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Demography, Phenology, and Factors Inf luencing Reproduction of the Rare Wildflower
Spalding’s Catchfly (Silene spaldingi i ) on the Zumwalt Prair ie
R.V. Taylor1, J. Dingeldein, and H. Schmalz
Final Report (March 2012)
Abstract
Spalding’s catchfly (Silene spaldingii S. Watson) is a long-lived perennial forb that is threatened by the loss of native grassland habitat throughout the Pacific Northwest. We studied a small population of catchfly plants on the Nature Conservancy’s Zumwalt Prairie Preserve in northeastern Oregon to understand key demographic parameters, timing of key life history states, and factors affecting reproductive success. We marked 100 individual plants and monitored them weekly across the growing season to obtain data on growth form, dates of emergence, flowering and fruiting, reproductive output, as well as prevalence of insect predation and ungulate browse. We found that a substantial and highly variable fraction of Spalding’s catchfly plants remained dormant each season. Plants that did emerge began leafing out in late May or early June and first flowering was observed by 16 July. Most emergent plants were reproductive and single-stemmed, though vegetative (i.e., sterile) plants were also common. Over three-quarters of emergent plants were affected by insect predation, ungulate browse, or other herbivory. Insect predators that destroy the ovaries of Spalding’s catchfly were found on nearly half of all plants. Few plants produced seed – less than one fifth of plants which produced buds went on to produce mature fruits. When plants did succeed in producing fruits, production was low with an average of only three fruits per plant. During 5 years of study we observed 11 seedling rosettes and documented the death of 16 plants. We recommend that land managers address excessive herbivory by elk and investigate the role that fire may play in regulating insect herbivory. Future monitoring of Spalding’s catchfly, incorporating measures of ungulate herbivory, insect predation, and mature fruit production is also recommended.
Introduction
Spalding’s catchfly (Silene spaldingii S. Watson) is a long-lived perennial wildflower that was
once abundant in the prairies of the inland Pacific Northwest. Population numbers have declined
due to conversion of grassland habitat to agricultural use (U.S. Fish and Wildlife Service 2007).
Spalding’s catchfly was listed as a threatened species under the U.S. Endangered Species Act
in 2001 (US. Fish and Widlife Service 2007). In northeastern Oregon, several significant
populations of Spalding’s catchfly persist in the grasslands of the Zumwalt Prairie with a large
population (>40,000 individuals) occurring on The Nature Conservancy’s Zumwalt Prairie
1 NE Oregon Regional Ecologist, The Nature Conservancy, 906 S River St, Enterprise, OR
97828 ([email protected]).
Taylor et al. Spalding’s catchfly demography
p. 2 The Nature Conservancy (March 2012)
Preserve (ZPP; Jansen and Taylor 2010; Figure 1). Larger populations are generally more
secure and less prone to extirpation than those that are smaller (Shaffer 1981), thus the sheer
abundance of Spalding’s catchfly on the Preserve confers some confidence regarding the
viability of this population. Long-term population viability, however, is contingent on many other
factors (Flather et al. 2011). Ultimately, long-term survival of the Spalding’s catchfly population
on the Zumwalt Prairie depends on recruiting a sufficient number of new individuals (i.e.,
seedlings) to replace plants which die from natural or other causes.
The Nature Conservancy initiated a monitoring program to detect long-term trends in the
abundance of Spalding’s catchfly on the ZPP in 2005 (Taylor et al. 2008, Jansen and Taylor
2009, 2010). Determining long-term trends in this species is challenging for several reasons.
First, Spalding’s catchfly is long lived and recruitment is episodic, thus changes in population
numbers may occur very gradually (Lesica 1997, Lesica and Crone 2007). Second, unlike most
perennial herbs, individuals of Spalding’s catchfly do not produce aboveground structures in
some years, remaining dormant throughout the growing season—a trait known as “prolonged
dormancy” (Lesica and Steele 1994). In long-term study in Montana, dormancy varied greatly
from year to year, with an average dormancy rate of 30% (Lesica and Crone 2007). Dormancy
rates measured at other sites, however, have been much lower. For example, in Hells Canyon
in Oregon, dormancy rates averaged <10% over 7 years of study (Hill and Gray 2006). Finally,
less conspicuous vegetative plants as well as rapidly senescent rosettes may evade detection.
All of these traits make it difficult to assess trends by simply counting plants each year (Lesica
and Steele 1994, Lesica 2008). An understanding of the prevalence of dormancy and how it
varies from year to year is essential in interpreting population monitoring data (Lesica and Rudd
2012).
The factors controlling the population dynamics of Spalding’s catchfly are poorly understood.
Population declines are proximally a consequence of a decrease in reproductive output or an
increase in mortality, both of which, ultimately, could be caused by many factors. For example,
Spalding’s catchfly relies on the pollination services provided by bumble bees (Bombus spp.)
without which seed viability and seedling growth are greatly reduced (Lesica 1993, Lesica and
Heidel 1996). Other possible causes decreased reproductive output may include grazing and
predation of floral structures by insects (US. Fish and Widlife Service 2007). Production of a
sufficient quantity of viable seeds, while necessary, is not sufficient for adequate levels of
recruitment. Low rates of seedling establishment, growth or survival may also drive population
declines. Changes in in fire regime (Lesica 1999), competition with invasive species (Huenneke
and Thomson 1995), and soil disturbance resulting from various land uses have all been
suggested as factors which may impede recruitment (US. Fish and Widlife Service 2007).
