Positive density dependence in seedlings of the neotropical tree species Garcinia macrophylla and Xylopia micans

Question: The Janzen‐Connell hypothesis predicts that herbivores and pathogens prevent seedlings from establishing in dense patches near adult conspecifics. Although many studies have investigated the Janzen‐Connell hypothesis, the environmental context – local or regional – in which juveniles establish is often overlooked. The objectives of this study were: (1) to evaluate Janzen‐Connell effects in contrasting environments, and (2) to incorporate microsite variation into the study of this hypothesis. Location: Pacaya‐Samiria Reserve, Peru. Methods: I assessed seedling performance of two tree species, Garcinia macrophylla and Xylopia micans, during one growing season. In an observational study, mortality and growth rates were regressed against distance to the nearest adult conspecific, conspecific seedling density, heterospecific plant density, and several abiotic variables in upland and floodplain forests. Field and shadehouse experiments were used to isolate distance‐ and density‐dependent effects. Results: Contrary to predictions, seedling survivorship increased in the presence of conspecific seedlings (Garcinia) and heterospecific understory plants (Garcinia and Xylopia) in the observational study. Survivorship in the field experiment, however, was unaffected by conspecific seedling density or adult proximity. In the shadehouse, Garcinia growth rates were highest in floodplain soils collected near adult conspecifics, but mortality was unrelated to the soil's habitat or proximity to an adult. Conclusions: The positive density dependence found in this study could have been produced by: (1) environmental factors that increase both density and survivorship, or (2) interspecific facilitation, if heterospecifics reduce herbivore or pathogen pressure on the focal species. Such interactions could help explain species coexistence in tropical forests.     

Testing for Janzen–Connell Effects in a West African Montane Forest

The Janzen–Connell hypothesis proposes that density dependent seed and seedling mortality, combined with increasing seed and seedling survival away from the parent tree, together promote regular spacing of species and thus α diversity. This hypothesis has rarely been tested in tropical Africa, and rarely in montane forests anywhere. We tested this hypothesis using a combination of field experiments and observations in the most floristically diverse dry submontane forest in Nigeria. We investigated distance effects on seedling herbivory, seedling survival and seedling height growth. We found a significant decrease in herbivory with distance from conspecific adult trees for all three species of experimentally planted seedlings (Entandrophragma angolense, Deinbollia pinnata and Sterculia setigera), and also for naturally occurring seedlings of Pouteria altissima but not of Newtonia buchananii or Isolona pleurocarpa. The relative density of large seedlings/saplings of P. altissima, N. buchananii and I. pleurocarpa increased significantly at greater distance from conspecific adult trees; however, we found no significant distance effect on survival or height growth over 3 mo for all three experimentally planted species. Taken together, our results are some of the first to show that Janzen–Connell effects occur on the African continent and in particular montane tropical forest and suggest that such effects may be pantropical.     

Enemies maintain hyperdiverse tropical forests

Understanding tropical forest tree diversity has been a major challenge to ecologists. In the absence of compensatory mechanisms, two powerful forces, drift and competition, are expected to erode diversity quickly, especially in communities containing scores or hundreds of rare species. Here, I review evidence bearing on four compensatory mechanisms that have been subsumed under the terms "density dependence" or "negative density dependence": (1) intra- and (2) interspecific competition and the action of (3) density-responsive and (4) distance-responsive biotic agents, as postulated by Janzen and Connell. To achieve ontological integration, I examine evidence based on studies employing seeds, seedlings, and saplings. Available evidence points overwhelmingly to the action of both host-generalist and host-restricted biotic agents as causing most seed and seedling mortality, implying that species diversity is maintained via top-down forcing. The overall effect of most host-generalist seed predators and herbivores is to even out the distribution of surviving propagules. Spatially restricted recruitment appears to result mainly, if not exclusively, from the actions of host-restricted agents, principally microarthropods and fungi, that attack hosts in a distance-dependent fashion as Janzen and Connell proposed. Near total failure of propagules close to reproductive conspecifics ensures that successful reproduction occurs through a scant rain of dispersed seeds. Densities of dispersed seeds and seedlings arising from them are so low as to generally preclude the operation of density dependence, at least during early ontogenetic stages. I conclude that Janzen and Connell were essentially correct and that diversity maintenance results from top-down forcing acting in a spatially nonuniform fashion.     

