Competition

 The effects of competition in nature can be observed through observation & experimentation.  Many organisms compete for limited resources and may defend territories to protect their access to these resources.

Hypothetical Example  Land Iguanas  3 species identical in nearly every way.  Color (green, blue, yellow)  Green ones find food faster

Unlimited Resources  If resources are plentiful, organisms have as much as they need and do not compete.  Fast growing/reproducin g cacti.

Limited Resources  When food resources are limited, some organisms may have an advantage over others.  When cacti grow slowly, not enough food for all.  Green iguanas find food faster, outcompete other colors.

Resources  Resources that can be limited include:  Food  Specific nutrients  Specific amino acids  Shelters  Nest sites  Water

Resources  If resources are unlimited, populations can grow rapidly.  Exponential growth  J shaped curve  In reality, resources are limited.

Competition  When their life-sustaining resources are limited, organisms are forced to compete with each other.  Sometimes those competitive interactions will be direct and obvious, while other times they will be indirect and subtle.

Niches  Niche : Summarizes environmental factors that influence growth, survival, and reproduction of a species.  An organism ’ s role within the environment.  Hutchinson defined niche as:  n-dimensional hyper-volume  n equals the number of environmental factors important to survival and reproduction of a species.

Niches  Fundamental niche – the entire area where a species could survive in the absence of biotic interactions.  Realized niche includes interactions such as competition that may restrict environments where a species may live.

Niche Overlap and Competition Between Barnacles  Connell examined the relationship between two kinds of barnacles – Balanus balanoides and Chthamalus stellatus.  Chthamalus is found in the upper intertidal.  Limited by competition with Balanus.  Balanus is found in the mid & lower intertidal.  Limited by exposure to air in upper intertidal.

Niche Overlap and Competition Between Barnacles  The fundamental niche of Balanus is equal to its realized niche.  The fundamental niche of Chthamalus is much broader than it ’ s realized niche.  Both are limited by predation in the lowest part of the intertidal.

Environmental Tolerance  All organisms have specific tolerances for a variety of conditions like temperature, pH, moisture level etc.  Mosquitos, for example, have a range of temperatures at which they survive and reproduce.

Niche and Environmental Tolerance  Where in Florida can the invasive tiger mosquito live?  Important abiotic factors:  Rainfall  Temperature

Fundamental Niche  The fundamental niche is everywhere the species could physically live.

Competitive Exclusion  Gause: Principle of Competitive Exclusion  Two species with identical niches cannot coexist indefinitely.  One will be a better competitor and thus have higher fitness and eventually exclude the other.

Competitive Exclusion  Gause developed a simple experimental system of two single-celled species of Paramecium growing in test tubes.  He found that both species were able to grow just fine if they were alone. However, as shown in the graph, one of the species, P. aurelia, would always out-compete the second species, P. caudatum, when they were grown together.

Competitive Exclusion  The yellow fever mosquito can also survive across all of Florida.  However, the Asian tiger mosquito has displaced it throughout most of the state.  Thus, the yellow fever mosquito's fundamental niche includes the whole state, but its realized niche is now restricted to small areas of the state, due to competition with the invading Asian tiger mosquito.

Classifying Competitive Interactions  Example – desert shrubs use water & nutrients in a halo around themselves.  Small plants can’t survive in this area.  Resource Competition – Exploitative Competition  While individuals are not actively fighting each other off, they indirectly compete because each individual obtains less of the resource due to the presence of other individuals.  Self-thinning

Resource Competition  Resource competition is not limited to plants.  The simulated iguanas also compete through resource competition, as do real mosquito larvae.  In both cases, the animals do not directly interact with each other, but because they use the same resource (food for iguanas, breeding sites for mosquitoes), they indirectly compete.

Interference Competition  Interference competition occurs when organisms compete directly for resources.  Aggressive interactions  Interference competition does not require a direct fight over a resource.  Some birds will attack and destroy eggs in nests near their own, presumably to reduce competition for resources.

