[Taxacom] a case of dispersal vs "everything everywhere"

Fri May 9 09:35:01 CDT 2014

-------- Original Message --------
Subject: [Aliens-L] Re Was didymo here all along?
Date: Fri, 9 May 2014 08:13:51 +0000
From: Shyama Pagad <s.pagad at auckland.ac.nz>
To: Aliens-L <aliens-l at list.auckland.ac.nz>

   Was didymo here all along?

By Jamie Morton
<http://www.nzherald.co.nz/jamie-morton/news/headlines.cfm?a_id=729>

New Zealand Herald

A new study by North American researchers has posed the suggestion that
river-fouling didymo may not have been introduced to New Zealand, but
was here all along. But New Zealand scientists have cast doubt on the
assertion, and back the widely-held view that the troublesome algae was
brought here. The microscopic pest, which can be spread by a single drop
of water, was first reported in New Zealand 10 years ago, in the Lower
Wairau River, and is now found in more than 150 South Island rivers.
So-called "rock snot" is especially worrisome in trout rivers because it
affects the insects they eat, and under New Zealand law, people are
legally obliged to prevent the spreading of didymo.

A study, led by Dartmouth College and published in the journal
BioScience, suggested the algae had been native to much of the world for
thousands of years, but conditions promoting visible growths were absent
or rare.

It suggested recent worldwide blooms of didymo are caused by a native
species responding to changing environmental conditions rather than by
accidental introductions by fishermen, or the emergence of a new genetic
strain, as has been widely believed. "Nuisance blooms have been reported
in rivers worldwide and have been hastily attributed to introductions,"
the researchers state. "However, evidence indicates that blooms are
probably not caused by introductions but, rather, by environmental
conditions that promote excessive stalk production by this historically
rare species."

Didymo blooms were hastily attributed to human introductions or the
emergence of new genetic strain because the absence of evidence was used
as evidence of absence in many locations, the study's lead author
Professor Brad Taylor said. "Even in locations where rock snot had been
recorded a century ago, this information was either ignored or the idea
of a new genetic strain was adopted."

Algal blooms are often caused by excessive phosphorus and other nutrient
inputs, but didymo bloomed because phosphorus was low. When nutrients
was rare, the algae produced long stalks that extended the cell into the
water above to access nutrients, creating thick mats covering the river
bottom. "The paradox of didymo blooms in low-nutrient rivers is not
really a paradox at all," Professor Taylor said.

"However, the idea that low phosphorus can cause an algal bloom is hard
for people to accept because we are all taught that more nutrients equal
more algae."

The study explained that other algae and bacteria responded similarly to
low nutrients, but rock snot blooms were unprecedented, making this
organism a good sentinel of what could be the new norm in many pristine
rivers worldwide. The new research suggested rock snot blooms had become
more common because of climate change and other human-caused
environmental changes that were decreasing phosphorus to levels that
promote the formation of didymo blooms in many remote, otherwise
pristine rivers worldwide.

New Zealand scientists have read the new findings with interest, but
were not convinced the pest had naturally evolved here. NIWA freshwater
ecocolgist Dr Cathy Kilroy said a response to environmental change was a
"highly likely" explanation for the expansion of didymo blooms in the
Northern Hemisphere. But she believed that in New Zealand, didymo was an
introduced pest. Research in New Zealand demonstrated that that didymo
blooms were caused by low dissolved phosphorus concentrations, she said.
But while the new study referred to processes that could and did lead to
declining phosphorus in North American streams, there was no evidence
that those processes apply to New Zealand streams, Dr Kilroy said.
"There has been no sign of a general decline in phosphorus in rivers,
even in undeveloped catchments." The South Island rivers presently
affected by didymo had very low phosphorus long before the discovery of
the first didymo blooms in 2004, and the subsequent rapid spread of
didymo in the South Island indicated transport by humans. "My view is
that the expansion of blooms in the Northern Hemisphere starting in the
late 1980s meant that didymo became much more common; ever increasing
international travel meant that it was inevitable that cells would
eventually find their way to New Zealand. "Once the cells were here, the
many low-phosphorus waterways in South Island provided perfect habitat
for blooms." She agreed that high phosphorus levels explained why no
didymo blooms had occurred in any North Island river. "Failure to detect
even a single didymo cell - despite continued surveillance - is more
puzzling," she said. "It seems likely that a combination of
environmental factors, such as temperature and water chemistry, prevent
the long-term survival of didymo in North Island waters."

Dr Marc Shallenberg, a zoology research fellow at the University of
Otago, said as didymo hadn't been previously reported from New Zealand
waters, it was assumed that didymo invaded the country and rapidly
proliferated, as many other invasive exotic organisms have done. "The
take home message of the article seems to be that environmental
conditions - and possibly recent changes in these conditions - play an
important role in blooms of didymo and in the proliferation of species,"
he said. "This is a sensible message and won't ruffle feathers with
scientists. "However, the authors have written their paper in a way that
appears to suggest that didymo blooms only occur in response to changes
to environmental conditions and not at all to recent invasions." While
their attempt to downplay the importance of invasion biology was
provocative, it was not convincing, he said.

Landcare Research phycologist Dr Phil Novis said the study's assertion
that didymo was not a recent introduction to New Zealand was
"problematic". "In my view, this is vulnerable to two potential errors:
treating the dispersal of didymo like that of any other microbe, and
treating New Zealand, with its isolation and human history, as a
continental country." Dr Novis said microbial biogeography - the study
of distribution patterns of microbial species - was a minefield because
it was impossible to prove conclusively that a microscopic species was
not present in a given country. "However, didymo is unusual. It is a
very large, easily recognised diatom, and paradoxically it is known to
be very sensitive to drying. "This sensitivity precludes the
long-distance dispersal that would have been required to transfer the
species to New Zealand in pre-human times.

"The authors concede that didymo has been known for some time in areas
where it does not bloom. "If it was not overlooked in those areas, why
so in New Zealand, when widespread and extensive sampling of river
periphyton has taken place here for decades?"

Dr Susie Wood, a senior scientist at the Nelson-based Cawthron
Institute, described the study as a "compelling article" but also
believed that didymo was a recent introduction. While it had been
demonstrated in New Zealand that not all problematic species are
necessarily new introductions, research by multiple agencies across
suggested this is not the case with didymo in New Zealand, she said. "As
the authors state - there is no fossil or confirmed historical records
of didymo in New Zealand. "Whilst the recent blooms in the Northern
Hemisphere might be a sign of environmental change rather than a new
introduction, I still believe that the evidence from Zealand, including
the way in which it spread across South Island rivers, suggests that it
was a recent introduction."

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Shyama Pagad
Program Manager
Invasive Species Information Management
University of Auckland

Program Officer
IUCN SSC Invasive Species Specialist Group

Tel +64 9 3737599 (X88624) DDI +64 9 9238624   Fax +64 9 3737042
email: s.pagad at auckland.ac.nz
skype: shyama.pagad

www.issg.org/
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The IUCN SSC Invasive Species Specialist Group aims to reduce threats
to natural ecosystems and the native species they contain by increasing
awareness of the impacts of invasive species; and of ways to prevent
their spread and, control or eradicate them.
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