Ecology and Population Management Flashcards

Trophic Levels and Energy Flow in Ecosystems

In every ecosystem, organisms are categorized into trophic levels based on how they obtain energy. Producers, also known as autotrophs, form the base of the food chain. These organisms utilize solar energy through the process of photosynthesis to convert it into chemical energy, effectively producing their own food. Following the producers are the Primary Consumers, which are herbivores that feed directly on the producers to transfer chemical energy from plants. Secondary Consumers consist of carnivores or omnivores that consume the primary consumers (herbivores). Tertiary Consumers are top predators that feed on other consumers within the ecosystem.

Energy transfer within these food chains follows a specific sequential flow. Initially, solar energy is captured by producers and converted into chemical energy. Primary consumers then obtain this energy by eating the producers, which is subsequently passed to secondary consumers. This process continues until it reaches the top consumers, who receive the smallest amount of the original energy captured from the sun. Decomposers play a critical role by obtaining energy from dead organisms and waste products, ensuring the cycling of nutrients.

The 10% Rule and Energy Loss

The transfer of energy between trophic levels is inherently inefficient. On average, only $10\%$ of the energy is transferred to the next trophic level, while the remaining $90\%$ is lost to the environment. This energy loss primarily occurs through several mechanisms, including movement, heat generated from cellular transport, and waste products. Because of this drastic reduction in available energy at each progressive level, higher trophic levels have significantly less energy available to them, which typically results in short food chains.

Consequences of Removing Trophic Levels

The removal of specific groups within a food web can lead to catastrophic ecological consequences. If producers are removed, less energy enters the ecosystem, causing herbivores to lose their food source. This leads to a collapse of energy flow at the first trophic level, a crash in herbivore populations, a decline in carnivores and omnivores due to prey loss, and a total collapse of the food web with a corresponding drop in biodiversity.

Removing secondary consumers causes a loss of predation pressure on herbivores, leading to a population increase among primary consumers. These herbivores then overgraze the producers, causing plant populations to decline. Eventually, the scarcity of producers causes the herbivore population to crash due to starvation. If primary consumers are removed, plants may grow unchecked initially, but secondary consumers lose their main food source. This may lead to the extinction of some carnivores or forced migration due to starvation, while plant overgrowth alters the habitat structure, potentially allowing new invasive species to establish themselves.

If the top predator is removed, mid-level predators and herbivores increase in number. This leads to the overhunting of smaller prey and overgrazing of vegetation. As vegetation collapses, the lack of root structures to hold the earth leads to an increase in soil erosion.

Ecological Succession: Primary, Secondary, and Fire-Related

Primary Succession occurs after major catastrophic events such as volcanic eruptions or tsunamis that leave an area bare of living organisms, a state known as nudatation. The process begins with lichen, which is usually the first species to colonize the bare area. Lichen helps establish soil so that mosses can grow. As these plants die and are broken down by bacteria, fungi, and invertebrates, nutrients are added to form a simple community. Weathering and soil accumulation allow grasses and small plants to grow. These are generally r-selected species, whose numbers decline once more competitive plants arrive. Eventually, a new community forms as species establish themselves and animals migrate back in.

Secondary Succession occurs after a disturbance like fire or flooding. Unlike primary succession, the soil and some organisms often remain. This allows organisms to recolonize the area and regain equilibrium much faster than in primary succession. The process essentially skips to the soil formation stage. r-selected species rapidly recolonize the area, leading to a faster community build-up.

Succession specifically after a fire is a unique process where most fires leave the soil intact, allowing ecosystems to recover quickly via surviving seeds and roots. Pioneer species, which are fast-growing plants and weeds, colonize first to stabilize the soil using nutrient-rich ash. This is followed by seral stage development, where shrubs, grasses, and young trees outcompete early colonizers, increasing the ecosystem's structure and biodiversity. Eventually, the environment returns to a mature forest unless interrupted by another disturbance.

Population Estimation: The Mark-Recapture Method

The Mark-Recapture method involves taking a random sample to estimate the overall abundance of a species. The process involves three steps: first, capturing animals randomly using methods like cage trapping, pitfall traps, or mist nets; second, marking and releasing them using non-harmful identifiers like paint, ear tags, or radio collars; and third, recapturing a sample later to count how many individuals are marked.

