Population Ecology Explained: Insights into Species Interactions and Growth

Objectives

This blog post provides readers with the following objectives. The reader will be able to:

o     Explain the terms, population density, population growth, immigration, emigration, birth rate and death rate.
o     Outline the population sampling techniques
o     Identify the factors which affect population size.
o     Distinguish between density dependent and density independent factors that affect population size.
o     Explain the concept of biological pest control.
o     Outline the advantages and disadvantages of biological pest control.
o     Explain the concept of ecological succession.
o     Explain how organisms in aquatic and terrestrial habitats are adapted to their habitat

POPULATION DYNAMICS

Population dynamics is the study of short-term and long-term changes in the size and age composition of populations, and the factors influencing these changes. It deals with the way populations are affected by birth and death rates, and by immigration and emigration.

Population Size

Population Size the population size (denoted by N) is the number of individual organisms in a population.

Population Density

Population Density is a measurement of population per unit area or unit volume i.e. the number of individuals per unit area.

Population Density = Number of individuals

Area

Population Growth

Population Growth is the change in a population over time, or the change in the number of individuals in a population per unit time. Population growth is determined by four factors, births (B), deaths (D), immigrants (I), and emigrants (E).

The population growth of a period can be calculated in two parts, natural growth of population (B-D) and mechanical growth of population (I-E).

Growth can be both positive and negative i.e. growth can be increasing or decreasing. The factors which limit the growth of population are collectively called environmental resistance and can be grouped into abiotic and biotic factors.

Population Growth Rate

Population Growth Rate is the rate at which the number of individuals in a population increases in a given time period.

Factors Affecting Population Size

o Natural disaster

o Emigration

o Food

o Oxygen

o Disease

o Immigration

o Birth rate

o Competition

o Water

o Death rate

o Predation

o Overcrowding

o Accumulation of toxic waste

o Light availability

Some of these factors are density dependent, others too are density independent

Density-dependent and -independent factors

Density Dependent Factors

They are density dependent because these factors relate to the size or density of the population. These include predation, epidemic, disease, parasitism, competition, food availability, accumulation of toxic waste. These factors lower the population and eventually stabilize it at a particular point.

In some populations, the rate of increase in numbers, the population growth rate, decreases as the density increases. For example, in studying the dynamics of bacteria or yeast, the population is seen to increase very quickly at first and then begins to decrease. The initial period of rapid growth may be due to the supply of adequate growth conditions, such as suitable temperature, food materials and oxygen. As growth progresses, poisonous waste products of metabolism from the organisms build up in the growth medium and the supply of food oxygen decrease. This makes the environment less favorable for growth. The result is a decrease in the rate of population growth. This type of population is described as self-limiting: it is density dependent because the growth rate depends on the numbers present in the population. If a graph is plotted to represent how such populations change with time, a sigmoid or S-shape is obtained.

Density Independent Factors

These are factors which can affect dense or sparse population irrespective of the level of population. They include natural disaster such as floods, earthquake, fire, storms, and volcanic eruption.

Other population growth rates are density independent. Their growth curves appear J-shaped. There is an initial period of rapid increase in the population as a result of favorable growing conditions. This allows for a rapid initial growth until a peak value is obtained. Unless this rapid growth is checked by other factors in the environment, the increase continues indefinitely, until the food supply is exhausted. Beyond this point the rate of reproduction drastically decreases and the growth rate decreases. This type of population growth is termed density independent because the regulation of growth rate is not tied to the population density until the final crash.

Growth Curves

Populations have characteristic growth curves. Growth in a density-dependent population is described by a S-shaped or sigmoid curve. In a density-independent population, growth rate is described by a J-shaped or exponential growth curve.

Exponential growth curve

An exponential growth curve represents density- independent population growth as the regulation of growth rate is not tied to the population density until a final crash. Exponential growth cannot continue indefinitely because growth is restricted by environmental resistance such as food supply, accumulated waste products, increased competition and predation.

An exponential growth curve has two phases:

i. The lag phase during which growth is slow because the population is small and is becoming established.

ii. The exponential growth phase during which growth is accelerated as the population increases, until an environmental limit causes a population crash.

Common examples of populations exhibiting this type of growth are those that show 'boom and bust' cycles, such as algal blooms and some species of insect.

