 Energy flow in ecosystems is one of the important elements of understanding the landscape. A fundamental aspect of energy flow is the trophic structure of an ecosystem. Today we're going to explore the major sources of energy and the trophic structure of both aquatic and terrestrial ecosystems and their implications for understanding our living landscapes. The major sources of energy in aquatic and terrestrial ecosystems are quite often the plants, that's the food base. And so the primary production that occurs within the system, the photosynthesis, provides important energy. Leaves and needles that fall into stream and aquatic ecosystems from terrestrial ecosystems are another major source of energy. Large wood large wood from the trees provides important food resources and also major habitat components. And then below ground production the unseen productivity of roots and mycorrhizae and the whole assemblage of organisms beneath the soil and in the soil. These are the major sources of energy in both aquatic and terrestrial ecosystems. In an aquatic system inside the stream we find several sources of energy. The primary production by algae, the microscopic plants that we find covering every wetted surface, the vascular macrophytes growing in the stream and the mosses and liverworts. In stream ecosystems there are two major sources of energy, the eloxanus inputs that come in from outside the stream ecosystem. These include leaves and needles, both herbaceous plants, leaves from shrubs and trees alongside the stream, wood material coming in from the trees growing around the stream, and then the fine particulate material that rains in from the terrestrial ecosystem. This includes things like soil that erodes into the stream and insect frass or feces that fall in out of the trees. In addition the stream depends upon octoctanus inputs. Those are production sources of production from inside the stream ecosystem. Examples are algae, the microscopic plants that cover every single wetted surface in the stream, the mosses, and then the vascular macrophytes, which are really terrestrial plants evolutionarily that have secondarily invaded aquatic ecosystems. And so these plants have more structural components like their terrestrial cousins, and so they make a lower quality food than the algae or mosses, but still they can be extremely abundant in many streams and rivers. The other day I went out on the Willamette River with my crew to take a look at some of the food webs and trophic structure that we would see in a large river system. The energy flow in this river ecosystem is provided by three major sources of energy. One is the plant material growing in the river. Most of the plant material growing in the river is this fine film of algae growing on the surface of the rock. This little scum that you see covering every wetted surface in the river is photosynthetic plant material growing every day providing critical food resources for the food web in the stream ecosystem. Here we see large numbers of snails and aquatic insects that are scraping and feeding on this plant material. These insects and snails provide a food resource for higher trophic levels within the food web such as the fish. In addition to the algae growing on the surface of the rock we have a lot of vascular aquatic macrophytes, aquatic plants that are growing in the water that are macroscopic. These are not as good quality food as the algal material. They're a lot tougher to eat. They have less nitrogen. They're not as good food but they're still much better than some of the other food that's around and so during the late summer and early fall as they start to die they provide a large input of food resources for this riverine ecosystem. But in addition to the stuff that grows in the river one of the most important food resources is the organic material such as the leaves and sticks and twigs and seeds that we see coming in from the riparian forest. All of this is organic material too and it's also a very important energy source for the stream ecosystem. Insects feed on these leaves grow and become food for fish and salamanders and higher trophic levels. These leaves are broken down by microbes the bacteria and the fungi and as these microbes grow they provide an important food resource for the insects and they help break down and soften the leaf so it's more digestible. As we take a look at this stick it looks like it has lots and lots of little twigs or spines on it but these are actually the cases of aquatic insects that are feeding on the film growing on this twig and feeding on the organic material in the twig itself so we see a complex food web of algae aquatic macrophytes and all kinds of organic material coming in from the flood plain forest the leaves the sticks the seeds that provide the energy sources that support this complex riverine ecosystem. The plants and animals in ecosystems can be grouped into trophic levels based on their function or the types of material they consume. The major trophic levels in either a terrestrial or aquatic ecosystem are primary producers these are the plants the growing things that produce organic material herbivores which are animals that consume plants carnivores which are animals that consume other animals such as this chironomid larvae that you see here eating a small zooplankton or decomposers the microbes that break down organic material and so we can classify the kinds of organisms we find in ecosystem according to their function or the type of food that they consume. The trophic structure is broken down into primary producers which produce organic matter through photosynthesis. The herbivores that consume living plants the tritivores that consume non-living organic material or dead organic material and the carnivores that consume living animals. But another way to classify the organisms we find in an ecosystem instead of what they eat is a classification system based on how they eat or how they gather their food and this is called the feeding functional groups. Feeding functional groups are designated according to how the organism gathers its food. One of the major functional groups in aquatic ecosystems is called scrapers. Scrapers scrape the surfaces of substrates such as scraping algae from a rock in the stream. The shredders are the animals that tear large particles of food apart and then consume those particles such as an animal