Chapter 14 Notes—Species Interactions
Consumer-Resource interactions organize biological communities into food chains, along which food energy is passed through the ecosystem.
• Types: predator-prey, herbivore-plant, and parasite-host
• Energy and nutrients move up a food chain, but populations are controlled both from below by resources and from above by consumers.
• Mutualism—interaction between two species that benefit both
• Competition—two consumers share the same resource, reducing the availability of the resource to the other
• Commensalism—one benefits, the other is unaffected (i.e. birds building nest in tree)
• Amensalism—one suffers, the other is unaffected (i.e. elephant stomping on insect)
Types of consumers include:
• Predator—capture individuals and consume them
• Parasite—consumes part of its living prey organism (host)
• Parasitoid—larvae consume the tissues of living hosts, leading to its eventual death
• Herbivores—eat whole plants or parts of plants
• Detritivores—consume dead organic material
Resource organisms have many tactics to avoid being eaten by consumers, such as hiding, escape, producing defensive chemicals, or having defensive physical characteristics such as thorns.
Parasites are usually smaller then their hosts, and can live either on the outside or inside of their hosts. Those living inside a larger organism enjoy a benign physical environment regulated by their hosts. Virulence is a measure of the capacity of a parasite to invade host tissues and proliferate in them. Cross-resistance occurs when a parasite causes an immune response that causes other similar parasites to face a large number of antibodies stimulated by the first parasite. This can considerably moderate the effect of the subsequent parasite infections.
Hosts can recognize and destroy parasites often by inflammation responses and the production of antibodies by the immune system. This response takes time to develop, however, providing an opportunity for the parasite to multiply within the host.
Plants will use structural defenses (hairs, tough seed coats, sticky gums), an inherently low nutritional value of most plant tissues, and toxic compounds (i.e. secondary compounds) as defense mechanisms against herbivores. Secondary compounds include nitrogen compounds (indigestible structural compounds such as lignin), terpenoids (oils, latex, and resins), and phenolics (antimicrobial properties). Defensive chemicals can either be maintained at high levels in plant tissues at all times (constitutive defenses) or activated by herbivore damage (induced defenses). There can often be a trade-off between the production of defensive chemicals and plant growth.
Interactions between resource and consumer individuals can have indirect consequences for other species. These can be positive (as when a predator reduces a herbivore population, stimulating plant growth) or negative (as when two or more consumers use a single resource, with each one reducing the availability of the resource for the other).
Individuals of one species can facilitate the germination and growth of a second species—nurse plants. This is a common theme in the development of biological communities, as when initial colonizers of newly exposed soil provide shade and organic soil materials to allow other species to invade the area. This relationship can shift the species from being commensal to being competitors as the new plants grow and use up more water/nutrients or produce more shade.
In most cases, each party to a mutualism is specialized behaviorally or physiologically to perform a function lacking in the other. Mutualisms can be particularly important under stressful conditions. There are three types of mutualisms: trophic (specialized in complementary ways to obtain energy and nutrients), defensive (species receive food or shelter from their partners in return for defending those partners against their consumers), and dispersive (i.e. animals transporting pollen or seeds).