Predation Flashcards
Predation
Predation – consumption of one organism (prey) by another (predator) in which the prey is alive and attacked by the predator, excludes detritivory (consumption of dead matter)
>50% of species obtain energy by feeding on other organisms in a variety of types of interactions
All = exploitation = one organism benefits by feeding on and directly harming another
Taxonomic classification (3 levels) - carnivores consume animals, herbivores consume plants, omnivores consume from both trophic levels
Functional classification (4 levels) – true predators, grazers, parasitoids and parasites
True predators kill prey immediately after attack, may kill and consume many different prey types or individuals – e.g cheetahs, eagles, sharks, seed-eating rodents, krill-eating whales, aphid-eating coccinellids
Grazers attack large amounts of prey but only remove a part of each individual, harmful but rarely lethal,– e.g large herbivores like wildebeest, small herbivores like rodents and insects
Parasites consume part of their prey (host), harmful but rarely lethal in the short term, an intimacy of association, only one or few hosts in a lifetime – e.g tapeworm, liver fluke, plant galls
Parasitoids – a group of insects, mainly Hymenoptera, free-living adults lay eggs in/on other insects, larvae live within host until they emerge and destroy the host
Traits of true predators
Broad diets
Some forage throughout their habitat in search of food
Others sit-and-wait and remain in one place and attack prey that move within striking distance or enter traps
Some true predators conc foraging on whatever prey is most abundant and may tend to switch from one prey type to another, switching may occur because the predator forms a search image of the most common prey type and orients toward that prey or learning enables it to become increasingly efficient at capturing the most common prey
Traits of herbivores
Mostly small insects specialised on one type of plant part, most commonly leaves and feed on a narrow range of plant species
Adaptations to/for predation
Organisms have evolved a wide range of adaptations that help them capture food and avoid being eaten
Before 530 million years ago, when first macroscopic predators came about, the seas were dominated by soft-bodied organisms, within a few million years many prey had evolved defences, such as body armour and spines
Predators exert strong selection pressure on their prey – if prey are not well defended, they die
Physical defences – large size, rapid or agile movement, armour
Other species contain toxins and aposematic warning colouration that predators instinctively/learn to avoid
Sometimes there is a trade-off between behavioural and physical defences – e.g crabs use claws to crush snail shells but snails have evolved defences including thicker shell and reduced shell aspect ratio (ratio of shell height to width) and some can even detect crab odours and retreat when crabs are present
Herbivores exert selection pressure on plants
Plants respond – avoidance, tolerance, defences
Avoidance – on a large time scale, some produce huge numbers of seeds in some years and hardly any in other years (masting), plants hide in time from seed eating herbivores, then overwhelm them by sheer numbers
Tolerance – on a shorter time scale, producing leaves at times of the year when herbivores are scarce, compensation = growth responses that allow the plant to compensate for herbivory, removal of plant tissue stimulates new growth, removal of leaves can decrease self-shading, resulting in increased plant growth, removal of apical buds may allow lower buds to open and grow
For some plants, herbivory can be a benefit in some circumstances – e.g in field gentiants herbivory in the growing season results in compensation but does not later in the season, if too much material is removed, or there are not enough resources for growth, compensation cannot occur
Defences – plants have an array of structural defences, including tough leaves, spines and thorns, saw-like edges and pernicious hairs that can pierce the skin
Secondary compounds are chemicals that reduce herbivory
Some are toxic to herbivores, others attract predators or parasitoids that will attack the herbivores
Induced defences are stimulated by herbivore attack (secondary compounds and structural mechanisms)
Changes in a single gene can alter both the level of herbivory and the community of herbivores, also showed the power of combining molecular genetic techniques with ecological field experiments and being able to examine the effects of particular genes in a natural setting
Improvement in defence adaptations exert strong selection pressure on predators and herbivores
For any defence mechanism of a prey species, there is usually a predator with a countervailing offense
e.g cryptic prey may be detected by smell or touch instead of sight, some predators subdue prey with poison, some use mimicry, blending into their environment so prey or predators are unaware of their presence, some have inducible traits
Plant defences can be overcome by herbivores, many have digestive enzymes that allow them to tolerate plant toxins
Effects of predation of communities
Predation and herbivory effect ecological communities greatly – in some cases, causing a shift from one community type to another
All exploitative interactions have the potential to reduce the growth, survival, or reproduction of the organisms that are eaten
Herbivores can decimate food plants
Saprotrophs
Saprotrophs – organisms that make use of dead organic matter
Do not control the rate of availability of resources
Dependent on the rate of another force (senescence, illness, fighting, leaf-drop)
Decomposers (bacteria and fungi) and detritivores (animal consumers of dead matter)
Decomposers
Break down dead matter
Can include plant matter and animal waste (skin, fur, feathers) as well as dead bodies
Release of energy and mineral nutrients
Complex organism molecules are broken down to yield CO2, water and inorganic nutrients
Lower activity in low temperatures, poor aeration, scare soil water and acidic conditions
Detritivores
Consume dead matter
Taxonomically diverse group
In terrestrial environments classified according to size
Some animal communities are composed almost exclusively of detritivores – forest floor, shaded streams, deep oceans, caves
Recycling of organic matter
Very important ecosystem service providers
