Planet Earth/7i. Earth’s Ecology: Food Webs and Populations.

Species Habitat and NicheEdit

 
The cold barren landscape of the Svalbard Islands.

Across the barren ice tundra of the island of Spitzbergen in the Svalbard Archipelago, a lone fur clad figure hiked across the landscape. Located north in the Arctic circle, the Svalbard Archipelago is a cold foreboding place, inhabited by a scraggily few coal miners who make a scant living pulling out coal from ice covered mines to be shipped south. The islands are technically governed by Norway, but its loose inhabitants come from America, Russia and Germany to work the coal seams, with a tiny tourism industry. However, the islands are nearly unpopulated with so few human inhabitants making the islands ideal to study natural ecology, arctic soils and the biotic world in a place so remote that it has been left to its original natural state, with little human disturbance. This remoteness was what drove Julian Huxley, a professor of zoology at Oxford University to come to the island as part of his travels of the world. Julian Huxley was the grandson of the famed defender of evolution and prominent zoologist, Thomas Huxley. Following in his grandfather’s footsteps, Julian had studied zoology and traveled to Texas to start up a university program in the United States, but with the outbreak of World War I, he found himself enlisted and back in Europe during the war. The aftermath of World War I found him back in his native England, where he took the place of his mentor at Oxford, Geoffrey Smith, who had died during the battle the Somme in 1916. One of Julian Huxley’s pupils at oxford was Charles Elton, and he invited him to travel to remote island of Spitzbergen to study what he could of the native life.

Charles Elton was enthusiastic for the expedition, but many viewed the trip as foolish, since the remote arctic island was noted for being nearly barren of life, with only a few insects and plants, and few if any vertebrates. Nevertheless, Charles Elton accompanied Julian Huxley and started his career in understanding the ecology of life. Elton was influenced by the question of how animals make a living within a community of organisms. The islands have few terrestrial mammal species, including the Svalbard reindeer, Arctic fox, and sibling vole, and wandering polar bears that crossed over the ice feeding on aquatic seals. Attempts had been made to introduce other species, like musk oxen, but they did not survive the cold harsh winters.

Ecologists define two characteristics of every biological species. The first is the habitat of the species, which is the physical geography of where the organism lives, and second is the niche of the species, the way in which the species is able to survive and acquire food and nutrients to live on. An easy way to remember the difference is that the habitat of a species is equivalent to its home, while the niche is equivalent to its job. For example, the Arctic fox that lives on the Svalbard Archipelago, has a habitat across the Arctic tundra, but a niche as a predator of small voles and birds that live on the island.

 
Numbers of snowshoe hare (Lepus americanus) (yellow background) and Canada lynx (black line, foreground) furs sold to the Hudson's Bay Company

During his study of the life on the Svalbard Archipelago, Elton was particularly interested in the cycle of population growth and decline in voles, which appeared to rise and fall with food availability. He also noticed that populations of arctic fox appeared to follow the rise and fall in vole populations, since they were dependent on healthy populations of voles to survive. The expedition was funded by the Hudson Bay company, a company interested in fur trapping and understanding the population dynamics of fur bearing mammals. Following the expeditions and his studies, Elton was hired as a consultant for the company to look at the dynamics of population growth and decline in fur bearing mammals in Canada. This allowed Elton access to an enormous amount of historical data regarding the record of trappings of mammals by the Hudson Bay company going back decades. Analyzing the company’s archives, Elton discovered a ten-year cycle in the abundance of Canadian lynx and snowshoe hares. Snowshoe hare populations would rise, resulting in an increase in available food for Canadian lynx, leading to their increase in population, as predation increased resulting in snowshoe hare populations to decline, as the snowshoe hare populations declined the population of Canadian lynx would subsequently decline too, and predation would decrease, this would result in a slow increase in snowshoe hare populations. This oscillatory cycle of population dynamics between the two species was strong evidence for the interconnection of prey and predator populations through the availability of food. The story of Canadian lynx and snowshoe hares is more complex, as the two species are also influenced by the availability of other sources of food, but they illustrated to Charles Elton the importance of food chains or cycles, and the interdependence of species on each other.

Food Webs, Trophic Pyramids and Ecological CommunitiesEdit

 
An example of A) a Trophic Pyramid, and B) a Food Web.

A food cycle or food web is the interconnection of food sources into a graphical representation depicting a web illustrating which species are being consumed by other species in an ecological community. Another name for food web is consumer-resource system. You can broadly divide all life into either autotrophs or heterotrophs. Autotrophs are organisms that derive their energy and nutrients from the abiotic environment, for example many plants unitize the sun (photosynthesis), requiring only water, carbon dioxide and soil nutrients to grow. Heterotrophs are organisms that must derive their energy and nutrients from other organisms, for example deer that eat shrubs and grass. These divisions are also called a trophic level.

