DENSITY INDEPENDENT FACTORS
Density independent factors can affect a population no matter what it’s density is. For example: natural disasters, temperature, sunlight, human activities, physical characteristics and behaviours of organisms affect any and all populations regardless of their densities. Natural disasters such as droughts, floods, hurricanes and fires can be devastating to aquatic life. For example, a severe drought could lower the water levels of Lake Winnipeg and decrease its carrying capacity. Thus, the fish population would decrease. Temperature influences the activity and growth of organisms. Temperature also determines which type of organisms can live in a lake. Usually, the higher the water temperature, the greater the activity in a lake. However, all aquatic species have a preferred temperature range. If temperatures vary too much out of this range the species will either die or move to a different location. Temperature also influences the chemical properties of water. The rate of chemical reactions in the water increases as temperature increases. For example, warm water holds less oxygen than cool water, so even though there is more activity in warm water there may not be enough oxygen for the activity to continue for long periods of time. Sunlight can only penetrate to a depth of 30 meters in water. Thus most photosynthesis in aquatic environments occurs near the surface. This means that most plants cannot grow if they are at the bottom of a deep lake. Human activities can also affect population dynamics. For instance, lake sturgeon spawn in fast water and sometimes use the “tailraces” of hydroelectric dams. However, the water level in this location often drops suddenly and the eggs die because they become exposed. Physical characteristics of organisms can affect their population. Many organisms have adapted and evolved in order to increase their chance of survival. For example, some species of fish have colored markings to warn predators that they may be toxic. Or, some species use camouflage colors to help them hide and avoid being eaten. Behaviors of organisms can also affect their population. For example, some species migrate to find new food sources or to mate. Some organisms create societies or feeding territories. For instance, white bass live in schools and work together to drive emerald shiners to the surface for feeding. Some species may have mating or courtship behaviors that affect their population.
DENSITY DEPENDENT FACTORS
YELLOW PERCH IN LAKE WINNIPEG
2) From the previous article “Yellow Perch in Lake Winnipeg”, identify and describe as many limiting factors as possible and classify them as density independent or density dependent.
3) Each of the statements below involves a situation that will affect the growth of a population. Classify each of the statements as DD (density dependent) or DI (density independent) and give a reason for your choice.
a. Rainbow smelt and yellow perch attempt to occupy the same area. The more aggressive smelt survive; the perch do not.
b. A severe flood brings a lot of sediment and silt into Lake Winnipeg. The turbidity of the lake increases greatly.
c. A drought decreases the water level in Lake Winnipeg. The carrying capacity of the lake decreases. d. Due to the introduction of rainbow smelt, Lake Winnipeg becomes crowded and some fish species do not survive.
e. Since northern pike prey on yellow perch, an increase in the perch population causes an increase in the pike population.
f. Many fish die due to an increase in water temperature.
g. Due to over-fishing, the number of walleye in Lake Winnipeg decreases.
h. A population is growing quickly when parasites cause disease to spread quickly.
i. Since lake sturgeon migrate long distances to spawn, many do not survive the trip