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Viability of a Florida Scrub-jay (Aphelocoma coerulescens) Population in north-central Florida

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Viability of a Florida Scrub-jay (Aphelocoma coerulescens) Population in north-central Florida

Abstract

Exceedingly limited breeding range, dispersal ability, and habitat con-nectivity tend to characterize species as at risk of extinction. A species endemic to Florida, the Florida Scrub-jay (Aphelocoma coerulescens; scrub-jay), exemplifies these characteristics, which have contributed to its alarming, roughly 90% population decline since the 1800s. To help stop or reverse this ongoing decline, land managers should communicate scrub-jay population trends to facilitate conversations about the effectiveness of respective management regimes within and between metapopulations and to identify regions in particular need of conservation focus. We report scrub-jay population survey data at Seminole State Forest (SSF) in north-central Florida between 2008 and 2016. Our data suggest a population high in 2008 (n = 137), a low in 2015 (n = 90), and 97 individuals in 2016. Using these data, we modeled a simple population viability analysis which can be easily replicated, and interpreted with caution , by those with similar count data. Our analysis indicated a 0.78 extirpation probability of scrub-jays at SSF within 100 years. Relatively small, simulated increases in population growth substantially decreased this probability. Given that SSF scrub-jays likely constitute the largest portion of their metapopulation, continued and perhaps increased support for management efforts at SSF may be required for metapopula-tion persistence. Our results highlight the importance of population monitoring with respect to validating current, and identifying the need for future, management efforts.
97
Florida Field Naturalist
PUBLISHED BY THE FLORIDA ORNITHOLOGICAL SOCIETY
Vol. 49, No. 3 September 2021 pageS 97-154
Florida Field Naturalist 49(3):97–109, 2021.
VIABILITY OF A FLORIDA SCRUB-JAY (Aphelocoma
coerulescens) POPULATION IN NORTH-CENTRAL FLORIDA
Dustin E. BrEwEr1 anD ralph risch2
1Department of Biology, Institute for Great Lakes Research,
Central Michigan University
2Florida Forest Service
Email: dustinbrewer92@yahoo.com
Abstract.—Exceedingly limited breeding range, dispersal ability, and habitat con-
nectivity tend to characterize species as at risk of extinction. A species endemic to
Florida, the Florida Scrub-jay (Aphelocoma coerulescens; scrub-jay), exemplifies these
characteristics, which have contributed to its alarming, roughly 90% population de-
cline since the 1800s. To help stop or reverse this ongoing decline, land managers
should communicate scrub-jay population trends to facilitate conversations about the
effectiveness of respective management regimes within and between metapopulations
and to identify regions in particular need of conservation focus. We report scrub-jay
population survey data at Seminole State Forest (SSF) in north-central Florida be-
tween 2008 and 2016. Our data suggest a population high in 2008 (n = 137), a low
in 2015 (n = 90), and 97 individuals in 2016. Using these data, we modeled a simple
population viability analysis which can be easily replicated, and interpreted with cau-
tion, by those with similar count data. Our analysis indicated a 0.78 extirpation prob-
ability of scrub-jays at SSF within 100 years. Relatively small, simulated increases in
population growth substantially decreased this probability. Given that SSF scrub-jays
likely constitute the largest portion of their metapopulation, continued and perhaps
increased support for management efforts at SSF may be required for metapopula-
tion persistence. Our results highlight the importance of population monitoring with
respect to validating current, and identifying the need for future, management efforts.
Key words: avian, corvid, population trends, Seminole State Forest, wildlife man-
agement
For species that are highly habitat-specialized, habitat loss is
currently the most common ultimate cause of extinction (Fahrig 1997,
Owens and Bennett 2000). Proximate causes for extinction other
98 FLORIDA FIELD NATURALIST
than habitat loss include stochastic (random) processes regarding
demography (e.g., births and deaths) and the environment (e.g.,
precipitation patterns, natural disasters; Simberloff 1995, Whitlock
2000). Species with a small population size, and especially those
species that naturally inhabit small geographic ranges (i.e., range-
restricted species), are particularly vulnerable to these proximate
causes when suitable habitat area within those ranges has been
substantially reduced (Harnik et al. 2012). Such habitat reduction
can further isolate metapopulations (i.e., generally isolated groups of
conspecific individuals between which dispersal occasionally occurs).
