Lecture 4 Review 1) Liebig’s law of the minimum says the growth of a population of organisms will increase until the supply of a critical resource becomes limiting 2) Results of intraspecific competition are reduced body size or growth and reduced fitness 3) The competitive exclusion principle says when two species compete for identical, limited resources, one will eventually eliminate the other

Lecture 4 Review 4) The two types of interspecific competition are interference and exploitation 5) Character displacement-through time two closely related species tend to become distinct morphologically and therefore use different portions of limiting resources 6) Connell’s experiments on barnacles

Lecture 4 Review 7) In competitive release the niche of the competitively-inferior species expands in the absence of the competitively-superior species 8) In character displacement two competing species diverge in a trait that reduces the strength of interspecific competition

Review Questions Define or compare/contrast: intra- vs. interspecific competition; the competitive exclusion principle; Gause; exploitative vs. interference competition; niche breadth Explain how character displacement can take place over evolutionary time Describe Connell’s classic experiment Explain competitive release

Predation

Predation – one species feeds on another  enhances fitness of predator but reduces fitness of prey (+/– interaction)

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Charles Elton (1942) British ecologist who studied mammalian population data. He concluded that oscillations were common and suggested that predators regulate prey populations. Cycles caused by predator –prey interactions prey predators

Lotka – Volterra Predation model Mathematicians who modeled predator-prey interactions wondered: - to what extent do predators cause these cyclic fluctuations (are they ½ responsible? Completely responsible?) - do predators keep prey populations below K? (If so then no reason to believe completion is important because resources would not be limiting) - if predators are so efficient, why don’t the prey populations go extinct?

Predator-Prey Interaction Model Changes in prey population N = # of prey r1 = intrinsic rate of increase in prey populations k1 = constant that measures the ability of the prey to escape predators (0-1) P = # of predators r1N =density independent growth - k1 PN = reflects the negative effect of predators on prey

Predator-Prey Interaction Model Changes in predator population dN dt r P k NP = - + 2 r2 = death rate in predator population k2 = constant that measures the ability of the predators to capture prey (0-1) P = # of predators - r2P = negative of density independent growth (drag on predator populations Assumption: no density dependent effects (no carrying capacity, no competition) only predation

Predator-Prey Interaction Model This model did produce an oscillation between prey and predator populations (thus, it appeared to reflect natural situations). However, more complicated models showed that they were math. unstable. Gause : 1st test of model – observed two species of protozoans (prey and predator) grown on an oat medium. Predator always totally consumed the prey, then starved to death.

Predator-Prey Interaction Model Gause : By adding sediment to the oat medium (habitat complexity) for hiding places for the prey (paramecium). Predators starved to death, then the prey populations increased dramatically. Gause concluded that the cycles seen in nature are the result of constant migration, because he could not get coexistence in his experiments.

Carl B. Huffaker (1950’s) Insect ecologist Experimented with predators and their prey species that fed on commercially important citrus crops. He concluded that Gause’s experiments were too simple to reflect nature. Studied predator and prey mite spp. on oranges. Prey fed on oranges and predators fed on prey. Lab arenas had oranges in rectangular trays and densities of predators/prey were manipulated. Also increased habitat complexity by adding rubber balls and vasoline barriers. oranges only: predators ate prey and then starved to death. oranges, balls and vasoline: complexity allowed coexistence

C. S. Holling (1960’s) Conducted studies on the components of predatory interactions (acts among individual organisms) Worked with vertebrates and invertebrates (entomologist, concerned with the outbreaks of insects in forests which denuded trees.) Functional response – relationship between prey density and the rate at which an individual predator consumes prey Numerical response- increase in predator numbers with increases in prey abundance

(for individual predators) Components of Functional Responses (for individual predators) Rate of successful search a. ratio of the speed of predator to prey b. size of the field of reaction of the predator (distance at which the predator can perceive the prey) c. success rate of capture (does a predator get a prey every time he encounters it)

Why are predators size-selective? Encounter frequency: Encounter of large prey is higher than small prey Reaction distance —how close to the fish does a prey item have to be for the fish to see it and react to (eat) it? Pumpkinseed Lepomis gibbosus Confer and Blades 1975 (L&O)

Two types of Ceriodaphnia It is not just size that matters, it is overall visibility Zaret 1972 Two types of Ceriodaphnia Big eye Small eye Fish always took the big-eye form. Artificially made small-eye morph more visible by feeding them india ink. Predation rate increased

