The Effect of Environmental Stress on Vulnerable Streams (The Streamers)
This topic submitted by Mark Momper, Jesse Murphy, Elizabeth Nellums, Craig Riffle (firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com) at 6:28 PM on 12/10/02.
Additions were last made on Wednesday, May 7, 2014. Section: Cummins
The degradation of natural habitats is a phenomenon familiar to us all; there are few ecosystems left in the world that have not been affected by man in some way. Human practices like strip mining, logging, and even agriculture are devastating to the land that is used, and this directly affects every living organism on that land. Many scientists believe that the overall consequence of habitat degradation will be an effect called homogenization of species. This is the term used to describe what happens when one or two species become dominant and replace a formerly diverse food web. There are many ways for homogenization to occur; invasive species can enter an ecosystem and wipe it out, or changes in the ecosystem itself can select against all but a few of the species; this is seen when environmental factors like acid rain or temperature changes alter the environment significantly. If the pH of a stream goes down, for example, all but the most acid-resistant species will be eliminated. The stream will have only a tiny percentage of the diversity that it originally supported. Human eings can directly alter ecosystems by polluting them, or by building up around them. Sometimes even small changes, such as the loss of one species, can have huge repercussions because ecosystems are so interconnected. For example, the loss of a seemingly insignificant type of toad due to light pollution can send shockwaves through the ecosystem; the mosquitoes are no longer kept in check, so their larvae support larger populations of dragonflies, who in turn attract new species of birds, whose wastes cause nitrogen excess in the stream; weeds overgrow, oxygen is consumed, and eventually the stream becomes fetid and cannot support life. This may be an extreme example, but the principle is correct. In most cases, however, ecosystems can suffer damage to their health and not shut down. Its a good thing, too! Human beings have damaged so many ecosystems that we are depending on natures ability to sustain itself. Most ecosystems have been damaged to some extent, but are still capable of performing most of their functions in the environment. How, then, do we measure the health of an ecosystem? It is important to understand the status of our environment so that we can monitor it, in the hopes of keeping it healthy. Because of this necessity, scientists, use the diversity of life found in the ecosystem called biodiversity as the measure of stream health. A stream that supports many different species is considered healthier than one with only a few types of fish. Likewise, the numbers of individuals is also necessary to consider; a healthy stream is a wonderful ecosystem and should be literally teeming with life. If a stream has only small numbers of each fish type, it is taken as a sign that the stream is not healthy.
What is the relevance of ecosystem health? Besides the obvious that we all depend on nature for every single life process we perform it is also very important for us to be able to keep track of how we are affecting nature. We will never know that we are killing of streams with our light pollution until we have some way to measure the health of our streams. By the time that degradation is blatantly obvious such as when it is a fetid pool it is often too late the reverse the damage. This is why the measurement of biodiversity is a useful one. Effects on the diversity will occur long before any noticeable changes would develop. In our experiment, we termed such streams vulnerable, meaning that we knew their health was affected but did not know to what extent.
What we have implied, then, is that there is a certain point beyond which nature cannot recover, and even if we were to leave an ecosystem alone, if it was damaged to this extent, it would not be able to survive. Becoming a sort of dead man walking, the ecosystem might continue for some time but its eventual decline is inevitable. But what is this set point? How will we know when an ecosystem has crossed this line? Are there streams in our area that have already crossed it without our knowledge? We set out to find out.
Finding a stream that had previously been identified as vulnerable was easy; a study performed last year called the Fishbusters study had proven statistically that diversity was lower in a stream known as Collins run, which was affected by development in its watershed, than a neighboring stream in a park, Harkers run. The Fishbusters also gave us an empirical map of the diversity of these streams, which is what we needed; something to compare current findings to. (See the Fishbusters study for a more comprehensive report of their findings.) For the entire past year, the Ohio area has been struck with a semi-serious drought, (click here to see drought data from the Palmer Drought Index.) If, we reasoned, Collins run had fared worse in the drought than Harkers run, it would be evidence that the vulnerable Collins was damaged enough that natural environmental stress was killing it. This would prove the theory that a stream, once damaged, cannot always recover - even if relatively little new damage has been inflicted. Thus we began our study, calling ourselves the streamers to reduce the chance of acquiring a theme song.
