BIOL 203 Lecture 15 Competition Fall 2024 PDF

Lecture 15 Competition BIOL 203 November 27th, 2024 1 Learning objectives 1. Illustrate that competition occurs when individuals experience limited resources 2. Explain the theory of competition as an extension of logistic growth models 3. Describe how the outcome of...

Lecture 15 Competition BIOL 203 November 27th, 2024 1 Learning objectives 1. Illustrate that competition occurs when individuals experience limited resources 2. Explain the theory of competition as an extension of logistic growth models 3. Describe how the outcome of competition can be altered by abiotic conditions, disturbances, and interactions with other species 4. Distinguish among exploitation competition, interference competition, and apparent competition 2  Competition occurs Key Concept when individuals experience limited resources 3 Competition occurs when individuals experience limited resources Competition = an interaction between two individuals that has a negative impact on both Occurs when resources are limited (food, mates, nesting sites, etc.) Two categories: Intraspecific competition = competition within a species Interspecific competition = competition between species 4 The role of resources A resource is anything an organism consumes/uses that causes an increase in the growth rate of a population when it becomes more available E.g.) sunlight, water, N/P/K for plants; food, water, space for animals Non-consumable factors are not resources (e.g., temperature) 5 Renewable vs non-renewable resources Renewable resources are constantly regenerated (e.g., sunlight continuously generated) Non-renewable resources are not regenerated (e.g., space is typically a fixed-amount) 6 Renewable vs non-renewable resources Renewable resources can originate from outside the ecosystem where a competitor lives Competition can reduce resource abundance but not rate of resupply Resources do not respond to rate of consumption E.g.) dead leaves falling into a stream 7 Renewable vs non-renewable resources Renewable resources can originate from inside the ecosystem where a competitor lives Competition can reduce both resource abundance AND rate of resupply E.g.) predators eating prey reduce abundance for other competitors; lower prey numbers could lead to changes in growth rate of those populations 8 Renewable vs non-renewable resources Sometimes, supply rate of a renewable resource generated within an ecosystem indirectly affected by competitors E.g.) nitrogen cycling in terrestrial systems 9 Liebig’s law of the minimum Although consumers reduce abundance of resources, not all resources limit consumer populations E.g.) All terrestrial animals require oxygen, but increases won’t deplete supply enough that affects growth rate Liebig’s law of the minimum = a population increases until the supply of the most limiting resource prevents it from increasing further Populations are limited by the scarcest resources relative to demand 10 Liebig’s law of the minimum Amount of resource that limits population’s growth depends on resource E.g.) plants need more N/P than trace minerals Whichever resource reaches limiting value first will be the one that regulates population growth 11 Liebig’s law of the minimum If we know minimum amount of resource required for population to grow, we can predict best competitor Lower than other species; E.g.) diatoms and silica should be better competitor 1 μm 0.4 μm 12 Liebig’s law of the minimum Synedra can persist with lower amounts of silica, outcompetes Asterionella When two species compete for single limiting resource, the species that can persist at lowest resource level will win the competition 13 Interactions among resources Liebig’s law assumes each resource has independent effect on population growth If a given resource limits individuals/populations, increasing other resources won’t affect growth Not always the case 14 Interactions among resources E.g.) small balsam (Impatiens parviflora) Grown in two batches (with or without fertilizer) Also grew at varying light intensities Slight increase in growth of non- fertilized Significantly increased growth with fertilized 15 Interactions among resources At low light intensity, light was limiting resource At high light intensity, nutrients were limiting resource 16 The competitive exclusion principle When two species limited by same resource, they cannot coexist; one will persist while the other dies out = competitive exclusion principle Occurs because one species is better at obtaining resource or better at surviving when resource is scarce (e.g., Synedra vs Asterionella) 17 Competition among closely related species Darwin hypothesized that competition is most intense between closely related species Share similar traits; likely consume similar resources We often see closely related species living in same region occupying different habitats Why? 18 Competition among closely related species Hypothesis: since closely related species compete