Individuals

Behavior
Adaptation
Life History
Resource Allocation

Intraspecific Interactions

Behavior
Competition

Interspecific Interactions

Biodiversity
Competition
Niche
Predation

True predators
Parasitoids
Parasites
Herbivores

Abiotic-Biotic

Energy Flow
Nutrient Cycle

"The main aim of ecology is to describe, explain and understand the distribution and abundance of organisms."
(Begon, Harper and Townsend, 1999)

It is not possible to cover all aspects of ecology, or even all those aspects that students may have questions about, in this website. The following short descriptions of a select few principles are meant as introductions to some major topics in ecology. The hope is that students will become proficient and confident enough to do research and investigations on their own, utilizing other information resources, such as ecology texts and even primary ecology journals.

INDIVIDUALS

Individual organisms are the units that make up a species. Even though ecologists are most interested in the distribution and abundance of many organisms, it is the individual that is usually observed, counted, or measured. It is the individual that exhibits behavior (herds and schools act like a single entity at times), and the individual who survives or dies.

Behavior— Organisms, even plants, exhibit behaviors. These are actions, be they quick or slow, performed by an organism. Behavior among animals includes moving, sleeping, eating, mating, laying eggs, giving birth, lying in wait, grooming, and many others. Plants behave, too, just not in such obvious ways as most animals. Flowering, growing toward the light, and wilting temporarily are examples of plant behaviors. A behavior is any observable response of an individual or group to a stimulus.

Obvious behaviors are those associated with acquiring needed resources. These resources can be things such as mates, pollen, territory, food, nutrients, light, water, and shelter. Other behaviors involve avoiding predators or herbivores or parasites. Most behaviors ultimately serve the goals of survival and reproduction.

Adaptations— Species evolve over generations. An individual organism does not evolve. The true meaning of adaptation is change in organisms over many generations through the inheritance of genes, resulting in improved survival and reproduction. To refer to how an individual changes its behavior to survive, the correct term is acclimation.

Life History— All living organisms go through a life history, which are the phases of growth and development leading to reproduction and eventually death. How long or complex a life history is depends on the species. Some species live a very short time (hours) while others live for centuries. How long it takes to reach a particular stage of development varies from species to species and sometimes individuals within a species will vary greatly in developmental time. Ecologists often study life history to understand how and why an organism interacts with its environment.

Resource Allocation— Resource allocation is the term used to describe how much of a resource (e.g., nutrients, light, water) an organism devotes to different aspects of its growth and maintenance. For example, ingested nutrients can be allocated to growth of new tissue to increase size, or to egg production. There is only so much of a resource that can be used and it gets allocated to different functions. The allocated amounts vary with age of the organism. Young, immature individuals allocate all resources to growth, development and survival. As an individual ages and matures more resources are allocated to reproduction.

An organism grows and develops at a rate characteristic for its species, but variation does exist within a species and between species. Species can fall into one of two growth categories: indeterminate and determinate. Indeterminate growth, not having a set adult size, is common among invertebrates, fish, reptiles, amphibians and plants. Determinate growth is typical among vertebrates, where the adult size of individuals of a species is within a narrow range. These two growth patterns influence how an organism allocates its resources and thus impact its fecundity. Plants are easy organisms to study for resource allocation. By measuring the dry weight or biomass of the stems, roots and leaves of a plant, and counting or measuring the seed output (fecundity), allocation of resources can be observed.

Juvenile and adult mortality rates also influence resource allocation between growth and reproduction. Species with short life spans usually develop quickly, reproduce once or a few times, then die (See semelparity and iteroparity.) Some species with long life spans also reproduce only once and then die (e.g., century plant), but these are very slow growers. Long-lived organisms tend to suffer high juvenile mortality but once reaching adulthood are practically immune to predators. Reproduction usually begins late in life but occurs until death. The shorter the life span, the more resources are allocated to reproduction each year. The longer the life span, the more resources are allocated to growth each year.

INTRASPECIFIC INTERACTIONS

Intraspecific refers to interactions between individuals or groups within the same species. These interactions can be positive for both parties, negative for each or positive for one and negative for the other. For example, mating behavior between two cactus wrens, competition between two herds of desert bighorn sheep for water, or the cannibalism of a male black widow spider by a female spider. All of these are examples of interactions between members of the same species, with the mating behavior being positive to both, competition for water could be negative for each herd or positive for one and negative for the other, and the cannibalism being definitely negative for the male spider and positive for the female.

