Succession of Exotic and Native Species Assemblages within Restored Floodplain Forests: A Test of the Parallel Dynamics Hypothesis

Exotic plants pose a threat to restoration success in post‐agricultural bottomlands, but little information exists on their dynamics during succession of actively restored sites. We hypothesized that exotic assemblages would establish during succession and that their compositional trends during succession time would mirror those published for native species in other systems, with an early peak in herbaceous richness followed by a decline as woody species establish. In the summer of 2008, we sampled 16 sites across an 18‐year chronosequence of restored forests, with an additional four mature forest stands for comparison, within the Cypress Creek NWR, Illinois, U.S.A. We identified all vascular plant species and quantified canopy openness at three canopy strata, and soil texture and chemistry. Trends in exotic assemblages were significantly correlated with canopy openness at all strata. Richness of exotic and native herbaceous species was related to stand age and consistent with a Weibull regression model. Native and exotic herbaceous cover followed an exponential decay model. Woody native richness over time conformed to a logistic model; woody exotics exhibited no relationship with stand age and were present in sites of all ages. Our results indicate that although their rates of decline differ, herbaceous exotics and natives exhibit similar successional dynamics; therefore, herbaceous exotics may not pose a lasting threat to restoration success in reforested floodplains. Woody exotics can establish across a range of successional stages and persist under closed canopy conditions. Bottomland restorations are vulnerable to the invasion and expansion of exotic plant species even after canopy closure.     

Effects of Flow Restoration and Exotic Species Removal on Recovery of Native Fish: Lessons from a Dam Decommissioning

Flow diversion and invasive species are two major threats to freshwater ecosystems, threats that restoration efforts attempt to redress. Yet, few restoration projects monitor whether removal of these threats improve target characteristics of the ecosystem. Fewer still have an appropriate experimental design from which causal inferences can be drawn as to the relative merits of removing exotic fish, restoring flow, or both. We used a dam decommissioning in Fossil Creek, Arizona, to compare responses of native fish to exotic fish removal and flow restoration, using a before‐after‐control‐impact design with three impact treatments: flow restoration alone where exotics had not been present, flow restoration and exotic fish removal, and flow restoration where exotics remain and a control reach that was unaffected by restoration actions. We show that removal of exotic fish dramatically increased native fish abundance. Flow restoration also increased native fish abundance, but the effect was smaller than that from removing exotics. Flow restoration had no effect where exotic fish remained, although it may have had other benefits to the ecosystem. The cost to restore flow ($12 million) was considerably higher than that to eradicate exotics ($1.1 million). The long‐term influence of flow restoration could increase, as travertine dams grow and re‐shape the creek increasing habitat for native fish. But in the 2‐year period considered here, the return on investment for extirpating exotics far exceeded that from flow restoration. Projects aimed to restore native fish by restoring flow should also consider the additional investment required to eradicate exotic fish.     

Exotic plant invasion alters chaparral ecosystem resistance and resilience pre- and post-wildfire

Perturbations such as wildfire and exotic plant invasion have significant impacts on soils, and the extent to which invaded soils are resistant or resilient to these disturbances varies by ecosystem type. Replacement of shrublands by herbaceous exotics pre- and post-wildfire may drastically alter soil chemical and biological properties for an unknown duration. We assessed above and belowground resistance and resilience to exotic plant invasion both before and after a chaparral wildfire. We hypothesized that exotic plant species would change chemical characteristics of chaparral soils by altering litter and microbial inputs, and that controlling exotics and seeding native species would restore chemical characteristics to pre-invaded conditions. We additionally hypothesized that exotic plant species would slow succession above- and belowground, as well as recovery of post-wildfire chaparral structure and function. Plant species composition and soil nutrient pools and cycling rates were evaluated in mature and invaded chaparral pre- and post-wildfire. Exotic plant species were weeded and native species were seeded to assess impacts of exotic competition on native species recovery. Invasion did not impact all soil characteristics before fire, but increased soil C/N ratio, pH, and N cycling rates, and reduced NO₃-N availability. After fire, invasives slowed succession above- and belowground. Removal of exotics and seeding natives facilitated succession and resulted in plant composition similar to uninvaded, post-wildfire chaparral. The chaparral ecosystem was not resistant to impacts of invasion as indicated by altered soil chemistry and C and N cycling rates; however, short-term restoration led to recovery of extractable nitrogen availability indicating resilience of chaparral soils. This suggests that the permanence of exotic plant species, once established, represents a greater ecological challenge than exotic plant impacts on soils.     