Several assumptions must be met for mark-recapture results to be valid: marks must not affect the animal (e.g., make it more visible to predators); all animals must have an equal chance of being captured (avoiding "trap happy" or "trap shy" individuals); there must be sufficient time between sessions for marked animals to mix back into the population; births, deaths, and migration must not be significant; and marks must not wash off.

If these assumptions are violated, the population estimate ($n$) will be incorrect. If animals are "trap happy," the number of recaptured marked animals ($m$) will be higher than expected, leading to an underestimate of the total population ($n$). Conversely, if they are "trap shy" or didn't mix back in, $m$ will be lower, resulting in an overestimate of $n$. If marked animals die or marks wash off, fewer marked individuals will be caught ($m$ is lower), and the population estimate ($n$) will be inaccurately high.

Ecological Pyramids

Ecological pyramids provide a visual representation of the various relationships between trophic levels. A Pyramid of Energy shows the flow of energy, illustrating the dramatic decrease as one moves up the food chain according to the $10\%$ rule. For example, if producers have $1000\text{ units}$, primary consumers will have $100\text{ units}$, secondary consumers $10\text{ units}$, and tertiary consumers only $1\text{ unit}$.

A Pyramid of Numbers represents the actual count of organisms at each level. These can be inverted; for instance, a single large producer like a tree can support thousands of small insect consumers. A Pyramid of Biomass measures the total living matter present. These can also be inverted, such as in aquatic ecosystems where the total biomass of consumers (like sharks or fish) might temporarily exceed the biomass of the producers (plankton) at a specific point in time.

Invasive Species and Control Methods

Invasive species are introduced organisms moved by humans from their native locations to new areas. They often lack native predators, allowing them to increase unchecked and damage native ecosystems. Environmental management involves chemical control, such as pesticides or 1080 baits. 1080 baits are made from poisonous compounds found in native plants; because native animals evolved alongside these plants, they can safely consume small amounts, whereas invasive mammals are killed.

Biological control involves using living organisms to manage pests. To be successful, the biological agent must only target the pest species, decrease in number as the pest decreases, not compete with native species, and be reproductively self-sustaining. There are four types of biological agents: General predators (e.g., ladybirds eating aphids), Specialized predators (targeting only one pest), Microbial diseases (bacteria, viruses, or fungi), and Parasites that cause the death of the host.

Dryland Salinity

Dryland salinity is the process where underground salts are transported to the surface by a rising water table. When the salinity becomes too high, plants cannot grow. The primary cause is the removal of deep-rooted native vegetation. Deep roots normally pump water from deep underground to evaporate through leaves. Without these trees, the water table rises, dissolving salt in the soil. As the water reaches the surface and evaporates, the salt is left behind. This is exacerbated by extra precipitation and irrigation. To combat this, farmers can syphon off water or grow short-rooted crops between trees to stabilize the water table.

Population Dynamics and Measurement

A population is defined as a group of individuals of the same species living in the same place at the same time. In open ecosystems, individuals can migrate, affecting population size. In closed ecosystems, only birth and death rates change the population size. The formula for determining population growth is:

$\text{Growth} = (\text{birth rate} + \text{immigration rate}) - (\text{death rate} + \text{emigration rate})$

Factors affecting growth include age structure (too young or too old to reproduce), biotic effects (predation, competition, parasitism, and disease), and abiotic effects (sunlight for energy, temperature for metabolic rates, and water for plant growth). Biological strategies vary between species: r-selected species (e.g., rats, frogs) have shorter lifespans, high reproductive rates, and minimal parental care, often colonizing unstable environments. K-selected species live longer, breed later, have fewer offspring, and provide extensive parental care in stable environments.

Quadrats are square frames used to count species at random to estimate population size. The calculation for average density is:

$\text{Average density} = \frac{\text{total number of individuals counted}}{\text{area of each quadrat} \times \text{number of quadrats}}$

For example, if there are $65$ individuals found in $24$ quadrats that are $2\,m \times 2\,m$ (total area per quadrat is not explicitly stated as $4\,m^2$ but derived from $24 \times 2\,m^2$ in the example calculation), the math provided is: $65 \div (2 \times 24) = 1.4\text{ individuals per } m^2$.

Transects involve drawing a line through a community to determine species distribution. Following a random start, subsequent lines are placed evenly. Researchers record organisms intersecting the line at set intervals. Transects identify environmental gradients, which are gradual changes in communities based on abiotic conditions along the line.