Behavior— Organisms, even plants, exhibit behaviors. These are actions, be they quick or slow, performed by an organism. Behavior among animals includes moving, sleeping, eating, mating, laying eggs, giving birth, lying in wait, grooming, and many others. Plants behave, too, just not in such obvious ways as most animals. Flowering, growing toward the light, and wilting temporarily are examples of plant behaviors. A behavior is any observable response of an individual or group to a stimulus.

Obvious behaviors are those associated with acquiring needed resources. These resources can be things such as mates, pollen, territory, food, nutrients, light, water, and shelter. Other behaviors involve avoiding predators or herbivores or parasites. Most behaviors ultimately serve the goals of survival and reproduction.

Competition— Individuals of the same species require essentially the same resources. The requirements may vary depending on age and/or sex. When a resource is in limited supply, some individuals will either directly or indirectly compete for that resource, or limiting factor. Examples of possible limited resources are water, food, soil nutrients, light, space and mates.

In order for competition to occur, a resource must be in limited supply and the organisms competing for it must be essentially equivalent. Competition results in a decrease in fecundity among competing individuals and its effect is density dependent, meaning there is an increase in the probability of competition occurring as the density of individuals increases. The type and intensity of competition affect the population dynamics. As the number of individuals increases, either through birth or immigration, resources— let us take food as an example— may become limited. Even if individuals do not compete directly, through fighting for example, they will compete indirectly. As food becomes limited, individuals compete indirectly by having to expend more energy and time searching for food. This increase in energy use and searching time can lead to an increase in mortality and a decrease in fecundity. The competition for food can then affect aspects of the population, such as decreasing the overall number of individuals through death or fewer offspring. Other effects can be increased emigration, decreased growth or increased disease.

INTERSPECIFIC INTERACTIONS

Interspecific refers to interactions between individuals or groups of two or more different species. These interactions can be positive for all parties, negative for each or positive for some and negative for others. For example, predation of a pronghorn antelope by a mountain lion, competition between a herd of desert bighorn sheep and javelinas for water, or the pollination of a brittlebush flower by a bee. All of these are examples of interactions between members of different species, with the predation being negative for the pronghorn and positive for the lion, competition for water could be negative for each herd or positive for one and negative for the other, and the pollination is positive for both the bee and the brittlebush.

Biodiversity— Biodiversity is the measure of the number and/or relative abundance of species in a given geographical area. An area can have many different species, each at approximately equal numbers, many species with a few at high numbers and the rest at low numbers, or a few species with equal or unequal numbers. Measures of biodiversity are often used as indicators of the ecological health of an ecosystem. Low biodiversity in an area that should have many different species would raise a red flag to scientists. A sudden increase in a particular species would indicate that something unusual is happening in the ecosystem.

Competition— Interspecific competition is similar to intraspecific competition (above), except that it concerns competition between individuals of different species. The detrimental effects of the competition on both species may be more difficult to determine since the contest may be more unequal than usual. This means, since two different species are competing, it is unlikely they are as physically comparable to each other as two individuals of the same species. Still, to be termed "competition," both parties must have the potential to suffer from the interaction. If one species is overwhelmingly stronger than the other, competition does not exist. Competition between species occurs when the resource needs of the two species overlap. A fruit eating bird could compete with a fruit eating primate for the same resource, if the resource were in limited supply. Competition for space as a resource is common among plants and sessile animals, such as barnacles.

Niche— A species' niche is its place in its particular ecosystem. It is determined by all the spatial, chemical, physical and temporal factors needed for survival. No two species occupy exactly the same niche in a particular habitat. Determining a niche is very complex because it involves numerous environmental characteristics. A given species requires a certain range of temperature, humidity, light, food type, food size, sheltering places, interspecific interactions, and so on. When a species is not competing with others it is in its fundamental niche, the niche it can fully occupy. "Fully" means the species can use all the available and needed resources in that niche. When a species is in the presence of competitors it can fill only its realized niche, which is smaller than the fundamental niche. When two species compete for the same resources or niche, several outcomes are possible. One is that only one species wins, completely driving the other out of the niche or to extinction (e.g., non-native fish killing off native fish in a stream). Or, there may be overlap in the niche requirements of each species resulting in reduced niches for both species such that they co-exist in the habitat. In this case, the requirements for each species are close but not exactly the same. If there is enough difference, or one species is not able to completely out-compete the other, coexistence can occur.

