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Kevin Wood

How do you solve a problem like herbivory?

Wednesday 16th July 2014

Chalk streams, such as Dorset’s River Frome, are internationally renowned for their abundant and diverse flora and fauna. Their importance for wildlife is in part due to the aquatic plant community, dominated by stream water crowfoot (Ranunculus penicillatus ssp. pseudofluitans). These plants fulfil a wide range of key ecological roles, modify the river and provide suitable conditions for wildlife. As examples, aquatic plants slow the passage of water, increase and diversify the physical habitat available to other species, as well as providing food and cover from flow and predators. A healthy aquatic plant community is therefore vital to the conservation objectives of chalk streams, and consequently these plant communities are protected under the European Union Habitats Directive (92/43/EEC), which requires the UK to maintain or restore chalk streams with this plant community to favourable condition. Chalk streams are also home to large populations of mute swans (Cygnus olor), a native herbivorous bird; these large waterfowl can eat up to 4kg of fresh vegetation per day. We showed recently that large flocks of swans can cause substantial reductions in aquatic plant abundance and flowering in chalk streams. A key conservation aim is the maintenance of aquatic plant community in favourable condition, swan grazing presents a challenge; how do we resolve a conflict between a native, protected herbivore and its native, protected food plants?

Controlling the swan population, for example by translocating adults or removing eggs from nests, has been suggested as a possible way of preventing swan grazing damage in chalk streams. Given swans legal protection and popular status, it is vital to evaluate whether such methods could succeed before carrying them out. Recent population control of mute swan and black swan populations in the United States and New Zealand respectively faced widespread public opposition, legal challenges, and damaged relations between different stakeholder groups, in particular government wildlife officials, landowners and animal welfare groups. Thus we need to evaluate the chances of success of different methods of population control prior to avoid the rancour experienced during the schemes in the United States and New Zealand. Unfortunately, carrying out field trials to examine the effectiveness of population management methods requires more time, effort and money than managers typically have available. This is particularly true for long lived animals such as swans. Furthermore, as swans are protected and popular it can be difficult to get both legal and landowner consent to carry out such field trials. To get around these problems, we can use simulation models to predict the effects of different management methods in a fraction of the time, and with none of the practical difficulties associated with field trials. Managers can then conduct field trials field trials for only those population control strategies which were predicted to be most effective.

In a recent study, we built and tested a population model which could predict changes over time in the size of the swan population in the River Frome. We assessed the accuracy of our model by using it to predict historical changes in the swan population sizes of two chalk streams, the River Frome (Dorset) and River Wylye (Wiltshire); our model predictions closely matched the observed population sizes, being on average 109% and 91% of the observed size for the Frome and Wylye respectively. Satisfied that our model was able to give accurate predictions of changes in swan population size, we used it to predict the effects of three population management strategies on population size; (a) reductions in reproductive success (i.e. removing eggs), (b) removal of individuals (i.e. translocating adults), and (c) reduced reproductive success and removal of individuals combined.

Our model predicted that the River Frome swan population would increase by 54%, from 257 to 397 individuals, over 17 years in the absence of any population control measures. However, population control measures were found to be effective in preventing grazing damage only at very high levels of management. In order to reduce the Frome population below the level for which grazing conflicts have been previously reported managers would need to remove ≥60% of non-breeding individuals each year indefinitely. Given the large and widespread swan population in Britain, it is doubtful whether release sites could be found for the 100s of swans which would need to be translocated each year! Reducing reproductive success alone, even to 0 eggs per nest, failed to achieve the population reduction required. Thus, we found that removing ≥60% of non-breeding swans each year, or management which combined high levels of egg removal and non-breeder removal together, could reduce the swan population below the target threshold. However, these values represent a massive sustained demand of time, money and interference with the swan population, which make population control impractical as a management tool. Consequently, population control does not represent an effective management option for preventing swan grazing damage to aquatic plants in the River Frome catchment. The key reasons for the ineffectiveness of population management were high adult and juvenile annual survival probabilities (>0.7) and immigration from outside of the catchment, which both offset the removal of swans due to management.

Due to the substantial ecological, practical and ethical obstacles to population control which we highlighted in our study, alternative management strategies for the swan grazing conflict such as habitat alterations or feeding deterrents should be explored. The next stage of our research is to explore whether habitat management offers a more promising direction for resolving the conflict. For example, aquatic plants could be protected by the establishment of sacrificial feeding areas near to areas of overgrazing, as have been used for grazing conflicts between geese and agriculture. Strategies which deter or prevent animals from feeding, such as scaring, repellents and fencing may also be used to prevent grazing damage by swans. Both sacrificial feeding areas and feeding deterrents have been used to successfully alleviate swan grazing conflicts with agriculture, and so may be effective in chalk stream catchments such as the River Frome.

If you would like to read more about this study please see our published paper, which is free to download:
Wood, K.A., Stillman, R.A., Daunt, F. & O’Hare, M.T. (2013). Evaluating the effects of population management on a herbivore grazing conflict. PLoS ONE, 8: e56287.

You can read more about my research into interactions between plants and their herbivores by visiting my web page or by following me on Twitter.

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