● When `color{violet}("Darwin")` spoke of the struggle for existence and `color{violet}("survival")` of the fittest in nature, he was convinced that `color{violet}("interspecific competition")` is a potent force in `color{violet}("organic evolution")`.

● It is generally believed that competition occurs when closely related species compete for the `color{violet}("same resources ")` that are limiting, but this is not entirely true.

● Firstly, totally unrelated species could also compete for the `color{violet}("same resource.")`

● For instance, in some shallow South American lakes visiting `color{brown}("flamingoes ")` and resident `color{brown}(" fishes")` compete for their common `color{violet}("food,")` the `color{violet}("zooplankton")` in the `color{violet}("lake")`.

● `color{violet}("Secondly, resources")` need not be limiting for competition to occur; in `color{violet}("interference competition")`, the `color{violet}("feeding efficiency")` of one species might be reduced due to the `color{violet}("interfering")` and `color{violet}("inhibitory presence")` of the other species, even if resources (`color{violet}("food and space")`) are abundant.

● Therefore, `color{violet}("competition")` is best defined as a process in which the `color{violet}("fitness")` of one species (measured in terms of its `color{violet}("‘r’")` the `color{brown}("intrinsic rate of increase")` ) is significantly lower in the presence of another species.

● It is relatively easy to demonstrate in `color{violet}("laboratory experiments")`, as `color{violet}("Gause")` and other `color{violet}("experimental ecologists")` did, when resources are limited the competitively superior species will `color{violet}("eventually eliminate")` the other species, but evidence for such competitive exclusion occurring in nature is not always conclusive.

● Strong and `color{violet}("persuasive circumstantial evidence")` does exist however in some cases.



● The `color{brown}("Abingdon tortoise ")` in `color{violet}("Galapagos Islands")` became extinct within a `color{violet}("decade")` after goats were introduced on the island, apparently due to the greater `color{violet}("browsing efficiency")` of the goats.

● Another evidence for the `color{violet}("occurrence of competition")` in nature comes from what is called `color{brown}("competitive release")`.

● A species whose `color{violet}("distribution is restricted")` to a small `color{violet}("geographical area")` because of the presence of a `color{violet}("competitively superior")` species, is found to expand its `color{violet}("distributional range")` dramatically when the competing species is experimentally removed.

● `color{brown}("𝘊𝘰𝘯𝘯𝘦𝘭𝘭’𝘴")` elegant field experiments showed that on the `color{violet}("rocky sea")` coasts of Scotland, the larger and competitively superior barnacle `color{brown}(" barnacle 𝘉𝘢𝘭𝘢𝘯𝘶𝘴")` dominates the `color{violet}("intertidal area")`, and `color{violet}("excludes ")` the smaller barnacle `color{brown}("𝘊𝘩𝘢𝘵𝘩𝘢𝘮𝘢𝘭𝘶𝘴")` from that zone.



● In general, `color{violet}("herbivores")` and `color{violet}("plants")` appear to be more adversely affected by `color{violet}("competition")` than `color{violet}("carnivores.")`

● Gause’s `color{brown}("‘Competitive Exclusion Principle’")` states that two closely related species `color{violet}("competing")` for the same resources cannot co-exist indefinitely and the `color{violet}("competitively inferior")` one will be `color{violet}("eliminated eventually.")`

● This may be true if `color{violet}("resources')` are `color{violet}("limiting,")` but not otherwise.

● More recent studies do not support such `color{violet}("gross generalisations")` about competition.

● While they do not rule out the occurrence of `color{violet}("interspecific competition")` in nature, they point outthat species facing competition might `color{violet}("evolve mechanisms")` that promote co-existence rather than exclusion.

