BIOL 203 Predation and Herbivory Lecture Notes PDF

Lecture 14 Predation and Herbivory BIOL 203 November 20th, 2024 1 Learning objectives 1. Explain how communities are organized into food webs (Chapter 17) 2. Demonstrate how predators and herbivores can limit the abundance of populations (Chapter 13) 3...

Lecture 14 Predation and Herbivory BIOL 203 November 20th, 2024 1 Learning objectives 1. Explain how communities are organized into food webs (Chapter 17) 2. Demonstrate how predators and herbivores can limit the abundance of populations (Chapter 13) 3. Illustrate how populations of consumers and consumed populations fluctuate in regular cycles 4. Describe how predators and herbivores respond to food availability with functional and numerical responses 5. Explain how predation and herbivory favour the evolution of defences 2  Communities are Key Concept organized into food webs 3 Communities are organized into food webs We often categorize species in a community based on who eats who (what is a community?) Food chains = linear representation of how different species in a community feed on other Greatly simplify species interactions in a community 4 Communities are organized into food webs Food webs = complex and realistic representation of how species feed on each other in a community Include links among many species of producers, consumers, detritivores, scavengers, and decomposers 5 Trophic levels Food webs describe the feeding relationships of ecological communities Helps determine whether a species can exist in a community, whether it will be rare/abundant Species richness in food web often quite high; can be challenging to illustrate all the interactions To simplify, we often categorize species into trophic levels 6 Trophic levels Trophic levels = broad categories/levels in a food chain or food web of an ecosystem All organisms in a trophic level obtain energy in similar way Two main categories: Producers = autotrophic species that produce energy through photosynthesis or chemosynthesis (e.g., plants, phytoplankton) First trophic level in a food chain/web Consumers = heterotrophs that gain energy from consuming other organisms 7 Trophic levels Consumers can be broken up into several subcategories Primary consumers = eat producers Secondary consumers = eat primary consumers Tertiary consumers = eat secondary consumers Most commonly seen in aquatic systems 8 Trophic levels Some consumers eat dead organic matter Scavengers = eat dead animals Detritivores = break down dead organic matter/waste (detritus) into smaller particles Decomposers = break down organic matter into simpler elements/compounds that can be recycled through the system 9 Trophic levels Omnivores are difficult to place into a trophic level E.g.) crayfish eat plant matter (primary consumer), but can also eat other animals (secondary consumer) and can even feed on dead/decaying organic matter (scavenger/detritivore) 10 Trophic levels Within a trophic level, often group species that feed on similar items into guilds E.g.) Primary consumers can be grouped into leaf eaters, stem borers, root chewers, nectar sippers, bud nippers Members of a guild all feed on similar items but may not be closely related E.g.) Both hummingbirds and bees feed on nectar 11 Direct vs indirect effects Species can affect the abundance of another through direct effects = two species interact without the involving other species E.g.) predation, parasitism, competition, mutualism Many species are interconnected in a food web, and direct effects can have a ripple effect through a community When two species interact with one or more intermediate species = indirect effect When indirect effect initiated by predator = trophic cascade 12 Density-mediated indirect effects Indirect effects caused by density changes in an intermediate species = density-mediated indirect effect E.g.) Increased density of sea stars causes decline in mussels, opens up more space for barnacles 13 Trait-mediated indirect effects Indirect effects caused by changes in the traits of an intermediate species = trait-mediate indirect effects Commonly happens when predator causes prey to change its feeding behaviour E.g.) wolves in Yellowstone 14 Concept check What are trophic levels of communities? 