For many extant species particularly vulnerable to these pressures,
we do not know which metapopulations are viable (i.e., likely to
persist 100 years into the future) because of a lack of monitoring.
Metapopulations can be defined by the 99th percentile for the focal
species’ dispersal distance. Namely, all conspecific territories that can
be linked together via straight lines less than that distance (one line
between each adjacent territory) and that do not cross known hard
barriers (like large water bodies) constitute a metapopulation (Stith
et al. 1996).
The Florida Scrub-jay (Aphelocoma coerulescens; hereafter scrub-
jay), endemic to Florida, exemplifies vulnerability and exposure to
each of the above factors. Even before intensive anthropogenic land
transformation began in the 1800s, scrub-jay metapopulations were
relatively small, non-contiguous, and isolated because of extreme
sedentariness (most scrub-jays disperse less than 3 territory
widths [Woolfenden and Fitzpatrick 2020], a distance of ~1.2 km
at SSF [D. E. Brewer, Central Michigan University, pers. obs.]) and
dependence upon fire-mediated oak-scrub confined to sandy ridges,
river corridors, and coasts. It has been estimated that the range-
wide scrub-jay population has declined by more than 90% since the
1800s largely because of fire suppression and anthropogenic habitat
conversion (Woolfenden and Fitzpatrick 2020). Though there have
been extensive studies of scrub-jay biology at Archbold Biological
Station (ABS; e.g., Woolfenden and Fitzpatrick 1984) and at or in
the vicinity of Merritt Island National Wildlife Refuge (MINWR;
e.g., Breininger et al. 2006), relatively few studies have occurred
in other parts of Florida where at least 8 additional genetically
distinct scrub-jay groups occur (Coulon et al. 2008). These 10 genetic
groups, which likely reflect habitat connectivity prior to European
settlement, contain metapopulations that have become increasingly
isolated in the past 200 years because of anthropogenic habitat
conversion (Coulon et al. 2008, Woolfenden and Fitzpatrick 2020).
In the early 1990s, there were 42 extant scrub-jay metapopulations
(each separated by >12 km; Stith et al. 1996) and a total population
BrEwEr anD risch—ViaBility of a scruB-jay population 99
size (~4,000 family groups) that was approximately 25% less than 10
years before (Woolfenden and Fitzpatrick 2020). Scrub-jay population
declines have continued since the early 1990s in urbanizing areas
(e.g., Miller and Stith 2002) and even on lands managed to sustain
natural resources, including scrub-jays (Boughton and Bowman
2011). Many of the 42 metapopulations present in the early 1990s are
now extirpated (Coulon et al. 2008), though another intensive state-
wide survey is required to determine how many metapopulations
and scrub-jays currently exist. One recent estimate suggests that
there are currently between 6,000 and 9,000 scrub-jays remaining
(~3,000 family groups; BirdLife International 2020). Management
strategies such as prescribed fire and mechanical disturbance that
promote an early successional, scrub-vegetation state can be used
to preserve or grow scrub-jay populations (e.g., at ABS; Boughton
and Bowman 2011). Such management likely would help to guard
remaining metapopulations throughout the state against extirpation,
though studying population trends at numerous sites beyond ABS
and MINWR may help to identify where to focus conservation efforts
and how those efforts should be fine-tuned based on location. To
better understand scrub-jay metapopulations throughout the state,
however, existing datasets that document population trends must be
used effectively.