for same resource, natural selection favours differences in habitat use Competitive advantage in preferred habitat, disadvantage in competitor's habitat 19 Competition among closely related species First test of this hypothesis by Arthur Tansley, 1917 Heath bedstraw (Galium saxatile) – lives in acidic soils White bedstraw (G. sylvestre) – lives in alkaline soils Grown separately and together in both conditions 20 Competition among closely related species Both could grow separately in either condition, but grew better in home soil When grown together, led to competitive exclusion 21 Competition among distantly related species Competition can also occur between distantly related species if they consume a common resource E.g.) Desert ants and rodents Ants eat more of the smaller seeds Rodents more of the larger seeds 22 Concept check What does the competitive exclusion principle predict about the outcome of two species competing for the same resource or resources? Which likely has a stronger impact on population abundance/growth, intraspecific competition or interspecific competition? Why? 23  The theory of Key Concept competition is an extension of the logistic growth model 24 Competition for a single resources We can understand how populations of two species compete for a single resource by extending the logistic growth equation: 𝑑𝑁 𝑁 = 𝑟𝑁 1 − 𝑑𝑡 𝐾 Intraspecific competition 25 Competition for a single resources We can understand how populations of two species compete for a single resource by extending the logistic growth equation: 𝑑𝑁 𝑁 = 𝑟𝑁 1 − 𝑑𝑡 𝐾 To model interspecific competition, we need to consider the carrying capacity of the environment relative to the number of individuals of both species 26 Competition for a single resources We can understand how populations of two species compete for a single resource by extending the logistic growth equation: 𝑑𝑁1 𝑁1 + 𝛼𝑁2 = 𝑟1 𝑁1 1 − 𝑑𝑡 𝐾1 𝑑𝑁2 𝑁2 + 𝛽𝑁1 = 𝑟2 𝑁2 1 − 𝑑𝑡 𝐾2 27 Competition for a single resources α/β are called competition coefficients = variables that convert between the number of individuals of one species and the number of other individuals of the other species Think of it as unit conversion, but for different species 28 Competition for a single resources E.g.) Both rabbits and squirrels share a common food source, but rabbits need twice as much food per every squirrel Limiting resource can support 50 rabbits or 100 squirrels 50 𝑟𝑎𝑏𝑏𝑖𝑡𝑠 𝛼 (𝑟𝑎𝑏𝑏𝑖𝑡) = = 0.5 100 𝑠𝑞𝑢𝑖𝑟𝑟𝑒𝑙𝑠 100 𝑠𝑞𝑢𝑖𝑟𝑟𝑒𝑙𝑠 𝛽 (𝑠𝑞𝑢𝑖𝑟𝑟𝑒𝑙) = =2 50 𝑟𝑎𝑏𝑏𝑖𝑡𝑠 29  Zero population growth Competition for a single resources isocline (species 1) For species 1 equilibrium (i.e., zero growth) is reached when dN/dt = 0, or: 𝑁1 = 𝐾1 − 𝛼𝑁2 We can also calculate the x (all species 1) and y (all species 2) intercepts: 𝑁1 = 𝐾1 𝑁2 = 𝐾1 ÷ 𝛼 30  Zero population growth Competition for a single resources isocline (species 2) For species 2 equilibrium when: 𝑁2 = 𝐾2 − 𝛽𝑁1 x- and y-intercepts: 𝑁2 = 𝐾2 𝑁1 = 𝐾2 ÷ 𝛽 31 Predicting the outcome of competition We can overlay the two plots to see if one species will outcompete the other or if they can coexist Four possible outcomes 32 Predicting the outcome of competition Species 1 isocline If species 1 isocline > species 2 isocline Increase in species 1 Decrease in species 2 Continues until reaches equilibrium (open circle; all species 1) Species 1 persists; species 2 extinct Species 2 isocline 33  Species 2 isocline Predicting the outcome of competition If species 2 isocline > species 1 isocline Increase in species 2 Decrease in species 1 Continues until reaches equilibrium (open circle; all species 2) Species 2 persists; species 1 extinct Species 1 isocline 34 Predicting the outcome of competition If isoclines cross and K1 and K2 are the Species 2 wins most extreme points on axis Two equilibria Species 1 Outcome depends on combination of N1 wins and N2 One species will persist, the other will perish 35 Predicting the outcome of competition If isoclines cross, and K1 and K2 are the innermost points One equilibrium Species will coexist Coexistence most likely when interspecific competition weaker than intraspecific competition (i.e., when both α and β are less than 1) 36 Competition for multiple resources Species often compete for more than a single resource What happens when species compete for two resources and have overlapping niches? If species 1 is better able to sustain itself when both resources low (more competitive for either resource), will outcompete species 2 However, if species 1 is better when resource 1 is low, but species 2 is better when resource 2 is low, should coexist 37 Competition for multiple resources Do we see this? E.g.) Asterionella and Cyclotella diatoms (David Tillman) Both require silicate (SiO2) and phosphorus (P) Cyclotella can use SiO2 more efficiently Asterionella can use P more efficiently What happens when grown together at different concentrations of phosphorus and silicate? 