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Trophic Levels & Food Webs
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0:01
[Music]
0:13
Hello, I am Dr. William Schlosser, Washington State University, School of the Environment, and this is my classroom!
0:24
The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and will, in turn, be eaten themselves. The trophic level of an organism is the number of steps it is from the start of the chain to the subject s position. A food web starts at trophic level 1, with primary producers such as plants, then moves to herbivores at level 2,
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carnivores at level 3 or higher, and typically finish with apex predators at level 4 or 5. The path along the chain can form either a one-way flow or a food web. Ecological communities with higher biodiversity form more complex trophic paths. Consider trophic levels in the framework of ecosystem stability. Ask yourself, what happens when its structure is disrupted. This entire process is how living plants and animals capture energy
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from the planet s energy source, the sun. Photoautotrophs use energy from the sun, along with carbon dioxide and water, to form glucose and oxygen. These building blocks enable all players through the food chain to thrive and survive. In photosynthesis, solar energy is harvested and converted to chemical energy. Oxygen is released as a byproduct. Oh, us lucky animals to benefit from plants who make our oxygen and our food!
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A trophic level describes a group of species that obtain energy in similar ways, classified by the number of feeding steps by which the group is removed from primary producers, which are the first trophic level. Consider the rule of 10% to know how the cost of converting energy uses about 90% each time just to live, breathe, and collect the food you eat. That means only 10% of the original energy remains after each stage of conversion.
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The higher you travel up the pyramid, the less original energy is maintained. This is why that apex predator, like the wolf, must be supported by thousands of other primary carnivores, millions of herbivores, and billions of primary production plants.
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Multiple linked food chains form a food web. Consider how food webs interact to define the success of the populations living within an ecosystem. You must consider how productive the site is, in terms of plant life. This is the result of precipitation, growing days, and soils. These basic building blocks define how much solar energy is harvested and converted to chemical energy in the form of glucose using water and carbon dioxide.
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These are the energy source for the next step up in trophic levels to define how herbivores succeed. Without those photoautotrophs, there would be no carnivores because they cannot eat plants. They need a thriving herbivore platter to feed from. It is all interconnected.
3:35
[Coyote howling]
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Coyotes are the Shape Shifters of North America. This generalist fills the gaps few others can make. Their niche is broad and seldom left without options. Coyotes are facultative feeders, seeking a meal that oftentimes has only marginal energy benefits for them. Remember, just being able to eat something DOES NOT mean it is digested. Think about those berries in the coyote scatt. It looks like the coyote was eating berries from the bush, right? The canine did not eat the fruit,
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it ate the grouse who had its craw full of berries from breakfast. The coyote ate the bird and everything in the craw was ingested. It passed as feces mostly undigested. Coyotes are carnivores, not omnivores. You cannot change that biology just by wanting it.
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Coyotes are despised by farmers and most citizens alike. Known for attacking livestock and killing pets. The Aztec root word for coyote literally means trickster. However, even with this hate by the public, and many attempts to eliminate them, coyotes still thrive.
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Coyotes are mainly diurnal however those that live in suburbs and cities have become nocturnal in order to avoid humans. Coyotes can often be mistaken for wolves. The main difference between the two is that coyotes are much smaller than wolves, male coyotes getting to 18 to 44 pounds, while male wolves can get to 66 to 180 pounds. Although coyotes are similar to wolves, they have quite different lives. Wolves have a very organized pack structure this is not like coyotes.
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Coyotes can be solitary or form packs with others. These packs aren't very organized but in these packs, they can take down larger prey like deer.
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Coyotes are very smart they have used very strange strategies for hunting. One of them is to team up with the American badger. Coyotes and American badgers both hunt prairie dogs. Prairie dogs are small rodents that day extensive burrows underground. When a prairie dog sees a coyote, they will run into their burrows. Because coyotes are much faster and more agile than prairie dogs. Badgers, however, can dig prairie dogs out of their burrows
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but prairie dogs are much more agile than badgers. So when coyotes and badgers team up any prairie dog that runs into its burrow the badger will get but any prairie dog that tries to run from the badger the coyote will get. Using this strategy both the coyote and the badger get food in a symbiotic relationship.
6:35
Food webs and interaction webs look the same, mostly. Here, it is all about the way you consider interactions between levels. In the food web, producers make the original energy capture and they are fed on by the herbivores and omnivores. The arrows point in the direction of energy flow. Carnivores and omnivores get their energy by eating herbivores and plants, then carnivores get their energy from herbivore meat. These interactions continue until the top-level predator eats their prey.
