Population Dynamics of Spring Ephemerals
Introduction:
Spring ephemerals are small herbaceous plants that grow on the forest floor and are
native to the northeastern United States. They are unique because they grow at a time
period between the winter and summer cycles of growth. They are characterized by their
ability to grow at low temperatures usually after the first snow melt, and continue their
lifecycle until the development of the forest canopy (Muller 48). They thrive on the high
levels of resources such as water, sunlight and nutrients and low levels of competition
from other plants at this time of year (Muller 48). Spring ephemerals are an important
part of a northern hardwood ecosystem. Early spring is when high levels of valuable
nutrients such as nitrogen and potassium are lost from the ecosystem. One study found
that spring ephemerals play the role of soaking up and conserving these nutrients acting
as a “vernal dam” and thus conserving these vital nutrients in the system (Zak 651).
Another study found that with disturbance of habitat, spring ephemerals are not easily
able to recolonize an area due to the fact that they need low levels of competition and
high levels of resources (McLachlan 99). From these two studies one would conclude that
spring ephemerals are an important part of a northern hardwood forest ecosystem and that
it is worthwhile to study their population dynamics.
Matrix models are an important part of understanding the population dynamics of a
particular species, in this case of this study the spring ephemeral Cardamine concatenate.
A matrix model allows the population growth rate to be calculated with the assumptions
that the birth-death rate is constant and that resources are unlimited. It is also useful in
determining the life history strategy of a species, more specifically what proportions of
resources are devoted to growth and reproduction and which stage has the highest
influence on the growth rate.
The growth rate of spring ephemerals is what determines if the population is stable,
increasing or decreasing. Factors that may influence the population growth rate are
habitat characteristics such as amounts of sunlight, nutrients and water that are available.
Factors from species interactions such as pollination, competition and herbivory may also
have an effect on the population growth rate. In conducting this study our broad aim was
to understand the population dynamics of Cardamine concatenate. More specifically, we
tried to answer the questions; does Cardamine concatenate devote more resources to
growth or reproduction? Does pollination have an effect on population growth rate? Does
the density of a patch have an effect on growth rate? Our specific expectations were that
individual populations with more flowers would have more pollination and thus a higher
growth rate and that due to competition for resources, the smaller less dense patches
would have higher growth rates. With these questions in mind, we conducted an
experiment to better understand the population dynamics of spring ephemerals.
Methods:
Study system
This study was conducted at the Kalamazoo Nature Center in southwestern Michigan.
The system where the study took place was a maple beech forest in Kalamazoo County,
the most common type of forest in Michigan. The specific spring ephemeral investigated
in this study was Cardamine concatenate more commonly known as cutleaf toothwort.
Cutleaf toothwort is a perennial woodland plant that blooms in early or mid spring and is
native to eastern North America (wikipedia encyclopedia).
Data collection
Four different populations of cutleaf toothwort were selected for observation. A
quadrangle of area 0.25 meters squared was used to section off part of each population.
Observations were made as to the size of the patch, other spring ephemerals that were
present in the same patch, estimates of levels of sunshine, soil moisture and general
observations of the location such as near water or on a sloped hillside. Within the
quadrangle, the number of individual plants were counted and divided into three different
stages of growth: 1 leaf small plants with leaflet diameter of less than 3 cm, 1 leaf large
plants with leaflet diameter of more than 3 cm, and flowering plants. From there the
numbers of flowers (including buds) were counted and flowers with at least two petals
fully opened were observed to see if they were pollinated in a time span of ten minutes.
Matrix models
After data collection, the data was organized and placed into static life tables which
separated the different stages of growth and the number of individuals that were in each
stage. The life tables helped us to determine survivorship, distribution of growth and
reproduction. These static life tables constructed with the assumptions that the population
was in equilibrium, that there were constant birth and death rates, that the sample was
unbiased and that resources were unlimited. A life table was made for each of the four
populations as well as a life table for the total numbers from all of the populations. From
the life tables, matrix models were constructed. The matrices were constructed based on
the percentage of individuals surviving from the previous stage of growth in the life table.
In some cases we had more individuals in our final stage than in previous stages; we
interpreted these differences as individuals who had survived from the previous year. The
matrices were then used in the program RAMAS in order to collect data on the growth
rate of the population, the elasticities within the different growth stages, a stable
age/stable structure graph which showed the proportion of individuals in each stage of
growth and a trajectory summary predicting whether the population will increase,
decrease or remain the same over time based on the growth rate. We took one of our
matrices for a population and manipulated it based on a hypothetical scenario and
observed the consequences on population growth after changing one of the elasticity
values. In this manipulated model we were able to see how a scenario such as increased
herbivory or increased pollination could have a great effect on the growth rate of the
population.
Results:
Distribution of resources: growth vs. reproduction
A stable age/stable structure graph for all four populations was made in order to
investigate the proportions of individuals in each stage of growth. The difference between
stages is representative of the survivorship from one stage to the next. For the overall
population approximately 65% of individuals were in stage 1, 20% in stage 2 and 15% in
stage 3. The stable age/stable structure graph for the total population can be seen in
Figure 1.
Figure 1. Stable Age/Stage Structure graph showing the overall distribution of individual
plants from all four populations in each of the three stages of growth. The y axis
represents the proportions of individuals in each stage. The x axis represents the three
stages of 1 leaf small, 1 leaf large and flowering.
To investigate which stage the most resources were devoted to, the elasticity values were
used from our matrix models. Elasticities between stages 1-2 and 2-3 represent the
amount of resources devoted to growth, whereas the elasticity from stage 3-1 represents
the amount of resources devoted to reproduction. Data for the elasticities between all
stages in the matrix for each individual population and also the total population can be
seen in Table I.