The Shannon-Weiner index of diversity was invented by Claude Shannon and Norbert Weiner, neither of whom were ecologists. In fact, they worked for a phone company! They developed a mathmatical model of what was needed to set up a phone system, taking account of everything that was needed and everything that was had and making them into one number. The inventory mathmatical model was not sensative to the type of data that was inserted, so it was applicable to any system! The work of these mathamaticians was important for much more than they had anticipatead - science is like that!
These are the predictions for our study:
- Rainfall significantly affects the diversity in an ecosystem. Due to the drought this year, the fish diversity in both Collins and Harkers Run should be less compared to a non-drought year.
- As found in the Fishbusters study, the diversity in a damaged ecosystem is considerably less than in a relatively undisturbed ecosystem (2001). This should still hold true for this year.
- We predict that the diversity in the disturbed stream, Collins Run, will be more affected by this years drought than the undisturbed stream, Harkers Run, due to human influence.
Our hypothesis is that the diversity of both streams will be statistically lowered due to the drought, but that Collins Run, a stream that was affected by human development, will be statistically more damaged than Harkers Run, which flows through a preserve.
The purpose of our experiment is to measure the effects the recent drought has had, using diversity as an indicator of health. We will compare the data we collect to the research done by the Fishbusters lab group in 2001, which acts as our control, since it gives us the streams health when there was no drought. The Fishbusters lab group measured the diversity of fish populations in Collins and Harkers Run and then compared the data between the two.
40 percent of streams are too degraded to fully support aquatic life. The Missouri river has seen an 83% drop in commercial catch in 1947, according to Imperiled Waters, Impovershed Future. In North America there is 950 known freshwater species, 2% of which are extinct and 37% or which are imperiled. These losses are due to habitat degradation and the introduction of exotic species.(Freshwater Ecology, 2002)
We hear frightening statistics like these everywhere in our society, but we're still inclined to disbelieve that the environment is as damaged as it is. As a group, we learned much from the study of the effects of pollution and drought on streams. The impact of experiencing firsthand what we had only been told about before has given us new insight on how we need to interact with the environment in the future. If we continue to pollute habitats such as rivers and streams the effects will be devastating.
Damaged ecosystems are increasing in number, and studies such as ours can be used to assess the capabilities of these ecosystems to recover from the damage that have sustained through pollution and other human activities. Streams that are damaged start off with a disadvantage compared to healthy streams, and then fall further and further behind as environmental stresses (like drought) set in. It's almost like a "natural selection" of ecosystems, and a damaged stream is less fit, and doesn't survive. If this is true, what about places like Costa Rica or the coral reefs? Can we just leave them to recover on their own, or is this setting them up to die? Furthermore, many habitats are dependent on individual species, and should these species die the habitat will become damaged. On top of this effect many habitats are interdependent, the effect on one habitat can be felt through out them all. (Abramovitz, p. 5-10) This indicates that it is necessary to protect ecosystems at all costs.
The book Riparian Ecosystem Recovery in Arid Lands says this of ecosystems that have been disturbed; "Once the causes of decline are addressed, the ecological resiliency . . . takes over, often leading to the dramatic recovery of the system." (p 26) But is this really true? Can we really continue to depend on nature to recover time and time again from our repeated abuse or do we need to intervene?
Materials and Methods
Our design, which features a random sampling technique, avoids bias by using statistics to arrive at a conclusion. We try to capture every fish in the section of stream, which even if impossible should result in a good representative population. This is why we use the electro shocker as opposed to any other method, such as nets, to capture the fish; it guarantees a large, evenly distributed sample of the stream as a whole. After that it is a matter of mathematics to validate or disprove our thesis, because either the populations are statistically comparable in the face of the drought, or they are not.
Electro Fish Shocker
Rubber boots: Both full length and Knee-High
Multiple Fish Nets
The procedure for using the electro shocker and collecting the fish follows.
We test two different streams for our survey, an undisturbed stream and a disturbed one. We tested each stream twice over a large area.
- Everybody has several layers of warm clothing on, as well as long wader pants. These were attached to rubber boots and were actually quite important because they kept us from being shocked and from freezing in the cold stream water.
- The person who is going to wear the shocker puts it over their shoulders like a large backpack, while attempting to keep most of the weight of the machine balanced on their hips. This is a difficult job because if the person leans too far forward the engine floods. When it's all loaded up, we start the engine. We use 1.5 amps to stun but not kill the fish.