that tears leaf material apart and consumes it. The collector gatherers are animals that collect small particles that are smaller than their mouth size sweep them together and then consume them and then finally the predators are animals that engulf other animals so you see these four functional groups that we're talking about here the scrapers the shredders the collectors the predators are classified according to how they gather their food. Now be careful here because a lot of people including well-trained scientists confuse trophic structure with feeding functional groups they're not necessarily the same thing remember trophic structure is based on what they eat feeding functional groups on the other hand are based on how they gather their food and so you might assume that a shredder for instance is shredding leaf material and therefore would be a detritivore but you're making an assumption about what it's eating and it's only if that's true are shredders synonymous with detritivores but if for instance that shredder was not shredding a piece of detritus or dead leaf material but instead with shredding living algae large macroscopic filaments of algae it would still be shredding but now it's functioning as an herbivore in terms of trophic structure so remember trophic structures based on what they eat feeding functional groups are based on how they gather their food and you can make assumptions about what they're eating and relate the feeding functional groups to trophic structure but you're making an assumption and you always need to question that assumption now one of the characteristics of these trophic levels and the transfer of energy or the energy flow between trophic levels is the transfer from the food base up through different organisms in a food chain this transfer of energy from its food base to the higher trophic levels is known as a food chain and this is something that we all learned about probably in our early biology classes about the food chains and the importance of the transfer of energy this transfer of energy to the higher trophic levels is limited you do not get 100 percent efficiency of transfer along this food chain there's only a certain portion of the solar radiation that plants are able to use and then each step up the trophic levels from the plants to the herbivores to the carnivores each step of that trophic level can only transfer about 10 percent of the energy that's present at the next lower level for example herbivores typically consume anywhere from 10 to 20 percent of the energy that is present in the plant trophic level below them and each of the trophic levels above the herbivores can only assimilate about 10 percent of the energy that was present at the next lower trophic level so we see that you cannot have huge amounts of production and biomass at the very highest trophic levels because of this restriction of the energy that can be transferred there's only a certain amount of sunlight coming into the surface of the earth and then the plants can only use a certain part of that and based on the nutrient dynamics and availability and productivity there's a certain amount of biomass in plant material available to the herbivores to consume and then they can only take a small fraction only about 10 to 15 percent of that energy and transfer it to the next higher trophic level the primary consumers and so forth the food webs that we see in aquatic and terrestrial ecosystem can be extremely varied the plant materials differ greatly from grasses to shrubs to herbs to trees to cactus and we see the types of animals that are doing the consuming ranging from invertebrates to vertebrates such as we see in this example with the desert rodents a food web is actually more typical of what we see in a given ecosystem the trophic structure and the transfer of energy throughout an ecosystem is known as a food web and so instead of a simple chain of plant to herbivore to carnivore to higher level carnivores we see instead a web of energy flow going from different organisms with multiple types of feeding relationships going on and multiple sources of food just the same as you you don't have just one single type of food you eat many different kinds of food from many different trophic levels and the animals that are present in the ecosystem are doing the same thing so we get a complex structure in our ecosystems it is because of that complexity of a food web that we see the stability of ecosystems that when a given food resource for an animal disappears or declines it can make up for that by consuming another type of animal another portion of the food web and so it provides a type of stability in that complexity so the transfer of energy to higher trophic levels is generally limited there's only a certain amount of productivity that we can have at those extremely high trophic levels that's why the idea of creating trophy fisheries for instance or trophy hunting on ecosystems in which we're harvesting or we're hunting or fishing for a very high trophic level doesn't make a lot of sense in many cases because there's a limited amount of production that can go in to support that higher trophic level and so if you're in a pristine a ligatrophic low productivity crystal clear lake the idea of catching tons and tons of really large fish is practically impossible because the ecosystem cannot supply enough food to supply large numbers of large trophy fish to be taken off by fishermen so this this characteristic of food webs and the limitation of transfer of energy is an important concept to keep in mind as we manage natural resources food webs tend to be relatively simple with only three to four trophic levels present within any food web as we increase the productivity of a system we tend to see the potential for higher numbers of trophic levels but it is extremely rare to see anything more than about four trophic levels because of this fundamental characteristic of the efficiency of transfer of energy the other thing that happens is in terms of the strength of the connections the connectance decreases as we increase species richness as you have more species the connection between any two species will diminish because there are multiple connections that are possible providing a stability to that ecosystem and then lastly arm omnivores are relatively scarce typically food chains only have about one omnivore per major predators