Lecture week 13 (11-14) - Notes Competition and Niche Space.docx

Biol 114 Fall 2023

Lecture Week 13 (11/14)

Competition and Niche Space

Introduction
1. Ecologists have long believed that competition is pervasive in nature and is an important driving force in natural selection. Competition, both within and between species, allows for those with certain traits to be better competitors. Those that out-compete others have greater fitness, thus creating populations of better competitors.
2. Thus far in the semester we have considered what goes on within individuals, how genes/traits are passed from generation to generation, how genetic diversity changes within a population over time, how populations grow, and how new species are formed. Until now, we have not talked about the costs of interactions within species, nor have we covered how different species interact. These are the subjects of the next several lectures.
3. When considering the interactions between individuals, it is often useful to keep track of processes and results in terms of costs and benefits. Much of modern ecology is studied in the same way an economist or accountant may study the flow of money through use of a balance sheet, with costs and outputs in one column, and income in another. In fact, a recent, commonly used (it’s in its 7^th edition) ecology textbook by Robert Ricklefs is entitled The Economy of Nature. We would also do well by measuring interactions we see in this course in terms of costs and benefits. As a result, we will find it easier to learn the different types of interactions, understand their effects, and better understand why and how individuals enter into the interactions in the first place.
4. Commonly, community ecologists define various interactions by considering those who benefit from the interaction (+), versus those that sustain a cost (-) and those that do not see a benefit or a cost (0).
Competition
1. General Definition
  1. Competition is defined as individuals interacting where both sustain some cost (or potential cost). That is, it is a - / - interaction.
  2. Many get confused with this definition in that they believe that one member of the interaction benefits and the other has a cost (+/-), but this only refers to the ultimate payoff (the reason for the competition)
  3. The process of competition is costly to both. Think of what the costs and benefits would be if there was no second individual to compete with. The individual in question would achieve the benefit, and suffer no cost. Now, place the second party in the mix. The act of competing with the second individual for the resource comes with some cost for the first member.
  4. Competition is for resources of some sort (e.g., food, nesting sites, sunlight, mates, water, etc.).
  5. There are two general classifications of competition: Intraspecific (within the species) and Interspecific (between species) competition. Often, people believe interspecific competition is the fiercer of the two, assuming that when individuals of two separate species interact, there is a greater chance that the costs incurred will be greater. Those that believe this are the same that fall into the trap of believing in what is called naïve group selection thinking (Individuals do things for the good of the species, often sacrificing themselves so that the species may continue.). The thought is that interacting members of the same species will not inflict high costs on each other because of some desire to ensure the success of the species. However, consider that any competition that exists between different species is for a resource that includes only a small part of either one’s total requirements for survival. Intraspecific competition occurs between individuals with identical requirements (see niche space below). Therefore, you would expect that intraspecific competition could have potentially higher costs due to the complete overlap in requirements between individuals.
2. Intraspecific Competition (Competition between individuals within a species)
  1. In the Pop. Ecol. lectures we saw that slowing population growth at high population densities produces a sigmoidal pattern in which the population growth slows and eventually fluctuates around some average value of K (but remember, it’s actually always following K).
  2. Our assumption in that discussion was that intraspecific competition for some limited resource (or many different resources) caused the population growth to slow and fluctuate. The effect of intraspecific competition was then built into the logistic model of population growth.
  3. In fact, intraspecific competition is the major cause of density-dependent growth of a population. Increased competition for resources decreases resources availability for reproduction, making the population follow K (This is exacerbated by the GAS response we see in many animals (and maybe similar mechanisms in some plants). The General Adaptation Syndrome is a physiological-based response system where short-term gains of survival in high population density situations are paired with long-term costs. In short, the short-term benefit of “fight or flight”, when experienced frequently, carries with it the costs of decreased immunity and reproductive potential in the individual.

(The example below of the Spongy moth has been edited out of the lecture video. This example will be presented during lecture on 11/14.)