- The people who are acting as netters get on either side of the shocker, although not in front of him due to the risk or stirring up the silt.
- The person, who is wearing the electro shocker, keeps the wire tail in the water sets the shocking pole/net into the water. He or she then activates the devices. As the machine is activated it beeps it sends an electric pulse into the water. The person sweeps the shocker from side to side, stirring up the water and going around the rocks and banks.
- The shocker and his netters move up the stream slowly, trying to gather samples from each section. This is important because a length of stream contains different depths and temperatures, as well as different concentrations of rocks and refuse. We are trying to sample the stream as a whole.
- The netters attempt to net all of the fish that come to the surface, attracted and shocked by the electro-shocker. While it may not be possible to collect every fish, in the attempt to do so we should get a good representative random sample.
- The collected fish are deposited into buckets for later sorting.
- After the sampling is finished, everybody gets out of his or her gear and the identification process begins. The fish are first sorted according to species (this is where Donnas help is invaluable) and then counted. Several factors that determine the species of fish are the color, body shape, and the mouth type, features of the fins, and the arrangement and appearance of scales. Our results are included, see table.
- The fish are released back in the stream when data collection is complete. Electroshock should not kill the fish, although a small percentage of fatalities can occur.
The method for sampling invertebrates follows.
- One of our group members holds a sifter net. While other members of the group stir up the water, rocks and leaves along the stream bottom. - The disturbed water and material then flows through the net and is captured along with any invertebrates that had been attached to this material. - The sample is then stored in plastic jars for later counting. These processes of sampling are to be repeated in the other sample environments until each target area has been tested. Then, using statistical analysis such as the Shannon-Weiner test and a simple T-test, the data is compared both to each other and to the previous year. We need statistics to measure the difference between the streams this year and last year to prove that the drought has affected the streams, and compare the differences to prove our hypothesis that the damaged stream has fared worse in a difficult year. We will be using the Spearman Rack Test in Statview to illustrate the abundance of species. The Index of Dominance will be used to compare the health of the streams. Unhealthy creeks will have only a few or single species in domination as opposed to a more evenly dispersed population concentration. Shannon Index of General Diversity will be used to calculate the biodiversity of the populations within the two creek systems.
(For results of invertebrate sampling Click HERE)
Shannon Index of General Diversity:
H = -&Mac183;(ni/N)log(ni/N)
Ni = Importance value (number of individuals)
N = Total of importance values
Index of Dominance:
ni = number of individuals per species
N = total number of species gathered
- On October 15th, at approximately 8:15 we started our first sampling at Collins Run. We worked until approximately 11.
- Our next sampling time is set for October 21st, also a Monday morning at 8. It will test Harkers Run.
- On Wednesday the 23rd of October we return to the streams to do necessary tests and document the streams fully in digital photography.
- After this point, all time until the project is turned in is devoted to research. We plan to spend the week following this second sampling researching the land use around the streams and running our statistical tests, to draw conclusions.
- On October 24th we presented to the class!
- The final project due date: Tuesday, December 10th
-.Wild party scheduled for this week.
(For a comprehensive documentation of our sampling results, Clic k Here)(For a comprehensive list of the dominant and least-represented fish in both studies, Click Here)
Spearman Rank Test
The Spearman Rank Test compares the similarity between two streams, in relation to the species that are present and their numbers. It is a complicated, and can accurately show whether two streams are statistically similar.
The most important result of the Spearman Rank Test is the Rho Value. This value, between 0 and 1, compares the similarity of the two streams. Any value that is .95 or higher states that the two streams are statistically correlated. So, we ran Spearman Rank Tests to establish how similar the streams were to the previous years, and how similar Harkers Run and Collins Run were in each year. None of our Rho values were over .95, but some were pretty close, implying some correlation between the streams.
Harkers Past / Collins Past = .337
Harkers Present / Collins Present . = 878
Collins Past / Collins Present = .840
Harkers Past/ Harkers Present = .270
The Rho value between the two streams in the Fishbusters study is very low. This shows that the streams are not very similar, and there is very little correlation between the two. However, between our two streams now, there is a somewhat high Rho value, meaning that the streams are pretty similar. The Rho between past and present Collins Run was also very high, meaning it did not change very much from the past year.