the omnivores feed on species at multiple trophic levels and as a result can greatly modify the dynamics that we would expect from the trophic levels present within an ecosystem so how do we control primary production in ecosystems this consideration of the controls on the plant production in an ecosystem led to the development of a very interesting concept only about one half of the variation in primary production that we see around lakes around the world can be explained by the nutrient supply so that's saying that if you're trying to understand what controls the productivity of lake only about half of them can be explained by the supply of nutrients in that lake the nutrient control is known as the bottom-up effect you're controlling the structure of that ecosystem by the food resources the energy sources at the base of the food web and so this is a bottom-up effect on that food web an example of bottom-up control would be seen in this example here in bottom-up control we see that an increase in nutrients leads to an increase in phytoplankton the plant production in the trophic level above and then an increase in the phytoplankton leads to an increase in the herbivorous zooplankton or the microscopic herbivores that are feeding on those plants suspended in the water and an increase in those herbivorous zooplankters leads to an increase in the vertebrate zooplanktovores or the vertebrate animals like fish that are eating those microscopic invertebrates and then an increase in the vertebrate zooplanktovores leads to an increase in the pisivores or the vertebrates that are eating other fish or other vertebrates this would be a bottom-up control from nutrients leading to increase in plants to the herbivores to their consumers and finally the top-level carnivores this is an example of bottom-up control in a concept known as the trophic cascading concept in the trophic cascading concept another form of control is recognized and that's top-down control in top-down control a pisivore or top-level carnivore such as say a largemouth bass in a lake an increase in that carnivore can lead to a decrease in its prey items the vertebrate zooplanktovores such as a bluegill as the bluegill numbers would go down their prey items the zooplankton the microscopic invertebrates would increase and as these microscopic zooplankton increase their prey items the phytoplankton would decrease and as the plants decrease they have less demand for nutrients and we would see that the nutrient concentrations could be higher so in this example the introduction or the abrupt increase in a pisivore at the top of the trophic structure in this system could lead to a decrease in the plant material thereby explaining some of the variation that we see in primary production in these ecosystems so we have two different explanations for the control of the amount of plant production in these ecosystems one is bottom-up control in which nutrients are driving the productivity and trophic structure of the system but the other is top-down control in which a series of cascading effects down through the trophic structure determine the amount and the productivity and the structure of the plant communities and so this trophic cascading concept can be an important management tool as we consider how to manage lakes with similar nutrient levels but different food webs and so we can take these principles these fundamental ecological principles of community control and trophic structure and start to manage fisheries for both fisheries productivity and resources but also for water quality and the dynamics of a lake in a four trophic level system we might have a pisivore a fish that consumes other fish such as bass or a pike or salmon a zooplanktovore a fish that consumes zooplankton the herbivore like the zooplankton that feed on plants and phytoplankton the primary producers so with this top-down control as we talked about the rise in the pisivore biomass initiates a cascade this planktovore biomass declines the herbivore biomass increases the phytoplankton biomass decreases and so let's take a look at some of the organisms that might be responsible for these dynamics first of all the cladocerans or water fleas present in many given many different lakes around the world major herbivores and the presence of large cladocerans typically means that there's a very heavy predation pressure on the phytoplankton in a lake copepods are another major zooplankton that we find they tend to have much longer life histories they only reproduce sexually and so we tend to find them in more permanent waters and they have a variety of types of feeding habits from detritivores to herbivores to predators we also find rotifers that tend to be very small have very short generation times can reproduce either sexually or asexually and they can kind of explode they're the weedy species in the zooplankton communities and they can move in and respond very quickly to changes in an ecosystem so does this always work does the trophic cascading concept always explain the way that an ecosystem works well typically food webs are more complex than just a simple four level almost food chain with just one representative at each level as we talked about quite frequently we see that our we have food webs with many representatives at any trophic level rather than just one this means that they are more dynamic at any given trophic level and so we might see the change in one species being compensated for by an opposite change in another species at that same trophic level so sometimes we don't see the strong top down effect because the food webs are just simply much much more complex than this simple four level system time lags sometimes mean that the responses occur after the change in pysivore biomass or reproduction so there might be a lag so that you don't see the effect immediately when you get a change in the pysivores it may take some period of time that means that it will start to obscure the response that you might see in the lower trophic levels and then finally organisms can shift in their feeding habits and so and particularly as they go through different stages of their life history a young organism can have a totally different feeding habit than an older organism for instance small bass tend to feed on zooplankton and larger bass start feeding on vertebrates and eating other fish and become pysivorous so based on the life