The metapopulation that includes scrub-jays at Seminole State
Forest (SSF; 28.9 N, 81.5 W; metapopulation 18 in Stith et al. [1996])
in north-central Florida is >175 km to the north of ABS (27.1 N, 81.2 W)
and >70 km west-northwest of MINWR (28.3 N, 80.4 W). This distance
functionally precludes SSF scrub-jays from interbreeding with these
well-studied scrub-jay groups (Coulon et al. 2008). Scrub-jays at SSF
are genetically most similar to those at Ocala National Forest (ONF)
and in metapopulations elsewhere in the western part of north central
Florida, which are all a part of the largest genetic group—based on
geographic coverage—identified by Coulon et al. (2008; Fig. 1). Studies
at SSF could improve our understanding of scrub-jays in the under-
studied region wherein the property occurs and could be compared to
studies completed at ABS or MINWR to determine the applicability of
those studies to other metapopulations, and so optimize conservation
efforts. The population of scrub-jays at SSF has been monitored since
2008 and exemplifies an opportunity to effectively use an existing
dataset to better understand population trajectory. The viability of the
SSF scrub-jay population could possibly be used as an indicator for
the viability of the metapopulation that it prominently exists within,
though monitoring of populations outside of SSF would be required to
confirm this possibility. Determining scrub-jay population viability at
SSF could also indicate if management practices implemented there,
100 FLORIDA FIELD NATURALIST
or at locations where similar management techniques are being used,
should be modified to increase the likelihood of long-term scrub-jay
persistence. Additionally, demonstrating the usefulness of long-term
scrub-jay population data for projecting future population change could
inspire such analysis of similar datasets to help inform management
decisions. Therefore, our objectives were to report SSF population
survey data from 2008 to 2016 and to use only these data to estimate
population viability of the scrub-jay population at SSF using a simple
analysis that managers could easily implement elsewhere.
Figure 1. Sites where scrub-jays occur that are discussed herein: S = Seminole
State Forest, O = Ocala National Forest, M = Merritt Island National Wildlife
Refuge, and A = Archbold Biological Station. The locations of these letters (which
do not correspond to polygons) indicate the center of the sites described. Major
solid lines outlining polygons indicate the most biologically likely scrub-jay
genetic groups as identified by Coulon et al. (2008). Minor solid lines indicate
county boundaries and light gray shading indicates waterbodies.
BrEwEr anD risch—ViaBility of a scruB-jay population 101
MEthoDs
Study site.—Since being established in 1990 in Lake County, Florida, SSF has expanded
to include approximately 113 km2. Approximately 6 km2 of this area is xeric scrub domi-
nated by sand live oak (Quercus geminata), myrtle oak (Quercus myrtifolia), and sand pine
(Pinus clausa) currently in suitable condition (Woolfenden and Fitzpatrick 2020) for scrub-
jay occupation and reproductive success at population replacement levels (R. Risch, Florida
Forest Service, pers. obs.). Other land cover types that occur on the property, among which
scrub is interspersed, include upland pine forests (e.g., slash pine [Pinus elliottii], long leaf
pine [Pinus palustris]) and riparian vegetation (e.g., sabal palm [Sabal palmetto], pond pine
[Pinus serotina]) along Blackwater Creek and its tributaries within the Wekiva River Basin.
Existing scrub vegetation is maintained via prescribed fire. The greater SSF area (includ-
ing a state park and state reserve) is bordered on the east, south, and west by dense human
settlement including the Orlando metropolitan area. The southeastern part of ONF borders
SSF on the north and is where the nearest concentration of scrub-jays occurs, though the
approximate number of scrub-jays in the southeast part of ONF is unknown.
Population surveys.—Between 2008 and 2016, population surveys occurred on average
eight times per year (every six weeks) in March, April, June, July, September, October, and
December. Each year, the majority of the scrub-jays present were banded with unique com-
binations of colored bands and trained to approach the surveyor upon hearing a whistle
(80–90% of scrub-jays were typically banded at any time). The surveyor searched all suitable
scrub patches at SSF (i.e., those 5–15 years post-burn) for scrub-jays during a one-week pe-
riod during each population survey. Scrub patches outside of the 5–15 years post-burn range
were rarely used and surveyed only if scrub-jays were known to use those sites based on
daily work responsibilities of the surveyor at the property. Scrub-jays were attracted to the
playback of conspecific vocalizations broadcast by the same surveyor each year. The surveyor
counted both the number of all-purpose territories defended by at least one bird—these ter-
ritories tended to be spatially stable through time—and the number of scrub-jays. Surveys
typically lasted 2–20 minutes, depending on how quickly the surveyor identified individuals
documented at the territory during previous surveys. The surveyor did not conduct surveys
when aerial predators were present. During surveys, the surveyor broadcast conspecific au-
dio from locations throughout the focal territory until they observed the focal individuals.
