Interspecific Competition and Its Effects on Survival, Reproduction, Growth, and Population Size

Today…Competition Competition occurs both within and between populations. Competition within a population (intraspecific competition) is the interaction that limits logistic growth because resources are limited. Here we are interested in competition among different species for similar resources. This interaction is called interspecific competition. Interspecific competition is important because it can affect: 1. survival 2. reproduction 3. growth in individual body size 4. population size (and potentially population extinction)

Definition: “Competition is any use or defense of a resource by one individual that reduces the availability of that resource to other individuals” (Ricklefs 2007, p. 365) Competition has long been the subject of intensive research… Papers on competition have constituted 5-6% of the the ecological literature for over 60 years. (Jackson 1981) A more recent count (Keddy 1989) found papers about interactions in this ratio: competition:predation:mutualism 8: 5: 1

Competition occurs when: Two or more species (or individuals) use (share) the same resources… and Those resources are limited in abundance or availability (at least some of the time). To continue a theme… We know exotic predators can cause extinction of native prey. Can exotic competitors eliminate native species through intense competition? Keep this question in the back of your mind as we learn about competition and its effects.

The degree to which resources are shared often determines the intensity of competition. We describe the way in which species use resources as a key element of their niche. Species that share resources have similar ecological niches. Sharing implies niche overlap. Definition of the niche (from Ricklefs): “The ecological role of a species in the community; the ranges of many conditions and resource qualities within which the organism or species persists, often conceived of as a multidimensional space.”

Among the components we would need to evaluate to fully determine the niche of a species are: • a habitat component • environmental components, and • a food component • We measure the relative abundance of a population or species along axes that describe each of these aspects (and others) of the niche… • The resulting model, if it has only 3 dimensions, could be plotted. If it has more than 3 dimensions, it becomes a multi-dimensional niche as described by Hutchinson.

In one dimension … we measure how well a species does as some environmental condition varies. Where the species persists, it is tolerant of the conditions. The Law of Tolerance Optimum zone Zone of stress Performance Zone of intolerance Low High Gradient

Here’s what it might look like for a bird that mostly chooses to live among shrubs… E stands for ecotone

And here is real data - Niches in one dimension: optimum temperature ranges for net photosynthesis in species originating from various altitudes in the Alps (Pisek et al. 1973) Note local adaptive differences in the niches of B. pendula from different elevations.

However, one dimension is rarely enough to adequately describe a niche. Consider a number of bird species sharing the habitat space you saw just a moment ago. Now let’s add a niche dimension reflecting use of different food types… These niches don’t overlap. There would be no competition among these 3 species, because those that feed on the same food do so in different habitats..

When there is niche overlap, then competition can occur if the resource(s) in question are limiting. In this diagram, species 1 and 2 may compete…

The intensity with which populations or species compete is often related to the amount of their niches that is in overlap. These species may compete strongly…

Here’s how we can depict three-dimensional niches… Humidityy x Temperature zNutrient 3 axes (= 3 variables) There are an infinite number of dimensions possible

Typically, differences in competitive ability lead species to occupy different regions of their niche when living with competitors. Fundamental Niche (entire hypervolume: no competitors) Realized Niche (where a species occurs with competitors)

Fundamental Niche: - niche of a species in the absence of competitors - a niche that the species could potentially occupy Realized Niche: In the presence of competitors, the species is restricted to the actual set of conditions under which an organism exists.

Here’s an example from the beginning of the chapter that demonstrates how realized niches differ from fundamental niches… British ecologist Tansley (1871-1955) was the 1st to experimentally show competition between two closely related species The Tansleys visiting the University of Chicago

Two perennial, herbaceous plants (bedstraw) used in a common garden experiment Galium saxatile: normally found on acid soils in peat bogs G. sylvestre: normally found on limestone hills & pastures. Limestone makes the soils basic. Two species were grown alone and in a mixture with the other species on both soil types. Galium saxatile

RESULTS Ricklefs (2001, Fig. 19.1) G. saxatile alone grows best in acid soils as observed in nature When 2 are grown together, G. saxatile prevails on acid soils G. sylvester alone grows best in limestone soils as observed in nature When 2 are grown together, G. sylvester prevails on limestone soils Limestone Acid Soils

Tansley’s Conclusions: • the presence or absence of a species can be • determined by competition with other species • conditions of the environment affect the outcome • of competition • competition may be felt broadly throughout • the community • the present segregation of species may have • resulted from past competition …. • “the ghost of competition past”

What happens to population size when species compete? The result depends on their tolerances, their efficiency in using a resource they share and its abundance …

In the previous slide, species B reached its carrying capacity, but the population of species A would be depressed to a size significantly below its K. When both competing species persist, the result is called competitive coexistence. In many other cases, both species in a competitive interaction are depressed to a population size lower than they would have reached if growing alone. First, if growing alone…

Now, what do we observe when they grow together? This is one possibility. both species persist Another is that the species that is ‘weaker’ in competition is driven extinct by the interaction. one species driven extinct After theoretical predictions; there is experimental evidence:

Gause did experiments on competition between two Paramecium species. His observations were that one species, P. aurelia, outcompeted and drove the other, P. caudatum, to extinction. This result is called competitive exclusion.