15  Predators and herbivores can limit Key Concept the abundance of populations they consume (direct effects) 16 Predators and herbivores can limit the abundance of populations they consume (direct effects) Populations can be limited by what they eat and by what eats them Important to understand these interactions for management of crop pests, game populations, endangered species, etc. Also helps us understand the structure of ecological communities 17 Predator-prey interactions Predators often reduce prey populations below carrying capacity E.g.) Spiders and lizards in the Caribbean Small islands had only spiders, larger at spiders + lizards Small islands at 10x density of spiders 18 Predator-prey interactions To test if predation kept spider populations low, introduced a new species of spider to islands, track population size After 5 years, spiders 10x more abundant on islands without lizards 19 Mesopredators There are two levels of predators in many ecological communities: Mesopredators = relatively small carnivores that consume herbivores (e.g., coyotes, weasels, feral cats) Top predators = larger predators that consume both herbivores and mesopredators (e.g., cougars, wolves, sharks) Top predators often interfere with human activities, often leads to direct reduction/elimination → ~60% mesopredators increased range in North America as a result 20 Mesopredators Expansion in range and abundance of mesopredators has had dramatic effects on prey populations E.g.) Cownose ray and bay scallops Overharvesting of sharks has led to increase in rays and a dramatic decrease in scallop populations 21 Predator-prey interactions Sometimes, predators are accidently introduced to an area where they weren’t previously found = introduced species (exotic, non-native) If the population spreads rapidly with negative effects = invasive species E.g.) Brown rats in Haida Gwaii 22 Herbivore-producer interactions Herbivores can also have substantial impacts on species they consume Many also specialize of few species of producers E.g.) sea urchins in rocky shore communities Control algae populations, prevent any species from becoming too dominant 23 Herbivore-producer interactions The effects of herbivores can easily be observed E.g.) Sitka black-tailed deer in Haida Gwaii 24 Concept check How has the reduction of top predators had unintended consequences on the abundance of prey? 25  Populations of consumers and Key Concept consumed populations fluctuate in regular cycles 26 Populations of consumers and consumed populations fluctuate in regular cycles The abundance of predators/prey are often linked E.g.) Canadian lynx and snowshoe hare 9-10 year cycles 27 Creating predator-prey cycles in the lab Carl Huffaker Prey: six-spotted mite (Eotetranychus sexmaculatus) Predator: western predatory mite (Typhlodromus occidentalis) Created patchy habitat with oranges/rubber balls With no predators, prey mite reached population size of 5500-8000 With predators, prey mite wiped out 28 Creating predator-prey cycles in the lab Increased distant between patches, took longer but ultimately same result Made it even more difficult for predators to disperse and easier to prey to disperse to see if that could make both populations persist 29 Creating predator-prey cycles in the lab Prey can disperse using silk threads/wind → adding dowels for jumping off points Predators can only walk → added Vaseline maze to slow them down 30 Creating predator-prey cycles in the lab Population sizes began to regularly cycle Predator-prey can only coexist when prey have dispersal advantage Population cycles achieved when environment is complex, and predators cannot easily find scarce prey 31  Mathematical models of predator-prey cycles Lotka-Volterra model incorporates oscillations in the abundances of predator and prey populations Models rate of change in both prey/predator populations Where: N = prey population size 𝑑𝑁 P = predator population size = 𝑟𝑁 − 𝑐𝑁𝑃 r = intrinsic growth rate of prey 𝑑𝑡 c = capture efficiency Prey population growth Loss of prey to predators 32 Mathematical models of predator-prey cycles Growth rate of predators can also be modelled Where: 𝑑𝑃 N = prey population size = 𝑎𝑐𝑁𝑃 − 𝑚𝑃 P = predator population size 𝑑𝑡 a = efficiency of converting prey into offspring c = capture efficiency Births Deaths m = per capita mortality for prey 33 Changes in prey population We can use the Lotka-Volterra model to determine when prey populations are stable: 0 = 𝑟𝑁 − 𝑐𝑁𝑃 𝑟𝑁 = 𝑐𝑁𝑃 𝑃 =𝑟÷𝑐 So prey populations stable when number of predators equal ratio between prey’s growth rate and predator’s capture efficiency 34 Changes in prey populations We can also examine what causes prey to increase/decrease When: 𝑃 𝑚 ÷ 𝑎𝑐 𝑁 < 𝑚 ÷ 𝑎𝑐 𝑁 = 𝑚 ÷ 𝑎𝑐 Stable Predators Increase Predators Decrease 37 Trajectories of predator and prey populations # prey when predators stable # predators when prey stable Equilibrium isocline (zero growth isocline) = population size of one species that causes the population growth of another to be stable 38 Trajectories of predator and prey populations Joint population trajectory = simultaneous trajectory of predator and prey populations Join equilibrium point = point at which