Sigmoid Growth Curve

The sigmoid growth curve describes the growth of a density-dependent population. The growth rate depends on the number of individuals present within the population. The population becomes stable when the carrying capacity of the environment is reached.

Carrying capacity is the number of individual that the environment can support.

The sigmoid curve has four phases:
(i)       The lag phase where population density initially shows a slow increase.
(ii)     The exponential growth phase where growth accelerates.
(iii)    The deceleration phase where growth slowest down due to competition between individuals for limited resources.
(IV)    The equilibrium phase where little or no growth takes place because the carrying capacity of the environment has been reached   and the number of births and deaths are approximately equal.

The population will only grow when its size is lower than the carrying capacity of the environment. This has practical implications, for example, in fish production. The fish population can be maintained as a continuous source of food if the population size is maintained below the carrying capacity at the exponential phase of growth, where replacement individuals is highest. If we overfish, the population will be reduced to the lag phase and take longer to recover. To limit the growth of a pest, it is always better to reduce the carrying capacity of the environment rather than reduce the population size, which only allows exponential growth to begin again.

Population Characteristics

o Natality (Birth rate): this shows the rate at which new individuals are added to the population through reproduction.

Natality = Number of births

Number of adult individuals in the population

o Mortality (Death rate): the rate at which individual are lost through death in the population.

Mortality = Number of deaths

Number of adult individuals in the population

Where natality exceeds mortality, population increase until the carrying capacity is reached. The carrying capacity is the measure of the maximum population size a habitat can support. A populating may be termed an ageing population if most of its individuals are in the higher age group. A growing population is a population in which a greater proportion of its individuals are in the lower age.

o Immigration: it refers to the increase in population as new individual move permanently to join another population.

o Emigration: this is a situation by which the population size decreases as a result of movement of individuals to establish colonies outside the parent population.

Methods of Determining Population Size or Density

1. Direct Counting:

a. Quadrat: A quadrat is a square wooden frame which may be divided into small squares by means of string or wire fastened across the frame. The size of the frame depends on the area under study and the size of the organism being sampled. The quadrat is normally carefully thrown at random within the habitat and where it lands, the area of ground enclosed carefully studied. This normally involves counting the number of each species of plant within the frame. The plants partially enclosed are counted as half. The random is repeated several times.

b. Line transect: This is a method of sampling plant distribution and it is mostly used with the quadrat. A string is laid down in a straight line across all or part of the habitat. Sampling of vegetation is then carried out by placing quadrats at regular intervals along the string.

c. Point frame: a point frame may also be used in conjunction with a quadrat to find percentage cover. This is a piece of wood with ten holes drilled in it and large pins put through the holes. The quadrat is marked off into ten equal strips and the point frame is put down in each strip in turn. Where a plant touches a pin, it is identified and recorded. This means that 100 points will be touched in the quadrat, so percentage cover is easily obtained by this method. If, for example, a species of grass is touched 20 times, its percentage cover is 20%. The total number of percentages recorded (including that of bare ground if no plant is touched by a pin) should add up to 100%. The mean (average) percentage cover of a plant species over a certain area can be obtained by throwing the quadrat several times and finding the percentage cover in each quadrat. The mean percentage cover for a particular plant species can then be calculated as the sum total of the percentage cover in each quadrat, divided by the number of quadrats sampled.

d. Butterfly net: Butterfly net may be used to catch flying insects such as butterflies,

e. Sweep net: Insects on the ground and also some land vertebrates can be trapped with a sweep net.

f. Pooter: It used in collecting small animals e.g. ants, found among leaf litter, on vegetation and in cracks and crevices.

Ecological instrument for population sampling

g. Light traps: Light traps use an ultraviolet or black light to attract insects, especially moths, where they are collected in a trap or attracted to a white sheet and selectively identified and removed.

h. Pitfall trap: This is a way of sampling the populations of small animals (usually invertebrates) in the habitat. A simple trap is just a hole in the ground with straight sides, or a large jar with smooth and slippery sides buried in the ground containing food as bait. Traps are placed randomly within the habitat and are examined regularly (at least once a day). Any organisms in the trap are collected, identified, counted and then released. Large trap may be used to sample small animals and lizards.

