Ecology 1 Notes: - Serrano HS AP Biology



Ecology 1 Notes:

Ecology: Study of the relationship of living things (organisms) and their environment.

Review: A population is a group of organisms of the same species that live in the same area or are interbreeding and sharing genetic information.

In these set of notes the population is described.

Properties of Populations:

The individual of a population is transitory, but the population endures. Properties of populations include 1) patterns of growth with carrying capacity, 2) survivorship patterns, 3) age structure, 4) density and dispersion, 5) regulation of population size: a) density dependent factors, b) density independent factors, c) population cycles, d) life history patterns, e) asexual advantage, and f) consequences of life history patterns.

1) Patterns of Population Growth:

The rate of population increase is the increase in the number of individuals per given unit of time per individual present.

In the absence of net immigration or emigration, the growth rate increase is equal to the birth rate minus the death rate. The growth rate can be positive or negative. This property of a population per capita rate of increase is symbolized by the letter r. If the population increases at a constant rate, it shows an exponential curve. This can be described by the differential equation:

dN = rN

dt

r = per capita increase N = # of individuals present, t= time period. This type of growth is sometimes called logarithmic or log growth, presumably because the graph of log N vs. t is a straight line.

The exponential growth curve is shown by microorganisms in the lab, and also by the initial stages of an algal bloom. In nature, short-term exponential growth occurs when opportunistic species invade an area and use abundant local resources. Weeds and some insects are examples of opportunists.

Sometime a population may hit an environmental limit and crash to low levels.

The effect of carrying capacity:

For many populations, the environment determines the number of individuals in an area. A given environment can support only a limited number of individuals. Population size hovers around this number, which is known as the carrying capacity.

Limiting factors in the environment may vary seasonally. A simple way of showing the effect of the carrying capacity is through the following equation often called the logistic equation:

dN =rN (K-N)

dt K

r= per capita increase, and it is multiplied by the number of individuals present (N) at any one time. K= carrying capacity, the number of individuals the environment can support over a specified amount of time.

The model of population growth, represented by the S-shaped or sigmoid curve is called the logistic.

The (K-N/K) is a negative feedback term. If N is small, (K-N)/K is almost 1 and the curve, if graphed, is almost an exponential curve. As N, the number of individuals increases, then the value of K-N/K decreases and the growth curve slows. When N = K, growth stops. If the number of organisms, N, becomes greater than K, growth is negative, ie. the population decreases. Eventually the population stabilizes (it oscillates around a maximum size that the environment can support). The model of population growth, represented by the S-shaped or sigmoid curve is called the logistic.

2) Survivorship Patterns:

Mortality also affects population size. There are three basic survivorship patterns. These patterns often show up in combination in a population.

1) Type I: Most organisms die at the later stages in life. ie. Humans.

2) Type II: Mortality rate is the same for all ages, ie. hydra.

3) Type III: Most organisms die in the early stages of life, ie. oysters, where most die in the larval stage.

Number of reproductive episodes per lifetime:

Semelparity: One single and large reproductive episode.

Iteroparity: Repeated but smaller reproductive episodes.

3) Age Structure:

Mortality patterns affect age structure of a population. Age structure is the number of individuals of different ages in the population. Cohorts are the individuals in a population of the same age. A plot of cohort survivorship for the population yields the age distribution of the population.

4) Density and Dispersion:

Population Density: the number of individuals per unit area or volume.

Dispersion: The pattern of distribution of the organisms within the two or three-dimensional space.

There are three basic dispersion patterns:

a) Random: The spacing between individuals is irregular; individuals are spaced independently of one another.

b) Clumped: Individuals associate in patches. The presence of one individual increases the chance of finding another individual.

c) Regular: individuals are evenly spaced within an area.

A number of factors, biotic and abiotic, may affect the spatial distribution of a population. Dispersion patterns often rely upon the distribution of essential resources. These dispersion patterns are not fixed and can change either seasonally or in different parts of the life cycles.

Dispersion patterns may also depend upon the scale of observation. A local scale (few square meters) that shows a random distribution may reveal a clumped distribution on a larger scale.

5) The Regulation of Population Size:

Two influences on population size are limiting factors and population cycles.

Limiting Factors:

Specific limiting factors affect the size and density of a population. The most important factors are the organism's tolerance to factors such as light, temperature, available water, salinity, nesting space, and availability of required nutrients. If any requirement is in short supply, or any environmental factor is too extreme, growth of the population may not be possible.

a) Density-dependent and Density-independent factors:

Density Dependent Factors cause changes to the birth or death rate as the population density changes, usually causing an increase in the death rate as population density increases. For example, a depleted food supply may increase competition, which increases the mortality rate. Also predators may be attracted to an area with a higher concentration of prey.

b) Density Independent Factors are those that affect the birth rate or death rate regardless of the population size. For example, environmental disturbance such as a flood will cause a similar death rate in a large or small population.

c) Population Cycles:

Population fluctuations that peak regularly over the course of years are an indication of the complexity of population dynamics and are not well understood by population ecologists.

d) Life History Patterns:

This is a variable property of populations.