 
A trophic pyramid showing the relative abundance of organisms is dependent on their trophic level.

A trophic level is the number of steps along the food chain (or web) a species is from a primary producing autotroph. For example, snowshoe hares feed on grass and shrubs, and has a trophic level of 2, while the grass and shrubs are autotrophic, with a trophic level of 1. Canadian lynx that feed on the snowshoe hares have a higher trophic level of 3, since they feed on tropic level 2 animals. Trophic levels higher than 4 are rare, as the amount of energy and nutrients decreases with each level, making higher trophic levels susceptible to breaks in the food chain when a prey species drops in population size or disappears. One important addition to a trophic level scheme are organism that feed on dead organic matter, the decomposers, which occupy a unique strategy for food by living on waste or dead organic matter, and depend on this supply of organic matter to feed on for their population growth and decline.

 
An example of a food web in the Chesapeake Bay

Trophic levels can be depicted graphically into a trophic pyramid, which shows the decrease in total biomass with each level, as availability of food resources decreases with higher trophic levels that can support fewer individuals and less total biomass. For example, the total individual grass plants are far larger than the total number of individual deer feeding on the grass. And the total individual deer population is far larger than the total number of individual wolves feeding on the deer. The total number of individuals of each species is depend on the metabolism and physiology of each species, and how effective they are in the amount of food they required to live.

The idea of food webs, and the interconnection of species is a major concept in ecological idea of a biological community. A community is a group of organisms that live together in the same geographical location and depend on each other for survival. The concept of an ecosystem, induces the abiotic environmental into the concept of a biological community as well, including such influence as climate, weather, water and nutrient resources, availability of light, water depth, water salinity, terrain and geology. All of these factors influence the plants and animals within a biological community.

Ecological cascade effectEdit

 
Cownose rays (Rhinoptera steidachneri)

An ecological cascade effect is a series of cataclysmic extinction events that occur when a single species is removed from a community, or when an invasive species is introduced that significantly disrupts the community structure. For example, a study led by Ransom Myers and Julia Baum in 2007 found that when sharks are heavily fished and killed in the open ocean, a removal of the top predators (a keystone species) in a marine community, prey species of those sharks (such as the cownose ray) increase in population size. This increase puts additional pressure on bay scallops which cownose rays feed on. This results in a decrease of bay scallop populations due to the increase in predation, and this will ultimately mean that cownose ray populations will eventually decline as well once bay scallop populations are reduced and the number of cownose ray falls without this availability of its food source. This is an example of a cascade effect that quickly collapses a biological community.

 
The common lionfish (Pterois volitans).

Another example of an ecological cascade is a study done by Mark Albins and Mark Hixon (2011) on the introduction of the invasive lionfish to the Caribbean ocean. Lionfish are predatory fish with venomous spines that were introduced to the Caribbean waters sometime in the 1980s. They have few if any predators because of the venomous spines which protect them from larger fish. Since 2005 they have increased dramatically in population size and geographic range in the Caribbean beyond their normal population density in their native habitat in the South Pacific. This is due to the abundant availability of fish in the Caribbean ocean and absence of predators. Lionfish are known to eat large quantities of groupers, snappers and goatfishes as well as gobies, wrasses and basslets, which are important fish for the region. These fish population sizes are in steep decline because of the introduction of lionfish. As lionfish switch to other prey items, such as the parrotfish, the ecological cascade effect becomes more dire. Parrotfish feed on seaweed and algae that grow on coral reefs. By keeping the seaweed from overgrowing the surface of the coral reefs in Caribbean, any drop-in the populations of parrotfish will result in seaweed overgrowing coral reefs, causing them to die because of lack of sunlight. If parrotfish are over preyed upon by lionfish, the overgrowth of seaweed and algae on corals will result in a major extinction within coral reef communities, altering the Caribbean reefs ecosystem. Ultimately the lionfish will become so populated that they will deplete the native fish populations and subsequently fall in population size themselves because of the lack of food— leaving an ocean devoid of many of the previous native species. To combat the invasion of lionfish, active fishing and harvesting of these fish is encouraged.

Population EcologyEdit

 
Microtus, the vole

When Charles Elton returned to England, he became in charge of one of the most interesting studies of rodent population experiments, when his team turned their interest to vole (Microtus) populations. This was part of the Bureau of Animal Population at Oxford University, a group of scientists interested in understanding population dynamics of animals. In the 1920s it was noticed that large populations of voles were observed in western Scotland near the towns of Dunoon and Strachur feasting on newly planted trees, an area that was part of a reforestation project. These voles increased in population, and then suddenly without explanation would die out. There was little in the way of predators, as many of the foxes and carnivores had been removed from the region, although owls and other birds of prey frequently feed on the voles, there was little sign of increased predation. This cycle of increasing populations and then a sudden crash was puzzling to the scientists, since the availability of food seemed unchanged during these cycles. There was mystery, and many on the team suspected that there was something mysterious about the decline in populations that might be related spreading disease. In 1934, G. M. Findlay and A.D. Middleton published the results of their study.

