An invasive dandelion unilaterally reduces the reproduction of a native congener through competition for pollination

The impact of invasive alien species on native species is of increasing global concern. Invasive plants have various negative effects on natives through competition; however, relatively little is known about competition for pollination. The relationship between Japanese native dandelions (Taraxacum spp.) and invasive congeners may be a typical case of such an interaction. For example, native dandelions are being replaced by invasive congeners, especially in urban and suburban areas of Japan. To explain this phenomenon, we hypothesized that when natives are mixed with attractive invasives, natives may suffer from reduced seed set because invasives deprive natives of pollinators or because pollinators frequently move between species, resulting in interspecific pollen transfer. To test this hypothesis, we studied the effect of the invasive dandelion T. officinale on the pollination and seed set of the native T. japonicum using artificial arrays of monospecific and mixed-species plots as well as natural populations. Taraxacum officinale attracted more pollinator visits, perhaps because it produced more nectar than T. japonicum. The number of pollinator visits to T. japonicum was reduced when the congeners were grown together, and pollinators moved frequently between the two species. The proportion of seed set for T. japonicum was reduced in the presence of T. officinale in both artificial arrays and natural populations. These results support our hypothesis that interspecific competition for pollination plays an important role in the recent replacement of native dandelions by invasive congeners in Japan. Because invasive dandelions are apomicts, negative effects are incurred only by sexual natives. Thus, this system can be recognized as a rare case of interspecific interaction through pollination.     

Differential effect of inter- and intraspecific competition on the performance of invasive and native Taraxacum species

Inter- and intraspecific competitive abilities are significant determinants of invasive success and the ecological impact of non-native plants. We tested two major hypotheses on the competitive ability of invasive species using invasive (Taraxacum officinale) and native (T. platycarpum) dandelions: differential interspecific competitive ability between invasive and native species and the kin recognition of invasive species. We collected seeds from two field sites where the two dandelion species occurred nearby. Plants were grown alone, with kin (plants from the same maternal genotype) or strangers (plants from different populations) of the same species, or with different species in a growth chamber, and the performance at the early developmental stage between species and treatments was compared. The invasive dandelions outcompeted the native dandelions when competing against each other, although no difference between species was detected without competition or with intraspecific competition. Populations of native species responded to interspecific competition differently. The effect of kinship on plant performance differed between the tested populations in both species. A population produced more biomass than the other populations when grown with a stranger, and this trend was manifested more in native species. Our results support the hypothesis that invasive plants have better competitive ability than native plants, which potentially contributes to the establishment and the range expansion of T. officinale in the introduced range. Although kin recognition is expected to evolve in invasive species, the competitive ability of populations rather than kinship seems to affect plant growth of invasive T. officinale under intraspecific competition.     

Common alien plants are more competitive than rare natives but not than common natives

Success of alien plants is often attributed to high competitive ability. However, not all aliens become dominant, and not all natives are vulnerable to competitive exclusion. Here, we quantified competitive outcomes and their determinants, using response‐surface experiments, in 48 pairs of native and naturalised alien annuals that are common or rare in Germany. Overall, aliens were not more competitive than natives. However, common aliens (invasive) were, despite strong limitation by intraspecific competition, more competitive than rare natives. This is because alien species had higher intrinsic growth rates than natives, and common species had higher intrinsic growth rates than rare ones. Strength of interspecific competition was not related to status or commonness. Our work highlights the importance of including commonness in understanding invasion success. It suggests that variation among species in intrinsic growth rates is more important in competitive outcomes than inter‐ or intraspecific competition, and thus contributes to invasion success and rarity.     

Neighbour tolerance,not suppression,provides competitive advantage to non‐native plants

High competitive ability has often been invoked as a key determinant of invasion success and ecological impacts of non‐native plants. Yet our understanding of the strategies that non‐natives use to gain competitive dominance remains limited. Particularly, it remains unknown whether the two non‐mutually exclusive competitive strategies, neighbour suppression and neighbour tolerance, are equally important for the competitive advantage of non‐native plants. Here, we analyse data from 192 peer‐reviewed studies on pairwise plant competition within a Bayesian multilevel meta‐analytic framework and show that non‐native plants outperform their native counterparts due to high tolerance of competition, as opposed to strong suppressive ability. Competitive tolerance ability of non‐native plants was driven by neighbour's origin and was expressed in response to a heterospecific native but not heterospecific non‐native neighbour. In contrast to natives, non‐native species were not more suppressed by hetero‐ vs. conspecific neighbours, which was partially due to higher intensity of intraspecific competition among non‐natives. Heterogeneity in the data was primarily associated with methodological differences among studies and not with phylogenetic relatedness among species. Altogether, our synthesis demonstrates that non‐native plants are competitively distinct from native plants and challenges the common notion that neighbour suppression is the primary strategy for plant invasion success.     