Increases in mortality rates of Spalding’s catchfly could result from a variety of factors including
drought, excessive browsing, or disease (US. Fish and Widlife Service 2007). Thus, information
on rates of mortality and recruitment, along with key factors that regulate these processes is
essential in formulating effective management strategies for this species.
Study objectives
Our objectives for this study were to: 1) estimate rates of dormancy, mortality, fruit production,
and seedling recruitment for Spalding’s catchfly on the ZPP; and (2) assess levels of predation
Taylor et al. Spalding’s catchfly demography
p. 3 The Nature Conservancy (March 2012)
by insects and mammals. For this purpose we established six study plots on the Zumwalt Prairie
Preserve and visited them weekly throughout the growing seasons of 2006-2011. We
documented the growth and fate of all catchfly plants within the plots, from emergence to
senescence. Because our study required us to examine plants weekly and monitor their growth,
we also recorded data on phenology (i.e., timing of critical life-cycle events) including dates of
emergence, flowering, and senescence. We report here on the results of this study.
Methods
Study Area
The study was conducted at the Zumwalt Prairie Preserve (ZPP, lat. 45º 3’ N, long. 116º 6’ W)
located in Wallowa County in northeastern Oregon (Figure 1a, b). The 13,269-ha preserve is
owned and managed by The Nature Conservancy (TNC) and lies in the southwestern portion of
the Pacific Northwest Bunchgrass Prairie (Tisdale 1982). At 1,060-1,680 m elevation, the
preserve is dominated by native bunchgrasses, including Idaho fescue (Festuca idahoensis
Elmer), Sandberg bluegrass (Poa secunda J. Presl), prairie Junegrass (Koeleria macrantha
[Ledeb.] Schult.), and bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Löve).
Spalding’s catchfly occurs across the prairie uplands that make up the bulk of the western
portion of the preserve; the population size has been estimated at > 40,000 plants which occur
across 166 ha of prairie uplands (Figure 1b; Jansen and Taylor 2010). The study took place in
Harsin pasture, an area known to have a high abundance of Spalding’s catchfly (Figure 1c). The
study area has not been grazed by cattle (Bos taurus) since 2004 and has not experienced wild-
or prescribed-fire in recent history (>15 year). For this study we established six permanently
marked 2 m radius circular plots (12.6 m2). Plot locations were chosen subjectively (non-
randomly), each < 25 m distant of all others, and were approximately 50 m from a road to
provide ease of access.
The study area experiences cold winters and warm summers. Average daily minimum
temperatures in December and January, the two coldest months were -9° and -7° C respectively
(Zumwalt Weather Station, unpublished data, 2005-2011). In July and August, the warmest
months, average daily maximum temperatures were 26° and 25° C, respectively. Average
annual precipitation was approximately 36 cm, the bulk of which falls in two periods: May-June
and October-November (Hansen et al. 2010). Over the course of the study, precipitation during
the growing season (May-Aug) averaged 15.9 cm, with the summers of 2009 and 2010
receiving relatively high amounts of rain (22 and 23 cm respectively), and the summer of 2007
the lowest (9 cm). During the other two years of the study (’08 and ’11), precipitation was slightly
below the 5-year average (13 cm). Average daily growing season temperature was 13° C from
2007-11. The warmest year was 2007 (15 ° C) and the coolest years were 2010-11 (12° C).
Field Data Collection
From May-September in the years 2007-2011 we tracked the emergence, growth, flowering,
fruiting, and predation of all Spalding’s catchfly stems within the six study plots. Any non-rosette
stems within 20 cm of each other (measured at the soil surface) were assumed to belong to the
same plant (hereafter “putative genet”, or PG). Upon discovery, each PG was marked by
placing a steel nail in the ground approximately 5 cm from one of its stems to facilitate its
Taylor et al. Spalding’s catchfly demography
p. 4 The Nature Conservancy (March 2012)
relocation during subsequent visits. Rosettes were always considered to be distinct PGs (as
defined by Lesica and Crone 2007). The location of each PG was mapped by measuring its
distance and azimuth in relation to the plot’s center stake thus allowing us to make repeated
measures throughout the season. After emergence each stem was observed weekly until it
either senesced or could no longer be located. Nails were left in the ground from one season to
the next to facilitate location of early emerging stems (e.g., rosettes) and to avoid excessive soil
disturbance.
Growth Form, Phenology, and Predation
Weekly observations of growth form and
phenological phase (i.e., phenophase) were
made for each stem of each PG following
the methods of Lesica and Crone (2007)
with minor modifications (Box 1; Figure 2).
Because a stem may exhibit multiple
phenophases simultaneously, we recorded
only the most advanced phenophase for
each stem on each visit. For stems in the
dFr and S phenophases we also counted
the number of dehiscent fruits. The
maximum number of dehiscent fruits
observed for a stem across all observations
within a year was used as an estimate of
the total reproductive output for that stem.