Table I: Elasticity values between the stages in the life table for the four individual
populations and the total population.
Popn Elasticity from stage 1-2 Elasticity from stage 2-3 Elasticity from stage 3-1
1
0.3021
0.3022
0.3021
2
0.3123
0.3552
0.1590
3
0.1666
0.4331
0.4000
4
0.2104
0.2105
0.2105
Total 0.3246
0.3588
0.3114
Effect of pollination
One factor that was hypothesized to affect the population growth rate was pollination. If
the population had a higher number of pollinations, then the population should be
increasing at a higher rate than if there were a fewer number of pollinations. Another
expectation was that when the number of flowers increased so would the number of
pollinators visiting. Data collected from the four populations over a ten minute time span
on number of flowers, number of pollinators and growth rates can be seen in Table II.
Table II: number of flowers, number of pollinators visiting, number of flowers per
individual, number of pollinations per flower and calculated growth rate of the four
populations.
Popn # flowers
#pollinators
# flowers/indvidual #pollins/flower Growth
visiting
rate (λ)
1
22
2
9
.09
0.9941
2
4
4
11.75
1
0.9967
3
3
0
6.8
0
0.9987
4
2
0
8.73
0
0.9991
Effect of density
Another factor that may affect growth rate is the size of the patch that the population was
taken from and also the density of plants within the quadrangle. The expectation was that
larger, more densely populated patches would have lower growth rates than smaller more
sparsely populated patches. Data collected on patch size, number of individuals within
the quadrangle, number of individuals per patch and growth rate can be seen in Table III.
Table III: Patch Size, Number of Individuals in a Quadrangle and Growth Rate for Four
Populations of Cutleaf Toothwort.
popn
patch size (m2)
# of indvs/quadrangle
Growth rate (λ)
1
17.6
88
0.9941
2
283.7
64
0.9967
3
234
62
0.9987
4
48
22
0.9991
Discussion:
Distribution of resources: growth vs. reproduction
From the data collected it was determined that the majority of individual plants are in
stage 1, 1 leaf small and that the number decreases to stage 2, 1 leaf large and further
decreases to stage 3, flowering (Figure 1). This follows a typical rate of survivorship
within a population and that few individuals survive to the flowering or reproducing
stage. In the investigation of what proportion of total resources are used for growth and
reproduction, the overall values were fairly even when looking at the total population.
The elasticity from stage 3-1 (0.3114) was slightly lower than the elasticities from stage
1-2 (0.3246) and 2-3 (0.3588). This indicates that resources are distributed fairly evenly
between the three stages, the first two representing growth and the last one representing
reproduction; overall more resources are devoted to growth rather than reproduction.
This is most likely because spring ephemerals are perennial plants, they survive from
year to year and do not have to devote as much energy to reproduction as annual plants.
Effect of pollination
The data collected on pollination did not have a correlation with the growth rates. The
number of flowers also did not correlate with the number of pollinators visiting. It was
determined that the population with the highest growth rate had the lowest number of
flowers and pollinators visiting. As discussed previously, pollination may not be a strong
indication of growth rates because Cardamine concatenate was found to spend more
resources on growth and not reproduction.
Effect of density
One trend that was found was that the data collected for number of individuals per
quadrangle did correlate with the growth rate of those populations. It was found that the
higher the density of the quadrangle, the lower the growth rate. This could be due to
higher density quadrangles having fewer nutrients for each individual which was the
expectation. However, relatively speaking the growth rates for all four populations were
highly similar and no significant differences based on density having a strong correlation
with growth rate could be found.
Problems and assumptions of this study
In this study, no concrete conclusions were drawn. This was largely due to the problem of
sample size. Only four populations were observed and in order to draw a conclusion the
sample size would have to be much larger and also randomly selected, instead of selected
on the basis of convenience. Stronger conclusions could be made if the populations were
randomly selected and if the sample size were larger. One assumption that we made
while conducting this study and analyzing the data with matrix models was that the
populations were all in equilibrium with constant birth and death rates. Another strategy
that could be used would be to follow a single population over the course of many years
in order to determine exactly how many individuals went from one stage to the next. In
any case our questions would have more sound answers with a longer time frame and
more in depth study.
Implications for management
Many studies have shown the importance of spring ephemerals. This study attempted to
better understand the population dynamics of the species Cardamine concatenate in order
to know what effects population growth so that conservation measures could be taken.
Habitat destruction is something that causes the population of spring ephemerals to
decline, without these herbaceous plants ecosystems can lose valuable nutrients.
Prevention of deforestation either for lumber or agricultural means is the most important
method when it comes to preserving the habitat of these spring ephemerals.
Work Cited:
McLachlan, Stephanie M. Recovery Patterns of Understory Herbs and Their Use as
Indicators of Deciduous Forest Regeneration. Conservation Biology. Vol. 15, No
1 (Feb., 2001), pp. 98-110.
Muller, Robert N. The Phenology, Growth and Ecosystem Dynamics of Erythronium
americanum in the Northern Hardwood Forest. Ecological Monographs. Vol. 48,
No 1 (Winter, 1978), pp 1-20.
Wikipedia Encyclopedia. <http://en.wikipedia.org/wiki/Cardamine_concatenata> April
24, 2006.
Zak, Donald R. The Vernal Dam: Plant-Microbe Competition for Nitrogen in Northern
Hardwood Forests. Ecology. Vol 71, No. 2 (April, 1990), pp. 651-656.