Biodiversity, phenology and temporal niche differences between native- and novel exotic-dominated grasslands

Many exotic species have been introduced or have escaped into grasslands where they form ‘novel ecosystems’ of species with no evolutionary history of interaction. Novel ecosystems are good model systems for understanding how diversity maintenance mechanisms might differ between species with a history of interaction (natives) and species without a history (exotics) in cases where exotics originated from several continents. We tested for lower species diversity and richness in exotic grasslands and found a negative correlation between species diversity measures and proportion of exotic species across 15 grasslands in an observational study in Texas. We then planted 9-species mixtures of all native or all exotics under ambient or elevated summer precipitation to compare dynamics of diversity and to test if exotic species respond more strongly to altered resource availability. Species diversity was lower in communities of exotic than native species by the second year. Reduced diversity in exotic communities resulted from lower complementarity and higher temporal niche overlap among species and occurred in both ambient and irrigated plots. In general, summer irrigation had additive positive effects and did not interact with native–exotic status. Exotic species and communities had much earlier green-up during spring than natives, and altered inter-correlations among phenology variables. There were no differences in flowering dates. Taken together, our results suggest that rapid and synchronous growth may increase niche overlap among exotic species and reduce local diversity in exotic-dominated grassland communities. Earlier green-up by exotics may complicate attempts to ascertain relationships between phenology and climate. An increase in exotic species may cause earlier green-up regardless of any climate change effects and our results suggest that phenology networks should take a species-based rather than an ecosystem approach to evaluate green-up if the abundance of exotics increases within the time-frame in question. These differences between native and exotic species and communities should be considered in future management and restoration projects.     

Environmental Variability and Ontogenetic Niche Shifts in Exotic Plants May Govern Reinvasion Pressure in Restorations of Invaded Ecosystems

When restoring ecosystems dominated by exotic plants, reinvasion pressure, or the rate of new exotic recruitment following mature exotic removal, can vary broadly between similarly invaded habitats. Reinvasion pressure strongly influences restoration costs and outcomes but is difficult to predict. Ontogenetic niche shifts (ONSs, changes in niche breadth or position during development) in exotic species paired with interannual variation in abiotic conditions may decouple pre‐removal mature exotic density and average reinvasion pressure. Identifying such decouplings could improve restoration efficiency by informing site selection and management strategies, but requires estimates of average reinvasion pressure that mandate greater understanding of its principle drivers. We hypothesize that reinvasion pressure is predominantly driven by exotic propagule abundance and spatiotemporal availability of realized recruitment windows, which are periods of variable duration that permit exotic establishment from propagules. Realized recruitment windows are based on the “safe sites” concept but account for ONSs and are determined by abiotic conditions and interspecific interactions with recipient communities. Biotic resistance or facilitation may increase or decrease times required for establishment by influencing exotic growth rates or altering niche availability and may permit or preclude establishment in marginal abiotic conditions. We discuss factors influencing reinvasion pressure, basic approaches to estimate reinvasion pressure, and potential ways to increase management efficiency under different reinvasion pressure scenarios. Accurate estimates of reinvasion pressure could improve restoration efficacy, efficiency, and predictability in ecosystems dominated by exotic plants. We argue that greater theoretical and practical considerations of reinvasion pressure and ONSs are merited.     

Characterizing the microhabitats of exotic species in Illinois shale barrens

Like native species, exotics form part of the mosaic structure of plant communities. However, their role in these communities is unclear. Thus, we ask, are invading exotic species present in recognizable microhabitats within plant communities? We compared the microhabitat of exotic species and random locations to determine if exotic plants occupied a recognizable subset of habitats within three barren communities. Fourteen exotic species were identified. The factors that characterized their habitats varied between barrens and among species. Considered as a group, exotic microhabitats comprised a subset of available habitats within the barrens, specifically, edge habitats rather than the open conditions of the barrens interior. Soil properties such as low temperature or neutral pH, and high litter or woody species cover, were common factors associated with the occurrence of exotics. Individual exotic species occupied different habitats within those identified for exotics as a group, but the range of variation between the different species was not generally significant. Within each barren, exotics occupied a distinct subset of microhabitats. This distribution suggests that exotics are not widely dispersed within these areas. Site conditions may be restricting the establishment of exotics to specific microhabitats, and in addition, some exotics may be altering the microhabitats that they colonize. Therefore, management efforts toward maintaining open conditions in the barrens may also discourage further encroachment by exotics into these areas.     

