Radio-collared adult female common brushtail possum (Trichosurus vulpecula)
Ph.D., University of British Columbia, 1999
M.Sc., Queen's University, 1990
B.Sc., University of Toronto, 1988
Department of Zoology
University of Western Ontario
London, Ontario, N6A 5B7
CANADA
Phone: (519) 850-2533
FAX: (519) 661-2014
Possum Links (PhD Thesis, demographic model)
Clinchy, Haydon & Smith Simulations
C.V. in PDF (Adobe Acrobat) format
C.V. and website last updated December 7, 2001
Current Research
Synergistic effects of food and predation on the physiological ecology of songbirds
Numerous studies have shown that animals balance the risk of predation
against time
spent foraging. Synergistic effects of food and predation on
demography ought to be
the norm if such individual-level phenomena have population-level consequences.
Nonetheless, since population-level experiments on terrestrial vertebrates
are rare,
and bifactorial experiments are rarer still, synergistic effects of
food and predation on
demography have only recently been shown in mammals and have never
before been
demonstrated in birds. Over the past 2 years I have been collaborating
with
Prof. L.Y. Zanette (U. Western Ontario) in conducting a 4-year, 2x2,
manipulative
food addition plus natural predator reduction experiment on 14 populations
of
Song Sparrows (Melospiza melodia). Our results provide
the first evidence of
synergistic effects in birds: territories subject to the combined food
addition + low
predator treatment produced 1.5 times more young per season than would
be
expected if the effects of food addition and predator reduction were
independent
and additive. This synergistic effect is similar in magnitude
to those (1.5-1.9 > additive)
recently shown in mammals.
Prof. R. Boonstra (U. Toronto), a principal investigator on both of
the recent mammal
studies, hypothesized that the individual-level mechanism responsible
may be the
physiological stress effects induced by protracted hunger and chronic
risk of predation.
Unfortunately, this hypothesis could not be fully evaluated before
the termination of the
earlier experiments. The Song Sparrow experiment thus provides
a unique opportunity
to address this hypothesis.
This research unites my expertise with Prof. Boonstra’s. We are
collaborating with
Prof. Zanette and Prof. J.C. Wingfield (U. Washington), the world’s
leading expert on
physiological stress in songbirds, in comparing the levels of stress
experienced by adult
and nestling Song Sparrows subject to each of the four experimental
treatments
(high predator, unfed; high predator, fed; low predator, unfed; low
predator). Stress
levels are quantified by measuring corticosterone, glucose, and free
fatty acid
concentrations in blood samples. The consequences of chronic
stress should be
reflected in anaemia (poor condition), low white-blood-cell counts
(impaired immunity),
lower body weight, lower fecundity, greater fluctuating asymmetry(developmental
anomalies) in offspring, and poorer offspring survival.
Ph.D. Research
Does immigration "rescue" populations from extinction?
Metapopulation dynamics theory has grown by leaps and bounds in recent
years.
Metapopulation models suggest that immigrants often help "rescue" populations
from
extinction. This is one of the primary rationales behind movement
corridors for
conservation. Metapopulation dynamics are generally inferred
from the presence and
abundance of a target species in a survey of suitable habitat patches.
Such surveys
rarely include basic demographic data such as sex ratio and age structure,
and do not
attempt to measure immigration directly.
Working in collaboration with Dan Haydon and Andrew Smith, we have developed
simple stochastic models that simulate the effects of spatially correlated
disturbances
affecting spatially clustered populations. Such disturbances
can generate patterns of
patch occupancy consistent with the "rescue effect", even though dispersal
is not involved.
Similarly, patterns of patch occupancy consistent with the "rescue
effect" are to be
expected in any taxa, such as mammals, where there is male-biased dispersal.
Males
are more likely to be found in isolated patches, and isolated patches
consisting entirely
of males are obviously doomed to extinction. All this indicates
the necessity for a more
rigorous empirical approach to the study of metapopulation dynamics,
involving
direct measurements of the actual contribution of immigration to recipient
population
dynamics in the field.
I am presently completing a series of papers reporting results from
a spatially and
temporally replicated removal experiment conducted on common brushtail
possums
(Trichosurus vulpecula), in a landscape with no physical barriers
to dispersal.
I chose the common brushtail possum as a 'model' medium-sized marsupial
herbivore.
Among mammals, Australia's medium-sized marsupial herbivores have been
by far
the most adversely affected by recent human disturbance. Australian
species
account for roughly half of all mammalian extinctions in the past 200
years. More
than one-third of all studies on mammals cited in a recent review on
the efficacy of
movement corridors for conservation involved presence/absence surveys
of
Australian marsupial possums and gliders. Yet, my results show
that even when
movements of only one home range length (about 200 m) are defined as
dispersal,
immigration contributes only 1 % to the growth rate of recipient populations.
Parentage analysis using microsatellite DNA indicated that almost all
daughters
settled on or adjacent to their mother's home range. My results
suggest that
presence/absence surveys do not provide sufficient evidence as to whether
or not
immigration "rescues" populations from extinction.
Radio-collared adult female common brushtail possum (Trichosurus
vulpecula)
Model of possum demography in XLS (Microsoft Excel 97) format
Clinchy (1999, Ph.D. thesis)
in PDF (Adobe Acrobat) format
Clinchy, Haydon & Smith Simulations
This document provides the source code for the simulations discussed
in Clinchy, Haydon & Smith (Pattern does not equal process: what
does
patch ocupancy really tell us about metapopulation dynamics?)
