New perspectives on harvesting as one driver of ecosystem dynamics
Article first published online: 21 JAN 2008
© 2008 The Author
Journal of Applied Ecology
Volume 45, Issue 1, pages 1–3, February 2008
How to Cite
Milner-Gulland, E. J. (2008), New perspectives on harvesting as one driver of ecosystem dynamics. Journal of Applied Ecology, 45: 1–3. doi: 10.1111/j.1365-2664.2007.01455.x
- Issue published online: 21 JAN 2008
- Article first published online: 21 JAN 2008
- Received 13 December 2007; accepted 13 December 2007
If the set of papers featured in this Special Profile on Harvesting is anything to go by, applied ecological research on harvesting is entering a new era. We have already left behind single-species studies that take neither space nor time into account, but we have been slower to consider the role of harvesting as a driver of change in an ecosystem context. Such an accusation cannot be levelled at this set of papers. The studies cover a wide range of taxonomic groups and geographical regions, from cockles in the Netherlands through a medicinal herb in the Himalayas to bats in the Amazon.
A key lesson from these papers is that the ecological effects of harvesting cannot be assessed in isolation. Harvesting is one of a complex set of factors influencing the vital rates of harvested populations. As a prime example of this, Fordham et al. (2008) show how predation by introduced feral pigs Sus scrofa swamps the effects of subsistence harvesting of turtle Chelodina rugosa populations in Australia, such that populations without pig predation can support a moderate harvest rate, while predated populations are predicted to decline rapidly to extinction. Watson et al. (2007) also considered the combined effects of harvesting and predation on grey partridges Perdix perdix; in this case shooting had the dominant effect on population viability, but it was confounded by the presence of captive-reared red-legged partridges Alectoris rufa; the presence of the commoner species leads to density-independent shooting pressure on P. perdix unless harvesters explicitly avoid the non-target species.
Indirect effects of harvesting on non-target species are the subject of increasing concern. In this Special Profile, Edman et al. (2008) show that selective forestry leads to reduced abundance and fertility of two species of lichen in Canada, while Presley et al. (2008) have a similar message for forest-living bats in Amazonia; reduced-impact logging leads to a reduction in abundance of already-rare species, though it has less impact on more common species. Following the theme of the effects of fisheries on albatross species begun by Véran et al. 2007), Rolland et al. (2008) disentangle the combined effect of climate and fisheries on the black-browed albatross Thalassarche melanophrys. Importantly, they show that the relative effects of fisheries (both positive through feeding on bycatch and negative through direct mortality) and climate vary between the breeding and wintering sites – it is important to consider the different seasonal locations of migratory species separately, as the pressures of fishing and climate may act very differently on population dynamics in each.
This theme of spatial heterogeneity and the importance of considering the interaction between habitat and hunting is taken up by a number of authors – this is clearly an issue that can no longer be ignored. In 2007, Wynne & Cote addressed the issue for spotted spiny lobsters Panulirus guttatus, demonstrating that the effect of fishing pressure on lobster size and density is mediated by habitat quality. In this Special Profile, Gaoue & Ticktin (2008) show that the combined effects of pruning and bark-stripping from a valuable multi-use tree species, Khaya senegalensis, are more severe in the drier Sudanian region of Benin than in the Sudano-Guinean region. Presley et al. (2008) also highlight the interaction between habitat type and response to forestry among bat species, while Ghimire et al. (2008) show how the effects of harvesting on the Himalayan medicinal herb Nardostachys grandiflora are more severe in low productivity rocky areas than in meadows. These studies are all empirical; from the modelling perspective, Ling & Milner-Gulland (2008) also look at spatial heterogeneity. However, the modelling literature has not kept pace with our empirical understanding of the issue; their advance is to include space explicitly into the simple single-species population models that are still used for many sustainability assessments, and show how this small step towards realism is enough to alter substantially our predictions concerning the sustainability of harvesting systems. Clearly there is a long way to go before we can convert empirical findings into realistic predictive models of sustainability; Fordham et al. (2008) get closest to this in our Special Profile, while Rolland et al. (2008) recognize the need for predictive modelling based on their results to be carried out if we are to understand the implications of their findings for sustainability.
Another key issue identified by our authors is the importance of considering indirect effects of harvesting on vital rates other than direct mortality. Gaoue & Ticktin (2008), Edman et al. (2008) and Rolland et al. (2008) all address the effects of harvesting on fecundity rates of their study species, and all show that there is an effect – in the case of Khaya senegalensis, for example, the main effect of pruning and bark-stripping is on seed size and fruit production. This is also a theme that the Journal of Applied Ecology has addressed in previous issues; in 2006, Rijkers et al. found that tapping for frankincense had a profound effect on a tree's reproductive output.
So what are the main themes that the authors highlight in terms of policy responses? Although many of the recommendations are case-specific, the one that does crop up in a number of the studies is that it is important to manage at the landscape level; this is one conclusion of Ling & Milner-Gulland's (2008) modelling study, as well as of the empirical studies of Presley et al. (2008) and Edman et al. (2008), who both considered the effects of forestry on non-target species and showed – as have Barlow et al. (2007) before them – that timber extraction, however selective, has a profound effect on non-target species. Only by maintaining non-harvested stands of mature timber at the landscape level can susceptible species persist.
