Chapter 14 – Species Interactions
Consumer-resource interactions organize biological communities into food chains; consumers benefit individually and
increase their numbers; resource populations decrease; populations are controlled from above and below their place on the
food chain by consumers and resources, respectively.
Mutualism: (+/+) interaction where both parties involved benefit (e.g. bees and flower pollination). Competition: (-/-)
interaction where two species share a resource and reduce the availability of it for the other species. Commensalism:
(+/0) interaction where one party benefits and the other is unaffected (e.g. bird builds its nest in a tree). Amensalism: (-/0)
one party suffers and the other is unaffected (e.g. you unknowingly crush an insect underfoot). Symbiosis: species
interaction referring to individuals who live in close association, mutualistic or parasitic.
Types of consumers: predator, captures prey and consumes it, kills prey; parasite, attaches itself or lives within a host,
consuming parts of its body, usually doesn’t kill host; parasitoid, live and mature within a host, eventually killing the
host; herbivores, consume plant material, are either predators (when they kill the plant) or parasites (when they graze or
browse); detritivores, consume dead organic matter, commensalistic (0/+), important for nutrient recycling.
Resource populations adapt many evolutionary tactics, suited to their environment, to avoid being captured by consumers.
As the fitness of both depends on the delicate relationships between consumer and resource, evolutionary responses
constantly readjust the relationship. There is, however, a cost to resource populations when they modify their behaviour to
avoid being captured (they have less habitat, food resources, time of day they can be active, etc.).
Parasites may live on or within the bodies on their hosts. Some parasites cause deadly or debilitating illness while some
may cause only moderate inconvenience and others still may actually be beneficial to their hosts (e.g. Buchnera eats a
host’s carbohydrate reserves but produces essential amino acids). Most parasites have complicated life cycles to survive
whatever life they have outside of a host and complicated evasive tactics to keep from being detected and destroyed by a
host. Virulence: capacity of a parasite to invade host tissue (infectiousness). Cross-resistance: when the body develops
antibodies which function against several varieties of an illness (consider smallpox and cowpox).
Plants, while they cannot flee like many resource population, employ other tactics of defense. Among them are: low
nutritional value of their leaves, stems, etc., indigestibility, chemicals which may harm consumers and structural
deterrents (spines, thorns, etc.). Secondary components are used by plants not for metabolism, but for other purposes like
defense. There are three categories based on their chemical structure: nitrogen compounds, derived from amino acids;
terpenoids, oil, latex, resin; phenolics, many have antimicrobial properties. Plants may also maintain high levels of
defensive chemicals called constitutive defenses or produce them only when predated: induced defenses. Plants may
also produce defenses when chemically alerted by neighbouring plants (chemical communication). For many plants, there
is a high metabolic cost to produce these defense chemicals; however, there is likewise a high metabolic cost to
consumers to circumvent them.
It is important to consider not simply direct consumer-resource interactions, but also their indirect repercussions (e.g. a
predator consumes an herbivore, subsequently benefiting certain plants as there are fewer herbivores to consume them).
Competition often has a strong, indirect effect on interactions. Trophic cascade: illustrates that interactions are often felt
across multiple trophic levels.
It is important to consider that interactions are not fixed to one category, illustrated by competition-facilitation
continuums. Facilitation is where one species allows the livelihood of another species with no detriment to itself. These
interactions may become competitive as the facilitated species develops, e.g. nurse plants offer shade and protection to
other species as they grow, but once the protected species matures, it may compete for water and nutrients with its nurse
Mutualistic interactions are extremely important for nearly all species. Generally species are behaviourally or
physiologically modified to facilitate each other in complementary ways. There are three types: trophic mutualism,
where two species facilitate nutrient gathering or digestion for each other (e.g. bacteria that within human intestines eat
food and allow digestion); defensive mutualism, where one species receives food or shelter from a partner in return for
defending them from consumers (e.g. cleaner species unburden an individual of parasites while deriving nutrition from
said parasites); dispersive mutualism, where one species disperses its partners pollen, eggs, etc. while deriving nutrition
from the process, often, the species are not closely associated (e.g. pollinator interactions between plants and bees).