Herbivores limit the population size of big‐leaf mahogany trees in an Amazonian forest

The Janzen–Connell hypothesis proposes that specialized herbivores maintain high numbers of tree species in tropical forests by restricting adult recruitment so that host populations remain at low densities. We tested this prediction for the large timber tree species, Swietenia macrophylla, whose seeds and seedlings are preyed upon by small mammals and a host‐specific moth caterpillar Steniscadia poliophaea, respectively. At a primary forest site, experimental seed additions to gaps – canopy‐disturbed areas that enhance seedling growth into saplings – over three years revealed lower survival and seedling recruitment closer to conspecific trees and in higher basal area neighborhoods, as well as reduced subsequent seedling survival and height growth. When we included these Janzen–Connell effects in a spatially explicit individual‐based population model, the caterpillar's impact was critical to limiting Swietenia's adult tree density, with a > 10‐fold reduction estimated at 300 years. Our research demonstrates the crucial but oft‐ignored linkage between Janzen–Connell effects on offspring and population‐level consequences for a long‐lived, potentially dominant tree species.     

Dispersal and recruitment limitation in native versus exotic tree species: life‐history strategies and Janzen‐Connell effects

Life‐history traits of invasive exotic plants are typically considered to be exceptional vis‐à‐vis native species. In particular, hyper‐fecundity and long range dispersal are regarded as invasive traits, but direct comparisons with native species are needed to identify the life‐history stages behind invasiveness. Until recently, this task was particularly problematic in forests as tree fecundity and dispersal were difficult to characterize in closed stands. We used inverse modelling to parameterize fecundity, seed dispersal and seedling dispersion functions for two exotic and eight native tree species in closed‐canopy forests in Connecticut, USA. Interannual variation in seed production was dramatic for all species, with complete seed crop failures in at least one year for six native species. However, the average per capita seed production of the exotic Ailanthus altissima was extraordinary: > 40 times higher than the next highest species. Seed production of the shade tolerant exotic Acer platanoides was average, but much higher than the native shade tolerant species, and the density of its established seedlings (≥ 3 years) was higher than any other species. Overall, the data supported a model in which adults of native and exotic species must reach a minimum size before seed production occurred. Once reached, the relationship between tree diameter and seed production was fairly flat for seven species, including both exotics. Seed dispersal was highly localized and usually showed a steep decline with increasing distance from parent trees: only Ailanthus altissima and Fraxinus americana had mean dispersal distances > 10 m. Janzen‐Connell patterns were clearly evident for both native and exotic species, as the mode and mean dispersion distance of seedlings were further from potential parent trees than seeds. The comparable intensity of Janzen‐Connell effects between native and exotic species suggests that the enemy escape hypothesis alone cannot explain the invasiveness of these exotics. Our study confirms the general importance of colonization processes in invasions, yet demonstrates how invasiveness can occur via divergent colonization strategies. Dispersal limitation of Acer platanoides and recruitment limitation of Ailanthus altissima will likely constitute some limit on their invasiveness in closed‐canopy forests.     

When do localized natural enemies increase species richness?