However, Harkers Runs Rho value is very low; only .270. This shows that Harkers Run is not very similar at all to that of the previous year. Why is this so? Could it be that Harkers Run is just more affected by droughts than Collins, or is it that it has been somehow altered since the last year? When we sampled Harkers Run, they had placed large stepping stones across the river with heavy machinery. We went upstream of that, so that interference should have been negated from our study, but there might have been even more damage to the ecosystem further upstream that could have affected our results.
Shannon Index of General Diversity
Fishbusters: Harkers = .888
Fishbusters: Collins= .788
Streamers: Harkers = .790
Streamers: Collins = .908
Describes how diverse the stream is. 0=Not Diverse, 1=Very Diverse
The results show that Harkers Run became less diverse from the previous year, while Collins Run became more diverse. Also, the diversity between the two streams is reversed from what it was last year; this time Collins Run, the supposedly polluted stream, is more diverse than Harkers. This could be for a variety of reasons, which we will explain later in the Discussion section.
Index of Dominance
Harkers = .2135
Collins = .1649
Describes how much the stream is dominated by individual species.
This shows that Harkers is more dominated by fewer species than Collins is. This supports the findings from the Shannon Index of General Diversity, as a stream dominated by few species is less diverse.
Collins Run Comparison Graphs (past vs. present) --see below
Harkers Run Comparison Graphs (past vs. present) --see below
These pie charts are a more visual representation of the difference in diversity. Each pie chart shows the relative abundance of the species in each stream. Using these images, it is easy to see the diversity of each stream. The steams with the lower Index of Similarity have more of the pie taken up by individual species, while those we found to be more diverse have a greater spread of dominant species.
Redbelly dace (pnoxinus erythrogaster) was the dominant fish species of Harkers run in the Fishbusters study.
This fish typically inhabits tea-colored habitats, found in lakes and small to moderately large creeks, or in pools away from the main channel in larger streams. The diet of this species is broad, and includes zooplankton and invertebrates, although they feed primarily on algae. It is mainly a herbivore and feeds on algae such as diatoms, but some aquatic insects are also eaten.
The dace lives in schools, often in association with stonerollers and creek chubs, and usually lives near the stream bottom. It is a hardy and adaptable species.
The central stoneroller (compostoma anomalum) was the dominant species in Colins run when the Fishbusters tested it.
This is a native species and is generally, abundant; it is found in small creeks throughout much of Ohio. It is found primarily in the riffles and runs of the smaller to medium-sized streams but also occurs in the larger rivers of the state. Stonerollers are bottom feeders utilizing a variety of plant and animal matter. Its main foods are algae and bottom ooze. Its tolerant of moderately turbid waters and increased stream temperatures; in fact, we found out that stoneroller populations can explode in warm, nutrient rich streams characterized by abundant growths of green algae.
The creek chub (semtilus atromaculatus) was the dominant species in Harkers run in our study.
Chub's live in small clear to slightly muddy streams, over sand, gravel, or rock. Creek Chubs range extends over the entire Eastern and central United States with the exception of Florida. Laboratory experiments suggested that higher relative abundance of creek chub was one of several important factors in the observed niche shift of brook charr. (Magnan and Fitzgerald 1984), meaning that they are highly capable of outcompeting dominant species.
The spotfin shiner (notropis spilopterus) was the dominant species in Collins run when we tested it this year.
The spotfin lives in lakes and in small to moderately large creeks. It appears to be quite tolerant of silty and turbid conditions. It inhabits lakes and small to moderately large creeks. It favors swift flowing waters over shallow sand flats. It feeds near the bottom during the day and nearer the surface at dawn and dusk when the feeding is most intense. It eats insects, vegetable material, and some small fish, and is especially fond of flies and mosquitos.
So what does this all mean? Evaluating a stream by its dominant species is a valid way of seeing what the differences are between streams (using under-represented species isnt so much of a good idea; some of those had probably migrated away by the time we tested - it doesnt mean that theyre dead just because we didnt find them) By knowing the requirements of each fish type, we can assume that a stream whose population is dominated by this type must have excellent conditions. The Fishbusters never did a profile using fish types, but we used their data to assemble one (above.)
