1
What ecological significance do a population’s size, density, and distribution have?
INFOGRAPHIC 9.1
The number of individuals in a given area, such as an acre or a square mile, is a measurement known as __________ ___________.
Population density
The concept of minimum viable population:
predicts how many individuals can fit into a habitat.
describes the potential number of individuals if there are no predators.
describes the potential number of individuals if resources are limited.
describes the smallest number of individuals needed to ensure the long-term continuation of a particular population.
D
Why is it important for an ecologist to understand how a species she is studying is distributed within its ecosystem?
This helps her know how to search for individuals. It also serves as a starting point when assessing how the population is doing. For instance, there is no cause for alarm if individuals are not found throughout the habitat if normally they are found in a clumped distribution. On the other hand, if their distribution changes, this could signal a change in their environment. For example, if a normally randomly distributed plant appears to have a clumped distribution, then there might be something killing individuals in certain parts of the environment.
2
What determines a population’s growth rate? Under what conditions do we see exponential population growth? Logistic growth?
INFOGRAPHIC 9.2 and INFOGRAPHIC 9.3
Exponential growth of a population:
is often seen if a population reaches a new environment that is favorable.
is a J-shaped curve on a population graph.
occurs when most or all females reproduce at every opportunity and most or all offspring survive.
All of the above are true.
D
Consider the roach. Why does its population size never reach its biotic potential?
Resistance factors limit population size.
Females produce few offspring at a time.
Its tolerance limits are too broad.
Too many growth factors are present.
A
Kangaroo rats eat seeds and are eaten by coyotes. Under what conditions might the kangaroo rat population increase exponentially? Logistically?
The population size of kangaroo rats could increase if more food or nesting sites (or other limiting factor) became available or if their predator, the coyote, declined in number. Their growth would slow to logistic growth as their population size approached carrying capacity, (though as an r species they might overshoot and crash).
3
How do density-dependent and density-independent factors affect population growth?
INFOGRAPHIC 9.4
True or False: Fire is an example of a density-dependent factor.
FALSE
Lesser goldfinches are small, seed-eating birds. In cities, both wild hawks and domestic cats eat these birds. Discuss several density-dependent and density-independent factors, including both growth and resistance factors, that could affect their carrying capacity.
Density-dependent factors would include the amount of food and number of nest sites available (critical factors), and the possibility of being captured by hawks, cats or other predators, or the spread of a disease (resistance factors). Density independent factors would include weather (which could be either a growth factor if pleasant or a resistance factor if there is either a drought or a hard, long winter) and predation by cats (a resistance factor).
4
What are the life-history strategies of rand K-selected species, and how do they relate to population growth patterns and potential?
INFOGRAPHIC 9.5 and INFOGRAPHIC 9.6
What type of species are more likely to have a population overshoot carrying capacity and then die back?
species with low biotic potential
species with high parental care
r-selected species
K-selected species
C
Compare the life-history strategy of a deer mouse with that of a bear and identify each as either an r- or K-selected species.
Deer mice reproduce frequently, having large litters. They have a short life span, rapid growth, and are prey for many larger animals such as snakes, hawks, and coyotes. They are considered r-selected organisms. A bear is much larger and longer-lived, has only one or two offspring at a time and cares for them for a long period. Bears are omnivores, and are often predatory. They are considered K-selected organisms.
Beavers in some areas of Michigan and Minnesota live almost exclusively on slow-growing aspen trees and can harvest them faster than the trees can reproduce. Describe how these two populations—beaver and aspen trees—undergo population fluctuations, or boom-and-bust cycles.
As the beaver harvest too many aspen, the aspen population drops. The beaver are then limited by the lack of the aspen trees (a growth factor), and the beaver population drops. Without the severe (resistance factor) pressure of overharvesting, the aspen population recovers. With plenty of aspen trees around, the beaver population makes a comeback.
5
What are top-down and bottom-up regulation, and which is most important in determining the size of a population?
INFOGRAPHIC 9.7
True or False: If predators are present in a community, top-down regulation will always trump bottom-up regulation.
FALSE
In a bottom-up regulation scenario, the size of an elk herd is determined by:
the number of wolves in the area.
the number of wolf predators in the area.
the amount of grass, aspen, and other food sources.
All of these are examples of bottom-up regulation.
C
How can it be that a single population might sometimes be controlled from the top down and other times be controlled from the bottom up? Give an example of an instance in which an elk population would be controlled by top-down regulation and another example of its control by bottom-up regulation.
Ecosystems are complex assemblages of biotic and abiotic factors that interact. Any difference, i.e. different species present, differences in population sizes, seasonal changes, physical disturbance such as a fire, etc., can change the relative effect of predators or food supply on the elk population. For instance, when food is scarce, predator pressure is not the biggest source of mortality, starvation is (bottom-up regulation). When spring arrives or fire sweeps through and stimulates a burst of new plant growth producing ample food, the presence of predators will likely be the most important population control mechanism in place (top-down regulation).
175