Middleton
Island is currently inhabited by an introduced European feral Rabbit
Oryctolagus cuniculus population, which
originates from four domestic rabbits (3 females and 1 male) that were purposely
liberated on the island in the fall of 1954. An earlier (but unsuccessful)
introduction of feral rabbits took place during the fox farm era in the
beginning of the twentieth century. An interesting account of the first few
years of the successful establishment of the rabbits on Middleton has been
written by
O'Farrell (1965). In
the present these rabbits are the only land mammals on the island and during my
four visits between 2005 and 2010 they were abundant.
This blog entry presents and discusses a few of
the findings of a study that focuses on the effects the introduced rabbits have
on the remaining wildlife of Middleton Island. It was initiated by me in 2009.
Armand Biron (720th A.C.&W. Squadron) holding one of the
earliest of Middleton’s rabbits (photo: Armand Biron, 1960).
Introductions
of non-native species into island ecosystems are known to be a reason for
concern. The presence of Arctic Foxes on Middleton Island between 1890 and the
1930’s, for example, certainly was disastrous for the island’s wildlife, as it
clearly reduced the possibilities for many species. The effects of fox
introductions to Alaskan islands have been studied before (see i.e. Bailey 1993
for a review) and eradication projects, that have been taken place as part of
wildlife conservation and habitat restoration, have been very successful. Thus far, however, the effects of rabbit introductions on Alaskan islands have
been poorly studied. Currently, introduced rabbits occur only on a few Alaskan
islands, as most other introduction attempts in the past have been
unsuccessful. Only recently have people started to eradicate introduced rabbits
from an Alaskan island as an ideological way of nature conservation (
http://alaskamaritime.fws.gov/rabbits.htm).
This action’s proposal consisted of not a single scientific field study from
the relevant area, but was largely based on references from the southern
hemisphere (the other side of the world…).
The reason
for me to investigate Middleton’s rabbit population has been that, during my
first few visits, I did not notice many negative effects of their presence on
the remaining wildlife. Instead, I noticed several what seem to be some positive
effects of their occurrence on the island. This made me very curious.
One of the
things I wanted to find out was how many rabbits were present on Middleton
during my stays. For this I conducted regular counts throughout the summer
(early April – end of August) in all of the different habitat types that can be
distinguished on Middleton, at different times of the day. The rabbits produce
litters usually from the end of April until the beginning of August, but their
timing depends on climatic conditions and can vary notably.
Preliminary
results indicate that in 2009 (after the species dealt with a relatively mild
winter) their population contained about 3000 individuals at the start of the
summer, and after 2 - 3 litters about 12,000 individuals inhabited the island by
the end of August (Van Nus, personal records; results of the following year’s
count (2010) still need to be analyzed). O’Farrell estimated the population to
be between 3,600 and 3,700 rabbits in February 1961 and noted that winter
mortality can be high (O’Farrell 1965). In 2006 and probably to an even greater
extend in 2012, it also seemed that a previous winter had taken its toll,
decreasing rabbit numbers to a large extend and resulting in temporary disappearance
of the rabbits from parts of the island.
I also
wanted to find out more about the interaction of the rabbits with other animals.
For example the influence of the introduced rabbits on the predators: which of
the present species feeds on rabbits and to what extend are these dependant on
them? And what other prey items do these particular predators feed on? For this,
I conducted several diet studies among the main predatory species, of which the
Bald Eagle Haliaeetus leucocephalus and
the Snowy Owl Bubo scandiacus are the
most important, but a small number of other species is known to take rabbits on
Middleton (i.e. Roberts 1985 (for the Glaucous-winged Gull Larus glaucescens) and Van Nus 2006 (for the Northern Harrier Circus cyaneus) and these predators’ diets
were investigated as much as possible.
A poor photo of an immature Gyrfalcon Falco rusticolus on its recently captured prey. It got flushed from
its prey by the photographer, revealing the remains of an adult rabbit (photo:
Tim van Nus, April 2009).
Preliminary
results show that predation makes up an important part of mortality, but also climatic
conditions during the winter are of large importance to the annual rabbit survival, probably even more important than predation. The island gets sometimes
visited by hunters that shoot an unknown number of rabbits and the impact of
their actions remains unknown.
