C4.1 populations and communites

a group of organisms of the same species, living in the same place

Populations of the same species are isolated by geographic barriers such as mountains or oceans, or by reproductive isolation, which includes barriers like different mating times, habitats, or behaviors. These mechanisms prevent gene flow and can lead to the development of new species over time, Because of this, the characteristics of one population can differ from another population of the same species in a different environment.
In the theory of evolution, the population (not the species as a whole) is the unit that evolves over time.

a subset of the population that is being studied

In most species a full count is impossible or prohibitively impractical, so instead random sampling is used to produce an estimate of the population size. This is less accurate than a full count, but is often the best that can be done.

the natural discrepancy, or amount of error, between a sample statistic and its corresponding population parameter

Randomness is needed when sampling so that there is no bias for the most accurate results.

For sessile organisms, quadrat sampling is a procedure that involves placing a number of quadrats randomly around an ecosystem.

For motile organisms (most animals), the capture-mark-release-recapture method can be used to estimate population size.

Quadrat sampling occurs just by laying a grid on a surface where the sessile organism is, vs the capture-mark-release-recapture that takes a lot more process steps to track population sizes in their own natural habitats.

Key differences lie in relying on fixed space (sessile) versus dynamic capturee-capture (motile), with motile methods needing assumptions about migration, birth/death rates, and mark retention.

For any species of interest within the quadrat, the researcher might record:
- Presence/absence in the quadrat
- Number of individuals
- Percent ground coverage

Some precautions need to be taken for quadrat sampling:

1. Quadrat location must be either random or systematic (i.e. laid out along a transect) to avoid bias in the location choices

2. Quadrat sampling can't be used for motile species, since they may move in and out of the quadrat

1. Capture a number of individuals using nets, traps, or other methods

2. Mark them using non-toxic paint in a way that won't make them more visible to predators

3. Release the marked individuals (M) and wait some time for them to mix with the population

4. Perform a second round of capture and count the total number captured (N) and the number that have marks (R)

The population can be estimated with the formula (called the Lincoln index): Population size = (M x N)/R

- There is no immigration, death, or birth during the sampling period
- Marked individuals and unmarked individuals are equally likely to be captured in the recapture phase
- The marks don't rub off or wear off

The Lincoln index is an estimate of the population size.

M*N/R
M=initially caught and marked
N=total recaptured (marked + unmarked)
R=total recaptured (marked only)

he maximum number of organisms in a population that an ecosystem can support

for plants:
> water
> light
> soil nitrogen
for animals:
> water
> space for breeding
> food/territory to obtain food
> dissolved oxygen (the amount of oxygen available to living aquatic organisms)

Intraspecific competition is a density-dependent factor limiting population size-the more frog eggs there are in a pond, the more frogs will die of starvation. This is a type of negative feedback that has a stabilizing effect on population size.

A negative feedback system is one where a change in the system is resisted by the system in order to bring it back to a set point.

Density-dependent factors, like disease and resource competition, have a greater impact in denser populations. (i.e DD factors have an increasing effect with increasing population size)

Density-independent factors, like natural disasters, are not facilitated by higher population density. (i.e they have the same effect no matter the size of the population,)

Density-dependent factors include:
- disease
- competition
- predation

Density-independent factors include:
- food or nutrient limitation
- pollutants in the environment
- climate extremes, including seasonal cycles such as monsoons.
- catastrophic factors can also impact population growth, such as fires and hurricanes.

all species have the inherent ability to produce far more offspring than the environment can sustainably support, leading to a "struggle for existence" where many individuals die due to limited resources (food, water, space) or predation, making overproduction a fundamental driver of natural selection and evolution. (known as density-dependent factors)

When a population is significantly below the environment's carrying capacity, then population growth can be very close to exponential.
this includes:
- abundance in space
- abundance in nutrients
- lack of competition

