The case of the grey-crowned babbler
Understanding the value of restoration requires measuring change through time
Measure response variables that are meaningful
Counterfactuals are necessary: Compared to what?
Many resources are spent on restoring habitat to counter the impacts of land clearing and habitat degradation on wildlife populations. But individual projects involving restoration usually have severe time and resourcing constraints. These constraints mean that the effectiveness of these projects in achieving their long-term goals (of improving the population viability of particular species) is rarely assessed. As a result, many restoration programs cannot demonstrate their effectiveness.
So what can be done about this? Well, one approach is to call for all new programs to include monitoring. This may help with future work. But what of all the hard work that has been done in the past. Can we learn anything about past efforts?
This is a story of trying to learn from past investment. Since the early 1990s, Doug Robinson from Trust for Nature in Victoria has been leading efforts to restore woodland habitat in northern Victoria for the grey-crowned babbler, a long-lived, colonial-nesting, woodland bird, in decline in south-east Australia. The question was simple, has this restoration effort been effective in improving the population viability of grey-crowned babblers?
But the route to answering that question was not so straightforward. It involved:
- a revisitation survey (resurveying sites first surveyed some 13 years previously);
- a stratification of the surveys so as to include restored and unrestored sites;
- an investigation of the demographic parameter of babbler family-group size (which has direct impacts on local population growth since an extra bird in the group translates into about half an extra fledgling per breeding season per group);
- an accounting for habitat variation between sites and effects of distances from occupied sites; and finally
- a determination of whether differences between the two surveys weren’t due to imperfect detection or to simple year-to-year variation.
The key result from all this was that restoration was effective in stemming the decline in group size; whereas at unrestored sites, decline continued. The restoration meant roughly an extra bird per group which has an important effect on the reproductive success of the social group.
But the bottom-line assessment is only part of the story. The lessons that have emerged from this study are the importance of measuring change over time and the need to include counterfactuals – you need a reference or control to compare with. Measuring ‘meaningful’ performance variables (such as group size) by themselves without the comparison of what happened on unrestored sites yielded little information.
Studies such as this help us demonstrate that we can learn from past conservation interventions. It may not always be possible, but it’s worthwhile if the information is available over time. We showed that good use can be made of existing data, coupled with good design for collecting new data. This is important because we need to know the effectiveness of past investments.
In this case, the work was only possible because of Doug Robinson’s championing the cause. He led volunteers from Friends of the grey-crowned babbler group to conduct surveys. Funding from The Norman Wettenhall Foundation and Goulburn Broken Catchment Authority assisted and ultimately, many landholders and managers contributed to habitat restoration.
More info: Peter Vesk firstname.lastname@example.org
Vesk PA, D Robinson, R van der Ree, CM Wilson, S Saywell & MA McCarthy (2015). Demographic Effects of Habitat Restoration for the Grey-Crowned Babbler Pomatostomus temporalis, in Victoria, Australia. PLoS ONE 10: e0130153. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0130153