(for individual predators) Components of Functional responses (for individual predators) Time available for hunting verses other activities other activities necessary for an organism to carry on to reproduce is going to influence the amount of time available to hunt a. avoiding other predators b. time looking for a mate c. patrolling a territory

(for individual predators) Components of Functional Responses (for individual predators) 3. Time spent handling prey amount to time it takes to capture a prey after recognizing a potential food source a. pursuit of prey b. subduction of prey c. eating prey d. digesting prey 4. Hunger level – function of the size of the gut of the predator and the time spent in digestion and assimilation (rapid versus slow metabolsim)

Type I: Functional Response Linear increase; same assumptions as the Lotka-Votera growth models No examples of Type I functional response curve observed in natural systems

Type II: Functional Response Leveling off of # of prey eaten even though the # of prey increases (satiation of predators or time spent hunting prey) Holling found Type II curve with invertebrate predators

Type III: Functional Response Lag period: even as density of prey increases the # of prey eaten doesn’t increase dramatically (thought to result from the formation of search image by predators) These were all the results of laboratory experiments that Holling conducted. People later found a few examples of Type II and more examples of Type III.

Search image – when prey are rare there is no value in hunting for them. Only when the prey population increases above some threshold level does the predator form a search image and begin to recognize that prey item as a valuable food source. The predator then focuses on and exploits that food source heavily.

Numerical response – when there is a large increase in prey density, the predators present can become satiated as prey densities increase and the rate of prey eaten is not going to increase for each individual predators. However, if predators are added to the population increased exploitation of the prey can occur (due to immigration not reproduction)

Switching If predators exploit prey populations heavily and drive prey populations down, eventually prey densities will decline below some threshold value and predators will switch to another prey item. If switching occurs then more than one prey species can coexist (many studies have found switching to take place).

Murdoch (1960’s) Conducted the 1st switching experiments – examined gastropods that feed on mussels and barnacles and found that switching took place. -best candidate for switch occurred when the predator exhibited a weak preference for prey species

Prudent Predators Predators can drive prey pops. to extinction. But there is some optimal level of predation intensity that will maximize the # of predators without driving the prey extinct. It has been suggested that predators might “manage” prey populations and that this might explain why predators and prey usually coexist. Problem: individuals must cooperate with each other. But why not cheat? Evidence: predators can be prudent without “altruistic behavior” because exploitation of prey is determined by the ability of predators to capture them. And which individuals are usually removed from prey populations?

Effects of predation on morphology, distribution and abundance Change in size structure of prey population (if predator prefers the largest individuals in a prey population) Brooks and Dodson 1965 (over 1350 citations) Lakes in North America When fish introduced there were huge changes - predators preferred the larger zooplankton small zooplankton became dominant large phytoplankton become abundant

Effects of predation on morphology, distribution and abundance Decreases in overall diversity – if predators are very efficient at removing prey, they drive populations to extinction which reduces diversity Increase in diversity – in simple systems with few prey species, one of which is a dominant competitor. If a predator prefers the dominant competitor it can reduce the number of the dominant competitors, allowing the inferior competitors to exist. All three of these can occur in “ecological time” = one to a few generations

Paine 1966 Effect of Pisaster on intertidal assemblage 15 species coexist in intertidal Food web: Pisaster starfish the dominant consumer

Experimental Design 8 x 2 m Plots in intertidal Control Pisaster removal Monitored changes over one year

Results Control plot: no change Removal plot: 80% barnacles (3 months) Mussels starting to dominate (1 yr) Species diversity decreased 15 to 8 spp. Predicted mussels would dominate available space

Produced the concept of the Conclusions Pisaster interrupts successional process After removal, superior competitor dominates Produced the concept of the “keystone predator” Limitations: no replication; did not examine smaller fauna (can be very diverse)

Keystone species paradigm Pisaster become known as a “keystone species” Paine (1966) cited 850 times 1970 –1979 Defined as “a single native species, high in the food web … which greatly modifies the species composition and physical appearance of the system” (Paine 1969)

Is the keystone a useful concept? Paine intended the term as metaphor – he rarely used it Others picked up on it -particularly conservation biologists (conserving keystones to maintain diversity) Problems: - identifying them - can be context dependent - may overlook other important species Criticized by Hurlburt (1997) and others

Results Take home message Pisaster more important at exposed sites Other sites: diffuse predation, with strong effect shared among species “Keystone” effect context dependent Why? – low productivity Predation can regulate assemblage structure - Directly: influences prey distribution - Indirectly: can mediate competition Keystone concept – beware of generality Take home message

Effects of predation on morphology, distribution and abundance Morphological modifications – inference from observation a. protective devices (spines on sea urchins; strong shells)

Effects of predation on morphology, distribution and abundance Morphological modifications – inference from observation b. mimicry – organisms that resemble unpalatable species (usually because they contain toxic compounds)

Effects of predation on morphology, distribution and abundance Morphological modifications – inference from observation c. crypsis – organisms match the color and shading of their habitats. Believed this morphology shaped by predatory pressure over time.

