The Dragonflies
(Insecta: Odonata)
of the Columbia Basin, British Columbia:
Field Surveys, Collections Development and Public Education
The Effects of Human Activity on Dragonfly
Populations
Most changes in species occurrence and abundance
have not been documented in the Columbia Basin, and their causes
can usually only be assumed -- and we assume that most changes are
the result of human activities.
The most serious historic anthropogenic stress on
dragonfly populations has been the alteration of their freshwater
habitats. Most destructive has been the draining and filling of
marshes. This has not eliminated any species from the region, but
it has surely reduced the populations of many species, including
Enallagma boreale, E. cyathigerum, E. ebrium, Ischnura cervula,
I. perparva, Aeshna californica, A. constricta, A. interrupta, A.
multicolor, A. canadensis, Somatochlora semicircularis, Libellula
forensis, L. pulchella, Leucorrhinia intacta, Sympetrum corruptum,
S. costiferum, S. danae, S. internum, S. madidum, S. obtrusum, S.
occidentale, S. pallipes, S. vicinum and others.
Large hydroelectric and flood control dams have flooded
102,000 hectares of wetlands in the Columbia Basin, including many
wetlands along the Columbia, Canoe, Pend d'Oreille and Kootenay
rivers (Cannings and Cannings 1996). These reservoirs have probably
eliminated populations of almost all dragonfly species on the Basin
list. Dams not only inundate wetlands, lakes and streams upstream,
but completely alter the natural flow regime downstream. Since Montana's
Libby Dam eliminated the spring freshet of the Kootenay River through
the Creston Valley, the large marshes that remain along the river
are now artificially maintained in a series of diked impoundments,
and their insect communities are undoubtedly different from those
before the dam was built. The modification of the shores of the
Kootenay and Columbia rivers has eliminated much habitat for Argia
emma, Ophiogomphus occidentis and Macromia magnifica.
Many smaller, high elevation dams built to supply
water to lowland communities have flooded peatlands, ponds, shallow
lakes and slow streams, creating larger lake habitat and causing
a reduction in odonate diversity. This habitat loss has eliminated
populations of dragonflies such as Coenagrion interrogatum, Aeshna
sitchensis, A. subarctica, Somatochlora minor, S. franklini
and many others, and -- assuming shorelines lack extensive marshy
or peatland edges -- replaced them with a few species characteristic
of montane lakeshores: Aeshna eremita, A. umbrosa and Somatochlora
albicincta.
Fish are major predators of odonate larvae (Corbet
1962, Hilton 1987), and the energetic programs to release sport
fish into almost 2,000 lakes in the Montane Cordillera (many originally
fish-free) (S. Billings, pers. comm.) must have had a significant
effect on both the abundance of Odonata and the composition of the
communities in these lakes. No data is available to document this
supposition but, in an experimental study in North Carolina, ten
times as many dragonfly larvae were found within fish exclusion
cages as outside them (Morin 1984). The poisoning of aquatic communities
to prepare lakes for sport fish introductions also has likely had
a significant impact on dragonfly populations.
The aquatic communities of many systems that historically
contained fish have also been altered by the purposeful or accidental
introduction of non-native fish species. In the Columbia watershed,
16 species, a full 37% of the entire fish fauna, are introduced.
Some of these species not only eat many odonate larvae, but also
alter the habitat structure. Carp (Carpinus carpio), introduced
into the region in the early 1900s, subsequently destroyed or reduced
much of the native aquatic vegetation, including Lemna and
Potamogeton (Brooks 1973).
Destruction of natural lakeshore, especially in the
southern valleys, for the development of housing and swimming beaches,
has reduced habitat available for many lake dwellers, including
Gomphus graslinellus, Ophiogomphus occidentis and Macromia
magnifica.
Many species breeding in small, often temporary ponds
or spring-fed streams in grasslands and dry forest have been adversely
affected by cattle that trample and pollute these habitats. Argia
vivida is especially vulnerable to these effects; outside of
hotsprings, it only lives in a few tiny spring-fed streams, all
of which are potentially affected by the activities of livestock.
Hot springs are almost always modified by humans.
In the Columbia Basin Argia vivida is largely restricted
to the outlet streams of hot springs and the small populations are
vulnerable. The species has almost certainly been extirpated from
some of the developed springs around Fairmont and Radium, although
it still occurs where warm streams continue to flow from the springs,
for example, at Nakusp and Albert Canyon.
Although there is little evidence to support the
assumption, the extensive logging that has affected hundreds of
streams in the Basin has probably reduced the populations of dragonflies
such as Ophiogomphus occidentis, O. severus, Cordulegaster
dorsalis, Somatochlora minor, S. walshii and perhaps
even the rare Calopteryx aequabilis and Stylurus olivaceus.
Logging and associated road building can result in streams with
unstable flows, warmer water temperatures and higher silt loads,
all of which negatively affect dragonfly larvae. Logging has also
likely affected the community structure in peatlands, marshes and
lakes, especially at higher elevations.
Climate change will drastically affect present
Odonata distributions as lowland waters dry and water in general
becomes scarcer (Hebda 1997). Hebda (1995) outlines the characteristics
of several British Columbia Interior localities in the drier and
warmer periods that prevailed between 10,000 and 7,000 years ago.
A significant component of the diversity of Odonata in the Columbia
Basin lives in grassland ponds and marshes and small lakes in the
lowlands. Presumably, many of these habitats will disappear in any
drying trend. We do not know if dragonfly populations will be able
to shift northwards or to higher altitudes if suitable water bodies
develop there, but the relatively strong powers of dispersal of
many species, should be a major factor in their survival. Similar
problems will probably exist when climate change affects the extensive
dragonfly populations in montane and northern peatlands.
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