Community Ecology

COMMUNITY LEVEL ECOLOGY

Interspecific Interactions

Zooming out from a single population, you will see multiple populations in a given area. A community consists of multiple populations and how they interact with each other and their environment. Because these are interactions between two different species, they are referred to as interspecific ("between species") interactions. Some of these interactions are negative, some are positive, and some are neutral for any given organism.

COMMUNITY INTERACTIONS

Habitat

The habitat is a natural setting in which a certain organism lives and relies on its surroundings for survival, food, shelter, protection, and mating.

The physical and biological aspects of an organism’s habitat define it.

The nature of the soil, land availability, sunlight, temperature, and climatic variables are all physical factors. Food availability and the presence or absence of predators are examples of biological factors.

Niche

An ecological niche describes the functional position and role of an organism within its environment

An ecological niche will be comprised of various components, including:

The habitat in which the organism lives
The activity patterns of the organism (e.g. periods of time during which it is active)
The resources it obtains from the environment (e.g. food source, territorial boundaries, etc.)
The interactions that occur with other species in the region (e.g. competition / predator-prey relationships)

Types of Niches

Some species may not be able to occupy their entire niche due to the presence or absence of other species

The fundamental niche is the entire set of conditions under which an organism can survive and reproduce (i.e. where it can live)
The realized niche is the set of conditions actually used by a given organism after interactions with other species are taken into account (i.e. where it does live)

Competitive Interactions

Competitive interactions among populations are characterized by competition for food between members of the same population OR members of separate populations.

Competition is generally considered a -/- interaction between organisms.

Those species that outcompete the other better survive and pass on their traits, and are more likely to occupy a biological niche.

Positive Associations

1.Symbiotic Interactions

Symbiotic Interactions are described as long-term biological interactions between members of different species/populations.

There are three types of symbiosis:

Mutualism (+/+)
Commensalism (+/0)
Parasitism (+/-)

2. Predator-Prey Relationships

Predation is a biological interaction whereby one organism (predator) hunts and feeds on another organism (prey)
Because the predator relies on the prey as a food source, their population levels are inextricably intertwined
- If the prey population drops (e.g. due to over-feeding), predator numbers will dwindle as intra-specific competition increases
- If the prey population rises, predator numbers will increase as a result of the over-abundance of a food source

Predation & Herbivory

When a predator and a prey population interact, it is positive for the predator but negative for the prey. This is sometimes described as an exploitative (+/-) interaction. Herbivory is essentially the same interaction between an herbivore and a plant or alga.

Interspecific competition within a shared niche will typically prompt one of two responses:

Competitive exclusion – One species uses the resources more efficiently, driving the other species to local extinction
Resource partitioning – Both species alter their use of the habitat to divide resources between them (i.e. niche separation)

Competitive exclusion

One species uses the resources more efficiently, driving the other species to local extinction

The less well-adapted species will struggle to survive and reproduce
It will either be eliminated (die off) or migrate

In this example, the Chthamalus barnacles (light-colored) could live across the entire rock. However, when Balanus barnacles (blue) are present, they are limited to a much narrower realized niche at the top.

Resource Partitioning

Sometimes, the niche of one species partially overlaps with that of another species. This is simple to imagine in the context of food - two predators might consume the same prey species. If there is overlap between niches, there can be a number of results. One population might get entirely outcompeted and die out, leaving all those resources for the surviving members of the "winning" population. Another possibility is that they will share the resources and coexist in some form, constantly competing with one another in a state of equilibrium.

Community Structure

A community's structure is crucial to the role it plays and the services it provides to humans and the ecosystem as a whole. A crucial aspect of a community is its species diversity and the variety of different kinds of organisms that make it up. When defining a community's species diversity, you must explore its richness and relative abundance (evenness).

A. Species Richness

A community's species richness is the number of different species found therein. A community with only one, or very few, species (i.e. a cornfield) has a much lower species richness than a diverse community with many different species (i.e. coral reef).

1. Cornfield has low species richness 2. Coral Reef has high species richness

B. Relative Abundance (Species Evenness)

The second component of a community's species diversity is its relative abundance, or the proportion each species represents of all individuals in the community. A simpler way to describe this is simply how equally represented each species is within the community. This is sometimes referred to as species evenness as well.

In other words, on one extreme, a particular species might be overrepresented or underrepresented. On the other extreme, all species could be evenly distributed and represent an equal portion of the whole.

In order to visualize this concept, your textbook provides the following example communities, each consisting of four species: A, B, C, and D.

Community 1 has equal relative abundance for all species - each of the four species makes up 1/4 of the total number of individuals.

Community 2, however, is not evenly distributed. Species A represents 80% of the total individuals. This means that the other three species only represent 20% collectively.

In general, it is better for a community to be more diverse and evenly represented because relying on a single species can be dangerous for the community and the services it may provide. This will be discussed further in ecosystem disturbances.

There are exceptions in which it is basically unavoidable to have an overrepresented species, of course.

Species with Huge Impacts

Not all species are created equal within a community. Some species have larger impacts and are more pivotal to the health of the others.

Simpson's Diversity Index - Quantifying Diversity

Discussing a community's diversity in terms of its richness and relative abundance sounds simple in theory, but can be much more complex when looking at large communities. Furthermore, scientists like to quantify values when possible - why use two hundred words when a single number is "good enough"?

In reality, of course, a number cannot represent all of the nuances and intricacies of these communities, but that doesn't mean we don't try! One of the ways by which scientists attempt to simplify the diversity of a community is by using Simpson's diversity index.

Simpson's Diversity Index

To calculate a community's diversity using this metric, simply use the shown equation. for each species, you will find its relative abundance (n/N). Square this value and add it to the same for each of the other species present. Subtract this sum from 1 to find the Simpson's diversity index value for this community!

The closer to 1 the value is, the more diverse, according to this metric.

This equation mathematically takes into account both the richness and relative abundance, but understanding how is beyond the scope of this course. It's complicated and tedious, but this essentially makes it so that one can compare two communities very easily. Which one has a larger Simpson's diversity index? That one is more diverse.

It is important to note that there are other diversity metrics, but this is the one that you are responsible for in AP Biology. There are even sometimes other formats of this equation for Simpson's, so if you do extra practice outside of class, make sure you follow this equation!

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