Vocabulary:

• Competition:
  • Conditions:
    • More individuals produced than can possibly survive
    • Individuals compete for resource in short supply
• Intraspecific: between individuals of the same species
• Interspecific: between individuals of different species
• Scramble competition: all individuals in a given population experience the same density-dependent decrease in growth and reproduction as the intensity of competition increases
• Consumption competition: competition between organisms for a shared food resource
• Contest competition: Some individuals claim resources at the expense of other individuals (zero-sum game)
• Preemptive competition: competition for limited space between sessile organisms
• Territorial competition: behavioral exclusion of others from a specific area that is defended as a territory
• Encounter competition: non-territorial meetings between individuals that negatively affect either one or both species
• Competitive exclusion principle: two species competing for same resources as complete competitors (with the same niche requirements and traits) cannot coexist
• Resource partitioning: division of resources by coexisting species such that each species occupies a unique niche within the same area
  

Outline of Notes:

Types of Competition in Nature:

Competition presents itself in a variety of ways in nature, and can occur between individuals of the same species (intraspecific) or between individuals of different species (interspecific).

Types of Competition:

1. Consumption competition: competition between organisms for a shared food resource.

• • Can be either intraspecific or interspecific

2. Preemptive competition: competition for limited space between sessile organisms (i.e. plants, barnacles); the occupancy of one organism prevents the establishment of another.

• • Can be either intraspecific or interspecific
  • Example: Overgrowth– one organism grows directly over another, with or without physical contact (i.e. kudzu, overstory tree species shading out saplings).

3. Chemical Interaction: Chemical growth inhibitors/toxins inhibit or kill individuals of other species (ie. penicillin, allelopathy in plants).

• • Interspecific only (by definition)

4. Territorial competition: behavioral exclusion of others from a specific space that is defended as a territory.

• • Typically intraspecific (courtship)

5. Encounter competition: non-territorial meetings between individuals that negatively affects either one or both species.

• • Interspecific
     
• Territorial vs Encounter competition:
  • Both involve shared land or space
  • Encounter competition tends to be interspecific while territorial tends to be intraspecific
  • Difference in timespan: encounter competition tends to be short/discreet while territorial competition tends to be more long-term

Measuring Competition Experimentally:

• G. Gause, 1934: Experimental competition between two species of Paramecium
  • P. aurelia had higher rates of growth and higher tolerance of density than P. caudatum
  • Gause artificially limited resources in culture: both species thrive in separate containers, but P. aurelia persists better than P. caudatum when in a shared container
  • P. caudatum and P. bursaria can coexist because they utilize different resources
    
    

• D. Tillman, 1977: Experimental competition between two species of diatoms
  • Both species (Asterionella formosa and Synedra ulna) require silica to grow
  • S. ulna can reduce silicate to lower level than A. formosa can tolerate, so thrives in shared habitat

→ Both of these experiments provide evidence for the competitive exclusion principle:

• • Competitive exclusion principle: two species competing for same resources cannot coexist
    • The population with a larger population or higher reproduction rate typically wins

→ Interaction in competition for resources can magnify negative impacts:

• • Ex: Two species of plants competing for multiple resources at once
• Need to first understand healthy populations, and can then measure effects of competition for
  one resource at a time:
  • Common garden experiment
  • Either sharing root space or shoot space
• Then can experiment with both resources at once:
  • Ex: Both root and shoot competition

Changes in Competitive Ability:

• Measuring Relative competitive ability along environmental gradients:
  • Step 1: measure growth of each species alone
  • Step 2: measure growth in mixed plots
  • Success in individual growth does not always match under conditions of competition; the ultimate winner changes based on environmental conditions
    • This provides evidence for very specific niche spaces
• Physical stress and interaction restrict habitat usage:
  • Example: Chipmunk aggression is correlated with heat tolerance
    • The most aggressive chipmunk (Lodgepole chipmunk) has lowest physiological tolerance for heat and is the most aggressive
    • The chipmunk with higher heat tolerance (Least chipmunk) is the least aggressive
• Food competition:
  • Typically, a species with a wider range of food preferences can occupy a wider niche

Niche Differentiation and Resource Partitioning:

Niche differentiation via resource partitioning allows for coexistence. Examples:

• Annual plants partition resources via differing depth of roots
  • Deeper roots lead to more consistent water supply
• Wild cat species partition resources by the size of prey they consume (mammalian example of Darwin’s finches)
  • Smaller teeth = smaller prey
• Range expansion of coyotes may be a result of competitive release from wolves
  • Gray wolf habitat declined while coyote range expand into previously wolf-occupied territories