Aggregative Root Placement: A Feature During Interspecific Competition in Inland Sand-Dune Habitats

Segregation of roots is frequently observed in competing root systems. However, recently, intensified root growth in response to a neighbouring plant has been described in pot experiments [Gersani M, Brown J S, O'Brien E E, Maina G M and Abramsky Z 2001. J. Ecol. 89, 660–669]. This paper examines whether intense root growth towards a neighbour (aggregation) plays a role in competitive interactions between plant species from open nutrient-poor mid-European sand ecosystems. In a controlled field-competition experiment, root distribution patterns of intra- and interspecific pairs as well as single control plants of Corynephorus canescens, Festuca psammophila, Hieracium pilosella, Hypochoeris radicata and Conyza canadensis were investigated after one growing season. Under intraspecific competition plants tended to segregate their root systems, while under interspecific competition most species tended to aggregate roots towards their neighbours even at the expense of root development at the opposite competition-free side of the target. Preference of a root aggregation strategy over the occupation of competition-free soil in interspecific competition emphasizes the importance of contesting between individuals in relation to mere resource acquisition. It is suggested that in the presence of a competitor the plants might use root aggregation as a defensive reaction to maintain a strong competitive response and exclusive access to the resources of already occupied soil volumes.     

Shift in competitive ability mediated by soil biota in an invasive plant

Understanding the shifts in competitive ability and its driving forces is key to predict the future of plant invasion. Changes in the competition environment and soil biota are two selective forces that impose remarkable influences on competitive ability. By far, evidence of the interactive effects of competition environment and soil biota on competitive ability of invasive species is rare. Here, we investigated their interactive effects using an invasive perennial vine, Mikania micrantha. The competitive performance of seven M. micrantha populations varying in their conspecific and heterospecific abundance were monitored in a greenhouse experiment, by manipulating soil biota (live and sterilized) and competition conditions (competition‐free, intraspecific, and interspecific competition). Our results showed that with increasing conspecific abundance and decreasing heterospecific abundance, (1) M. micrantha increased intraspecific competition tolerance and intra‐ vs. interspecific competitive ability but decreased interspecific competition tolerance; (2) M. micrantha increased tolerance of the negative soil biota effect; and (3) interspecific competition tolerance of M. micrantha was increasingly suppressed by the presence of soil biota, but intraspecific competition tolerance was less affected. These results highlight the importance of the soil biota effect on the evolution of competitive ability during the invasion process. To better control M. micrantha invasion, our results imply that introduction of competition‐tolerant native plants that align with conservation priorities may be effective where M. micrantha populations are long‐established and inferior in inter‐ vs. intraspecific competitive ability, whereas eradication may be effective where populations are newly invaded and fast‐growing.     

Effect of arbuscular mycorrhiza on inter- and intraspecific competition of two grassland species

We were interested in the role of arbuscular mycorrhiza (AM) in the competition between plants of different sizes. A pot experiment of factorial design was established, in which AM root colonization and competition were used as treatments. Five-week-old Prunella vulgaris seedlings were chosen as target plants (i.e. plants whose response to competition was studied) and the following (13 replicates of each) were used as neighbours: (1) a large, 10-week-old P. vulgaris, (2) two P. vulgaris seedlings, and (3) a large, 10-week-old Fragaria vesca. In the experiment where small neighbours were grown together with small target plants, competition did not reduce target plant weight significantly, compared to the other two treatments. The competitive effects of large neighbours were significant, regardless of species (both older neighbours reduced the weights of target plants similarly), but there was a clear difference between intra- and interspecific competition when plants were mycorrhizal. In intraspecific competition with a large neighbour, the target plant shoot weight was reduced 24% when inoculated with AM. Thus, AM amplified rather than balanced intraspecific competition. In interspecific competition with old F. vesca, the shoot weights of target plants were 22% greater when inoculated with AM than when non-mycorrhizal. The results showed that, for given soil condition, AM might increase species diversity by increasing competitive intraspecific suppression and decreasing the interspecific suppression of small plants by larger neighbours.     

