Lecture 9: Interspecific Competition EEES 3050

Competition In the past chapters, we have been discussing how populations grow and what factors determine that growth. What happens when you put populations of more than one species together?

How do species interact? Competition Predation Herbivory Parasitism Disease Mutualism

Interspecific Competition Competition  When two species use the same limited resource to the detriment of both species. Assessment-some general features of interspecific competition Competitive exclusion or coexistence Tilman’s model of competition for specific resources (ZINGIs) Coexistence: reducing competition by dividing resources

Assessment mechanisms  consumptive or exploitative — using resources (most common)  preemptive — using space, based on presence  overgrowth — exploitative PLUS preemptive  chemical — antibiotics or allelopathy  territorial — like preemptive, but behavior  encounter — chance interactions

Modeling coexistence? Can we model the growth of 2 species? Remember logistic model? What is K? Now we add another factor that can limit the abundance of a species.  Another species.

Freshmen and donuts: an example There is a room with 100 donuts – what does a typical male freshmen do? First – eat several donuts. (A male freshman can eat 10 donuts) Second – rapidly tell friends  But not too many! Third – Room reaches carrying capacity at 10 male freshmen. So K=10 for male freshmen.

Freshmen and donuts: an example What happens if a male and female discover the room at the same time? First – eat several donuts. (A female freshman can eat 5 donuts) Second – rapidly tell friends  But not too many! Third – Room reaches carrying capacity at ? males and ? females. What is the carrying capacity?  It depends…

Lotka-Volterra Need a way to combine the two equations. If species are competing, the number of species A decreases if number of species B increases.  Such that: Where alpha is the competition coefficient Lotka-Volterra: A logistic model of interspecific competition of intuitive factors.

Freshman Example In a room we have 100 donuts.  Need 10 donuts for each male freshmen.  So K 1 = 10  Need only 5 donuts for each female freshmen.  So K 2 = 20  If room is at K 1 and 1 male leaves, how many females can come in? So,, where α = 0.5 And,, where B = 2

Possible outcomes when put two species together. Species A excludes Species B Species B excludes Species A Coexistence

Changes in population 1:

Yellow: both increase White: both decrease

Changes in population 2: Yellow: both increase White: both decrease

Yellow: both increase White: both decrease Green: Sp 1 increase Brown: Sp 2 increase

Tilman’s model Problems with Lotka-Voltera model?  No mechanism Dr. Tilman developed a model based on resource use.

1 – no species can survive 2 – Only A can live 3 – Species A out competes B 4 – Stable coexistence 5 – Species B out competes A 6 – Only B can live

Lab Experiments? Gause using yeast and Birch using beetles.  Results show both exclusion and coexistence It was hypothesized that the yeasts had enough differences to allow coexistence.  i.e. the requirements of the 2 species are slightly different.

Gause’s hypothesis: As a result of competition two similar species scarcely ever occupy similar niches…  Also called the competitive exclusion principle: “Complete competitors cannot coexist.” Niche – still controversy about the definition.  1) The role of a species in the community – Elton  2) a subdivision of the habitat. - Grinnell 1917.

Back to Competition Coefficient Competition coefficient:  Intensity of competition from species. In our original donut example:  α = 1/ β  Also, can read about Gause’s yeast populations in book.

Changes in population 1: K2K2 K2/βK2/β K1 = 10 K2 = 20 α = 0.5 β = 2

Back to Competition Coefficient Competition coefficient:  Intensity of competition from species. In our original donut example:  α = 1/ β However: in systems that are more complex  the coefficients are not necessarily reciprocals.  And carrying capacity may not be purely determined by resource being competed for.

Changes in population 1: K1 = 10 K2 = 17 α = 0.5 β = 3 K2K2 K2/βK2/β

Niche: Hutchinson Redefinition in 1958.

Niche: Hutchinson Redefinition in Two environmental variables, can produce an environmental space or a species niche. Can add many other environmental factors. n-dimensional hypervolume  Or a species Fundamental Niche However, because competition can limit this fundamental niche, what we witness in nature is the:  Realized Niche

Can 2 species exist in the same niche?

Observation:  Several types of warblers live in the same tree species. Hypothesis based on competition theory:  Warblers will use different parts/areas of the trees. Experiment:  No experiment conducted, but observations can be made to test hypothesis. Can 2 species exist in the same niche?

McArthur suggested competition to explain warbler patterns Ghost of competition past. So how do species coexist?  Different food resources, i.e. diet specialization What about plants?  Plants usually need same resources, water, nutrients, light.  What about phytoplankton?

How do phytoplankton live in the same location? Phytoplankton  Common pool of nutrients  Often large number of species  Same environment, i.e. amount of light, temperature.  In many bodies of water, nutrients are limited. Reasons?  Environmental instability  Non-equilibrium system.

Assumptions of competition theory life history characteristics of species are adequately summarized by the per capita growth rate of species; deterministic equations are sufficient to model population growth, and environmental fluctuations need not be considered; the environment is spatially homogeneous and migration is unimportant; competition is the only important biological interaction; and coexistence requires a stable equilibrium point.

How to determine if interspecific competition has occurred (or is occurring)? From Wiens (1989) 1. Need a checkerboard distribution 2. Species overlap in resource use 3. Intraspecific competition occurs 4. Resource is limited 5. One or more species is limited 6. Other hypotheses do not fit.

Example where criteria 1 and 2 fit. Feeds far from shore Feeds near shore

Test theory with plants Observation:  Plants all require light, nutrients and water.  Plant often found together. Hypothesis:  Competition between plants ought to be common.  Plants do worse with other plants. Experiment: Results:

Experimental design. Compare the growth of annuals and shrubs in the Mojave Desert. 2 experiments  Effects of annuals on shrubs  Effects of shrubs on annuals How? Effects of annuals on shrubs Effects of shrubs on annuals

Results: Annual had positive benefits from shrubs. Shrubs had negative benefits from shrubs.

Facilitation:  Where one or both species benefit (have a positive effect) due to the presence of the other species.

Resource utilization curves Species may evolve to minimize the impact of competition.

Character Displacement Definition:  In areas where species overlap, there has been a divergence between the two species, supposedly as a result of competition.

Example: Galapagos finches Theory:  According to displacement theory, species that are sometime found together will have a character that has changed compared to when the species are found by themselves. Observation:  There are three species of finches in the Galapagos that are sometimes found together and sometimes separate. Hypothesis:  There will be differentiation in bill size when species are on the same island: Test (Not truly an experiment):  Examine the bill size of three species. Results:

*Could we actually test this?

Four criteria for determining character displacement Change in mean value of the character in areas of overlap should not be predictable from variation within areas of overlap or areas of isolation. Sampling should be done at more than one set of locations Characters need to be heritable. Species must actually be competing for resource.

r vs. K selected species What do r and K refer to?  r – growth rate  K – carrying capacity r – selected  Species that remain in the growth rate stage for most of their existence. K – selected  Where organisms remain near the carrying capacity  Influenced more by competition.

Is it really competition? How else could these ideas be framed?  Conflict avoidance.  Not survival of fittest, but perhaps least noticeable. Best hider.  Path or least resistance