The Janzen–Connell hypothesis states that local species‐specific density dependence, mediated through specialist enemies of offspring such as fungal pathogens and insect seed predators, can facilitate coexistence of species by preventing recruitment near conspecific adults. We use spatially explicit simulation models and analytical approximations to evaluate how spatial scales of offspring and enemy dispersal affect species richness. In comparison with model communities in which both offspring and enemies disperse long distances, species richness is substantially decreased when offspring disperse long distances and enemies disperse short distances. In contrast, when both offspring and enemies disperse short distances species richness more than doubles and adults of each species are highly spatially clumped. For the range of conditions typical of tropical forests, locally dispersing specialist enemies may decrease species richness relative to enemies that disperse long distances. In communities where dispersal distances of both offspring and enemies are short, local effects may enhance species richness.     

Context‐dependent biotic interactions control plant abundance across altitudinal environmental gradients

Many biotic interactions influence community structure, yet most distribution models for plants have focused on plant competition or used only abiotic variables to predict plant abundance. Furthermore, biotic interactions are commonly context‐dependent across abiotic gradients. For example, plant–plant interactions can grade from competition to facilitation over temperature gradients. We used a hierarchical Bayesian framework to predict the abundances of 12 plant species across a mountain landscape and test hypotheses on the context‐dependency of biotic interactions over abiotic gradients. We combined field‐based estimates of six biotic interactions (foliar herbivory and pathogen damage, fungal root colonization, fossorial mammal disturbance, plant cover and plant diversity) with abiotic data on climate and soil depth, nutrients and moisture. All biotic interactions were significantly context‐dependent along temperature gradients. Results supported the stress gradient hypothesis: as abiotic stress increased, the strength or direction of the relationship between biotic variables and plant abundance generally switched from negative (suggesting suppressed plant abundance) to positive (suggesting facilitation/mutualism). For half of the species, plant cover was the best predictor of abundance, suggesting that the prior focus on plant–plant interactions is well‐justified. Explicitly incorporating the context‐dependency of biotic interactions generated novel hypotheses about drivers of plant abundance across abiotic gradients and may improve the accuracy of niche models.     

Distance- and density-dependent leaf dynamics of seedlings of a tropical rainforest tree

Parental distance and plant density dependence of seedling leaf turnover and survival was examined to investigate predictions of the Janzen–Connell hypothesis. The focal study species, Shorea macroptera is a canopy tree species in a lowland rain forest in peninsular Malaysia. We found that the peak of the distribution of plants shifted from 3–6 m to 6–9 m during the course of the change from seedling to sapling stage. The leaf demography of the seedlings was influenced by their distance from the adult tree and also by the seedling density. Although significant density- and distance dependence in leaf production was not detected, seedling leaf loss decreased with distance from the parent tree and with seedling density. Similarly, leaf damage was not found to be distance- or density-dependent, but net leaf gain of seedlings increased with distance from the parent tree. Although no significant distance- or density-dependence was evident in terms of leaf damage, significant distance dependence of the net leaf gain was found. Thus, we concluded that positive distance dependence in the leaf turnover of seedlings may gradually contribute to a shift in the distribution pattern of the progeny through reductions in growth and survivorship.     

Neighbour-regulated mortality: the influence of positive and negative density dependence on tree populations in species-rich tropical forests

Density‐dependent mortality has long been posited as a possible mechanism for the regulation of tropical forest tree density. Despite numerous experimental and phenomenological investigations, the extent to which such mechanisms operate in tropical forests remains unresolved because the demographical signature of density dependence has rarely been found in extensive investigations of established trees. This study used an individual‐based demographical approach to investigate the role of conspecific and heterospecific neighbourhood crowding on tree mortality in a Panamanian and a Malayan tropical forest. More than 80% of the species investigated at each site were found to exhibit density‐dependent mortality. Furthermore, most of these species showed patterns of mortality consistent with the Janzen–Connell hypothesis and the rarely explored hypothesis of species herd protection. This study presents some of the first evidence of species herd protection operating in tree communities.     

