An invasive frog, Eleutherodactylus coqui, increases new leaf production and leaf litter decomposition rates through nutrient cycling in Hawaii

A frog endemic to Puerto Rico, Eleutherodactylus coqui, invaded Hawaii in the late 1980s, where it can reach densities of 50,000 individuals ha⁻¹. Effects of this introduced insectivore on invertebrate communities and ecosystem processes, such as nutrient cycling, are largely unknown. In two study sites on the Island of Hawaii, we studied the top-down effects of E. coqui on aerial, herbivorous, and leaf litter invertebrates; herbivory, plant growth, and leaf litter decomposition rates; and leaf litter and throughfall chemistry over 6 months. We found that E. coqui reduced all invertebrate communities at one of the two study sites. Across sites, E. coqui lowered herbivory rates, increased NH₄⁺ and P concentrations in throughfall, increased Mg, N, P, and K in decomposing leaf litter, increased new leaf production of Psidium cattleianum, and increased leaf litter decomposition rates of Metrosideros polymorpha. In summary, E. coqui effects on invertebrates differed by site, but E. coqui effects on ecosystem processes were similar across sites. Path analyses suggest that E. coqui increased the number of new P. cattleianum leaves and leaf litter decomposition rates of M. polymorpha by making nutrients more available to plants and microbes rather than through changes in the invertebrate community. Results suggest that E. coqui in Hawaii has the potential to reduce endemic invertebrates and increase nutrient cycling rates, which may confer a competitive advantage to invasive plants in an ecosystem where native species have evolved in nutrient-poor conditions.     

Comparison of leaf litter ant communities in woodlands, lowland forests and montane forests of north-eastern Tanzania

Ants were extracted in Winkler bags from sifted leaf litter sampled in arange of forest and woodland types in and around Mkomazi Game Reserve innorth-eastern Tanzania, including the Eastern Arc Mountains of South Pare andWest Usambara. A total of 87 ant species were recorded, of which 32.2% were onlyrecorded from montane forests (1400–1850 m altitude), 6.9%only from lowland forest (540–810 m), 19.5% only fromwoodland (300–1080 m), and 16.1% in all three forest types.Of the 28 species recorded only from montane forests, 12 species were only foundin the Mkomazi forests, four only in the Pares and seven species only in theUsambaras. Sites of similar altitude grouped together in a cluster analysis, andspecies richness decreased with an increase in altitude. The lowland forest andclosed woodland sites did not form distinct communities. To ensure preservationof ant species, forests from a full range of altitudes need to be conserved.This study confirms the status of the West Usambara forests as having a highlyendemic biota, and the critical need to adequately conserve the remainingvestiges of montane forest within Mkomazi Game Reserve.     

Nitrogen in shoot litter,root litter and root exudates from nitrogen-fixingAlnus incana

Summary A pot experiment withAlnus incana (L.) Moench growing in sand was set up to compare the amounts of nitrogen released from plants shoot litter with that released below ground as root litter and/or root exudation. No nitrogen fixation by free-living microorganisms was found in the sand and the increased nitrogen content of the plant + soil system was therefore due to nitrogen fixation byFrankia in the alder root-nodules. Most of the nitrogen released from the plants was in the nitrogen-rich leaf and other shoot litter. Only small amounts of nitrogen were found in the drainage water from the pots and were recorded as increased nitrogen content of the sand.     

Disparate effects of plant genotypic diversity on foliage and litter arthropod communities

Intraspecific diversity can influence the structure of associated communities, though whether litter-based and foliage-based arthropod communities respond to intraspecific diversity in similar ways remains unclear. In this study, we compared the effects of host-plant genotype and genotypic diversity of the perennial plant, Solidago altissima, on the arthropod community associated with living plant tissue (foliage-based community) and microarthropods associated with leaf litter (litter-based community). We found that variation among host-plant genotypes had strong effects on the diversity and composition of foliage-based arthropods, but only weak effects on litter-based microarthropods. Furthermore, host-plant genotypic diversity was positively related to the abundance and diversity of foliage-based arthropods, and within the herbivore and predator trophic levels. In contrast, there were minimal effects of plant genotypic diversity on litter-based microarthropods in any trophic level. Our study illustrates that incorporating communities associated with living foliage and senesced litter into studies of community genetics can lead to very different conclusions about the importance of intraspecific diversity than when only foliage-based community responses are considered in isolation. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effects of leaf litter on woody seedlings in xeric successional communities