Artificial camouflage Decorator crabs put algae on their backs, which increases their survival In areas with Dictyota algae, crabs use this species for decoration, but rarely food

Optimal Foraging Important because increased food intake results in - types of feeding behaviors that would maximize food intake rate. Important because increased food intake results in larger and healthier organisms with more energy for growth and reproductive output. (maximize fitness) Is taking the largest prey item the best strategy?

With abundant prey, bigger is better 2. Optimal foraging —take the prey that provides the greatest energy return for cost of capture/handing. Werner and Hall (1974) Ecology With abundant prey, bigger is better Fed fish choice of three sizes of Daphnia magna

Elner and Hughes (1978) Used predatory green crabs (Carcinus maenas) and mussels. Several different sizes of mussels were offered (small, medium and large). Feeding trials measured the amount of energy gained/unit time.

Manipulation: Fixed the proportion of different sized mussels but varied the overall abundance (# of each size class in a given area) Observed: Proportion of each size class eaten under different abundances. Conclusion: The crabs are foraging in a size-selective manner AND they get more selective at higher abundances However, they still sample unprofitable size classes

Goss-Custard (1977) Studied size selection of worms eaten by redshanks (Tringa totanus). Redshanks ate prey of 7mm more than any other size even though prey of 8mm were more common. Thus, worms were eaten in proportion to their net energy benefit --not in relation to their abundance. Also…. When large #’s of worms available birds were selective When small #’s of worms were available all were consumed (take what you can get)

Why may organisms not follow predictions of Optimal Foraging Theory The idea that organisms maximize energy uptake is an assumption – other factors may be involved (e.g., predator avoidance). An organism may not maximize energy consumption because of the need to minimize predation risk. 2. Organisms may not be able to detect all available prey. 3. Caloric value may not account for all needed resources (essential vitamins). Optimal Foraging needs to be thought of as a concept not a theory.

Inducible versus Constitutive defenses A bryozoan makes spines when placed in contact with a predatory nudibranch. A hydrozoan, Hydractinia, produces defense stolons armed with nematocysts when in contact with another colony.

Inducible Defense: The conical (right) and bent (left) forms of the acorn barnacle Chthamalus anisopoma. The animal develops the bent form if predatory snails are present.

Threat of predation leads to: Mytilus edulis (Blue mussel) Threat of predation leads to: Thicker shells Leonard et al (1999) Smith & Jennings (2000) Larger adductor muscle Reimer & Tedengren (1996) Increased gonad ratios Reimer (1999) Increased byssus volume Cote (1995) Mytilus edulis also undergoes inducible changes when exposed to predator effluent however it is unlikely that you will ever see it pictured in a textbook. The mussel on the left was exposed to water-borne cues from the sea star Asterias for 3 months and the one on the right was a control. Now any differences you perceive between these mussel shell are probably not the result of phenotypic plasticity However in the presence of predators blue mussels are known to: 50

Predation: Indirect Effects Non-lethal effects Injury by browsing predators Trait-mediated indirect interactive effects (TMII) Risk averse foraging More shelter dwelling in the presence of predators Can produce larger effects than consumption does Trophic cascades

Predation: Indirect Effects Non-lethal effects Injury by browsing predators Trait-mediated indirect effects (TMII) Risk averse foraging More shelter dwelling in the presence of predators Can produce more dramatic effects than actual predation does Trophic cascades 54

Dugongs can modify the structure of seagrass beds through their foraging Tiger sharks cause dugongs to change habitats, which can affect seagrass communities

Predation: Indirect Effects Non-lethal effects Injury by browsing predators Trait-mediated indirect effects (TMII) Risk averse foraging More shelter dwelling in the presence of predators Can produce more dramatic effects than actual predation does Trophic cascades 56

Trophic Cascade in Kelp Forests When the keystone sea otter is removed, sea urchins overgraze kelp and destroy the kelp forest community. Figure 5.15b

Emergent Multiple Predator Effects (MPEs) Types of interactions among predators (Soluk and Collins, 1988): Neutral: predators do not affect one another’s rates of prey consumption Negative (interference): combined prey consumption less than neutral values  MPE Positive (facilitation): combined prey consumption greater than neutral values  MPE

Parasitism A two species interaction in which one species (parasite) lives in or on a second species (host) for a significant period of time and obtains its nourishment from it.

