What do boar in the UK, porcupines in Israel, and bettongs in Australia have in common? They all eat geophytes – but is this a good or a bad thing?
Geophytes and diggers
Geophytes are plants that have starchy underground storage organs, also known as bulbs, tubers or corms. As they dig for these nutritious roots, digging animals create holes that trap litter, water and nutrients and can become hotspots for productivity and seed germination. Geophytes have co-evolved with digging animals, and in some cases seem to have formed symbiotic relationships where the diggers rely on the tubers for food, but the plants rely on the digging to stimulate germination. This process is so important that digging animals are often referred to as ‘ecosystem engineers’.
In the Negev desert in Israel, porcupines are known to consume at least 18 different species of geophytes, some of which show a clear response to the diggings and regenerate either from seed or from parts of the plant that are left undamaged. In the harsh desert environment the digs may be particularly important, and some species appear to almost completely rely on digs to provide the conditions required for germination (Gutterman 1987).
But what happens to the geophytes if these digging animals go extinct? And what if we reintroduce them to areas where they have been absent for a long time?
When boar were reintroduced to the UK in the 1980s after a 700 year absence, it was noticed that they were rooting up large patches of bluebells. Bluebells are highly valued for their beautiful spring displays, so this disturbance is widely seen as negative. However, boar have been absent from the UK for so long that it’s impossible to know how the ‘natural’ system would have worked. A study by Sims et al. (2014) found that when boar were excluded from rooted areas, the bluebells were able to recover very quickly, which was thought to be partly due to a beneficial effect on germination – so perhaps the bluebells will actually benefit in the long term. On the other hand, the boar could actually increase diversity by reducing the dominance of bluebells and encouraging other species. Whether this is a good or bad thing is at least partly determined by the values humans place on the ecosystem (aesthetics, diversity, function), something that must always be considered with any reintroduction.
Eastern Bettongs (Bettongia gaimardi) disappeared from mainland Australia in the 1920s, after decades of being hunted as agricultural pests and predation by feral cats and foxes. Many of the other small digging mammals that used to be common in eastern Australia have met a similar fate, leaving very few animals to fill the important ‘ecosystem engineering’ role. The loss of these digging animals is thought be contributing to a decline of ecosystem function.
Three years ago, around 30 Eastern Bettongs were translocated from Tasmania to a predator-proof sanctuary in the ACT, and since then the population has increased to over 300 individuals. Bettongs mainly feed on native truffles, but as their numbers increased, researchers started to notice that the bettongs seemed to be targeting a particular species of geophyte – a native lily called ‘Early Nancy’ (Wurmbea diocia).
In some areas, populations of Early Nancies were now fields of digging that look more like the work of pigs than small marsupials!
Since my PhD is looking at the impact that the Bettongs are having on the ecosystem, this seemed like a good place to start. Is this predation by Bettongs having a negative effect on the Early Nancy population? Or will the Early Nancies benefit from increased germination over time? Is there any evidence of a symbiotic relationship between Bettongs and Early Nancies? And how many Bettongs is too many?
Early Nancy monitoring
To answer some of these questions, over the next 2 years I will be monitoring populations of Early Nancies to see how many seedlings emerge each year, how many plants flower and seed successfully, and how many are dug up by bettongs. There are small bettong exclusion areas within the reserve that will act as controls. I have set up 5 monitoring sites, each with 5 quadrats inside the exclusion area and 5 outside.
In July, when the seedlings were just emerging, I counted and marked every plant in the quadrats and took photos. The quadrats are permanently marked using pegs and tags at opposite corners, so that we can return to the same locations each time. The plant markers are not permanent, but we can use the photos to compare the locations of plants over time.
In September, I came back and again marked every plant – this time with different colours indicating seedlings (yellow), flowers (red), and death by bettong, either through digging or burial (blue). This week I will return to count how many have managed to seed.
In the 2 months between the first and second monitoring, about 15-20% of the Early Nancies within the quadrats had been eaten by bettongs. If the predation continues at this rate, it won’t be long before all the Early Nancies are gone! However, there were also plenty of new seedlings, and there seemed to be more seedlings in the areas with bettongs than the exclusion areas. This would suggest that the digging does have a positive impact on germination, but we won’t know for sure until we have followed the changes over several years.
Gutterman, Y. (1987). Dynamics of porcupine (Hystrix indica Kerr) diggings: their role in the survival and renewal of geophytes and hemicrytophytes in the Negev Desert highlands. Israel Journal of Botany, vol 36. pp 133-143.
Sims, N., John, E., Stewart, A. (2014). Short-term response and recovery of bluebells (Hyacinthoides non-scripta) after rooting by wild boar (Sus scrofa). Plant Ecology, 215: 1409-1416.