Species Interactions

Survival for living organisms is not just responding to physical environmental factors. It also involves interactions with other organisms. Two species may interact as predator and prey or parasite and host. In this interaction, one species benefits (+) and the other is negatively (-) affected. A predator is typically larger than its prey and kills it, whereas a parasite is typically smaller than its host and usually does not kill it or does not kill it immediately.

PREDATION

Early observations indicated that predator- prey relationships were cyclical. This idea gained support from analyses of fur trapping records of the Hudson's Bay Company. The number of furs purchased at the Company's forts was meticulously recorded, for well over 100 years.

An analysis of the numbers of snowshoe hares, and one of their main predators, the lynx, shows predator-prey cycle. Peaks and valleys can be easily observed at roughly 8-10 year intervals.

Predation can be a strong agent of natural selection. Prey defenses can be a stabilizing factor in predator-prey interactions. 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.

Predation can have far-reaching effects on biological communities. A starfish is the top predator upon a community of invertebrates living in tidally inundated rock faces in the Pacific Northwest. The rest of the community includes mollusks, barnacles and other invertebrates, for a total of 12 species. When the the starfish were removed, 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 predator, keeping 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.

DEFENSE AGAINST PREDATORS With predators always on the lookout for a meal, prey must constantly avoid being eaten. Any adaptation the prey uses adds to the chances of survival for the species. Some adaptations are defense mechanisms which can give the prey an advantage against enemies.

There are three ways animals avoid falling prey to a predator. The first is very direct and comes naturally. Animals can use speed as a very effective means of escaping predators. You can't eat what you can't catch!

A second defense mechanism is camouflage. One form, cryptic coloration, allows the animal to blend in with its environment to avoid being detected. It is important to note that predators also use cryptic coloration to avoid detection by unsuspecting prey.

In the snowy environment of the Arctic, the polar bear is white to avoid being noticed as it approaches the seal, and the seal pup is white to avoid being noticed by the bear.

A female katydid blends with the tropical vegetation in the lowland Amazon rain forest of Peru. Her wings mimic the mottling of the surrounding leaves.

A scorpionfish rests immobile on a coral reef in the Philippines, 60 feet below the surface, camouflaged against the colorful tapestry of the reef.

Trickery can also be used as a defense. False features that appear to be enormous eyes or appendages can serve to dissuade potential predators. Mimicking an animal that is dangerous to a predator is another effective means of avoiding being eaten.

Munching on a plant stem in Costa Rica's Monteverde Cloud Forest Reserve, this Xylophanes caterpillar has such tiny eyes that you would need a hand lens to see them. The red "eyes" and pointed "stinger" are both fake.

This tasty nymph of an insect in the family Alydidae assumes the fraudulent guise of an ant, and forages on plants in open areas of tropical rain forest in Costa Rica where birds could easily find it. Unless an experienced bird calls its bluff, it may owe its survival to its anatomical mimicry of an ant

The final defense is physical or chemical combat. Some animals' physical features make them a very undesirable meal. Porcupines make it very difficult for predators with their extremely sharp quills.

Chemical features can be just as effective. We all know what happens when a skunk is scared! The dart frog also uses chemicals (poisons secreted from its skin) to deter attackers. Any animals that eat these small frogs are likely to get very sick or die.

Monarch larvae are specialist herbivores, consuming only host plants in the milkweed family. Milkweed makes them poisonous to most vertebrates and provides monarchs with an effective chemical defense against many predators.

The Viceroy butterfly. This is the well known mimic of the more common Monarch butterfly. Because they look like the bad tasting monarch butterflies, they are often avoided by predators

PARASITISM

This Caribbean soldierfish is host to the parasitic isopod attached to its head, between its eyes. The fish has no way of removing the isopod which feeds on his body tissues.

Human Parasities: roundworms (intestinal cavity); Tripanosoma gambiense causes African sleeping sickness; Human liver fluke ( a flatworm)

Lampreys on Great Lake fish The opened up hinge area of the Purple Hinged Rock Scallop. Cliona celata is a boring sponge which bores as a parasite into mollusc and barnacle shells

Birds are well known for their parental care, patiently incubating their eggs and then bringing food to their young until they are old enough to look after themselves. However, certain birds, known as "brood parasites," lay their eggs in the nests of other birds and do not provide any parental care for their own offspring. Care that the "hosts" provide to the young parasites is care denied to their own young. This often has a detrimental effect on the reproductive success of the hosts and may affect their population numbers as well.

Brown-headed Cowbirds are one of the most well known examples of a brood parasite. It hosts are species such as warblers, tanagers, vireos, and thrushes. The populations of many of these birds have been declining, partly due to parasitism by cowbirds. Cowbirds affect the breeding success of their hosts in two ways: 1.female cowbirds remove host eggs from the nest 2. nestling cowbirds compete with the host nestling. Cowbird nestlings are usually much larger and more aggressive than nestlings of host species.

SYMBIOTIC RELATIONSHIPS

MUTUALISM Mutualism is when two organisms of different species "work together," each benefiting from the relationship. One example of a symbiotic relationship is that of the oxpecker (a kind of bird) and the rhinoceros or zebra. Oxpeckers land on rhinos or zebras and eat ticks and other parasites that live on their skin. The oxpeckers get food and the beasts get pest control. Also, when there is danger, the oxpeckers fly upward and scream a warning, which helps the symbiont

Goby fish and shrimp

The shrimp digs and cleans up a burrow in the sand in which both the shrimp and the goby fish live. The shrimp is almost blind leaving it vulnerable to predators when above ground. In case of danger the goby fish touches the shrimp with its tail to warn it of imminent danger. When that happens both the shrimp and goby fish quickly retract into the burrow.