Blog-style Summary:

So you’ve heard about competition, right? We do it all the time to win a prize or defend a title, but how does competition look in the animal kingdom? Competition arises when populations meet certain conditions, as described by Darwin:

1. 1. More individuals are produced in a population than can survive
   2. These individuals compete for access to limited resources in the environment

 When members of the same species compete between themselves, it is called intraspecific competition. An example of this would be when mockingbirds chase each other to claim territories. When two different species compete, it is called interspecific competition. This would be, for example, when bluejays and sparrows compete for space at a bird feeder. Figures 5 and 6 show additional examples of these competition types.

Now that you know the two broad classifications of competition, let’s take a look at the more specific types of competition. There are many possible consequences of competition. The competitive exclusion principle states that two species competing for the same limited resources cannot coexist, therefore subsequent changes in population dynamics will occur. In scramble competition, the intensity of competition is inversely correlated with a decrease in growth and reproduction. In other words, as the fighting over resources becomes more intense, individuals can’t afford to grow as much or reproduce as much. For example, frogs with explosive-breeding patterns exhibit scramble competition as all of the males have access to and compete to fertilize a female; typically, the largest frog wins and is able to mate while the smaller frogs do not. Even cows in a pasture exhibit scramble competition because they are all essentially competing for the grass. If you increased the number of cows, there would be less grass available per cow, resulting in decreased fitness. For plants, they all compete for water and sunlight (equally available for all to use), but some will be better at capturing and utilizing these resources to grow quicker and shade out or outcompete those plants that are not as good at utilizing the resources.

In contest competition, the claiming of limited resources by some individuals results in other individuals having decreased fitness. The gains of some individuals are balanced with the losses of other individuals. Think about territorial species. Mockingbirds, for example, have several types of territories including feeding and breeding/nesting territories. They defend the desirable space, so once males or breeding pairs have claimed the best quality spaces, all the other birds must choose less desirable places to nest, which can have an impact on their reproductive success. Mockingbirds also defend territories for food resources, so they claim areas with the best or most abundant food which causes other birds to have less available food, decreasing their fitness. So if species competing for the same resources can’t coexist, how are species not going extinct left and right? There is a concept called resource partitioning, which means species divide up resources into separate niches. This can come in the form of living in different areas of habitat, being active at different times of day, only eating certain parts of plants, etc. Having different niches in an ecosystem is incredibly important, as without niches an ecosystem could not survive. If every species was competing for the same resources, the ecosystem would be in chaos at all times. Also, think about if we all did the exact same job and needed the exact same resources; we wouldn’t get very far, would we? Niches provide special “jobs” for each species and help combat this possible chaos. Not to say that there is no competition, but niches can help reduce it as much as possible.

        The resources that individuals compete over vary greatly between environments and thus the type of competitive interactions that organisms have can fall under many different categories. When the competition is over a shared food resource, it is called consumption competition. Think of dropping a piece of meat off the side of a boat in shark-infested waters. When the sharks eat the meat, they compete to see who can eat it first. When the competition is between sessile organisms over the occupation of limited space it is called preemptive competition. When lichen attach to the side of a tree (or another surface), no other lichens can live where that original lichen attached, and thus lose out on the resources available. Overgrowth competition is when one organism grows over another. Kudzu takes over entire forest sections and steals the sunlight needed to live, killing the plants it grew over. The above mentioned types of competition can be either interspecific or intraspecific. Chemical competition is interspecific since it involves the chemicals or toxins of one species preventing the growth or killing another species. Take the plants in the genus Salvia for example; they release volatile compounds that suppress the growth of other plants around them, which is why the soil is often bare around them. The last two types both involve competition over shared land but territorial competition (think fights between different wolf packs of the same species) is often intraspecific and involves behavioral exclusion from a defended territory while encounter competition (think about a hyena and a vulture fighting over some roadkill) is a short, interspecific, non-territorial meeting that negatively affects one or both parties involved.

        
So how do we measure competitive ability? One easy way is to measure the growth of species alone and then measure their growth when in a mixed plot. Competitive abilities change along the environmental gradient. There can be a different “winner” under the conditions of each specific niche. A common garden experiment involves two different plant species being grown in the same environment and the size of the two plant species are compared.