Voles were captured in live traps to determine population size of the voles in the study area, and recorded over several years. Some of these individual voles captured in the live traps were sent back to the lab to be kept in captivity, and observed. When the population suddenly started its declined, many of the traps became empty, as the population fell. The few voles captured during the population decline period were taken into captivity, but subsequently died even captivity only after a few short days later. Study of the dead bodies of the voles showed signs of lesions and cysts in the brains, related to a parasite called Toxoplasma. Toxoplasma is a single celled eukaryote with a complex life cycle that infects mammalian tissues. Once in a host, the parasite proliferates in the tissue as tachyzoites and eventually forms cysts, called bradyzoites. These cysts proliferate mostly in the brains of mammals, and can alter the behavior of the animal and can cause death. A sick or dying mammal is susceptible to predation, and the parasite can be transferred to a predator, most often domestic cats. Once in the digestive track of a cat, the parasite changes into sexually reproducing merozoites in the digestive track, that can result in abundant oocysts (eggs). These oocysts are shed from the digestive track into the soil, when the cats defecate. These oocysts, which can survive in the outside environment, if ingested can hatch and infect a new host.

 
The life cycle of the Toxoplasma parasite.

Food webs are more complex with the introduction of parasites, such as Toxoplasma. As vole populations increase, there was likely increased predation by house cats, resulting in the surrounding soil to become enriched in Toxoplasma. The increase in the parasites in the soil, likely meant that more voles were infected by the parasite, which made them more susceptible to illness, death, and predation. At peak population, voles were impacted by two forces, the increase in cat population and predation, but also the increase in parasites due to the increase of cats shedding oocysts into the soil. This unbalance, results in a very sudden drop in vole populations. Parasites, and other disease-causing microorganisms should not be excluded from food webs, as they can contribute to dramatic changes in population sizes, that often remain hidden to researchers.

OverpopulationEdit

We can think of overpopulation as the moment in time when the available resources are inadequate for the population to continue to grow. Post peak population decline can be sudden, as in a crash, or it can be protracted, such as a stable plateau or slow decrease over several generations. The rate of fall depends on the patchiness of the resources, and the exponential rate of the decline in the necessary resources for the organisms to survive on. In most stable long-term ecosystems, overpopulation episodes are rare, given the natural inclination for a dynamic equilibrium to be achieved due to the negative feedbacks that stabilize populations of a biological organism overtime. Ecosystems can become susceptible to overpopulation episodes when they are disturbed from their stable conditions, like the introduction of invasive species, or alteration of land use, as well as changes to the physical environment, such as climate changes. These episodes can result in significantly altered environments that can collapse an ecosystem into fewer trophic levels, decrease species richness and biodiversity. Recovery of such events are prolonged, often requiring thousands to millions of years of slow recovery to begin to see increased biological diversity and speciation in those environments. The heterogeneity of the environment also helps partition populations, increasing the likelihood of geographic speciation events in the long-term recovery process.

Overcrowding and Self Organizing Behaviors.Edit

 
Data collected on mice populations by Colhoun in his "Universe 25" experiment. The Y axis is the log of the population size which increased quickly in Phase B, slowed during Phase C, and started to collapse during Phase D. The X-axis is time.

One of the interesting questions explored by science, is whether there is a limit to the density of a growing population. If an organism is given access to unlimited food and resources at what point do they decline. Geographic space itself can be thought of as a hidden natural resource in overcrowded populations, which can lead to population crashes as easy as loss of food availability or other natural resources. Most species have optimum space requirements, which may become limited during overpopulation events. In the 1960s, John B. Calhoun conducted a series of experiments starting with domesticated Norway rat populations, but following with experiments on domesticated mice populations that were allowed to grow in an enclosed limited space, in the absence of predation and any food and water scarcity the population grew. The population grew quickly for the first several generations, then begin to plateau rising more slowly toward a peak of 2,200 within an 9-foot square enclosure, with a density of 20 mice per square inch of floor space, although the enclosure contained tunnels and other partitions. After this peak density, the population suffered a rapid decline, with low reproduction, fighting between individuals, cannibalism and other maladapted behaviors. The surviving population was unable to recover, leading to a complete collapse of the colony of mice. The experiments demonstrated that geographic space is an important natural resource for species, and the limitations of geographic space can result in population decline, equally or more severely as that of food and other resources. In a natural population, organisms will maximize the availability of space to their preferred life style, and self-organize within local groups or isolated units depending on the nature of the niche and habitat requirements, as well as the inner species behavior of the organism.