2. Capture-Mark-Release- Recapture Method

A number of animals of a certain species are captured either using a stout net, an empty bottomed box or any other suitable method. The animals captured are then marked with a non-poisonous dye or nail vanish and released into the same area of the habitat from which they were removed. Enough time is then allowed for the marked animals to mix randomly with the rest of the population in the habitat. A second sample is then taken in exactly the same way as the first sample and the number of marked animals recaptured is recorded.

methods of determine population size and population density

The value obtained is called the Lincoln index. This method works best with animals whose movements are limited by geographical barriers e.g. animals in a pond (aquatic)

N/B:

o Marking the animals does not impede the movements.
o Marking the animals does not increase the chances of their being detected by predators.

o It is also assumed that changes in the population due to death, birth, immigration and emigration are insignificant during the period of sampling.

BIOLOGICAL PEST CONTROL

Biological control is a method of controlling pests using other living organisms (or their natural enemies). It relies on predation, parasitism, herbivory, or other natural mechanisms. Natural enemies of pests, also known as biological control agents, include predators, parasites and pathogens.

Predators

Predators are mainly free-living animals that directly feed on other animals or preys. Examples

o Ladybugs are voracious predators of aphids, scale insects and small caterpillars.

o Dragonflies are important predators of mosquitoes. In water, the dragonfly larvae eat mosquito larvae, and on land, adult dragonflies capture and eat adult mosquitoes.

o Tilapia can be used to control mosquito larvae and pupae in stagnant water.

Parasites/Parasitoids

Parasitoids are insects that lay their eggs on or inside the body of another animal or insect host. The hatched newborns feed off the body, eventually killing the host. Example

o Wasps attack caterpillars and a wide range of insects including greenfly, whitefly, cabbage caterpillars

Pathogens

Pathogenic micro-organisms include bacteria, fungi, and viruses. They kill or weaken their host. Microbial insect diseases occur naturally, but may also be used as biological pesticides. Examples.

o Bacillus thuringiensis is widely used to control moth, butterfly and beetle. The bacteria available in sachets of dried spores are mixed with water and sprayed onto vulnerable plants such as fruit trees.

o Trichoderma species are used to manage certain plant pathogens.

Advantages of Biological Control

o Biological control is specific, so destroys target organisms.

o The host are not able development resistance against them
o It is cheaper than chemical pest control
o Environmental pollution is reduced

o Biological agents can multiple on their own and spread

Disadvantages of Biological Control

o The agents may attack non-target species if its host-specificity is not properly checked

o It requires intense supervision because the agents may become pests themselves
o Control agents that are not restricted to a single species can cause changes in ecological niche which may directly affect the ecosystem

o It is slow as it takes times to multiply and spread

ECOLOGICAL SUCCESSION

Ecological Succession is a unidirectional, gradual change in vegetation, soil or animal community with ecological time. Succession may be initiated either by formation of new, unoccupied habitat (e.g., a lava flow or a severe landslide) or by some form of disturbance of an existing community (e.g. fire, severe wind throws, logging).

Types of Ecological Succession

Primary Succession

Primary Succession begins on an area that has not been occupied by a community. It occurs on bare, lifeless substrate, such as rocks, or in open water, where organism gradually move in and change it nature. Pioneer or opportunistic organisms are the first organisms to occupy newly exposed area such as rock or an area which has been disturbed by a disruption. Typical pioneers in a succession include lichens on rocks or grasses in a ploughed field. Pioneer organisms modify the environment and create conditions which are favorable for more advanced organisms to colonize.

On bare (mineral-poor soil) lichens grow first forming small pockets of soil. Acidic secretion from the lichens breaks down the substrate. Mosses then colonize these soil particles and build up enough nutrients for shrubs to take hold.

Secondary Succession

Secondary Succession begins on an area where a community has previously existed but has been destroyed by natural disaster such as fire, floods or destruction by human. Secondary succession has a higher level of production of biological material at a faster rate than primary succession. When natural disaster occurs, the damaged ecosystem is likely to recover in a series of successional stages that eventually result in a stable system similar to the original one that occupied the area.

Succession ends in a stabilized community and ecosystem called the ecological climax.