We investigated the effect of leaf litter on the establishment of Eucalyptus incrassata, a mallee eucalypt. It has been suggested that litter accumulation may hinder seedling establishment, and that the removal of litter may be one of the mechanisms through which fire enhances recruitment. We conducted factorial experiments testing the effects of three kinds of leaf litter on E. incrassata seeds and seedlings at three contiguous sites with different land use histories. One site was an uncleared E. incrassata open mallee woodland (Mallee site), one a cleared area that had been ungrazed for about five years (Pasture site) and the third an area of mallee rolled some 40 years ago and permitted to regenerate (Regrowth site). Litter had no effect on emergence of planted E. incrassata seeds, but emergence differed between sites. Overall, the percentage of seeds that germinated and emerged was substantial (mean 35.2% ± 25.9%). Seedling shoot biomass did not differ between sites or litter treatments. Although seedlings grown in Pasture litter suffered higher mortality rates, overall mortality rates were low (mean 13.2% ± 15.5%), suggesting that leaf litter has little effect on recruitment rates during winter and spring. We conclude that leaf litter does not affect emergence or growth in young E. incrassata seedlings during winter and spring, when most establishment occurs. Our results emphasize the difficulty in predicting litter effects on recruitment.     

Food-web modification by an invertebrate predator in the Great Salt Lake (USA)

Summary During unusually wet years the salinity of the Great Salt Lake (Utah) decreased from above 100 g/L to 50 g/L. This allowed the predaceous insect Trichocorixa verticalis to invade the pelagic region of the lake and reach a mean summer density of 52/m³. Concurrent changes in the pelagic ecosystem were: a decrease in the dry biomass of the previously dominant filter-feeding brine shrimp Artemia franciscana from 720 to 2 mg/m³, the invasion of three other zooplankton taxa, a 10 × decrease in community filtration rate, a 20 × increase in chlorophyll a concentration, a 4 × decrease in water clarity and perhaps a decrease in soluble nutrients. Trichocorixa abundance was also inversely correlated with the abundance of Artemia along a salinity gradient in the lake's estuary. In a 9-d microcosm experiment Trichocorixa preyed on nauplii and decreased the total density of Artemia from 103 to 6/L. The reduction in Artemia allowed protozoans to increase 10–100 ×. Changes in chlorophyll and clarity were consistent with those observed in the lake. These results suggest that invertebrate predation may be an important factor structuring simple food webs such as those found in moderately saline lakes.     

Unadapted behaviour of native,dominant ant species during the colonization of an aggressive,invasive ant

Among the factors driving the invasive success of non-indigenous species, the “escape opportunity” or “enemy release” hypothesis argues that an invader’s success may result partly from less resistance from the new competitors found in its introduced range. In this study, we examined competitive interactions between the little fire ant Wasmannia auropunctata (Roger) and ant species of the genus Pheidole in places where both are native (French Guiana) and in places where only species of Pheidole are native (New Caledonia). The experimental introduction of W. auropunctata at food resources monopolized by the Pheidole species induced the recruitment of major workers only for the Guianian Pheidole species, which were very effective at killing Wasmannia competitors. In contrast, an overall decrease in the number of Pheidole workers and no recruitment of major workers were observed for the New Caledonian species, although the latter were the only ones able to kill the Wasmannia workers. These results emphasize the inappropriate response of native dominant New Caledonian species to W. auropunctata and, thus, the importance of enemy recognition and specification in the organization of ant communities. This factor could explain how invasive animal species, particularly ants, may be able to successfully invade species-rich communities.     

Tree leaf litter composition and nonnative earthworms influence plant invasion in experimental forest floor mesocosms

Dominant tree species influence community and ecosystem components through the quantity and quality of their litter. Effects of litter may be modified by activity of ecosystem engineers such as earthworms. We examined the interacting effects of forest litter type and earthworm presence on invasibility of plants into forest floor environments using a greenhouse mesocosm experiment. We crossed five litter treatments mimicking historic and predicted changes in dominant tree composition with a treatment of either the absence or presence of nonnative earthworms. We measured mass loss of each litter type and growth of a model nonnative plant species (Festuca arundinacea, fescue) sown into each mesocosm. Mass loss was greater for litter of tree species characterized by lower C:N ratios. Earthworms enhanced litter mass loss, but only for species with lower C:N, leading to a significant litter × earthworm interaction. Fescue biomass was significantly greater in treatments with litter of low C:N and greater mass loss, suggesting that rapid decomposition of forest litter may be more favorable to understory plant invasions. Earthworms were expected to enhance invasion by increasing mass loss and removing the physical barrier of litter. However, earthworms typically reduced invasion success but not under invasive tree litter where the presence of earthworms facilitated invasion success compared to other litter treatments where earthworms were present. We conclude that past and predicted future shifts in dominant tree species may influence forest understory invasibility. The presence of nonnative earthworms may either suppress of facilitate invasibility depending on the species of dominant overstory tree species and the litter layers they produce.     