Science, Art, or Application—the "Karma" of Restoration Ecology

The present state of restoration ecology is far away from Bradshaw’s “acid test for ecology.” The conclusions drawn from the series of papers in this issue and from the Jena workshop suggest some directions in which the field may progress. More attention must be paid to the degraded state, which should be evaluated by its specific features and carefully analyzed before any restoration plan is laid down. Restoration goals have to be realistic, which includes the appreciation of globally changing conditions, resulting in a paradigm‐shift toward “forward‐restoration.” Basically, the transition from the degraded state conditions to the target state is a kind of succession that is manipulated by the application of goal‐orientated and system‐specific disturbances. Whenever possible, restorationists should step back and make use of naturally occurring succession, which requires a sophisticated restoration strategy, involving flexible management responses, multiple alternative target states, robust measurements for the restoration progress, and careful long‐term monitoring. The unique feature of restoration ecology is the involvement of socioeconomic decisions, and conceptual frameworks for ecological restoration have to implement the specific links to natural succession. To bridge the gap between ecological theory and on the ground restoration, it is essential that restoration practice is translated into the vocabulary and thinking of basic ecology. If all these aspects are integrated, ecological restoration as an application—and restoration ecology as an applied science—may develop into an acid test for our understanding of interactions between people and their environment, rather than for pure ecology.     

Setting Effective and Realistic Restoration Goals: Key Directions for Research

Restoration ecology has made significant advances in the past few decades and stands to make significant contributions both to the practical repair of damaged ecosystems and the development of broader ecological ideas. I highlighted four main areas where progress in research can assist with this. First, we need to enhance the translation of recent advances in our understanding of ecosystem and landscape dynamics into the conceptual and practical frameworks for restoration. Second, we need to promote the development of an ability to correctly diagnose ecosystem damage, identify restoration thresholds, and develop corrective methodologies that aim to overcome such thresholds. This involves understanding which system characteristics are important in determining ecosystem recovery in a range of ecosystem types, and to what extent restoration measures need to overcome threshold and hysteresis effects. A third key requirement is to determine what realistic goals for restoration are based on the ecological realities of today and how these will change in the future, given ongoing changes in climate and land use. Finally, there is a need for a synthetic approach which draws together the ecological and social aspects of the issues surrounding restoration and the setting of restoration goals.     

Some issues and options for the conservation of native biodiversity in rural New Zealand

Summary For the 70% of New Zealand under private ownership, native biodiversity conservation has to occur within a landscape that must also provide a productive return to land owners. Recent New Zealand legislation, especially the Resource Management Act 1991, promotes sustainable management on private land by allowing for the economic and cultural well-being of local communities while providing for the protection of natural resources including native biodiversity. We suggest that, to effectively conserve native biodiversity in rural landscapes, we need to consider four key issues: (i) what might be realistic goals for native biodiversity conservation; (ii) how might we better arrange different land uses to meet both native biodiversity and production goals; (iii) what is the optimum arrangement of native biodiversity; and (iv) how native biodiversity conservation can improve productive returns to land managers. Options to enhance native biodiversity conservation include a variety of incentives (e.g. management agreements, financial incentives and regulatory systems) and onsite management options (e.g. remnant management, restoration plantings, weed and pest control, use of native species for commercial and amenity purposes, use of exotic species to facilitate native biodiversity). The importance of taking a landscape-based rather than a paddock-based approach to management is emphasized.     

The roles of exotic grasses and forbs when restoring native species to highly invaded southern California annual grassland

Many semi-arid shrublands in the western US have experienced invasion by a suite of exotic grasses and forbs that have altered community structure and function. The effect of the exotic grasses in this area has been studied, but little is known about how exotic forbs influence the plant community. A 3-year experiment in southern California coastal sage scrub (CSS) now dominated by exotic grasses was done to investigate the influence of both exotic grasses (mainly Bromus spp.) and exotic forbs (mainly Erodium spp.) on a restoration seeding (9 species, including grasses, forbs, and shrubs). Experimental plots were weeded to remove one, both, or neither group of exotic species and seeded at a high rate with a mix of native species. Abundance of all species varied with precipitation levels, but seeded species established best when both groups of exotic species were removed. The removal of exotic grasses resulted in an increase in exotic and native forb cover, while removal of exotic forbs led to an increase in exotic grass cover and, at least in one year, a decrease in native forb cover. In former CSS now converted to exotic annual grassland, a competitive hierarchy between exotic grasses and forbs may prevent native forbs from more fully occupying the habitat when either group of exotics is removed. This apparent competitive hierarchy may interact with yearly variation in precipitation levels to limit restoration seedings of CSS/exotic grassland communities. Therefore, management of CSS and exotic grassland in southern California and similar areas must consider control of both exotic grasses and forbs when restoration is attempted.     