Comparison of Herbivores and Herbivory in the Canopy and Understory for Two Tropical Tree Species1

The Janzen–Connell model of tropical forest tree diversity predicts that seedlings and young trees growing close to conspecific adults should experience higher levels of damage and mortality from herbivorous insects, with the adult trees acting as either an attractant or source of the herbivores. Previous research in a seasonal forest showed that this pattern of distance‐dependent herbivory occurred in the early wet season during the peak of new leaf production. I hypothesized that distance‐dependent herbivory may occur at this time because the new foliage in the canopy attracts high numbers of herbivores that are limited to feeding on young leaves. As a consequence, seedlings and saplings growing close to these adults are more likely to be discovered and damaged by these herbivores. In the late wet season, when there is little leaf production in the canopy, leaf damage is spread more evenly throughout the forest and distance dependence disappears. I tested three predictions based on this hypothesis: (1) the same species of insect herbivores attack young and adult trees of a given plant species; (2) herbivore densities increase on adult trees during leaf production; and (3) herbivore densities in the understory rise during the course of the wet season. Censuses were conducted on adults and saplings of two tree species, raribea asterolepis and Alseis blackiana. Adults and saplings of both species had largely the same suite of chewing herbivore species. On adults of Q. asterolepis, the density of chewing herbivores increased 6–10 times during leaf production, but there was no increase in herbivore density on adults of A. blackiana. Herbivore densities increased 4.5 times on A. blackiana saplings and 8.9 times on Q. asterolepis saplings during the wet season, but there were no clear trends on the adults of either species. These results suggest that the potential of adult trees as a source of herbivores on saplings depends on the value of new leaves to a tree species' herbivores, which may differ across tree species.     

Elephant‐induced damage drives spatial isolation of the dioecious palm Borassus aethiopum Mart. (Arecaceae) in the Pendjari National Park,Benin

Spatial patterns (SP) of treefall by elephants is known to be clustered across landscapes as a result of food selection and group foraging. Yet, few studies have explicitly elucidated how elephant pressure (EP) alters SP and tree‐to‐tree distance of tree species especially for dioecious plant species, at stand scale. Using the pair‐correlation function and distance to the nearest neighbour on spatial data from five plots of 1–1.5 ha, this article compared SP of damaged and undamaged individuals and tree‐to‐tree distance of the dioecious palm Borassus aethiopum Mart. in stands of low versus high EP in the Pendjari National Park. We tested the hypothesis that high EP would modify SP and results into isolated adults. Nested ANOVAs were used to compare distances. The overall SP of individuals did not vary, but distance among living adults was twofold extended in stands of high EP. The Janzen–Connell escape hypothesis is supported by our data for ungrazed saplings. The study concluded that increasing EP reduces density and induces spatial isolation of adults that may increase pollination failure and threat persistence of B. aethiopum.     

Invertebrate Seed Predators are not all the Same: Seed Predation by Bruchine and Scolytine Beetles Affects Palm Recruitment in Different Ways

Because most invertebrate seed predators are host‐specific, they are usually expected to produce Janzen–Connell patterns. This expectation was fulfilled for Astrocaryum but not for Allagoptera, depending on the effects of bruchine and scolytine predators on the seeds of these palms. Thus, the mere existence of invertebrate predation is not sufficient for generating Janzen–Connell; what matters is seed mortality, which varies between predators and between plant species for the same predator.     

Indirect interactions among tropical tree species through shared rodent seed predators: a novel mechanism of tree species coexistence

The coexistence of numerous tree species in tropical forests is commonly explained by negative dependence of recruitment on the conspecific seed and tree density due to specialist natural enemies that attack seeds and seedlings (‘Janzen–Connell’ effects). Less known is whether guilds of shared seed predators can induce a negative dependence of recruitment on the density of different species of the same plant functional group. We studied 54 plots in tropical forest on Barro Colorado Island, Panama, with contrasting mature tree densities of three coexisting large seeded tree species with shared seed predators. Levels of seed predation were far better explained by incorporating seed densities of all three focal species than by conspecific seed density alone. Both positive and negative density dependencies were observed for different species combinations. Thus, indirect interactions via shared seed predators can either promote or reduce the coexistence of different plant functional groups in tropical forest.     