This process also occurred at scrub patches that the experienced surveyor believed had a
reasonable chance of containing scrub-jays but where recent, previous surveys had not in-
dicated scrub-jay presence. The surveyor re-visited territories where they had previously
documented scrub-jays but did not observe scrub-jays during the first survey; they re-visited
the territory at least twice to confirm absence. This same process (two re-visits) occurred
for missing breeding individuals, and for missing helpers, at least one re-visit occurred (the
scrub-jay cooperative breeding system is described by Woolfenden and Fitzpatrick [1984]).
The surveyor completed all surveys in a one-week period to minimize the risk of double-
counting due to movement of un-banded individuals. Further, the scrub-jays that were pres-
ent tended to approach the surveyor, which minimized the likelihood of missing individuals.
We are therefore confident that our counts of scrub-jay population size at SSF are accurate.
We chose to report data only from the March population survey herein, and to base our
analysis on these data, because this survey was the least likely to miss un-banded individu-
als such as juveniles (yet to be trained to approach the surveyor), incubating or brooding
individuals, or recent immigrants.
Population viability analysis (PVA).—Using the counts of scrub-jays in March be-
tween 2008 and 2016, we calculated population change (λ), which indicates how the SSF
population size (N) changed from one year (t) to the next (t+1):
λ = Nt+1
Nt
102 FLORIDA FIELD NATURALIST
A λ value for 2008 of 0.91, for example, indicates that between March of 2008 and
March of 2009, the total number of scrub-jays at SSF declined by 9%.
We used a discrete model to predict temporal population change:
Nt+1 = λ × Nt
We used this model to project population change 100 years into the future based on
random selection of a λ value for each year for 10,000 populations. We randomly selected
the λ values from a log-normal distribution (the best fit for the observed λ values) that
was structured based on the mean and standard deviation (SD) that we calculated using
the natural log of each observed λ value from 2008 to 2015. Each simulated population
began at the mean population size observed from 2008 to 2016 (107). Though we did
not model environmental and demographic stochasticity explicitly, we assumed that the
natural variation in λ that we simulated accounted for the majority of these factors (ef-
fects of future climate change, however, are absent).
We estimated probability of extirpation for the SSF population based on the propor-
tion of the 10,000 simulated populations that declined to below 3 individuals during the
100-year simulation. We chose this threshold for extirpation because less than 3 indi-
viduals would be smaller than the mean observed family size at SSF during our study
period (2.8) and would indicate that breeding success had likely ceased or would soon.
We also estimated how different long-term means of yearly λ values for the SSF
population might affect extirpation probability and population size. To do so, we system-
atically increased the mean λ value from the observed 0.96 by iteratively adding 0.01,
0.02, 0.03, and 0.04 to all observed yearly (2008 to 2015) λ values, calculated the natural
log for each of those sets of derived values, and then calculated the mean and standard
deviation to conduct a PVA as described above. Then we determined which artificially
increased mean λ values (of 0.97, 0.98, 0.99, and 1.00) resulted in at least 95% of the
simulations not ending in extirpation (i.e., N 3 after 100 years). We conducted analy-
ses in R (v. 3.6.2; R Core Team 2019). Variability reported is standard deviation unless
otherwise noted.
rEsults
Between 2008 and 2016, mean population size in March at SSF
was 107.22 ± 16.47 (Fig. 2). The population size ranged from a high in
2008 of 137 to a low in 2015 of 90 and ended at 97 individuals in 2016
(Fig. 2). During the study period, the 8 λ values for population size had
a mean of 0.96 ± 0.11 and tended to decline temporally (6 of 8 λ values
indicated a decline; Fig. 3). Mean number of territories in March was
38.56 ± 4.82 (Figs. 2, 4) and the 8 associated λ values for territories
had a mean of 1.0 ± 0.16 (4 indicated a decline; Fig. 3). The λ values
for number of scrub-jays and number of territories did not tend to vary
synchronously with respect to degree or direction of change (Fig. 3).
