18 Species Interactions Shorter
Plant Defenses
Mechanical Defenses:
Thorns and spines protect against herbivory.
Tough plant fibers deter herbivores.
Silica found in grasses and palms adds hardness.
Chemical Defenses:
Plants produce secondary metabolites that are not part of primary energy-generating pathways.
Alkaloids:
Examples include nicotine (tobacco), morphine (poppies), cocaine (coca), and caffeine (coffee).
Often bitter or toxic.
Phenolics:
Example: lignin in wood and tannins in leaves.
Terpenoids:
Found in peppermint; can deter herbivores.
Examples of Defensive Features:
Thorns on rose stems provide a mechanical barrier.
Alkaloids in tobacco act as chemical deterrents.
Phenolics in tea provide taste and health benefits.
Terpenoids in peppermint offer flavor and potential herbivore resistance.
Host Plant Resistance
Definition:
The ability of plants to prevent herbivory through chemical or mechanical defenses.
Agricultural Significance:
Understanding plant defenses helps in improving crop resistance.
Commercial development of resistant plants may take long and can increase vulnerability to other pests.
Current Usage:
Around 75% of U.S. cropland utilizes pest-resistant varieties.
Example:
Bt corn is genetically modified to produce Bt toxin from Bacillus thuringiensis, effective against specific insect pests.
Bt Toxin Mechanism
Activation of Toxin:
Toxins are activated by gut enzymes of insect larvae.
Ingestion Process:
Crystals and spores are consumed, damaging midgut membranes, leading to starvation or septicemia.
Mechanism:
Activated toxins bind to gut receptors, causing ion and small molecule leakage, leading to death.
Herbivore Adaptation to Plant Resistance
Detoxification Pathways:
Oxidation:
Catalysis of secondary metabolites into alcohol by mixed-function oxidases in mammals' and insects' livers and midguts.
Conjugation:
Combines oxidized products with additional molecules to produce inactive compounds for excretion.
Parasitism in Ecology
Definition:
One organism benefits while the host is usually not killed outright.
Types of Parasites:
Some remain attached for life, while others have complex life cycles requiring multiple hosts.
Behavioral Manipulation:
Many parasites can alter host behavior to facilitate their transmission.
Parasitic Flowering Plants
Holoparasites:
Lack chlorophyll, entirely reliant on host plants for water and nutrients.
Example: Rafflesia arnoldii, known for its large flower, lives mostly within its host.
Hemiparasites:
Can photosynthesize but still depend on hosts for water and minerals.
Example: Mistletoe (Viscum album), capable of parasitizing trees.
Classification of Parasites
Host Range:
Monophagous: Feed on a few related hosts.
Polyphagous: Feed on many different species.
Size:
Microparasites: Multiply within host cells (e.g., bacteria, viruses).
Macroparasites: Live in hosts but release juveniles outside (e.g., tapeworms).
Living Site:
Ectoparasites: Reside on the exterior (e.g., fleas).
Endoparasites: Reside inside the host (e.g., bacteria).
Effects of Parasites
Prevalence:
Parasites may outnumber free-living species significantly.
Impact:
Removal of parasites can lead to increased host population densities, e.g., in studies with blue tit birds affected by parasitic blowfly larvae.
Mutualistic Relationships
Mutualism:
Both species benefit from the interaction (e.g., pollination).
Commensalism:
One organism benefits while the other is unaffected (e.g., seed dispersal via barbs).
Types of Mutualism
Resource-based Mutualism:
Both species gain resources (e.g., leaf-cutting ants and their fungus).
Defensive Mutualism:
One species provides protection in exchange for food/shelter (e.g., ants and aphids).
Dispersive Mutualism:
Involves transport of pollen/seeds with food rewards (e.g., plants and pollinators).
Commensalism Examples
Organsims that benefit without affecting their host, such as epiphytes or cattle egrets following livestock to catch stirred insects.
Cheating in Commensalism:
Some plants mimic flowers to attract pollinators without offering nectar.
Interactions Among Ecological Processes
Predation and Competition:
Predators reduce competition by affecting competitive species.
Parasitism and Competition:
Parasites may impact sympatric species, affecting interspecific interactions through indirect effects.
Bottom-Up and Top-Down Control
Bottom-Up Control:
Plant quality and abundance affect herbivores and their predators.
Top-Down Control:
Predators and parasites influence prey populations.
Trophic Cascade:
The presence or absence of any level in the food chain can impact all other levels.
Bottom-Up Control Evidence
Energy Transfer Properties:
Energy conversions are not 100% efficient, resulting in less energy at higher trophic levels.
Nitrogen-limitation Hypothesis:
Herbivores respond to nitrogen content; plants with higher nitrogen availability yield larger herbivore populations.
Top-Down Control Evidence
Natural enemies like predators regulate prey populations, offering insights through predator removal/addition studies, e.g., wolf reintroductions in the U.S. market.