Fire Effects on Forest Habitats: An Individual Based Model

1. Fire Effects on Forest Habitats: An Individual Based Model Byron Roland andIstvan Karsai, Department of Biological Sciences, East Tennessee State University, Johnson City, TN, 37614 Abstract HYPOTHESES Fires are interesting in that they are both deadly and yet necessary in forest habitats. Fires kill animals and trees and allow for rebirth of a forest. State Parks sometimes use controlled fires to destroy old worn down areas of forest to allow for regrowth of newborn trees and new wildlife. We are studying the fires effects on forest habitats. These forest habitats are commonly ravaged by fires and we use a program called Netlogo, which is an agent based modeling environment, to model this ecosystem. In this study the interesting dynamics between the plants, animals, and fires were mimicked as well as the consequences of forest destruction. For the sake of simplicity we assumed a very basic life history using birth, growth, reproduction, and death for both of the animals and trees. These animals and trees are also prone to death by fires. The dispersion of the fire is dependent upon the place of initiation and the amount of fuel available, which corresponds to the density of the trees. We addressed a few problems specifically: -How the strength and frequency of the fires will affect the survival of the animal population? -How the connectedness of the animals to the vegetation affect their survival? -How the dependency of the animals on trees to reproduce will affect their reproduction rate? -How does fire strength (number of initiation points) affect the ecosystem dynamics? Null Hypothesis : The dynamics of the forest ecosystem will not be affected by the fire. Hypothesis 1: The fires will have a considerable effect on the forest ecosystem. Prediction 1: If true, we predict that the animals will die out as a direct result of the fires. Prediction 2: If true, we predict that the ecosystem will change dynamically. The perturbation of the fires are compensated by fast repopulations. Prediction 3:High density of the trees will cause massive fires and, as a consequence, there will be a massive decrease in population size. Fig 5. The number of burned animals over a period of 100 years. Notice the population size of animals decreasing as the fire strength increases. Figs 6. The average annual growth rate of the animals with no fires, notice as the number of initial animals increases the average growth rate decreases, this is because the animals are limited in their breeding based on the number of animals currently present around them (carrying capacity). Figs 1-3. Simulations of the ecosystem. These screenshots show the fires burning the animals, also the animals breeding only around the trees and the trees getting burned by the fires. Modeling • We modeled a simple ecosystem using an agent-based approach. In an agent-based model individuals interact with each other and these local interactions drive the system via simple rules. • For simplicity we assumed the following: • The habitat is closed • The organisms do not have an explicit metabolism • The animals birth depends on a coverage of the trees and a low density of other animals nearby • The animals death depends upon their age and accident (fire burn) • The regeneration of the forest depends upon the density of the trees • Occurrence of fires are random in position as well as strength • The fires kill all trees and animals they land on. • Once every year the fires return with a random strength (1-10). • Animals and plants breed once every year • The animals move some amount (as defined by the user on the slider in Fig 4.) every month. • The trees reproduce every year. • The Netlogo interface (Fig 4.) includes controls that setup and run the program as well as other variables in the form of slider controls and a couple plots that update continuously. Using our model, 20 simulations were run in each case to predict how the animal population was able to survive the fires and habitat destruction (Figs 1-3). Fig 7. The number of burned organisms as the function of fire strength. Notice That most animals burned to death while the fire strength was medium in size (5). Fig 8. Total number of live animals, trees, and the number of fires as the function of the fire strength. Notice the optimal fire strength for developing the largest fires is roughly medium in size (5) (see also Fig 7). RESULTS & CONCLUSIONS • This study shows that the fires have considerable effects on the forest ecosystem. These fires resulted in strong population size decrease and sometimes drove the population to extinction. • Fires caused large habitat destruction when the density of the trees reached a high population size. After a big fire the diminished habitat was re-colonized with trees. During the re-colonizing the occurrence of large or medium sized fires were rare. These larger fires emerged after the re-colonization generated more dense forests. • The results of our studies agree with all predictions of the alternative hypotheses. The fires caused large population degenerations followed by a restoration period. These processes caused strong fluctuation in both populations. • It was also found that the survival of the forest habitat is dependent upon the density of the trees, the life of the animal, and the random occurrence of fires. • New variables will be included to approach more realism in the model. For example, estimating wind speed and direction as well as temperature and movement patterns of the animals. We can also add the possibility of the animals moving towards the trees rather than moving randomly so as to look for an appropriate breeding habitat. Fig 4. Netlogo interface, right panel shows the habitat and the agents. Left top is the program setup, left bottom is the monitors, the left monitor shows the population size throughout the years as the program is running, and the right monitor shows the number of dead organisms of animals and trees.