The phenophase of a PG was determined
by scaling up stem level data; a PG was
assigned the phenophase of its most
phenologically advanced stem (Box 1) with
one exception: a PG was considered
senesced only when all of its constituent
stems had succumbed to the S
phenophase.
We also recorded, by stem, any signs of
insect predation or ungulate browsing. A stem was considered to have been predated by insects
if there was any evidence that a bud, flower, or immature fruit had been significantly damaged
by an insect. Indicators of insect predation were: presence of an insect or larvae, chewed holes
in any part of the reproductive growth, and presence of frass in or adjacent to floral receptacles
(Figure 3). We did not track insect damage to stems or leaves. A stem was considered browsed
by ungulates if it was severed and had chew marks indicative of cattle, elk (Cervus elaphus) or
mule deer (Odocoileus hemionus) herbivory. The predation status of a PG on given date was
determined by scaling up the stem level data. A PG was considered depredated by insects or
browsed if any of its component stems exhibited signs of depredation.
Box 1. Growth forms and phenological stages of Spalding’s
catchfly plants observed during weekly visits to the study
plots (ordered from least to most advanced).
Dormant (D) – a plant which produces no above ground
structures. These are not observed in the field but are
inferred from observations in prior and subsequent years.
Non-reproductive
Rosette (rV) – a vegetative (i.e., non-reproductive) plant
that does not elongate into nodes and internodes.
Multi-nodal vegetative (mnV) – a plant that elongates into
nodes and internodes but lacks reproductive structures
(buds, flowers, fruits) on any of its stems.
Reproductive
Bud (B) – a multi-nodal plant having at least one unopened
flower.
Flowering (Fl) – a multi-nodal plant having at least one
flowering stem, i.e., a stem having at least open flower.
Immature fruiting (iFr) – a multi-nodal plant with at least
one stem having at least one immature (i.e., “green”) fruit.
Dehiscent fruiting (dFr) – a multi-nodal plant having at least
one mature fruit (i.e., a dried open capsule with reflexed
teeth).
Senescent (S) –a plant whose stems and leaves have
completely dried and turned brown.
Taylor et al. Spalding’s catchfly demography
p. 5 The Nature Conservancy (March 2012)
Demography: Dormancy, Recruitment, and Mortality
Analysis of inter-annual demographic transitions required that we: (1) estimate how many actual
genets (hereafter “plants”) occurred within the plots across the entire 5 year study period; (2)
determine a spatial location for each plant; and (3) assign each field observation of a PG and its
constituent stems to a plant.
We first mapped all PGs observed within plots using the field-recorded azimuth and distance to
plot center using a Geographic Information System (GIS, ArcMap 9.2) and overlaid PG locations
for all 5 years of our study (Figure 4). We then used the ArcMap buffer tool to determine which
PGs, observed in different years, were within 20 cm of each other. We provisionally assigned
those PGs (and their constituent stems) to the same plant. The geographic location of each
plant was determined by averaging the X and Y coordinates of the PGs belonging to it. Rosettes
were always considered distinct from non-rV PGs, regardless of proximity (Lesica and Crone
2007). A PG > 20 cm from any other PG was generally treated as a distinct plant unless strong
evidence existed for its inclusion in another plant. For example, in a few cases, a non-rV plant
appeared where none had been observed for 3 years and this coincided with disappearance of
another PG that had occurred nearby for the previous 3 years. In this case we assessed
whether errors in field mapping of the PG could have resulted in our overestimating the spatial
separation of that PG from others. In cases where the assignment of a PG to a plant was not
possible due to conflicting evidence, we removed it from our demographic analysis (n=25).
Using this procedure we identified a total of 100 distinct plants. The greatest distance between
any single-year PG location and the GIS-derived location of the plant to which it was assigned
was 26 cm; only 8 were >15 cm.
The growth form attained by a plant in a
given year was determined by scaling up
observations made of the stems belonging
to the PG corresponding to that plant. We
set the growth form to be equal to that of
the most advanced phenophase observed
for any stem. Four growth forms were
recognized: D, Rv, mnV, and reproductive
(B, Fl, iFr, and dFr; Box 1). For example, in
2007 plant P17 consisted of three stems:
one stem emerged as mnV and later
disappeared; a second stem emerged,
produced buds, was browsed and lost all
reproductive parts and then senesced; a
third stem emerged, flowered, and
eventually produced dehiscent fruits. Plant P17 would thus be classified as having attained the
Reproductive growth. Transitions from one growth form to another and rules for assessing
mortality and recruitment were determined using rules described in Box 2. Estimates of
dormancy and mortality are sensitive to assumptions made regarding the number of years in
which plants can remain dormant. Based on previous work we assumed that dormancy lasted 1-
3 years (Lesica 1997, Lesica and Crone 2007).
Box 2. Rules used to determine demographic transitions
1. Rosettes were always presumed to be seedlings
(Lesica and Crone 2007).