Intra and interspecific competition among invasive and native species during early stages of plant growth

Plant competition is a primary ecological process limiting grassland restoration success. Appropriate restoration techniques require an understanding of the degree to which intra and interspecific competition control invasive and native plant growth. The objective of this study was to determine how the intensity of intra and interspecific competition changes during early stages of plant growth. Two invasive (Bromus tectorum and Taeniatherum caput-medusae) and two native (Pseudoroegneria spicata and Poa secunda) species were grown in a diallel competition experiment, either alone or in 1:1 binary combinations and exposed to two levels of N (no N or 400 mg N kg⁻¹ soil added) in a greenhouse. Total biomass for each species was quantified over four harvests and competitive effects were calculated. Our results show that the relative magnitude of intra and interspecific competition changes through time. Intraspecific competition was intense for native species at the initial harvests and therefore important in contributing to the outcome of final size of native species seedlings. Interestingly, bluebunch wheatgrass imposed interspecific competition on annual grasses at the first two harvests and appeared to be a better competitor than Sandberg’s bluegrass. We found that fast growing invasive species became more competitive compared to slow growing native species with increasing N and appear to establish a positive feedback mechanism between size and resource uptake. Opportunities to improve restoration success exist from determining the optimum combination of density, species proportion, and their spatial arrangement in various ecosystems and environments.     

Predicting the current distribution and potential spread of the exotic grass Eragrostis plana Nees in South America and identifying a bioclimatic niche shift during invasion

Eragrostis plana (Poaceae) is a perennial grass introduced from South Africa to the state of Rio Grande do Sul in southern Brazil. Currently, it is considered an invasive grass in several regions of the world, including South America, where it has caused negative ecological and socio‐economic impacts. Ecological niche models, using bioclimatic variables, are often used to predict the potential distribution of invasive species. In this study we prepared two bioclimatic models for E. plana using the Genetic Algorithm for Rule‐set Production, the first based on data from its native region (South Africa) and the second on data from both the native and invaded (South America) regions. We then projected each model onto South America to identify regions vulnerable to invasion by the species, and compared our results with available records of the species in South America. Finally, we explored the model's predictions for the existence of a bioclimatic niche shift during the invasion process of E. plana in South America, using multivariate statistical analysis. The model created with native distribution data was only able to predict (with highly suitable habitat) the region of introduction of E. plana in South America. However, the current distribution, as well as the region of introduction of the species, was reliably predicted by the model created with data from both native and invaded regions. Our multivariate analysis supports a hypothesis of bioclimatic niche shift during the invasion process of E. plana in South America.     

Direct and feeding‐induced interactions between two rice planthoppers, Sogatella furcifera and Nilaparvata lugens: effects on dispersal capability and performance

Abstract. 1. A series of laboratory experiments was conducted to explore the effects of inter‐specific interactions, both direct interactive effects and those induced through previous feeding, on the dispersal capability (proportion of macropterous adults) and performance (development time and survival) of two wing‐dimorphic planthoppers, the whitebacked planthopper Sogatella furcifera and the brown planthopper Nilaparvata lugens, two pests of rice throughout Asia. 2. An asymmetric effect of inter‐specific crowding on dispersal capability was detected between the two planthoppers. With density controlled, the proportion of macropterous adults in N. lugens was higher when raised in mixed populations with S. furcifera than in pure cultures comprised of conspecifics, suggesting that interspecific effects on dispersal capability are stronger than intraspecific effects. In contrast, interspecific effects on macroptery (%) were weaker than intraspecific impacts for S. furcifera. 3. This trend was parallelled by adverse competitive effects on survival, with interspecific effects stronger than intraspecific impacts for N. lugens and intraspecific effects stronger for S. furcifera. 4. Crowding also affected the incidence of macroptery indirectly through feeding‐induced changes in plant physiology. Rearing N. lugens on plants fed on previously by heterospecifics resulted in a higher incidence of macroptery in females and protracted development than for N. lugens raised on plants fed on previously by conspecifics. By contrast, the intraspecific effect of previous feeding was stronger than the interspecific effect in S. furcifera, with higher macropter production and prolonged development occurring on plants exposed previously to conspecifics. 5. The results suggest that interspecific interactions between herbivorous insects, both direct and via feeding‐induced changes in plant physiology, can have negative consequences for performance and survival and promote the production of flight‐capable adults that can disperse.     