Predation ‚ Predation, the act of consuming another organism, is a general term for the activities of four types of predators: true predators, parasitoids, parasites and herbivores. True predators are the ones we usually think of in the context of predation— animals that consume other animals. Parasitoids are insects that are free-living as adults but live off their host when in the larval (immature) stage. The adult female parasitoid lays an egg or eggs on an insect host. The eggs hatch, the larval parasitoids develop on or in the host body, usually totally consuming it. By this time, the parasitoid is ready to pupate and become a free-living adult. Parasites are either plants or animals that live in close association with a host organism for most of their lives. They take nutrients from the host but in most cases do not kill the host. Some parasitic species use different host species at different stages of the parasite's life, going from one host to another to complete the life cycle. Herbivores are animals that eat plants. Some eat the whole plant, thus acting like a true predator; others eat only parts of plants and do not kill the plant, acting like a parasite.

Predation is a large, complex area of study in ecology. Related areas of study include coevolution, specialization, food preference, effects on fitness, distribution in time and space of predators and prey, optimal foraging and other topics. How predator and prey interact is not as simple as just observing the two species of interest, but that is always a useful beginning point. Each also interacts with its abiotic environment and the other organisms in the community. This leads to complex multiple interactions. Prey have evolved over time to avoid predators, while predators have evolved to better capture their prey. This coevolution has often led to specialized characteristics of both prey and predator.

ABIOTIC - BIOTIC INTERACTIONS

Abiotic refers to non-living materials such as soil, water, rocks, and other inorganic compounds. Abiotic also encompasses climatic conditions of wind, rain, drought, and light. Energy sources such as sunlight and chemicals are also abiotic components of an ecosystem. Biotic refers to all living organisms. Ecologists study the interaction between the living and the non-living environment because organisms are dependent upon the abiotic environment for various needs. The abiotic environment strongly dictates where a species can live, how long it lives, what its abundance and distribution will be and what other species will coexist with it.

Energy flow— Energy cannot be created or destroyed, it can only be converted into other forms (First Law of Thermodynamics). Energy is the driving force behind all forms of life, and thus, all ecological levels, from an individual to a species, population, community, ecosystem or biome. The overriding source of energy is in the form of photons from the sun. Specialized green plant pigments called chlorophyll collect photons. When conditions are right, photosynthesis occurs, changing carbon dioxide and water to glucose and water. As in any system, energy is needed to drive the synthetic process. Carbon dioxide, water and light are common everywhere, but photosynthesis only occurs within green plants because the conditions and structures within the plant cells make the probability of the chemical reaction happening increase.

The actual chemical reaction is: 6CO2 + 6H2O ------------ > C6H12O6 + 6O2

Light energy

These six molecules of carbon dioxide plus six molecules of water and light energy yield one molecule of sugar in the form of glucose and six molecules of oxygen. Glucose, a simple sugar, is the basic molecule of biological energy used by living organisms for growth, development and reproduction.

Once light is used by a plant to make glucose, the energy trapped in the molecules of glucose used by the plant to provide energy and structural material for growth and maintenance. Some of the trapped energy is lost to the atmosphere during cellular growth and maintenance. The rest remains in the plant structures. Some of this energy becomes part of another organism if the plant is eaten by an herbivore, a primary consumer. Energy passes through the food chain each time an organism is consumed by another, until the trapped energy is completely used or released back to the atmosphere.

Nutrient cycling— A nutrient is anything that an organism takes in for the purpose of growth and reproduction. Nutrients include water, proteins, amino acids, vitamins, carbohydrates, fatty acids, and minerals. Plants and animals require many of the same minerals in varying amounts. Essential minerals are nitrogen, phosphorus, sulfur, potassium, calcium, magnesium and iron. Other elements are required in small amounts, called trace elements. Plants get most of their nutrients from the soil and from the photosynthetic process. Animals must consume their nutrients.

Nutrients are used, assimilated, released and used again. This is the nutrient cycle. Most ecosystems are in balance between the pool of nutrients available to be used and those currently in use by an organism. Nutrients are released to the environment through death, partial body loss (for example, in the case of plants, a broken branch), excrement, and leaching by water. Many elements are made available to organisms through the action of detritivores. Detritivores consume and break down organic material into its smaller elemental components, such as nitrogen, calcium, and phosphorus. How long the process takes and the exact pathways followed vary greatly among minerals.