Competition may also restrict habitat usage. For example, different chipmunk species have different heat tolerances and level of aggressiveness, so the least heat tolerant and non-aggressive species will inhabit the top of a hill, more aggressive and mid-heat tolerant but aggressive species will inhabit the middle of the hill, and the most heat-tolerant and non-aggressive species will inhabit the bottom. So, the most heat-tolerant species could live at any elevation as it can survive any heat, however, it is restricted to the bottom of the hill because the mid-heat tolerant species is very aggressive and competitive. This heat-tolerant chipmunk inhabits a special niche, in which it tailors where it lives based on the species around them! If the aggressive species of chipmunk further up the hill moves farther up, or experiences a decrease in population, these heat-tolerant chipmunks can make use of that new range and move upwards.

You might think, are there any specific disciplines where knowledge of competition is important? Of course! Conservation biology is a discipline that would focus heavily on competition. What are some implications within that discipline? Well, the biggest implication is the fact that when resources are limited and competition is present, the “weaker” species could possibly go extinct, evolve to use another resource, or improve and strengthen its competitive abilities by evolving certain mechanisms. A grand issue that involves competition is the conservation of native species, due to the accidental or intentional import of invasive species. Competition between native and invasive species can disrupt the delicate balance of the ecosystem where the invasive species is introduced. For example, lake trout that is native to the Great Lakes have become invasive to Yellowstone Lake after its introduction by humans (NWF, 2021). This species competes with the native cutthroat trout for habitat space and food, which would be considered consumption competition. Problems could arise with invasive species competition if a species comes into an environment and out-competes a native species and their numbers decline due to lack of resources, but the predator of that native species does not want to or cannot feed on the new invasive species. This will cause the predators to become malnourished, their numbers would then decrease, and the ecosystem will be severely disrupted.

 Competition is relevant in the discipline of phylogeny and evolution in that we now know that competition contributes to the success of a species. Consumption competition between two different birds of the same species can cause changes in selective pressures that may favor a certain advantageous characteristic, such as a longer beak to dig or pick out food. These selections caused by competition may be of interest to evolutionary biologists. Using past models of extinctions we can learn about the fate of certain species and possibly protect them from competition that could lead to their demise.

Spotlight on NC:

Rock, paper scissors, shoot! As fun of a competitive game that is, distribution of resources in nature does not work the same way. Competition in nature only occurs if individuals compete for a short supply of resources AND if there are more individuals being produced than what could survive. Once those two conditions are met, let the “games” begin! When you compete in nature, you can either compete with the same species (intraspecific), or with a different species (interspecific).

There are generally 6 types of competition in nature: consumption, preemptive, chemical, territorial, overgrowth, and encounter competition. Consumption competition is when two living things compete between a shared food resource. This type of competition can either be intraspecific or interspecific, such as two white-tailed deer eating acorns from the same tree. Preemptive competition occurs when species compete for a limited space. This is typically between sessile organisms such as plants or barnacles. These organisms can not move to find other food sources, therefore their established occupancy prevents others from using that space. Overgrowth competition occurs when one organism grows over another to reach the needed resources. A very common example of this would be Kudzu plants (Pueraria sp.)! Kudzu is typically found in many disturbed forests, forested edges, fields, and roadsides [#1]. North Carolina has this species all throughout the state.

Chemical competition is when species compete by using chemical compounds; these compounds can inhibit growth or even kill another species. Territorial competition is when two of the same species compete for specific space that they defend. Red wolves are a very common example as the pack will defend their territory against other red wolves. Encounter competition is similar to territorial competition by parties using a shared resource (space), however, encounter competition involves two different species competing for that space. Encounter competition is also during a shorter time frame use of the land and does not defend it for long-term benefits.

Competition can also be categorized by resource accessibility. The accessibility of a resource may not only impact survival but reproduction as well. Gray squirrels are a perfect example of this and they are the most common species of squirrel in eastern North Carolina. They are so abundant that they are the most commonly observed mammal in the state as well [2]. With an abundance of individuals comes an abundance of competition. Gray squirrel populations are heavily regulated by the availability of their resources, especially the availability of tree nuts.

When there is not enough food to go around, a lot of gray squirrels will not make it through winter, and the ones that do often produce fewer young [2]. When resources are limited a species will experience one of two different types of competition.
On the left side of figure 10 you can see an example of scramble competition.
Scramble competition refers to a species who is affected equally by a lack of resources due to the resources being scrambled throughout their environment. In this case, all of the gray squirrels would experience an equal lack of sufficient tree nuts. On the right, you can see that the tree nuts are not scattered equally throughout the environment so these squirrels would experience contest competition. Contest competition would mean that some of the squirrels gain access to more tree nuts at the expense of the other squirrels. In this case, the squirrels with more access to tree nuts will live through the winter and produce many offspring, while the squirrels with less access will most likely not make it through the winter.

