Community Ecology, pt. 1
Principles of Ecology Week 5
Photo of Roseate Spoonbill by Joshua J. Cotten on Unsplash
Structured discussion about semester project
Groups of 3 people each, discussion for ~20 minutes.
Nominate a facilitator, a recorder, and a reporter
Each person to discuss:
What idea(s) did you have going into this project?
From your literature search, do you feel that your idea was too narrow/broad/just-right/other?
Among the primary literature (peer-reviewed articles in formal scientific journals), is there a good mix of “basic” ecology and “applied” environmental/social science?
What are some ways to connect material from class to your semester project?
What was one paper/idea/resource that surprised you from your search?
What are your next steps for (a) selecting a focal community from your options or (b) getting more information on the community you have selected?
Video of a Veery by Dan O’Brien on Macaulay Library
Whole-class discussion
The reporter should be prepared to share:
For each group member, what ecological community is (currently) the top candidate as the focus of the semester project?
For 1-2 group members, what are some challenges that you discussed that might make progress on this community difficult?
Principles of Ecology
Commuity ecology introduction/overview
“The organism as the subject and object of evolution”
- “Organisms are both the consumers and the producers of the resources necessary to their own continued existence.”
White pine trees in New England make such a dense shade that their own seedlings cannot grow up under them, so hardwoods come in and take their place.
On the other hand, organisms may make an environment more hospitable to themselves.
Grazing animals actually increase the rate of production of forage, both by fertilizing the ground with their droppings, and by stimulating plant growth by cropping.
Beavers create ponds by felling trees and building dams; indeed, a significant part of the landscape in northeastern United States has been created by beavers.
The takeaway is that the activity of any organism changes the environment in which it lives.
- The movement of an earthworm through soil aerates the soil
- The transpiration of a tree cools the atmosphere
- An organism may add or subtract resources that are available to other organisms
- In some cases, an individual will actually enter into the lives of other organisms
This can in turn shape the dynamics of other species.
Community ecology
- The study of how interactions between species shape ecological communities
- Vast subject, given the wide variety of interactions that can unfold in nature
Photo of the Everglades Basin from Getty Images
Brainstorm a list of ways in which species might interact with one another in this setting
Direct and Indirect interactions
For an hourlong documentary about hummingbirds
https://www.youtube.com/watch?v=yc8oRjt7jpkDirect and Indirect interactions
Direct and Indirect interactions
Direct and Indirect interactions
Conceptual model of a pollination network from MacGregor et al. (2023), Ecological Entomology
Direct and Indirect interactions
Conceptual model of a plankton network in temperate from Merz et al. (2023), Nature Climate Change
Approaches to community ecology
- Reductionism
- Study a complex system by breaking it down into component interactions
Photo from Smithsonian’s Tropical Research Institute
e.g. In a forest that houses hundreds of tree species, how do two interact with each other?
Approaches to community ecology
- Reductionism
- Study a complex system by breaking it down into component interactions
- e.g. In a forest that houses hundreds of tree species, how do two interact with each other?
- Where else do biologists take a reductionist approach?
Approaches to community ecology
- Holism
- Historically, some ecologists have favored a view of communities as “superorgansims”
- “Emergent properties” that are only evident when we study complex, messy networks
- Where else do biologists take a holistic approach?
Approaches to community ecology
In this class, we will explore both approaches…
- Reductionist approaches, e.g.
- Under what conditions can a pair of species that competes with one another for the same resources coexist?
- How do predator–prey interactions shape population dynamics?
- Under what conditions can a pair of species that competes with one another for the same resources coexist?
- Holistic approaches, e.g.
- How do disturbances (re)shape the structure of ecological communities?
- Under what conditions do interactions between species change from competitive to mutualistic?
Approaches to community ecology
Ecological communities as composites of pairwise interactions
- What are the ways in which two species can interact?
Nature of species interactions
| Sp 1 effect on Sp 2 |
Sp 2 effect on Sp 1 |
Shorthand |
|---|---|---|
| Benefit (+) | Benefit (+) | |
| Harm (–) | Harm (–) | |
| Benefit (+) | Harm (–) |
Nature of species interactions
| Sp 1 effect on Sp 2 |
Sp 2 effect on Sp 1 |
Shorthand |
|---|---|---|
| Benefit (+) | Benefit (+) | Mutualism |
| Harm (–) | Harm (–) | Competition |
| Benefit (+) | Harm (–) | Predation |
Nature of species interactions
| Sp 1 effect on Sp 2 |
Sp 2 effect on Sp 1 |
Shorthand |
|---|---|---|
| Benefit (+) | Benefit (+) | Mutualism |
| Harm (–) | Harm (–) | Competition |
| Benefit (+) | Harm (–) | Predation Herbivory Parasitism |
Nature of species interactions
| Sp 1 effect on Sp 2 |
Sp 2 effect on Sp 1 |
Shorthand |
|---|---|---|
| Benefit (+) | Benefit (+) | Mutualism |
| Harm (–) | Harm (–) | Competition |
| Benefit (+) | Harm (–) | Predation Herbivory Parasitism |
| Neutral (0) | Benefit (+) | Commensalism |
| Neutral (0) | Harm (–) | Ammensalism |
To further complicate matters, the same pair of species can have different interactions under different conditions
Nature of species interactions
Biological examples of mutualisms
Other examples?
Nature of species interactions
Biological examples of competition
Nature of species interactions
Competition can also be less overt – “apparent” competition
Predation, herbivory, and parasitism
Unifying theme: one species benefits at the cost of the other . . .