Competitive abilities of native grasses and non-native (Bothriochloa spp.) grasses

Old World Bluestems (OWB), introduced from Europe and Asia in the 1920s, recently have begun to raise concerns in the Great Plains. Despite suggestion in the late 1950s that OWB were weedy and negatively impacted biological diversity, they were widely introduced throughout the Great Plains for agricultural purposes. Anecdotal evidence suggests that OWB exhibit invasive characteristics that promote competitive exclusion of native species. The objective of our study was to quantify the competitive abilities of two OWB species (Caucasian bluestem; Bothriochloa bladhii (Retz.) S.T. Blake (= Bothriochloa caucasica (Trin.) C.E. Hubb.) and yellow bluestem; Bothriochloa ischaemum (L.) Keng) with three native grass species (big bluestem (Andropogon gerardii Vitman), little bluestem (Schizachyrium scoparium (Michx.) Nash), and sideoats grama (Bouteloua curtipendula (Michx.) Torr.)). A greenhouse target-neighbor study was conducted to assess both interspecific and intraspecific competition. A total of 480 pots (4.4 l) filled with native soil was used with all pair-wise combinations of species and four density treatments (six replications). Vegetative tiller height, above- and belowground biomass were measured at the end of 16 weeks. Both of the OWB significantly inhibited at least one growth parameter of the three native grass species, while most of the native species did not inhibit growth of either OWB species. Growth of B. ischaemum was enhanced when grown in association with S. scoparium. Based upon the results of our study of OWB competitive superiority and previous research, many of the characteristics possessed by OWB are found to be in common with known invasive species. Hence, we propose that two OWB are competitively superior to three common native prairie species providing them with the ability to invade and threaten the native grasslands of the Central and Southern Great Plains.     

Competition between co-dominant plants of the Serengeti plains depends on competitor identity,water, and urine

Kyllinga nervosa (Steud.) and Sporobolus kentrophyllus (K. Schum.) are co-dominant plants of the Serengeti short-grass plains, Tanzania. The plains are characterized by seasonal and sporadic rainfall and currently support in excess of 1.5 million migratory ungulates. The interactive effect of simulated bovine urine and water availability were tested on the competitive interactions of these species in the laboratory. Sporobolus kentrophyllus was a superior competitor to K. nervosa over the tested treatment levels with respect to growth and reproductive effort. Sporobolus kentrophyllus exhibited rapid growth in response to urine addition, leading to a significant species × urine interaction while reduced growth by K. nervosa in response to low water availability explained the significant species × water interaction and is likely explained by K. nervosa's shallow root system. Kyllinga nervosa, however, appears to be more tolerant of low nitrogen conditions based on its similar growth with and without the urine treatment. The effect of intraspecific competition on total biomass was similar for S. kentrophyllus and K. nervosa. Competition resulted in increased size differences (asymmetry) for K. nervosa and for the interspecific competition treatments compared to the size differences observed for plants grown individually (in absence of competition).Total reproductive biomass was reduced most by competition with S. kentrophyllus, irrespective of target species. The water treatment did not influence reproduction while the urine treatment significantly increased reproductive biomass and interacted with target species, competitor species, and yielded a three-way urine × target × competitor species interaction.Results suggest that codominance of these two species in the Serengeti is regulated by water availability, nitrogen input from grazers, and local neighbor identity.     

大别山五针松种内和种间竞争强度

通过对安徽省岳西县大别山五针松群落内的53株对象木及2079株竞争木的调查,运用Hegyi单木竞争模型分析了大别山五针松的种内和种间竞争强度。结果表明,大别山五针松的种内和种间竞争强度分别为17.11%和82.89%,说明竞争主要来自种间。大别山五针松的伴生种较多,种内与主要伴生种间的竞争关系为短柄枹大别山五针松种内黄山松满山红茅栗金缕梅紫茎四照花灯台树鹅耳枥。竞争强度随对象木胸径的增大而减小,当对象木的胸径小于25cm时,所受到的竞争强度较大;当胸径在25cm以上时,竞争强度变化很小,二者符合幂函数关系(CI=AD-B),所得的预测模型能很好的预测大别山五针松种内和种间竞争强度。     

Elevated nitrogen deposition may advance invasive weed,Solidago canadensis,in calcareous soils

Aims Change in nitrogen (N) availability regulates phosphorus (P) acquisition and potentially alters the competition among native species and invasive weeds. This study determines how current and projected N deposition affect the growth, the intraspecific and interspecific competitive ability of native and invasive plants in calcareous soils with low P availability.     

