Population Ecology

Despite this initially going up on a Valentine's Day, I have to regretfully inform you that population ecology doesn't really hinge itself on love. It's mostly about survival here, sorry!

But we can still bask in the notion that most penguins do mate for life. And penguins are already just so dang cute. So there's your lovey-dovey warm fuzzies for today :)

Now that that's out of the way...populations. We probably don't think about this concept too much as humans. It's not that the principles of population don't apply to humans as a species, just that we've taken such a command over the planet that we've successfully subverted a lot of what other species just have to deal with. We'll get to how humans have done this in a later post, but for now, what we're going over is largely with regards to non-human animals and, to an extent, plants.

Edit: For a deeper look into human population, check our our blog post Human Population Defiance.

Let's figure this out!

Population Curves

In population ecology, we have 2 distinct types of population growth that are used to define how a given species deals with survival in its environment. Now, while these patterns are indeed different, it's good to realize that most species treat these patterns as extremes, generally falling somewhere in the middle. In short, the patterns are 2 sides of the same spectrum.

On one end of the spectrum, we have the exponential growth model, also called J-Curve due to the shape of its graphical representation. J-Curve populations lean heavily into high reproduction rates coupled with generally short life expectancy. A lot of insects fall into the most extreme representations, but small mammals (such as mice) fit neatly into this population model. The general thought here is that the species can put most of its energy into reproduction to ensure species stability over generations, though this comes at a cost dealing with environment variability (not to mention the high mortality rate alluded to above).

Species in this model undergo genetic changes much more rapidly, so their adaptability comes more in the form of subsequent generations being readily adapted to an environmental shift than actively adapting during one generation. The scope of the species environmental domain is also generally quite small, making the species particularly susceptible to density-dependent challenges, one of the factors we'll cover in a bit.

There are 2 primary population models, J- and S-Curve, commonly known for their graphical representations, which happen to look like their respective English letters

The other side of the spectrum is the logistic growth model, also known as S-Curve, again due to its shape when graphically represented. S-Curve populations rely a lot more on raising their young than J-Curve populations. The general idea here being that the species has a higher rate of survival when the new generation is fostered to live both longer and "taught" to deal with its environment. Large mammals, like elephants, are at the extreme of this spectrum, but some shorter-lived examples such as birds of prey fit well into this model.

Species in this model generally preside over a larger environmental domain, and the time parents take to rear their offspring allows for them to learn how to adapt to that environment. The parental generation exerts an enormous amount of energy (not to mention time) giving their next generation the best chances of living to a point of reproducing in the same manner. Given the time it takes to fit this model, large, quick shifts in the environment can force the species out of their expected habitat.

Density-dependent factors limit population growth based on the size of the population, while density-independent factors are generally external forces

Factors

We mentioned briefly the concept of factors earlier, but it's probably a good idea to cover them in a bit more detail just to understand what can influence and/or stifle a population's full potential. These factors are called density-dependent and density-independent.

Density-dependent factors limit population growth based on the density of the species itself. This might sound a bit strange at first. Think of how difficult it is to move in, let's say, a crowded concert venue. Your environment, the venue, is a completely viable place for a certain amount of people, but the greater the density of the population within the room, the harder it is to move around and enjoy the concert. Instead of this concept being about space to move, consider the space as things to eat. Populations gradually work to these types of thresholds, which put a ceiling in place for that species in the environment that they reside. Density-dependent factors can range from livable space and viable food sources to an increase in the chance of being preyed upon (the more of the species, the more of the predator to that species will come to the environment).

Density-independent is a little more straightforward. These are the factors that can happen regardless of population density. Maybe a storm comes through a forest and lightning strikes a tree. That tree was home to an ecosystem of species that no longer have their environment. This challenge was introduced not by the density of the species in the ecosystem, but brought upon by an external force. Though, while the event did not come about due to density, it's likely that events like this have an effect on the environment, usually shifting it so dramatically that current population levels may become inviable, revealing a new ceiling for density-dependent issues to rise. Separating the event from the result is important to understanding what causes population shifts.