Clinchy, Haydon and Smith Source Code
in PDF (Adobe Acrobat) format
Selected Publications
____________________________________________________________________
Clinchy, M., Krebs, C. J. and Jarman, P. J. 2001.
Dispersal sinks and handling effects:
interpreting the role of immigration in common
brushtail possum populations.
Journal of Animal Ecology, 70:
515-526.
1. An evaluation of the potentially adverse effects of measurement
must be made before
concluding that one is dealing with a 'dispersal
sink'.
2. We conducted a spatially and temporally replicated removal
experiment on common
brushtail possums (Trichosurus vulpecula)
in uniformly suitable old-growth eucalypt
forest in south-eastern Australia, that was
designed to address the question: does
immigration 'rescue' populations from extinction?
3. Despite taking precautions to minimize potential harm, analyses
indicated some
evidence of an adverse effect of handling
on the survival of pouch-young and strong
evidence of effects on adult survival.
In addition, symptoms of stress associated with
handling observed at our site, corresponded
to symptoms reported in connection with
the long-term (15 + years) trapping study
on possums conducted by Efford et al. in the
Orongorongo Valley (OV) of New Zealand.
4. Initial projections from a demographic model indicated that
the resident population
at our site was not replacing itself (births
< deaths), suggesting that the site was a
dispersal sink. This was inconsistent
with the fact that the site was in prime habitat.
Moreover, the measured rate of true immigration
in response to experimental removals
was not sufficient to maintain the population
density. Using data from Efford (1998),
our model confirmed his suggestion that the
OV site also appears to be a dispersal
sink for possums, despite being in prime habitat.
5. When otherwise undiagnosable deaths among adults were assumed
to be due to
handling and 'right-censored' (excluded),
the projection was that the resident
population at our site was stable (r
= 0), and therefore not in need of 'rescue' by
immigration. Similarly, when survival
estimates for the OV site were corrected by
the same amounts, the projection was that
the population at that site was also stable.
6. Most vacancies created by our experimental removals were filled
by neighbouring
residents that expanded their ranges into
the removal areas. We suggest that the
artificial 'removal' of residents as a consequence
of deaths due to handling, may often
induce an influx of such apparent immigrants,
thereby giving the impression that
immigration is 'rescuing' populations from
extinction.
____________________________________________________________________
Clinchy, M., Haydon, D. T. and Smith, A. T. 2001.
Pattern does not equal process:
what does patch occupancy really tell us about
metapopulation dynamics?
American Naturalist, accepted for publication
September 2001.
Patch occupancy surveys are commonly used to parameterize metapopulation
models.
If isolation predicts patch occupancy this is generally attributed
to a balance between
distance-dependent re-colonization and spatially independent extinctions.
We investigated
whether similar patterns could also be generatedby a process of spatially
correlated
extinctions following a unique colonization event (analogous to non-equilibrium
processes
in island biogeography). We simulated effects of spatially correlated
extinctions on patterns
of patch occupancy among pikas (Ochotona princeps) at Bodie,
California, using randomly
located ‘extinction discs’ to represent the likely effects of predation.
Our simulations
produced similar patterns to those cited as evidence of balanced metapopulation
dynamics.
Simulations using a variety of disc sizes and patch configurations
confirmed that our results
are potentially applicable to a broad range of species and sites.
Analyses of the observed
patterns of patch occupancy at Bodie revealed little evidence of ‘rescue
effects’ and strong
evidence that most re-colonizations are ephemeral in nature.
Persistence will be
overestimated if static or declining patterns of patch occupancy are
mistakenly attributed to
dynamically stable metapopulation processes. Consequently, simple
patch occupancy
surveys should not be considered as a substitute for detailed experimental
tests of
hypothesized processes, particularly when conservation concerns are
involved.
____________________________________________________________________
Johnson, C. N., Clinchy, M., Taylor, A. C., Krebs, C. J., Jarman,
P. J., Payne, A.,
and Ritchie, E. G. 2001. Adjustment
of offspring sex ratios in relation to availability
of resources for philopatric offspring in
the common brushtail possum. Proceedings
of the Royal Society of London, Series
B, 268: 201-205.
The Local Resource Competition (LRC) hypothesis predicts that where
philopatric
offspring compete for resources with their mothers, offspring sex ratios
will be biased in
favour of the dispersing sex. This should produce variation in sex
ratios among populations
in relation to differences in the availability of resources for philopatric
offspring. However,
previous tests of LRC in mammals have used indirect measures of resource
availability,
and have focussed on sex ratio variation among species or individuals,
rather than local
populations. Here we show that the availability of den sites predicts
the offspring sex ratio
in populations of the common brushtail possum. Female possums defend
access to dens,
and daughters but not sons occupy dens within their mother's range.
However, the
abundance of possums on our study areas was determined principally
by food availability.
Consequently, in food-rich areas with high population density the per
capita availability of
dens was low, and the cost of having a daughter should have been high.
This cost was
positively correlated with male biases in the sex ratio at birth.
____________________________________________________________________
Clinchy, M. 1997. Does immigration "rescue" populations
from extinction?
Implications regarding movement corridors
and the conservation of mammals.
Oikos, 80: 618-622.