history stage or the type of species that's out there you may get different responses in feeding habits and so sometimes we don't see the patterns as strongly there are several case studies about how removal of zooplankton from lakes usually by poisoning can affect the structure when we've done these experiments we've seen quite frequently that if you eliminate the zooplanktivorous fish that there is an increase in the zooplankton and the phytoplankton in the chlorophyll decline and the second disc disc transparency increases a second disc is a disc that we lower into the water the deeper that you can lower and still see the disc the more clear the lake will be and so as the phytoplankton and the chlorophyll decrease the clarity of the water or the transparency of the water will increase and so in this case of top down effects as we change the abundance of the zooplankton through changing its consumer we can see that the phytoplankton decreases and actually the lake becomes more clear the management implications are all of this are that you can stock pysivores or a bivorous zooplankton so that you can get the kind of top down effect that you would want to decrease the primary production that you might have a problem with in a utrophic lake if a lake had too many nutrients if the nutrient loading was excessive and everything was being done to try to reduce that loading but it still had not had the full outcome that was anticipated or desired then one of the possibilities is to start to manage the trophic structure so that you have intense predation pressure on that phytoplankton community so the lake will clear up even more and so you would have to understand trophic cascading to know how you might modify the abundance of organisms at higher trophic levels to increase those consumers those zooplankton herbivores that are feeding on the plants and so trophic cascading can be an important management tool that's being used in streams and lakes of the midwest to start to manage for trophic structure and productivity it represents a blend of fisheries biology and limnology and water quality management and so you have to understand the dynamics of the fisheries the trophic structure the dynamics of a lake system and how water quality is determined to manage an ecosystem using this concept in some cases this type of biological management can substitute for engineering solutions or hard fixes to try to control excessive growths of algae or phytoplankton now let's shift from lakes to rivers for a moment one of the major concepts that came out to understand the trophic structure and the energy flow in streams and rivers was a concept known as the river continuum concept it recognizes that there are patterns of physical processes from headwaters to large rivers very predictable patterns and that the patterns of ecological processes also change in a predictable fashion from small stream to large river and that these continua of physical processes and ecological processes occur within river networks and as we get down to large rivers they are connected to their very important flood planes and the dynamics of a river system can be viewed from small headwater stream all the way down to large floodplain river through this continuum of physical processes and ecological processes now very simply stated the river continuum concept says that streams represent a continuum of physical chemical and biological characteristics extending from headwaters to large rivers this has been debated and the utility of this concept has been debated now does it make any sense well let's take a very simple look at a river system from a small headwater stream to a large river but in a small headwater stream that's so small that you could step across it you know that you're going to have a small channel it will be relatively shallow it will be very simple for any kind of forested vegetation around that stream if that's what's appropriate for that landscape to shade that stream and so not very much light is going to reach the stream and lots and lots of leaf material will come falling in as a result we will see large amounts of alachanus organic matter coming into that stream and very little alachanus organic matter so lots of leaves and needles lots of wood very little algal production because it's going to be dark and as a result the community will be dominated by shredding feeding functional groups that are shredding a lot of that leaf material the scrapers will be a relatively small part of it because there's not much algal production and the carnivores will be there eating the shredders and some of the collectors that are feeding on their leftover materials now so as we see in a headwater stream there's a dominance of alachanus inputs the shading by the canopy reduces the primary production the aquatic invertebrates are dominated by organisms that shred this alachanus organic material and the collectors that feed on the small bits and pieces but as we go to larger streams how would you think it would change think about it the stream gets wider what's going to happen to the canopy above the stream it's going to open up you're going to get more sunlight reaching the stream you get more sunlight you get more photosynthesis the algae growing in the stream will be more productive and so we start to get more alachanus organic material and we get slightly less alachanus organic material because that influence from the side of the stream is now being diluted across this wider stream bed and so we start to get the combination of both alachanus and alachanus organic material so algae and leaves both become important so we see an increase in the importance of the scraping functional groups a decrease a slight decrease in the importance of the shredders the collectors are still important because everybody's messy and dumping lots of stuff around and so the collectors are still out there collecting all the fine particles and then the carnivores are ready to go in and feed on all these animals that are doing their thing so we see that we get significant inputs of both alachanus and alachanus organic material that the aquatic invertebrates are dominated by the scrapers and the collectors but if we go to a large river a really really wide river system the alachanus inputs are limited to this fringe of trees growing along the edge of the river and so we see the autachanus input or the algal