Commensalism
One species benefits; the other is unaffected
e.g. Remora hitching a ride with a sea turtle
Community ecology
How competitive interactions shape biodiversity
| Sp 1 effect on Sp 2 |
Sp 2 effect on Sp 1 |
Shorthand |
|---|---|---|
| Benefit (+) | Benefit (+) | Mutualism |
| Harm (–) | Harm (–) | Competition |
| Benefit (+) | Harm (–) | Predation Herbivory Parasitism |
| Neutral (0) | Benefit (+) | Commensalism |
| Neutral (0) | Harm (–) | Ammensalism |
Competition
Why are we starting with this?
- Common process within and between species
- Basis for natural selection
- Conceptually simple - can focus within a guild
- Guilds can be roughly defined as groups of organisms that rely on overlapping resources and environmental conditions, e.g. herbacious plants; raptors (birds of prey), etc.
- Long history of study in ecology
Competition
Key question: What effects can competition have on biodiversity patterns?
Relatively low species richness
Relatively high species richness, especially of native flowering plants
Given that species compete for shared resources, why do we observe more coexistence here…
… but less here?
Key question: What effects can competition have on biodiversity patterns?
Competition
Did we study any form of competition in the Population Ecology unit?
Competition is simply the reduction in fitness with higher densities of individuals
Intra-specific competition
We learned about logistic growth as
\[\frac{dN}{dT} = rN\bigg(1-\frac{N}{K}\bigg)\]
But we can readily express it as \[\frac{dN}{dt} = rN(1-\alpha N)\]
where \(\alpha\) is \(\frac{1}{K}\)
When two species compete, the growth rate is also affected by the presence of a second (competing) species. How to account for this?
Incorporating Inter-specific competition
Given that intraspecific competition is modeled as
\[\frac{dN}{dt} = rN(1-\alpha N)\]
How to can extend this model to incorporate interspecific effects?
First, we need to account for the fact that there are two species: \(N_1\) and \(N_2\).
That means we have a system of equations: \(\frac{dN_1}{dt}\) and \(\frac{dN_2}{dt}\)
If species 1 is growing alone, its equation is:
\[\frac{dN_1}{dt} = r_1N_1(1-\alpha_{11} N_1)\]
Incorporating Inter-specific competition
If species 1 is growing alone, its equation is:
\[\frac{dN_1}{dt} = r_1N_1(1-\alpha_{11} N_1)\]
If species 2 is growing alone, its equation is:
\[\frac{dN_2}{dt} = r_2N_2(1-\alpha_{22} N_2)\]
Incorporating Inter-specific competition
\[\frac{dN_1}{dt} = r_1N_1(1-\alpha_{11} N_1) ~~~\text{ and} ~~~~ \frac{dN_2}{dt} = r_2N_2(1-\alpha_{22} N_2)\]
We can modify these to add the effects of the other species:
\[\frac{dN_1}{dt} = r_1N_1(1-\alpha_{11}N_1 - \alpha_{12}N_2)\]
\(\alpha_{11}\) is the competitive effect of species 1 on itself, \(\alpha_{12}\) is the competitive effect of species 2 on species 1, and \(N_i\) is the density of species \(i\)
Incorporating Inter-specific competition
\[\frac{dN_1}{dt} = r_1N_1(1-\alpha_{11}N_1 - \alpha_{12}N_2)\]
\[\frac{dN_2}{dt} = r_2N_2(1-\alpha_{21}N_1 - \alpha_{22}N_2)\]
How fast a population grows depends on
- intrinsic growth rate (\(r_i\)),
- the size of the population (\(N_i\)),
- the strength of intra-specific competition (\(\alpha_{ii}\)),
- the abundance of competitors (\(N_j\)),
- and the strength of interspecific competition (\(\alpha_{ij}\))
Getting familiar with the models
Consider what happens when a species is growing alone:
\[\frac{dN_1}{dt} = r_1N_1(1-\alpha_{11}N_1 - \alpha_{12}N_2)\]
Collapses down to logistic growth
\[\frac{dN_1}{dt} = r_1N_1(1-\alpha_{11}N_1)\]
Getting familiar with the models
\[\frac{dN_1}{dt} = r_1N_1(1-\alpha_{11}N_1 - \alpha_{12}N_2)\]
\[\frac{dN_2}{dt} = r_2N_2(1-\alpha_{21}N_1 - \alpha_{22}N_2)\]
What determines the strength of competition?
The size of \(\alpha\) reflects “how competitive” interactions are:
Strong competition (high \(\alpha\)) happens when each additional individual of a species strongly reduces fitness of other individuals
In other words, \(\alpha_{12}\) are determined the degree to which an individual of species \(2\) changes the environment in a way that suppresses individuals of species \(1\)
What makes intra-specific competition different from inter-specific competition?
Why are \(\alpha_{ii}\) and \(\alpha_{ij}\) different from one another?
When the species have highly overlapping niches, intra-specific and inter-specific \(\alpha\)s are very similar: \(\alpha_{ii} \approx \alpha_{ij}\)
When two species have highly distinct niches, intra-specific competition is very low: \(\alpha_{ij} \approx 0\)
What are the equlibrium conditions of the model?
\[\frac{dN_1}{dt} = r_1N_1(1-\alpha_{11}N_1 - \alpha_{12}N_2)\]
\[\frac{dN_2}{dt} = r_2N_2(1-\alpha_{21}N_1 - \alpha_{22}N_2)\]
- Have to solve this as a system of equations