Parasitism (cont’d) Viruses, bacteria and fungi Protozoa, arthropods, helminths (nematodes, cestodes, trematodes, and acanthocephala) Parasites are ubiquitous and should probably be considered in every ecological study (but aren’t)

Parasitism cont’d Parasite classifications Ectoparasites- live attached to or embedded in the external body surface (gills, body walls etc) of an organism Endoparasites lie within the body, and may occupy circulatory vessels or internal organs

Parasitism (cont’d) Parasite benefits, the host loses Effects on host Reduced feeding efficiency Depletion of food reserves Reduced reproduction Lowering of disease resistance

Isopods

Isopods 64

Fish Lice (Branchiurans) 65

Acanthocephalans In fish intestine

Behavior and parasitology 67

Parasites exploit natural patterns of host behavior to maximise transmission 68

This is especially important for parasites with an indirect life-cycle The types of host behavior that can be exploited by parasites is variable, but usually involves feeding / foraging. This is especially important for parasites with an indirect life-cycle The Gasterosteus -Schistocephalus system Stickleback Free-swimming coracidium Copepod sp. 69

Ex 2: A tale of two fishes… Atlantic halibut Hippoglossus hippoglossus Diurnal forager Rests during night (sand) Infected by the worm: Entobdella hippoglossus Common sole Solea solea Nocturnal forager Rests during day (sand) Infected by: E. soleae Parasites are closely related, but cannot successfully infect the ‘wrong’ host 70

Sole Halibut E. Hippoglossus hatching E. Soleae hatching Parasites lay sticky eggs that adhere to sand particles, near their potential hosts Eggs of E. hippoglossus and E. soleae exhibit opposite hatching periodicity, which match host activity patterns 71

We know that host behaviour can influence parasite infections Therefore…..variation in host behaviour patterns can create variation in parasite infection levels

Hosts can evolve behavioral resistance as a response to infection threat 73

Behavioral resistance ‘the first line of defence’ Behavioral Defense Prevention better than cure. Least energetically demanding defense. But there are also….. Structural Defenses Skin of Red Sea cling fish can produce enough anti-parasite mucus to cover its entire body in a few minutes. Crinotoxic fishes (sedentary) have epidermal toxins that protect against parasites. Immunological Defenses Immune defense is impt, BUT energetically expensive, with negative effects on growth, repro and maintenance. 74

Herbivory Herbivory is a special case of predation herbivory differs from predation in that the prey cannot move (remember that much herbivory in the ocean is really predation)

The importance of herbivory to nearshore ecosystems It is the first step in the transfer of energy in nearshore food webs It provides a major trophic link for the cycling of nutrients within these food webs It often affects the productivity and structure of plant communities

Plant Community Shifts due to Herbivory Increases prevalence of species with: Low nutritive value (low nitrogen) Chemical Defenses (secondary compounds) Structural Defenses (calcareous skeletons) Shifts in functional groups (from erect fast growers to prostrate slow growers) mutualisms between grazers and host plants

Why is the world green? Why don’t herbivores consume more of the plants that are available to them? Maybe herbivores aren’t food limited (predators control herbivore density) alternatively the plants are not really as available as they appear to us, either because they have chemical or morphological defenses (spines) or they are nutritionally inadequate

Herbivory: Secondary compounds Secondary compounds are not part of primary metabolic pathways so they must be synthesized at some cost to the plant The primary function of these compounds is controversial: some view them as waste products of plant metabolism that are only coincidentally toxic others suggests they are so costly as to have evolved specifically to thwart herbivores

Herbivory: Secondary compounds cont. Types of chemical defenses: quantitative examples include tannins and resins which occupy as much as 60% of the plants leaf dry weight these compounds are thought to deter specialized herbivores qualitative comprise < 2% of a plants leaf dry mass examples include alkaloids and phenols deter generalist herbivores

Herbivory:Secondary compounds (2) The amount of energy invested in a plant depends on the vulnerability of the tissue growing shoots and leaves are more heavily defended than old leaves usually compounds are concentrated near the surface of the plant Because these compounds are expensive to produce some plants have the ability to turn the production of these compounds on and off (induced defenses) in as little as 12 hours after a bout of herbivory only studied for a few marine species

Secondary compound tradeoffs Reduced competitive capability for the defended plant Grazer may seek out defended plants to sequester these compounds for their own defense Some grazers are well equipped to defeat chemical defenses

