Bad Neighbors

Hidden effects of habitat restoration on the persistence of pollination networks (2022) Gaiarsa & Bascompte, Ecology Letters, https://doi.org/10.1111/ele.14081

Image credit: dronepicr, CC BY 2.0, via Wikimedia Commons

The Crux

It’s no secret that the world is undergoing a biodiversity crisis. This comes not only from climate change and human land use, but also invasive species – non-native species that cause harm to native ecosystems. Specifically, there are seven times more invasive species now than there were 75 years ago. Because of how many there are, and just how fragile ecosystems have become, it’s important to know what effects that invasive species have.

Ecological restoration (see Did You Know?) is one effective solution that can be used to mitigate the biodiversity crisis. Reestablishing native species can often help with this restoration, as does removing invasive species, but it usually requires human intervention. By removing these invasive species, the idea is that the native species will be released from competition and benefit from better access to necessary resources.

Yet to monitor invasive species removal, you need long-term data on population persistence, which is very difficult (logistically and financially) to collect. Understanding how the removal of invasive species benefits restoration requires not only measuring how such removal benefits ecosystem function, but also how it can benefit population persistence in the long term. Today’s authors wanted to understand how the removal of an invasive species benefited local community resilience.

Did You Know: Ecological Restoration

Ecological restoration involves trying to restore an ecosystem to a previous condition. If restored, all functions, processes, and community dynamics return to a semblance of their former states, if not a complete return. For example, beavers are ecosystem engineers that can create wetland habitat by damming up a stream. If beavers return to a habitat they were removed from, this contributes to ecological restoration by once again creating a wetland, which provides homes for a number of other species.

What They Did

The authors utilized data from 64 plant-pollinator networks (i.e., interacting groups of plants and their pollinators) on the island of Mahé in the Seychelles. These 64 sites were distributed across eight sites (8 networks per site), and there were two different treatments. The first treatment was the “restored” treatment, where invasive plant species were removed, and the “unrestored” treatment, where the invasive plants were left alone.

Using this data, the authors then constructed a model to understand the population dynamics of the plant species in each network. This model tracked how plant abundances changed over time due to interactions with competitors and with mutualistic pollinators (insects that depend on the flower for nutrients). The authors also characterized community structure using the nestedness metric, which is a measure of the distribution of species across locations. Think about a Russian nesting doll, where the bigger doll holds a number of smaller dolls. If an area has a high nestedness value, it means that there are multiple, smaller areas within that larger area that have a subset of the species found within the larger area. Nestedness can increase species persistence, so relating the nestedness to the restoration status of a given system could be informative.

What They Found

Surprisingly, there was no clear effect of restoration on the persistence of plant-pollinator communities. That is, neither the plants nor pollinators benefited from the removal of invasive species. So, the authors then grouped all of their data by season, late season and early season. They found that plant species persistence was higher in the restored sites, and this effect was stronger in the late season. The opposite was true for the pollinators, with their persistence being higher in the late season at the unrestored sites.

For the nestedness measure, the authors found that nestedness had no effect on plant persistence, and this did not vary between the restored and unrestored sites. However, nestedness did increase the persistence of pollinators, and this effect was greater in the unrestored sites.

Pollinators like this bee may benefit from invasive plant species, despite the harm that these plants impose on other, native plants (Image credit: Adventures of KM&G-Morris, CC BY-NC-ND 2.0)

Problems?

The scale of this study is impressive, but the temporal nature of the data is not as robust. The authors measured community dynamics within ~7 months of removing the invasive species, which is a very short amount of time. I am not an expert in this system, and 7 months may be enough to detect an effect in these plants and pollinators, but to know how invasive species removal is supposed to help population persistence I am curious how these populations will look in 5-10 years.

So What?

The results from this study reveal an important, hidden effect of invasive species. While invasive plants are harmful to the native plants, they can actually benefit the native pollinators. This means that restoration efforts must carefully consider the ramifications of invasive species removal, which would involve population assessment before and after any restoration efforts.


Dr. Adam Hasik is an evolutionary ecologist and Zuckerman Postdoctoral Fellow interested in the ecological and evolutionary dynamics of host-parasite interactions who has successfully invaded Israel. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

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