After reading through all of the examples and definitions of competition, you may be thinking to yourself that there is always a winner and a loser, or in the case of scramble competition there are many losers. In most cases that is true. The competitive exclusion principle states that two species who live in the same environment and have the same niche cannot coexist. However, that is not always a realistic statement because of context dependence. Many environmental factors such as temperature, pH, salinity, and humidity can play a role in influencing survival, growth, and reproduction in species. Because of environmental stressors, there are rarely ever two species with the same exact niche.

In some cases, species will avoid competition altogether. In order to do this they will have to share resources or use the resources differently; this is called resource partitioning. There are many different species that participate in resource partitioning, including many species of warblers. In the 1950s, American ecologist Robert H. MacArthur discovered that five species of insectivorous warblers divided their feeding time into 5 different regions of conifer trees [3]. He found that the warblers developed different feeding habits and techniques to avoid competition with the other warblers.

Competition can take many different forms and have a variety of outcomes and consequences. Understanding species interactions is important because it will allow us to make more knowledgeable decisions when it comes to ecosystem management and conservation efforts.

Featured Ecologists:

Akana Noto, Phd

Dr. Akana Noto’s research focuses on the effects of biodiversity within species relationships (plant-plant and plant-consumer). They are most interested in researching what causes species interactions to vary across space and in different spatial scales. Their research uses field and greenhouse experiments, genetic tools, and mathematical modeling to investigate how biodiversity impacts competition and other ecological processes. Akana’s findings indicate that intraspecific diversity at different trophic levels can affect species interactions, so variation within species is important for understanding how species interact within communities. This is important for plant-consumer communities that are dominated by few plant species because diversity plays a huge role in how species interact. Another important finding from Akana’s research is shown in the BioRender. Sea level rise is a threat caused by climate change, so Akana investigated how plant-plant interactions in salt marshes would be affected by tidal inundations. They found that sea level rise increases competition in salt marsh plant communities because of higher water and lower salinity levels from tidal inundation. This means plant diversity in salt marshes is likely to decrease as sea levels rise because of increased competition. Akana’s research findings are important for understanding species diversity and interactions and the impact of environmental changes on salt marsh ecosystems.

Robin Wall Kimmerer, Phd

Dr. Robin Wall Kimmerer is a plant ecologist and distinguished professor who focuses on mosses and sweetgrass, and using methods of the indigenous people to heal the environment and humans’ relationships with the environment. Another important part of the majority of her research was maintaining natural elements of the environment that are important to indigenous people.

One topic that Dr. Kimmerer has conducted research on is the potential ability to restore populations of Sweetgrass which is culturally important to indigenous people in the Northeastern United States. Dr. Kimmerer and her team conducted this study with members of the Mohawk Nation of the Haudenosaunee who are familiar with the use of Sweetgrass in basketry. Four treatments (Sweetgrass alone; Sweetgrass with existing, old-field vegetation; Sweetgrass with a cover crop of Hairy vetch; and Sweetgrass with a cover crop of Annual ryegrass) were utilized in the study and the results showed that there is great potential for Sweetgrass restoration. The different species that were selected for the treatments were cover crops which helped with effective weed suppression. The Sweetgrass planted with no competition and with Hairy vetch was successful. This study connected to the ecological concept of competition from class. Many native plants that indigenous people have relied on for centuries are threatened by non-native species that can outcompete natives. Additionally, as the world has become more urbanized, species have encountered more disturbances which also favors invasive species. An important implication of this study is that these cover crops can be effective treatments to restore the populations of Sweetgrass, which is very important to many indigenous people. Another implication is that working with indigenous people, like Dr. Kimmerer did in this study, can be very helpful to finding solutions to ecological problems such as this one. I think it is important to highlight this aspect of her work as it can be helpful for conservation efforts in the future.

Student contributors:

Note Outline: Heather Moonier, Hannah Cooper, Devin Adas, Molly Crilly

Blog Style Summary: Jason Loher, Caitlyn Poplaski, Magigan Clossick, Maddie Townsend

Spotlight on NC: Skylar Cole, Mary Grace Ussary

Featured Ecologists: Kyla Marze, Christina Conrad