Population Ecology Summary PDF

# Population Ecology - Ecology is the study of interactions of organisms with their physical and biological environments and how these determine the distribution and make up of populations within an ecosystem. - Population ecology is concerned with changes in the size of a population, and the facto...

# Population Ecology - Ecology is the study of interactions of organisms with their physical and biological environments and how these determine the distribution and make up of populations within an ecosystem. - Population ecology is concerned with changes in the size of a population, and the factors that regulate these changes. ## Levels of Organisation - **Individuals:** An individual form of life, such as a bacterium, protist, fungus, plant, or animal, composed of a single cell or a complex of cells that are capable of growing and reproducing. - **Species:** A group of closely related organisms that are very similar to each other and are usually capable of interbreeding under natural conditions to produce fertile offspring. - **Population:** A group of organisms of a species that occupy the same area and can breed freely with each other. - **Community:** A group of **different** species that inhabit and interact in a particular area. - **Ecosystem:** Made up of groups of different species (populations) that interact with each other and with the environment. - **Biosphere:** The part of the Earth where living organisms are found. ## Population Size - The total number of individuals in a population. - It can increase and decrease over time with a change in one or more of the following population parameters: - **Natality:** Immigration - **Mortality:** Emigration ## Population growth - **Exponential (J):** Extinction due to overpopulation. - **Logistical (S):** Carrying capacity reached, species is acclimatising. ## Carrying Capacity (K) - The max. number of individuals of a particular population that the environment can support at any given time. - A combination of the **limiting factors** allows a population to be self-regulating: the dips/drops allow the environment to regenerate. - Lag phase - Exponential/accelerating growth phase - Decelerating growth phase - Stabilisation phase (fluctuates around carrying capacity) ## Limiting Factors - Anything that constrains a population's size and slows or stops it from growing. There are two types: ### Density Dependent - Limit the growth of a population as a direct result of its density - **Example:** Limited food/water, disease, limited space, predation, inter/intraspecies competition. ### Density Independent - Limits the size of a population regardless of its density. - **Example:** Changes in rainfall, temperature, natural disasters, floods, fires, humidity, acidity, salinity, drought, earthquakes. ## Estimating Population Size ### Direct Methods - **Census:** Counting every individual - **When?** Suitable for populations with organisms large enough to be seen (elephants, rhinos, whales) in an area not too large. - Suitable for populations where individuals are slow-moving (snails, tortoises) / stationary (plants) / fixed (barnacles, mussels) ### Indirect Methods - **Quadrat Method:** Counting individuals in small areas (quadrats) then using the numbers to calculate pop. Size - **Formula:** Total population = Average no. of organisms per quadrat sampled × Size of the whole habitat - **Precautions:** Choose random quadrats, distribution may not be uniform, size of the quadrat, round down final answer, use for slow/sedentary populations. - **Mark-recapture method:** A known no. of individuals is captured, marked, released, then after a suitable amount of time passes another sample is captured, and no. of marked ones are counted. - **Formula:** P = Total caught in 1st capture × Total caught in 2nd capture ÷ Total marked in 2nd capture. - **Precuations:** Short time between captures so there aren't births/deaths, marking must not affect individuals' movement/behaviour/visibility to predators, closed population, ensure individuals do not become "trap shy" or "trap happy," marks must not come off between captures. ## Predator/Prey Relationship - The role of predators in the environment is to help create and maintain greater diversity within an ecosystem. - **How?:** - Regulate the abundance/distribution of prey species. - Increase biodiversity, prevent a single species from becoming dominant. - Keep the prey population genetically fit by removing Sick/injured/weak individuals. - Provide food for scavengers. - The feeding relationship between predator/prey determines the size of the populations through a negative feedback loop: - **Increase in prey** allows prey numbers to increase. - **Increase in predators and predation** occur, causing a cyclical pattern. - Predator numbers lag behind prey numbers. - **Decrease in prey** results in a decrease in food available for predators. - **Decrease in predators** as they need time to breed after the prey. - **Note:** You will never have more predators than prey, only 10% of energy flows from 1 tropic level to the next ### Example: - **Aphid/Ladybird:** Ladybirds lay eggs amongst aphid colonies. Both adults and larvae from hatched eggs feed on aphids, reducing the aphid population. This in turn, reduces food source for ladybirds, causing their population to decrease. The aphid population then recovers and increases again. - **Zebra/Lion:** They depend on each other for survival. Zebra live in herds and have strategies to avoid being preyed on by lions. Lions use cooperative hunting to catch zebra. Lions prey on many species; would not be drastically affected