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island located in western Lake Superior, about 22 km from the
Minnesota/Ontario shoreline. The island and surrounding islets and waters comprise Isle Royale
National Park. The Canada lynx (Lynx canadensis) is a 7-15 kg wildcat indigenous to the boreal
forests of northern North America. The species is listed as threatened in the contiguous 48 states per
the Endangered Species Act. It is widely believed that Canada lynx historically occupied Isle
Royale, but have been essentially absent since the 1930s. The island supports a prey base—
specifically, snowshoe hare (Lepus americanus)—that might support a lynx population. As directed
by National Park Service policies, we evaluated the ecological feasibility of reintroducing Canada
lynx to Isle Royale.
Lynx were present on Isle Royale, apparently in great abundance, when explorers and settlers of
European descent first reached the island. A harvest of lynx was underway by the 1890s. Reported
harvests include a minimum of 48 lynx taken in the winter of 1903-04 and 67 lynx in the winter of
1916-17. We conducted a Population Viability Analysis (PVA) of the reported and assumed harvest
rates from the 1890s-1930s. Although there are several possible reasons for the extirpation of lynx in
the 1930s, we conclude that over-harvest was likely the primary cause. There were periodic reports
of lynx on Isle Royale from the 1960-80s; however, those animals were likely immigrants from the
mainland that did not persist. There is no evidence of a resident population since the 1930s.
Based on known snowshoe hare densities at Isle Royale, reported lynx densities in Minnesota and
Ontario, and other scientific information we concluded that Isle Royale can support about 30 lynx on
average. However, we acknowledge that the mean carrying capacity might be substantially more or
less than 30 lynx due in part to the unique characteristics of Isle Royale, so we evaluated long-term
viability across a range of population sizes. Furthermore, we varied the carrying capacity over time
as Canada lynx populations in northern regions often go through a 10-year cycle in abundance. For
example, our baseline carrying capacity of 30 lynx fluctuated from 15 to 45 animals every decade.
However, some southern lynx populations show little if any cyclicity so we also evaluated noncyclical
We conducted a PVA of a potential lynx reintroduction to assess the persistence of the population.
When we parameterized the PVA with demographic rates reported from mainland studies our
cyclical model indicated a 0.36 chance of population persistence for 100 years. A non-cyclical
model indicated a 0.73 probability of 100-year persistence. A sensitivity analysis of the model
indicated that assumed levels of inbreeding depression had a significant impact on long-term
viability. For example, when we disabled inbreeding depression the probability of persistence
increased to 0.96 and 1.00 for the cyclical and non-cyclical scenarios, respectively. When we
modeled the addition of 2 unrelated lynx into the population every 10 years the probability of
persistence increased to ≥ 0.98 for both models, even when inbreeding depression was enabled. In
historical times lynx might have immigrated to Isle Royale every 10 years or so via an ice-bridge
connecting the mainland to the island. However, we found that the probability of ice-bridge
formation in a winter has declined from 0.8 in 1959 to 0.1 in 2013, apparently due to climate change.
As a result, to mitigate for the loss of natural ice-bridge connectivity managers might have to
periodically introduce new lynx to the island to avoid inbreeding depression.
However, our baseline models might underestimate lynx viability on Isle Royale because the
mortality rates used in our models came from mainland lynx populations exposed to human harvest,
vehicle collisions, and other anthropogenic mortality. Those mortality factors would not occur at Isle
Royale. When we excluded anthropogenic mortality the 100-year persistence at Isle Royale
increased to ≥ 0.99 for both the cyclical and non-cyclical models, even without augmentation and
with inbreeding depression enabled. Furthermore, Isle Royale has other conditions beneficial for
lynx that could not be reasonably incorporated into our models, such as the absence of competitors
like coyotes (Canis latrans).
Conversely, climate change could deteriorate future conditions for lynx on Isle Royale. We did not
parameterize climate change into our PVA due to the very speculative nature of the phenomenon;
however, other studies have conducted region-wide analyses of climate change impacts on lynx and
they indicate that the 100-year changes at Isle Royale would be minor. We therefore conclude that a
lynx reintroduction to Isle Royale would likely be successful assuming appropriate management;
however, due to the unique characteristics of the island and model uncertainties a lynx reintroduction
to Isle Royale should be accompanied by a rigorous monitoring and adaptive management program.
Our PVA suggests that an initial release of 20 lynx would be most efficient as larger releases showed
a negligible improvement in persistence. Released animals should come from southern Ontario as
that population is not listed as threatened and the region has a similar habitat type and prey base to
Isle Royale. The initial release, and subsequent augmentations should they be deemed necessary,
would likely be most effective if they occurred at the ascending phase of the 10-year lynx-snowshoe
hare cycle. The next such period would be around 2018-21. However, source lynx might be most
available during the next population peak in Ontario, projected to be around 2021-23.
Reintroducing Canada lynx to Isle Royale appears warranted and feasible, especially if a
reintroduction is accompanied with post-release monitoring and adaptive management. There would
likely be little opposition to a lynx reintroduction at Isle Royale due to the protected status of the
island, its isolation from development and commercial land uses, the low risk of impacts to other
park resources, and the charismatic nature of Canada lynx. A reintroduction of lynx to Isle Royale
would greatly contribute to our understanding of lynx ecology and conservation as the island is a
unique laboratory that does not have the anthropogenic mortality factors and altered mammalian
population that compromise many mainland lynx populations. As a result, Isle Royale would
continue its long history and legacy of being a leader in ecological research.
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- Location: Quad Cities, IL