The combination of these two factors-exponential growth potential and limited resources-means that a population growth curve takes a sigmoid (S-like) shape with three phases:
- Exponential phase: population increases more and more rapidly
- Transitional phase: population growth slows as limiting factors have increasing effect
- Plateau phase: population remains close to carrying capacity

See slide 21

Using a logarithmic scale for population growth plots compresses large numbers, making it easier to visualize exponential trends and compare growth rates across different population sizes, as exponential growth appears as a straight line, highlighting consistent percentage increases rather than absolute changes. It effectively handles wide value ranges (e.g., 10 to 100,000) on one graph, showing that equal vertical distances represent equal multiplicative changes (e.g., doubling), not equal additive changes, making relative growth rates comparable.

To study duckweed in an experimental setting, it can be grown in petri dishes or beakers filled with water.
Possible independent variables to test include:
- pH
- Light intensity
- Nitrate or phosphate concentration (fertilizer)
- Salinity

Yeast: Measure optical density (absorbance) over time in a sugar solution using a spectrophotometer to get growth rate, or count cells on a hemocytometer.
Duckweed: Count the number of fronds (plants) in a dish or beaker with a set amount of water/nutrient solution weekly.

To compare observed and expected population growth curves of yeast or duckweed, plot the population size over time for both datasets. The observed growth curve can then be compared to the expected growth curve based on mathematical models or theoretical predictions, evaluating factors such as exponential growth, logistic growth, or deviations from expected patterns.

Intraspecific relationships refer to interactions between individuals of the same species within a population or community. These interactions can include mating, competition for resources, cooperation, territoriality, and social hierarchies.

Competition can be a powerful force affecting the abundance of populations. Competition will reduce the amount of available resources to each species. In most of the cases the effects of competition are asymmetrical or unequal. That is, one species is harmed more than the other.

Cooperation in a population occurs when individuals work together to achieve mutual benefits such as increased foraging efficiency, defense against predators, reduce intraspecific competition or care for offspring. This can enhance survival and reproductive success for individuals involved, promoting population stability and contributing to the overall resilience of the community.

- Brown pelicans nest near the ocean, and with one another for space
- Competition for space for sessile species, like barnacles or oysters.
- Same population seeks the same types of food in the same territory
- Competition for mates is violent, but attracting a mate is about being the most beautiful. ex: the peacock.
- Territorial species will fight other members of the same species when their territory is invaded.

Cooperation:
- Emperor penguin colonies huddle together to conserve heat.
- Travelling in flocks; allows cranes to conserve energy while flying.
- Predator and prey species can both benefit from forming herds (elk form herds) or packs (wolves hunt in packs) — hunting and defense.

the group of all populations living in the same area and interacting with one another.

A coral reef is a complex community with many interactions between populations. Most corals have photosynthetic unicellular algae called zooxanthellae living inside their cells.

Mutualism: when two species have a close association that benefits them both.

Interspecific competition: an interaction in which two species are competing for the same resource.

Predation: a feeding relationship in which a consumer kills and eats another consumer species.

Herbivory: a feeding relationship in which a primary consumer eats part or all of a producer.

Parasitism: when two species have a close association that benefits one species at the expense of the other.

Pathogenicity: a relationship where one species lives inside another and causes a disease in the host.

Mutualism: flowering plants produce nectar to attract pollinators. They feed, then pollinate for the plant.

Interspecific competition: trees in a forest are competing for space and light, even if they are different species.

Predation: lions hunting zebras

Herbivory: broad-tailed hummingbird eating nectar from honeysuckle.

Parasitism: mosquitos visit a host briefly, take a blood meal, then leave.

Pathogenicity: viruses like SARS-CoV-2 virus (causes covid) can take over cells and use their ribosomes and enzymes to reproduce.

In the root nodules of Fabaceae, there is a symbiotic relationship with the mutualistic bacteria such as Rhizobium fix atmospheric nitrogen, providing the plant with usable nitrogen in exchange for carbohydrates.