38 Competition for multiple resources Hypotheses: At low ratio of SiO2:P (silicate-limited), Cyclotella dominates At high ratio of SiO2:P (phosphorus-limited), Asterionella dominates At intermediate ratios, both species coexist (limited by different resource) 39 Competition for multiple resources Silicate limited Intermediate Phosphorus limited 40 Concept check In the competition equations of population growth, why do we need to include competition coefficients? Under what conditions do we predict stable coexistence of two species competing for two resources? 41  The outcome of competition can be altered by abiotic Key Concept conditions, disturbances, and interspecific interactions 42 The outcome of competition can be altered by abiotic conditions, disturbances, and interspecific interactions In nature, competition occurs under many different environmental conditions and other types of interactions (e.g., predation/herbivory) These factors can alter the competition coefficients or even prevent a species from reaching carrying capacity Can affect outcome of competition 43 Abiotic conditions Ability to persist when resources scarce important to winning competition Can be overwhelmed by ability to persist under harsh abiotic conditions 44 Abiotic conditions E.g.) Chthamalus and Semibalanus barnacles Semibalanus better competitors for space Chthamalus better at surviving desiccation Trade offs 45 Disturbances Disturbances can also impact competitive interactions E.g.) Many ecosystems rely on frequent low- intensity fires to clear areas Fire suppression has led to changes in community compositions as plants that would be removed by fire persist and outcompete other species 46 Predation and herbivory Presence of predators/herbivores can affect competition Frequently, we see trade-offs between competitive ability and predator/herbivore defences Most competitive organisms usually most susceptible to predation/herbivory E.g.) Animals that move a lot may be able to eat more food, but more conspicuous to predators 47 Predation Studies of aquatic communities show that trade off between competitive ability and ability to avoid predation causes a reversal in the outcome of competition 48 Predation E.g.) Tadpoles and predatory newts Added 0-8 predatory newts to outdoor tanks with tadpoles, measured tadpole survival Active spadefoots win when no predators Docile peepers win with more predators 49 Herbivory Herbivores have a similar effect on competition E.g.) goldenrod Can grow to >1m, shade out shorter plants Specialist beetle, outbreak every 5- 15 years 50 Herbivory Plots sprayed with insecticide led to dominance of goldenrods In other plots, goldenrod eaten by beetles, led to smaller goldenrods and increase in survival of other plant species 51 Concept check How can abiotic conditions alter the outcome of competition? What is the underlying trade-off that allows predators and herbivores to reverse the outcome of competition? 52  Competition can occur through Key Concept exploitation, interference, or apparent competition 53 Competition can occur through exploitation, interference, or apparent competition Exploitative competition = individuals consume and drive down abundance of a resources such that other individuals cannot persist Indirect interaction Interference competition = competitors defend resources but do not immediately consume them Direct interaction Apparent competition = species appear to compete for a resource but limit each other due to some other mechanism 54 Interference competition: aggressive interactions Aggressive interactions occur between species of animals E.g.) long-legged ants and red harvester ants Compete for seeds, other insect-prey Long-legged ants emerge early in morning and plug entrances of red harvester ant nests, takes several hours to unplug Minimal competition during this time 55 Interference competition: allelopathy In allelopathy, organisms use chemicals to interfere with their competitors Can be phenotypically plastic E.g.) tall goldenrod Release polyacetylenes to inhibit growth of competitors 56 Apparent competition When two species have a negative effect on each other through an enemy (predator, parasite, herbivore) = apparent competition Looks like competition but actually occurs because of some other factor Occurs when organisms share a resource but don’t compete for that resource 57 Apparent competition E.g.) ring-necked pheasants and grey partridge Live in same area, similar diets Parasitic nematode infects both; few harmful effects on pheasant, but significant impacts on partridge 58 Apparent competition Sometimes, apparent competition occurs because one species facilitates enemy of another species 59 Apparent competition E.g.) sage and grasses in California Bare zone around sages, though to be allelopathy Cages around sage excluded rodents, caused increase in grasses around sage 60 Concept check How is interference a form of competition? Why is allelopathy considered a form of interference competition? 61 Next class Guest lectures: Ecological genetics 62