(4/15) Week 13 - Interspecific Exploitation Notes

Interspecific Exploitation (predator/prey)

Introduction

• Strong links exist between populations, especially between herbivore and plant, predator and prey, and parasite/pathogen and host.
• These interactions increase the fitness of one individual (herbivore, predator, parasite, or pathogen) while reducing the fitness of the other (plant, prey, or host).
• These relationships are termed exploitative interactions, summarized as a + / - relationship.
• The number of exploitative interactions between species is much greater than the number of species.
  • All species are food for other species and hosts to parasites and pathogens.
  • Example: Lake Okeechobee, Florida, has 500 species linked by approximately 25,000 exploitative interactions (50x the number of species).
• Consumption includes herbivory, parasitism, and predation.
• Case study: Moose on Island Royale
• Population graphs of moose and wolves show similar patterns but out of phase, resembling the Lotka-Volterra Model of Predator-Prey Interactions.
  • The model predicts that as prey increases, they become more available to predators.
  • Predator numbers increase, consuming prey at a greater rate than prey production, leading to a decrease in prey numbers.
  • Predator reproduction lags behind prey consumption, causing predator numbers to continue increasing even after prey numbers decrease (population inertia).
  • Predator numbers decline due to lack of food, eventually allowing the prey population to rebound.
  • When the prey population is large enough, predators start finding prey at a rate great enough to cause an increase in predator numbers, and the whole cycle repeats itself.

Consumption

Herbivores

• Herbivory involves the consumption of plant material by an animal, which may reduce plant fitness (+/-) or increase it (+/+).
  • Mutualism (+/+) involves eating fruits and dispersing seeds, or eating nectar and dispersing pollen.
• Exploitative herbivory involves an animal eating various plant parts (seeds, fruit, flowers, leaves, stems, bark, roots, sap).
• Most herbivores restrict their foraging to specific areas of a plant.
• Herbivory in this lecture refers to the general consumption of plants by animals.
• Food is generally more available for herbivores than carnivores because plants are more available as food than animals.
• High-energy plant foods (seeds, fruit, flowers) are the least available, while low-energy plant parts (wood, bark, branches) or moderate-energy parts (leaves, grasses, algae) are more abundant.
• Herbivore digestive tracts are longer than those of carnivores due to the need to digest complex plant components.

Carnivores

• Carnivory involves the killing and consumption of other animals.
• Prey may be partially or entirely consumed.
• Predators hunt and eat when they are confident they are not being pursued themselves.
• Witnessing predation requires remaining unnoticed by both the predator and its prey.
• Animals represent a higher quality food item (protein, fat) than plants.
• Mechanical and chemical breakdown of animal tissue requires fewer and simpler digestive specializations.
• Carnivore digestive systems have a relatively lower biomass compared to herbivore digestive systems.
• Specializations for carnivory include those for catching, subduing, and killing prey.
  • Examples: claws, talons, refined senses, large brains, specialized teeth, venom glands, and morphological adaptations for speed and prey capture.
• Predators utilize a specific search image (visual and chemical focus on the most abundant/rewarding prey type).
• Search images increase predator success by allowing them to notice small parts of hidden or camouflaged prey.
• Searching for multiple prey types simultaneously leads to confusion and reduced success.

Parasites

• Parasitism typically does not lead to the death of an individual.
• Parasitism is the relationship where one individual (the parasite) lives in (endoparasite) or on (ectoparasite) another organism (the host).
• The host is usually not killed but experiences reduced fitness.
• It is not in the parasite’s best interest to kill its host, as it depends on it for food and protection.
• Parasites and parasitoids are not the same.
• Parasites generally do not kill their hosts.
• Parasitoids behave like parasites but typically kill their host.
• Parasitoids often include insects that lay eggs in other insects, with the larvae consuming the host as they grow.

Defenses

Introduction

• Prey actively avoid predators and have evolved defenses.
• Plants have also evolved defense strategies.
• Behavioral ecologists distinguish between standing (constitutive) and inducible defenses.
• Standing defenses are permanently present in the prey, regardless of predator presence.
• Inducible defenses appear as a result of predator pressure, directly (e.g., herbivore eating a plant) or indirectly (e.g., predator presence).