equilibrium isoclines for both predator and prey meet Both populations stable 39 Trajectories of predator and prey populations When small predator but large prey population: Both populations increase 40 Trajectories of predator and prey populations When large predator and large prey population: Predators increase Prey decrease 41 Trajectories of predator and prey populations When large predator but small prey population: Both decrease 42 Trajectories of predator and prey populations When small predator and small prey population: Predators decrease Prey increase 43 Trajectories of predator and prey populations 44 Trajectories of predator and prey populations Join equilibrium point = point at which equilibrium isoclines for both predator and prey meet Both populations stable 45 Concept check When predator and prey populations cycle, why do the predators of small prey cycle faster than the predators of larger prey? Based on the predator-prey population equations, why is the prey population stable when rN = cNP? 46  Predators and herbivores respond Key Concept to food availability with functional and numerical responses 47 Predators and herbivores respond to food availability with functional and numerical responses Lotka-Volterra model relies on simplified version of nature Does not include time delays, density dependence, does not incorporate real behaviour To be more realistic, must consider functional responses and numerical responses 48 Functional responses Functional response = relationship between density of prey and an individual predator’s rate of food consumption Concept also applies to herbivores and producers Three potential responses 49 Type I functional response Predator’s rate of prey consumption increases linearly with an increase in prey density until predator is satiated E.g.) web-building spiders Functional response used by Lotka-Volterra model 50 Type II functional response Number of prey consumed slows as prey density increases and then plateaus (satiation) Incorporates handling time = amount of time it takes to capture and subdue prey E.g.) shore birds eating crabs 51 Type III functional response Prey consumption increases very slowly at low prey density, rapid consumption at moderate density, slowing consumption at high density (what growth curve does this look like?) Three factors account for initial slow uptake of prey 52 Type III functional response First, at low prey densities, prey can hide in refuges where they are safe from predators Predators can only consume prey once all refuges taken 53 Type III functional response Second, at low prey densities, predators are less practiced at locating and catching prey As prey numbers increase, predators learn to locate/identify particular species for effectively = search image Learned mental image that helps the predator locate and capture food 54 Type III functional response Third, prey switching may occur at low densities Occurs when one species is very rare, so predator prefers another species that is more abundant (what type of foraging strategy is this related to?) Can switch back to first species if their numbers increase 55 Type III functional responses in nature No search image E.g.) Backswimmer (Notonecta glauca), isopods (Asellus aquaticus), and larval mayflies (Cloeon dipterum) First offered both species, then manipulate mayfly proportions 20- 80% At low densities, only 10% successful captures At high densities, 30% successful 56 Numerical responses A numerical response is a change in the number of predators through population growth or population movement due to immigration/emigration Predator populations grow slowly relatively to prey, but mobile species can move rapidly when prey densities change E.g.) During outbreaks of spruce budworm, populations of bay-breasted warblers (Dendroica castanea) increase dramatically Rapid increase in predators has ability to regulate prey abundance 57 Concept check What causes the differences between a predator exhibiting a type II versus a type III functional response? What can we conclude about the importance of handling time in predators that exhibit a type I functional response? 