2. A non-rosette plant (mnV, B, Fl, iFr, dFr) in a location
where no plant had been observed in the previous 1-2
years was considered to have been dormant in the
preceding years.
3. If a plant was observed one year but not in the
following 3 consecutive years it was assumed to be
deceased (Lesica 1997), unless an observation was
made of a non-rV plant in the same location in the
fourth year. In this case the plant was assumed to
have been dormant for 3 years.
Taylor et al. Spalding’s catchfly demography
p. 6 The Nature Conservancy (March 2012)
Statistical Analyses
As this was primarily a descriptive study, statistical analysis was limited to descriptive statistics.
Plants, as determined by the GIS analysis described above, were also the basis of all analyses
of phenology, fruit production, browse by ungulates, and insect predation. Stem level data was
scaled up to plants as follows. The phenophase of a plant on a given date was set to the most
advanced phenophase observed on any of its component stems on that date. For a given date,
a plant was considered to have been browsed or depredated by insects if any of its component
stems had been browsed or depredated. The total number of mature fruits produced by a plant
was set to the sum of the maximum number of fruits observed on each of its component stems.
For each plant we determined the first and last dates for which each phenophase was observed
within each year and averaged these across years to determine average phenology. Rates of
browse and insect predation were determined by dividing the total number of emergent (i.e.,
non-dormant) plants which emerged in a given year by the number affected by each type of
predation in that year.
Results and Discussion
Phenology and Growth Forms
We observed a total of 299 stems belonging to 100 distinct Spalding’s catchfly plants across 84
weeks over the course of the five years of our study. The average date of emergence of non-rV
plants was 24 May (± 6 days SD; min =16 May 2008; Figure 5); first buds were observed 5 Jul
(± 7 days; min = 25 Jun 2009 ) and the first open flowers were observed 16 July (SD = 11; min =
28 June 2007; Figure 5). Seedling rosettes emerged approximately 10 days later non-rV plants,
though sample sizes were small and difficulties in detecting rosettes may have precluded them
from being noted at the earliest opportunity. The average date of peak flowering, that is, the day
on which the greatest number of Fl plants was observed, was 25 July (± 8 days). Reproductive
plants continued to flower and fruit for 4-7 weeks. The average number of days for which the B,
Fl and iFr phenophases were observed were 42 (± 10 SD) , 29 (± 10), and 35 (± 16) days,
respectively. The first mature fruits were observed 14 Aug (± 4 days; min = 8 Aug 2007). Fl and
iFr phenophases occurred earlier in the year in 2007, which was relatively warm and dry; than in
the two cooler/wetter years (2010, 2011) of the study. The typical growing season for Spalding’s
catchfly, calculated as the average number of days from first emergence to last senescence
was 118 days (± 23 days; Appendix 1).
Each year we observed reproductive (B, Fl, iFr, dFr) and mnV growth forms but rV growth forms
(i.e., seedlings) were observed in only 4 of the 5 years. Reproductive plants comprised the
majority (61%) of non-dormant plants whereas sterile, non-rosette plants (mnV) were the
second most common growth form (32%; Figure 6). The relative abundance of mnV plants
varied from year to year with the highest fraction observed in 2011. Non-rV plants were
predominantly single stemmed (81%) while those with two or three stems made up 17% and 1%
of our observations, respectively. A single plant had four stems which was the maximum
number of stems observed.
Non-rosette plants senesced as early as 10 June and in a typical year 50% of plants had
senesced by 3 August (Figure 5; Appendix 1). Rosettes senesced earlier; the average date of
Taylor et al. Spalding’s catchfly demography
p. 7 The Nature Conservancy (March 2012)
senescence was 14 July (± 13 days). Because rosettes and senesced plants are difficult to
detect in the field, our findings suggests that field surveys conducted for the purpose of
monitoring population trends of Spalding’s catchfly on ZPP, which are performed during the
period of peak flowering, likely miss a substantial fraction of plants. This bias has been noted by
others and should be considered in the planning and interpretation of future monitoring efforts
(Hill and Gray 2006, Lesica and Rudd 2012).
Flowering and Fruit Production
Seventeen of the 100 plants produced mature fruits during our study. Of those, 15 succeeded in
doing so only once while 2 plants did so in 2 of the 5 years. Within a year only 15% (±7% SD) of
reproductive plants advanced to the dFr phenophase (Appendix 2). Reproductive failure
occurred at several key stages in catchfly’s life cycle: 28 ± 6% produced buds but failed to
flower; 30% ± 18% flowered but failed to fruit; and 26 ± 9% produced immature fruits but never
produced seeds. A total of 54 mature fruits were produced over the 5 year period. The number
of fruits produced by dFr plants was highly skewed: 69% produced only 1 or 2 mature fruits, and
the maximum number of fruits produced was 8. Fruit production also varied greatly across
years: the lowest fruit production (3) was observed in 2007, a relatively warm/dry year, while the
highest (21) was in 2011, which was much cooler with high rainfall. Transitions from the B to Fl,
Fl to iFr, and iFr to dFr phenophases took 16 (± 4 SD), 11 (± 4), and 18 (± 4) days, respectively
(Figure 5; Appendix 1). The low rates of mature production are likely a consequence of high
rates of ungulate browse and predation of flowers and fruits by insects (see section Insect
Predation and Ungulate Browse).