Restoring Tropical Dry Forest Communities: Effects of Habitat Management and Outplantings on Composition and Structure

We sought to increase the conservation value and ecological resilience of a disturbed woodlot on protected land in suburban Miami‐Dade County, Florida, by restoring a local tropical dry forest community. These efforts included adding 26 “novel native” tropical hardwood hammock species in different SR and density treatments, and conducting regular habitat management actions including exotic biomass removal. We monitored a variety of community composition and forest structure variables over 2 years to assess the success of our restoration efforts and the relative roles of habitat management versus native outplantings in achieving those outcomes. Habitat management proved influential to changing forest structure, while both habitat management and outplantings impacted changes in community composition, at least in the short term. Habitat management and outplantings in combination, however, allowed us to successfully (1) increase the number of native species and decrease the number of exotic species, (2) increase the number of protected plant species on the site, and (3) alter the community composition and forest structure of the site from that of a highly disturbed woodlot to that of a typical Miami Rock Ridge tropical hardwood hammock. Our success in meeting these restoration goals in just 2 years is one such example where simple native outplanting and exotic control projects can produce large returns with minimal resources in the form of time, money, and manpower. Finally, restoring regrowth sites or other remnant habitats may prove an efficient and effective way to conserve biodiversity and basic ecosystem processes in close proximity to metropolitan areas .     

Options for enhancing forest biodiversity across New Zealand's managed landscapes based on ecosystem modelling and spatial design

In most regions of the world removal of environmental stress facilitates regeneration of native plants and habitats. However, in many of New Zealands modified landscapes, exotic species are likely to respond first to any reduction in stress because these fast-growing species are prevalent in local vegetation and dominate seed banks. Given the trend in agriculture towards intensive management on larger units, the indigenous character in New Zealand landscapes is being marginalised and there is the risk that further reduction in visibility of native vegetation may be perpetuated by a growing familiarity and identification with ubiquitous exotic species. Alternative landscapes, based on an understanding of ecosystem processes, need to be explored if biodiversity goals set by international convention and national resource management law are to be achieved. This study provides a set of predictions and pathways, backed by field observations, to underpin a restoration strategy at patch to landscape scales. A forest model, LINKNZ, is employed to simulate species succession under New Zealand conditions. The incorporation of disturbance regimes and species dispersal processes in the model permits a wide range of scenarios to be investigated encompassing indigenous forest development, exotic species interactions with indigenous forest ecosystems, management of mixed introduced-indigenous forests, and landscape dynamics. The results illustrate an approach that identifies potential biosecurity threats and provides additional options for integrating nature and production in New Zealands rural and urban landscapes.     

Properties of native plant communities do not determine exotic success during early forest succession

Considerable research has been devoted to understanding how plant invasions are influenced by properties of the native community and to the traits of exotic species that contribute to successful invasion. Studies of invasibility are common in successionally stable grasslands, but rare in recently disturbed or seral forests. We used 16 yr of species richness and abundance data from 1 m² plots in a clearcut and burned forest in the Cascade Range of western Oregon to address the following questions: 1) is invasion success correlated with properties of the native community? Are correlations stronger among pools of functionally similar taxa (i.e. exotic and native annuals)? Do these relationships change over successional time? 2) Does exotic abundance increase with removal of potentially dominant native species? 3) Do the population dynamics of exotic and native species differ, suggesting that exotics are more successful colonists? Exotics were primarily annual and biennial species. Regardless of the measure of success (richness, cover, biomass, or density) or successional stage, most correlations between exotics and natives were non‐significant. Exotic and native annuals showed positive correlations during mid‐succession, but these were attributed to shared associations with bare ground rather than to direct biotic interactions. At peak abundance, neither cover nor density of exotics differed between controls and plots from which native, mid‐successional dominants were removed. Tests comparing nine measures of population performance (representing the pace, magnitude, and duration of population growth) revealed no significant differences between native and exotic species. In this early successional system, local richness and abundance of exotics are not explained by properties of the native community, by the presence of dominant native species, or by superior colonizing ability among exotics species. Instead natives and exotics exhibit individualistic patterns of increase and decline suggesting similar sets of life‐history traits leading to similar successional roles.     

