disturbance on forest floor microbial communities

Forest floor microbial communities play a critical role in the processes of decomposition and nutrient cycling. The impact of cultivation, contamination, fire, and land management on soil microbial communities have been studied but there are few studies of microbial responses to the effects of tropical storms. The Canopy Trimming Experiment was executed in the Luquillo Experimental Forest of Puerto Rico to decouple two prominent effects of a hurricane—canopy opening and debris deposition on the forest floor—on forest biota and processes. We studied the independent and interactive hurricane effects of canopy openness and debris deposition on the relative abundance and diversity of microorganisms in soil and leaf litter using ester link fatty acids methyl esters (EL-FAME) analysis, and terminal restriction fragment length polymorphism (TRFLP) profile. Non-metric multi-dimensional scaling analysis of soil FAME showed soil microbial community composition was significantly different between pre- and post-hurricane periods including in the unmanipulated plots and among blocks, but there was no significant separation among treatments. This shows that there are strong spatiotemporal dynamics in the structure of soil microbial communities which masked hurricane effects (canopy opening and deposition of green debris). The degree of difference among treatments decreased with time in soil which suggests that our study may have started too late after the manipulations and therefore missed the effects of canopy opening and debris addition. This reflects the resilience of the soil microbial communities. The richness of soil bacterial TRF’s however, showed a significant positive response to added debris. Neither fungal nor bacterial NMDS clusters for leaf microbial communities showed significant grouping by treatment, time or litter cohorts. Significant differences were observed through time for fungal diversity in green leaves and for both bacterial and fungal diversity in senesced leaves. Senesced leaves microbial succession apparently stopped when both the canopy and debris were removed, and there was a suggestive trimming by time interaction which reflects the susceptibility of the leaf litter microbial community. Our findings contribute to the understanding of how microbial community structures can be affected by hurricane disturbances and forestry management practices that remove canopy and debris from the forest floor, and shows the need to analyze the microbial community immediately after the disturbance. Short-term changes in microbial communities due to forest disturbances can have significant implications for litter decomposition, soil organic matter accumulation, nutrient cycling, and food web dynamics in tropical forests. All of these factors should be taken into consideration when selecting the appropriate forest management practice.

Effects of loss of algal canopies along temperature and irradiation gradients in continental Portugal and the Canary Islands

We examined the hypothesis that the loss of canopy-forming macroalgae from rock pools would have stronger effects on associated algal assemblages along gradients of harsher cli- mate conditions (i.e. higher sea and air temperatures and irradiation) occurring in continental Por- tugal and the Canary Islands. The study was carried out at 'cool' and 'warm' regions in both sys- tems (N and SW Portugal and Lanzarote and El Hierro Islands, respectively), using a comparative approach involving the same manipulative experiment simultaneously conducted at multiple locations. Canopy-forming macroalgae were experimentally removed, and the structure (abun- dance and distribution) of associated algal assemblages (in terms of morpho-functional groups) was compared between treated and unmanipulated plots 6 and 12 mo after removal of the canopy. In both continental Portugal and the Canary Islands, warmer conditions were associated with a larger Euclidian distance in the structure of assemblages between canopy-removed and unmanip- ulated plots 6 mo, but not 12 mo, after the removal treatment. Responses of morpho-functional algal groups varied depending on the system, in addition to the environmental gradient. Although the present findings could not be unequivocally ascribed to causal effects of climate conditions alone, they indicate that the effects of algal canopy loss were enhanced along gradients of harsher environmental conditions at even supposedly very different continental and island systems over the short term. The studied systems were potentially able to recover about 1 yr after the distur- bance, but this might not happen under current or predicted scenarios of natural or anthropogenic disturbances able to drive a persistent loss of foundation species.

Effects of intensity and seasonal timing of disturbances on a rocky intertidal benthic community on the southern coast of Korea

AbstractThe effects of intensity and timing of disturbances on recovery of marine benthic organisms were investigated on a rocky intertidal shore in Gwangyang Bay, Korea. We hypothesized that the recovery pattern of the benthic community structure would be affected by disturbance intensity and season. Twenty‐eight permanent plots were set up, with disturbance intensity (cleared plots and sterile plots) and seasonal disturbance (fall 1999 and spring 2000) incorporated into the experimental design. To monitor natural seasonal variation in benthic community abundances, we established seven permanent unmanipulated plots. Turf‐forming algae were observed in the unmanipulated plots throughout the experimental period, whereas green algae and invertebrate presence varied with season. In the disturbance‐intensity experiment, turf‐forming and green algae were dominant in cleared plots. The highest coverage of sessile organisms was observed in sterile plots, which exhibited the highest species richness because of their relatively low macroalgal coverage. Seasonal effects of disturbance were an important factor in the recovery pattern of benthic organisms under high disturbance intensity. Coverage of green algae was higher in sterile spring plots than in sterile fall plots; this result was attributed to low spatial competition, as the disturbances occurred just before green algal blooms. On the other hand, the abundances of barnacles and bivalves were highest on sterile fall plots, as these organisms were suppressed by green algal blooms in other periods. These results indicate that the effects of disturbance intensity on benthic community recovery patterns can be influenced by season of disturbance.