Dormancy, Mortality, and Recruitment
Plants often transitioned from one growth form to another across years (Figure 7; Appendix 3).
Transitions from D to a reproductive plant (16%) and vice-versa (11%) were most common.
Dormant plants were more likely to reemerge as reproductive plants (64%) than as mnV plants
(36%). Reproductive plants were twice as likely to attain a reproductive growth form the
following year than revert to the mnV form; mnV plants, however, were equally likely to re-
emerge as reproductive or mnV forms.
Of the 100 plants considered in our demographic analyses, an average of 58% (± 14% SD) of
plants emerged in a given year while 42% (± 14%) remained dormant (Figure 6; Appendix 2).
Three-quarters of dormancy events lasted 1 year and 20% were of 2 year duration. We
observed two instances of plants remaining dormant for 3 years. Only 8 plants were observed
as emergent in all 5 years of the study. The fraction of dormant vs. emergent plants varied
greatly from year to year and was approximately congruent to measures of density derived from
counts done across the entire population of the Zumwalt Prairie Preserve. That is, the high
dormancy rates we observed in 2007 and 2010 correspond with the lowest measures of density
observed in that broader scale study (Jansen and Taylor 2010), suggesting that the factors
controlling dormancy of Spalding’s catchfly operate at the scale of the population rather than the
patch. Similar to the population of Spalding’s catchfly studied by Lesica and Crone (2007), the
current year’s weather does not appear to influence dormancy in the catchfly population of the
Zumwalt Preserve. The two years of highest dormancy (2007 and 2010) had nearly opposite
weather during the growing season, the former having the warmest and driest conditions of any
Taylor et al. Spalding’s catchfly demography
p. 8 The Nature Conservancy (March 2012)
year of the study and the latter being the coldest and wettest. The high rate of dormancy we
observed in 2010 is also consistent with their finding that dormancy is more likely the year
following a wet summer. Likewise, the low rates of dormancy in 2011 might be explained by the
high number of dormant plants the previous year (Lesica and Crone 2007).
We estimated that 16 of the 100 plants we observed died during our study with the majority of
those last observed in 2008 (Figure 6; Appendix 2). Mortality estimates are very limited from this
study due to its short duration. We could confidently assess mortality only for plants which were
observed as non-dormant in 2007 and/or 2008, as our criterion for death ( > 2 years with no
emergence) could not be applied to plants observed in the last 3 years of the study. Any plant
which emerged after 2008 and was not observed subsequently was assumed to be dormant but
this method of determining dormancy may include some plants that will not reemerge. Annual
survivorship estimates were 96% and 84% in 2007 and 2008, respectively, which are similar to
than the approximately 90% rate reported by Lesica (1997).
In most years seedlings were uncommon; a total of 18 were observed over 5 years. Consistent
with the findings of Lesica and Crone (2007) the abundance of rV plants was highly variable
across years. No rosettes were observed in 2007 and a maximum of 11 were observed in 2011.
Annual recruitment – calculated as the average number of rV plants divided by the number of
non-rV plants – at 5% (± 5% SD). Although our estimate of the total number of recruits slightly
exceeds our estimate of deaths, it is important to note that we could estimate mortality only for
the 2007-8 and 2008-9 periods. Some plants that we categorized as dormant in 2010 and 2011
will likely not reemerge and would, with continued observation, eventually be reclassified as
mortalities. Thus, we believe that the small population studied here probably declined slightly
across the 2007-11 period.
Insect Predation and Ungulate Browse
Predation by ungulates, insects, or other agents (i.e., plants were found pulled up or could not
be located) was observed on an average of 76% (± 5% SD) of emergent plants each year
(Table 1). Insect damage to reproductive structures affected 47% (± 18%) of all emergent plants
annually. Plants in the B phenophase were most susceptible to insect herbivory (56% of
observations). The most common insect predator we observed was the larvae of Oregon gem
moth (Noctuidae: Heliothis oregonica) which we identified by rearing a moth through
metamorphosis into adult form (Figure 3). Several moth species are known to be common
predators of other Silene species and some may also serve as pollinators (Kephart et al. 2006).
Insect predators and evidence of their damage were found on plants at different phenological
stages but was most commonly observed on plants having unopened buds, open flowers and
immature fruits. Predation on catchfly flowers and fruits by larvae virtually always destroys the
affected flower (RV Taylor and J Dingeldein, pers. obs.). Thus the high rates of predation by
these insects have important consequences for the Zumwalt population. Insect predation has
been shown in other studies to decrease overall reproductive success of other plant species
with consequences for population viability (Vickery 2002). Insect herbivory rates were
substantially lower in 2011 (21%) compared to other years so it may be that in certain years low
moth abundance provides an opportunity for catchfly plants to produce ample numbers of
mature fruits. Although insect predation on the flowers and fruits of Spalding’s catchfly by moth
Taylor et al. Spalding’s catchfly demography
p. 9 The Nature Conservancy (March 2012)
larvae has been noted previously (US. Fish and Widlife Service 2007, Youtie 2009), this is the
first measure of the prevalence of this phenomenon.
