Population Growth Dynamics Environmental Science

Environmental Science: Population Growth Dynamics

Population Growth Dynamics: Environmental Science

Population growth generally occurs in five major phases -- the lag phase, the exponential phase, the stationary phase, the overshoot phase, and the death phase. The change in the size of the snow goose population over the years is a perfect demonstration of how the process of growth flows through these five phases. This text discusses the specific events that occur in each of these five phases of population growth.

Population Growth Dynamics

Define exponential and logistic growth patterns. How are they similar? How are they different?

Exponential growth rate is said to exist when the rate of growth is proportional or equal to the existing amount, such that the larger the existing amount, the greater the growth rate (Gilewski & Norton, 2008). This, in terms of population growth, refers to a situation where the birth rate is constant and is not restricted by diseases or scarcity of resources. In other words, there are sufficient resources to support the continuous doubling of the population. In case, however, the growth of the population is restricted by such factors as disease or resource scarcity, the growth pattern ceases to be exponential and is then referred to as logistic growth. In other words, logistic growth pattern exists when growth is restricted by scarcity of resources. Thus, both logistic and exponential growth patterns are characterized by increases that are proportional to the existing amounts; however, whereas logistic growth includes competition and resource limitations, exponential growth is characterized by an affluence of resources and lack of competition. Another fundamental difference between the two is that in logistic growth, the growth rate is faster at the beginning and then slows down in progressive periods as resources become scarce and the level of competition increases; in exponential growth, however, the rate is slow at first, but increases as the population grows larger.

ii) Define and describe the processes that are believed to be taking place in a population when it is in the:

a) Lag Phase - in this phase, there is very little or no growth at all as the bacteria are still synthesizing bio-chemicals and adjusting to the new environment. The only noticeable feature at this stage is the growth in the size of the bacteria. How long the phase lasts depends on the conditions of the environment and the health status of the bacterial cells.

b) Exponential phase -- in this phase, the bacteria are well-accustomed to the environment, and as such, there is rapid growth, characterized by the doubling of bacterial cells within every specified length of time.

c) Stationary Phase/Carrying capacity -- the carrying capacity refers to the maximum population size that an environment can support without imposing a strain on resources. In this stage, growth levels off and the rate of cell division becomes proportional to that of cell death. The death rate becomes equal to the birth rate, causing stagnation.

d) Population Overshoot: at this stage, the population has surpassed the environment's carrying capacity, and the rate of cell death is just slightly more than that of cell division. This is particularly because following the rapid division of cells in the exponential phase, competition increases and toxic byproducts are released at an increasingly high rate. The result is that growth slows down as it becomes harder to survive the toxins or even find food.

e) Population Crash: at this stage, the death rate is extremely high, causing a steady decline in the size of the population. Toxicity in the environment increases as more and more cells die.

III) Describe the environmental conditions that contribute to exponential and logistic growth, and the requirements necessary to sustain the growth pattern indefinitely. Which of the two patterns is more realistic?

Exponential growth only exists if natural resources are infinitely available - for plants, this would include sufficient space to grow, and an affluence of nutrients, sunlight and water; whereas for animals, they would include an affluence of mates, nesting space, shelter, food and water. In order for this type of growth to be sustained, these resources must increase at the same rate as the population; otherwise, competition would develop and the rate of growth would level off (logistic growth). Logistic growth, however, can be sustained indefinitely at carrying capacity if death rates and birth rates are changed such that the carrying capacity is maintained. Of the two models, the logistic one is more realistic for two major reasons -- first, resources will always be limited; land, for instance, will never increase along with the population in reality as the exponential model suggests. It would be unreasonable to assume, therefore, that resources could grow proportionally with the population. Secondly, we cannot assume that grows occurs continuously -- growth will only occur when conditions are favorable. Fish, for instance, will often report higher rates of growth during summer as compared to winter -- the rate of growth cannot be constant through favorable and unfavorable seasons.

IV) Explain how the birth rate influences each of the following phases of growth:

a) Lag Phase -- at this stage, members of the population are still adjusting to the situation, synthesizing enzymes and RNA, and storing nutrients. The cells are not yet mature, and are, therefore, unable to divide to form new cells. The birth rate, therefore, is nil and no population growth is experienced.

b) Exponential Phase -- here, members have already adjusted to, and are effectively accustomed to the population. Resources are available in abundance, providing ample opportunities for the cells to divide extensively. The birth rate is highest at this stage as competition is still very low.

c) Stationary Phase -- Owing to the massive growth experienced in the exponential phase, the level of competition for resources increases, and the available resources become scarce. The reduction in resource availability causes the death rate to rise (through the process of natural selection) as the birth rate falls. The decline in the birth rate is spurred by the fact that food, water, mates and other resources are no longer easily available, and hence the rate of cell division is minimized. The rates will fall until the birth rate is equal to the death rate such that no further growth is experienced.

d) Population Overshoot -- scarcity of resources and increasing toxicity in the environment causes more and more cells to die, as less and less cells are born. As such, the birth rate falls below the death rate such that there is negative growth.

e) Population Crash -- the high death rates cause high toxicity in the environment such that cells are no longer dividing but mutating to make themselves better able to survive the changes in the environment. The birth rate falls to extremely low levels (sometimes to as low as zero) such that the size of the population decreases until the carrying capacity is reached, then it levels off.