Experimental support for food limitation of a short‐distance migratory bird wintering in the temperate zone

The Winter Food Limitation Hypothesis (WFLH) states that winter food abundance is a dominant source of population limitation of migratory birds. Evidence is accumulating that long-distance migratory birds wintering in tropical climates have high overwinter survival probabilities and that winter food limitation mainly affects their fitness nonlethally by limiting energetic reserves necessary for successful reproduction. In contrast, the relative roles of direct mortality vs. indirect effects caused by food limitation have not been investigated thoroughly on short-distance migratory birds wintering in temperate zones, where they face thermal challenges. We performed the first test of the WFLH for a temperate migratory bird in the wild on the Swamp Sparrow (Melospiza georgiana), with a replicated plot-wide food supplementation experiment. In contrast to tropical, but consistent with other temperate-wintering migrants, Swamp Sparrows on unmanipulated plots showed relatively low apparent survival across the winter. Following food addition, birds (1) immigrated to experimental plots, which subsequently supported approximately 50% higher abundances, (2) experienced increases of within-season apparent survival of 8-10%, depending on age/sex class, and (3) had higher-scaled mass index values, all supporting winter food limitation. The last two findings are interrelated because birds with higher scaled mass had higher survival probabilities, further supporting direct effects of winter food limitation. Food limitation of fat reserves might also have indirect effects on reproductive success by limiting migration timing and survival during migration. Increases in scaled mass were higher in females, suggesting that they are disproportionately affected by food limitation, possibly through competition. Based on Robust Design Modeling, we found no support for emigration prior to food addition, indicating that our estimates of mortality are unbiased.

SETTLING BEHAVIOR OF THE DESERT ISOPOD HEMILEPISTUS REAUMURI IN RESPONSE TO VARIATION IN SOIL MOISTURE AND OTHER ENVIRONMENTAL CUES

ABSTRACT We report the results of three studies, one observational and two manipulative, that examine the settling behavior of isopods in response to soil moisture and other cues. In a census of isopod settling patterns in twenty-five 5 × 10 m plots arranged on a 200 m2 grid, isopods settled more densely in plots with more successful old burrows from the previous season, and in plots with more shrubs. Isopod settling was not correlated with soil moisture at 10 and 30 cm depths. In a manipulative experiment carried out over two years, isopods settled more densely in 3 × 3 m outdoor pens that received 20 mm of supplemental water in February than in unwatered plots. In a second experiment, 50 mm of water was added to open plots, and two densities of artificial holes were added as well. Isopods settled more densely in plots with added water, and settled much more densely in plots with artificial holes. Water did not significantly affect isopod survival or offspring weight. Unmanipulated plots with higher dens...

Previous disturbance enhances the negative effects of trampling on barnacles

Communities are influenced by multiple stressors that can interact in surprising ways, yet trampling studies typically ignore other sources of disturbance. While there is some evi- dence that disturbed communities may resist trampling because of species shifts, we lack an understanding of how previous disturbance interacts with trampling in monospecific stands. Fur- thermore, recent studies emphasized experimental trampling additions with assumed ecological realism. We monitored barnacle cover in unmanipulated and experimentally disturbed plots at 2 sites separated by only 300 m and with disparate levels of human visitation. During the summer, barnacle cover increased by 6 and 15% at the less-visited site but decreased by 5 and 4% at the heavily visited site (unmanipulated and disturbed plots, respectively). Interestingly, the influence of site on barnacle cover was greater in disturbed than in unmanipulated plots. New recruits rep- resented a greater proportion of the barnacles at the heavily visited site. Neither plot slope nor predator abundance differed between these sites. To isolate the influence of trampling on barna- cles, we monitored barnacle cover in caged, human exclusion plots and in uncaged controls at both sites. Summer barnacle cover increased in all treatments except in uncaged plots at the heav- ily visited site. During 9 paired surveys coinciding with the exclusion experiment, as many as 70 humans (km �1 shoreline h �1 ) entered the intertidal zone of the heavily visited site but no one did so at the less-visited site. Given that (1) visitation was the dominant factor explaining differences in barnacle cover between the 2 sites, and (2) caging enhanced barnacle cover only at the heavily visited site, we conclude that previous disturbance mediated the effects of human trampling on barnacle populations, with more disturbed plots displaying greater trampling vulnerability.

