Presence of an invasive plant species alters pollinator visitation to a native

Impacts of invasive plants on animal behaviour

The spread of invasive species is a threat to ecosystems worldwide. However, we know relatively little about how invasive species affect the behaviour of native animals, even though behaviour plays a vital role in the biotic interactions which are key to understanding the causes and impacts of biological invasions. Here, we explore how invasive plants - one of the most pervasive invasive taxa - impact the behaviour of native animals. To promote a mechanistic understanding of these behavioural impacts, we begin by introducing a mechanistic framework which explicitly considers the drivers and ecological consequences of behavioural change, as well as the moderating role of environmental context. We then synthesise the existing literature within this framework. We find that while some behavioural impacts of invasive plants are relatively well-covered in the literature, others are supported by only a handful of studies and should be explored further in the future. We conclude by identifying priority topics for future research, which will benefit from an interdisciplinary approach uniting invasion ecology with the study of animal behaviour and cognition.

Invasion of a dominant floral resource: effects on the floral community and pollination of native plants

Through competition for pollinators, invasive plants may suppress native flora. Community-level studies provide an integrative assessment of invasion impacts and insights into factors that influence the vulnerability of different native species. We investigated effects of the nonnative herb Lythrum salicaria on pollination of native species in 14 fens of the eastern United States. We compared visitors per flower for 122 native plant species in invaded and uninvaded fens and incorporated a landscape-scale experiment, removing L. salicaria flowers from three of the invaded fens. Total flower densities were more than three times higher in invaded than uninvaded or removal sites when L. salicaria was blooming. Despite an increase in number of visitors with number of flowers per area, visitors per native flower declined with increasing numbers of flowers. Therefore, L. salicaria invasion depressed visitation to native flowers. In removal sites, visitation to native flowers was similar to uninvaded sites, confirming the observational results and also suggesting that invasion had not generated a persistent build-up of visitor populations. To study species-level impacts, we examined effects of invasion on visitors per flower for the 36 plant species flowering in both invaded and uninvaded fens. On average, the effect of invasion represented about a 20% reduction in visits per flower. We measured the influence of plant traits on vulnerability to L. salicaria invasion using meta-analysis. Bilaterally symmetrical flowers experienced stronger impacts on visitation, and similarity in flower color to L. salicaria weakly intensified competition with the invader for visitors. Finally, we assessed the reproductive consequences of competition with the invader in a dominant flowering shrub, Dasiphora fruticosa. Despite the negative effect of invasion on pollinator visitation in this species, pollen limitation of seed production was not stronger in invaded than in uninvaded sites, suggesting little impact of competition for pollinators on its population demography. Negative effects on pollination of native plants by this copiously flowering invader appeared to be mediated by increases in total flower density that were not matched by increases in pollinator density. The strength of impact was modulated across native species by their floral traits and reproductive ecology.

Disturbance-dependent invasion of the woody weed, Calotropis procera, in Australian rangelands

Plant invasions are threats to biodiversity and ecosystem processes that have far reaching ecological and economic impacts. Understanding the mechanisms of invasion essentially helps in developing effective management strategies. Rubber bush (Calotropis procera) is an introduced milkweed that invades Australian beef production rangelands. Its establishment is often associated with disturbances caused by pastoral management practices. We examined whether or not rubber bush (1) outcompetes native grasses, (2) can invade intact rangeland, and (3) if disturbance facilitates rubber bush establishment and spread in grassy rangelands. We measured the competitive response of different densities of Mitchell grass (Astrebla pectinata) individuals and the competitive effects of associate rubber bush seedlings in an additive common garden experiment. Replicated field exclosure experiments, under grass-dominated and tropical savanna woodland conditions examined the effect of increasing levels of disturbance on rubber bush seedling emergence. The dominant native Mitchell grass was a stronger competitor than rubber bush when grown together under greenhouse conditions, whereby root and shoot biomass yields were more restricted in rubber bush compared with Mitchell grass. This finding was corroborated in the field exclosure experiments at both sites, where seedling emergence increased 5-fold in seeded and highly disturbed plots where superficial soils were turned over by treatments simulating heavy grazing and trampling by cattle or machinery. Emergence of rubber bush seedlings in seeded plots that were undisturbed, clipped and grazed was minimal and rubber bush seedlings did not survive the seedling stage in these plots. These results demonstrate that disturbance to the superficial soil stratum affects the ability of rubber bush seeds to successfully establish in a microsite, and high levels of soil disturbance substantially increase establishment. Thus, rubber bush is a poor competitor of Mitchell grass and does not invade intact grassland. Consequently, rubber bush invasion is disturbance-dependent in the vast Australian rangelands. The spread of this weed may be arrested by management practices that minimise disturbances to grass cover.

Pollination of a native plant changes with distance and density of invasive plants in a simulated biological invasion

PREMISE OF THE STUDY

Effects of an exotic plant on pollination may change as the invasive increases in density. Quantity of pollinator visits to a native may increase, decrease, or change nonlinearly, while visit quality is likely to decrease with greater interspecific pollen movement. How visit quantity and quality contribute to the effect on reproductive success at each invasion stage has not been measured.

