Living with Weeds

Many conservation managers and researchers have pointed out that eradication of invasive species is too expensive, too dangerous, or currently impossible.

In addition, with climate change now rapidly coming upon us, many conservationists are rethinking their approaches to manage for biodiversity and ecosystem processes.

Amongst the expected changes are:

• Changes in the Timing of Seasonal Life-Cycle Events
• Range Shifts
• Food Web Disruptions
• Threshold Effects
• Pathogens, Parasites and Disease
• Extinction Risks.

In addition:

• Climate change can alter where species live and how they interact, which could fundamentally transform current ecosystems.
• Impacts on one species can ripple through the food web and affect many organisms in an ecosystem.
• Mountain and arctic ecosystems and species are particularly sensitive to climate change.
• Projected warming could greatly increase the rate of species extinctions, especially in sensitive regions.

These are discussed more fully by the US EPA (1). See also the Climate Institute’s: Invasive Species & Climate Change (2).

The key messages from C.S.I.R.O. Research (3) are that:

• Ecological change will be widespread and potentially very significant
• Biodiversity will be affected by climate change in many different ways
• There will be much spatial variation in ecological change

The NSW Department of Environment (4) points out that:

Current climate changes are more serious than previously due to the rate of change in atmospheric greenhouse gas levels and temperatures, and because ecosystems are already stressed by other human impacts.

The most vulnerable ecosystems include coastal ecosystems, alpine areas, rainforests, fragmented terrestrial ecosystems and areas vulnerable to fire or low freshwater availability.

Species that could become endangered or extinct include those living near the upper limit of their temperature range (for example, in alpine regions); those with restricted climatic niches; and those that cannot migrate to new habitats due to habitat fragmentation or lack of alternatives.

Changes in carbon fluxes are also expected: it remains uncertain which species will be favoured by the elevated CO2 concentrations. Increasing aridity in many areas could decrease soil carbon storage and additional loss of soil carbon through our industrial agriculture, forestry, and ‘weed’ management strategies augments global warming.

So how do we maximise biodiversity conservation in this changing environment? How do we view ‘weeds’?

Novel Ecosystems /Conciliation Biology/Retaining weeds in damaged Ecosystem Restoration

Most ecosystems are now sufficiently altered in structure and function to qualify as novel systems, and this recognition should be the starting point for ecosystem management efforts. …. Attempts to return systems to within their historical range of biotic and abiotic characteristics and processes may not be possible, and management activities directed at removing undesirable features of novel ecosystems may perpetuate or create such ecosystems. Management actions should attempt to maintain genetic and species diversity and encourage the biogeochemical characteristics that favor desirable species. (5)

Some invasive species can help ecosystems adapt to global change. Sometimes weed removal may have unforeseen negative consequences or their use in restoration is desirable. The ‘conciliatory approach’ asks what can we do to reduce negative consequences and accentuate the positive. Many researchers have listed the potential conservation values which should be considered before eradication has begun.

Positive values include:

• Invasive species are a functional, structural, and compositional part of ecosystems and may maintain ecological integrity.
• Some invaders can increase ecosystem resilience to climate and land cover changes.
• they can maintain ecosystem processes such as productivity, carbon storage, or nutrient cycling in conditions of climate change or land cover transformations

Zavaleta et al. (6) advise that food web and functional role frameworks be considered and integrated into a holistic process of assessment and restoration to avoid unexpected detrimental ecosystem changes from weed eradication. This is echoed by other researchers when considering the role of weeds in ecosystem processes like erosion (7) or the role that weeds may have in providing resources for threatened species.

D’Antonia et al. (8) advocate consideration of exotic species in the greater context of community structure and succession. This requires an understanding of the potential transience of exotics at a site and the ecological role particular exotics might play in changing processes that influence the course of succession to set removal priorities and develop realistic management goals.

Scott Carroll (9) outlines the need for the retention of novel organisms from an evolutionary viewpoint: they may slow rates of resistance evolution, promote evolution of indigenous biological control, cultivate replacement services and novel functions, and managing native–non-native coevolution.

To sum up, Schlaepfer et al. (10) suggest that a more meaningful definition of an invasive species would be one for which there is a net negative effect.