production and the macrophytes and the mosses being extremely important because there's a tremendous amount of solar radiation reaching the stream and then in really large rivers we also get phytoplankton growing in the water column and then the alachanus material is less important and so in this large river system with all of the fine particles the collectors become a very major feeding functional group in the large river system feeding on the fine particular material that's raining in from upstream and also the phytoplankton that's being produced in the water column so we see in large rivers a shift toward planktonic primary production or plants growing in the water column we see a dominance of the fine particular organic material because all the material that's being processed upstream comes down as it process it continues to get smaller and smaller and smaller and the aquatic invertebrates are dominated by collectors feeding functional groups that feed on these fine particles now one of the things that has often been overlooked in this little story or vignette that we gave you is the importance of flood plains because extensive floodplain development means that you start to get lateral processes on the floodplain and so the forest or the terrestrial ecosystem can be extremely important in the productivity of large rivers through floodplain dynamics this is not the same as the influence of the terrestrial ecosystem in the small headwater stream which is just the dominance of inputs or the control of inputs directly to the surface this is the dynamics of the floodplain during floods and high flows a subject that we will return to in the river continuum concept we commonly assume that the stream side area will be dominated by wooded forest and to the extent that your region or landscape is dominated by a forested ecosystem the kinds of trajectories or kinds of patterns that we just described would be expected but that's not always the case all regions and all streams don't necessarily have forested riparian areas there are many natural non forested riparian systems such as grasslands such as wet meadows in high alpine areas such as tundra streams there are many streams around the world that do not have naturally wooded riparian systems and in this case for the appropriate application of the river continuum concept you would have to adjust your predictions for the type of terrestrial interactions you would expect with the terrestrial ecosystem and its characteristics with that aquatic ecosystem so one of the misconceptions has been that the river continuum concept says that all streams around the world are wooded it doesn't really say that it just says you have to assume the appropriate type of terrestrial ecosystem but at the very heart of it the river continuum concept just gives you a model to say what would you expect for a certain size river with a certain type of terrestrial interaction in a river system with water flowing downhill that's what you would expect you may not see it and that's the value of the concept to order your thinking and give you a concept and then you go out in the field and see if the real world really works that way one of the ideas that we've been kicking around within the river continuum concept has been the term riparian so what do we mean by riparian in riparian areas there are many definitions of riparian the simplest definition of riparian is a legal definition that it just means property that is adjacent to a stream or a stream site and a simple definition also is just a linear definition of a riparian area it's the edge along a stream and so the only riparian consideration is that linear extent we can consider kind of an area or a planar perspective of riparian systems and that would be the surface area on which riparian vegetation might grow but actually all of these are rather limited more ecologically relevant definition would be a functional definition that sees a riparian area not as a edge not as a surface area but actually as a three-dimensional zone of influence that extends above a stream laterally away from a stream under the stream in the hyper-rig zone upstream and downstream linearly so it's three-dimensional along a river network and then finally we can consider a structural definition of riparian areas that sees them as mosaics of geomorphic surfaces that are created and maintained by disturbance the primary disturbance being flooding and the geomorphic surfaces and terrestrial plant succession create the riparian area I'd like to make a point here about the term riparian even within this video you may have heard the term riparian ecosystems and I'm going to point out to you why I would prefer not to use the term riparian ecosystems even though it seems to make a lot of sense the term riparian is important in management because riparian areas are one of the most complex diverse productive pieces of the landscape riparian areas really are ecotones what is an ecotone that's a basic ecological term that refers to interfaces between different ecosystems so an ecotone is the gradient between two adjacent ecosystems and an example in terms of a stream and an upland forest this riparian area is this gradient very sharp gradient in physical and biological properties of the terrestrial ecosystem as it grades into the stream ecosystem and the stream of ecosystem as it grades into the wet areas in the terrestrial system and so it is rich and diverse because of those very sharp gradients because of its ecotone because it has two ecosystems coming together and that's why the concept of riparian ecosystem overlooks that importance of the richness of the two ecosystems and their interface now within these ecosystems there is another component that I would like to discuss and that's the role of salmon carcasses now I don't want to make too big a deal about the salmon carcasses because there are many parts of the world that never had huge amounts of salmon and those portions that did quite often they've been greatly decreased in recent years but it is important to recognize in terms of trophic structure and nutrient dynamics and energy flow that organisms and their movement across the landscape can have tremendous impacts on productivity an example from the northwest are the carcasses of salmon as salmon come back to spawn they die after they lay their eggs and so these carcasses of the spawning salmon