METHODS

We simulated four stages of invasion by Brassica nigra by manipulating the neighborhood of potted plants of the native Phacelia parryi in a field experiment. Stages were far from the invasion, near the invasion, intermixed with the invasive at low density, and intermixed at high density. We measured pollinator visitation, conspecific and invasive pollen deposition, and seed set for P. parryi at each stage.

KEY RESULTS

Native individuals near invasive plants and within areas of low invasive density showed greatest seed production, as expected from concurrent changes in conspecific and invasive pollen deposition. Those plants experienced facilitation of visits and received more conspecific pollen relative to plants farther from invasives. Native individuals within high invasive density also received frequent visits by many pollinators (although not honeybees), but the larger receipt of invasive pollen predicted interference with pollen tubes that matched patterns in seed set.

CONCLUSIONS

Pollinator visitation was highest when exotic plants were nearby. Detrimental effects of heterospecific pollen deposition were highest at high exotic density. Our study quantified how reproduction benefits from near proximity to a showy invasive, but is still vulnerable when the invasive reaches high density.

Nonrandom Composition of Flower Colors in a Plant Community: Mutually Different Co-Flowering Natives and Disturbance by Aliens

When pollinators use flower color to locate food sources, a distinct color can serve as a reproductive barrier against co-flowering species. This anti-interference function of flower color may result in a community assembly of plant species displaying mutually different flower colors. However, such color dispersion is not ubiquitous, suggesting a variable selection across communities and existence of some opposing factors. We conducted a 30-week study in a plant community and measured the floral reflectances of 244 species. The reflectances were evaluated in insect color spaces (bees, swallowtails, and flies), and the dispersion was compared with random expectations. We found that co-existing colors were overdispersed for each analyzed pollinator type, and this overdispersion was statistically significant for bees. Furthermore, we showed that exclusion of 32 aliens from the analysis significantly increased the color dispersion of native flowers in every color space. This result indicated that aliens disturbed a native plant–pollinator network via similarly colored flowers. Our results demonstrate the masking effects of aliens in the detection of color dispersion of native flowers and that variations in pollinator vision yield different outcomes. Our results also support the hypothesis that co-flowering species are one of the drivers of color diversification and affect the community assembly.

Indirect effects of herbivory on plant-pollinator interactions in invasive Lythrum salicaria

PREMISE OF THE STUDY

Herbivory can affect a plant's fitness in a variety of ways, including modifying the biotic interactions of the plant. In particular, when herbivory influences floral display, we hypothesize that pollinator visitation will be altered accordingly. Here we studied the indirect effects of feeding by two beetles, Neogalerucella calmariensis and N. pusilla, released as a biological control, on plant-pollinator interactions and fitness in the invasive plant, purple loosestrife (Lythrum salicaria).

METHODS

Two herbivory treatments (ambient and simulated) were applied to plants in a naturally occurring population of purple loosestrife. During flowering, traits of plants in the treatment and control groups were recorded. Data on pollinator visitation behavior was then collected after intense larval herbivory had ended.

KEY RESULTS

Plants exposed to herbivory treatments produced more flowers and inflorescences but flowered significantly later than those in the control group. Moreover, we found a significant, positive association of herbivory with the number of flowers probed by bumblebees and with the number of times a foraging pollinator moved among inflorescences on a single plant. No differences in female fitness (fruit or seed production) were detected.

CONCLUSIONS

We conclude that herbivore-mediated differences in floral display traits impacted pollinator visitation behavior. However, as we discuss, differences in pollinator visitation did not translate into detectable differences in female reproductive success. We postulate that herbivory could influence other unmeasured aspects of fitness, such as seed quality or the number of seeds sired.

Evidence for pollen limitation of a native plant in invaded communities

Animal-pollinated invasive species have frequently been demonstrated to outcompete native species for pollinator attention, which can have detrimental effects on the reproductive success and population dynamics of native species. Many animal-pollinated invasive species exhibit showy flowers and provide substantial rewards, allowing them to act as pollinator 'magnets', which, at a large scale, can attract more pollinators to an area, but, at a smaller scale, may reduce compatible pollen flow to local native species, possibly explaining why most studies detect competition. By performing pollen limitation experiments of populations in both invaded and uninvaded sites, we demonstrate that the invasive plant Lythrum salicaria appears to facilitate, rather than hinder, the reproductive success of native confamilial Decodon verticillatus, even at a small scale, in a wetland habitat in southeastern Ontario. We found no evidence for a magnet species effect on pollinator attraction to invaded sites. Germination experiments confirmed that seeds from invaded sites had similar germination rates to those from uninvaded sites, making it unlikely that a difference in inbreeding was masking competitive effects. We describe several explanations for our findings. Notably, there were no differences in seed set among populations at invaded and uninvaded sites. Our results underscore the inherent complexity of studying the ecological impacts of invasive species on natives.