Competition has the potential to structure communities, either by excluding species, affecting the size of populations of competitors, or by changing the distribution of species within space. Studying the impact of competition is challenging as the absence of competition currently could be the result of competition in the past driving niche partitioning, or could be evidence that competition never existed.  Two questions that researchers have focussed on have been:

1. How prevalent is current competition in natural communities?
2. Are the patterns in existing communities due to competition or due to mere chance?

The discussion of the Lotka-Volterra model provides an oversimplified view of the world of competition — and suggests that more often than not, competition should lead to exclusion.  However, we routinely see communities comprised of similar species — today we’ll talk about several mechanisms that can lead to coexistence despite Lotka-Volterra dynamics.

1. • Habitat and temporal heterogeneity can prevent superior competitor from ever excluding the inferior
   • frequent disturbance can permit a weaker competitor to persist via metapopualtion-like dynamics
   • r-selected species can take advantage of ephemeral gaps before exclusion by K-selected species
   • Aggregated distributions can intensify intra- compared to inter-specific competition
   • Competition can drive selection for changes in characteristics that minimize competition (e.g. character displacement)

The second homework assignment is due on 18 October by 5 PM.  The files are here, a word document with the questions, and an excel worksheet use to complete the questions:

Assignment 2

Homework 2 worksheet

Paper for section 2

Paper for section 3

In this lecture we’ll consider study designs that are used in ecology and the challenges that arise from studying patterns and processes that necessarily occur at scales that are bigger than any laboratory.

1. • Scientific method — holds for ecology as well as for other disciplines — though scale makes it tough!
   • Natural variation — Observable phenomena differ from place to place for many, many reasons.  We can only characterize differences relative to variation — i.e. is it out of the ordinary?
   • Statistical significance — refers to differences that are unusual relative to the expected degree of variation (where expected variation depends on how we study the system)
   • Studies in ecology often trade off controllability for realism.  The scale of ecology means that advancing understanding often requires a combination of types of studies to see if patterns remain consistent.

Since I won’t be around to deliver this in person, I’ve recorded the lecture here:

https://prezi.com/tkpmgpi34tcr/ecological-methodology/

Click through the prezi to hear my recorded audio for each slide.

Last time, we hinted at how potential competitors can coexist by avoiding competition — now we’ll formalize that idea in the concept of the “realized niche”. But that begs the question — does competition still happen if all species are just avoiding it?

We can categorize the outcomes of interspecific competition with the simple mathematical model first put forth (independently) by Alfred Lotka and Vito Volterra which predicts 4 possible outcomes of two species interactions.  Using the original logistic model as a foundation, we can derive the Lotka-Volterra model and use it to consider the impacts of one species on another.

1. • The Lotka-Volterra model is an extension of the simple logistic growth model
   • Isoclines define combinations of species that are functionally equivalent (“iso” means equal) with respect to resource use
   • For the L-V model, these isoclines give resource equivalent combinations of species 1 and 2, of the form e.g. “species 1 can tolerate 20 conspecifics OR 15 conspecifics + 8 of species 2”
   • 4 possible outcomes of competitive interactions: Species 1 “wins”, Species 2 “wins”, Coexistence, “it depends”
   • Coexistence occurs when species are more limited by intraspecific competition that by interspecific competition
   • When interspecific competition is more limiting than intraspecific competition, then the outcome depends on the starting composition of the community (i.e. how many of Species 1 and Species 2)

For your viewing pleasure, here’s a stirring description of the Lotka-Volterra model by yours truly.

In this second section of the course we will start building up ecological complexity to consider the consequences of interactions among species — competitors, consumers, parasites, mutualists.

1. • There are many types of interspecific interactions, which can be characterized in terms of the costs and benefits to each of the players.
   • Competition is an interaction in which both species suffer net negative consequences
   • Competition drives resource levels down for all competitors, relative to single species communities. Competitive exclusion may occur If one competitor can drive resources below the niche limits of the other.
   • Avoidance of competition can drive species distributions
   • We must interpret distribution in the light of the avoidance of competition
   • The realized niche is the combination of conditions and resources that allow a species to survive, grow, and reproduce in the presence of other species that may be harmful