Caddisflies of the Yukon - Department of Biological Sciences ...

Caddisflies of the Yukon 823
was much larger than Nearctic Beringia and also extended to lower latitudes; therefore, it
would have supported more species of plants and animals, including Trichoptera (see under
Ecological Considerations). By contrast, glaciation in North America isolated the Alaska –
Yukon peninsula, largely if not entirely closing off exchange of species with lower latitudes
(above). Consequently, the 2 species pools must have been imbalanced when the Bering land
bridge was exposed. We infer that whatever dispersal followed the opening of the Bering
land bridge would necessarily have been weighted in favour of a Palaearctic-to-Nearctic
movement; in a group-to-group comparison, not only would there have been more species
in Palaearctic Beringia, but also there probably would have been more vacant ecological
niches in Nearctic Beringia because of extinctions caused by glaciation. For mammalian
species, predominantly eastward dispersal across the Bering land bridge has been inferred
from evidence for the Pleistocene, but movement at about the same level in each direction
during the Pliocene (Vermeij 1991). These conclusions are consistent with the concept that
dissimilar patterns of glaciation in North America and Asia are correlated with asymmetric
interchange of species. Moreover, global analysis of past biotic interchanges leads to the
general conclusion that the resistance of a biota to invasion is reduced by previous extinction
of species in that biota (Vermeij 1991).
Superimposed upon these general factors influencing the interchange of animals and
plants between North America and Asia are the special requirements of aquatic insects. Only
the Odonata have the capacity for active dispersal over longer distances between the aquatic
habitats of larvae. Adult Trichoptera are subject at best to passive dispersal by strong winds;
even then, ovipositing females would have to be relocated in the vicinity of appropriate
aquatic habitats. Dispersal by wind over the narrow sea barrier of the present-day Bering
Strait seems at least as likely as dispersal across long distances overland; but there is little
supporting evidence for successful dispersal across Bering Strait, nor is there evidence for
post-Pleistocene intercontinental interchange of Trichoptera by way of the Aleutian Island
chain (e.g. Karlstrom and Ball 1969). Even so, several species of Trichoptera evidently did
reach Greenland by dispersal at high latitudes over sea barriers from distant glacial refugia
(see category I, Greenland Trichoptera). Beringia during much of the Pleistocene is depicted
as arid and steppe-like with the muskeg and wet tundra, now so typical, greatly diminished
in extent (Schweger et al. 1982). Thus, Nearctic Beringia, the corridor between Asia and
North America, would seem to have provided a different mix of habitats and probably more
restricted habitats for aquatic insects than are now available. Moreover, the glacier-fed water
courses were probably similar to glacial streams of today that support few aquatic insects
because they carry heavy loads of suspended glacial debris which are deposited to form
unstable and largely unproductive substrates, shifted by braided channels as the volume of
meltwater fluctuates. Unglaciated upland parts of Beringia, however, probably had streams
that were similar to present-day upland streams in tundra regions. These streams would have
joined the lowland river systems that traversed the exposed coastal shelf (Fig. 16), forming
a network of freshwater systems that is believed to have facilitated an exchange of fish
species between Asia and North America (Lindsey and McPhail 1986). Because of the high
biological diversity of Trichoptera, however, the availability of freshwater habitats across
Beringia is only one aspect of the issue; another aspect is the extent to which these freshwater
habitats coincided with the requirements of the species that had access to them. These issues
are explored under Ecological Considerations.
We infer that the Bering land bridge of Pleistocene time would have been a difficult
passage for many aquatic insects because the cold dry climate was not well suited to dispersal
of the adult insects, and because freshwater habitats would have been localized, unsuitable,