V) Describe how carrying capacity is affected by the biotic and abiotic features of the ecosystem. Is it accurate to describe carrying capacity as a parameter that stays constant over time?

Biotic factors are the living things that indirectly or directly affect an organism's ability to reproduce or even survive in a particular environment. They include such factors as predation, disease and parasitism. Abiotic factors, on the other hand, are the chemical and non-living physical factors that influence an organism's ability to thrive in a particular environment. They include pollutants, dissolved gases, PH level, temperature range, light intensity and so on. Both biotic and abiotic factors work to restrict the size of the population to ensure that the carrying capacity is not surpassed. Abiotic factors such as oxygen shortages and water pollutants, for instance, limit the size of aquatic species by causing the intolerant ones to die such that the numbers left are enough to be sufficiently supported by the existing oxygen levels (Williams, 2000). Moreover, plants need light of the correct duration, intensity and wavelength in order to survive -- inappropriate light could cause the intolerable species to die until such a point when the carrying capacity for that particular environment under the prevailing conditions is reached.

The biotic factors work in an almost similar way -- predators, for instance, can essentially limit the numbers of a prey species such that there are fewer animals competing for space and resources such as water when the same are at low levels. Moreover, the greater the size of a population, the greater the possibility of a disease spreading and killing some of its members until the numbers can be sufficiently supported by the existing resources. Carrying capacity, therefore, is a function of the existing resources, and since the level of resources is constantly changing, the carrying capacity of a particular environment also keeps changing. It would, therefore, be inaccurate to describe carrying capacity as a parameter that stays constant over time. A perfect example is that of a grazing field -- if one begins with 10 heads of cattle on a sizeable piece of land, resources will be affluent and readily available, and one could place the land's carrying capacity as 10; however, as pasture becomes less and less available, competition would develop, and the land would no longer be able to support 10 heads of cattle -- the carrying capacity then would reduce to say 5 heads of cattle because resources are less readily available.

Part Two: Lesser Snow Goose Population

VI) Describe the two major ecosystems used by snow geese during their annual biological and migratory cycle. Outline how each of these ecosystems affects the carrying capacity for the North American population overall

The snow geese's annual migratory cycle is characterized by two different ecosystems -- the coastal marsh ecosystems in the high arctic during summer and the marshes along the coastal Gulf of Mexico during winter. The warm conditions in the arctic make food and other abiotic factors such as lighting and warmth readily available, making the birds able to feed heavily and store sufficient fat reserves for successful netting. Moreover, the warm climate of the arctic provides suitable temperatures for young ones to hatch and grow. It is estimated that approximately four eggs are laid by each adult female and incubated for 23 days, which only implies a relatively high birth rate and carrying capacity for the population. At the beginning of fall, however, the birds begin their migration to the second major ecosystem in the Gulf of Mexico. Here, the environmental conditions are not as conducive -- temperatures are extremely low, making it difficult for the birds to obtain food, water, and other resources. The high density of the population opens up opportunities for diseases to spread faster. Predation, particularly by humans, also becomes an issue as the birds infest private hayfields along the St. Lawrence River in search of food (Batt, 1998). As a result, mortality rates are high, and consequently, the population's carrying capacity falls substantially by the time they begin their migration back to the arctic during spring. This is a perfect demonstration of how carrying capacity changes with changes in ecosystems and resource availability.

VII) Snow Geese Estimated Population over Time

Labels: 1941-1975 -- lag phase (very minimal growth)

1975-2000 -- exponential phase

2000-2003 -- stationary phase

2003-2013 -- overshoot phase (fluctuations slightly below and above the carrying capacity)

Evidently, the death phase is missing from the representation -- if the death phase were presented, we would have seen a steady decline in numbers after 2003; however, all we see are slight fluctuations above and below the carrying capacity of 3,200. The lack of the death phase could be explained by the snow goose's ability to migrate into more favorable conditions before a complete crash or dieback can take place.

VIII) Describe the key factors that have contributed to the changes in the lesser snow goose population over the years

The continuous growth in the population of snow goose can be attributed to a number of factors. First, the establishment of the National Wildlife Refuges and state refuges in the 1970s to restore and protect wetland habitat for migrating and breeding wetland-dependent birds significantly reduced mortality resulting from poaching (U.S. Fish and Wildlife Service, 2015). With a reduced risk of human predation, wetland-dependent birds such as the snow goose have been able to thrive and multiply in numbers significantly. A second contributing factor is that of climate amelioration in the arctic, one of the birds' major ecosystems (U.S. Fish and Wildlife Service, 2015). This has been characterized by a general warming trend that has reduced the risk of reproductive failures, leading to an increasingly high carriage capacity in the Arctic ecosystem. Finally, there is the issue of shift in nesting range -- the distribution of wetland-dependent birds has shifted significantly since the 1930s (U.S. Fish and Wildlife Service, 2015). Owing to this, the birds' breeding ranges in the Arctic have moved south to areas with more favorable and less severe climate -- these warmer climates have made it easier for the birds to reproduce and thrive.