if zebra went extinct but if lion went extinct it would impact zebra and the other species they hunt. - **Fish/Shark:** Sharks eat fish efficiently and prey on the old, sick, or slow. Since they have a different immune system to the fish, they can eat the sick ones, preventing spread of disease and strengthening the prey's gene pool. Sharks are a Keystone species, they have a disproportionately large effect on the environment relative to their abundance. - **Note:** The longer the generation time, the longer it takes for fluctuations to occur. (Time it takes for an organism to reach sexual maturity). ## Food Webs - Interconnected food chains within an ecosystem. - **Autotrophs (producers) --(energy transferred)---> Heterotrophs (herbivores) --(energy transferred)---> Carnivores** - **1st trophic level:** 10% of energy transferred. - **2nd trophic level:** 10% of energy transferred. - **3rd & 4th trophic levels:** 1% of energy transferred - **Keep ecosystems balanced:** The removal of a species has a large impact on the others. - **Biomass:** The mass of one species all together. Decreases as you go up the trophic levels. - **Human activities (pollution, habitat destruction, over-fishing, hunting) have reduced numbers of top predators.** ## Competition - When two or more species/populations compete for **the same resources that have become limited.** - **What?** Light, space, water, food, shelter. ### Types of Competition - **Interspecific:** Between 2 or more different species with similar ecological niches - Includes: Competition for territory/space, food water. - Can lead to competitive exclusion - One species uses the resources more effectively than the other: outcompetes it, reaches carrying capacity. - **Intraspecific:** Between individuals of the same species. - Includes: Competition for resources as well as mates, which leads to a stronger gene pool. - The most intense as members of the same species have the same ecological niche. ## Ecological Niche - Refers to all the conditions, biotic and abiotic, necessary for a species to survive and reproduce, as well as the role of that species in the ecosystem. ### Example: - **Species A:** Outcompeted species B. - **Species B:** Has in turn gone extinct. - **Note:** Direct competition only begins when resources become limited. ### Competitive Coexistence - When 2 species are able to use the resources differently - They are not in direct competition, and can coexist. - This is called resource partitioning. ## Resource Partitioning - The evolutionary process whereby species with similar requirements living in the same habitat have specialized traits that enable them to utilize the resources differently, creating separate niches to reduce interspecific competition and make coexistence possible. - **How are resources partitioned?:** By using the resource: - **At different times (temporal):** Eg, one species feeds during the day, the other at night. - **In different parts of the habitat.** - **In different parts of the same plant.** ### Strategies Among Plants - **Stratification:** - **Emergent layer:** Exposed to maximum light intensity. These plants have small leaves to allow light through to lower layers as well as to prevent wind from knocking them over. - **Canopy layer:** - - **Understory:** Shorter trees growing in the gaps between larger ones - **Undergrowth:** Large leaves + Lots of chlorophyll in order to absorb the maximum amount of light. ### Strategies Among Animals - **Coexisting Herbivores: Giraffe and kudu:** Are both browsers in savannah areas. Both eat from a wide range of trees, and giraffe eat from the top while kudu eat from the bottom. - **Coexisting Shorebirds:** Shorebirds partition resources both vertically and horizontally. This is seen in their feeding habits and morphological adaptations. *Pickers* look for food and forage whereas *Probers* have long bills that they stick into the sand to feel for prey. - **Coexisting Predators: Lion and leopards:** Avoid competition by hunting at different times (Lion early morning/night, leopard day), hunting different prey (Lion: medium-sized mammals, leopard: small mammals), hunting in different areas of the habitat. ## Ecological Succession - A predictable pattern of gradual change over time in the types of species in an ecosystem in a previously uninhabited area (primary) or following a disturbance (secondary). ### Primary Succession - **Pioneer Stage:** Begins with lichen. Fungus absorbs H₂O and minerals from the rock. Algae uses these to make food. Fungus uses the food and grows. This causes erosion of the rock, it breaks down into soil. The lichen then dies, adding minerals to the soil. Now other pioneer species begin to establish such as masses, herbs, weeds, ferns, grasses, annuals, insects, small birds, rodents, decomposers, and detritivores. - **Intermediate Stage:** The conditions are better, allowing for more biodiversity. The soil is thicker and more nutritious, and can hold more water. - **Plants:** Small trees and large shrubs. More shade, lowers temperature and provides shelter. - **Animals:** Larger herbivores (antelope, hares) and smaller carnivores (snakes, wildcats). - **Climax Community:** The semi-stable end point of succession (some smaller changes will always happen). Large and varied plant species. Large herbivores and carnivores. The types of organisms depend on environmental factors: - **Hot and Dry:** Semi desert - **Hot and Wet:** Tropical rainforest ### Secondary Succession - **Pioneer Stage:** There is already soil present. Pioneer species establish such as herbs, weeds, annuals, grasses, ferns, perennials, and climbers. - **Intermediate