provide nutrients that are important for the food web that supports the young salmon as they rear before they go back out to the ocean so the productivity of many species of salmon has been linked to the abundance of the carcasses of the adults as they spawned to start that next generation but it goes beyond that people have linked the productivity of other parts of the ecosystem such as in this example the forest beside the stream researchers have found that the growth rates of riparian trees in areas with carcasses are higher than riparian areas nearby in stream reaches without carcasses and so the movement of salmon carcasses into the terrestrial system either by flooding or by consumers such as bears and raccoons dragging those carcasses are basically moving nutrients to fertilize the riparian forest and so we see these strong links between trophic structure and nutrient dynamics and the energy flow that starts to shape the productivity and function of ecosystems many other ecosystems have similar movements of organisms that result in these concentrations of carcasses of body parts of waste materials that are important nutrients to supply and support the younger generations in those ecosystems now one of the things that's often overlooked in management are floodplain systems and we find in streams and rivers and lakes and for that matter terrestrial ecosystems floodplains are some of the most ecologically rich and highly productive pieces of the landscape floods do many many things the role of floods includes creating aquatic habitats shaping those habitats through forming pools by eroding and depositing riffles creating complex habitats and accumulations of large wood uh supporting and regenerating and renewing floodplain vegetation and then nutrient exchange nutrients exchange from the river to the floodplain and from the floodplain to the river and so floods play many many important roles in streams one of the important aspects of floods is that a flood is a time in a stream in which it has energy imagine what it takes to sculpt a stream to shape a pool to deposit a river when will the stream have the energy to dig that pool and deposit that river it will be during a flood and so during that flood you have the high velocity that has the energy to pick up sediment to transport it downstream to deposit it in rivers creating the shallow areas and shaping the aquatic habitat so if you want to understand what shapes habitat in a stream you have to understand a flood if you want to destroy a stream you eliminate floods because you eliminate the process that shapes the habitat this moving water is an extensive force that not only shapes the habitat but also cleans the silt out of the gravels and deposits this fine particular material out on the adjacent margins and floodplain and that's one of the reasons that floodplains are so productive is that floods move the silt out of the stream and deposit up on the floodplain and that's one of the reasons why many stream fish deposit their eggs in the gravel such as spawning trout and salmon putting their eggs in the gravel that's a bummer of a life history stage if you have a lot of silt down in those sediments consuming the oxygen all the eggs are going to die but thanks to floods cleaning that out and keeping an open trickling gravel environment you have an environment that's well oxygenated well protected and a good place to build a nest for laying your eggs we see the habitat in any stream being shaped by this complex history of floods interacting with the topography with the stream bed with the vegetation creating a very complex habitat we see this also in large rivers and when we were out on the Willamette River recently with our crew we stopped off in a side channel along the Willamette River to take a look at the importance of these side channel habitats and floodplains in these high flood events and how they start to influence the life that we see in a large river system off channel habitats like alcoves and side channels as you see here are extremely important habitats for aquatic ecosystems they provide rich food supply in terms of the deposits of organic material that they contain and the riparian forests that line them and they also serve as critical refuges during high flow events during floods rivers turn into high velocity very turbulent habitats and these off channel habitats provide low velocity sanctuary and hiding places during these floods many times the public and landowners are concerned when trees start falling from the banks into the river but actually from the river ecosystem point of view that's a really good thing this large wood provides important habitat and food resources and provides refuge for the aquatic organisms in the river in a small stream large wood like this would be stable and persist for many years even decades or centuries but in a large river as we see here the high flows are going to be adequate to move it around and we'll find it at the heads of side channels like this wood or up on flood plains and it still provides a very important habitat and ecological function as we can see here this large wood captures the smaller wood and provides retention and all up the bank we see the large wood providing complex habitat and some of it has a large root watt which is kind of an anchor and helps stabilize it and provide persistent habitat and so the wood here in the river can stabilize the sediments provide complex habitats slow down the velocity and provide low velocity habitats create complexity provide food resources because there's algae and microbes growing on the surface of this wood insects feeding on that fish feeding on the insects an entire food web associated with this large wood and so when we see wood in a large river like this or when we see the forest start to fall over into the river it's the start of a very important ecological function in riverine ecosystems one of the interesting things about floods is that anytime a flood occurs in any given region the news media quickly sweeps out they stick a microphone in everyone's face and say tell us how bad it is and they want to hear about the death and destruction and how horrible it is but actually for the stream of river or even the lake adjacent to that stream of river that flood is an important ecological event and provides many very positive functions and you try to tell people about this