Mean scrub-jays per territory in March was 2.79 ± 0.39, tended to
decline temporally (5 of 8 transitions), and also did not tend to vary in
the same direction as number of territories through time (Fig. 4). We
did not detect a significant correlation between number of territories
and number of scrub-jays per territory (r = −0.288, df = 7, P = 0.45),
though sample size and power were low such that an undetected
negative relationship is possible.
BrEwEr anD risch—ViaBility of a scruB-jay population 103
The range of λ values observed between 2008 and 2015 resulted
in an extirpation probability within 100 years of 0.78 (Fig. 5). Upon
systematically increasing the mean of yearly λ values for our study
period, we found that mean λ values of 0.97, 0.98, 0.99, and 1.00 resulted
in extirpation probabilities of 0.44, 0.14, 0.02, and 0.00, respectively
(Fig. 5). With increasing λ, projected final populations sizes tended to
increase and median year of extirpation tended to occur later (Table 1).
Discussion
Based on the generally declining number of scrub-jays that
occurred at SSF between 2008 and 2016 (Figs. 2, 3), our PVA indicated
that the SSF population has a 0.78 extirpation probability within
the next 100 years (Fig. 5) if the range of λ levels that we observed is
representative of future conditions. The number of territories at SSF
were relatively constant during the study period (Figs. 2, 4) compared
to the period between 1992 and 2010 when the number of territories
more than doubled (Boughton and Bowman 2011). Both number of
Figure 2. Variation in number (#) of scrub-jays (gray) and number of territories
(black) at Seminole State Forest between 2008 and 2016 during the March
population survey.
104 FLORIDA FIELD NATURALIST
territories and number of scrub-jays per territory did, however, tend
to decline during the study period and generally varied between
years in opposite directions (Fig. 4). We also observed that between-
year changes in total scrub-jays and territory numbers at SSF often
varied in opposite directions (Figs. 2, 3). The cooperative breeding
system of scrub-jays (Woolfenden and Fitzpatrick 1984), in which some
juveniles remain helpers on parental territories, could explain both of
these patterns (Figs. 2, 3, 4). Fewer scrub-jays per territory would be
expected when there are more territories resulting from dispersal and
vice versa (Fig. 4). Opposite trajectories in overall scrub-jay population
and territory number (Figs. 2, 3) could indicate an oscillation between
productive years when habitat is saturated (juveniles survive because
of favorable conditions but do not disperse because of lack of suitable
habitat available) and mortality-mediated habitat availability, which
facilitates territory establishment by former helpers. Productive years
and saturated habitat might result in larger territories with more
Figure 3. Variation in population change (lambda) values for number (#) of
scrub-jays (gray) and number of territories (black) at Seminole State Forest
between 2008 and 2015, based on March population surveys. The horizontal
dashed line represents no growth. The lambda values for number of scrub-jays
and for number of territories tended to vary in the opposite direction.
BrEwEr anD risch—ViaBility of a scruB-jay population 105
scrub-jays per territory (and more scrub-jays overall, fewer territories),
whereas habitat vacated by deceased breeders might result in
colonization by former helpers that establish smaller, less scrub-jay-
dense territories (and fewer scrub-jays overall, more territories).
Regardless, it is clear that the SSF scrub-jay population, and
perhaps the metapopulation to which it belongs, requires modified
management action to improve conditions to attain a reasonably low
extirpation probability (e.g., < 0.05, acceptable risk) within the next
100 years. An additional 18 km2, approximately, of xeric-scrub could
be restored at SSF as habitat for scrub-jays. Even moderate expansion
of area managed as scrub-jay habitat within SSF boundaries could
possibly, if targeted around patches with high scrub-jay densities
(Breininger et al. 2006), be sufficient to increase the mean λ value on
the property from 0.96 to 0.99 or greater and so significantly reduce
extirpation probability (Fig. 5). Our simulations suggest that such
a modest increase in λ could reduce extirpation risk to acceptable
Figure 4. Mean number of scrub-jays per territory at Seminole State Forest
(gray; left y axis) and number of territories (black; right y axis) during March
population surveys from 2008 to 2016. The number of territories and number of
scrub-jays per territory tended to vary in the opposite direction.