The full path of Janzen-Connell effects: genetic tracking of seeds to adult plant recruitment

The Janzen-Connell (J-C) model (Janzen 1970; Connell 1971) has been a dominant yet controversial paradigm for forest community dynamics for four decades, especially in the tropics. With increasing distance from the parent plant, the density of dispersed seeds decreases and, because of a reduced impact of distance- and density-responsive seed and seedling enemies, propagule survival increases, resulting in peak recruitment at some distance from the parent and little recruitment near adult conspecifics. This spacing generates gaps near adult trees for the recruitment of heterospecifics, enhancing species coexistence and species richness. Field studies, primarily focused on seeds and young seedlings, have repeatedly demonstrated increasing survival with increasing distance from parents or decreasing density of propagules (e.g. Clark & Clark 1984; Gilbert et al. 1994; Swamy & Terborgh 2010). Yet a meta-analysis of distance-dependent propagule survival failed to support a general pattern of survival increasing with distance from adult conspecifics, suggesting that there is no need for further experimental tests of the J-C hypothesis in terms of diversity enhancement-results are species-specific, not general (Hyatt et al. 2003). However, a lack of consistent experimental results is not surprising. The outcome of tests of the hypothesis can vary as a function of many factors that can affect successive recruitment stages differently (Schupp 1992; Hyatt et al. 2003; Swamy & Terborgh 2010). This highlights a critical gap-a full test of the J-C model requires data demonstrating that effects carry over to recruitment of new reproductive adults, yet few studies have gone beyond early stages. There is strong inferential evidence that adult trees can show the imprint of J-C effects (e.g. Nathan et al. 2000; Howe & Miriti 2004), and focal individual modelling has clearly demonstrated that J-C effects can operate from sapling through adult stages in a significant number of species (Peters 2003). It is likely that such results are not unusual, but there have been few attempts to demonstrate J-C spacing at the adult stage. In this issue of Molecular Ecology, Steinitz et al. (2011) studied the Mediterranean pine Pinus halepensis (Aleppo pine) and combined a unique situation with an innovative approach to provide the most elegant demonstration yet that adult recruits are spaced further from parents than expected from the initial seed distribution, clear evidence of a J-C effect carrying over to reproductive adults. A major advancement of this study is that it incorporates estimates of the initial patterns of seed dispersal and parentage analysis of adult-offspring relationships, illustrating the value of combined field and genetic approaches.     

Spatially variable propagule pressure and herbivory influence invasion of chaparral shrubland by an exotic grass

Although numerous studies have identified mechanisms that either resist or facilitate biological invasions, few studies have explicitly tested how resisting and facilitating mechanisms interact to drive invasion success. In California, USA, undisturbed Mediterranean-type shrublands have resisted invasion by the perennial tussock grass Cortaderia jubata. In some cases, however, this resistance has been spectacularly breached even in the absence of large-scale disturbance. I tested the hypothesis that these invasions are facilitated by local reductions in the strength of biotic resistance. I evaluated invasive success using C. jubata seed and seedling additions at different microhabitats: the edge of a chaparral stand, under shrub canopy at different distances from the stand edge, and in canopy gaps within the stand. When left exposed to mammalian herbivores, seedling survivorship decreased sharply from nearly 40% on the stand edge to zero just 10 m into the stand. When transplants were protected from herbivory, however, distance from the edge had no significant influence on transplant survivorship. Seedling emergence was also greater on the edge and in canopy gaps than under the canopy, but these differences were not caused by differences in herbivory. The flux of invasive propagules reaching the soil surface was immense and greater along the edge and within gaps than under the stand canopy. Mirroring these patterns, naturally occurring seedling abundance declined dramatically with distance from the stand edge, and seedlings were far more common within stand gaps than would be expected given gap frequency within the stand. Despite strong biotic resistance to invasion within the stand, the cover of C. jubata has increased 20% over the last 9 years. These results suggest that the relative amount of susceptible edge habitat and the supply of invasive propagules can facilitate invasion even in the face of strong local biotic resistance.     