Bacteria are the limiting food resource for Paramecia. The species that ‘wins’ draws the level of food available down to an abundance insufficient for the ‘losing’ species. Another example of competitive exclusion comes from your text (p.371). Here is a picture of citrus scale. It a serious and damaging pest in California citrus orchards.

It was an example of biological control. Three different parasitoids were introduced to control it. They show clear evidence of geographical competitive exclusion…

An aside: Remember Leibig’s law of the minimum (Justus von Leibig, 1840). It says that there is, in the end, some single resource, the limiting resource, whose supply no longer meets the population’s need for it. Does this law always apply? No! We came up with an alternative before. Here the evidence of competition brings the simple interpretation of the ‘law’ into question. Frequently resources interact in affecting population growth. Your text provides an example: Peace and Grubb (1982)…

Two resources may enhance the growth of the consumer more than the sum of each resource. • Peace & Grubb 1982 New Phytol. 90:127-150. • Designed a 2 (fertilized) x 4 (light level) • factorial experiment to examine growth • of Impatiens • Both factors are resources need by plants: • N & P for protein synthesis • C assimilated by photosynthesis • for carbohydrates • Interesting interaction effects http://www.botany.com/ impatiens.html

The two fertilizers used were nitrogen and phosphorus. Both, considered separately, enhanced plant growth (dry weight), but together they increased growth compared to controls much more than either alone. The two fertilizers are synergistic. Next, add the effect of light intensity…

Fertilized plants use light resources more efficiently than unfertilized plants Ricklefs (2001, Fig. 19.4)

You may think (logically) that the intensity of competition should lessen as resources become more abundant. This is the view of some prominent ecologists, e.g. P.J. Grubb and David Tilman. However, an opposing view, that the intensity of competition increases with resource abundance, also has important adherents, including J.P. Grime and Paul Keddy. The explanation of that counterintuitive view is that at low resource abundance (lack of water or mineral nutrients) plants are more widely spaced, and that will limit the intensity of competition both for soil resources and light. Opposing views can be tested experimentally…

Here’s evidence that competition can increase with resource abundance (in a desert annual grass)… Water is the resource controlled in this experiment. As standing crop increased (evidence of greater water availability, which would seemingly reduce competition in a desert environment) the intensity of competition instead increased. Why? (Hint: competition occurred above ground)

The opposing view was tested by David Tilman in studies of competition among three species of prairie grasses. Plots were fertilized with ammonium nitrate at three levels: low, medium, and high. The intensity of competition did not vary much. What changed was the nature of the competition… At low nutrient level, the competition among below ground parts (roots) was fierce for the limiting nitrogen. At high nutrient level, the competition was high among the much larger above ground plant parts (stems and leaves). The limiting factor became sunlight and space.

Back to the main theme – competition… The competition between Paramecium species for food is called exploitative competition or exploitation. Definition: A competitive interaction in which each species consumes the shared resource, thus reducing the amount remaining to be used by the other species. It occurs when individuals competeindirectlythrough their mutual effects on shared resources. They don’t ‘fight’ with each other; they use resources to their individual best ability.

There is an alternative form of interspecific competition that is called interference competition. Definition: Interference occurs when one species sequesters or defends a resource without consuming it. It interferes with access to the resource by the other species. Usually the interference occurs by means of antagonistic behaviour. This is direct interaction. It is only likely when it is profitable for one species to defend resources. There is no evidence that it occurs spitefully.

Previous examples (competition between P. aurelia and P. caudatum orbetween species of Galium or bedstraw) are occurrences of exploitative competition. An example of interference competition: Competition for space frequently occurs by means of interference. Allelopathy among plants provides classic examples. The text describes the protection of space around individual shrubs of sage (Salvia spp.) in California. Allelopathy occurs by leaves producing volatile terpenes (or other inhibitory compounds) that wash and evaporate off into the air and soil beneath and around the plants. The result is a bare zone and an inhibition zone around the plants.

sage bare zone suppressed zone

A Review before going on: Interspecific competition occurs when two species share resource(s) and those resources are available only in short supply. Resource sharing is evident in niche overlap among competing species. Competitive interactions can take the form of exploitative or interference interactions. The classic experiments demonstrate occurrence of competitive exclusion or the persistence of both species in competitive coexistence.

Interspecific Competition and Its Effects on Survival, Reproduction, Growth, and Population Size