We must not forget that harvesting pressure is a product of human decision-making. If we are to manage harvested populations, and mitigate the effects of harvesting on other components of the ecosystem, then we have to understand the incentives that harvesters face. In some cases, harvesting is managed, in which case we can model the optimal strategy that managers should follow in order to meet their objectives. Hauser & Possingham (2008) consider the importance of learning in adaptive management of harvested populations, and show that the time horizon over which managers operate is a key determinant of their willingness to experiment and learn about the system; when time horizons are short they may choose a lower-revenue, safer management option over a riskier but ultimately more profitable option involving learning.
In many cases, the management regime is less clearcut, and is a product of social and cultural constraints. Milner et al. (2006) demonstrated this clearly for red deer Cervus elaphus populations around Europe. However, in many of the papers in this Special Profile, the harvesting regime is taken as an externally determined given, and the ecological effects flowing from that regime are the focus of interest. If ecology is truly to have management relevance, the management regime itself has to be considered as part of the system dynamics. Unless we understand the drivers behind the harvesting regimes in different locations, we cannot intervene effectively to promote sustainability. Often we confine our insights on these issues to qualitative discussion, as Gaoue & Ticktin (2008) have done, or give insights as to sustainable harvesting levels as Fordham et al. (2008) have done, rather than considering how these can be achieved within the prevailing institutional context. Our understanding of the effects of fishing on albatross population dynamics is now far superior to our understanding of the reasons why fishing vessels are distributed as they are, and hence the underlying causes of the interactions between humans and these non-target species. Even when harvesting is done by governments or large corporations (e.g. the timber industry), there are social and economic drivers that determine the location and type of management employed.
Ghimire et al. (2008) touch on some of these issues and highlight the importance of understanding the social context of harvesting – they worked closely with traditional harvesters in their study, mimicking the effects of the careful and ecologically well-informed harvesting techniques used locally to minimize harm to the plants. They highlight the importance of including the harvesters in any management planning, as well as the dangers of the increasing levels of uncontrolled commercial harvest that do not respect the sustainability of the resource. However, the issue is most comprehensively addressed by the last paper in the Special Profile, by Swart & van Andel (2008). These authors show us how very tightly linked ecological research and public discourse can be, when the subject is one of intense political and social interest. They suggest that we as scientists need to consider not just the soundness of our science, but the context in which we work, and how best to interact appropriately with society, such that our work informs rather than obfuscates key debates. There are many examples of the social relevance of the work that we feature in the Journal (e.g. the link between badgers and bovine TB in the UK; Jenkins et al. 2007). One aim of the Journal in future years is to raise the profile of research that effectively links real-world management with the highest quality of ecological research, demonstrating how previous research results have fed back to inform management, which in turn leads to new scientific insights (broadly, the paradigm of adaptive management, cf. Armstrong et al. 2007). Harvesting is one area in which this feedback between science and management can be particularly productive. This Special Profile demonstrates the cutting edge of harvesting research within applied ecology, and I hope you find it as stimulating to read as I have.
- 2007) Using adaptive management to determine requirements of re-introduced populations: the case of the New Zealand hihi. Journal of Applied Ecology, 44, 953–962. , & (
- 2007) The value of primary, secondary and plantation forests for fruit-feeding butterflies in the Brazilian Amazon. Journal of Applied Ecology, 44, 1001–1012. , , , & (
- 2008) Effects of selection cutting on the abundance and fertility of indicator lichens Lobaria pulmonaria and L. quercizans. Journal of Applied Ecology, 45, 26–33. , & (
- 2008) Indigenous harvest, exotic pig predation and local persistence of a long-lived vertebrate: managing a tropical freshwater turtle for sustainability and conservation. Journal of Applied Ecology, 45, 52–62. , & (
- 2008) Impacts of bark and foliage harvest on Khaya senegalensis (Meliaceae) reproductive performance in Benin. Journal of Applied Ecology, 45, 34–40. & (
- 2008) Demographic variation and population viability in a threatened Himalayan medicinal and aromatic herb Nardostachys grandiflora: matrix modelling of harvesting effects in two contrasting habitats. Journal of Applied Ecology, 45, 41–51. , , , & (
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- 2008) When does spatial structure matter in models of wildlife harvesting? Journal of Applied Ecology, 45, 63–71. & (
- 2006) Temporal and spatial development of red deer harvesting in Europe: biological and cultural factors Journal of Applied Ecology, 43, 721–734. , , , , & (
- 2008) Effects of reduced-impact logging and forest physiognomy on bat populations of lowland Amazonian forest. Journal of Applied Ecology, 45, 14–25. , , & (
- 2006) The effect of tapping for frankincense on sexual reproduction in Boswellia papyrifera. Journal of Applied Ecology, 43, 1188–1195. , , & (
- 2008) Combined effects of fisheries and climate on a migratory long-lived marine predator. Journal of Applied Ecology, 45, 4–13. , & (
- 2008) Rethinking the interface between ecology and society. The case of the cockle controversy in the Dutch Wadden Sea. Journal of Applied Ecology, 45, 82–90. & (
- 2007) Quantifying the impact of longline fisheries on adult survival in the black-footed albatross. Journal of Applied Ecology, 44, 942–952. , , , & (
- 2007) The relative effects of raptor predation and shooting on overwinter mortality of grey partridges in the United Kingdom. Journal of Applied Ecology, 44, 972–982. , , & (
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