Parasite loss and introduced species: a comparison of the parasites of the Puerto Rican tree frog, (<Emphasis Type="Italic">Eleutherodactylus coqui</Emphasis>), in its native and introduced ranges

The Puerto Rican frog, Eleutherodactylus coqui has invaded Hawaii and reached densities far exceeding those in their native range. One possible explanation for the success of E. coqui in its introduced range is that it lost its co-evolved parasites in the process of the invasion. We compared the parasites of E. coqui in its native versus introduced range. We collected parasite data on 160 individual coqui frogs collected during January-April 2006 from eight populations in Puerto Rico and Hawaii. Puerto Rican coqui frogs had higher species richness of parasites than Hawaiian coqui frogs. Parasite prevalence and intensity were significantly higher in Hawaii, however this was likely a product of the life history of the dominant parasite and its minimal harm to the host. This suggests that the scarcity of parasites may be a factor contributing to the success of Eleutherodactylus coqui in Hawaii.     

桂林岩溶石山檵木群落不同恢复阶段凋落物层酶对凋落物分解的影响

该文选取桂林岩溶石山檵木群落不同恢复阶段(灌木阶段、乔灌阶段和小乔林阶段)作为研究对象,探究凋落物层酶对凋落物分解速率的影响。结果表明:不同恢复阶段凋落物经1 a分解后,凋落物剩余率分别为灌木阶段(59.58%)、乔灌阶段(61.79%)和小乔林阶段(62.02%)。不同恢复阶段凋落物分解速率随演替的进行而减小。3个不同恢复阶段凋落物层多酚氧化酶、脲酶、蔗糖酶活性均在12月份最低,多酚氧化酶活性均在3月份最高,脲酶和蔗糖酶活性均在6月份最高。3个恢复阶段纤维素酶活性变化规律趋势一致,均在6月份酶活性最高,灌木阶段纤维素酶活性在3月份最低,乔灌阶段和小乔林阶段纤维素酶活性均在9月份最低。3个不同恢复阶段的凋落物层酶活性在不同时期均表现为蔗糖酶脲酶纤维素酶多酚氧化酶。不同恢复阶段凋落物层酶活性对凋落物分解速率影响不同。灌木阶段凋落物层蔗糖酶活性与分解速率呈显著正相关(P 0.05),乔灌阶段脲酶活性与分解速率呈显著正相关(P 0.05),小乔林阶段各酶活性与分解速率相关不显著。蔗糖酶、脲酶和多酚氧化酶是影响灌木阶段凋落物分解速率的重要因素,脲酶、纤维素酶和多酚氧化酶是影响乔灌和小乔林阶段分解速率的重要因素。     

不同林龄和密度对马尾松人工林凋落叶养分变化的影响

该文选择广西南宁市横县镇龙林场的4种林龄(幼龄林、中龄林、成熟林和过熟林)和4种密度(低密度林、中低密度林、中高密度林和高密度林)马尾松人工林共8种林分作为研究对象,分析了未破碎和破碎两个不同降解阶段的凋落叶C、N、P含量及其生态化学计量学特征。结果表明:(1)不同林龄中,凋落叶初始C、N含量在过熟林和成熟林中较高,P含量没有显著变化,且C∶N比值和C∶P比值从幼龄林到成熟林逐渐升高,说明较高林龄马尾松对N和P重吸收较低,而较低林龄马尾松对N和P重吸收较强,需要较大。(2)不同密度林中,随着林木密度的增加,凋落叶初始C含量逐渐升高,N含量无显著变化,P含量降低;高密度林凋落叶的初始C∶P比值和N∶P比值较高,说明高种植密度下马尾松可能对N和P养分的需求较大,P重吸收较强。(3)不同林龄和不同密度马尾松林的破碎凋落叶C含量、C∶N比值、C∶P比值和N∶P比值比未破碎凋落叶的低,N和P含量较高,说明凋落物在降解过程中出现N和P养分的富集现象。(4)中林龄和较高种植密度的马尾松破碎凋落叶与未破碎凋落物的C含量差值最大,C∶N比值和C∶P比值较低,说明这两种林分的凋落叶C的降解速率可能较大。上...     