Plant defenses

• Between 15% and 70% of terrestrial plants are eaten by herbivores annually, varying by ecosystem.
• Plant defenses prevent the consumption of 100% of plants.
• There are costs associated with possessing defense strategies.
• Mechanical defenses:
  • Tough epidermis: seed shells and bark on branches/trunks.
  • Entanglement devices: thick waxy cuticle and plant hair on leaves/stems that entangle and deter small herbivores.
  • Piercing devices:
    • Cutting edges in “cutgrass” (silica).
    • Spines and needles in deciduous plants and cacti.
  • Polymers within tissues: Cellulose and Lignin are indigestible by herbivores on their own.
• Chemical defenses:
  • Secondary metabolites: derivatives of plant metabolism that deter herbivory.
  • Secondary metabolites deter common herbivores and may not affect other herbivores.
  • Spices (cinnamon, ginger, paprika, oregano) contain chemicals evolved to avoid herbivory but are palatable to humans in small quantities.
  • Phenolics (tannins, flavonoids): reduce protein digestion, slow growth, and block cell division.
  • Terpenes (mints, citrus): block ion transmission across membranes, cause dermatitis, and interfere with animal hormone action.
  • Alkaloids (morphine, codeine, caffeine, cocaine): block ion channels, interfere with neurotransmission, inhibit enzymes, and cause dizziness/vomiting.

Animal Defenses

• Mechanical/behavioral defenses of prey:
  • Retaliation: porcupine quills, stingray spine, zebra kick.
  • Startling behavior: underwing of some moths with hind wing eye spots looking like owl face.
  • Deflection of attack to non-vital area: 4-eyed butterfly fish with eyespots on the tail.
  • Large size: elephant, buffalo
  • Death feigning: most carnivores avoid eating aged flesh to avoid toxins/disease of rotted flesh. Example: Hog-nose snake, opossum
  • Fleeing: quickly as deer, antelope; Retreat to burrow in ground hogs.
  • Crypsis: becoming hard to locate
  • Clustering: Safety in numbers and confusion effect
• Chemical defenses:
  • Nausea induction: skunk scent, hydrogen cyanide production by ladybugs.
  • Monarch butterfly example.

Host defenses

• Consider the Nasonia vitripennis case study.

Counter adaptations

Herbivores

• Selection of plant parts with fewer defense mechanisms.
  • Example: Gray squirrels eating acorns.
• Tough tongue, mouth, and gut.
  • Examples: cactus-eating tortoises, wood rats, and giraffes who eat the prickly underside and outer crown leaves of Acacia trees.
• Mastication apparatus and grinding mills to breakdown tough seed coats and resistant tissue:
  • Examples: grinding teeth, continually growing teeth, gizzards in birds, regurgitation of cud and re-mastication in many hoofed mammals and rodents.
• Countering chemical defenses:
  • Cellulase evolution: bacteria, protists, and fungi evolved the enzyme cellulase to breakdown cellulose into glucose.
  • Alkaline pH: Spongy moth caterpillars have an alkaline stomach, preventing tannins from binding to protein.
• Form a mutualism with another species that can digest the defense:
  • Internal gut symbiosis of animals with bacteria and protists that breakdown cellulose.
  • Examples: Ruminant herbivores, hoofed mammals, rodents, some birds, and termites have microbial partners in their stomachs or caeca.
• Eat sub-toxic amounts of plants and move on to another species.
• Eat plants before induced defenses are activated and then abandon the plant.
  • Example: Gypsy moth larvae eat enough oak leaf material before oak trees increase tannin levels.

Carnivores

• Overcoming prey mobility:
  • Social carnivory (lions).
  • Pursuit: short distance (cats) or long distance (wolves) dichotomy of adaptations.
  • Luring (angler fish lure and snapping turtle tongue).
  • Sit and wait (praying mantis) and surprise pounce/grasp/bite.