and they just don't believe it can't be true but actually we've already talked about how it shapes the habitat but one of the questions is can the organisms survive the intensity of the flood here's an example from one of the streams we've studied in the cascade mountains of oregon and we see a graph here of a stream from the late 1980s through 1998 and in the winter of 1996 we had a flood in oregon and in this particular stream it filled up the active channel and had about two meters of water flowing above the active channel extending out onto the flood plain this is a 10 gradient stream this is a steep stream really high velocity the flood was ripping and so you would expect the fish to be washed downstream or wiped out but what we see here is the this is the point this is the population of cutthroat trout that were present in a tear cut and old growth stream prior to the flood the flood occurs right here where you see the white arrow and then this is the population of cutthroat trout that we observed in the stream the summer after the flood we had more fish not less and so how in the world could that be well two things happened first of all the adult fish survived the flood extremely well the populations of adult trout in the old growth forest did not change at all they were the same as the populations of adult trout the year before not only that but about 40 of the fish that we tagged the previous summer were still present in the same 150 meter reach and so not only did they survive the flood they stayed right where they had been the year before and then in addition to that we saw four times as many young fish the fry the flood had scoured the silt out of the sediments allowing the adults that had survived quite well to spawn and lay their eggs in a much better spawning environment and so we got a population increase instead of a population decrease now in other streams that have been converted into pipes and there was no flood refuge we saw that the trout populations decreased abruptly and so where we have intact functional riparian systems and flood plains these communities and populations are well adapted to deal with the natural disturbances but where we modify it and simplify it they're not as able to deal with the natural disturbance regimes that occur this has led to a concept known as the flood pulse concept the pulsing of river discharge is one of the major forces that controls the biota in river flood plains these pulses of flooding are very very predictable think about rivers like the mississippi the amazon you know exactly when they're going to be flooding and the organisms that have evolved in those ecosystems have experienced that flooding regime for years after years and they've evolved life history strategies so that they take advantage of this flooding event so these flood plains are areas that are periodically inundated by these lateral overflows of the river into the flood plain surfaces and by the direct precipitation or rainfall on those flood plains or the groundwater coming up during the flood events this environment causes causes the biota to respond with unique adaptations and processes that are characteristic to these flood plain systems in many of these river systems the reproduction of the fish is tied to the flood cycle so that right at the peak of the flooding or immediately after the flooding the fish spawn lay their eggs so their young can produce and take advantage of the nutrients that were brought in during flooding in some of the amazon river systems and tropical river systems that have extensive periods of flooding we see that the river fish actually even harvest food on the flood plain and they have special adaptations such as fish that eat on fruit on the trees and so as the trees are flooded the fish swim among the branches eating on fruit and they have special adaptations and mouth parts that allow them to feed on these flooded fruits of the flood plain forest within this concept of the flood pulse we have this idea of the transition in inundation this has been termed the aquatic terrestrial transition zone it's an area that alternates between being aquatic and terrestrial this moving literal zone or moving wetted zone in the inshore edge is called the aquatic terrestrial transition zone and so we get a transition from low flow to high flow and the communities and habitats and characteristics of that transition zone differ sharply from the part that's permanently wetted or the part that's permanently dry so the flood pulse concept works in to some degree in all rivers it was developed first for long duration flooding rivers like the amazon river that flood for months on end and it's an important concept in managing these tropical rivers and understanding their dynamics but we also have streams throughout much of temperate north america and other regions that have short duration floods the organisms in these systems can still be adapted to these short duration floods and have very unique life history characteristics that take advantage of these flooding events and even flash flood environments in the desert southwest where the flood systems are dominated by abrupt and very short flash floods many of the native minnows spawn immediately in the reseeding waters of those flash floods and so they take advantage of that one event lay their eggs and get their young in in that period in between that flash flood and the next flood period so when we stop and think about a flood and a stream ecosystem that leads us into a concept in ecology known as disturbance ecology so what is a disturbance well this has caused a lot of debate among ecologists and early on in many of these studies people started finding that their ecosystems were dependent upon disturbance not only did they kind of tolerate it but they had to have it pretty much like the example that I just gave you of a river that if you don't let it flood it's going to silt in and become less productive and actually the absence of flooding is a great detriment to that ecosystem and the presence of that disturbance or flooding keeps it vibrant and alive and renews it and so people started looking at it and they said well how do you define disturbance then because it's not just a bad event so what is a disturbance one of the best definitions of disturbance that has been developed is one that was developed by White and Pickett and they stated that disturbance