Rewilding

“Rewilding is large-scale conservation aimed at restoring and protecting natural processes and core wilderness areas, providing connectivity between such areas, and protecting or reintroducing apex predators and keystone species. Rewilding projects may require ecological restoration or wilderness engineering, particularly to restore connectivity between fragmented protected areas, and reintroduction of predators where extirpated.” (Wikipedia)

It’s a term, coined by activist Dave Foreman and broadened by Michael Soulé and Reed Noss in the 1990s. In his TED talk (11) George Monbiot explains ‘rewilding’ has two meanings.

The mass restoration of ecosystems through attention to “trophic cascades” — the ecological processes that start at the top of a food chain and tumble down to the bottom, affecting the entire ecosystem in the process.

The rewilding of places humans live—restoring some of the fauna that we’ve wiped out through hunting and habitat destruction.

In another wonderful TED talk (12) Monbiot outlines the way in which the reintroduction of a top predator, wolves, in Yellowstone National Park not only controlled deer populations and thus vegetation browsing but had major effects on landscape and stream processes – “bare valleys reverted to aspen and willow; birds and beavers alike flourished; beavers’ dams created habitats for otters, muskrats, fish, frogs and reptiles; and on and on. The wolves altered the rivers themselves. The Yellowstone wolves demonstrated that a single species, when allowed to pursue its natural behaviour, can transform an entire ecosystem”.

But what are the limits to rewilding? In Monbiot’s view, it’s not about controlling nature but letting it find its own way. There are a few necessary actions, like reintroducing absent plants and animals and pulling down fences, but in his view rewilding is not a teleological progression, with a correct endpoint or ideal ecosystem. “It lets nature decide,” he says.

An accidental rewilding occurred with the Spanish invasion of Central America when the horse, which had become extinct in America 12,000 years ago, was reintroduced. Species like the Gourd Tree (Crescentia alata), with large hard fruits, had lost their dispersal agent with the extinction – the fruit rotted where it fell. With the reintroduction, the fruit were again more widely distributed (13).

[PHOTO of Crescentia fruit??]

Janzen and Martin (14) list the traits of fruit distributed by megafauna. There are echoes in Australia where, the Endangered Small-leaved Tamarind (Diploglottis campbellii) has a very restricted natural distribution in northern NSW. According to the NSW Office of Environment and Heritage the dispersal mechanisms of its large seed are unclear. However it is possible that this too is a ghost of evolution. Fossil evidence from the Pleistocene shows that the now extinct cassowary (Casuarius lydekkeri) once extended far south of the present range of North Queensland Cassawary (Casuarius casuarius) to the Wellington Valley of New South Wales (15, 16).

The autumn 2016 edition of Nature (Vol 60, no.1) illustrates successful rewilding projects and some of the concerns involved in Europe, on oceanic islands, in Australia.

Managed Relocation/ Assisted Migration

Intentional moving of species threatened by climate change is actively being discussed as a conservation approach. Hallfors et al. (17) have usefully distinguished ‘assisted migration’ from other translocations through three aspects:

1. It is directional and based on a prediction of the potential future distribution of the biological unit;
2. It is limited to translocations as a way to overcome temporal or spatial dispersal limitations; and
3. It is used to mitigate threat caused directly or indirectly by anthropogenic climate change.

“Significant controversy has developed around the idea of assisted colonization since it was first put forth in the scientific literature in 2007. At one extreme of the debate are those who argue that human management of species range shifts should be avoided at all costs due to the high risks of colonizing species becoming invasive. At the other extreme are those who believe that the expected benefits of assisted colonization, primarily in terms of prevented extinctions, may often outweigh the risks. ….The science is clear that climate change will drive many species extinct, and a traditional, land-preservation ethic will not prevent extinctions. Those wary of moving species instead suggest expanding networks of habitat corridors, allowing species to naturally migrate into newly suitable areas. Under the rates of climate change projected for the coming decades, however, even perfectly connected habitats will probably be insufficient. Species that cannot naturally keep pace with shifting climates will be at risk regardless of habitat connectivity. Evidence suggests that slowly evolving and slowly dispersing species (including species that are dispersal-limited due to habitat fragmentation) will decline or go extinct in the absence of assisted colonization programs.”