Interference Competition - Allelopathy  Plants can interfere with each other directly, and a common way they do this is through releasing poisons that prevent other species from growing.  Allelopathy

Interference Competition - Allelopathy  Examples of Allelopathy  Black walnut  Eucalyptus  Bracken fern

Interference Competition - Territoriality  Some organisms defend limited resources by actively excluding others that would potentially use that resource.  Territoriality

Interference Competition - Preemption  Preemption is interference competition that occurs when an individual prevents other individuals from occupying a location by occupying the space first.

Direct or Indirect Competition?

Intraspecific Competition  The strength of competition is greatest when individuals share all of the same limited resources. This occurs most often among individuals of the same species within a population.  Intraspecific Competition

Intraspecific Competition  The strength of intraspecific competition factors strongly into whether a population will grow or shrink.  But how strongly?  Ecologists use population growth models to quantify the strength of intraspecific competition and to predict how competition will influence changes in population size over time.

Intraspecific Competition  When resources are unlimited, populations grow exponentially.  dN/dt = r N

Intraspecific Competition  Resources are never unlimited for long.  As populations grow, they use more resources.  Ultimately, resources become limited and individuals in the population must compete with each other to obtain the resources they need to survive and reproduce.  As a population grows, intraspecific competition reduces birth rates and increases death rates, and the population growth rate drops below its maximum possible growth rate.

Survival of the Fittest (Fattest)  Individual mosquito larvae grow slower at high densities, they also die at a faster rate (presumably because they do not get enough to eat), and those that survive take longer to pupate and hatch into adults.

Carrying Capacity  Increased population density results in slower growth for individual larvae.  The slower individual growth rates translate into slower growth of the population as a whole.  Eventually, the density of larvae in the tire limits individual growth to the point that population growth completely stops.  A population's carrying capacity is the maximum size it can reach when constrained by limited resources.

Carrying Capacity  At carrying capacity, the resources available to a population are exactly equal to the resources required to sustain that population.

Limiting Resources  Population growth is not always limited by food.  The limiting resource can be space, chemical nutrients, sunlight availability, or some other factor.

Logistic Growth  The growth of the mosquito population, with its carrying capacity (K) determined by intraspecific competition, can be represented using the logistic growth equation.

Interspecific Competition  If the individuals are different species, it is known as interspecific (b etween species) competition.

Interspecific Competition  The different strains of bacteria in your body form their own ecological communities.  The bacteria living in your gut compete with each other via exploitation for food (sugars, fats, and proteins from the food you eat).  They also compete preemptively for space, since the surface on the inside of your intestines where they adhere is limited.  And they compete directly by releasing chemicals that act as poisons.

Interspecific Competition  Some strains of bacteria benefit us directly, such as those that aid digestion. But there are also strains that benefit us indirectly, including those that outcompete pathogenic (harmful) strains.  What attributes would make a strain of probiotic bacteria an effective competitor?

Probiotics vs Pathogens  In the simulation, probiotics use twice the resources of pathogens.  The average effect of a probiotic bacterium on a pathogenic bacterium was twice as strong as the effect of a pathogen on another pathogen.

Competition Coefficients  Ecologists quantify the relative competitive strengths of different versus same species competitors with competition coefficients.  A competition coefficient (α) is the per capita effect of one species on the population growth of another species.

Lotka Volterra  Lotka-Volterra competition models are based on the logistic equation of population growth – the s-shaped curve.

Lotka Volterra  Four possible outcomes of the Lotka- Volterra competition equations:  Species 2 eliminated.  Species 1 eliminated.  Either species 1 or species 2 eliminated, depending on starting conditions.  Both species coexist.

Lotka Volterra  In general, LV predicts coexistence of two species when, for both species, interspecific competition is weaker than intraspecific competition.

Paramecia Lab Experiments  Gause demonstrated resource limitation with Paramecium caudatum and Paramecium aurelia in presence of two different concentrations of food - Bacillus pyocyaneus.

Paramecia Lab Experiments  When grown alone, carrying capacity was determined by intraspecific competition.  When grown together, P. caudatum quickly declined.  Reduced resource supplies increased competition.

Flour Beetle Experiments  Tribolium beetles infest stored grain products.  Park studied interspecific competition between T. confusum and T. castaneum under varied environmental conditions.

Flour Beetle Experiments  Two species of flour beetle grown in containers with and without parasites.  With parasites, T. c onfusum has a competitive advantage over T. castaneum, but this is reversed when parasites are removed.

