4.1 Reasons for decline
It is unclear whether the apparent declines in muskrat abundance that we
have demonstrated are a result of a broad-scale underlying cause or
site-specific factors.
In recent studies of muskrat occurrences in Lake Ontario-St. Lawrence
River coastal wetlands it was found that water-level regulation of that
system had a negative influence on muskrat abundance (Greenhorn et al.,
2017; Toner et al., 2010). While the two Great Lakes that our study
sites occur on are not subject to such tightly controlled water levels
as Lake Ontario, both Lake Erie and Lake Huron have experienced record
high and extended low water levels over the duration that our
investigation spans (NOAA, 2020), primarily brought on by weather
extremes and climate variability over the past few decades (Gronewold
and Rood, 2019). The presence of water, either through excessively high
water levels or periods of drought, has been considered the greatest
selection pressure affecting muskrats (Bellrose and Brown, 1941;
Errington, 1963; Virgl and Messier, 1996; Proulx and Gilbert, 1983;
Ahlers et al., 2015; Ward and Gorelick, 2018). Essentially, the water
level in a marsh is suitable for muskrats when it is deep enough to
maintain travel routes and allow access to houses and feeding areas
underwater, even in cold winters when ice cover can be very thick, yet
shallow enough to permit the growth of emergent aquatic vegetation. It
is also important for muskrats that any changes in water level occur
gradually, so that houses are neither flooded out nor left high and dry.
We have not conducted a detailed analysis of water level patterns at
Pelee and Matchedash in relation to muskrat occupancy, therefore this is
an avenue that warrants closer examination.
Closely related to water levels is the hypothesis that habitat change
has led to dramatic reductions in muskrat numbers. For example, Ward and
Gorelick (2018) suggested that the loss of critical wetland habitat due
to drying is the primary driver responsible for the decline of muskrats
in the Peace-Athabasca Delta over the past half-century and potentially
across their entire native range. While the amount of wetland habitat on
the landscape available to muskrats is certainly an important factor
influencing their population levels, the sizes of the wetlands in our
study area have not changed appreciably over the timespan of our
investigation. Province-wide, Ontario has experienced significant loss
of wetlands over the past two centuries, but most of this loss (70% of
original wetland area) occurred between the time of European settlement
(circa late 1700s) and 1982 (Ontario Biodiversity Council, 2010). Only a
relatively small additional amount of wetland area loss (3.8%) occurred
between 1982 and 2014 (Ontario Biodiversity Council, 2015). Thus, it is
clear that the vast majority of wetland loss in Ontario occurred prior
to the recent period of muskrat decline that we have demonstrated.
Therefore, it seems unlikely that a loss of habitat area is a major
factor driving the recent declines we have seen in muskrat populations
in Ontario over the past 30-50 years.
A more likely factor responsible for muskrat decline in Ontario is a
change in the overall quality of muskrat habitat, particularly in the
structure and composition of wetlands that were previously occupied in
large numbers by muskrats. Proulx and Gilbert (1983) demonstrated that
muskrats prefer to occupy areas of marsh with a 1:1 interspersion of
emergent vegetation and open water, a structural pattern of habitat
patchiness often referred to as a “hemi-marsh”. Yet there is evidence
around the Great Lakes basin that the structural diversity of wetlands
has declined. The research of Wilcox et al. (2008) has shown that Lake
Ontario coastal wetlands have experienced a significant decrease in
emergent marsh habitat heterogeneity and the amount of emergent-open
water edge in the past 50 years since regional water level control was
implemented. At the Point Pelee Marsh, one of our two study sites,
Markle et al. (2018) demonstrated that both marsh habitat diversity and
open water connectivity have declined since 1931, with the most
significant reductions occurring between 1959 and 2015. Similarly, at
our other study site, the Matchedash Bay-Gray Marsh, air photo
interpretations by Taylor et al. (2015) demonstrate a large increase in
the areal coverage of dense emergent vegetation along with a
correspondingly large decrease in the extent of aquatic habitat between
the years of 1973 and 2008. These reductions in habitat structural
quality coincide with the time periods over which we have demonstrated a
decline in muskrat abundance at both sites.
The research of Markle et al. (2018), Wilcox et al. (2008) and others
(e.g., Farrell et al., 2010, Wilcox et al., 2003) also demonstrate
changes to the compositional quality of Great Lakes coastal wetlands
over the past 50 years that coincide with the decline in muskrats that
we report here. Not only have they demonstrated class-level habitat
changes, such as the expansion of dense emergent cattail marshes at the
expense of meadow marsh and other shallow mixed marsh communities, but
they have documented dramatic increases in the prevalence of invasive
emergent marsh species such Phragmites australis subsp.
australis, Typha angustifolia and Typha X glauca . In particular,
the hybrid species of cattail (Typha X glauca) has increased
significantly in extent, displacing both the native broad-leaved cattail
(Typha latifolia ) and other aquatic plant species, and is now the
dominant emergent species in most of our coastal wetlands. How such
changes in both the structure and composition of coastal wetlands may be
driving muskrat decline is a question that deserves further exploration.
Several other explanations for the decline in muskrat populations are
possible but seem less likely for a variety of reasons. Overharvest by
trappers is often postulated as the reason behind furbearer population
declines, however the demographic characteristics of muskrats makes them
resilient to overharvest (Boutin and Birkenholz, 1987). Even when heavy
harvest does occur in a wetland, the muskrat’s high reproductive output
and dispersal ability usually enables a population to rebound relatively
fast. As well, one of our study sites (Pelee) has been closed to muskrat
trapping since 1959, therefore muskrat harvest levels have had no direct
influence on the population dynamics there for at least 50 years. At our
other study site, local long-time trappers have reported low harvests
and great difficulty finding muskrats for many years now (M. Dunlop,
personal communication, March 2014).
Predation and disease are important natural aspects of muskrat ecology
that can affect their abundance over the short-term, but which typically
balance out over the long-term. Errington (1963) suggested that
significant mortality of muskrats due to predation usually only occurs
when muskrats are already vulnerable as a result of disease or habitat
changes such as loss of cover, drought/flooding, or freeze-outs. As
well, muskrat populations typically respond to predation mortality by a
variety of compensatory processes, such as increased reproduction and
survival among the remaining individuals (Errington, 1951). Mink
(Neovison vison ) are considered the most important predator of
muskrats (McDonnell, 1983), but there is evidence from harvest data of
long-term mink population decline in Ontario (Gorman, 2007) and no
evidence of local mink population booms at our sites (personal
observations). Disease outbreaks among muskrats are typically localized
and usually occur when a muskrat population is under some form of
stress, such as overcrowding, food shortage or drought (Errington,
1963). Also, as with predation, the effects of disease on muskrat
population size are often short-term because of compensatory responses
in other sources of mortality or in reproduction. We are unaware of any
documented outbreaks of disease in muskrats at either of our study sites
or elsewhere in Ontario over the time period that our study covers
(Ganoe et al., 2020).