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Unit 11

Connecting with Networks

11.6 Ecosystems

LINKS IN THE FOOD CHAIN

For most of this unit we have been focusing mainly on physical, human-made networks, such as our power grid, the Internet, and the nation's highway system. We have also looked briefly at intangible networks, such as webs of social connections. Until now, we have neglected a particular group of networks that are more fundamental and important than any of those created by people: ecosystems.

One common aspect of ecosystems is the food chain. A food chain describes how energy gets transferred through a chain of organisms, beginning with photosynthetic microorganisms such as algae, to consolidate in apex predators, such as a great white shark, and then to be dispersed by scavengers, only to re-enter the system at the bottom again.

food chain

A food chain provides a convenient way of obtaining a rough approximation of what happens in an ecosystem. A better approximation is available through the food web. Food webs take into account that most members of ecosystems interact with more than just one other member, or neighbor. In a food web, nodes represent species, and edges represent predator-prey relationships, or alternatively, mutually beneficial, or symbiotic, relationships.

food web Item 2919 /Neo Martinez, UNITED KINGDOM TROPHIC WEB (2005). Courtesy of Neo Martinez A food web for a British forest.


food web Item 2920 / Neo Martinez, CARIBBEAN REEF TROPHIC WEB: IMAGE 1 (2005). Courtesy of Neo Martinez. A food web for a Caribbean reef.


Food webs are examples of directed graphs, because certain relationships are "one-way streets." Sharks, for example, may eat otters, but otters do not usually eat sharks. Such a relationship would be represented by an edge that has some directionality.

directionality

Alternatively, remoras are fish that tend to attach themselves to sharks and feed off of scraps, bacteria, and feces. This is a mutually beneficial, or symbiotic, relationship: the shark gets a good cleaning and the remora gets a free ride and free food. Species that live in symbiosis such as this would be represented in a graph by nodes that are connected by two edges, one traveling each way.

symbiosis

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UNINTENDED CONSEQUENCES

Ecosystems in nature portray dynamic equilibrium; predator and prey populations are constantly changing in response to one another. For this reason, any realistic model has to incorporate some sort of dynamics. It is critical to study what happens when certain nodes become diminished in their influence or are removed entirely from a network. Because ecosystems are typically made up of many different species that interact in complicated ways, the consequences of removing one or more nodes can be hard to predict.

A famous example of the unpredictable consequences of removing a key node from an ecosystem occurred on the West Coast of North America in the 19th century. Throughout the 1800s, Russia controlled what is now Alaska and had considerable influence along the entire west coast of Canada and the northwest coast of what is now the United States.

Russian traders were especially interested in the pelts of both river and sea otters to be used in making warm clothing for withstanding the cold Russian winters. They paid trappers very handsomely for any and all otter pelts. As a result, the trappers scoured the rivers, streams, and coastlines for otters. By the year 1900, the otters had been hunted to the brink of extinction, effectively removing them from the ecosystem of which they were a well-connected member.

otter

Whenever a species is removed or disappears from a network, its prey tend to benefit, and its predators tend to suffer. This causes ripple effects that can rapidly spread to affect other nodes (species) in different ways. In the case at hand, otters prey heavily on sea urchins. With the otters out of the picture from the over-hunting, the sea urchin population began to boom up and down the coast.

urchin eating kelp

As it turns out, a favorite food of the urchin is kelp, a form of algae that grows into large stalks, creating underwater forests that serve to hide and protect all manner of other organisms, especially juvenile fish. The exploding population of sea urchins feasted voraciously on the kelp, especially upon the vulnerable spots where the stalks anchor to rocks. Under pressure from the increased consumption by the urchin predation, the kelp forests very rapidly began to disappear, and along with them the precious juvenile fish habitat.

With diminishing cover, the young fish were especially vulnerable to predation. This eventually led to the collapse of certain fisheries along the coast. These consequences were ultimately attributable to the removal of the otters from the ecosystem. When governing authorities realized what had happened, otters became a protected species. They have since slowly regained some of their numbers, which has in turn resulted in the rejuvenation and expansion of some of the kelp forests along the coast.

The difficult task of understanding the many different interactions in an ecosystem is made even more difficult by variations in complexity. Some ecosystems are quite simple, such as those found at high elevations, where only a few of the hardiest, best-adapted species can survive. Other ecosystems, such as those found in tropical rain forests, may have millions of member species and are extraordinarily complex. A major question in ecology is whether or not complexity in an ecosystem increases its stability.

It might seem obvious that, the more nodes and edges a network has, the less likely it will be that the entire network or a large portion of it falls into dysfunction at the removal of a random node. However, as we saw in our discussion of random, small-world, and scale-free networks, different structures behave differently when randomly disrupted. Recall that removing nodes from a randomly connected network tends to lead rapidly toward disconnection.

On the other hand, removing a few nodes from a scale-free network usually has little effect, due to the presence of its highly connected hubs. Removing a hub, however, can be catastrophic.

Do real ecosystems behave as random graphs, small worlds, or scale-free networks? Real-world food webs tend to have different qualities of all of these types of structure. For example, the idea of keystone species, a species whose presence or absence directly and strongly affects the stability of the entire system, is closely related to the highly connected hubs of scale-free networks.

One final note: because species play very different roles in their ecological networks, their form and behavior is often closely related to their connectedness. This is why, for example, when snorkelling you will commonly see many small and medium-sized fish, less commonly a few large fish, and very rarely a shark. The same goes for terrestrial creatures. Deer sightings are a quite common occurrence all over the country, but visual reports of bears, wolves, and mountain lions are relatively rare. A chief reason for this is that being a large predator requires expending a large amount of energy hunting herbivores and growing the teeth and claws required to kill and eat them.

At each step in a food chain or web, a certain amount of energy is lost. Sunlight falls on autotrophs, who convert it to sugar with a certain efficiency through the process of photosynthesis. Nonetheless, not all of the sun's energy gets converted. The creatures that consume these primary producers convert their sun-made sugars into body-mass via enzymatic processes that have a certain efficiency. However, not all of the "sun energy" stored in the autotrophs is captured. Consequently, after passing through just two levels of the food web, the energy that started with the sun is only a fraction of what it was when it arrived on the surface of the earth. The larger an animal's mass, the more energy it has consumed, because the amount of energy that strikes the earth is fixed, this means that there should be fewer large animals than small ones. Furthermore, because large predators are a step above large herbivores in the hierarchy, it stands to reason that there should be still fewer of them.

Understanding how the different species with which we share our planet interact requires an understanding of how the structure of networks affects the roles and importance of the network members or elements. Networks such as ecosystems are constantly changing, putting pressures on the species that comprise them to adapt or die. In this sense, dynamic networks can be thought of as one of the fundamental engines of evolutionary change.

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