Crustose lichens cling to the bare, exposed surface of metamorphic rock in the Colorado Rockies, where only the hardiest pioneer organisms can survive

Leafcutter ants carry leaf fragments to their underground nest in the lowland rain forest of La Selva Biological Station, Costa Rica. The leaves will become food for the symbiotic fungi cultivated by the ants, which in turn provide food for the ants in the form of filaments swollen with nutrients.

In the tunnels of the nest interior, members of the colony bring leaf fragments back to feed the fungal garden (light-colored substrate), and soldiers guard the queen, nearly hidden at right.

Avoiding antibiotic resistance for 50 million years: a pathogenic fungus, Escovopsis attacks the fungal garden of leafcutter ants. Some leafcutters carry on their bodies a living bacterial colony (white patches on ant) which produces an antibiotic that controls the growth of the pathogen. An evolutionary "arms race" keeps the antibiotic effective.

Labroides dimidiatus inspecting the mouth of a giant moray eel

The anglerfish uses its bioluminescent capability in its hunt for food. The fish dangles an illuminated pod to lure prey close enough to be snatched. The bioluminescence for the lure is due to presence of bacteria, that are endosymbionts. In a related adaptation, anglerfish are dull gray, dark brown or black, and are thus not visible either in their own light or in that of similarly luminescent prey.

COMMENSALISM

Anemone fishes (sometimes called clownfishes) are tropical, reef fishes from the Pacific and Indian Oceans. These fishes are unusual because they have a close relationship with sea anemones. Sea Anemones consists of a hollow cylinder surrounded by a crown of tentacles. The tentacles are equipped with specialized cells called nematocysts. Nematocysts are shaped and function like small harpoons and contain a poison sufficient to paralyze or kill small fish and other reef inhabitants.

The anemone fish lives among the forest of tentacles of an anemone and is protected from potential predators not immune to the sting of the anemone. The anemone fish is protected from the sting of the anemone tentacles by a substance contained in the mucous on its skin. the anemone treats the fish as part of itself and does not sting it

A critical phase in the life cycle of plants is the proper dispersal of its seeds. One adaptation to increase dispersal is the evolution of recurved spines on the seeds or seedpods to attach the seeds to the fur of passing vertebrates who carry the seeds away from the parent plant. The plant benefits from the relationship by the dispersal of its seeds. The vertebrates are not affected except, perhaps, by being annoyed.

Interspecific competition The interactions among species influence the number and kinds of species that exist within a community. Competition limits the number of species that can coexist. Two species that have similar niches compete very strongly. The competitive exclusion principle states that only one of the species can exist; the other is out-competed and dies out.

Interspecific competition occurs in two ways: 1. Interference competition: involves actual fighting over resources. 2. Exploitative competition: consumption of resources by individuals of one species limits availability to another species.

RESOURCE PARTIONING Species that share the same resources can coexist if their niches are not too similar. Species evolve differences in what they eat or where they feed such that coexistence is possible. This is referred to as resource partitioning.

Character displacement Populations of the same species that compete for the same resources can decrease competition by exploiting different parts of their habitat. This will often lead to physical changes in the populations as they become increasingly adapted to a particular niche. This can lead to speciation and increased biodiversity

Ants live on acacia trees and are able to feast on the sugar produced by the tree. The tree is protected by the ants' attack on any foreign insects that may harm the tree. This is an example of : a) parasitism b) commensalism c) mutualism d) symbiosis e) competition

Which of the following relationships would be symbiotic? a) commensalism only b) mutualism only c) parasitism only d) commensalism and mutualism e) commensalism, mutualism, and parasitism

Which of the following is an example of a mutualistic relationship? a) Lichen. b) Barnacles that grows on a rocky surface. c) Tapeworm in the stomach of a mammal. d) Cattle egrets feast on insects that are aroused into flight by cattle grazing in the insects' habitat. e) An African bird benefits from sitting in shade underneath an elephant.

A form of species interaction in which one of the species benefits while the other is unaffected is called a) parasitism b) mutualism c) commensalism d) symbiosis e) competition

Symbiotic relationships frequently develop between organisms that live in close physical proximity to each other. Female cowbirds often lay their eggs in the nest of wrens, who unwittingly raise the young cowbirds as their own, This relationship is an example of a) commensalism b) parasitism c) mutualism d) competition e) nihilism

Protozoans living in the intestine of a termite secrete enzymes that digest cellulose, providing digestive end products of value to both organisms. This symbiotic relationship could be described as a) mutualism b) commensalism c) parasitism d) saprophytism e) competition

The leaves of mistletoe plants photosynthesize, but the roots of the mistletoe plant absorb nutrients from living oak trees. The symbiotic relationship is best described as a) mutualism b) commensalism c) parasitism d) saprophytism e) competition

Interaction between two species in which one feeds on the other is a) competition b) a community c) an ecosystem d) predation e) symbiosis

Interaction between two species as both attempts to use the same environmental resources is a) competition b) a community c) an ecosystem d) predation e) symbiosis

In large natural ecosystems, competition between two species over time will usually result in a) each species occupying a slightly different niche. b) equal numbers of each species persisting for a long time. c) death of all members of one species within a short time. d) hybridization between the two species, resulting in a third species. e) None of these are correct.

Which statement is NOT true about parasitism? a) The host is generally larger than the parasite. b) An efficient parasite usually kills its host. c) Smaller parasites often live as endoparasites within the body of the host. d) Ectoparasites are attached to the outside of the host's body by specialized organs. e) Some organisms and all viruses are obligate parasites and must live inside a host

A keystone species is a) an organism that acts as a commensal in a host. b) a predator that destroys many different species in a community. c) a mimic that has the same appearance as another, poisonous species. d) a prey species that must be present or the predator species will die off. e) a species whose removal causes major shifts in other species in the community.