Population As a Function of Age to Reproductive Maturity Among European Buckthorn

Nathan Turk, Jose Lopez, Tom Ryan

For 322

Lab #3: writeup

October 6, 2005

Population as a Function of Age to Reproductive Maturity Among European Buckthorn

Abstract:

Altering the age at which European Buckthorn achieves reproductive maturity (M) will affect the population demography of that population. We expected to find how increasing or decreasing M will cause carrying capacity to be reached in a shorter or longer amount of time, respectively. A total of 5 different values were selected for the age of reproductive maturity, including the default value of 5 years. Carrying capacity was reached more quickly when maturity ages were decreased, and time to carrying capacity was increased as age of maturity increased. When age of reproductive maturity was set to 10 years the species did not survive. Our results agreed with our hypothesis since the population of European Buchthorn reached carrying capacity in a shorter expand of time when the M was lower and in a longer expand of time when M was increased. But these was only true for a certain amount of time because if M is increased too much (10 years), the population would not achieve maturity so they all died before reaching carrying capacity.

Introduction:

Plant population and structure are ever going processes that have a major impact in all terrestrial environments. That is why the study of plant population changes over time, or plant demography, is such an important part of ecology. Many factors and their interactions are the causes of the changes in population structure of forests. These factors include environmental conditions, resource availability, competitors, and disturbance (Barbour, Burk, Gillam, and Schwartz, 1994). The outcome of these population dynamics can be changes in the death and birth rates, age structure, the carrying capacity and the reproductive rate, among variables. The object of this study was to see how different factors could affect the demography of a population. For this, we altered the age at which European Buckthorn achieves reproductive maturity (M) using the POPSIM growth simulator program. From this we expected to find how the amount of time until carrying capacity is reach is reduced, when M is reduced, and is increased, when M is increased.

Methods:

Within the POPSIM simulations, the time taken for European buckthorn to reach carrying capacity of the site was altered by changing the age of reproductive maturity of individuals within this stand. The default time to reproductive maturity is 5 years, for this experiment we altered it to have the values of 1 year, 3 years, 7 years, and 10 years. Although altering this aspect of the buckthorn’s growth should not affect the actual numerical carrying capacity, an unexpected glitch within the programming created a differentiation between the maturity levels.

Results:

At 1-year maturity the carrying capacity was reached within approximately 6 years (fig. 1). At 3-year maturity the carrying capacity was reached at approximately 24 years. Five-year reproductive maturity, the default for this program, reaches carrying capacity at approximately 35 years. When the European buckthorn’s reproductive maturity was changed to 10 years, carrying capacity was never reached, mortality was too high for the species to perpetuate itself under these specific conditions (fig.2).

Discussion

European Buckthorns’ population explosion when just one year was required for sexual maturity (fig. 1) was a result of simultaneous regeneration. Each year, new generation of shrubs reproduced side-by-side with their parents, their parents’ parents, and so on, producing an exponential population growth with the addition of each new generation. Team Gray Van’s data indicated that lowering the survival rate of mature individuals delayed the population’s attainment of carrying capacity, and thus indicated a way in which exponential growth’s most negative ecological component—full realization of the invasive species’ population potential (Myers & Bazely, 2003)—was managed. However, Team Gray Van also found that lowering the survival rate (qs factor) to 1 precluded carrying capacity status entirely, since seedlings experienced mortality before they could reproduce. Terminal mortality also occurred when our shrubs’ age to reproductive maturity was set to 10 years (fig. 2), because no trees persisted long enough to produce a second generation.

Trees that do survive, all groups’ results show, exhibit growth like that of our exponential model, just that it’s spread over increasing increments as mature trees’ and seedbank seeds’ survival rates decrease, or when age to reproductive maturity increases. For example, Team Gray Van’s population of qs.5/qz.9 (moderate seed survival/high mature tree survival) grew from less than 100 in year 10 to 800 in year 20 and full site capacity, approximately 4500, in year 30. Buckthorns’ population size maintenance depends on the shrub’s ability to produce a constant number of seeds (Randall & Marinelli, 1996), which may be why Team Xtreme’s reduction of the number of females able to produce these seeds delayed full site capacity longer (approximately 70 years) than our increasing of age to reproductive maturity could (56 years) in POPSIM trials.

Conclusions

Early attainment of reproductive maturity is important in a species’ invasiveness because there are more individuals to ensure that the population is established. The longer the time between seed establishment and growth into a reproducing individual, the greater the risk of mortality before reproduction. For invasive species such as European buckthorn, late maturation is an evolutionary disadvantage, our experiment shows. Considering the magnitude of seeds that invasive species must produce to accomplish exponential growth, prevention efforts could benefit from scrutinizing harvests of mulches, feeds, commercial seeds and other harvestable items in natural environments in which invasive species’ seeds could infiltrate. Another context in which knowledge of reproductive maturity can be applied is with timeframes regarding when to enact measures to prevent invasive species’ establishment. The earlier the age to reproductive maturity, the sooner the establishment prevention should take place, our results show.

References

Randall, John M, Janet Marinelli. 1996. Invasive Plants. Brooklyn Botanic Garden, Inc. New York: pp. 10 & 64.

Myers, Judih H, Dawn R. Bazely. 2003. Ecology and Control of Introduced Plants. Press Syndicate of the University of Cambridge. Cambridge: 89-96.

Barbour, M.G., S. H. Burk, W.D., M. W. Schwartz. 1994. Terrestrial Plant Ecology. New York, NY: Addison Wesley Longman, Inc.