Rabbits are
often accused of preventing some burrow-nesting seabird species from nesting on
the island, such as Storm-petrels Oceanodroma
sp. or Ancient Murrelets Synthliboramphus
antiquus (i.e. see Audubon’s site description for Middleton Island: http://iba.audubon.org/iba/viewSiteProfile.do?siteId=3282&navSite=state).
However, there seems to be no scientific data to support this.
I am
studying this by using an extensive database of historical bird records, which
covers pretty much the whole of Middleton’s recorded history. In combination
with my recent inventory work and the work of others, by now I know relatively
well which species nest or have nested on the island, which are seen on or
around the island and when, and which are not seen. Neither Storm-petrels nor Ancient Murrelets
are known to nest or have nested on Middleton but both have been recorded
irregularly around the island in summer. These species nest in extensive
colonies in the surrounding area of Middleton, and are abundant.
A beached Ancient Murrelet in 1960. This is the second oldest
record of this species on or around Middleton that I could find (photo: Armand
Biron).
Besides
these aspects, there are several other things concerning Middleton’s rabbit
population that I’m studying, which I will not go into detail about here, except
for one. Introduced rabbits are often thought to cause habitat degradation and
negatively affect the indigenous flora, indirectly affecting other wildlife.
Therefore I took the opportunity to study the rabbit’s influence on the
island’s vegetation, which I thought would be interesting enough to present some
parts of in this post.
Middleton
Island’s vegetation has been studied before by Thomas (1957) who identified 116
plant species. Thomas’ study was part of the 1956 biological reconnaissance of
Middleton. Thomas conducted his study in June 1956, less than two years after
the introduction of the first four rabbits. Because of winter mortality, in the
spring of 1956 their population was reduced to five females and two males. The
summer population in 1956 was estimated at 200 animals (O'Farrell 1965). I therefore suspect the
influence these rabbits had on the vegetation at that time was still minimal
and Thomas’ work still largely represents the pre-rabbit situation.
In 2010 I
conducted a vegetation inventory and managed to identify 105 plant species, excluding
a number of species (about 20) which up to this point have remained unidentified.
Now, this would seem to indicate that the rabbits grazing activity has probably
not decreased the island’s vegetational diversity, but unfortunately things are
a bit more complicated. Due to the island’s uplift during the 1964 Earthquake (which
created a whole new supra-tidal area around the island) a number of new species
can be expected to have shown up on the island during the following decades. At
the same time, the uplift may have influenced the vegetation of the older
island parts, as this got further out of the reach of the ocean. When not
taking into account the newly created formations, the number of species
identified on the pre-1964 parts of the island (98) is lower than was found by
Thomas in 1956. Some species that I could not relocate on the older formations,
however, were found in the post 1964 areas instead, indicating that ‘succession’
(or the uplift) may be an issue. Additionally, it appears that the number of
invasive species has increased during the recent decades. And finally, I must
point out that I probably was not carrying the same expertise as Thomas’ in
1956 (but certainly not unusual with vegetation inventories). None the less, at
this point I believe it would be safe to say that the rabbits have NOT caused
any serious decrease in the diversity of the vegetation of Middleton Island,
about 57 years after their introduction.
But what are
the effects of the rabbit’s grazing activity and what would the island look
like without them? In order to study this I erected several exclosures: small fenced off areas that are inaccessible
to the rabbits. The idea was (and still is) to keep the rabbits out for a
period of many years in order to be able to monitor vegetation development
without the presence of rabbits. These exclosures were made from metal poles
and 50 cm high chicken wire that was pinned into the ground, and they vary in
size from about 2 x 2 m for the single smallest, to about 4.5 x 4.5 m for the
largest. Three exclosures were erected in the summer of 2009 and another two
during the following summer. The exclosures are distributed over different vegetation
types and/or formations.
Exclosure 1 in the north-end of the island in the spring of its
fourth year in 2012 (photo: USGS crew 2012)
Rabbits are
not the only herbivore on the island. There is a large population of Canada
Geese Branta canadensis present on the island throughout most of the year, numbering just over
2000 birds (adults and young) in late June 2008. After a fast increase from the 1970's, their population stabilized in the mid 1990's around this number (Petrula et al 2008). These
geese will also influence the vegetation, but could technically have access to the
vegetation within the exclosures if they want to (and they are allowed to). Due to the exclosures’ sizes, however, I doubt they ever did thus far. The geese are not very common around three of the five exclosures and I did not
see any signs that these had regularly been feeding in or in the vicinity of
the remaining two (no, or hardly any, fecal pellets of them were found there during
my visits).