Theories & Discourse

Ever since identifying patterns in nature, biologists and ecologists have tried putting proofs to those patterns. It's no surprise that humans do this, we really like categorizing things, but even some of the most cited theories have been heavily debated as recently as the late 1990's, with discourse still active on the topics today. Let's take a look at some of those theories to get a better idea for the justification of these curves and factors.

r/K Selection

As modern ecology started taking form, the r/K selection theory took hold. Introduced in 1967 and popularized in the 1970's, it introduced the base concept that there were 2 strategies held by species, one producing a high quantity of offspring at the expense of parental investment (the r-strategists) and the other producing limited offspring with an increased amount of parental investment (K-strategists). These concepts both align with the general shape of the J- and S-curves we described earlier, but also lack a nuance necessary to understand how and why species select themselves into these strategies.

r/K Selection theory may only paint the one-to-one understanding of population curve and reproduction, but is based on the concept of trade-offs that permeates population theory to this day

Life History Theory

Understanding the underlying concept of trade-offs introduced in the r/K selection model is still integral in all theories regarding population ecology. Taking that concept a step further is the paradigm of life history theory. Through this lens, we can take into account several traits that factor into survivability affordances gained by species from those selected strategies. These traits include:

Size at birth
Growth pattern
Age and size at maturity
Number, size, and sex ratio of offspring
Age- and size-specific reproductive investments
Age- and size-specific mortality schedules
Length of life

An example of a tool used to take a closer look at some of these traits is the Leslie Matrix, a model used to zone in on how species population patterns change as the species ages.

Plants and animals both experience population trade-offs based on their environment

C-S-R Triangle

Another thing to take into consideration is differences in population strategies based on kingdom. Or rather, can population strategies apply differently to plants and animals? While the r/K selection theory has widely been attributed to both plants and animals, there have been other theories put forth specific to plants.

One of the prevailing theories is the C-S-R Triangle, also called the Universal Adaptive Strategy Theory. The theory revolves around 2 factor gradients, disturbance and stress, with the 3 letters in the triangle representing strategies: Competitors, Stress-tolerators, and Ruderals.

Competitor strategies are seen widely in environments with low stress and low disturbance levels. These areas are perfect for many plants, like wetland cattails, thus they are in the most competition for available resources. Stress-tolerators do as their name implies, capable of surviving high stress and low disturbance environments. These areas are generally harsh in their lack of available resources, and plants will devote resources to highly defensive needs, as with pine trees with their needle-like leaves and cone-protected seeds. Ruderals deal with the opposite of stress-tolerators, dealing with low stress and high disturbance. In the anthropocene, there's no shortage of areas disturbed by humans, left with abundant resources for plants willing to deal with that disturbance to thrive, like joe pye weed around rail- and road-sides. The 4th "quadrant" (high stress and high disturbance) is deemed impossible for any plant to take hold.

And that all makes up a solid basis of population ecology. All life on Earth follows these principles of trade-offs, trying to find the strategy that makes the most sense for each species. As with most things in nature, these strategies exist in a beautiful chaos, where there is no one-or-the-other approach. There's a lot of nuance involved as far as what options are available to which species, and some may exhibit characteristics of multiple strategies if it results in higher survivability. So when you're out on a stroll, taking in the natural sites around you, maybe now you'll be able to pick up on the differences in how certain species act, react, and interact with themselves and one another through the lens of population theory. There's so much to understand and unwrap as you learn how everything does its best to thrive and survive.

~ And, as always, don't forget to keep wondering ~

Sources
* r/K Selection
Pianka, E.R. (1970). "On r and K selection". American Naturalist. 104 (940): 592–597. doi:10.1086/282697. (https://www.journals.uchicago.edu/doi/10.1086/282697)

* Life History Theory (as cited on wikipedia)
Flatt, T., & Heyland, A. (Eds.). (2011). Mechanisms of Life History Evolution : The Genetics and Physiology of Life History Traits and Trade-Offs. Oxford, GB: OUP Oxford.

* Leslie Matrix
https://en.wikipedia.org/wiki/Leslie_matrix

* Plant strategies
Grime, J.P. (1977). "Evidence for the existence of three primary strategies in plants and its relevance to ecological and evolutionary theory". American Naturalist. 111 (982): 1169–1194. doi:10.1086/283244. (https://www.journals.uchicago.edu/doi/10.1086/283244)