● One such `color{violet}("mechanism")` is `color{brown}("Resource partitioning.")`

● If two species compete for the same resource, they could `color{violet}("avoid competition")` by choosing, for instance, different times for feeding or different `color{violet}("foraging patterns.")`

● `color{brown}(" MacArthur")` showed that five closely related species of `color{brown}("warblers ")` living on the same tree were able to avoid competition and co-exist due to `color{violet}("behavioural differences")` in their color{violet}("foraging activities.")`

● Considering that the `color{violet}("parasitic mode")` of life ensures free `color{violet}("lodging")` and `color{violet}("meals")`, it is not surprising that`color{violet}(" parasitism")` has evolved in so many `color{violet}("taxonomic groups")` from `color{violet}("plants")` to higher vertebrates.

● Many parasites have evolved to be `color{brown}("host-specific ")` (they can parasitise only a single species of host) in such a way that both host and the parasite tend to `color{brown}("co-evolve")`; that is, if the `color{violet}("host evolves")` special `color{violet}("mechanisms")` for rejecting or resisting the parasite, the parasite has to `color{violet}("evolve mechanisms")` to counteract and neutralize them, in order to be successful with the same host species.

● In accordance with their life styles, parasites evolved `color{violet}("special adaptations")` such as
`star` The loss of `color{brown}("unnecessary sense organs")`
`star` Presence of `color{brown}("adhesive organs or suckers ")` to cling on to the host
`star` Loss of `color{brown}("digestive system ")`
`star` High `color{brown}("reproductive capacity.")`

● The life `color{violet}("cycles of parasites")` are often complex, involving one or two intermediate `color{violet}("hosts")` or `color{violet}("vectors")` to facilitate parasitisation of its `color{violet}("primary host.")`

● The `color{brown}("human liver fluke ")` (a trematode parasite) depends on two intermediate hosts (a `color{brown}("snail ")` and a `color{brown}("fish")`) to complete its life cycle.

● The malarial `color{violet}("parasite")` needs a vector `color{violet}("(mosquito)")` to spread to other hosts.

● Majority of the `color{violet}("parasites harm")` the host; they may reduce the `color{violet}("survival, growth")` and `color{violet}("reproduction")` of the host and reduce its `color{violet}("population density.")`

● They might render the host more `color{violet}("vulnerable")` to `color{violet}("predation")` by making it `color{violet}("physically weak")`.

● Parasites that feed on the `color{violet}("external surface")` of the `color{violet}("host organism")` are called `color{brown}("ectoparasites.")`

● The most familiar examples of this group are the `color{brown}("lice on humans ")` and `color{brown}("𝘵𝘪𝘤𝘬𝘴 𝘰𝘯 𝘥𝘰𝘨𝘴.")`

● Many `color{violet}("marine fish")` are infested with `color{violet}("ectoparasitic copepods")`.

● `color{brown}("𝘊𝘶𝘴𝘤𝘶𝘵𝘢,")` a parasitic plant that is commonly found growing on `color{violet}("hedge plants,")` has lost its `color{violet}("chlorophyll")` and `color{violet}("leaves")` in the course of `color{violet}("evolution.")`

● It derives its nutrition from the `color{violet}("host plant")` which it `color{violet}("parasitises.")`

● The `color{violet}("female mosquito")` is not considered a parasite, although it needs `color{violet}("our blood")` for `color{violet}("reproduction.")`

● In contrast, `color{brown}("endoparasites ")` are those that`color{violet}(" live inside")` the `color{violet}("host body')` at different sites (liver, kidney, lungs, red blood cells, etc.).

● The life cycles of `color{violet}("endoparasites')` are more complex because of their `color{brown}("extreme specialisation.")`

● Their `color{violet}("morphological and anatomical")` features are greatly simplified while `color{violet}("emphasising")` their `color{violet}("reproductive potential.")`

● `color{brown}("Brood parasitism ")` in `color{violet}("birds")` is a fascinating example of `color{violet}("parasitism")` in which the `color{violet}("parasitic bird")` lays its eggs in the nest of its host and lets the host incubate them.

● During the course of `color{violet}("evolution, the eggs")` of the `color{violet}("parasitic bird")` have evolved to resemble the host’s egg in size and colour to reduce the chances of the `color{violet}("host bird")` detecting the foreign eggs and ejecting them from the nest.