Stage:** The conditions are better, allowing for more biodiversity. The soil is thicker and more nutritious, and can hold more water. - **Plants:** Small trees and large shrubs. More shade, lowers temperature and provides shelter. - **Animals:** Larger herbivores (antelope, hares) and smaller carnivores (snakes, wildcats). - **Climax Community:** The semi-stable end point of succession (some smaller changes will always happen). Large and varied plant species. Large herbivores and carnivores. The types of organisms depend on environmental factors: - **Hot and Dry:** Semi desert - **Hot and Wet:** Tropical rainforest ## What Factors Determine the Endpoint (Climax) of a Community? - **Rainfall:** The amount of rainfall determines the biome of the climax community - **Draining of wetlands:** Destroys the wetland and affects all of the communities downstream - **Overgrazing:** If one species of grass is eaten more than others then the others will become dominant - **Climate change:** Can cause climates to become drier or wetter - **Invasion by aliens: **Outcompete indigenous plants as animals do not eat them. ## Social Organization - Resources and activities divided amongst the group. - Improves the survival and reproductive success of an individual. - **How?** ### 1. Avoid Being Attacked by Predators - Prey form large groups. While this makes them more visible, it decreases the amount of prey captured by predators through: - The group's many "eyes and ears". - **Dilution effect:** greater the number of individuals, the greater the survival chance of the individual. - **Confusion and distraction effect:** Prey scatters in all directions, preventing the predator from singling out its prey. - **Vulnerable animals:** Stay in the middle of the group for protection - **During migration**: The inexperienced are given guidance and protection ### 2. Hunting Collectively to Find Food - **African wild dogs** are highly social animals with complex methods of communication that they use when hunting. - They prey on animals larger than them such as antelope, zebra, and warthog, and are successful 8/10 times. - They hunt in packs of up to 15 individuals. - **How do they hunt?** - They detect their prey and chase it at a fast run. - They are tireless hunters; they can chase their prey for 1+ hours - When their prey tires, they immobilize it by grabbing its limbs before efficiently killing it. - The hunters eat the entire kill to prevent other stronger predators from taking it; They then regurgitate it back at the den, where the whole pack shares the food – the young eating first and the other dogs who did not hunt (dominant breeding, sick, injured, old) still getting food. ### 3. Protecting Resources and Regulating Population Size - **Wild dog packs** have a dominant breeding pair who mate for life and prevent the other females from breeding. When the females reach sexual maturity at 18-24 months they leave the pack to join another, while the males remain for the rest of their lives (around 11 years). - **Benefits of this:** Dominant pair keeps the pack under control - Raising the pups and caring for the sick/injured is a group task. - Subordinate members benefit too as they have access to the group's shared resources. ### 4. Dividing Labour Among Individuals - **Most animals living in social groups have division of tasks/labour within the group.** - **Each individual has a role to play, contributing to the success of the whole group.** - **Eusocial animals:** Which need the colony to survive - Characterised by a dominant breeding pair / single female (queen), a strict caste system, individuals which need the colony to survive. ### Example: Termites - **Reproductive:** Alates, the winged termites which founded each colony. - King and Queen, the alates who lost their wings and have burrowed underground. The queen lays eggs continuously which hatch into nymphs that will grow into other castes. The queen lays thousands of eggs weekly and can have an abdomen up to 10cm long. The king continuously mates with the queen. - **Young reproductives:** Which can become new alates or supplementary reproductives if the king and queen die. - **Workers:** Majority of the colony. They can be male or female. They make tunnels, build the mound, forage for food, look after eggs and nymphs, and feed members of the colony. Digest cellulose then pass the partially digested food to the colony regurgitated or as fecal pellets. - **Soldiers:** Make up 5% of the colony. Develop huge biting / squirting mouthparts to defend the colony. - **Ratio of the castes is regulated through the pheromones secreted by the reproductives and soldiers which inhibits the nymphs from developing into these 2 castes if there are too many.** ### Example: Honeybees - **The Queen:** The only fertile female and starts the colony. Has nuptial flights in which all of her eggs are fertilised by drones. - **Worker Bees:** Sterile females (diploid) - reproductive parts modified to form a stinger. These bees have different roles as they age: - **Feed larva** after the eggs hatch for a week (nurse phase) before sealing up the larva in its cell (pupal stage). After another week they emerge as adult bees. - **Workers begin building comb cells** by secreting wax. This is where nectar and pollen is stored. - **They leave the hive to forage for nectar and pollen**. They also defend the hive. - **Drones:** “Flying gametes,” male (haploid) bees. Their only role is to reproduce. They are the ones who are kicked out if the hive is overcrowded. - **Unfertilised eggs:** Drones. - **Fertilised eggs:** Worker bees (fertilisation done by the queen).