Ecological niche modelling of an invasive alien plant and its potential biological control agents

Invasive alien plants are of concern in South Africa. Pompom weed (Campuloclinium macrocephalum) is currently invading the Grassland and Savannah biomes of South Africa and is likely to continue spreading in the southern African sub- region. Two possible biological control agents (Liothrips tractabilis and Cochylis campuloclinium) have been identified for control of pompom weed. We used ecological niche modelling to predict which areas in southern Africa are likely to be suitable for pompom weed and the two potential biological control agents. The overlap between areas predicted to be highly suitable for pompom weed and areas suitable for the biological control agents was assessed. Methods of reducing sampling bias in a data set used for calibrating models were also compared. Finally, the performance of models calibrated using only native range data, only invaded range data and both were also compared. Models indicate that pompom weed is likely to spread across a greater region of southern Africa than it currently occupies, with the Savannah and Grassland biomes being at greatest risk of invasion. Poor overlap was found between the areas predicted to be highly suitable for pompom weed and those areas predicted to be suitable for the biological control agents. However, models of the potential distribution of the biological control agents are interpreted with caution due to the very small sample size of the data set used to calibrate the models. Models calibrated using both native range and invaded range data were found to perform best whilst models calibrated using only native range data performed the worst. There was little difference found between models that were calibrated using spatially reduced (selecting only one record per 30 min grid cell) and randomly reduced (randomly selecting 50% of available records) biased data sets.     

Population Dynamics and Potential for Biological Control of an Exotic Invasive Shrub in Hawaiian Rainforests

Introduction of biological control agents to reduce the abundance of exotic invasive plant species is often considered necessary but risky. I used matrix projection models to investigate the current population dynamics of Clidemia hirta (Melastomataceae), an invasive shrub, in two rainforest stands on the island of Hawaii and to predict the efficacy of hypothetical biological control agents in reducing population growth rates. Stage-structured matrix models were parameterized with field data collected over 3 years from 2906 C. hirta plants in a recently invaded forest with an open overstory (Laupahoehoe) and 600 plants in a less recently invaded forest with a closed canopy (Waiakea). Asymptotic population growth rates (λ) for both populations in all years were greater than one, demonstrating that both populations were growing. Composite elasticities were high for the seedling life-history stage and fecundity, and near-term demographic elasticities suggested that changes in seedling survival would have the largest effect on population size in the short term. However, simulations showed that almost 100% of seedlings or new recruits produced per reproductive adult would have to be destroyed to cause populations to go locally extinct under current environmental conditions. Herbivores or pathogens that decrease survival across all vegetative stages by 12% at Waiakea and 64% at Laupahoehoe were projected to cause the populations to decline. Thus, biocontrol agents that reduce survival of multiple life-history stages rather than seed production should be pursued to control C. hirta in Hawaiian rainforests.     

三工河流域琵琶柴群落凋落物对土壤有机碳固定的影响

土壤有机碳是土壤碳库的重要组成部分,对生态系统生产力和全球碳循环有着重要作用。采用凋落物收集器和DIRT法(添加和去除凋落物法)研究三工河流域两处不同琵琶柴群落凋落物的产量、现存量、凋落物处理对土壤有机碳的影响。结果表明:群落1和群落2的凋落物产量季节变化趋势相同,均呈"N"型变化,在10月份达到最大值,7月或8月份达到次大值。凋落物现存量随季节均呈现"W"型变化,在10月份达到最大值,最大值分别为30.65 g/m~2和57.87 g/m~2。土壤有机碳随土壤深度的增加均逐渐降低,群落1和群落2分别下降了61.73%—62.39%和18.24%—25.84%。与对照处理相比,去除凋落物处理(NL)的群落1和群落2土壤有机碳分别降低了6.97%和18.38%;添加凋落物处理(DL)的土壤有机碳分别增加了19.64%和13.66%;去除凋落物处理(NL)的群落1和群落2土壤有机碳储量分别为1007.36 kg/hm~2和709.30 kg/hm~2,添加凋落物处理(DL)的土壤有机碳储量分别为1197.88 kg/hm~2和1010.78 kg/hm~2。双因素方差分析表明群落1的土层深度和三种处理对土壤有机碳的交互作用不显著,群落2的交互作用显著。回归分析显示:土壤水分、电导率、pH、容重和温度是导致两琵琶柴群落土壤有机碳不同的主要生态因子。相对较高的土壤pH和盐分含量抑制了凋落物的分解,导致凋落物现存量较高、土壤有机碳含量低;相对较高的土壤含水量和较小的容重,有利于土壤生物的活性和土壤有机碳的矿化,导致土壤有机碳含量降低。     