Alternatives:

Two extreme types: r- selected (prodigal) and k- selected (prudent)

Characteristics:

|r-selected |k-selected |

|many young |few young |

|small young |large young |

|rapid maturation |slow maturation |

|little or no parental care |intensive parental care |

|reproduction-once ('big bang') |reproduction-many times |

Early or late Reproduction:

Breeding early or late can affect population growth.

A short life expectancy strategy is to reproduce early.

A long life expectancy strategy is to put energy into growing first and then reproducing.

e) The Asexual Advantage:

Asexual organisms increase the number of individuals in a population quickly. For example, some plants have runners that are able to grow and cover large areas. These new plants root and have continual support of the parent plant as they grow. Survivorship is higher in plants produced from runners than those produced from seed.

Parthenogenesis is another form of asexual reproduction. Parthenogenesis is the development of an organism from an unfertilized egg. Parthenogenesis always results in female offspring.

ie. Dandelions form flowers and sometimes functionless pollen grains.

Parthenogenesis also occurs in some invertebrates, fish, amphibians, and reptiles.

f) Consequences of Life-History Patterns:

Opportunistic organisms appear to lead risky lives, but these populations can recuperate their numbers quickly following a population crash because a population can be built from only a few organisms.

Populations composed of long-lived, slow to mature individuals have an increased probability of long-term survival. However, they are slow to recuperate their numbers when population size is reduced.

Ecology 2 Notes:

A community is composed of all of the populations of organisms inhabiting a common environment and interacting with one another. These interactions can be classified as competition, predation, and symbiosis (commensalism, mutualism, and parasitism).

Here how the populations interact with each other is discussed.

There are two hypothesis on how the types of species ended up in an area: 1) Individualistic Hypothesis: species happened there by chance. The area has what the species needs, so they stay. 2) Interactive Hypothesis: there is an assemblage of closely linked species. We’ll focus on the interactive hypothesis.

Competition:

Interaction between individual organisms of the same species (intraspecific competition) or different species (interspecific competition) that use the same resources that are present in limited supply. As a result of the competition, the overall fitness (reproductive success) of one or both competitors may be reduced.

There are two types of competition:

1) Interference Competition: In animals this involves overt fighting face-to-face interactions. In plants it may take the form of secreting toxins that harm or restrict the growth of potential competitors.

2) Exploitative Competition: removing a resource and leaving less for competitors.

Organisms with more similar needs have more intense competitive interactions.

Principle of Competitive Exclusion was developed in 1934 by Russian biologist G.F. Gause. This states that if two species are in competition for the same limited resource, one of the species will be more efficient in utilizing or in obtaining access to the resource. That species will eliminate the other species in situations in which they occur together.

The Ecological Niche: The total environment and way of life of all the members of a species in a community, or a specific role of a species within a community is termed its niche. The niche includes physical factors, such as temperature and moisture and biological factors, such as food availability, competitors, or predators, behavioral habits such as seasonal activities and movements. These factors determine species and their interactions with the environment.

Fundamental Niche refers to the physiological limits of tolerance of the organism; the organism occupies the niche in the absence of interactions with other organisms. Realized Niche is the portion of the fundamental niche actually used by an organism and is determined by physical factors and interactions with other organisms.

Resource Partitioning: If two similar species are living in the same area, then their niches are expected to be different in some way. These species may appear to be competing for the same resources, but upon close examination one would find that their respective uses of the resources would differ in some way, thus lessening competition and avoiding competitive exclusion.

In some cases resource partitioning is thought to be due to ongoing competition for resources. In other cases, the partitioning is thought to have occurred because of competitive interactions in the past, leading to different adaptations that enabled the two species to coexist.

Character Displacement is a phenomenon in which species that live together in the same environment tend to diverge in those characteristics that overlap.

Winner Takes All: Competition may not only wipe out individual organisms, but competition between species may lead to the elimination of a species from an area.

Predation:

Predation is the eating of live or freshly killed organisms. To do this predators use a number of techniques and foraging strategies. These strategies are under intense selective pressures. If an organism can procure prey, it reproduces more. If prey can avoid predators successfully, then the prey is more likely to have offspring.

The Arms Race: Predators and prey are involved in an ever-escalating arms race.

