Organisms and Populations -3 Flashcards
define predation
Organisms which catches and kills another species for food.
* E.g.- Tiger and deer, sparrow eating any seed.
eoclogical role of predators:
- Predators act as ‘conduits’ for energy transfer across trophic levels and play other important roles.
-They keep prey populations under control.
But for predators, prey species could achieve very high population densities and cause ecosystem instability.
-When certain exotic species are introduced into a geographical area, they become invasive and start spreading fast because the invaded land does not have its natural predators.
- Biological control methods adopted in agricultural pest control are based on the ability of the predator to regulate prey population.
- Predators help in maintaining species diversity in a community by reducing the intensity of competition among competing prey species.
example of ecological role of predator: helps control of invasive exotic species
The prickly pear cactus introduced into Australia in the early 1920’s caused havoc by spreading rapidly into millions of hectares of rangeland.
Finally, the invasive cactus was brought under control only after a cactus-feeding predator (a moth) from its natural habitat was introduced into the country.
example of ecological role of predator: helps maintain species diversity
In the rocky intertidal communities of the American Pacific Coast, the starfish Pisaster is an important predator.
In a field experiment, when all the starfish were removed from an enclosed intertidal area, more than 10 species of invertebrates became extinct within a year because of interspecific competition.
why are predators said to be prudent
If a predator is too efficient and overexploits its prey, then the prey might become extinct and following it, the predator will also become extinct for lack of food.
This is the reason why predators in nature are ‘prudent’.
what are mechanisms evolved by prey species to avoid predators
- Prey species have evolved various defenses to lessen the impact of predation.
- Some species of insects and frogs are cryptically-coloured (camouflaged) to avoid being detected easily by the predator.
- Some are poisonous and therefore avoided by the predators.
- The Monarch butterfly is highly distasteful to its predator (bird) because of a special chemical present in its body.
- The butterfly acquires this chemical during its caterpillar stage by feeding on a poisonous weed.
who are predators for plants? why are plants at an disadvantage?
- For plants, herbivores are the predators.
- Nearly 25 per cent of all insects are known to be phytophagous (feeding on plant sap and other parts of plants).
- The problem is particularly severe for plants because, unlike animals, they cannot run away from their predators.
morphological/ chemical defences against predators by plants
- Thorns (Acacia, Cactus) are the most common morphological means of defense.
- Many plants produce and store chemicals that make the herbivore sick when they are eaten, inhibit feeding or digestion, disrupt its reproduction, or even kill it.
- The weed Calotropis produces highly poisonous cardiac glycosides, which is why you never see any cattle or goats browsing on this plant.
-A wide variety of chemical substances that we extract from plants on a commercial scale (nicotine, caffeine, quinine, strychnine, opium, etc.) are produced by them actually as defenses against grazers and browsers.
Define competition
- Competition is best defined as a process in which the fitness of one species (measured in terms of its ‘r’ the intrinsic rate of increase) is significantly lower in the presence of another species.
-When Darwin spoke of the struggle for existence and survival of the fittest in nature, he was convinced that interspecific
competition is a potent force in organic evolution.
Misconceptions About Competition
It is generally believed that competition occurs when closely related species compete for the same resources that are limiting, but this is not entirely true.
i) Unrelated Species Competition:
- Totally unrelated species could also compete for the same resource.
- For instance, in some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton in the lake.
ii) Non-Limiting Resources:
- Resources need not be limiting for competition to occur.
- In interference competition, the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of the other species, even if resources (food and space) are abundant.
Laboratory Evidence of Competition
- It is relatively easy to demonstrate competition in laboratory experiments.
- Gause and other experimental ecologists showed that when resources are limited, the competitively superior species will eventually eliminate the other species.
competitive exclusion in nature
- Evidence for such competitive exclusion occurring in nature is not always conclusive, but strong circumstantial evidence exists in some cases.
The Abingdon tortoise in Galapagos Islands became extinct within a decade after goats were introduced on the island, apparently due to the greater browsing efficiency of the goats.
Gause’s Competitive Exclusion Principle
- Gause’s principle states that two closely related species competing for the same resources cannot co-exist indefinitely, and the competitively inferior one will be eliminated eventually.
- This may be true if resources are limiting, but not otherwise.
- More recent studies do not support such generalisations about competition.
- While they do not rule out the occurrence of interspecific competition in nature, they point out that species facing competition might evolve mechanisms that promote co-existence rather than exclusion.
Evidence of Competition in Nature: Competitive Release
- A species whose distribution is restricted to a small geographical area because of the presence of a competitively superior species is found to expand its distributional range dramatically when the competing species is experimentally removed.
- Connell’s field experiments showed that on the rocky sea coasts of Scotland, the larger and competitively superior barnacle Balanus dominates the intertidal area and excludes the smaller barnacle Chathamalus from that zone.
-In general, herbivores and plants appear to be more adversely affected by competition than carnivores.
Mechanism of Co-existence: Resource Partitioning:
- One mechanism to promote co-existence is resource partitioning.
- If two species compete for the same resource, they could avoid competition by choosing, for instance, different times for feeding or different foraging patterns.
- MacArthur showed that five closely related species of warblers living on the same tree were able to avoid competition and co-exist due to behavioural differences in their foraging activities.
