Variation in pollinator-mediated plant reproduction across an urbanization gradient

Urban environment decreases pollinator availability, fertility, and prolongs anthesis in the field bindweed (Convolvulus arvensis Linnaeus, 1753)

Urbanization alters the natural environment, with broad negative impacts on living organisms. Urbanization can also disrupt plant-pollinator networks by reducing the abundance and diversity of invertebrates. Firstly, I investigated whether the field bindweed (Convolvulus arvensis) is an obligatory entomophilous plant because previous reports were ambiguous. Secondly, I investigated how the obligatory entomophilous plant, field bindweed, responds to urbanization by comparing the flowering duration (anthesis) and the reproductive success of field bindweeds in urban and rural populations. Unlike cross-pollinated flowers and controls, flowers experimentally prevented from pollination and self-pollinated flowers did not produce seeds, suggesting that the field bindweed is self-incompatible and obligatory entomophilous. The abundance of urban pollinators was 5-6 times lower than the abundance of rural pollinators, and flies (Diptera), beetles (Coleoptera) and moths (Lepidoptera) were significantly more negatively influenced by the urban environment than hymenopterans (Hymenoptera). Urban plants showed significantly longer anthesis duration and lower reproductive success than rural plants. Illuminance and low pollinator abundance were negatively associated with the duration of the anthesis, but relative humidity did not affect the anthesis. Prolonged duration of the anthesis may be an adaptation to pollinator scarcity because more prolonged flowering increases the likelihood of pollination. Future research should unravel whether the longer anthesis of urban flowers is determined by behavioral plasticity or by the evolutionary selection of plants with a genetically determined longer anthesis.

Is urbanization a driver of aboveground biomass allocation in a widespread tropical shrub, Turnera subulata (Turneroideae - Passifloraceae)?

Plant biomass allocation is mainly affected by the environment where each individual grows. In this sense, through the rapid global expansion of impermeable areas, urbanization has strong, albeit poorly understood, consequences on the biomass allocation of plants found in this environment. Nevertheless, the comprehension of biomass allocation processes in urban shrubs remains unclear, because most studies of urban ecology focus on tree species. This is an important gap of knowledge because a great part of urban vegetation is composed of shrubs and their association with trees have positive impacts in urban ecosystem services. In this study, we explored the ecological and potential selective pressure effects of an urbanization gradient on the biomass allocation patterns of aboveground organs of Turnera subulata, a widely distributed tropical shrub. We have demonstrated that, for certain reproductive organs, biomass allocation decreases in locations with higher urbanization. Unlike expected, the biomass of vegetative organs was not affected by urbanization, and we did not observe any effect of urbanization intensity on the variance in biomass allocation to vegetative and reproductive organs. We did not record urbanization-mediated trade-offs in biomass allocation for reproductive and vegetative organs. Instead, the biomass of these structures showed a positive relationship. Our data suggest that urbanization does not result in radical changes in biomass allocation of T. subulata, and neither in the variation of these traits. They indicate that the ability of T. subulata to thrive in urban environments may be associated with life history and morphological mechanisms. Our findings contribute to the understanding of shrub plant responses to urbanization and highlight urbanization as a potential factor in resource allocation differences for different structures and functions in plants living in these environments.

Variation in flower size and shape of Impatiens capensis is correlated with urbanization in Montreal, Canada