The impact of herbivores on plant communities How much plant production is taken: in macroalgae on reefs and phytoplankton in some estuaries virtually all of the Net Aboveground Primary Production (NAPP) is consumed by herbivores in seagrass and mangroves usually less than 30% of NAPP is consumed (but more in past). Bucktooth Video

Population Interactions Competition (--) when both species suffer from an association Predation (+-) when one benefits and one suffers Commensalism (+0) when one species benefits from another Amensalism (-0) when one species negatively affects another Mutualism (++) when both species benefit from another

Mutualisms Both species benefit One species provides nutrition while the other provides either protection or cleaning services Examples include: Clownfish-anemone Giant clams/corals-zooxanthellae Goby-shrimp Decorator crabs-sponges, tunicates and anemones Deep sea worms-sulphur metabolizing bacteria

Mutualisms Classifications of mutualisms Obligatory At least of the species can not live in the absence of the other Facultative Each species can survive singly but quality of life is much less

Mutualisms How did mutualisms evolve? Most think they develop as a result of some intense negative interaction (parasitism or predation) Organisms had two options Escape the interaction Adapt to the interaction

Mutualisms Benefits of mutualisms Each species grows, survives, or reproduces at a higher rates in the presence of the other

Mutualisms Mutualisms less important when resources are plentiful They are more common in stressful environments Benefits maybe density-dependent

Cleaner Stations An experiment conducted with cleaner fishes and larger predators Cleaners feed on ectoparasites In some cases parasites within the mouth When cleaners are removed parasite infestation increase within a very few hours Cleaner stations are sites of very high species richness

Cleaners in Hawaii http://www.youtube.com/watch?v=OMFiI2at1p4

Another example gobies and snapping shrimp

Another example Sea anemone - protection against predators Clown fish highly evolved to survive cnidarian nematocysts Mucus - thicker & lacks sialic acid groups which trigger nematocyst discharge.

Algal-Invertebrate Mutualisms Found in protozoans, sponges, cnidarians, asciidians, flatworms, and mollusks. Alga generally lose motility during symbioses and may lose cell walls Animal hosts may change behavior – e.g. Cassiopeia, Convoluta, Tridacna Relationships seeming to be mutualisms (trading food for food, protection, oxygen, carbon dioxide) Vertical or horizontal transmission possible

Algal-Invertebrate Symbioses, cont.

Population Interactions Competition (--) when both species suffer from an association Predation (+-) when one benefits and one suffers Commensalism (+0) when one species benefits from another Amensalism (-0) when one species negatively affects another Mutualism (++) when both species benefit from another

Commensalism Facultative commensal e.g. – barnacles

The Remora fish (Echeniedea) has its dorsal fin modified as a sucker-like attachment organ. It attaches to the sides of larger fish and turtles using them as transport hosts but in addition, obtains food fragments dropped from the host.

Trophic amensalism Amensalism is the opposite of mutualism, and occurs when one organism alters the environment such that another type of organism cannot live there. Deposit feeding organisms cause bioturbation, making suspension feeding more difficult or impossible. This is amensalism among trophic levels, or Trophic amensalism.

Ghost shrimp burrows Prevent clams from living because the loose sediment around burrows clogs their feeding apparatus

Population Interactions Competition (--) when both species suffer from an association Predation (+-) when one benefits and one suffers Commensalism (+0) when one species benefits from another Amensalism (-0) when one species negatively affects another Mutualism (++) when both species benefit from another

Back to the Rocky Intertidal Early work by Connell (1970) Conducted a 9 year study of barnacles and predatory whelks (San Juan Island, WA) 103

Observations Juveniles barnacles Adult barnacles Predatory Whelks 104

Model Results Model: Lower limit of adults caused by predation H1: Excluding predators in low areas leads to presence of adults Experiment: predator exclusion cages on a pier piling Results Midshore level: excluding whelks resulted in an adult population Lowshore: small whelks got into cages 105

Conclusion Predation controls lower limit of barnacle population Contrast with Connell (1961), where competition controlled lower limit of Chthamalus Reason for difference? - predation reduced density below which competition could occur - space not limiting 106

Menge (1994) Effects of Pisaster under different conditions Transplanted mussel clumps Hypothesis: Pisaster will consume mussels at all locations where it is present 107

Experimental Design Boiler Bay Strawberry Hill Sheltered Exposed Pred No pred Pred No pred Pred No pred Pred No pred Turf Bare Replicates (n=5) 108

Reaction distance translates to overall volume searched, which influences vulnerability of the prey Reaction distance = radius of sphere Longer radius = higher encounter rate