From our results, it seems like the algae levels in Harkers run were lower by the time we tested it; thats why creek chubs (which eat insects) have gained ascendancy over the slime-eating redbelly dace. Algea levels fluctuate with nitrogen levels, causing a reduction of oxygen in the water when the eutrophication is high. However, there is not a satisfactory answer in this data as to why the stream would have lost diversity ecosystem is considerably less than in a relatively undisturbed ecosystem (2001). This should still hold true for this year. - We predict that the diversity in the disturbed stream, Collins Run, will be more affected by this years drought than the undisturbed stream, Harkers Run, due to human influence. Also, this was our stated thesis: " Our hypothesis is that the diversity of both streams will be statistically lowered due to the drought, but that Collins Run, a stream that was affected by human development, will be statistically more damaged than Harkers Run, which flows through a preserve." Now that the data is in, let's look at the ramifications: Unfortunately, it appears that our results don't agree with most of our predictions. The first part of the first prediction, that the streams would be measurably affected by the drought, was the only section that was supported, and it was a pretty safe bet. Rainfall really does affect a stream. There were less fish found overall this year than in the year prior to the drought. However, the diversity of each stream changed in different ways. Harkers Run, as predicted, became less diverse compared to last year. Conversely, Collins Run, the polluted stream, became more diverse. There is a theory to explain these results. According to some scientists, environmental stress can increase diversity by "shaking up" the ecosystem. Dominant species can be dethroned and formerly suppressed species can fill the niche. Meanwhile, new ecosystems can be made within the stream that support unique forms of life. This does not always ha! ppen, as it is well documented that degraded streams support less life.
Our second prediction was also proved inaccurate, due to the same reasons mentioned above. Despite our predictions streams in the year following a drought are not less diverse than in the past. The damaged ecosystem was found to be more diverse this year than the healthy one. One factor that could be at work is the actual preservation of the stream itself. Although we did not take this into account, when we arrived at the park, we found tire tracks and other evidence of disturbance in our supposedly undisturbed stream! Apparently the park management hadbuilt in stepping-stones for some reason. This was a serious detriment to our study, so we ended up moving far upstream of the damage site - but still, it has to be acknowledged that our point of reference, a so-called 'invulnerable' stream, has been compromised. Perhaps this explains its low level of diversity in our study. Also, it is unfortunate for our purposes because now we ! are not testing the exact same section as the Fishbusters study tested last year. We do not believe, however, that this discrepancy invalidates our findings.
The third prediction, which stated that the proportional diversity after the drought would be lower, was unfortunately incorrect as well - and this was basically our hypothesis! In fact, the results were the total opposite! The diversity was higher this year than last year, even after a drought, and the diversity of the 'vulnerable' stream is higher than the protected one! Oh well - the important thing is what is learned in the process, which is always the point of science. A proven hypothesis may be valuable for the purpose of pride, but a faulty one is just as likely to contribute to science. Even great scientists can never predict nature!
So what do these findings mean for the purpose of environmental degradation? Apparently, that old mother nature is tougher than we thought! She takes a beating and ends up using it to her own advantage - and lucky for us, too! Hopefully we will be able to depend on nature to recover from our foolishness until we develop methods that are sustainable.
Another possible conclusion is that biodiversity may not be a good indicator of stream health. Since we found good diversity of both invertebrates and fish, that should translate to two decently healthy streams. But that's not really the case - Collin's run had garbage floating in it, for goodness sakes, and a giant slimy pipe, and we documented at least one invasive species (the little clam)! Perhaps better ways of monitoring stream health need to be discovered, or perhaps better ways already exist and we should use them instead of Shannon-Weiner. This implies that we should continue to be careful about nature (which is a pretty good idea, in my opinion.)
Although our results seem to devalue the Fishbusters' conclusion, this does not indicate that there were flaws in that study, or ours - rather, it suggests that in the year between 2001 and 2002, some pretty interesting things happened! We may not have established our hypothesis, but it can't be said that the results were inconclusive. Things changed! Species that were formerly dominant no longer are, and species that were under-represented were not even found in our study. Meanwhile, some new species were found that were not recorded in their study - this indicates that they have multiplied in the interim. The most curious thing is, none of our research indicated anything significant about the species that have changed. For example, creek chubs, which were not highly rated in the Fishbusters' study, had become hugely dominant by the time we sampled them. But our research indicates that creek chubs are found in a range of water typ! es, polluted and unpolluted both, and no conclusions can be drawn from their presence. It would have made sense if the new up-and-coming fish had been more tolerant of pollution, but we could not establish this link with any of our findings. I guess this just goes to show that nothing is as cut-and-dry in the field as it would be in theory!