Urbanization is changing the conditions in which many species live, forcing them to adjust to these novel environments. Floral size and shape are critical traits for the reproduction of plants pollinated by animals as they are involved in the attraction of pollinators and in efficient pollination. Variation in size and shape could be affected by urbanization via its modification of the abiotic environment (habitat fragmentation, water availability, temperature, soil properties), or via its impact on the biotic environment of plants (pollination, herbivory). Although numerous studies have assessed the impact of urbanization on pollinator communities and many plant traits, few have investigated its impact on floral size and shape while quantifying the proportion of the total urbanization effect that is due to biotic interactions. In this study, we tested if urbanization and pollinator visitation rates affect the flower shape of the spotted jewelweed, Impatiens capensis. We quantified the size and shape of flowers in frontal and profile views using geometric morphometrics for 228 individuals from six populations from the region of Montreal, Canada. Pollinator visitation rates were estimated at each site and the main pollinators were found to be bumblebees, honeybees and hummingbirds. We found that floral size and shape are significantly correlated with urbanization as measured by the amount of vegetation in the surrounding environment of the plants (mean normalized vegetation index, NDVI) and by the visitation rates of bumblebees and honey bees. Partitioning of the total flower shape variation suggests that urbanization affects flower shape through abiotic factors and via its impact on pollinator visitation rates. While further studies from other cities are necessary to confirm the role of urbanization in shaping the floral shape of I. capensis, these results support the idea that urbanization could affect flower shapes.

Urbanization and a green corridor do not impact genetic divergence in common milkweed (Asclepias syriaca L.)

Urbanization is altering landscapes globally at an unprecedented rate. While ecological differences between urban and rural environments often promote phenotypic divergence among populations, it is unclear to what degree these trait differences arise from genetic divergence as opposed to phenotypic plasticity. Furthermore, little is known about how specific landscape elements, such as green corridors, impact genetic divergence in urban environments. We tested the hypotheses that: (1) urbanization, and (2) proximity to an urban green corridor influence genetic divergence in common milkweed (Asclepias syriaca) populations for phenotypic traits. Using seeds from 52 populations along three urban-to-rural subtransects in the Greater Toronto Area, Canada, one of which followed a green corridor, we grew ~ 1000 plants in a common garden setup and measured > 20 ecologically-important traits associated with plant defense/damage, reproduction, and growth over four years. We found significant heritable variation for nine traits within common milkweed populations and weak phenotypic divergence among populations. However, neither urbanization nor an urban green corridor influenced genetic divergence in individual traits or multivariate phenotype. These findings contrast with the expanding literature demonstrating that urbanization promotes rapid evolutionary change and offer preliminary insights into the eco-evolutionary role of green corridors in urban environments.

How does pollen production of allergenic species differ between urban and rural environments?

Pollen production is one plant characteristic that is considered to be altered by changes in environmental conditions. In this study, we investigated pollen production of the three anemophilous species Betula pendula, Plantago lanceolata, and Dactylis glomerata along an urbanization gradient in Ingolstadt, Germany. We compared pollen production with the potential influencing factors urbanization, air temperature, and the air pollutants nitrogen dioxide (NO2) and ozone (O3). While we measured air temperature in the field, we computed concentration levels of NO2 and O3 from a land use regression model. The results showed that average pollen production (in million pollen grains) was 1.2 ± 1.0 per catkin of Betula pendula, 5.0 ± 2.4 per inflorescence of Plantago lanceolata, and 0.7 ± 0.5 per spikelet of Dactylis glomerata. Pollen production was higher in rural compared to urban locations on average for B. pendula (+ 73%) and P. lanceolata (+ 31%), while the opposite was the case for D. glomerata (- 14%). We found that there was substantial heterogeneity across the three species with respect to the association of pollen production and environmental influences. Pollen production decreased for all species with increasing temperature and urbanization, while for increasing pollutant concentrations, decreases were observed for B. pendula, P. lanceolata, and increases for D. glomerata. Additionally, pollen production was found to be highly variable across species and within species-even at small spatial distances. Experiments should be conducted to further explore plant responses to altering environmental conditions.

