What is a natural freshwater ecosystem: invasions, aliens and novel ecosystems

Derwent Water in the English Lake District: the last remaining native habit of the vendace (Coregonus vandesius) which is predated by introduced populations of the non-native ruffe (image: wikicommons)

At first glance, defining a ‘natural’ ecosystem seems relatively straightforward.   However, on closer inspection, an understanding of the ecological, climatic and cultural histories of a landscape muddies the waters of identifying a set ‘natural’ ecosystem state.  Global ecosystems are in constant flux, influenced by both natural processes and human influence over thousands of years, which makes defining a natural state for their conservation or restoration difficult.

Writing in Freshwater Biology in January, David Strayer discusses the effects of biological invasions of ‘alien’ species into global freshwaters, creating new and ‘novel’ ecosystems.  Such invasions are becoming increasingly common, with species spread in ship ballast water (e.g. the round goby), through the aquatic trade (e.g. Canadian pondweed), deliberate introductions, and angler’s bait buckets (e.g. the introduction of ruffe in Scottish lochs).

Zebra mussels (image: wikicommons)

There are numerous economic, environmental and cultural services and benefits resulting from the conservation of biodiverse and distinctive freshwater ecosystems – see this short interview with BioFresh partner Dr. William Darwell for more details.   Allowing biological invasions to occur unchecked may lead to ecosystems becoming dominated by highly adaptable ‘generalists’ like the zebra mussel or the round goby, potentially at the expense of ecosystem diversity and function (dubbed the “Homogecene”).

Round goby (image: wikicommons)

Given that current ecological thought stresses how ecosystems are in a constant state of flux, it could be argued that we accept biological invasions as natural processes thathave occurred countless times in the past.  As Stephen Jay Gould describes, on an evolutionary timescale species are only native to a particular area through a series of chance events, and aren’t necessarily better adapted to their habitat than ‘non-native’ species.  However, the issue isn’t that biological invasions are still occurring – it is that their rate and magnitude is being significantly increased by human influence.

Returning then to the idea of what constitutes a ‘natural’ ecosystem, Strayer’s paper raises a number of important points.

The composition of global ecosystems are being increasingly reshuffled and altered.    Any attempt to conserve or restore a ‘natural’ ecosystem must be aware of these processes.     Strayer’s research suggests that freshwater ecosystems will continue to be flooded with new invasive species in the future, creating new ecosystem assemblages with no historical equivalent to guide their management.  His conclusion states “the best solution to the management problems caused by alien species will be to work aggressively to cut the arrival rates of new invaders”.

Canadian pondweed (image: wikicommons)

For Richard Hobbs and colleagues, biological invasions make us re-assess a stark distinction between ‘natural’ and ‘non-natural’ ecosystems – moving away from an over reliance on excluding alien species at all costs, instead focussing on appropriately managing emerging ecosystems .  This may seem like a defeatist approach, but as Hobbs and colleagues suggest, we are faced with an unprecedented rate of ecosystem change, making exclusion of new invasive species extremely difficult.

Responding to the comments on a draft of their paper, they conclude dryly that:

“one reviewer commented that the examples[of novel ecosystems] are ecological disasters, where biodiversity has been decimated and ecosystem functions are in tatters, and that ‘it is hard to make lemonade out of these lemons’.

Our point is, however, that we are heading towards a situation where there are more lemons than lemonade, and we need to recognize this and determine what to do with the lemons”

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How far do biological invasions make us re-assess what we term a ‘natural’ ecosystem?

Do you agree with Hobbs and colleagues that we should focus our conservation efforts on appropriate management of new and novel ecosystems rather than relying solely on excluding new invaders?

As ever, we welcome your thoughts.

More information:

• Richard Hobbs and colleagues on the prospect for new and ‘novel’ ecosystems
• Petr Pysek and colleagues discuss the role of different pressures on ecosystem invasions in Europe
• Free access Ecology and Society article: Fryirs and Brierley (2009) “Naturalness and place in river rehabilitation”
• DAISIE: Delivering Alien Invasive Species Inventories for Europe
• IUCN / SSC: Invasive Species Specialist Group