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The study of ecology
Ecology is the study of interactions among organisms and their relationship with the environment.
It is possible to study ecology at different levels. From the micro to the macro levels, there exists:
• organism;
• population;
• community;
• ecosystem;
• biosphere.
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Ecology concepts: the habitat
Every organism presents two defining characteristics that define it: habitat and niche.
The habitat is the particular environment in which an organism lives.
The tropical forest is a habitat for thousands of plants, insects and birds.
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Ecology concepts: the niche
A species’ niche is the set of physical and biological conditions it requires to survive, grow, and reproduce.
A niche is therefore partly defined by the resources available in the environment.
A plant’s niche includes how much direct sunlight it can tolerate and the type of soil on which it thrives; for an animal, niche involves elements such as the food it eats and whether it is active at night or during the day.
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Describing a population
A population is a group of individuals of the same species living in a habitat for a relatively long period.
Populations can be described using the following aspects:
• the size – the number of individuals in the population;
• the density – the number of individuals per unit area;
• the distribution in the environment – the pattern of dispersal of individuals across the area;
• the age distribution – the number of individuals in different age groups.
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Population dynamics
Demography is the study of changes in a population’s density and structure – also known as population dynamics .
Birth and death rates are the main factors influencing changes in the size and structure of a population, along with migration.
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Patterns of population growth: exponential growth
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Exponential growth is a type of population growth that accelerates over time.
It is typical of conditions with unlimited resources and no limiting factors.
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Patterns of population growth: logistic growth
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Logistic growth occurs when the density of a population reaches the carrying capacity of the environment; growth slows down, resulting in an S-shaped curve.
It is influenced by limiting factors.
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Density-dependent factors affect population size
Population size can be influenced by factors that depend on how many individuals from that population live in a given habitat; these are densitydependent factors , such as competition, predation and parasitism.
In these cases, mortality increases as the density of the population increases.
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Density-independent factors affect population size
Also abiotic factors, like hurricanes, floods and fires, can influence population size; these are density-independent factors , which are not influenced by population dynamics.
In these cases, mortality does not depend on density.
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Life history patterns: r -strategy species
In order to survive in different environmental conditions, species adopt different strategies.
Opportunistic species (r-strategist)
• Exponential population growth followed by periodic falls in population size;
• Small individuals with short life spam;
• Many offspring, but with relatively low probability of surviving to adulthood.
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Life history patterns:
K -strategy species
Equilibrium species (K-strategist)
• Logistic population growth;
• Larger individuals with long life spam;
• Fewer offspring, but with relatively high probability of surviving to adulthood.
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Life history patterns and the environment
Opportunistic species, like rabbits and dandelions, are adapted to a fluctuating environment.
Equilibrium species, like gorillas and oaks, are adapted to a relatively stable environment.
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Interactions of populations in a community
A community is an assemblage of populations of different species, living in the same area and interacting with each other and with the environment.
There are different types of interactions:
• competition
• predation
• parasitism
• mutualism
• commensalism
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Competition
Competition occurs when two or more species use the same resource. The result of competitive interactions depends on the availability of the resource.
According to the competitive exclusion principle, no two species can occupy the same ecological niche at the same time.
Resource partitioning decreases competition between two species.
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Predator-prey interactions /1
Predation occurs when individuals of a species (predators) feed on individuals of a different species (prey).
More prey
More predators
Fewer prey
Fewer predators
Predator-prey interactions affect the dynamics of both populations: more predators result in fewer prey; when the number of prey declines, predators decrease too and prey increase again.
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Predator-prey interactions /2
Camouflage of a Shield Mantis
(also called Leaf Mantis) in the
Peruvian Amazon.
Prey have evolved defences to escape predators: senses, speed, protective body parts, and chemicals.
Some species are able to deceive predators by camouflage, cryptic coloration, and mimicry.
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Parasitism
Monotropa uniflora, also known as the ghost plant, does not contain chlorophyll. Instead of generating energy from sunlight, it is parasitic to some mycorrhizas.
In parasitism , a population receives benefits at another’s expense. It occurs when individuals of a species consume only part of the tissues of individuals of another species, usually without killing them.
Some parasites are pathogens and they cause a disease in the host.
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Commensalism and mutualism
Commensalism benefits one species without causing damage to the other.
For example, scavengers have a commensalistic interaction with some predators.
Mutualism is an interaction between species that results in benefits for both.
Pollination is a typical example of mutualism between the plant and the pollinator.
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Ecosystems
Ecosystems are composed of:
• Biotic components – living organisms;
• Abiotic components – - resources, like sunlight and nutrients, as well as conditions such as soil, water, temperature and wind.
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Trophic structure of the ecosystems
The trophic structure of an ecosystem describes the food relationships inside a community.
It presents different trophic levels , that when taken together, form a food web .
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Flux of energy through ecosystems
Primary producers: autotrophs
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Primary consumers: herbivores heterotrophs
Secondary consumers: carnivores heterotrophs
Tertiary consumers: carnivores that feed on other carnivores
The main source of energy for the Earth is the Sun.
In the flux through different trophic levels , energy is consumed and dissipated in the form of heat.
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Chemical cycling through ecosystems
Chemical nutrients, like energy, pass from one trophic level to the next until decomposers return them to the environment where producers can take them up again.
Unlike energy, matter does not dissipate.
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Ecological pyramids
An ecological pyramid is the graphic representation of the number of organisms, biomass or energy, content of trophic levels.
Tertiary consumers
Secondary consumers
Primary consumers
Producers
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