Already
within a year after the first exclosures had been placed it became notable that,
during the summer, the rabbit’s grazing activity was aimed primarily at flowers,
but also at seeds and fruits of the plants. At closer examination in 2010 it
appeared that all of the flowering plants that were found within the exclosures
could also be found elsewhere on the island. Some of them were very common and
others relatively rare.
An abundance of flowering Yellow Monkey-flowers Mimulus guttatus within exclosure 3. The
Yellow Monkey-flower grows abundant just around the exclosure too, but there
their flowers get eaten by rabbits (photo: Tim van Nus, June 2010).
Strangely, in
a few cases it got noticed that some of the plants that were found flowering
within the exclosures were only incidentally found flowering elsewhere, but notably
only in the middle of the area containing the highest rabbit density! This was
the case, for example, with a single Yellow Paintbrush
Castilleja unalaschensis growing in
exclosure 1 at the island’s north end. Though this species is stated as being fairly
common by Thomas (1957), this was only the second time I observed it on the
island. Remarkably, a few days after this finding, I also observed a small
stand (3 - 4 plants) growing in the center of the island, at a location where
rabbits are extremely abundant, and mainly feed on Annual Bluegrass Poa annua.
Since the summer of 2010 there is a single Yellow Paintbrush to be found flowering in
the middle of exclosure 1 (photo: USGS crew 2012).
In 2010 Yellow Paintbrush was also noticed growing in the A.C.&W. station, the area containing the island’s
highest rabbit density (photo: Tim van Nus, July 2010).
During 2011
and 2012, when I did not visit Middleton, the exclosures were monitored by
others, and a number of photos got taken of each exclosure, once during spring
and once during summer (the beginning of July). Thus far, not all the
exclosures have shown such a dramatic difference. Two exclosures placed in
areas with a relatively low rabbit density, up till now have shown only little
differences with their surrounding vegetation.
Three years after its construction, the largest exclosure
(#5), in the southern upland of Middleton Island, still shows no clear difference in vegetation
structure compared to the surrounding vegetation (photo: USGS crew 2012).
After
analyzing these photos as much as possible, I noticed a similar pattern taking place as during the
first two growing seasons of the project. However, one thing that stands out at the moment is
the presence of a yet unidentified purple flower that appears to be growing
well within exclosure 3. Unfortunately, from the photos I can’t manage to
identify it, and I don’t know whether I’ve listed it before in a ‘non-flowering
state’.
Suddenly there were these yet unidentified purple flowers
occurring in exclosure 3. A Shooting-star sp?? (photo: USGS crew 2012).
But, with
only four years after the first exclosures were placed, there are still new
developments expected to be taking place within them, so what is presented here
will be very preliminary. A continuation in monitoring the exclosures would be
very helpful, and so would a further increase in the number of exclosures be.
Thus far, the observations indicate that:
- Although already a colorful place, without the
presence of the rabbits Middleton would probably host a lot more flowers.
- The rabbits’ occurrence probably changed
abundances of some plant species, but yet there is no clear evidence that
plant species disappeared due to the rabbits' presence.
- The rabbit’s grazing pressure varies along the
location and is absent in some locations.
- The rabbit’s grazing pressure varies between
years and is absent in some locations in some years (probably allowing some
plant species to flower irregularly or incidentally).
- Due to repeated grazing, a large number of plant
species occurs in a sort of ‘dwarf state’; they remain low and will never
flower, yet they can be very abundant.
- Certain plant species are preferred by the
rabbits and when these are growing abundant they ignore certain other plants
that would readily be eaten in other locations on the island.
Throughout the summer the introduced Annual Bluegrass
Poa annua appeared to be the rabbits’
favored food item and the highest rabbit densities are found on and in the
vicinity of short grazed foraging patches containing this species, primarily on
and near the island’s infrastructure. These areas are also the place where the
highest winter survival occurs, possibly allowing the species to remain on
the island during the more severe winters. It remains unclear whether the establishment of humans on the island ultimately created a life-line for the rabbit population throughout the following decades, but
I am more and more getting that impression.