106 FLORIDA FIELD NATURALIST
levels at SSF and perhaps for the entire metapopulation, given that
most of its scrub-jays are assumed to exist in SSF. Further, modifying
the frequency of prescribed fire or mechanical removal of larger
trees in existing scrub could improve the quality of habitat and so
increase scrub-jay numbers (Breininger et al. 2014, Woolfenden and
Fitzpatrick 2020). We did not conduct habitat surveys during this
study; therefore, we do not know the primary driver of population
change at SSF. Regardless, our results suggest that conservation
funds meant to benefit scrub-jays would, in conjunction with habitat
surveys, likely be well-spent on conducting management activities
at SSF.
Figure 5. Projected extirpation probabilities during a 100-year period as
a function of different mean values of population change (lambda) for the
Seminole State Forest scrub-jay population. The mean lambda value observed
between 2008 and 2015 was 0.96 and extirpation probabilities are based on the
proportion of 10,000 simulations that resulted in extirpation.
BrEwEr anD risch—ViaBility of a scruB-jay population 107
A limitation of our PVA is that it did not explicitly consider
environmental or demographic stochasticity or density dependence
but rather relied on the variation in observed λ values to encapsulate
such processes. Regarding catastrophes, we believe that our approach
is justifiable given that SSF is distant from the coast, which causes
the catastrophic effects of hurricanes to generally be avoided.
Epidemics, however, can substantially increase extirpation probability
(Breininger et al. 1999) and exemplify a stochastic event that may not
have occurred during our study period that could occur in the future.
Similarly, wildfires could also occur and may increase in frequency
and severity because of climate change. We believe, however, that the
simplicity of our model is advantageous in that only population survey
data are required, which, in many cases, is all that managers who
wish to determine the trajectory of their population possess. As with
all PVAs (Lacy 2019), but especially with respect to ours because of its
simplicity, caution should be used when interpreting our results which
are meant as a rough guide rather than an exact prediction about how
SSF scrub-jay population size may change in the future. If scrub-jays
are monitored for longer periods of time, the accuracy of analyses such
as ours will likely improve.
Given that many of the remaining scrub-jay metapopulations are
currently likely declining (Coulon et al. 2008, Boughton and Bowman
2011), it is crucial to identify which of these metapopulations could
feasibly be conserved, which metapopulations require management
modification, and which metapopulations could benefit from habitat
acquisition. Monitoring efforts, such as Audubon Florida’s Jay Watch
community science (i.e., citizen science) program, can produce accurate
population survey data (Miller et al. 2015) and could be expanded to
other sites throughout Florida. These or similar data could be used to
complete periodic viability analyses, as demonstrated by the current
study, for each remaining metapopulation to help inform decision
Table 1. Mean final scrub-jay population size and standard error (SE), median
years until extirpation, and years until first extirpation based on 10,000 simu-
lations for different mean values of population change (λ). Each simulation
had a duration of 100 years. The mean λ value observed at Seminole State For-
est between 2008 and 2015 was 0.96.
λ
Mean final
population size (SE)
Median years until
extirpation
Years until first
extirpation
0.96 1.67 (0.04) 75 28
0.97 6.44 (0.11) 85 37
0.98 20.02 (0.31) 88 41
0.99 55.26 (0.78) 89 49
1.00 150 (2.13) 89 65
108 FLORIDA FIELD NATURALIST
makers about where limited conservation resources would be most
effectively allocated. Though he used a more complex (individual-
based, spatially explicit) viability analysis than the one described
herein, Stith (1999) completed an analysis of metapopulation viability
previously. Another such effort could help to identify areas of current
conservation need and also help managers to identify which scrub-jay
populations are being managed in a way that can sustain scrub-jays
and should be emulated. Our approach could feasibly be incorporated
into a state-wide metapopulation analysis by providing a simple
process by which to use site-specific scrub-jay population trend data
that might otherwise remain largely unused. It will be important,
however, to consider not just changes in λ but also to identify what
factors (e.g., habitat management, genetic diversity) are influencing
that metric of population change.
