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Population Dynamics of Spring Ephemerals Introduction


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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 visiting

# flowers/indvidual

#pollins/flower

Growth 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.


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