Offspring performance and recruitment of the pioneer tree Acacia caven (Fabaceae) in a fragmented subtropical dry forest

The process of habitat fragmentation results in the breaking apart of originally continuous habitats, causing multiple changes in biotic and abiotic interactions. Alterations in resource availability and in mutualistic and antagonistic plant–animal interactions may impact plant offspring quantity and quality. Currently, several old fragmented systems evidence a process of flora homogenization, where shade‐tolerant species are replaced by pioneer light‐demanding species. Notably, the relationship between quantity and quality parameters of plant offspring production and the successful recruitment of pioneer species in fragmented forests has been poorly explored. Here, we assess population size, sapling recruitment and offspring performance of one of the most widespread tree species of subtropical South America, the native pioneer Acacia caven (Fabaceae). Population size of adults and saplings increased from small to continuous forests, whereas the sapling recruitment per adult tree (sapling/adult ratio) showed no significant differences among forests of different size. Seedling performance was negatively related to forest area and population size, implying potential superior competitive ability of seedlings produced in smaller populations compared to larger ones. Our results show that A. caven is resilient to habitat fragmentation effects, which may be ascribed to a set of advantageous ecological traits such as outcrossing, massive flowering, generalist pollination, drought resistance, rapid growth and re‐sprouting. Thus, this pioneer tree benefits from the availability of vacant sites and resources released by declining plant populations of other species, eventually becoming the dominant species in fragmented habitats. Pioneer native plant species with ecological traits such as A. caven may represent the silent successful survivors and new colonizers of fragmented habitats, the ubiquitous landscapes of the future.     

A test of the escape and colonization hypotheses for zoochorous tree species in a Western Amazonian forest

In order to assess the importance of seed dispersal (escape and colonization hypotheses), I used transplant experiments for seeds and seedlings of 5–11 plant species with fleshy fruits in a lowland tropical forest (Tinigua National Park, Colombia). I controlled seed density, distance to parental tree, and habitat type. I monitored seed removal, seedling survival, and seedling growth during the first year of development for an average of 554 seeds and 169 seedlings for each species. I supplemented the experimental results with measurements of natural recruitment. I found little support for the escape hypothesis during the seed and seedling stages. For six species that showed differences in seed removal associated with distance, five showed highest removal away from, than close to parent trees, suggesting predator satiation. Seedling survival during the first year was not consistently associated with low densities and long distances from parent trees. For the majority of species, seedlings did not survive flooding in low basins, and there was growth advantage for most plant species in canopy gaps. These differences imply advantages for seed dispersal to adequate habitats, as predicted by the colonization hypothesis. In contrast to experiments, strong negative distance-dependent effects were evident when analyzing natural recruitment patterns. The ratio between saplings and seedlings was higher away from parent trees for the species with enough recruitment to be analyzed and this suggests that a negative distance-dependent effect may also occur after seedling establishment. This pattern is suspected for several other species, but an analysis with some of the other most common trees showed a variety of negative, neutral, and positive distance dependent effects. This study emphasizes the importance of long-term studies to asses the role of seed dispersal.     