- 1. Example of intraspecific competition: Spongy Moth explosion.
    1. In the summer of 2012, much of the NE U.S. saw a significant increase in the spongy moth (Lymantria dispar). As the summer wore on, the larvae (caterpillars) pupated; the hatched adults mated, and the females laid masses of eggs (each mass containing several hundred eggs) on virtually every tree in the region.
    2. In May/June of 2013, the young caterpillars that hatched from these eggs began feeding and molting.
    3. The results were dramatic: Large patches of forest began to take on a winter appearance with their skeletons of bare branches. For example, in a documented 72-hour period, a 50-ft beech tree and a 25-ft white pine tree were completely defoliated (spongy moth caterpillars really don’t do well developmentally when eating white pine). In fact, the infestation was so heavy that many trees were completely defoliated before the caterpillars could complete their larval development.
    4. The result: a massive die-off of the animals; very few succeeded in completing metamorphosis.
    5. Here, then, was a dramatic example of how competition among members of one species for a finite resource — in this case, food — caused a sharp drop in population size.
    6. The effect was clearly density-dependent. In comparison, the lower population densities of the previous summer had permitted most of the animals to complete their life cycle.
  2. One possible outcome of intraspecific competition is that superior competitors will have greater fitness than lesser competitors will. This sort of “fine tuning” of a species trait (the better ability of individuals to gain resources) can cause an eventual specialization among the species (the species can only use this resource because they are not capable of using others, having lost their ability through evolutionary time). For example, many flowers can only be pollinated by one particular species of insect. This relationship may have resulted from competition within the ancestral pollinating population. Those that were better at obtaining the resource were more fit. Eventually, only those pollinators that could perform at a high level would get the resources. The continued optimization of the behavior over evolutionary time comes with the cost of not being able to effectively compete/use other resources that may require different behaviors to obtain.
1. Interspecific Competition and Niche Space
  1. Niche- Summarizes the environmental factors that influence the growth, survival, and reproduction of a species.
    1. A species’ niche consists of all the factors necessary for its existence – approximately when, where, and how a species makes its living.
    2. The word “Niche” has been around for a long time (referring to a recessed place in a wall where one could display items of interest), and has been a part of Biology for almost 100 years.
    3. The term “niche”, in a biological sense, developed over several decades, but it was within interspecific competition that the importance of niche space was fully realized.
  2. A single paper in 1957 by G.E. Hutchinson described what ecologists consider to be the most complete way of thinking about the niche concept. Hutchinson defined the niche as an n-dimensional hypervolume, where n equals the number of environmental factors important to the survival and reproduction of the species. Rather than considering niche as a one- or two-dimensional attribute of a species, niche became multidimensional, having more than one or two environmental requirements. Although it may be easier at times to study one or two contributing factors to a species’ success, we do need to be aware that each species is a complex organism with multiple, complicated interconnections with its environment.
    1. Hutchinson’s hypervolume, which specifically defines the accumulated factors permitting a species to survive and reproduce, is what we call the fundamental niche of a species. The fundamental niche defines the physical conditions under which a species might live, in the absence of interactions with other species (the abiotic environment).
    2. Hutchinson also realized that interactions, such as competition, may restrict the environments in which a species may live. He referred to those more restricted conditions as the realized niche. The realized niche is the actual niche of a species that is restricted through its interactions (whether due to competition, predation, disease, and many others) with other species.
    3. Do you think it is at all possible to completely describe Hutchinson’s n-dimensional hypervolume niche for any one species?
    4. Example of niche-overlap: Coyotes and Wolves
  3. Interspecific Competition
    1. Interspecific Competition- Competition between individuals of different species for the same resources.
    2. Interspecific competition can lead to an Evolutionary Arms Race, where the adaptations of one species are overcome by the selection for traits within another species, which is countered by the evolution of new traits in the first to overcome adaptations by the second.
    3. His study of interspecific competition in 1934 led G.F. Gause to describe the Competitive Exclusion Principle.
      1. The Competitive Exclusion Principle states that two species with identical niches cannot coexist in that niche indefinitely.
      2. The costs (both in the competition itself and the loss of potential resources) of competition are great enough that if 2 species have identical niches, the better competing species will exclude the lesser competitor from the niche.
      3. The species that is out-competed will likely go extinct from the area. If the niches do not completely overlap (that is, if the variation around niche space for the 2 species results in some little area of non-overlap), the lesser competitor MAY alter its niche space to better fit that which does not overlap with the other species (see realized niche above). Sometimes both competitors will shift their niche space.
  4. If complete competitors drive each other to local extinction, how different do two species have to be to coexist and in what features do they usually differ? The first question is quite complicated, since it varies by situation, with each species occupying highly complicated niche spaces. The latter, however, is more straight-forward.
    - 1. The term resource partitioning describes the differentiation of niches, both in space and time that enables similar species to coexist in a community.
      2. Example: MacArthur’s warblers.
      3. There are several different requirements in which species can compete. One can look at a series of comparisons among many species across these niches to determine the niche types that are more commonly competed for to see where resource partitioning has taken place. We can think of resource partitioning as reflecting the results of past competition. So by understanding where species, that are otherwise similar, are particularly different, we can see where in the history of their multi-dimensional niche spaces competition was the greatest. This one difference between species is referred to as resource segregation.
      4. Example: Lack’s passerines: Among passerines (small perching birds) sharing fairly similar lifestyles, most segregation takes place by habitat preference, followed by (in decreasing order) body size, feeding habit, geography, and wintering range.

Mechanisms of Competition
1. Ultimately, competition mechanisms fall into one of two categories:
  1. Scramble or exploitative competition- where the competing individuals target the resource, and there may be no direct interaction between the two.
  2. Interference competition- where individuals target the competitor, and not necessarily the resource.
  3. Examples: which type of competition is it: scramble or interference?
    1. Dung beetles
    2. Dung beetles
    3. Barnacles
    4. Hyenas vs. scavengers
    5. Salvia sp. vs. other plants.