is any relatively discrete event in time that disrupts either the ecosystem or the community or the population structure so it can it can affect any one of those organizational levels of the biological system either the ecosystem or the community or the population structure but it also changes the resources the substrate availability or the physical environment around the organism or community and so we see that a disturbance is relatively discrete it's not a gradual change it's fairly abrupt in time it's it disrupts now i didn't say it decreases it could actually disrupt it by increasing so a sudden dumping of nutrients into a system could be a disturbance but it would cause an increase in production so a disturbance is discrete it disrupts the ecosystem community or population structure and it changes the resources the substrate availability or the physical environment now so from this definition how will disturbances change the community structure that we see in an ecosystem one of the concepts that relates disturbance to community structure is the intermediate disturbance hypothesis in this relationship we see we relate the magnitude or the frequency or the intensity of disturbance along the x-axis here to the number of species that we find in an ecosystem the diversity or species richness at extremely high levels of disturbance we find relatively low numbers of species as we decrease the magnitude or intensity or frequency of disturbance we get an increase in the number of species such that we get our highest richness at some intermediate level of disturbance and then the richness declines as we decrease the magnitude or frequency or intensity of disturbance so why would we get the highest richness of species at an intermediate level of disturbance the explanation for this has two parts first of all at really high levels of disturbance there aren't going to be very many species that can tolerate those high levels of disturbance and so it's only made up of those species that can resist the intense disturbance regimes as you start to make the disturbance less frequent or lower in magnitude more species can exist in that environment and so we get an increase in species richness but that still leaves us with the problem of why would we get a decrease in species richness with decreasing disturbance and that's where biotic interactions kick in as we decrease the magnitude of disturbance the physical environment is playing less of a role in modifying or reducing the distribution of organisms and so we start to see organisms using up the available resources and starting to compete for resources so at low levels of disturbance competition between organisms between populations for limited resources means that the the higher or superior competitor is going to win out over another species and will start to see a decrease in richness through the process of competition so as a result we see that the disturbance regime that is natural to an ecosystem starts to shape the structure of communities and the flow of energy that we would expect in that ecosystem if we modify disturbance regimes then we would expect to modify the structure of communities that are present within these ecosystems another major disturbance that we find in ecosystems is the introduction of exotic species or introduced species these are species that have not evolved within the communities and so the interactions can be particularly strong these organisms may or may not be well adapted to the natural disturbance regimes in the southwest we found that the introduction of species at first seemed optimistic because the native species were better adapted to flooding than the introduced minnows and the minnows were washed out by the floods but further experimentation showed that they were able to start to learn and so these introduced species did start to learn how to deal with disturbance as they experienced it more and more so as we modify our landscapes it makes the habitats more available to be occupied by these introduced species from other regions now it's not just introduced fish such as carp that you see here in this picture in river systems we see that there are patterns of the abundance of native species and the introduction of exotic species in the Willamette river basin for example we see that in the headwater portion of the main stem approximately 96 percent of the species that were encountered were native fish species and only four percent were introduced but as we head downstream and become more urbanized and we have more of the riparian and flood plain forest converted to agriculture residential use and urban use the proportion of native fish species decreases and the proportion of introduced or exotic species increases and so these alien species soon become more than two thirds of the fish community and only about a third or less of the fish that we find in the urban areas down near portland or the mouth of the Willamette river are the native fish species and so either the introduction of the exotic species or the habitat degradation that occurred along this main stem river led to a replacement of native species by exotic fish species now this doesn't occur just in fish and there are major concerns about introductions of exotic plants and animals in both terrestrial and aquatic ecosystems another example that we commonly find in many of our systems is the introduction of exotic amphibians bullfrogs for instance have been introduced far outside their native range and have replaced many of the native amphibians or led to the declines of native native amphibian populations and so the introduction of organisms into an environment may seem like a good idea for a particular use but quite often you're putting a new character into the mix of the community that has evolved in a given region and there's always a great danger that you may disrupt critical community interactions by introducing exotic species only by understanding the flow of energy through these ecosystems can we start to manage the landscape whether we're talking about forests or grasslands streams or lakes urban areas or farms it is this flow of energy and the structure of the community that start to shape the processes that determine the landscape land uses can be developed to conserve ecological function and to restore the productivity and community structure required in a healthy and dynamic landscape