58  Predation and Key Concept herbivory favour the evolution of defences 59 Defences against predators To understand prey defences, need to understand predator hunting strategies Can be categorized as either: Active hunting = predator spends most time moving and looking for prey (e.g., insectivorous birds) Ambush hunting (sit-and-wait) = predators lie in wait for prey to pass by (e.g., chameleon) 60 Defences against predators Hunting is a series of events: 1. Detect prey 2. Pursue prey 3. Catch prey 4. Handling prey 5. Consuming prey Prey have evolved defences to stop predators at various points 61 Behavioural defences Common behavioural defences include alarm calling (common in birds/mammals), spatial avoidance (reduce chance of contact), and reduced activity (less likely to be noticed) E.g.) Tadpoles and predatory dragonflies 62 Crypsis Another way to avoid detection is through camouflage = crypsis 63 Structural defences Some prey use mechanical defences to reduce predator’s ability to capture, attack, or handle prey E.g.) quills on porcupines Some can be phenotypically plastic, only produce when predator present (why not all the time?) E.g.) Water fleas can chemically detect predators, develop spines to deter consumption 64 Structural defences Some species can change their overall body shape in response to predators E.g.) crucian carp (Carassius carassius) Develop large, hump (muscle) in presence of predators, allows them to accelerate faster 65 Chemical defences Prey can also use chemical defences to deter predators E.g.) skunks Also includes toxins E.g.) monarch butterfly Some produce powerful attacks E.g.) bombardier beetles 66 Chemical defences While effective, many species communicate their chemical defences before predator attacks Many develop conspicuous patterns/colours = aposematism (warning colouration) Predators learn to avoid markings 67 Mimicry of chemical defences When predators avoid aposematic species, they tend to avoid all species that look similar If a palatable species resembles an unpalatable species = Batesian mimicry 68 Mimicry of chemical defences Sometimes, several unpalatable species evolve similar warning patterns/colouration = Müllerian mimicry E.g.) Poison dart frogs (Ranitomeya spp.) 69 Costs of defences against predators Many types of defences against predators are costly Behavioural defences such as spatial avoidance can result in reduced feeding/increased crowding Mechanical defences are energetically expensive to produce Costs can become so high that growth/reproduction is reduced even if prey aren’t being consumed 70 Costs of defences against predators Chemical defences also probably costly to produce (though we aren’t sure) E.g.) Ladybird beetles Aposematic, produce alkaloids that make them unpalatable Only beetles that ate large amounts of food could produce carotenoids (red pigment), alkaloids 71 Counter adaptations of predators Predators can also adapt to prey defences (evolutionary arms race; what does this resemble?) When two or more species affect each other’s evolution = coevolution E.g.) porcupine, bobcats, and wolverines Learned to flip porcupines upside down to avoid spines 72 Counter adaptations of predators Some predators evolved to handle toxins produced by prey E.g.) cane toads (Bufo marinus) and black snakes (Pseudechis porphyriacus) Snakes exposed to cane toads the longest had the lowest reduction in swimming speed 73 Defences against herbivores Selective pressures from herbivores has also caused the evolution of defences against herbivory in producers Some defences are induced by herbivore attacks (phenotypic plasticity) Others fixed Some herbivores have evolved counter-adaptations; can become so good they specialize only a single species 74 Structural defences Plants have evolved a number of structural defences Sharp spines, prickles Wooly layer of hair (trichomes) make them difficult for insects to get past 75 Chemical defences Wide-variety of chemical defences in plants Sticky resins, latex compounds, alkaloids (caffeine, nicotine, morphine), tannins Many herbivores have developed tolerance E.g.) Tahitian noni, “vomit fruit” 76 Tolerance to being eaten Instead of evolving defences, some plants just tolerate herbivory Rapid re-growth of consumed tissues E.g.) When animals eat plants, often just eat top meristem (region of the plant where most growth occurs) Lower meristems experience increased growth to compensate 77 Cost of herbivore defences Researchers have investigated whether plant defences come at a cost of reduced fitness If trait is phenotypically plastic, can compare fitness in induced and non- induced plants 78 Cost of herbivore defences E.g.) tobacco (Nicotiana sylvestris) Respond to herbivory by producing chemical defences (includes nicotine) Induced plants produced fewer seeds than non-induced plants 79 Concept check What are four general ways that prey have evolved to reduce their risk of being killed? Why does natural selection favour counteradaptations to prey defences? What are three ways in which plants have evolved to reduce their risk of being killed by herbivores? 80 Next class Competition (Chapter 15) Next week: Seminar 3 – Changes in Communities Poster presentations See Moodle for presentation order Email me your posters at least 24 hours in advance of your seminar 81