CONCEPTS IN BIOLOGY
PART IV. EVOLUTION AND ECOLOGY
17. Population Ecology
17.5. Categories of Limiting Factors
Limiting factors can be placed in four broad categories:
1. Availability of raw materials
2. Availability of energy
3. Production and disposal of waste products
4. Interaction with other organisms
Availability of Raw Materials
The availability of raw materials is an extremely important limiting factor. For example, plants require magnesium to manufacture chlorophyll, nitrogen to produce protein and water to transport materials and as a raw material for photosynthesis. If these substances are not present in the soil, the growth and reproduction of plants are inhibited. However, if fertilizer supplies these nutrients, or if irrigation supplies water, the effects of these limiting factors can be removed, and a different factor becomes limiting. For animals, the amount of water, minerals, materials for nesting, suitable burrow sites, and food may be limiting factors.
Availability of Energy
The availability of energy is a major limiting factor, because all living things need a source of energy. The amount of light available is often a limiting factor for plants, which require light as an energy source for photosynthesis. Because all animals use other living things as sources of energy and raw materials, a major limiting factor for any animal is its food source.
Accumulation of Waste Products
The accumulation of waste products can significantly limit the size of populations. It does not usually limit plant populations, however, because they produce relatively few wastes. However, the buildup of high levels of self-generated waste products is a problem for bacterial populations and populations of tiny aquatic organisms. As wastes build up, they become more and more toxic, and eventually reproduction stops, or the population dies out. For example, when a few bacteria are introduced into a solution containing a source of food, they go through the kind of population growth curve typical of all organisms. As expected, the number of bacteria begins to increase following a lag phase, increases rapidly during the exponential growth phase, enters a deceleration phase, and eventually reaches stability in the stable equilibrium phase. However, as waste products accumulate, the bacteria drown in their own wastes. When space for disposal is limited, and no other organisms are present that can convert the harmful wastes to less harmful products, a population decline, known as the death phase, follows (figure 17.10).
FIGURE 17.10. Bacterial Population Growth Curve
The rate of increase in the size of the population of these bacteria is typical of population growth in a favorable environment. When the environmental conditions worsen as a result of an increase in the amount of waste products, the population first levels off, then begins to decrease. This period of decreasing population size is known as the death phase or decline phase.
In small pools, such as aquariums, it is often difficult to keep organisms healthy because of the buildup of ammonia in the water from the animals’ waste products. This is the primary reason that activated charcoal filters are used in aquariums. The charcoal removes many kinds of toxic compounds and prevents the buildup of waste products.
Interaction with Other Organisms
Organism interactions are also important in limiting population size. Recall from chapter 16 that organisms influence each other in many ways. Some interactions are harmful; others are beneficial.
Predation, parasitism, and competition tend to limit the growth of populations and their maximum size. Parasitism and predation usually involve interactions between two different species. Thus, they are extrinsic limiting factors. Competition between members of different species (interspecific competition) is an extrinsic limiting factor. Competition among members of the same species (intraspecific competition) is an intrinsic limiting factor and often is extremely intense.
On the other hand, many kinds of organisms perform beneficial services for others that allow the population to grow more than it would without the help. For example, decomposer organisms destroy toxic waste products, thus benefiting populations of aquatic animals. They also recycle the materials that all organisms need for growth and development. Mutualistic relationships benefit both populations involved. The absence of such beneficial organisms is a limiting factor.
Often, the population sizes of two kinds of organisms are interdependent, because each is a primary limiting factor of the other. This is most often seen in parasite-host relationships and predator-prey relationships.
A study of the population biology of the collared lemming (Dicrostonyx groenlandicus) in Greenland illustrates the population interactions between lemmings and four predators. Lemmings have a very high reproductive capacity, producing two or three litters per year. However, their population is held in check by four predators. Three—the snowy owl, the arctic fox, and the long-tailed skua (a bird that resembles a gull)—are generalist predators, whose consumption of lemmings is directly related to the size of the lemming population. They constitute a density-dependent limiting factor for the lemming population. When lemming numbers are low, these predators seek other prey.
