Hedges for Biodiversity

Background

Leiden University was founded in 1575 and at its 15^th anniversary it was gifted its own botanical garden by the city council. From the very start, both medicinal and ornamental plants were grown here for education and research. We know quite well which plants were cultivated at that time thanks to the Libri Picturati, a stunning collection of historical water colours ¹. The species depicted were painted between 1565 and 1569 on request of botanist Karel van Sint-Omaars. One of his pupils was Charles d’Ecluse, the first prefect of Hortus botanicus Leiden. With help of the first hortulanus, Dirk Outgaertszn Cluyt, medicinal plants like spurge laurel (Daphne mezereum) were shown to the students and citizens of Leiden. Nowadays, this plant species is highly endangered in the Netherlands and part of a national conservation project. 

Pictured below: Fruiting spurge laurel (photo by Willem Braam) and Clusius garden in 1712 (illustration by Pieter van der Aa).

In the first century of its existence, highlights of the living collection consisted of exotic ornamentals such as tulips and hyacinths, that were grown in a tight 40-square-meter space. During the next centuries, the botanical garden slowly expanded to its current size of 4,5 hectares. Its living collection nowadays encompasses over 6000 plant species from all over the world. Many of these, such as Laburnum (Laburnum anagyroides) from central and southern Europe, have been at this botanic garden right from the start. After the use of chemical pesticides was banned, this species was embraced by the local foodweb. Nowadays, local bumblebees visit the flowers, its leaves are consumed by local leaf miners, and its seeds by beetles ². It is just one of the many exciting examples of exotic plants, that are being detected by native insects and other animals to become part of a newly evolving urban ecosystem. 

Pictured below: Laburnum (Laburnum anagyroides) with buff-tailed bumblebee (Bombus terrestris) and broom seed beetle (Bruchidius villosus) in Hortus botanicus Leiden (photos by Duco de Vries and Udo Schmidt).

A unique foodweb

Due to climate change and a switch to nature-inclusive garden management, contacts between native animals and exotic plants intensified. Many animals can now survive here and not only as adult but also as egg, caterpillar, or nymph. As a result, a unique foodweb is starting to evolve.

Plants are at the heart of this foodweb. They are the producers and turn water and carbon dioxide into sunlight and sugars with the help of oxygen. Herbivores such as insects and snails eat plants. Carnivores such as newts, spiders, birds, wasps, and bats eat herbivores. Omnivores, such as beetles, eat both plants and animals. Fungi and worms are the detrivores: they break down all dead organic material and return nutrients to the soil.   The first chains of this unique foodweb were detected with the naked eye. By now, hundreds of species have been discovered in and around the plant collection during national garden counts. The first new interactions among species from all over the world, that did not co-evolve together, have been documented ^3-5.

Multimodal monitoring

To detect and quantify all species present and correlate these with types and frequency of garden maintenance, we need more than just a microscope, hand lens or binoculars. That is why we study the emerging food web using wild camera’s, audio-moths, and DNA-analyses. Data captured are being processed through automated species detection in collaboration with the Leiden Institute of Advanced Computer Science (LIACS), Naturalis Biodiversity Center, and others. Computer science students develop algorithms and interfaces to identify species and connect undescribed species to yet unexplored branches of DNA-based phylogenies. Colleagues of the Mathematical Institute offered their help with describing the food web with models to better understand interactions among players. 

Pictured below: Wildlife camera (Reolink) capturing a treecreeper (Certhia brachydactyla) foraging on branches of one of the monumental snow pears (Pyrus nivalis) in Hortus botanicus Leiden (photographs by Barbara Gravendeel and Rogier van Vugt).

The need to reconnect with biodiversity

Hortus botanicus Leiden has been developing into an amazing hotspot of biodiversity over the past four and a half centuries. Sadly, this cannot be said for the surrounding city of Leiden. Due to ongoing urbanization and climate change, the areas around the botanic garden slowly changed into an unhealthy urban heat island. As a result, the citizens of Leiden became alienated from nature. We have the ambition to prevent endangered plant species from going extinct by reconnecting its 260.000 annual visitors with biodiversity as a healthy and enriching experience. 

Promoting smelling wellness

Plants produce volatile defense compounds when attacked by herbivores, such as mealybugs or aphids. Surrounding plants, that are not yet attacked, detect the volatile compounds, and start to produce their own defense compounds. While walking through a botanic garden, you inhale all these volatile compounds. They give your immune system a boost and reduce the amounts of stress hormones ⁶. A walk of just half an hour is already sufficient. To make our visitors more aware of the invisible chains in the food web that stimulate plants to produce healthy volatile compounds, we are developing interactive games. By engaging our visitors in these games, we hope to make them more aware of the added value of nature-inclusive gardening and biodiversity on their own health. 

Pictured below: Hortus Spotter game developed by MSc student Teun Bergsma.