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The top-level predator is where no higher predator is found. No creature feeds on the representatives on this heap. This is the wolf in the forest who has no natural predator feeding on them. Now consider the interaction web. This is all about how populations interact within a community. We still have the interaction line moving up the food web, but this is to show an interaction we know already. This is about those horizontal interactions; they show competition and
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other kinds of interspecific interactions. These are non-trophic interaction events.
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These direct interactions happen between representatives of two different species. It might be a symbol to show forage or habitat competition between a deer and an elk. It might be competition between a cougar and a coyote as they hunt rabbits. They are in direct competition but are from different species. Indirect interactions begin the understanding of how species interact across trophic levels. Consider how the cougar and coyote interaction effects rabbits.
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If two populations are targeting those bunnies, the rabbit population may begin to drop. With fewer rabbits, their plant diet, clover, might blossom to spread into more territory. Those are indirect interactions brought to light on a secondary animal species, rabbit, or plant habitat as clover populations. This happened with the now familiar sea otter who fed on urchins, who in turn was feeding on kelp. When the sea otter was repurposed into fashionable
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hats, the indirect interaction was seen in deteriorating population of kelp. Removing the Otter has the Indirect Interaction to clearcut the Kelp forest.
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Food chains are all about the direct interactions. When I talk about a grasshopper and its successes in the food chain, I am not talking about the fox. That is an indirect interaction, but only when we term it in the interaction web.
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But when we switch the channel to Trophic Levels, we explore how ENERGY is transferred through the system. This transferal is completely dedicated to attention about the network. HOW does energy flow, on what route, and what are the congestion points or additions of passing lanes? Look for these bottlenecks and what changes they effect.
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This IS the Food Web. Arrows show the direction of ENERGY TRANSFER. A food web is a diagram that shows all the pathways of energy flow in a community. The food web is like a food chain in that it depicts organisms which eat each other. Here we identify who is on the platter, and who holds the menu.
10:05
See these screens to understand how many ways we use these connections to describe how species and populations interact. A significant ecological theme pivots around trophic level discussions. These interactions all integrate how energy is transferred and how indirect interactions create opportunities or remove linkages. These can become game changing events.
10:32
Going back to the food web, recognize we do not see those non-trophic interactions. Interaction webs allow us to watch ecological networks. They are representations of the interactions that occur between species within a community. The interactions include competition, mutualism and predation. Network properties of particular interest include stability and structure. Any action or influence that species have on each other is considered a biological interaction.
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These interactions between species can be considered in several ways. One such way is to depict interactions in the form of a network, which identifies the members and the patterns that connect them. Specie interfaces are considered primarily in terms of trophic interactions, which depict which species feed on other species.
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Ecological networks that integrate non-trophic interactions are being built. The type of interactions they contain are classified into six categories: mutualism, commensalism, In, amensalism, antagonism, and competition. Observing and estimating the fitness costs and benefits of species interactions can be problematic. The way interactions are interpreted affects your conclusions. Be inclusive in your
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investigations and how you arrange results. Reveal new knowledge as the benefit of discovery.
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Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a trophic level in a food web is suppressed or released. A bottom-up system concentrates attention on how resources (space and nutrients) influence higher trophic forms. You stop plant growth, and all up-stream participants starve. Game over, all stop. A top-down flow is a trophic cascade where the top consumer, that is the apex predator,
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controls the primary consumer population and their influence on all lower trophic forms.
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We explored site differences about how precipitation, temperature, and soil nutrients are delivered through time to each site, and now we explore how plants and animals respond to these opportunities. These are the natural plant communities which support the native herbivore populations and all those carnivores in competition to feed on them all. Humans are part of this interaction mix. We can either support stability or admit our role in ecological uncertainty.
13:18
Oftentimes we put this into the context of NPP, that is Net Primary Productivity. This is gross primary productivity minus the rate of energy loss to metabolism and maintenance. In other words, it's the rate at which energy is stored as biomass by plants or other primary producers and made available to the consumers in the ecosystem. All participants are involved in the NPP balance. This defines what successes will be made and lost. It is a running life trek, and a death march.
13:55
We have talked about the role the first three varieties serve in the Keystone Species category, but what about Salmon? Although they are the species that is affected by the loss of wolves, which increased elk herbivore population size, who excessively fed on hardwood trees along rivers. This allows river channels during spring freshens to migrate wildly after salmon spawning has happened, which leads to a DROP in salmon survival.
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Eggs left high and dry do not hatch to become alevin, then fry, then smolt.
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This places salmon into a trophic cascade where the salmon are a Keystone Species which provides key nutrients to river systems, feed for bears, eagles, coyotes, and other salmon feeding species. In the river, salmon are the top level predator. Salmon feed on other fish, insects, crawfish, worms, and sometimes by my fishing lure! Although beavers do not feed on salmon, they rely on a productive river ecosystem to sustain their habitat. This is where the insects thrive and
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where calm water loving species establish. Beavers make the pools while salmon add their nutrients to the smorgasbord of prosperity. All is interconnected. Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a trophic level in a food web is suppressed.
