Forest Ecology Lab 3
Tree Demography Computer Simulation Exercise
Team Grey Van: Eric North, Andrew Testo, James Werner
10/6/05
Abstract
Computer analysis allows a simulation of possible growth potential for plant
species. By manipulating different factors that lead to growth, a person can determine
what will happen in a species when certain conditions are met. In this experiment
computer simulation was used to predict what will happen to European buckthorn when
changes are made to survival of seeds in the seed bank or the survival of mature trees.
We found out that if the seed bank survival is low then the population will die out. This
is untrue with mature trees. If the mature tree population is low it will just take longer for
European buckthorn to recover. In order to keep this species at a low population one
would have to import exotic species or use herbicides to control the European buckthorn
population.
Introduction
Invasive species can be a difficult challenge to natural resource managers.
Because many invasive species have no natural predators or diseases in the area they are
introduced, their populations often go unchecked and they are able to out-compete the
local fauna. In order to control the population of an invasive, or any species, it is
important to understand that species demography. Often the most effective means of
controlling a species is limiting its reproductive ability. The two most important factors
in tree species demography in respect to reproduction are the survival of mature seed-
producing trees and the survival of those seeds in the seed bank. In this experiment a
computer simulation was run for the survival of European buckthorn over time, while
changing the amount of living mature trees and surviving seeds in the seed bank. It is
expected that reducing the number of seeds will be more effective than removing mature
trees in reducing the persistence of European buckthorn in the ecosystem. This is
hypothesized because the combined reproductive potential of seed is far greater than that
of allowing the existing mature trees to persist.
Methods
A computer simulation of European buckthorn was utilized in order to check
effects on population when plant variables were manipulated. The population was
manipulated in two ways to see how final population would change. The mortality rate of
seeds in the seed bank and mortality rate of mature (5 year old) female buckthorn were
varied over a 75 year period. Five different comparisons were used to see the final
population. These five manipulations included 10% survival of seeds in seed bank (QS)
and 90% survival of five year old mature females (QZ), 50% survival QS and 90%
survival QZ, QS survival 90% and QZ survival 90%, QS 90% survival and QZ 50%
survival, and QS 90% survival and QZ 10%. The was no need for comparing site quality
of any sort as this was one plot the entire run, and no conditions other than survival rate
were recorded.
Results
Fig. 1
Population
Population Trends in European Buckthorn
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
time
qs .1 qz.9
qs .5 qz.9
qs.9 qz .9
qs .9 qz .5
qs .9 qz .1
10
20
30
40
50
60
70
Year
qs=survival of seed bank (% as decimal)
qz=survival of 5 year-old mature female trees (% as decimal)
Fig 1 shows that with only 10% of seeds surviving in the initial seed bank, the population
does not recover. Whereas, when only 10% of 5 year-old mature trees are initially left,
the population recovers but shows fluctuations.
Discussion
Our results concur with our original hypothesis showing that reducing the
amount of seeds in the seed bank more effectively reduces the persistence of European
buckthorn. When ten percent of the seed bank survives initially, and survival rate of
mature females is unchanged (90% survival) the population of buckthorn is effectively
eliminated. When half of the seed bank survives there is a larger lag time in recovery, but
the population eventually reaches carrying capacity. When the seed banks survival is
90% and the female mature trees population survival is at 10%, the population goes
through fluctuations. These fluctuations can be explained by the 5 year period the
program assumes it takes a tree to mature. The graph “jumps” up as this new cohort
reaches maturity and reproduces. However, this would likely not happen in nature
because a trees ability to reproduce is less dependant on age than it is on a minimum size.
Conclusion
Through this study it was found that the most effective method of reducing the
population of European buckthorn was destroying the seeds in the seed bank, supporting
the original hypothesis. The most probable management application for these findings, in
an attempt to reduce the population of the invasive buckthorn, would likely be an
herbicide application which impedes germination or the production of seeds by mature
trees. However, it would be likely that non-target species would be affected. It may be
possible to introduce an herbivorous exotic species or disease which effectively reduces
the number of seeds in the seed bank; however that may lead to a new invasive species
problem.