Spatial arrangements affect suppression of invasive Alternanthera philoxeroides by native Hemarthria compressa

It has been hypothesized that differences in spatial arrangements change the relative frequency of intra- and interspecific encounters between plant species. Manipulating spatial arrangement may play a role in invasive plant suppression when native species are used as competitors against introduced species. In this study, a replacement series experiment was performed to investigate the effects of intraspecifically random and aggregated spatial arrangements on interactions between the native plant Hemarthria compressa and the invasive plant Alternanthera philoxeroides, to test the possibility and effectiveness of H. compressa in suppressing A. philoxeroides. When both species were planted in intraspecifically random spatial patterns, H. compressa had a competitive advantage over A. philoxeroides at relative densities of 2:2 and 3:1. However, aggregation increased the strength, and therefore the cost, of intraspecific competition in H. compressa, resulting in lower biomass production, which reduced its effectiveness as an interspecific competitor. As the relative density of H. compressa in mixtures decreased, plants allocated more biomass to belowground parts, but fewer interspecific encounters lowered its inhibitory effects on A. philoxeroides. The results not only confirm that the frequency of conspecific and heterospecific encounters can influence competitive outcomes, but also suggest that a reduction in the degree of spatial aggregation in H. compressa and an increase in its relative densities may be essential to increase the suppression of A. philoxeroides.     

Effects of clonal fragmentation on intraspecific competition of a stoloniferous floating plant

Disturbance is common and can fragment clones of plants. Clonal fragmentation may affect the density and growth of ramets so that it could alter intraspecific competition. To test this hypothesis, we grew one (low density), five (medium density) or nine (high density) parent ramets of the floating invasive plant Pistia stratiotes in buckets, and newly produced offspring ramets were either severed (with fragmentation) or remained connected to parent ramets (no fragmentation). Increasing density reduced biomass of the whole clone (i.e. parent ramet plus its offspring ramets), showing intense intraspecific competition. Fragmentation decreased biomass of offspring ramets, but increased biomass of parent ramets and the whole clone, suggesting significant resource translocation from parent to offspring ramets when clones were not fragmented. There was no interaction effect of density x fragmentation on biomass of the whole clone, and fragmentation did not affect competition intensity index. We conclude that clonal fragmentation does not alter intraspecific competition between clones of P. stratiotes, but increases biomass production of the whole clone. Thus, fragmentation may contribute to its interspecific competitive ability and invasiveness, and intentional fragmentation should not be recommended as a measure to stop the rapid growth of this invasive species.     

喜旱莲子草和接骨草竞争对模拟增温的响应北大核心CSCD

研究外来入侵植物与本地植物种竞争对气候变暖的响应,对于预测未来气候变化背景下入侵植物的入侵趋势、理解其入侵机制以及筛选生态替代种具有重要的意义。以入侵我国的外来植物喜旱莲子草(Alternanthera philoxeroides)和本地植物种接骨草(Sambucus chinensis)为材料,通过两种植物单栽、纯栽和混栽,采用红外辐射加热器模拟增温,研究了两种植物竞争对模拟增温的响应。结果表明:(1)在模拟增温期间(2013年5–12月),增温组空气平均温度比不增温组提高了0.47℃,相对湿度降低了1.87%;(2)混栽的喜旱莲子草除根冠比与单栽无显著差异外,其余各生物量和根系形态指标均显著低于单栽喜旱莲子草;无竞争、种间竞争和种内竞争三种竞争间,接骨草除根冠比、细根与总根生物量比、比根长和比根表面积无显著差异外,其余指标均呈现无竞争>种间竞争>种内竞争的趋势;(3)无竞争、种间竞争和种内竞争三种条件下,喜旱莲子草各指标在增温和不增温处理间差异均不显著,而接骨草总生物量和根生物量在无竞争和种间竞争条件下增温处理均显著低于不增温处理,在种内竞争条件下则相反;(4)增温使接骨草的相对拥挤系数降低,接骨草对温度升高反应敏感,而喜旱莲子草则表现出一定的适应性。由此推测,在中度遮阴陆生生境中,接骨草有望成为喜旱莲子草生物替代控制的材料。     

Seedling competition between native cottonwood and exotic saltcedar: implications for restoration