We are also disturbed by the finding that the healthy stream became less diverse. This stream is in a preserve and should be protected. Why has it become less healthy? Can it all be attributed to the fairly minor addition of stepping-stones? Or did we disprove diversity as an indicator of health - and if so, why has it become so established in the scientific community? Something doesn't fit!
If we could continue this study there is a lot we could do. First, we could fix some of the flaws in this version! For example, we regret that our study was a little later in the year than the Fishbusters - as it gets colder, fish go to deeper streams. I wish we could redo the Fishbusters, and test invertebrates too - since we are doubtful of the use of fish diversity alone. Also, we would like to examine some other measures of health - perhaps water toxicity, or generational survival, or something. If we extended the reach, it would be interesting to test the surrounding area, such as trees and the bank, to see how the changes in those affect the stream health. Currently, we are hypothesizing that small-scale differences, such as erosion of the bank, might directly affect the stream more than larger scale ones like acid rain, in the short run. I don't think that a time span of one year was enough to accurately evaluate the long-t! erm future of the stream. But in conclusion, according to our findings, Collins run's future is not as dire as we had feared! In fact, it seems to be well capable of weathering the stress of the environment - but for how much longer? How much more can we damage it, before it does start to loose its health? Or is the damage already lurking somewhere in the pages of our data, and we were just unable to uncover it? These questions are pressing and someday they deserve to be solved.
- Abramovitz, Janet N., Imperiled Waters, Impoverished Future: The Decline of Freshwater Ecosystems, Worldwatch Institute, 1996 ¨¢ We will use this source for information about environmental degradation and other such information.
Brezonik, Patrick L. et. al., Freshwater Ecosystems, National Academy Press: Washington, D.C. 1996 ¨¢ We will use this source to see what we should expect to find in the streams.
Briggs, Mark K., Riparian Ecosystem Recovery in Arid Lands: Strategies and References, University of Arizona Press, Tuscon, TX 1996 ¨¢ About the recovery of damaged ecosystems, which is what the stream is.
Cairns, John, Functional Testing of Aquatic Biota for Estimating Hazards of Chemicals, American Society for Testing and Materials, Ann Arbor, MI 1989 ¨¢ Further information regarding chemical tests.
Dodds, Walter K., Freshwater Ecology: Concepts and Environmental Applications, Academic Press, London UK, 2002 ¨¢ Information on freshwater ecology and information on electrofishing
Downes, Barbara J., Monitoring Ecological Impacts: Concepts and practice in flowing waters, Cambridge University Press: 2002 ¨¢ Information on human effects on flowing water
Hill, Ian R., Freshwater Field Tests for Hazard Assessment of Chemicals, CRC Press, London, England 1994 ¨¢ Used for information on the various testing techniques we want to use
Isom, Billy G. ed., Rationale for Sampling and Interpretation of Ecological Data in the Assessment of Freshwater Ecosystems, American Society for Testing and Materials, 1986 ¨¢ Information on gathering and interpreting data for our experiment
Lind, Owen T., Handbook of Common Methods in Limnology, The C.V. Mosby Company, Saint Louis 1974 ¨¢ This has information about how the streams healths' can be measured
Myers, Wayne L., Survey Methods for Ecosystem Management, Wiley-Interscience Publication, New York, NY 1980 ¨¢ Describes the various sampling systems and statistical analysis that we could use.
Trautman, Milton B., The Fishes of Ohio, Ohio State University Press, Columbus, OH 1981 ¨¢ Will be useful for identifying the various fish
William, D. Dudley, The Ecology of Temporary Waters, Timber Press: Portland, Oregon 1987 ¨¢ This source describes the affects of drought on both temporary and more permanent bodies of water.
Online Resources and Journals:
Weekly Palmer Drought and Crop Moisture Data Products Explanation, Palmer Drought Data , 9/8/02
JSTOR Journal of Applied Ecology, jstor.org , 9/8/02
Journal of Fish Biology, ohiolink journals , 9/8/02
Microsoft Terraserver Imagery, terraserver.homeadvisor , updated daily
Also the fishbusters study, which can be found on the sylabus!
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