The fourth lemming predator is the short-tailed weasel (Mustela ermina); it is a specialist predator on lemmings. The weasels are much more dependent on lemmings for food than are the other three predators. The weasels mate once a year, so their population increases at a slower rate than that of the lemmings. However, as the weasel population increases, it eventually become large enough that it drives down the lemming population. The resulting decrease in lemmings leads to a decline in the number of weasels, which allows for the greater survival of lemmings, which ultimately leads to another cycle of increased weasel numbers (figure 17.11). Outlooks 17.1 provides an example of how interactions between humans and leatherback turtles have negatively affected turtle populations.
FIGURE 17.11. Population Cycles
In many northern regions of the world, population cycles are common. In the case of collared lemmings and short-tailed weasels in Greenland, interactions between the two populations result in population cycles of about 4 years. The graph shows the population of lemmings per hectare and the number of lemming nests occupied by weasels per hectare. The researchers used the number of nests occupied by weasels as an indirect measure of weasel population size.
Marine Turtle Population Declines
Throughout the world, all seven species of marine turtles are endangered. There are several characteristics of their life history and reproductive biology that contribute to this problem. Although there are some differences among the species, in general they are slow to mature and there is high mortality among the young.
Some details about the leatherback turtle (Dermochelys coriacea) will help to describe why marine turtles are in trouble. Leatherback turtles are the largest of the marine turtles that are in the range of 1-2 meters in length and between 250 and 700 kg in weight. They reach sexual maturity between 6 and 10 years of age. Once a female turtle reaches sexual maturity she will nearly always return to the beach from which she hatched to lay her eggs. She generally arrives at the beach at night at high tide and drags herself up the sandy beach where she digs a hole in the sand with her rear flippers and deposits up to 100 eggs. She then covers the eggs with sand and leaves. She may lay eggs several times during the breeding season, but it may be two to three years before she returns to lay more eggs.
The eggs hatch after about 60 days. At night the hatchlings crawl up through the sand and migrate down the beach to the ocean. If they are successful in reaching the water they still need to live to maturity. Although large adult turtles have few predators, the hatchlings and small turtles are prey to many birds, mammals, crabs, and fish. In addition, terrestrial mammals may find and eat the eggs. Survival is low. Some researchers estimate that about 1 hatchling per 1,000 makes it to adulthood.
Several kinds of human activities reduce survival. Although marine turtles are a protected species worldwide, in many parts of the world people dig up the eggs of the turtles or capture turtles for food. In addition, the building of resorts and other urban development alters the beaches traditionally used by the turtles. Even the lights associated with urban development are a problem. In particular, the hatchlings rely on light stimuli to guide them to the water. In the absence of human lights, the surface of the water reflects light, which the hatchlings follow to the water. If there are other sources of light, the hatchlings may orient on lights that lead them away from the water. Human fishing activity also results in the death of many young and adult turtles. If turtles become entangled in nets or are caught on a submerged fishing hook, they will be unable to surface to breathe and will drown. Even discarded plastic in the ocean is a problem—plastic bags resemble jellyfish that are the primary food of the turtles. The turtles eat, but cannot digest them, and the bags block their digestive system, causing death.
So, a combination of factors contributes to the endangered status of the leatherback turtle:
1. Sandy beaches are required as a suitable nesting habitat and they need to be isolated or protected from human predation on the eggs and adults.
2. Mortality of the hatchlings and young is very high due to a combination of natural predation and accidental capture by fishers.
3. Many turtle nesting beaches are also desirable as recreation sites. New resort facilities and the lights associated with them lead to inadvertent deaths of the hatchling turtles.
Female leatherback turtle fitted with a tracking device
17.5. CONCEPT REVIEW
12. List four kinds of limiting factors that help set the carrying capacity for a species.
13. Describe an example of how waste products can limit the size of a population.
14. Give an example of how interaction between two species of organisms could allow for populations to be larger than if the interaction did not occur.