Ungulate browse was observed on an average of 32% of plants (± 15% SD) and 18% (± 15%
SD) of plants had at least one stem pulled from the ground or otherwise disappeared prior to
senescence (Table 1). The study area is excluded from cattle grazing, mule deer numbers are
low, and elk scat was frequently observed in the plots (J Dingeldein, pers. obs.). We thus
suspect that elk are the primary cause of the browsing and pulling of plants. Browsing may be
one cause for low fruit production in this population of Spalding’s catchfly. It should be noted
that the rate of browsing was higher in the last two years of the study than in the initial three and
that elk numbers in the area increased sharply over the course of our study; numbers increased
from 1400 to 3500 individuals over the 5 years of this study (Oregon Department of Fish and
Wildlife, unpublished data). Browsing of Spalding’s catchfly by cattle has been identified as a
possible threat for populations occurring where livestock are pastured (US. Fish and Widlife
Service 2007). However, a study of browse rates in areas having different cattle stocking rates
found little evidence that cattle consume significant numbers of this species (Cullen and Taylor
2010).
Conclusions and Management Implications
Spalding’s catchfly has a complex life history that includes periods of prolonged dormancy
(Lesica 1997, Lesica and Crone 2007). Our study confirms that a significant fraction of catchfly
plants on the Zumwalt Prairie are dormant each year which complicates the task of assessing
population trends for this species (Lesica and Steele 1994) and should be taken into
consideration in designing future monitoring efforts (Lesica and Rudd 2012). Specifically, we
believe it is necessary to either (1) estimate dormancy for each year based on random plot
locations and adjust observed values to account for the fact that only plants which emerged
could be counted; or (2) mark plants at fixed plot locations and merge observations across
years to estimate the total number of plants alive within a fixed (e.g., 3 year) multi-year period.
The Spalding’s catchfly population we observed for this study experiences very high rates of
ungulate predation and insect predation of flowers and fruits. High rates of predation coupled
with low seed production and viability result in low reproductive output that may be ultimately
affect the viability of the Spalding’s catchfly population on the ZPP. Of particular concern is the
recent increase in the elk population of the Zumwalt Prairie area. Predation by ungulates nearly
always results in total reproductive failure as the flowers or developing fruits are consumed.
Given the current population of approximately 3500 elk, it is likely that Spalding’s catchfly
populations currently experience unprecedented levels of wild ungulate herbivory in this area.
We encourage land managers to continue to explore opportunities to decrease the numbers of
elk in the area with the expectation that this would yield benefits for Spalding’s catchfly.
It is difficult to speculate as to the cause of the high insect predation levels that we observed on
Spalding’s catchfly plants on the ZPP compared to other locations, thus we can offer no specific
management advice to abate this potential threat. Rather, we encourage further investigation
into this phenomenon. One specific avenue of research might investigate the fire ecology of
Spalding’s catchfly with relation to insect herbivory. Inhabiting an area that experienced frequent
fire for millennia, Spalding’s catchfly has been shown to tolerate fire and to have increased
Taylor et al. Spalding’s catchfly demography
p. 10 The Nature Conservancy (March 2012)
recruitment success following burning (Lesica 1999). Furthermore, insect predation on
grassland wildlfowers has been shown in other studies to be influenced by fire. Vickery (2002)
found that populations of northern blazing star (Liatris scariosa var.novaeangliae) which were
excluded from fire had high rates of predation by moth larvae and virtually no reproduction.
Sites where prescribed fire was implemented temporarily reduced seed predation from
approximately 90% to 16% in the year following the burn (Vickery 2002). We believe that
research into the role that fire may play in insect predation of Spalding’s catchfly should be a
high priority.
Even when Spalding’s catchfly does succeed in producing seed, the quality of seed may be low.
A study of germination rates of seeds collected from the Zumwalt population found that only 9%
of seeds germinated (Taylor and DeBano 2012). Using our data on fruit production along with
estimates of seed production and seed viability, we estimate that each Spalding’s catchfly plant
produces on average approximately 1 viable seed per plant per year. The cause of low
germination rates is not known, but pollination limitation is one possibility (Gonzales et al. in
review). Managing for an abundance of pollinator insect species is one way to reduce the
likelihood that low seed viability leads to declines in Spalding’s catchfly populations.
The demographic analysis of Spalding’s catchfly in this study is insufficient for a robust
estimation of population viability. The relatively short duration of this study, high variation in the
production of seedling rosettes, and periods of prolonged dormancy all conspire to diminish the
accuracy and precision of our estimates of recruitment and mortality, which are essential for
robust population modeling (Menges 2000). Furthermore, it may be that the small patch of
plants studied here is not representative of the greater population of Spalding’s catchfly on the
ZPP. Rather than continuing the demographic studies of Spalding’s catchfly, we recommend
instead that a carefully designed population monitoring program track trends in the numbers of
this plant into the foreseeable future. Monitoring should include some measure of fruit
production, insect predation, and ungulate herbivory, the factors which were identified by this
study as posing the greatest threat to the Spalding’s catchfly population on ZPP. Should
declines become apparent through long-term monitoring, the knowledge gained through this
study should be used to formulate future research priorities to understand and address the
causes.