The tendency for an ecosystem to reach a stage where it stays in equilibrium (stable) is an example of Homeostasis. The climax community last for hundreds or thousands of years unless again disrupted.

THE STUDY OF SPECIFIC HABITATS

o Aquatic Habitats

Marine ecosystems
Freshwater ecosystems

o Terrestrial Habitats

Tropical Rainforest
Savanna
Desert
Arboreal habitat (on top of trees)

Aquatic Habitats

An aquatic ecosystem is an ecosystem in a body of water. The two main types of aquatic habitat are marine and freshwater.

Adaptation: is the possession of structural and functional features which enable an organism to live successfully in its habitat.

Adaptations of Animals to Aquatic Habitat

· Possession of gills for gaseous exchange

·         Possession of swim bladder for buoyancy
·         Streamlined shape for easy movement
·         Lateral line for detection of vibration
·         Fin for movements
·         Slimy body for easy movement
·         Sucker for attachment onto vegetation
·         Webbed limbs for swimming e.g. frog

· Special coverage like shell, scales, waxy coat to protect against decay.

Marine

They are distinguished from freshwater habitats by the presence of dissolved compounds, especially salts, in the water. They generate 32% of the world’s net primary production. Marine ecosystems are made up of seas, oceans, lagoons and estuarine.

Classes of organisms found in marine ecosystems include brown algae, dinoflagellates, corals, cephalopods, echinoderms, and sharks.

Estuaries or Brackish are coastal ecosystems where sea water and fresh water meet. The freshwater comes from rivers and streams and runs off from the land when it rains. The animals commonly found in estuaries include crab, mollusks, shrimps, bivalves.

Adaptations of Organisms to Marine Habitat

· Starfish use tube feet whiles mollusks use specialized foot for digging into the sand.

·         Many crabs and annelids (worms) are active burrowers.
·         Some animals avoid drying up by withdrawing into a protective shell.
·         Algae (seaweeds) have holdfasts for attachment to rocks.
·         Some algae have air filled bladders which enable the branches to float.
·         Some seaweed has slime around the body which protects it from drying out.
·         White mangrove has pneumatophores for gaseous exchange.
·         Possession of waxy cuticles on the leaves to prevent wetting.
·         Most aquatic plants have reduced conducting and strengthening tissues.

· Stomata are on the exposed surface of the leaves to increase the transpiration rate.

Freshwater

Freshwater ecosystems contain 41% of the world’s known fish species. Some organisms live on the surface of the water. Examples include microscopic algae such as diatoms, desmids, blue green bacteria and Chlamydomonas. These organisms form the Phytoplankton. Phytoplanktons are microscopic producers. Other free-floating organisms are spirogyra, water lettuce (Pistia) duckweeds (Lemma), salvania (aquatic ferns).

Microscopic free-floating animals or Zooplankton include crustaceans such as Cyclops, Daphnia and freshwater shrimps. Zooplanktons are the primary consumers of the ecosystem. They provide food for freshwater fish, e.g. Tilapia and tiger fish. Insects include water boatman, water beetle, pond skaters, mosquito larvae and pupae, and dragonfly larvae.

There are three basic types of freshwater habitats:

o Lentic: slow-moving water, including pools, ponds, and lakes.

o Lotic: rapidly moving water, for example streams and rivers.

o Wetlands: areas where the soil is saturated or inundated for at least part of the time.

Adaptations of Plants to Freshwater Habitat

· Presence of waxy cuticle to repel water droplets.

·         Leaves have large surface area for maximum water absorption.
·         Plants that are submerged have thin cuticle permeable to water and mineral salts.
·         Submerged plants have flexible stem to withstand the action of water currents.
·         Leaves, roots or stems have large air space for floatation or buoyancy.
·         Floating leaves have on its upper surface stomata for gaseous exchange.
·         Some plants produce seeds that can float.
·         some plants have leaves that float atop the water, exposing themselves to the sunlight

Examples: water lily, Elodea, Water lettuce, Lemma, duckweed.

Adaptations of Animals to Freshwater Habitat

o Possession of gills for gaseous exchange

o Possession of swim bladder for buoyancy
o Streamlined shape for easy movement
o Lateral line for detection of vibration
o Possesses lomotory structures such as cilia, pseudopodia flagellates or fin for movement.

o Slimy body for easy movement
o Sucker for attachment onto vegetation
o Webbed feet for swimming.