Altered hydrology of southwestern United States rivers has led to a decline in native cottonwood (Populus deltoides). Areas historically dominated by cottonwood have been replaced by invasive saltcedar (Tamarix chinensis). Restoration of historic hydrology through periodic flooding of riparian areas has been a means of restoring native species. However, due to similarity in germination requirements of cottonwoods and saltcedars, flooding may create an unwanted increase in the number of saltcedar seedlings. Therefore, we evaluated competitive aspects of these co-occurring species in an extant riparian habitat in the arid southwestern US. We measured effects of competition between cottonwood and saltcedar seedlings and among cottonwood seedlings during the first growing season following seedling establishment in 360, 0.5 × 0.5-m plots at the Bosque del Apache National Wildlife Refuge, New Mexico. We used five interspecific density treatments and five intraspecific density treatments. Cottonwood seedling biomass and height were twice that of saltcedar seedlings across all density treatments. As density of cottonwood increased, intraspecific competition increased in severity and biomass of cottonwood seedlings decreased. At 4 plants/0.25 m², cottonwood seedlings had the greatest biomass; whereas, survival was highest at 10 plants/0.25 m². Our results support greenhouse studies and suggest that if favorable germination conditions are established for cottonwood in floodplains, saltcedar seedlings that cogerminate could be outcompeted by native cottonwood seedlings.     

Using coloured roots to study root interaction and competition in intercropped legumes and non-legumes

Aims Root interactions between neighbour plants represent a fundamental aspect of the competitive dynamics in pure stand and mixed cropping systems. The comprehension of such phenomena places big methodological challenges, and still needs clarification. The objectives of this work were (i) to test if a species with coloured roots can be used to examine the interaction in a legume-non-legume intercropping system; (ii) to verify the importance of initial root growth on the successive root development of mixture component plants; (iii) to test if the root interaction in the shallow layers has consequences for deep root growth and (iv) to compare the effect of intraspecific and interspecific competition on root development and biomass growth.Methods A detailed study on root growth and interaction was carried out using rhizotron tubes where two legume species were grown in pure stands or were intercropped with red beet, a variety of Beta vulgaris L. with clear red roots. Within the rhizotrons, the three species were grown either without competitors, with two plants of the same species to measure intraspecific competition or with one legume and one red beet plant to study interspecific competition. The use of mixtures where one component has clearly coloured roots, together with several scalar measurements of root depth and proliferation, allowed the measurement of the root system of each species when grown in the mixtures.Important findings The use of rhizotron tubes coupled with species with coloured roots represented a valuable method to study the belowground interaction in mixed cropping systems. The initial root growth was a very important feature for the subsequent dominance of a species and it was not related to seed dimension. Initial root growth was also important because the root interactions in the shallower soil layers were found to influence the root growth in deeper soil. The root system of the red beet showed much faster and deeper growth than that of the legumes, and made red beet the dominant component in the mixtures while the legume root system was confined to the shallower soil layer. Intraspecific competition was well tolerated by the legumes, but it was limiting for the highly competitive red beet. The outcome of root interaction between neighbour plants was confirmed to be species-specific as it changed according to the intensity of the competitive effect/response of each species of the mixture: both legumes were slightly affected by the intraspecific and highly affected by interspecific competition while red beet was more affected by intraspecific competition but strongly dominant when intercropped with legumes.     

Intra‐ and interspecific interactions between aphidophagous ladybirds: the role of prey in predator coexistence

Cannibalism (CANN) and intraguild predation (IGP) may provide energy and nutrients to individuals and eliminate potential competitors. These negative competitive interactions could also affect the coexistence of predatory species. The co‐occurrence of aphidophagous ladybird species in crops creates opportunities for CANN and IGP, especially when aphids become scarce. The Lotka–Volterra model predicts the coexistence of two species if intraspecific competition is stronger than interspecific interference interactions. Cycloneda sanguinea L. and Eriopis connexa (Germar) (both Coleoptera: Coccinellidae) coexist in sweet pepper crops in La Plata (Argentina) consuming mainly Myzus persicae (Sulzer) (Hemiptera: Aphididae). The present study used laboratory experiments to estimate levels of CANN and IGP by adults and larvae on eggs, and by adults on larvae, in both the presence and absence of prey (i.e., M. persicae), to explain the effect of prey on coexistence of these two predators. Levels of CANN by C. sanguinea and E. connexa were high in the absence of aphids, and decreased when prey was present. Intraguild predation was bidirectional and asymmetric. Adults and larvae of E. connexa were more voracious IG predators of C. sanguinea than vice versa, the former being the stronger IG predator and interference competitor. Eriopis connexa always won when larvae of the same instar were compared, whereas the larger larva always won when larvae were of different instars, regardless of species. In the presence of prey, CANN by both species decreased, but IGP by E. connexa on C. sanguinea remained high, suggesting that E. connexa could displace C. sanguinea via interspecific interference competition. Other factors potentially affecting the coexistence of C. sanguinea and E. connexa in sweet pepper crops are discussed.