The Florida Scrub Working Group approach currently being
employed helps to facilitate the communication required to achieve
such state-wide goals. When possible, those seeking advice about
scrub-jay habitat management techniques should prioritize consulting
land managers at properties where scrub-jays within the same
metapopulation (Stith et al. 1996), or at least the same genetic group
(Coulon et al. 2008), have been successfully managed. For example,
managers of scrub-jay habitat in the Northeast Florida Scrub Working
Group region may benefit more from communicating with the managers
at SSF than with managers in other parts of the state given regional
variation in scrub types and re-growth rates. This collaborative,
biologically informed approach among managers based on which
metapopulation or genetic group is being managed, rather than being
based solely on which species is being managed, could both improve
our ability to conserve scrub-jays and serve as a model for conserving
other sedentary, range-restricted species.
acknowlEDgMEnts
We thank Florida Forest Service employees Joe Bishop (Forestry Supervisor II) and
Mike Martin (Forester) for their work at Seminole State Forest and for their years of
support for and assistance with scrub-jay management efforts. Karl E. Miller provided
helpful advice during manuscript preparation. Kevin L. Pangle provided R code in sup-
port of the population viability analysis, and helpful instruction. Two anonymous review-
ers helped to improve the manuscript. This research was supported by the Earth and
Ecosystem Science PhD program at Central Michigan University.
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Within the range of the Florida scrub-jay (Aphelocoma coerulescens), the only bird species endemic to Florida, there was a need for a population risk assessment that considered the impacts of declining habitat availability and declining fire frequency associated with rapid human population expansion. We developed a population risk model to examine influence of population size, catastrophes (epidemics and hurricanes), and habitat management scenarios on Florida scrub-jay populations. Extinction risk declined rapidly when initial population sizes increased from 20 to 100 breeding pairs. Hurricanes increased extinction risk for coastal populations by 10-30% compared to inland populations. Our results suggested that habitat in poor condition was unlikely to support a population for more than a few decades. Poor habitat quality conditions were common throughout the species' range because of fire suppression or inadequate fire management. Habitat management was more effective than habitat restoration because population recovery occurred slowly after restoration and only if habitat was restored to optimal conditions. Sensitivity analyses showed that fecundity and survival of experienced breeders without helpers (adult nonbreeders) were the most important model parameters. Slow recovery rates occurred because helpers were the only rapid source of colonists in restored habitat and because an absence of helpers reduced breeder survival and fecundity. Small population sizes and habitat degradation make the Florida scrub-jay vulnerable to rangewide decline and extinction unless habitat is protected, restored, and managed to maintain optimal conditions.
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Population viability analysis (PVA) has been used for three decades to assess threats and evaluate conservation options for wildlife populations. What has been learned from PVA on in situ populations are valuable lessons also for assessing and managing viability and sustainability of ex situ populations. The dynamics of individual populations are unpredictable, due to limited knowledge about important factors, variability in the environment, and the probabilistic nature of demographic events. PVA considers such uncertainty within simulations that generate the distribution of likely fates for a population; management of ex situ populations should also take into consideration the uncertainty in our data and in the trajectories of populations. The processes affecting wildlife populations interact, with feedbacks often leading to amplified threats to viability; projections of ex situ populations should include such feedbacks to allow for management that foresees and responds to the cumulative and synergistic threats. PVA is useful for evaluating conservation options only if the goals for each population and measures of success are first clearly identified; similarly, for ex situ populations to contribute maximally to species conservation, the purposes for the population and definitions of sustainability in terms of acceptable risk must be documented. PVA requires a lot of data, knowledge of many processes affecting the populations, modeling expertize, and understanding of management goals and constraints. Therefore, to be useful in guiding conservation it must be a collaborative, trans‐disciplinary, and social process. PVA can help integrate management of in situ and ex situ populations within comprehensive species conservation plans.
