Native plants and pollinators in the city: the case of Abutilon grandifolium in Buenos Aires

Honey bee visiting Abutilon grandifolium

by Cáceres et al.

Urban green spaces can be important places for plants and insects. Parks, reserves and even small public squares can provide important spaces for plants, insects and the ecological interactions that connect them. In Buenos Aires, urban green spaces now include native plants but we still know relatively little about how these plants reproduce and which insects visit their flowers.

In this study, we worked with Abutilon grandifolium, a native shrub with orange-yellow flowers. We studied plants growing in two urban green spaces: an ecological reserve and a public square planted with native species to recreate local habitats. We studied its floral biology, reproductive system and flower visitors to understand how this plant reproduces and how it interacts with pollinators in the city.

We found that A. grandifolium can produce fruits and seeds by self-pollination, meaning that it does not strictly need pollinators to reproduce. Even so, its flowers attracted several insects, especially bees and native halictid bees.

Honey bees were the most frequent visitors, ut they did not always act as good pollinators: in many cases, they collected nectar without touching the reproductive parts of the flower. Native halictid bees, instead, moved inside the flower and touched both the pollen and the stigmas, making them particularly important pollinators. Other insects, such as large bees, hoverflies and beetles, also visited the flowers, although less frequently. By observing their behaviour and analysing where pollen was carried on their bodies, we were able to distinguish visitors that likely contributed to pollination from those that mainly collected floral rewards without effectively transferring pollen.

An interesting result was that the public square supported a community of floral visitors similar to that of the ecological reserve. In fact, visitor richness was even higher in the square. This suggests that small urban green spaces planted with native species can help maintain pollination processes and may act as stepping-stones for biodiversity within the city.

Our results show that bringing native plants back into urban landscapes is more than a matter of beauty or gardening. These plants can help create living networks between flowers and insects, supporting pollinators and ecological interactions even in the middle of a large city.

Read the scientific article in JPE here. 

Misconceived experiments may yield valuable insights: Simulated florivory has unpredictable consequences for plant reproduction in Knautia arvensis (Caprifoliaceae)

By Ollerton et al.

Sometimes science does not go the way you plan. This study began as an experiment to test whether the showy outer florets of Knautia arvensis (Field Scabious) help attract pollinators. These enlarged “ray florets” sit around the edge of the flower head and make the inflorescence more visually conspicuous. The original question was simple: if these ray florets are reduced in size, do pollinators visit the flowers less often, or spend less time on them?

The answer turned out to be: not really. But that “failed” result became interesting in a different way.

In 2001, flower heads of Knautia arvensis growing at Bradlaugh Fields in Northampton (UK) were experimentally altered. Some were left untouched as controls, some had only the tips of the ray florets cut off, and others had most of the outer ray-floret tissue removed. The plants were then observed to see how three main groups of pollinators responded: bumblebees, butterflies and other day-flying Lepidoptera, and hoverflies. Nectar production and seed production were also measured.

Experimental manipulation of Knautia arvensis inflorescences, showing a Control inflorescence (left), an inflorescence with a Small cut (centre), and an inflorescence with a Large cut (right).

The cutting treatments did not significantly change how often pollinators visited the flowers. Bumblebees and hoverflies were generally more frequent visitors than butterflies, but all three groups continued to visit and pollinate the manipulated flowers. Nor did the treatments clearly affect how long insects stayed on the flower heads. In other words, reducing the ray florets did not make the flowers obviously less attractive to their pollinators.

However, the treatments did have effects that were not expected at the time. The most severe cutting treatment reduced nectar quality. Nectar volume stayed about the same, but nectar concentration fell substantially in the large-cut flowers. Once the analyses accounted for missing nectar data and differences in volume, the large-cut flowers also contained less sugar overall. This suggests that the plant may have responded to damage by reducing its investment of resources in those flower heads.

The most striking effect was on seeds. Cutting the ray florets did not significantly reduce the number of seeds produced, nor the proportion of flowers that set seed. But the seeds from the small-cut and large-cut treatments were roughly one fifth lighter than seeds from uncut control flower heads.

This matters because seed number is not the whole story of plant reproduction. A plant may produce the same number of seeds, but if those seeds are smaller and less well provisioned, their chances of germinating, establishing, and surviving may be reduced. Larger seeds often produce stronger seedlings, especially in competitive grassland environments. So the damage may have affected reproductive quality rather than reproductive quantity.

With hindsight, the study is therefore best understood not as a successful test of whether ray florets attract pollinators, but as an accidental experiment in florivory - the eating or damaging of flowers by animals. Florivory is common in nature but is less studied than leaf herbivory. Many studies show that damage to flowers can reduce pollinator visits and lower seed production. This experiment suggests a subtler possibility: floral damage may leave pollination apparently unaffected, while still changing how the plant allocates resources to nectar and seeds.

The broader lesson is that archived or apparently inconclusive experiments can still be valuable. Data that once seemed difficult to interpret can become meaningful when viewed through a different ecological framework. Here, an experiment that did not answer its original question nevertheless revealed something important: damage to floral tissues can alter plant resource allocation in ways that are not obvious from pollinator behaviour or seed number alone.

A remaining mystery is what the ray florets of Knautia arvensis actually do. In this experiment, they did not have a strong effect on attracting or retaining pollinators. They may still contribute to the overall visual display of the flower head, or they may have a more subtle role that was not detected here. Alternatively, they may not be strongly adaptive in this species. Further experiments would be needed to resolve that question.

Overall, the study shows that plant responses to floral damage can be complex and unpredictable. Pollinators may continue to visit damaged flowers, and seed numbers may remain stable, but the quality of the plant’s reproductive output can still decline. That makes florivory an important but underappreciated influence on plant reproduction.

Read the scientific publication in JPE here!