Rabbits are most abundant on or near the island’s
infrastructure, where they maintain short-grazed foraging patches. Here they
are also most easily seen (photo: Tim van Nus, August 2009).
Now, does
this mean that the rabbits are causing habitat degradation and are negatively
affecting the indigenous flora? I don’t believe so. Thus far it seems their
presence results only in changes in the vegetation, and from a vegetational
point of view I believe these changes are not necessarily a bad thing.
And whether
the rabbits are indirectly affecting other wildlife… Yes, certainly they do! The
reduction of flowers, seeds and fruits will likely have an effect on the
island’s insect world and therefore indirectly affect food supply of some birds
species. According the Audubon the “Heavy grazing by the hares must also make
the island less attractive as nesting habitat for some shorebirds, such as
Short-billed Dowitchers Limnodromus
griseus” (http://iba.audubon.org/iba/viewSiteProfile.do?siteId=3282&navSite=state), but again I did not managed to find any scientific data
that can support this. (A single pair of this species is recorded to have
likely nested on Middleton Island for a period of several years during the late
1990’s (Van Nus, unpublished data)).
Whether the overall
effect of the rabbits’ grazing is negative remains yet to be seen; on Middleton
the locations with the highest rabbit densities coincide with the highest
densities (record densities!) of savannah sparrows, which, in combination with
the presence of juvenile rabbits in late summer, seems to allow a pair of
Northern Harriers to nest there irregularly (Van Nus 2006).
This blog
entry would get a bit too complex to present and discuss other findings, but I
can ensure you that, after examination, at this point for every negative aspect
of the rabbit’s occurrence I can come up with a positive. In my opinion, the
assumed but not proven absence of Storm-petrels and/or Ancient Murrelet on
Middleton, due to the presence of rabbits, gets easily compensated by the
remarkable appearance of Snowy Owls on Middleton. During each winter, a number
(1 – 35+) of these owls resides on Middleton and 99% of their diet consists of
rabbits (Rausch 1958, O’Farrell 1965, Van Nus,
unpublished data 2005 – 2006). Immature birds, usually 1 - 2, are resident to the island in the summer (well away from their breeding range), although irregularly. In 2005, 96% of the diet samples that were collected from
two immature individuals throughout the summer, contained the remains of only rabbits
(Van Nus 2006).
I remember
that the birdwatchers that paid organized summer-day visits to the island during my stays were
all pretty excited about putting the Snowy Owl on their list. I believe that these owls
are one of the many things that make Middleton Island unique, and this could
also count for the rabbits, which certainly deserve more attention in the future,
and not from hunters but from scientists, before the opportunity is lost.
Thanks to the rabbits Middleton Island can contain a Snowy
Owl population year-round. Unique for southern Alaska! (photo: Brian Guzzetti,
May 2005).
Acknowledgements
I want to
thank everybody that helped out with constructing and placing the exclosures in
2009 and/or 2010, as well as the people that helped monitoring the exclosures
during the following two years. I thank Scott Hatch (USGS) for letting me use
the chicken wire and a few stakes, and the FAA for the leftover metal poles.
Literature
Bailey, E.P. 1993. Introduction of foxes to
Alaskan Islands – History, effects on Avifauna, and eradication. United States
Department of the Interior, Fish and Wildlife Service. Resource publication
193. Washington DC.
O'Farrell, T.P. 1965. The Rabbits of Middleton
Island, Alaska. Journal of Mammalogy, Vol. 46, No. 3, pp. 525-527.
Thomas, J.H. 1957. The vascular flora of Middleton
Island, Alaska Contributions from the Dudley Herbarium, 5:39-56.
Rausch R. 1958. The occurrence and distribution
of birds on Middleton Island, Alaska. The Condor 60: 227-241.
Roberts, B. 1985.
Glaucous-winged Gulls prey on feral rabbits on Middleton Island, Alaska.
Murrelet 66:24.
Petrula, M.J., Rothe, T., Rosenberg, D. &
Crowley, D. 2008. Canada Goose Survey on Middleton Island – 2008. Alaska
Department of Fish & Game. Unpublished report.
Van Nus, T. 2006. The simple diet of a Northern
Harrier Circus cyaneus hudsonius pair at Middleton Island in the Gulf of
Alaska. De Takkeling 14: 68-77.