Acknowledgements
The US Fish and Wildlife service supported this work through a grant to RVT (Agreement #
F10AC00090). We thank Peter Lesica for his pioneering work on the study of Silene spaldingii
upon which this study rests heavily. Dana Ross deserves a round of applause for his assistance
in identifying the catchfly predator Heliothis oregana (Noctuidae) and Vincent Jansen lent a
strong and solid hand in our GIS analysis of plant locations. Finally, we thank the members of
The Nature Conservancy for their support of conservation science at the Zumwalt Prairie and
beyond.
Taylor et al. Spalding’s catchfly demography
p. 11 The Nature Conservancy (March 2012)
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Lesica, P. 2008. Detection error associated with observing Silene spaldingii at four sites in Montana and
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Lesica, P. and E. Crone. 2007. Causes and consequences of prolonged dormancy for an iteroparous
geophyte, Silene spaldingii. Journal of Ecology 95:1360-1369.
Lesica, P. and B. Heidel. 1996. Pollination biology of Silene spaldingii. Unpublished report, Montana
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Lesica, P. and N. Rudd. 2012. Guidelines for Monitoring Trend of Silene Spaldingii Populations in Key
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Lesica, P. and B. Steele. 1994. Prolonged dormancy in vascular plants and implications for monitoring
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Tables
Table 1. Predation on Spalding’s catchfly plants by insects and ungulates and other agents (i.e.,
pulled). The number of plants affected by each type of predation along with the fraction of
emergent plants affected is provided. Averages are shown for all years.
2007 2008 2009 2010 2011 Avg ± SD
Insect, Browsed, or Pulled
# plants affected 28 39 36 26 45 34.8 ± 7.9
% emergent 80% 74% 84% 72% 71% 76.2% ±
5.4%
Insect damage
# plants affected 20 31 28 15 13 21.4 ± 7.9
% emergent 57% 58% 65% 42% 21% 48.6% ± 17.8%
Browsed
# plants affected 4 11 12 15 31 14.6 ± 10.0
% emergent 11% 21% 28% 42% 49% 30.2% ± 15.3%
Pulled
# plants affected 10 9 17 1 4 8.2 ± 6.1
% emergent 29% 17% 40% 3% 6% 18.8% ± 15.3%
Taylor et al. Spalding’s catchfly demography
p. 13 The Nature Conservancy (March 2012)
Figures
Figure 1. (a) The Zumwalt Prairie (red) is a large remnant of the once extensive Pacific
Northwest Bunchgrass biome. (b) Within the Zumwalt Prairie, a large population of Spalding’s
catchfly (Silene spaldingii; red points) occurs on The Nature Conservancy’s Zumwalt Prairie
Preserve (bright green). (c) The locaiton of the 6, 2m radius plots used for this study (triangle).
Taylor et al. Spalding’s catchfly demography
p. 14 The Nature Conservancy (March 2012)
Figure 2. Three growth forms of Silene spaldingii: rosette, multi-nodal vegetative, and flowering
(clockwise from top left).
Taylor et al. Spalding’s catchfly demography
p. 15 The Nature Conservancy (March 2012)
Figure 3 . A common insect predator of Silene spaldingii, the larvae of the noctuid moth
Heliothis oregonica is often observed feeding on the immature ovaries and stamens of catchfly
plants on the Zumwalt Prairie. Shown here is the larvae (left) and the adult moth (right).
Figure 4 . Spatial locations of Spalding’s catchfly plants observed in one plot over 5 years of
study. Triangles indicate locations of putative genets observed weekly in the field; gray circles
are plant locations determined by mapping multiple years of observations in a Geographic
Information System. Triangles of the same color indicate putative genets that were determined
to belong to the same plant.
Taylor et al. Spalding’s catchfly demography
p. 16 The Nature Conservancy (March 2012)
Figure 5 . Phenology of Spalding’s catchfly on the Zumwalt Prairie Preserve (2006-11). Red
ovals indicate the mean first observed date of a phenophase across all years and blue
rectangles indicate the mean of the last observation of that phase. Bars indicate 95%
confidence intervals.
100 150 200 250 300
Senesce
dFr
iFr
Fl
Bud
Emerge
May Jun Jul Aug Sep Oct Apr
Taylor et al. Spalding’s catchfly demography
p. 17 The Nature Conservancy (March 2012)
Figure 6 . Number of dormant and emergent (reproductive, vegetative, and rosette seedlings)
Spalding’s catchfly plants observed each year of the study. The height of each bar represents
the total estimated population size for that year. The number above each bar indicates the
number of plants estimated to have died between that and the prior year (available only for
2008-9).
0
10
20
30
40
50
60
70
80
90
2007 2008 2009 2010 2011
Nu
mb
er
of
Pla
nts
Dormant
Reproductive
Vegetative
Recruits
Taylor et al. Spalding’s catchfly demography
p. 18 The Nature Conservancy (March 2012)
Figure 7. Inter-annual demographic and growth form transitions of Spalding’s catchfly on the Zumwalt Prairie Preserve (2007-11).