An ecological analysis of the riparian vegetation for improving the riverine ecosystem management: the case of Lombardy region (North Italy)

This research aims at finding the potential plant successions in riparian vegetation along gravel-bed streams of Lombardy (Northern Italy), investigating the influence of exotic species and proposing a more sustainable and efficient management planning for habitats conservation. To characterise the riparian vegetation, 65 phytosociological relevés were carried out along four rivers in Lombardy (Pioverna, Staffora, Ogliolo and Grigna). We evaluated a series of ecological indexes to assess the ecology of plant communities. A clustering analysis detected five different plant communities characterised by specific ecological traits. On the basis of such results, we discuss about a model of plant succession describing: (i) the typical native species succession representing the riverine forests of the study areas; (ii) a succession with moderate presence of exotic species; and (iii) a succession dominated by alien species with an unclear development. To preserve the natural heritage and landscape with a small rate of alien species, it is important to plan vegetation management actions, which limit the presence of new empty space. For containing the exotic species, mechanical and chemical measures could lead to good results, but they can be expensive and cause adverse effects. Otherwise, biological control, combined with other actions, has the potential to bring results with low environmental and economic impact. Ecological restoration measures using fast-growing and spreading native species could restrict alien plant colonisation. In this regard, a better comprehension of the invasive alien plants behaviour is necessary, especially in terms of their competition mechanisms. Nevertheless, the application of long-term vegetation management of river ecosystems is crucial for a continuous monitoring and for addressing the goals of the 2030 global agenda concerning biodiversity conservation.

     

A plant traits approach to managing legacy species during restoration transitions in temperate eucalypt woodlands

Degraded communities often contain a subset of the species that comprised the predisturbance community. These represent an important legacy of the predisturbance state, yet restoration treatments may be detrimental to them. This study examined the potential of leaf traits and life form to predict whether restoration treatments can maintain legacy swards of Austrostipa bigeniculata (hereafter Austrostipa) while controlling exotic annuals in temperate eucalypt woodlands. Treatments included carbon addition to reduce soil nitrate, both with and without burning or pulse grazing to deplete exotic seed pools. We compared leaf traits of Austrostipa with a native grass (Themeda triandra) known to be advantaged, and 8 exotic annual species known to be disadvantaged by these treatments. Leaf traits indicated potentially greater negative impacts of carbon addition on exotic annuals compared to Austrostipa, and on Austrostipa compared to Themeda, suggesting a net restoration benefit. Similarly, burning or pulse grazing is expected to have little negative impact on perennial resprouting grasses (hemicryptophytes; Austrostipa and Themeda) compared with annual exotics (therophytes) with short‐lived seed banks. Treatment responses were largely consistent with predictions: treatments that significantly reduced exotic annuals had no net disadvantage to Austrostipa swards despite significant reductions in Austrostipa seedling growth with carbon addition. Indeed by Year 3, Austrostipa mortality in untreated plots led to 46% lower Austrostipa abundance than in treated plots at one site, potentially due to litter build‐up or other mechanisms. We conclude that plant traits provide a useful framework for designing restoration transitions that retain native legacy species while controlling exotics.     

The Effect of a Disturbance Corridor on an Ecological Reserve

The effect of a pipeline corridor constructed through an ecological reserve in Southern California was investigated by assessing plant species composition and soil chemistry. A homogeneous plant community comprised primarily of exotic annuals was found along the entire length of the corridor. This community has low similarity to the adjacent native plant communities. Soil organic matter was significantly less on the disturbed corridor than in contiguous undisturbed areas. Both available nitrogen and extractable phosphorus values were greater in the disturbed corridor. By contrast, total nitrogen was significantly higher outside the pipeline. The more labile litter of the exotic annuals allows increased mineralization along the corridor than does the more recalcitrant litter of the native perennial shrubs in the undisturbed areas. Once established, the weedy exotic annual litter may completely turn over organic matter and nitrogen, favoring the persistence of the weedy annuals. These exotic annuals appear to be moving into three of the native communities - grassland, coastal sage, and oak woodland - that have less organic matter and a more open plant canopy. Poor restoration efforts can lead to the establishment of such exotics, subsequent invasion into the surrounding undisturbed habitat, and degradation of the reserve.     