Examples: Amoeba, Euglena, Mosquito larva, Frog, Tilapia, Crab, shrimp, Crocodiles.

Adaptation of animals to freshwater habitat

Adaptation of insects to freshwater habitat

Terrestrial Habitats

Terrestrial habitats vary according to the type of vegetation. In the forest large trees are abundant; grasslands or savannah contain more grasses than trees and desert have little vegetation. The type of vegetation depends on the physical factors such rainfall, temperature and soil type. The distribution of organisms in different habitat depends partly on the vegetation and partly on climate factors.

Tropical Rainforest Habitat

The tropical rainforest is hot and it rains a lot, about 80 to 180 inches per year. Heavy rainfall increases the risk of flooding, soil erosion, and rapid leaching of nutrients from the soil. This abundance of water can cause problems such as promoting the growth of bacteria and fungi which could be harmful to plants. The distribution of organisms depends on the amount of sunlight that is made available to plant. Trees are the abundant plants. The rainforest is very thick, and much sunlight is not able to penetrate to the forest floor. The crown of rainforest trees forms three layers, storeys or canopies. Trees above 30m are called emergent. The middle layer is formed by trees about 30m high. The lower canopy, are trees about 15m high. Carpeting the forest floor is the palms, shrubs and herb layer made up of wildflowers, mosses, and ferns. Fallen leaves, twigs, and dried plants cover the ground, decompose, and help add nutrients to the topsoil.

The forest is rich with epiphytes and climbers. The epiphytes or aerophytes depend on trees for support only.

Examples of ground animals are tortoise, lizard, gaboon viper, mongoose, rat and mouse.

Adaptations of animals to rainforest habitat

Adaptations of Plants to Tropical Rainforest

o   green leaves use to trap sunlight for photosynthesis
o   drip tips and waxy surfaces allow water to run off, to prevent bacteria and fungi growth
o   buttresses, prop and stilt roots help hold up plants in the shallow soil
o climbers have hooks and tendrils to provide attachment and support
o   flowers on the forest floor are designed to lure animal pollinators since there is relatively no wind on the forest floor to aid in pollination
o   plants have shallow roots to help capture nutrients from the top level of soil

o epiphytic (orchids) have aerial roots that cling to the host plant and to absorb water

Adaptations of Animals to Tropical Rainforest

o   Termites can climb, burrow and digest cellulose.
o   The forest is the source of food and shelter to birds and other organisms.
o   Insects such as ants or butterfly live in trees and build their shelter with plant leaves.
o   Spiders construct webs among the branches of trees to trap insects
o Chameleons have narrow bodies and constant changing color, makes them inconspicuous amongst the foliage.
o   Millipedes are mostly herbivores and have two pairs of legs for each body segment, enabling them to climb well.

o Frogs have sucker-like toes that make it possible to climb up trees.

Savanna (Grassland) Habitat

A savanna is a rolling grassland scattered with shrubs and isolated trees, which can be found between a tropical rainforest and desert biome. Not enough rain falls on a savanna to support forests. There are actually two very different seasons in a savanna; a very long dry season and a very short wet season.

In West Africa there are three types of savanna zones; Guineasavanna, Sudan savanna and Sahel savanna. The change that occurs on passing from one zone into another is due to decreasing rainfall, increasing drought and annual burning of vegetation for farming purposes. Guinea savanna experience the highest rainfall followed by the Sudan and finally the Sahel zone.

Majority of savanna faunas (animals) are herbivores. Examples are mice, rat, zebra, antelopes, giraffe and elephants. Other organisms include ants, termites, pigeons, lizards and snakes.

Adaptations of Animals to Savanna Habitat

o   Many burrow underground to avoid the heat or raise their young.
o   Some migrate to deal with the lack of food during the dry season.
o   Animals (e.g. giraffe and ostrich) are fast runners to escape predators
o   They have well sense of smell which help them to detect their prey.

o Ruminants such as cattle and goat have stomachs that are adapted for storage of food for a long time.