The weight of each arrow is proportional to the number of individual plants that underwent the indicated transition. For abbreviations
see Box 1.
Taylor et al. Spalding’s catchfly demography
p. 19 The Nature Conservancy (March 2012)
Appendices
Appendix 1 – Average dates of first and last observations of growth forms and phenophases for Spalding’s catchfly plants on the Zumwalt Prairie Preserve (2007-11). See Box 1 for abbreviations.
mnV B Fl iFr dFr S
Yr First Last First Last First Last First Last First Last First Last
2007 31-May 28-Jun 4-Jul 1-Aug 28-Jun 25-Jul 18-Jul 16-Aug 8-Aug 16-Aug 10-Jul 22-Aug
2008 16-May 15-Aug 4-Jul 22-Aug 18-Jul 29-Aug 1-Aug 22-Aug 15-Aug 4-Sep 18-Jul 12-Sep
2009 21-May 18-Jun 25-Jun 13-Aug 16-Jul 19-Aug 23-Jul 9-Sep 13-Aug 24-Sep 11-Jun 30-Sep
2010 28-May 8-Jul 8-Jul 26-Aug 22-Jul 19-Aug 29-Jul 23-Sep 19-Aug 29-Sep 3-Jun 20-Oct
2011 25-May 28-Jul 14-Jul 18-Aug 28-Jul 11-Aug 4-Aug 25-Aug 18-Aug 1-Sep 14-Jul 14-Sep
Avg 24-May 13-Jul 5-Jul 16-Aug 16-Jul 14-Aug 27-Jul 31-Aug 14-Aug 8-Sep 29-Jun 19-Sep
Taylor et al. Spalding’s catchfly demography
p.20 The Nature Conservancy (March 2012)
Appendix 2 – Growth forms, dormancy, recruitment and mortality
For each year the table below provides the number of plants of each growth form (see Box 1)
and maximum phenophase observed each year. Estimates of the number of deaths and
recruitment by year are also shown.
2007
2008
2009
2010
2011 Avg ± SD
Emergent 35
53
43
36
63 46 ± 11.9
Non-reproductive 17
14
8
11
45 19 ± 14.9
rV 0
2
1
4
11 4 ± 4.4
mnV 17
12
7
7
34 15 ± 11.2
Reproductive 18
39
35
25
18 27 ± 9.7
B 5
7
11
8
6 7 ± 2.3
Fl 7
22
8
5
2 9 ± 7.7
iFr 4
6
13
6
6 7 ± 3.5
dFr 2
4
3
6
4 4 ± 1.5
Dormant 48
29
27
38
22 33 ± 10.3
Mortality - 3 - 13 - NA - NA
4 ± 6.2
Recruits + 2 + 1 + 4 + 11
5 ± 4.5
Total Plants 83
81
58
78
96 79 ± 13.7
% Emerged 42%
65%
74%
46%
66% %59 ± %13.8
% Dormant 58%
35%
26%
54%
34% %41 ± %13.8
Taylor et al. Spalding’s catchfly demography
p. 21 The Nature Conservancy (March 2012)
Appendix 3 – Year to year transition probabilities (2007-11)
(a) Number of plants
Yrs 1->2 1->D 1->M 2->2 2->3 2->D 2->M 3->2 3->3 3->D 3->M D->2 D->3 D->D Total
2007-08 0 0 0 4 5 6 2 1 13 3 1 7 21 19 82
2008-09 0 0 2 1 6 2 3 5 19 7 8 1 10 17 81
2009-10 0 1 0 3 2 2 0 2 12 21 0 4 10 12 69
2010-11 1 3 0 5 1 1 0 13 8 4 0 15 9 13 73
Total 1 4 2 13 14 11 5 21 52 35 9 27 50 61 305
(b) Frequency
Yrs 1->2 1->D 1->M 2->2 2->3 2->D 2->M 3->2 3->3 3->D 3->M D->2 D->3 D->D Total
2007-08 0% 0% 0% 5% 6% 7% 2% 1% 16% 4% 1% 9% 26% 23% 100%
2008-09 0% 0% 2% 1% 7% 2% 4% 6% 23% 9% 10% 1% 12% 21% 100%
2009-10 0% 1% 0% 4% 3% 3% 0% 3% 17% 30% 0% 6% 14% 17% 100%
2010-11 1% 4% 0% 7% 1% 1% 0% 18% 11% 5% 0% 21% 12% 18% 100%
Total 0% 1% 1% 4% 5% 4% 2% 7% 17% 11% 3% 9% 16% 20% 100%
(c) Matrix
From
D mnV rV Reproductive
To
D 20.0% 3.6% 1.3% 11%
rV 0.0% 0.0% 0.0% 0.0%
mnV 8.9% 4.3% 0.3% 7%
Reproductive 16.4% 4.6% 0.0% 17%
M 0.0% 2% 1% 3%
M = Mortality; 1 = rV; 2 = mnV; 3 = B, Fl, iFr, dFr; See Box 1 for more information.