Urbanization shapes phenotypic selection of fruit traits in a seed-dispersal mutualism

Urbanization is currently one of the trademarks of the Anthropocene, accelerating evolutionary processes and reshaping ecological interactions over short time scales. Species interactions represent a fundamental pillar of diversity that is being altered globally by anthropogenic change. Urban environments, despite their potential impact, have seldom been studied in relation to how they shape natural selection of phenotypic traits in multispecies interactions. Using a seed-dispersal mutualism as a study system, we estimated the regime and magnitude of phenotypic selection exerted by frugivores on fruit and seed traits across three plant populations with different degrees of urbanization (urban, semiurban, and rural). Urbanization weakened phenotypic selection via an indirect positive impact on fruit production and fitness and, to a lesser extent, through a direct positive effect on species visitation rates. Our results show that urban ecosystems may affect multifarious selection of traits in the short term and highlight the role of humans in shaping eco-evolutionary dynamics of multispecies interactions.

Pollen limitation of native plant reproduction in an urban landscape

PREMISE

Evidence suggests that bees may benefit from moderate levels of human development. However, the effects of human development on pollination and reproduction of bee-pollinated plants are less-well understood. Studies have measured natural variation in pollination and plant reproduction as a function of urbanization, but few have experimentally measured the magnitude of pollen limitation in urban vs. non-urban sites. Doing so is important to unambiguously link changes in pollination to plant reproduction. Previous work in the Southeastern United States found that urban sites supported twice the abundance of bees compared to non-urban sites. We tested the hypothesis that greater bee abundance in some of the same urban sites translates into reduced pollen limitation compared to non-urban sites.

METHODS

We manipulated pollination to three native, wild-growing, bee-pollinated plants: Gelsemium sempervirens, Oenothera fruticosa, and Campsis radicans. Using supplemental pollinations, we tested for pollen limitation of three components of female reproduction in paired urban and non-urban sites. We also measured pollen receipt as a proxy for pollinator visitation.

RESULTS

We found that all three plant species were pollen-limited for some measures of female reproduction. However, opposite to our original hypothesis, two of the three species were more pollen-limited in urban relative to non-urban sites. We found that open-pollinated flowers in urban sites received less conspecific and more heterospecific pollen on average than those in non-urban sites.

CONCLUSIONS

These results suggest that even when urban sites have more abundant pollinators, this may not alleviate pollen limitation of native plant reproduction in urban landscapes.

The impact of urbanization on outcrossing rate and population genetic variation in the native wildflower, Impatiens capensis

Cities are one of the fastest growing ecosystems on the planet, and conserving urban biodiversity is of primary importance. Urbanization increases habitat fragmentation and may be particularly problematic for native plant species which often exist in small, remnant populations in cities. We studied the effects of urbanization on Impatiens capensis, a self-compatible native wildflower, which is an important nectar and pollen source for native bees and hummingbirds. We sampled I. capensis from six populations located in urban and rural habitats in Toronto, Ontario, Canada. We sequenced the DNA of 43 families (N = 86 individuals) using genotype-by-sequencing to obtain 5627 single nucleotide polymorphisms. From each parent and offspring, we estimated individual outcrossing rates, population-level genetic diversity and genetic structure among populations. We found that 95% of plants were outcrossed, and populations were genetically differentiated, where urban populations contained a subset of the genetic variation found in rural populations. Urban populations exhibited lower genetic diversity than rural populations, and we detected a relationship between population census size and habitat on genetic diversity. Despite high outcrossing rates, our results suggest that urbanization reduces the genetic diversity of I. capensis populations, potentially increasing the vulnerability of these populations to long-term population declines and extirpation in response to urbanization.

Ecological Interactions, Environmental Gradients, and Gene Flow in Local Adaptation

Despite long-standing interest in local adaptation of plants to their biotic and abiotic environment, existing theory, and many case studies, little work to date has addressed within-species evolution of concerted strategies and how these might contrast with patterns across species. Here we consider the interactions between pollinators, herbivores, and resource availability in shaping plant local adaptation, how these interactions impact plant phenotypes and gene flow, and the conditions where multiple traits align along major environmental gradients such as latitude and elevation. Continued work in emerging model systems will benefit from the melding of classic experimental approaches with novel population genetic analyses to reveal patterns and processes in plant local adaptation.