Vegetation structure,species life span,and exotic status elucidate plant succession in a limestone quarry reclamation

An understanding of the processes involved in plant succession is pivotal in achieving an effective site restoration. In a former limestone quarry (northeastern Italy), we explored the effects of a technical reclamation on the plant community using changes in cover of vegetation layers and two sensitive plant traits (i.e. exotic status and life span), with a chronosequence approach. Four reclaimed areas of different ages (from 8 to 35 years old) and natural vegetation in the surroundings were investigated with seven permanent plots each, for a total of 35. Changes in vegetation layers and species richness of both exotic status and life span were analyzed by generalized linear (mixed) models. Relations with plant community assembly were also considered, using a multivariate approach. Both vegetation layers and plant traits were affected by the age of reclaimed areas, evidencing the main changes in plant succession. Annual and exotic species decreased toward the mature stages of reclamation and target vegetation, whereas overall plant diversity (species richness) was stable. Our findings show that both vegetation layer changes and plant traits can be used to assess the degree to which reclamation efforts produce results that approach the restoration of a natural vegetation reference. Implementation of management practices aimed at favoring native perennial species (e.g. appropriate seed mixtures, mowing, tree, and shrub planting) could limit weed‐control efforts, representing a reasonable trade‐off between biodiversity promotion and invasive plant control.     

Long‐term outcomes of forest restoration in an urban park

Creating, restoring, and sustaining forests in urban areas are complicated by habitat fragmentation, invasive species, and degraded soils. Although there is some research on the outcomes of urban reforestation plantings during the first 5 years, there is little research on longer term outcomes. Here, we compare the successional trajectories of restored and unrestored forest sites 20 years after initiating restoration. The sites are located within the Rodman's Neck area of Pelham Bay Park, in the northeast corner of the Bronx in New York City (NYC), U.S.A. Compared with unrestored sites, we saw improvements in species diversity, greater forest structure complexity, and evidence of the regeneration and retention of native tree species in restored sites. In addition, we found differences in restoration outcomes depending on the level of intervention: clearing exotic shrubs and vines and planting native trees and shrubs improved tree diversity and canopy closure to a greater extent than clearing exotics alone, and the mechanical removal of invasive plants after the native plantings further improved some measures of restoration, such as tree species diversity and native tree regeneration. The results of this study suggest that the goal of a sustainable forest ecosystem dominated by native trees and other plant species may not be achievable without continued human intervention on site. In addition, these results indicate that the restoration approach adopted by NYC's reforestation practitioners is moving the site toward a more desirable vegetative community dominated by native species.     

The alien flora of Europe: a taxonomic and biogeographic review

Abstract. A geographic and taxonomic overview of the non-indigenous plant species of Europe, based on the ‘Flora Europaea’ is given. The flora of Europe includes 1568 species which have either expanded their ranges within Europe under human influence (naturalized European species) or are of non-European origin (exotic species). The latter group consists of 580 species (37%) which form a diverse group in terms of their taxonomic composition and geographic origin. The exotics are represented by 113 families, of which the Compositae, Rosaceae and Gramineae are most important. The ratio of species to families is low. Most exotic species in Europe originate from the Americas and Asia. Countries of southern Europe have a higher relative number of exotics in their non-native flora than northern ones. The species-range size distribution differs between naturalized European and exotic species; the latter are on average more widespread than the naturalized.     

Additive effects of exotic plant abundance and land-use intensity on plant–pollinator interactions

The continuing spread of exotic plants and increasing human land-use are two major drivers of global change threatening ecosystems, species and their interactions. Separate effects of these two drivers on plant–pollinator interactions have been thoroughly studied, but we still lack an understanding of combined and potential interactive effects. In a subtropical South African landscape, we studied 17 plant–pollinator networks along two gradients of relative abundance of exotics and land-use intensity. In general, pollinator visitation rates were lower on exotic plants than on native ones. Surprisingly, while visitation rates on native plants increased with relative abundance of exotics and land-use intensity, pollinator visitation on exotic plants decreased along the same gradients. There was a decrease in the specialization of plants on pollinators and vice versa with both drivers, regardless of plant origin. Decreases in pollinator specialization thereby seemed to be mediated by a species turnover towards habitat generalists. However, contrary to expectations, we detected no interactive effects between the two drivers. Our results suggest that exotic plants and land-use promote generalist plants and pollinators, while negatively affecting specialized plant–pollinator interactions. Weak integration and high specialization of exotic plants may have prevented interactive effects between exotic plants and land-use. Still, the additive effects of exotic plants and land-use on specialized plant–pollinator interactions would have been overlooked in a single-factor study. We therefore highlight the need to consider multiple drivers of global change in ecological research and conservation management.