Predator -prey relationship
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This South American cactus became a widespread nuisance in Australia, making large areas of farmland unusable. When the moth, which feeds on this cactus, was introduced, it rapidly brought the cactus under control. Some years later both moth and cactus were rare, and it is unlikely that the casual observer would ever think that the moth had accomplished this.
Once the cactus became sufficiently rare, the moths were also rare, and unable to find and eliminate every last plant. Inadequate dispersal is perhaps the only factor that keeps the cactus moth from completely exterminating its principal food source, the prickly pear cactus.
Prey defenses can be a stabilizing factor in predator-prey interactions. Predation can be a strong agent of natural selection. Easily captured prey are eliminated, and prey with effective defenses that are inherited rapidly dominate the population.
Examples include camouflage in the peppered moth, and prey that are nocturnal to escape detection. Bats capture moths in flight, using sonar to detect them; some moths are able to detect incoming sonar, and take evasive action. Perhaps seriously unbalanced system simply disappear, and those that persist are ones in which the predator is not "too effective", likely because the prey has adaptations to reduce its vulnerability.
The availability of a second prey type -- an alternate prey -- can be stabilizing or destabilizing. Often a predator eats more than one prey.
If a predator switches between prey A and B on the basis of their frequency, it will eat A when B is rare and B when A is rare. The prey should exhibit mild oscillations, and the predator should fluctuate little. This would stabilize prey abundances.
However, if one prey species is abundant and the predator is unable to reduce its numbers, the result might be the maintenance of a continuously high predator density. Such an abundant predator might then eliminate a second prey species.
This is a destabilizing effect of an alternative prey. The hare-caribou-lynx relationship in Newfoundland is a complex example of such a destabilizing effect. Complex Interactions in Ecological Communities Predation can have far-reaching effects on biological communities. A starfish is the top predator upon a community of invertebrates inhabiting tidally inundated rock faces in the Pacific Northwest.
The rest of the community included mollusks, barnacles and other invertebrates, for a total of 12 species not counting microscopic taxa.
Relationships Between Organisms
The investigator removed the starfish by hand, which of course reduced the number of species to Soon, an acorn barnacle and a mussel began to occupy virtually all available space, out competing other species. Species diversity dropped from more than 12 species to essentially 2. The starfish was a keystone predatorkeeping the strongest competitors in check. Although it was a predator, it helped to maintain a greater number of species in the community. Its beneficial impact on species that were weak competitors is an example of an indirect effect.
When non-native species exotics invade an area, they often create "domino" effects, causing many other species to increase or decrease. The rainbow trout, beautiful, tasty, and beloved by anglers, has been purposefully spread to virtually all parts of the world where it can survive. In New Zealand, it has out-competed the native fishes, which now are found only above waterfalls that act as barriers to trout dispersal.
Because it is a more effective predator than the native fish species, the invertebrates that are prey to the trout are reduced in abundance wherever trout occur. Algae, which are grazed by the invertebrates, increase because of reduced grazing pressure. This is an example of a trophic cascade. Introduction of the opossum shrimp to Flathead Lake, Montana, is yet another example of complex interactions in ecological communities.
These linkages are the prime movers of energy through food chains and are an important factor in the ecology of populations, determining mortality of prey and birth of new predators.
Mathematical models and logic suggests that a coupled system of predator and prey should cycle: So the time, the horizontal axis is time. The vertical axis is population. And so let's just, in our starting point, let's say that our prey is starting out at a relatively high point.
Let's say we're right there in time, and let's say for whatever reason, our predator population is relatively low. So what do we think is going to happen here? Well, at this point, with a low density of predators, it's gonna be much easier for them for find a meal, and it's gonna be much easier for the prey to get caught. So since it's more easy, it's easier for the predators to find a meal, you can imagine their population starting to increase. But what's going to happen is their population is increasing.
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Well, it's gonna be more likely that they're gonna, they prey is gonna get caught. There's gonna be more of their hunters around, more of their predators around. So that population is going to start decreasing all the way to a point where if the population of the prey gets low enough, the predators are gonna have, they're gonna start having trouble finding food again, and so that their population might start to decrease, and as their population decreases, what's gonna happen to the prey?
Well, then, there's gonna be less predators around, so they might be able to, their population might start to increase. And so I think you see what's happening.
The predator and prey, they can kind of form this cyclic interaction with each other. And what I've just drawn, this is often known as the predator-prey cycle.
And I just reasoned through that you can imagine a world where you can have the cycle between predator and prey populations. But you can also run computer simulations that will show this, and even observational data out in the field also shows this.
One of the often cited examples is interactions between, between the snowshoe hare, which would be the prey in this situation, and the Canadian lynx, which would be the predator, the predator in this situation.
And you see a very similar cycle to what I just drew, kind of just reasoning through it, and this, right here, is actual data. You see the passage of time here, and this is a long passage of time. We're starting in the early 's going all the way to the early-mid 's.