Light limitation creates patchy distribution of an invasive grass in eastern deciduous forests

Species interactions and their indirect effects on the availability and distribution of resources have been considered strong determinants of community structure in many different ecological systems. In deciduous forests, the presence of overstory trees and shrubs creates a shifting mosaic of resources for understory plants, with implications for their distribution and abundance. Determination of the ultimate resource constraints on understory vegetation may aid management of these systems that have become increasingly susceptible to invasions by non-native plants. Microstegium vimineum (Japanese grass) is an invasive annual grass that has spread rapidly throughout the understory of forests across the eastern United States since it was first observed in Tennessee in 1919. M. vimineum occurs as extensive, dense patches in the understory of eastern deciduous forests, yet these patches often exhibit sharp boundaries and distinct gaps in cover. One example of this distributional pattern was observed relative to the native midstory tree Asimina triloba (pawpaw), whereby dense M. vimineum cover stopped abruptly at the drip line of the A. triloba patch and was absent beneath the A. triloba canopy. We conducted field and greenhouse experiments to test several hypotheses regarding the causes of this observed pattern of M. vimineum distribution, including allelopathy, seed dispersal, light limitations, and soil moisture, texture, and nutrient content. We concluded that light reduction by the A. triloba canopy was the environmental constraint that prevented establishment of M. vimineum beneath this tree. Whereas overstory tree canopy apparently facilitates the establishment of this shade-tolerant grass, the interaction of overstory canopy with midstory canopy interferes with M. vimineum by reducing the availability of sunflecks at the ground layer. It is likely that other midstory species influence the distribution and abundance of other herb-layer species, with implications for management of understory invasive plant species.     

Comparative study of the mass loss rate of moss litter in boreal and subalpine forests in relation to temperature

The mossHylocomium splendens shows a very wide distribution in the Northern Hemisphere and may be useful as an indicator of climatic change on a global scale. We aimed to establish a convenient method to estimate the annual rate of litter mass loss of this species. The rate was calculated from the annual litter production rate and the amount of litter accumulated in the field. The litter production rate was estimated by analysis of the moss shoot growth. The rates calculated by this method tended to be larger than estimates obtained by the litter bag method. Using this method, we examined the difference in the litter mass loss rate along the altitudinal and latitudinal temperature gradients. The moss samples were collected from three boreal forests in Canada and four subalpine forests in Japan. At the subalpine sites, the annual rate of litter mass loss was within the range of 10–24% and tended to be smaller with increasing altitude. The rates in the boreal sites were similar to those in the subalpine sites despite lower mean annual temperatures. A significant log-linear relationship was observed between the annual mass loss rate and the cumulative value of monthly mean air temperatures higher than 0°C (CMT). Nitrogen concentration of the litter was positively correlated with mean annual air temperature. Site to site variation in the annual mass loss rate was largely explained by CMT and nitrogen concentration of the litter.     

Biomass and litter dynamics in a Melaleuca forest on a seasonally inundated floodplain in tropical,northern Australia

Litterfall from a Melaleuca forest was investigated as part of chemical cycling studies on the Magela Creek floodplain in tropical, northern Australia. The forest contained two species of tree, Melaleuca cajaputi and Melaleuca viridiflora, with a combined average density of 294 trees ha^–1. The M. viridiflora trees had diameter breast height measurements ranging from 11.8 to 62.0 cm, median class 25.1–30.0cm and a mean value of 29.2±1.0 cm, compared to 13.0 to 66.3 cm, 30.1–35.0cm and 33.5±1.0cm for M. cajaputi trees. A regression model between tree height, diameter breast height and fresh weight was determined and used to calculate average tree weights of 775±1.6kg for M. viridiflora and 1009±1.6kg for M. cajaputi, and a total above-ground fresh weight of 263±0.3t ha^–1. The weight of litter recorded each month on the ground beneath the tree canopy ranged from 582±103 to 2176±376 g m^–2 with a monthly mean value of 1105±51 g m^–2. The coefficient of variation of 52% on this mean indicates the large spatial and temporal variability in litter distribution over the study site. This variability was greatly affected by the pattern of water flow and litter transport during the Wet season. Litterfall from the trees was evaluated using two techniques - nets and trays. The results from these techniques were not significantly different with annual litterfall collected in the nets being 705 ± 25 g m^–2 and in the trays 716±49 g m^–2. The maximum monthly amount of litterfall, 108 ±55g m^–2, occurred during the Dry season months of June–July. Leaf material comprised 70% of the total annual weight of litter, 480±29 g m^–2 in the nets and 495 ± 21 g m^–2 in the trays. The tree density and weight of litter suggest that the Melaleuca forests are highly productive and contribute a large amount of material to the detrital/debris turnover cycle on the floodplain.     