Competition and Niches  Competition can restrict species to their realized niches.  But if competitive interactions are strong and pervasive enough, they may produce an evolutionary response in the competitor population.  Changes fundamental niche.

Competition in a Complex Environment  The logistic growth and Lotka-Volterra competition equations provide a logical framework to study intraspecific and interspecific competition.  The equations are particularly good models for predicting results of competitive interactions under tightly controlled laboratory conditions.

Competition in a Complex Environment  However, natural environments change over time and across space.  This inherent variability impacts species interactions, making it difficult to describe natural populations using a small number of variables (e.g., N, K, α).  Species often coexist in nature where simple models of competition would predict competitive exclusion.

Field Study Examples  Studying competition outside of the lab can be challenging.  Chance events, weather, and even experimenters themselves can add noise to the results, obscuring patterns.  Competitive relationships can also change over time and from place to place.

Field Study Examples  A simple way to document competition is to identify places where two species seem to compete, remove one of them, and then examine what happens to the other.

Field Study Examples  Rodents and ants compete for annual seeds in Arizona deserts.  The results indicate that ants and rodents are the primary consumers of grass seeds in these deserts, and that they compete with each other for the seeds.

Common Garden Experiments  One theory for coexistence within niches is that our interpretation of species' niches is often wrong.  Without experimentation, it can be difficult to tell exactly how species are using local resources.

Common Garden Experiments  Heath bedstraw is found growing in acidic peat soil, while a related species, slender bedstraw is found in more basic limestone soils.  Both plants grew in both kinds of soil, although they both grew better on the soil in which they are found in nature.

Common Garden Experiments  If seeds from both species were planted in a pot of peat soil, heath bedstraw grew faster than slender bedstraw, eventually blocking all the light, and the slender bedstraw died.  Conversely, if he planted both seeds in limestone soil, he got the opposite result—slender bedstraw outgrew and excluded heath bedstraw.

Common Garden Experiments  Two species of lizard, Sceloporus merriami and Urosaurus ornatus occur in each of the three isolated plots of land.  Both species are small diurnal insectivores, and they appear to compete with each other.

Common Garden Experiments  The top plot contains S. merriami only, the middle plot contains U. ornatus only, and the bottom (control) plot contains both species.  Initial lizard densities reflect naturally occurring densities when temperature and humidity are average.

Common Garden Experiments  Is there evidence for competition between these two species?  Rainy?  Drought?  Environmental factors can shift the competitive interaction so it favors one species under certain conditions and another species under different conditions.

Hypotheses for Competitive Coexistence  Variability within and between local patches can also favor one species over another in a very small area or time window.  Spatial and/or temporal variability (or heterogeneity)

Hypotheses for Competitive Coexistence  Two competing species may evolve in ways that reduce the strength of competition between them through opposing shifts along some trade-off.  Character Displacement

Hypotheses for Competitive Coexistence  Another way that competing species can coexist is if competition is weak due to predation or other ecological interactions in the community.  If predation reduces population sizes for multiple competing species in an area, then resources might never become limiting and competition might not be important.

Mosquitos in a Complex Environment  What might explain the patches of coexistence for these competing species?  Both species can survive in Florida’s temperatures.  Perhaps there is an interaction between temperature or rainfall and the competitive interaction between the two mosquito species.

Mosquitos in a Complex Environment  Common Garden Experiment  Hot/Wet  Hot/Dry  Cool/Wet

Mosquito Abundance & Competition  Asian tiger mosquitos have a competitive advantage.  The two species still coexist in some areas.  Incomplete picture…

Mosquito Abundance & Competition  Another possible explanation for the pattern of competitive exclusion in some areas and coexistence in others is a trade-off between competitive ability and colonization ability.

Mosquito Abundance & Competition  Colonization is important because the stagnant water habitats appear and disappear as rain falls and then evaporates.

Mosquito Abundance & Competition  Lotka-Volterra coexistence occurs where the yellow fever mosquito is still found: in hot, dry, and urban areas of Florida.  It turns out that the eggs of the Asian tiger mosquito are more susceptible to desiccation in hot, dry environments than the eggs of the yellow fever mosquito.