Mycorrhizae are a common form of mutualistic relationship. Orchids form mutualistic relationships with mycorrhizal fungi, the fungi forms thin filaments known as hyphae, which interact with the roots of the plants hyphae greatly increase the surface area of the root systems of the plants, increasing the amount of water and mineral ions that can be absorbed by the plant roots. Thus the fungi assist in nutrient uptake from the soil and receive organic carbon from the orchid roots.

Hard corals host mutualistic zooxanthellae, photosynthetic algae that provide the corals with essential nutrients and oxygen while receiving shelter and nutrients in return.

Endemic species: native organisms that are restricted in a particular geographic region because they wont thrive without those specfic conditions, such as a mountain range, lake or island, among others.

Invasive species: an introduced, nonnative organism (disease, parasite, plant, or animal) that begins to spread or expand its range from the site of its original introduction and that has the potential to cause harm to the environment, the economy, or to human health.

Invasive species can directly compete with endemic species for resources, potentially reducing their access to food, habitat, and other essential resources within their realized niche. Additionally, invasive species may introduce predation, disease, or other negative interactions that further constrain the habitat and resource availability for the endemic species, leading to changes in their distribution, abundance, and overall fitness.

Invasive cats in Australia compete with endemic species for food

Cats prey on endemic mammals, birds, and reptiles

Reduced resources lead to declining endemic populations

Competition and predation disrupt community balance

The methodology of using a chi-square test to assess interspecific competition in a community involves comparing observed frequencies of species interactions to expected frequencies under a null hypothesis of no competition.

Limitation:
assumptions about independence of observations, requires a large sample to be accurate and cannot show the strength of thes associations.

Direct experimentation to assess the presence of interspecific competition in a community involves manipulating the presence or abundance of species and observing the resulting changes in population dynamics or ecological interactions. By comparing experimental treatments with control groups, researchers can directly infer the role of competition in shaping community structure and dynamics.

No association:
- The presence of one species has no impact on the presence of the other
- Indicates no close interaction (null hypothesis)

Positive association:
- The two species are more likely to be found together
- Could indicate mutualism

Negative association:
- The two species are less likely to be found together
- Could indicate competition

To conduct a chi-square test of association between species in a community, first create a contingency table that tabulates the observed frequencies of interactions or occurrences between the species. Then, calculate the expected frequencies under the null hypothesis of independence, and use the chi-square test statistic to determine if the observed frequencies significantly deviate from expected values, indicating a potential association between the species.

Since predators and prey have a direct feeding relationship with one another, their population sizes are closely related.
As a prey population increases, so can the predator population.
As a predator population increases, more prey will be consumed, driving the population down.
Predation is therefore a density-dependent factor controlling prey populations.

In the Arctic, the population of arctic foxes and lemmings demonstrate an oscillating cycle; when lemming populations surge due to abundant food, arctic fox numbers subsequently increase due to ample prey availability. As the foxes prey upon the lemmings, their population declines, leading to a decrease in fox numbers, allowing lemming populations to recover and restart the cycle.

When a higher trophic level influences a lower trophic level in an ecosystem, this is called a top-down control on the population. By eating prey, predators lower the prey population.
A bottom-up control involves a population being limited by the resources available in lower trophic levels. When prey numbers decrease, there is less food for predators, so the predator population also decreases.

one plant species releases biochemicals that inhibit the growth or germination of their competitors.

The common sunflower produces chemicals in its roots and seed coats that damage the roots of other plant species nearby.

chemicals produced by bacteria and fungi that inhibit or kill other microrganisms

Penicillium: a genus of fungi (saprotrophic) that, to prevent competition for food, secretes penicillin (only when food supplies are scarce). penicillin works by interfering with the cross-linking of peptidoglycan molecules in the cell walls of Gram-positive bacteria, causing their cell walls to become weak, eventually it will burst and die.