Shifts in Microbial Biomass and the Bacteria: Fungi Ratio Occur Under Field Conditions Within 3 h After Rainfall

Increases in the frequency of soil drying and extreme precipitation projected by climate models may have important consequences for soil microbial community composition. However, the microbial response may occur over short time scales not captured by traditional sampling methods. Following a 2-year rainfall exclusion experiment in a pine forest ecosystem, we used phospholipid fatty acid profiling to measure the hourly, daily, and weekly-scale response of soil microbial biomass and the bacteria/fungi ratio to a precipitation event. We compared this response to the rewetting of un-manipulated plots. Within 3 h of watering, we detected increases in fungal and bacterial biomass of 125% and 66%, respectively, in un-manipulated plots, but only small increases in biomass within drought plots. We detected a decrease in the bacteria/fungi ratio in un-manipulated plots and an increase in this ratio in the drought plots. This surprising result was likely caused by root mortality (resulting from the previous 2-year rain exclusion) and an increase in ammonium pools in the drought plots, both of which could have suppressed fungal growth. Whereas past research suggests that soil microbes are resistant to drying-rewetting stress and to changes in annual precipitation patterns, here we show that microbes are sensitive to soil drying, but highly resilient, recovering within hours or days of a rain event. We propose that more emphasis be placed on hourly-scale field measurements of soil microbial community structure in future climate change studies.

Herbivores on a dominant understory shrub increase local plant diversity in rain forest communities

Indirect effects of trophic interactions on biodiversity can be large and common, even in complex communities. Previous experiments with dominant understory Piper shrubs in a Costa Rican rain forest revealed that increases in herbivore densities on these shrubs caused widespread seedling mortality as a result of herbivores moving from Piper to seedlings of many different plant genera. We tested components of the Janzen-Connell hypothesis by conducting focused studies on the effects of specialist and generalist Piper herbivores on local seedling diversity. Whereas specialist herbivores are predicted to increase mortality to neighboring seedlings that are closely related to the source plant, true generalists moving from source plants may cause density-dependent mortality of many species, and possibly increase richness if new species replace abundant species that have been thinned by herbivores. Therefore, we hypothesized that seedling richness would be greater in understory control plots created in patches of Piper that had normal densities of generalist herbivores compared to plots from which we removed generalist herbivores manually from all Piper shrubs. After 15 months, generalist-herbivore-removal plots had > 40% fewer seedlings, > 40% fewer species, and 40% greater seedling evenness, on average, than control plots with generalist herbivores intact. Using a complementary approach in unmanipulated plots in four forests, we used path analysis to test for a positive association between seedling diversity and herbivore damage on Piper species. In unmanipulated plots, for both generalist and specialist herbivores, our data were significant fits to the causal model that Piper herbivores decrease evenness and increase plant species richness, corroborating the experimental results. Because herbivores changed how individuals were apportioned among the species and families present (lower evenness), one interpretation of these associations between herbivores on Piper shrubs and local seedling richness is that high seedling mortality in dominant families allowed the colonization or survival of less common species. If interspecific or apparent competition allowed for a relative increase in species richness, then the Janzen-Connell hypothesis may extend its predictions to generalist seedling predators. We speculate that apparent competition may explain some of the deviations from neutral model predictions, especially at small scales.

Evaluation of restoration effectiveness: community response to the removal of alien plants

Plant invasions are a key cause of biodiversity loss and motivate many restoration programs worldwide. We assessed restoration success of an invaded forest in the Azores using two complementary experimental designs: a before-after control-impact (BACI) design compared a restored and a control (unmanipulated) site over three years, while a control-impact (CI) design evaluated the short-term effects of restoration on restored-control replicated pairs. In both designs, a food web approach was used to evaluate both structural and functional aspects of the restoration. Two years after removing alien plants from the BACI design, there were increases in the abundance of native seeds (110%), herbivorous insects (85%), insect parasitoids (5%), and birds (7%) in the experimental plot compared to the unmanipulated plot. In the CI design, five experimental plots were weeded and paired with five adjacent unmanipulated plots. Immediately following the removal of alien plants within the experimental plots, there was a significant decrease in native plant species, likely attributed to the effect of disturbance. Nevertheless, the production of native seeds increased by 35% in year 1, and seed production of the focal endemic plant, Ilex perado (holly), increased 159% in year 2. Weeding increased the survivorship and growth of seedlings transplanted into the plots, particularly those of alien species. Both experiments provide evidence of the positive effects of weeding cascading through the food web from native plants to herbivorous insects, insect parasitoids, and birds. Two aspects that could prove critical to the outcome of restoration programs deserve further attention: most bird-dispersed seeds were alien, and weeding favored alien over native seedling growth.