But firstly… what are saproxylic invertebrates?
Saproxylic invertebrates are those that rely on dead and decaying wood for at least part of their life cycle. And in the UK there are over 2000 different invertebrate species in Britain which are dependent on this dead an decaying habitat, this represents about 7% of the entire invertebrate fauna – wood decay is hugely important!
Saproxylic invertebrates often have exacting requirements which is why many have become so rare and they are often unable to move long distances to find new habitat if the one they are in disappears or becomes unsuitable. This means that not only do they rely on the continuity of the habitat they are in, but also on having a landscape with stepping-stones and corridors of similar habitat for them to be able to disperse. Any population that is isolated is more vulnerable to local extinction, which alongside a decline in suitable habitat is one of the reasons this group of species is now so rare.
What did the survey show?
Last spring and summer we set up several flight interception traps on suitable veteran trees across the Common. The traps were emptied every few weeks and the invertebrates were sent off to the highly skilled entomologist Dr Jonty Denton. From a qualifying list of 139 saproxylic species (including data from previous surveys) Jonty was able to calculate the Index of Ecological Continuity (IEC) and the Saproxylic Quality Index (SQI); these are two systems that have been derived for the assessment of site quality for nature conservation.
Fig. 1. Dead wood sites in London District ranked according to IEC
As you can see from the table above, Barnes Common ranks joint 5th when compared to other sites that have been assessed in the London district. Sites with an IEC of over 25 are considered of national importance, so Barnes is well above this threshold!
27 of the species found have a conservation status with 5 being listed in the British Red Data Book (this is a database for recording endangered and rare species) and another 4 being proposed for Red Data Book status, based upon their rarity.
Why is Barnes Common so good for saproxylic invertebrates?
The Common is not rich in ancient and veteran trees and so is not a site you would immediately associate with having an important saproxylic beetle fauna. However, the proximity to Richmond and Bushy Parks, which are of international importance for their saproxylic invertebrates, has most likely acted as a source for many of these species to colonise suitable habitat across the Common.
What will we do with this survey data?
These surveys are imperative to informing our management of the site – at the end of the day, you can’t protect what you don’t know you have! By understanding the requirements of these species, whether as an assemblage or, when they are very rare, as individuals, we can act appropriately and ensure that their habitat requirements are met both in the present and in the future. These surveys also show the need to make sure we do everything we can to link all suitable habitats together, so we do not end up with isolated pockets of species that can go extinct.
What can we all do to help saproxylic invertebrates?
It’s all about the deadwood! It is so important to retain as much dead wood as we can. Whether this is standing dead trees, parts of living trees that are decaying, split, or cracked, fallen dead wood and underground dead wood. Decaying and dead wood does not necessarily mean that a tree is in poor health.
As well as deadwood habitat many saproxylic species, when adults, need nectar rich flowers to feed on for this part of their life cycle. So, encouraging wildflowers, particularly umbellifers such as wild carrot, cow parsley, and hogweed will also help. Hawthorn is also an incredibly important nectar source for these invertebrates. So, if you’re lucky enough to have a garden, please do try and help these species by either encouraging some of these plants and retaining any deadwood habitat you may have!