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The combined effects of habitat quality, breeder experience and sociobiology on population demography are poorly understood. Natural fire regimes, which influence habitat quality and sociobiology, have been replaced by controlled fire management in most ecosystems. Managing fire mosaics (vegetation at different ages since fire) can be important to sustain species in fire-maintained habitats, but requirements are usually poorly defined. Source-sink theory provides a foundation to quantify such habitat heterogeneity, but source-sink applications are largely focused on modeling. We quantified how habitat quality, breeder experience and non-breeding adult helpers affected Florida scrub-jay (Aphelocoma coerulescens) recruitment to describe source-sink heterogeneity within local populations. We used 22 years of census data of uniquely marked Florida scrub-jays to measure recruitment at 36 sites and combined that data with habitat-specific survival to characterize habitat-specific demography. To define habitat quality at the territory scale, we used static habitat features (soils, scrub oak cover) and dynamic habitat states (shrub heights and open sandy patches) that resulted from fire mosaics. Habitat quality most affected recruitment followed by the presence of helpers; fire mosaics, described by habitat states, determined whether territories functioned as strong sources, weak sources or sinks. Subdividing landscapes into habitat states allowed quantification of the fire mosaic at the territory scale and population scale, as the proportions of habitat states can predict local population growth rates. Our approach provides an example of how characterizing habitat quality at the territory scale, relative to source-sink categories, can explain habitat heterogeneity within local populations and inform fire management.
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In their review of the recent ''explosion of spatially explicit theory'' in ecology Kareiva and Wennergren (1995) suggest a number of emerging principles for species conservation. One of these principles is that how habitats are arranged in space can mitigate the risks of species extinctions from habitat loss. I tested this by estimating the relative importance of habitat loss and habitat spatial pattern (fragmentation) on population extinction, using a simple, spatially explicit simulation model. Results indicate that the effects of habitat loss far outweigh the effects of habitat fragmentation. I therefore suggest that, in fact, details of how habitats are arranged cannot usually mitigate the risks of habitat loss. Conservation efforts should be aimed foremost at stopping habitat loss and at habitat restoration.
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The delimitation of populations, defined as groups of individuals linked by gene flow, is possible by the analysis of genetic markers and also by spatial models based on dispersal probabilities across a landscape. We combined these two complimentary methods to define the spatial pattern of genetic structure among remaining populations of the threatened Florida scrub-jay, a species for which dispersal ability is unusually well-characterized. The range-wide population was intensively censused in the 1990s, and a metapopulation model defined population boundaries based on predicted dispersal-mediated demographic connectivity. We subjected genotypes from more than 1000 individual jays screened at 20 microsatellite loci to two Bayesian clustering methods. We describe a consensus method for identifying common features across many replicated clustering runs. Ten genetically differentiated groups exist across the present-day range of the Florida scrub-jay. These groups are largely consistent with the dispersal-defined metapopulations, which assume very limited dispersal ability. Some genetic groups comprise more than one metapopulation, likely because these genetically similar metapopulations were sundered only recently by habitat alteration. The combined reconstructions of population structure based on genetics and dispersal-mediated demographic connectivity provide a robust depiction of the current genetic and demographic organization of this species, reflecting past and present levels of dispersal among occupied habitat patches. The differentiation of populations into 10 genetic groups adds urgency to management efforts aimed at preserving what remains of genetic variation in this dwindling species, by maintaining viable populations of all genetically differentiated and geographically isolated populations.
Accuracy assessment of a Jay Watch post-reproductive survey of Florida Scrub-jays
  • K E Miller
MillEr, k. E., c. a. faulhaBEr, anD j. o. garcia. 2015. Accuracy assessment of a Jay Watch post-reproductive survey of Florida Scrub-jays (Aphelecoma coerulescens).
Ecological basis of extinction risk in birds: habitat loss versus human persecution and introduced predators
  • K E Miller
  • B M Stith
MillEr, k. E., anD B. M. stith 2002. Florida Scrub-jay distribution and habitat in Charlotte County. Center for Avian Conservation, inc., Gainesville. owEns, i. p. f., anD p. M. BEnnEtt. 2000. Ecological basis of extinction risk in birds: habitat loss versus human persecution and introduced predators. Proceedings of the National Academy of Sciences 97:12144-12148.