Adaptation of mammals to Savanna habitat

Adaptations of Plants to Savanna Habitat

Vegetation must survive the long period of drought and the fires during the dry season.

o   Grass grows quickly up to 130cm high during the short rainy season.
o   Baobab tree has thick bark and store water in the trunks to withstand fire and drought
o   Trees lose their leaves through the dry season to conserve moisture.
o Trees have few leaves so less moisture is lost through evaporation and transpiration
o   Leaves are small, waxy and thorny to reduce moisture loss.

o Trees have long roots so as to extract water from the deep ground

Adaptations of Plants to Dry Terrestrial Habitat

·         Has thick bark for fire resistance
·         Have short life cycle to survive unfavorable conditions
·         Succulent leaves or stem has water storage tissues
·         Presence of thick waxy shiny cuticles or hairs on leaves to reduce water loss
·         Sunken stomata or leaf rolling to reduce water loss
·         Roots are deep seated for tapping water from soil
·         Leaves are modified into spine or thorns for reduction of water loss or

· Seasonal shedding of leaves to reduce water loss.

Examples: Bryophyllum, Acacia, Euphorbia, Baobab, Casuariana, Aloe, catus.

The Desert Habitat

Desert organisms live in an environment that can be very harsh. Major problems include an average 10 to 12 hours hot sunshine a day, a scarcity of water and often of food, and predators. The soil is often sandy or rocky and unable to hold much water. Winds are often strong, and dry out plants. Plants are exposed to extreme temperatures and drought conditions. Animals respond to such conditions and have evolved mechanisms to deal with various extremes.

Adaptations of Plants to Desert Habitat

o   Succulent plants store water in their stems or leaves
o Some plants have no leaves or small seasonal leaves that only grow in rain season, to reduce water loss. Leafless plants conduct photosynthesis in their green stems.
o   Long root systems spread out wide or go deep into the ground to absorb water
o   Plants have a short life cycle to survive unfavorable conditions
o   Leaves with hair help shade the plant, reducing water loss.
o   Some leaves turn throughout the day to expose a minimum surface area to the heat.
o   Spines to discourage animals from eating plants for water

o Waxy coating on stems and leaves help reduce water loss.

Examples: Cacti, Euphorbias, xerophytes

Adaptations of Desert Animals

¨      They stay in the shade of plants or rocks or by burrowing underground.
¨      Many are nocturnal: they are active and hunt at night when it is cool.
¨      They do not have sweat glands and pass only small amounts of concentrated urine.
¨      Large ears help dissipate excess body heat on hot days
¨      Thick fur helps insulate the body from the cold desert nights.
¨      Thick skin helps to prevent drying out in the desert.
¨      A nictitating membrane on the eye wipe to remove sand from the eyes

¨ Enlarged appendages increase surface area which promotes heat loss.

Examples: kangaroos, rats, snakes, lizard, arachnids, a few birds and insects.

The Arboreal Habitat

Most animals live in trees and are termed as arboreal. These include grasshopper, butterfly, ants and spiders. Among reptiles are chameleons, gecko and snakes such green mamba. Common birds include African grey parrot, wood owls and several species of hornbills. Mammals include bats, monkeys (such as mona and colobus monkey), squirrels, pangolins and leopards.

Plants Adaptations to Arboreal Habitat

Ø Some plants are parasitic, extracting nutrients from the host plant e.g. mistletoe

Ø Epiphytes have aerial roots for absorbing moisture and nutrients from humid air

Ø Woody stems are very flexible to permit bending, twisting, and coiling.

Ø Epiphytic orchids have thick, waxy leaves for retaining water and allowing excess water to drip off.

Ø Climbers have hooks and tendrils to provide attachment and support.

Animals Adaptations to Arboreal Habitat

o Termites can climb, burrow and digest cellulose
o Insects such as ants and butterfly live in trees and build their shelter with plant leaves.
o Spiders construct webs among the branches of trees to trap insects.
o Chameleons have narrow bodies and constant changing color, makes them inconspicuous amongst the foliage.
o Moths can mimic insects that predators won't eat, such as wasps
o Frogs have sucker-like toes that make it possible to climb up trees.
o Birds have strong legs, with toes adapted for grasping.
o The long, curved claws of monkeys help them to cling on to branches, while their tails allow them to balance on narrow branches.

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