Wikipedia

Emma Marris (18) outlines the growing awareness by both ecologists and citizen-naturalists of range shifts of species and the barriers which some then face. Marris covers some of the dilemmas faced around this issue. John Measey points out that, though there are serious ethical and practical dilemmas,

“There is good reason to consider letting plants and animals remain where they were introduced intentionally or unintentionally, particularly when they are threatened with extinction in their historic native ranges. Monterey pine is an example of a tree that has been planted outside its native range that is now threatened with extinction in its native range. Ironically, the fossil record in California where it is now eradicated proves that Monterey pine was once more widely dispersed.” (19).

The International Union for the Conservation of Nature (IUCN) provides best practice guidelines for introductions of threatened species for conservation both within their previous range and outside it. (20)

Learn to like/love your weeds

“In the end, we really have little choice. Climate influences and species introductions will continue into the foreseeable future no matter what humans decide to do or not do. We will continue to devote considerable resources to try to protect our ecosystem services from threats from particularly harmful invasive species. However, in many instances, rather than trying to manage the endless introductions, we would do better to try to manage our attitudes toward them. Although we do have some ability to shape the direction and combined impacts of climate and species introductions, ultimately, like other species, we are going to have to adapt and accommodate these changes into our lives, economies, and cultures.” (Mark Davis, 21)

References

1. US EPA websiteThe future of climate change.
2. Szyniszewska, Anna, n.d. Invasive Species & Climate Change. Climate Institute.
3. C.S.I.R.0. website. Implications of climate change for Australia’s biodiversity.
4. NSW Department of Environment website. Climate change impacts on biodiversity.
5. Seastedt, T.R., Hobbs, R.J., Suding, K.N., 2008. Management of novel ecosystems: are novel approaches required? Abstract Front Ecol Environ, 6.
6. Zavaleta, Erika S., Hobbs, Richard J. and Mooney, Harold A.,2001 Viewing invasive species removal in a whole-ecosystem context. TRENDS in Ecology & Evolution Vol.16 No.8.
7. Shafroth, P.B., Cleverly, J.R., Dudley, T.L., Taylor, J.P., Van Riper III, C., Weeds, E.P. Stuart, J.N., 2005. Control of Tamarix in the Western United States: Implications for Water Salvage, Wildlife Use, and Riparian Restoration. Environmental Management, Volume 35, Issue 3, pp 231-246.
8. D’Antonio Carla, and Laura A. Meyerson, 2002. Exotic Plant Species as Problems and Solutions in Ecological Restoration: A Synthesis Restoration Ecology, 10: 703–713.
9. Carroll, Scott P., 2011. Conciliation biology: the eco-evolutionary management of permanently invaded biotic systems. Evolutionary Applications, Blackwell Publishing Ltd., 4 pp 184–199
10. Schlaepfer, M.A., Sax, D.f., Olden, J.D. 2011. The Potential Conservation Value of Non-Native Species. Conservation Biology, Volume 25, No. 3.
11. Monbiot, G.,2013. A walk on the wild side: 7 fascinating experiments in rewilding. TED talk.
13. Monbiot, G.,2014. Video: How wolves can alter the course of rivers. TED talk.
14. Barlow, Connie 2000. The Ghosts of Evolution. Basic Books.
15. Janzen, Daniel H. And Martin, Paul S., 1982. Neotropical Anachronisms: The Fruits the Gomphotheres Ate. Science, Vol 215.(PDF)
16. Miller, A. H., 1962. The history and significance of the fossil Casuarius lydekkeri. Records of the Australian Museum 25(10): 235–238.(PDF)
17. Hallfors, M.H., Vaara, E.M., Hyvarinen, M., Oksanen, M., Schulman, L.E., Siipi, H., Lehvavirta1, S., 2014. Coming to Terms with the Concept of Moving Species Threatened by Climate Change – A Systematic Review of the Terminology and Definitions. PLOS ONE 2014 vol 9, issue 7.
18. Marris, Emma, 2011. Rambunctious Garden: Saving Nature in a Post-Wild World. Bloomsbury USA.
19. Measey, John 2014. Comments on Opinion: its time to stop thinking that all non-natives species are evil by Emma Marris, National Geographic
20. International Union for the Conservation of Nature 2012. IUCN Guidelines for Reintroductions and Other Conservation Translocations.(PDF)
21. Davis, M.A. 2013. Climate Vulnerability: Understanding and Addressing Threats to Essential Resources. (Ed. In chief R.A.Pielke Sr.) Invasive Plants and Animal Species: Threats to Ecosystem Services, Ch. 4, Academic Press, Elsevier Inc.

Follow