Evaluation of the field impact of an adventitious herbivore on an invasive plant,yellow toadflax,in Colorado,USA

Although some introduced plants arrive into their new range without their generalist and specialist herbivores, for others, their herbivores arrive prior to, with, or after the introduction of the plant, reestablishing the link between natural enemies and invaders in the introduced range. Research documenting the effects of adventitiously introduced herbivores on their target plants in the introduced range, and the mechanisms by which those effects occur, can provide insight into potential biological weed control. We studied the effects of an accidentally introduced beetle Brachypterolus pulicarius on the growth and reproduction of its host, the invasive plant Linaria vulgaris (yellow toadflax), growing under field conditions across multiple years and sites in western Colorado, USA. We found that feeding by B. pulicarius on L. vulgaris was variable among 3 years (2002–2004) and across eight local sites. Part of the variation in damage was explained by ramet density; sites with greater ramet density experienced a higher proportion of damage. In an observational study across 2 years, damage was positively correlated with estimates of sexual reproduction, ramet growth, and clonal shoot production. However, opposite trends were observed in an experiment; damage by B. pulicarius decreased estimates of sexual reproduction. Differences between the results of the observational and experimental studies were likely driven by selective feeding by B. pulicarius on larger ramets. Nonetheless, the ability of B. pulicarius to control established L. vulgaris population growth remains uncertain under the environmental conditions we studied. In both the observational and experimental study, B. pulicarius did not affect L. vulgaris survival, and we found no evidence that established L. vulgaris populations were seed limited, suggesting that reductions in seeds may not translate into demographic changes in heavily infested populations. Interactions among insect foraging behavior, individual plant responses to damage, and the demographic consequences of seed input may help to explain the varying degrees to which herbivores affect plants and populations in this and other systems.     

Existence of an endothelio-endothelial placenta in the insectivore,Suncus murinus

Summary The interhemal membrane of the chorioallantoic placenta in the insectivore Suncus murinus was investigated by means of electron microscopy. In the interhemal membrane the syncytiotrophoblast clearly intervened between the hypertrophied maternal endothelium and the fetal endothelium by day 20 of pregnancy. Although the syncytiotrophoblast showed a sieve-like feature from day 20 to 24, it was distinctly continuous. The syncytiotrophoblast, however, became discontinuous in most areas of the labyrinthine zone after day 24, and finally both projections of the maternal and the fetal endothelium contacted each other. These findings indicate the focal existence of an endothelio-endothelial condition within an otherwise endothelio-chorial placenta.     

Interactions in a tritrophic acarine predator-prey metapopulation system V: Within-plant dynamics of Phytoseiulus persimilis and Tetranychus urticae (Acari: Phytoseiidae, Tetranychidae)

To investigate the relative contributions of bottom-up (plant condition) and top-down (predatory mites) factors on the dynamics of the two-spotted spider mite (Tetranychus urticae), a series of experiments were conducted in which spider mites and predatory mites were released on bean plants. Plants inoculated with 2, 4, 8, 16, and 32 adult female T. urticae were either left untreated or were inoculated with 3 or 5 adult female predators (Phytoseiulus persimilis) one week after the introduction of spider mites. Plant area, densities of T. urticae and P. persimilis, and plant injury were assessed by weekly sampling. Data were analysed by a combination of statistical methods and a tri-trophic mechanistic simulation model partly parameterised from the current experiments and partly from previous data. The results showed a clear effect of predators on the density of spider mites and on the plant injury they cause. Plant injury increased with the initial number of spider mites and decreased with the initial number of predators. Extinction of T. urticae, followed by extinction of P. persimilis, was the most likely outcome for most initial combinations of prey and predators. Eggs constituted a relatively smaller part of the prey population as plant injury increased and of the predator population as prey density decreased. We did not find statistical evidence of P. persimilis having preference for feeding on T. urticae eggs. The simulation model demonstrated that bottom-up and top-down factors interact synergistically to reduce the density of spider mites. This may have important implications for biological control of spider mites by means of predatory mites.