15:27
Gulls and crabs feed on sea stars; killer whales feeds on otters. There are a lot of interactions and most pivot on meals. In the kelp, sea urchin, sea otter example, the carnivore (sea otter) feeds on the herbivore (sea urchin) which indirectly has a positive effect on the primary producer (kelp) that is eaten by the herbivore (sea urchin). At equilibrium, this is how the system developed. Remove the sea otter and changes happen. This is the trophic cascade, seen as kelp
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populations diminish and those species depending on kelp forests also diminish.
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That is a cascade. In the sea star, mussel, barnacle example: the trophic cascade occurs when the carnivore (sea star) feeds on mussels creating space for other species to occupy, thus increasing the diversity of the ecosystem.
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In ecology, Autogenic succession is driven by the biotic components of an ecosystem. Of course, these all happen within the responses to the abiotic realm, but it is the biotic players that hold the crux of this movement. Beavers are a case of ecosystem engineers. These autogenic engineers are defined as species that modify the environment by mechanically changing living or nonliving materials. Allogenic engineers change the environment by transforming
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living or nonliving materials from one physical state to another, via mechanical or other means. Keystone species as ecosystem engineers may affect ecosystem processes, such as nutrient cycling, and thereby ecosystem functioning. Beavers are the original model for ecosystem engineers; in the process of clearcutting trees and damming streams, beavers alter their ecosystem extensively. The addition of a dam will change both the distribution and the abundance of many organisms in the area.
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We need to work on terminology concerning Keystone Species. There is an apex predator, like the wolf, who has no natural predators feeding on them. These wolves are keystone species. But being at the top of the food chain is a rare incidence. The beaver is an ecosystem keystone species, but not THE Keystone Species. We call them keystone engineers. Their role is significant, but beavers are not predators at all. They are herbivore rodents, like the squirrel and porcupine.
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We still call them a keystone species, just, they do not carry the banner of THE Keystone Species.
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An additional example is that of woodpeckers or other birds who create holes in trees for them to nest in. Once these birds are through with them, the holes are used by other species of birds or mammals for housing. Ecosystem Engineers using mainly the hardwood trees of the ecosystem make dams and therefore lakes. Woodpeckers make nests, store food, and create mutualistic opportunities across the ecosystem.
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These players make biodiversity flourish. It happens in the water, along the riparian zones, as bird nesting and propagation expands. Recognize riparian zone plant diversity to accept the reality of the species you cannot see here: bees, bats, snakes, and ladybugs.
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It is all made possible because of these ecosystem engineers.
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Allogenic succession is driven by the abiotic components of the ecosystem. For us humans, these can be the most inconvenient disruptions we experience. It is the volcano erupting, landslides breaking free, floods backing up water where we built a house or where we put down a road. This is the earthquake rattling these lands. All of these happen because of simple mechanics and natural processes that made things like
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Pangea break apart on that plate tectonics theme Alfred Wagner talked about not so long ago. Non-anthropogenic climate change is one of those allogenic instances. This is what I call out as a critical instance of how we consider allogenic climate change versus anthropogenic induced climate change. They are happening together, and I challenge all higher-thinking minds to embrace the ability to express the global climate warming we expect to
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happen as we depart this current global ice-age we are still experiencing from the effects of human induced greenhouse gas emissions. Obviously, they are simulcasting. Be honest to describe and quantify them under separate headings and with specific amounts. Allogenic succession that is the result of expected and natural climate change can happen over hundreds-of-thousands of years. The Anthropocene has only been officially named for about 70 years.
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Trophic cascades happen more often than many of us realize. When it lands firmly where our attention is focused, we see animal habits and even their existence disappears. As these transition from the biotic realm to capture an abiotic trend, the game changing event is marked. This happened with the removal of wolves from the Olympic National Park and the Roosevelt elk began to spend all their time in the lush riparian zone.
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With wolves in the area, no successful elk would rest on the shore where they only hear the river roaring all around. That is s meal platter ready to eat. But Olympic National Park wolves have been gone since 1940, so no risk, right?
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Elk populations fed on the hardwood vegetation so much that rivers are rerouted. Unintended salmon populations are feeling the pain. Quinault Indian Nation declared in 2020 that blueback salmon are in the fourth year of minimal native spawners returning to their Nadal spawning lands. This is the trophic cascade slapping us in the face. As it happens across most fields of ecology, we find interrelatedness between all the issues we discuss.
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We started with the food chain idea, linked it to food webs, and straight into trophic levels. The theme of energy transfers became paramount to see how biotic interactions pivot on simple things like animals eating a meal. [Music]
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