Daylight shrubbery with thieving hummingbirds

A new study from Brazil by Pedro Amaral Anselmo and colleagues reveals that non-native plants in urban environments make important contributions to supporting hummingbird pollinators throughout the year. The findings highlight the complex ecological roles of non-native species and have implications for urban conservation efforts. One of the most surprising findings is that many of the interactions were ‘illegitimate’.

The researchers spent a year observing hummingbirds visiting flowers across a large urban university campus in Belo Horizonte, Brazil. They watched closely to document how the birds interacted with each plant species. The interactions were classified as either “legitimate” or “illegitimate.”

Legitimate visits occurred when the hummingbird acted as a pollinator by properly contacting the flower’s reproductive structures, potentially enabling pollen transfer. These visits could lead to successful pollination and reproduction. Illegitimate visits happened when the hummingbirds stole nectar from the flowers without pollinating them.

Illegitimate visits could be nectar theft, where they took nectar through the flower opening without touching pollen-bearing structures. An alternative was nectar robbery, where the birds damaged flower parts to access the nectar. Theft and robbery allow hummingbirds to take nectar without assisting the plant’s reproduction. Surprisingly, over half of the more than 1300 recorded visits were illegitimate nectar theft or robbery, suggesting opportunistic nectar acquisition by the hummingbirds in the city.

The researchers analysed the pattern of interactions between hummingbirds and plants as a network. Generalised networks occur when species interact with many partners randomly rather than having specialised relationships. For example, a generalised hummingbird-plant network would mean that each bird species visited many plant species indiscriminately. This contrasts with a specialised network where certain hummingbirds consistently preferred particular flower species.

Amaral Anselmo and colleagues found that the urban hummingbird-plant network was highly generalised, with a lack of specialisation between particular birds and blooms. The hummingbirds visited diverse flowers opportunistically. This type of generalised network is typical of urban environments, where plant and animal communities tend to consist of generalist species that interact randomly rather than having coevolved specialised relationships. The prevalence of generalised interactions matched the opportunistic illegitimate visits by the hummingbirds.

The study revealed that non-native plant species played a major role in the urban hummingbird-plant network. Eleven of the seventeen plant species observed were non-native. The non-native plants were abundant at the site and produced a total of 16 times more nectar compared to native species over the course of the year-long study.

On average, the non-native and native plants produced similar amounts of nectar per month when each species was considered individually. However, because the non-natives were much more numerous, they contributed the bulk of food resources sustaining the local hummingbird population. Although native plants are often promoted for supporting urban wildlife, at this location, introduced species provided the majority of essential hummingbird nectar.

Another attraction of non-native plants is due to how native and non-native plants supply nectar over time. The native species tended to have concentrated flowering peaks, with bloom periods clustered into one or two parts of the year. In contrast, the non-native plants generally had extended flowering durations. While native species flowered intensively at certain times, non-natives continued producing flowers and nectar for longer stretches. This meant the non-native plants filled seasonal gaps when few native blooms were available.

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Although we observed flowering peaks of native plants in different months, there were some periods of the year with few or even without native flowers. This highlights that native plants were not sufficient in providing resources during the whole year in our study area.

So the introduced species provided essential resources when the native plants failed to offer nectar. This complementary phenology suggests non-native plants can play an important role in ensuring year-round nutrition for urban pollinators.

Amaral Anselmo and colleagues question if there are hazards in extensively using non-native plants to support pollinators. A successful coloniser can go on to become invasive. Here, they point out the complexity of the interactions, particularly the significance that so many of the interactions are illegitimate.

From the perspective of pollinator conserva­tion, the non-native plants’ role in complementing nectar supply during periods of resource scarcity is positive, and it may be beneficial that pollinators making use of these resources illegitimately do not contribute to their reproduction and potential spread. In this respect, it is important to consider that supporting diverse groups of animals in urban environments and the ecosystem function they provide is a major goal in urban greening plans.

Amaral Anselmo et al. 2023.

It doesn’t follow that people should plant non-natives without consideration if they’re aiming to have a positive effect with urban greening. Still, Amaral Anselmo and colleagues’ results do support other recent research showing that thoughtful planting can complement native plants in helping wildlife.

READ THE ARTICLE
Anselmo, P.A., Cardoso, J.C.F., Siqueira, P.R. and Maruyama, P.K. (2023) “Non-native plants and illegitimate interactions are highly relevant for supporting hummingbird pollinators in the urban environment,” Urban Forestry & Urban Greening, 86(128025), p. 128025. Available at: https://doi.org/10.1016/j.ufug.2023.128025.