Escape from Predation: Prey often escapes predators without engaging in combat. Some are masters of camouflage. Others use hiding places like burrows. Still others use temporal strategies. Plants can produce physical structures, thorns, and chemicals that are distasteful or harmful.

Predation and Population Dynamics: For many populations predation is the main cause of death. However, if predation reduces the number of prey at a certain age group, then it can alter the structure of a population and promote adjustments in life history patterns.

Predation on large herbivores kills animals in poor physical condition. In some cases, predators do limit their prey species.

Most predators have more than one prey species. When one prey species decreases, predation on the other species increases. The availability of prey is a major determinant of carrying capacity of the predator population. In many situations, when the prey population increases, so do the predators. When the predator population increases, the prey number decreases, bringing down the number of predators. Why do we see this?

There are three hypotheses:

1) When the population of prey decreases, predators have no food, and thus the population of predators decreases. When the population of predators decreases, the prey population increases.

2) Prey populations may undergo a regular cycle, ie. A regular 10-year cycle.

3) Predator populations may undergo a regular cycle.

Predation and Species Diversity:

Predators can affect the number and types of species. Predators can eliminate species in an area, but can also maintain species diversity. Predation often keeps a species that would normally dominate an area in check. Without predators, one species could competitively eliminate other species.

Symbiosis:

Symbiosis is the close and long term association between organisms of two different species.

There are three types of symbiotic relationships:

1) Parasitism occurs when one species benefits and the other species is hurt. This can be considered a special form of predation in which the predator is smaller than the prey. Parasitic diseases are more apt to kill the very young, the very old or the disabled. However, if a parasite kills a host, it too dies.

2) Mutualism occurs when both species benefit from the relationship. Examples include the evolution of the eukaryotic cell, mycorrhizae, nitrogen fixing bacteria, and lichens.

3)) Commensalism is when one species benefits, and the other neither benefits nor is harmed, ie. The remora-shark relationship.

Community Composition and the Question of Stability:

What determines the number of species in a community? What factors underlie the changes in community composition that are revealed by close observation to occur with the passage of time?

Island Biography Model:

There is a balance between the rate at which new species immigrate to an island and the rate at which species already present become locally extinct. The number of species is in equilibrium between those two rates. The composition of the species is in non-equilibrium. When a species becomes extinct, it may be replaced by a different immigrating species.

There are two variables in the Island Biography Model of species diversity: island size and the distance from a source of flora or fauna.

The Intermediate Disturbance Hypothesis:

Among the most diverse communities are the tropical rain forests and coral reefs. It is now thought that their diversity is a function of the frequency and magnitude of the disturbances.

Disturbances can take many different forms. After the disturbance, immature forms of species invade the open area. These species are species that live in close proximity to the open area and are reproducing at the time of the disturbance. If disturbances occur frequently, the species that can invade, mature, and reproduce quickly will be successful.

According to the intermediate disturbance hypothesis, as the intervals between disturbances increases, so does species diversity. However, if the interval between the disturbances continues to increase, the species diversity decreases. What causes this decline is the COMPETITION between species for resources. Also, predation and diseases would affect the community over time.

Ecological Succession:

This process occurs when the intervals between disturbances is long. The photosynthetic organisms are usually the first to recolonize. These organisms are usually replaced by other photosynthetic organisms. As the photosynthetic organisms change, the subsequent animal life changes.

ie. In a field, which is abandoned, seeds from other vegetation bombard the field. These will germinate and grow. Some plants will dominate the community. Eventually, a tree community will replace these. These trees may be replaced by others trees.

There are two types of succession: primary and secondary. Primary succession begins in a lifeless area. The wind carries moss spores and lichen fragments to a rocky area. These two types of organisms can build soil for the following plant life (first the herbaceous monocots and dicots move in, then the woody plants and finally the woody dicots). In secondary succession, some type of disturbance had cleared the existing life (so the herbaceous monocots and dicots move in first…).

The Facilitation Hypothesis:

States that the sequence of photosynthetic organisms is so regular and predictable that succession can be viewed as analogous to the development of a simple organism. At each stage, the organisms prepare the way for the next organism. Ultimately, the community would reach a 'mature' stable state, which is a CLIMAX COMMUNITY.

The Inhibition Hypothesis:

A more recent hypothesis, which states that early species inhibit rather than help other organisms to colonize. Eventually, other invaders replace the earlier colonists. These recent invaders may in turn inhibit others from colonizing.

The Tolerance Hypothesis states that the earliest colonists neither help nor hinder colonization by later species. The dominant species at any time is the species that can best tolerate the existing physical conditions and available resources.

Experiments support all of the hypotheses.

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