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Interspecific Interactions
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.
https://botany.one/2016/03/leaf-mechanical-traits-insect-herbivory/
Predation & Herbivory
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 but between an herbivore and a plant or alga.
Competition & Niches
Competition & Niches
Competition is an interesting interspecific interaction because it is negative for both species (-/-). Both individuals expel effort and energy to compete for resources. If no one benefits, why does it occur? In order to explore this, it is important to understand what a niche is.
An organism's ecological niche is simply its role in its environment. A niche is an amorphous term that encapsulates what an organism eats, where it lives, how often it mates, etc. An organism's fundamental niche describes all of its potential niche (all the food it might eat, all the places it may live, etc.). Its realized niche, however, is the portion of its fundamental niche that it actually fulfills. Why would an organism not live up to its full potential niche?
https://eco-intelligent.com/2016/09/28/habitat-and-niche/
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 an coexist in some form, constantly competing with one another in a state of equilibrium.
Urry, L., Cain, M., Wasserman, S., Orr, R., & Minorsky, P. (2020). Campbell Biology in Focus, AP Edition (3rd ed.). Pearson Education.
Urry, L., Cain, M., Wasserman, S., Orr, R., & Minorsky, P. (2020). Campbell Biology in Focus, AP Edition (3rd ed.). Pearson Education.
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.
This is an example of resource partitioning, in which the resource (space, in this case) is shared and the species coexist.
Symbiotic Relationships
Symbiotic Relationships
Not all interspecific interactions fall into competition, predation, or herbivory, however. There is a classification known as symbiotic relationships, categorized by the close and hyperspecific interactions between species. Some of these relationships benefit both individuals (mutualism), some benefit one party but has no effect on the other (commensalism), and some are positive for one and negative for the other (parasitism).
These are exemplified by the following organisms in your textbook:
Mutualism (+/+)
Mutualism (+/+)
Some acacia trees in Central & South America have hollow thorns to house stinging ants (Pseudomyrmex). The ants protect the trees in exchange for receiving food from the plant.
Commensalism (+/0)
Commensalism (+/0)
Cattle egrets (Bubulcus ibis) often follow herds of large organisms such as African buffalo (Syncerys caffer) and eat the insects they stir up as they move. The egrets benefit but the buffalo are unaffected.
Parasitism (+/-)
Parasitism (+/-)
The parasitoid wasp Cotesia congregata cocoons cover this tobacco hornworm (Manduca sexta). The wasp derives its nourishment from the hornworm plants, resulting in a parasitic relationship.
Community Structure
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, 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. 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).
https://www.dreamstime.com/photos-images/corn-field.html
https://www.worldatlas.com/articles/what-are-the-biggest-threats-to-coral-reefs-across-the-world.html
B. Relative Abundance (Species Evenness)
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.
Urry, L., Cain, M., Wasserman, S., Orr, R., & Minorsky, P. (2020). Campbell Biology in Focus, AP Edition (3rd ed.). Pearson Education.
Species with Huge Impacts
Species with Huge Impacts
Not all species are created equally within a community. Some species have larger impacts and are more pivotal to the health of the others therein.
Foundation Species
Foundation Species
Foundation species have strong influence on their communities because of their large size and abundance. Removing this species is extremely detrimental to the community, but there are many individuals.
Keystone Species
Keystone Species
Keystone species have strong influence on their communities despite their lack of abundance. Removing a keystone species can result in the loss of the entire community.
Ecosystem Engineers
Ecosystem Engineers
Ecosystem engineers create or dramatically alter their environment, and therefore, their communities. Many communities only thrive because of the structures built by these species.
Simpson's Diversity Index - Quantifying Diversity
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.
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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