Strong distance-dependent effects for a spatially aggregated tropical species

A fundamental aspect of the Janzen–Connell Hypothesis (JCH) is that distance- and density-dependent mortality reduce the local dominance of species and cause regular rather than random or aggregated spatial patterns. Despite this explicit linkage between process and pattern, very few studies have explored how JCH processes translate into the spatial distributions of adult populations. In field experiments, we assessed germination, mortality and growth of conspecific and heterospecific seedlings beneath and away from Esenbeckia leiocarpa, a highly aggregated tropical tree species. We also investigated the effects of vertebrates using exclosures in the field, and the effects of pathogens in a soil sterilization shadehouse experiment. Germination of conspecifics underneath Esenbeckia was reduced by 64 % and mortality increased 28–123 % when compared to seedlings growing under other tree species; 99–100 % of Esenbeckia seedlings died under conspecifics. Heterospecifics were much less affected by Esenbeckia canopies. However, we found no evidence that either vertebrate herbivores or soil pathogens affected seed germination and seedling performance. Although many tropical tree species are aggregated, our results are the first to demonstrate strong negative distance-dependence for an aggregated species and one of the few to explore germination in the context of the JCH and show differences between conspecifics and heterospecifics, thus suggesting a broader role for Janzen–Connell processes as determinants of the distribution and abundance of tree species in tropical forests.     

Tree seedling richness,but not neighborhood composition,influences insect herbivory in a temperate deciduous forest community

Insect herbivores can serve as important regulators of plant dynamics, but their impacts in temperate forest understories have received minimal attention at local scales. Here, we test several related hypotheses about the influence of plant neighborhood composition on insect leaf damage in southwestern Pennsylvania, USA. Using data on seedlings and adult trees sampled at 36 sites over an approximately 900 ha area, we tested for the effects of total plant density, rarefied species richness (i.e., resource concentration and dietary‐mixing hypotheses), conspecific density (i.e., Janzen–Connell hypothesis), and heterospecific density (i.e., herd‐immunity hypothesis), on the proportion of leaf tissue removed from 290 seedlings of 20 species. We also tested for the effects of generic‐ and familial‐level neighborhoods. Our results showed that the proportion of leaf tissue removed ranged from zero to just under 50% across individuals, but was generally quite low (<2%). Using linear mixed models, we found a significant negative relationship between insect damage and rarefied species richness, but no relationship with neighborhood density or composition. In addition, leaf damage had no significant effect on subsequent seedling growth or survival, likely due to the low levels of damage experienced by most individuals. Our results provide some support for the resource concentration hypothesis, but suggest a limited role for insect herbivores in driving local‐scale seedling dynamics in temperate forest understories.     

Stronger effect of gastropods than rodents on seedling establishment,irrespective of exotic or native plant species origin

Experimental evidence about how generalist consumers affect exotic plant invasions is equivocal, but most tests have been limited to few plant species, single herbivore guilds, and single locations. Using a seed‐addition experiment, we studied effects of gastropods and rodents on recruitment success of 37 exotic and 37 native plant species affiliated to three different functional groups (i.e. grasses, legumes and non‐legume herbs). We replicated our seed addition x herbivore exclusion experiment at multiple grassland sites, located within a few km of each other in two regions, coastal central California (USA) and southern Saxony–Anhalt (Germany). The two study regions differed in climate, land‐use, invasion history and species pools which allowed us to disentangle general from context‐specific effects. In both regions, herbivory by gastropods had a stronger impact on the proportion of recruited seedlings and the proportion of recruited species than rodent herbivory, but this effect was much more pronounced in California than in Germany. Especially, seedling recruitment of non‐legume herbs and legumes suffered from gastropod herbivory. Contrastingly, the effect of rodents was negative at the German sites and positive at the Californian sites, likely driven by context‐specific differences in the rodent assemblages. Across both study regions, exotics had higher seedling recruitment than natives, indicating that higher recruitment success constitutes an inherent feature of exotic species. After two years, more exotic than native species established at grassland sites in California while the opposite was true for the German grassland sites. Consistently across regions, native and exotic species did, however, not differ in their response to herbivory, suggesting that generalist consumers suppress recruitment and colonization of plant species irrespective of their origin. Our results demonstrate the importance of a multi‐species, multi‐site approach to separate general responses of exotic and native plants to generalist herbivory from local, regional or species‐specific peculiarities.