Exotics, Ecosystems, and Populations

Warren Coleman, John Craig, Marianne Donohue, Maynard Hiss, Barb Houren, Kate Moran, Linda Tyson, Jackie Wilson


Warren Coleman - Peacock Bass

An interesting way to view the idea of multiple stable states (at least to me) is to consider the changes in an aquatic system when a non-native or exotic species is introduced. In some systems, the introduction of an exotic does create an obvious (to human perception) change in the balance of organisms, while in others, the introduced exotic species may not create any perceivable changes. Perhaps the other way to think about the potential for change, or a shift in stability, is whether the potential existed in the system prior to the introduction. That is, is it the introduction of the species or the system that is the main cause of a shift, if it occurs or does not occur. I suppose some examples might help build my case here.

The Peacock Bass, a cichlid found in South America, is a prized gamefish of anglers. In 1984, this fish was introduced into the canal system in Dade County Florida. The introduction was justified on the grounds that 1) it would help control the overblown population of blue tilapia (another exotic) that native largemouth bass could not control 2) it would be geographically limited to the canal system due to temperature and salinity tolerances and 3) it would create a sport fishery and therefore economic revenue from anglers. Not to get bogged down in all the interesting issues one could talk about here, but to deal with the original question...what happened??

More than ten years after the introduction, it seems that the peacock bass is flourishing in canal systems all over Dade and Broward counties. The aquatic system, which is largely manmade canals, has definitely changed. Electroshocking data suggests that there are as many largemouth bass as ever before, but that they are less competitive than the peacock bass. Therefore, anglers tend to catch more peacock bass versus largemouths. Also, the biomass of the blue tilapia has been reduced as suspected, however so has the native population of panfish, such as bream/bluegill. I believe that overall, the total biomass of the aquatic systems has shifted toward one where the main predator is the peacock bass. However, since we are only talking about ten years worth of history to base explanations upon, what direction is the fishery moving in?

This is hard to say because I doubt that the system has reached another stable state yet. There may come a time when the largemouth bass population shows a definite decline due to the absence of small panfish consumed by the peacock bass. Largemouth bass generally do not eat the tilapia unless they are very small, where the peacock bass will take on bigger fish. Therefore, the largemouth and the panfish may get squeezed out in some areas. That's just a feeling...probably something that would take quite a while to observe if it ever did occur.

Perhaps another example might be, and again humans are directly involved, the trout streams in southern Appalachia. The native trout is the brook trout, a char, which thrives only in clear, clean and cold water. It is the most susceptible of the species of trout to polluted water, high temperature and imitations of insects from sly anglers. Today, there is probably less than 5% of the original water that still contains these trout. The causes are 1) extensive harvest during the late 19th century and early 20th century 2) the introduction of brown trout (from overseas) and rainbow trout (from the western U.S.) and 3) the pollution of streams from runoff and directly from the contents in rain water.

What you can find today though, are streams that contain good populations of browns and rainbows, primarily because they are more hearty than the brook trout, especially browns. These non-native species displaced the brook trout early in the century and I would say, have led to a new stability regime. Brook trout today are found only in very remote, high altitude, clear headwater regions. The populations that exist do so probably only because of natural barriers, such as waterfalls, that prevented the introduction of browns and rainbows. So, yes, I think there is another stable state with respect to the trout populations. One dictated primarily by the competitiveness and natural limiting factors of the different species. If you do consider, though, the effects of pollution, sedimentation, and angling pressure it would complicate the picture, however, it would favor the brown trout. So purely as a case of introducing non-native species, the composition of trout in southern Appalachia is in a new stability regime.

I hope Loren reads this and can correct me if I am wrong. He knows a few things about trout and aquatic I hear.

Barb Houren - Myrica faya in Hawaii

MYRICA FAYA in Hawaiian lava fields MYRICA FAYA , through a mutualism with an endosymbiont, is a nitrogen fixer which has invaded Hawaiian lava fields. This area is characteristically an open-canopied forest.

There is discussion on the effect of this nitrogen fixing shrub on the overall availability and concentration of nitrogen in an already limited environment. Studies so far do not indicate that any alterations of nitrogen have had major effects on other species abundances, although the shrub is a factor in direct competition with other species. (Levin, 1995) But could the argument be looked at on a much larger time scale.

This shrub, an exotic, was able to invade young volcanic soils. Volcanic eruptions are examples of large natural disturbances. Concern now is how these nitrogen fixing shrubs may deplete the nitrogen available and limit other species. Was there any investigation though into the condition of the system before the shrub was detected? Was the current system already in a decline and ripe for the invasion of the nitrogen-fixing shrub which could prosper in the recently altered environment? If we think on large, extended time scales this could be another stable state of an environment which may remain relatively "stable" for long periods of time between eruptions which then extremely alters the environment. Is this long-term ecological resiliency within this environment if we use the definition of ecological resilience as the amount of disturbance that can be sustained before a change in system control and structure occurs? Though the system may be in the process of being altered in part due to this shrub, what will the future hold for this system if the time between another eruption is hundreds of years away; will the shrub hold out or be pushed out as the soil constituency changes? Another question still not determined is whether this will alter the environment substantially, and if so will it be due to nitrogen or due to competition for other resources?

I realize I am most likely just spouting at the mouth, but I choose to approach this assignment as a mental exercise. The examples I located to illustrate multiple stable states were not novel, and I would have just been rewording someone else's previous discussion. So, I thought I would pose a few thoughts and questions on an interesting question involving system knowledge, scale, and resilience(or stability).

Linda Tyson - Exotics in the Everglades

Everglades National Park, popularized by Marjory Stoneman Douglas' Book "The River of Grass", has become a virtual "Sea of Exotics". Twenty-five percent of species encountered in a recent park survey were deemed non- indigenous species (NIS). The potential change away from the status quo that may result from these added species are of great concern to park managers. The success of these invasions are often associated with human induced changes that have deviated natural conditions away from those that support a perceived stability. Reduction of variability in the system through human control of water and fire appears to have reduced the resistance of the natives to invasion. Management goals his

While south Florida systems are adapted to natural disturbance the interplay of NIS invasions and anthropomorphic perturbations have led to system responses that are neither resistant or resilient and have resulted in "new" communities. Prior to the introduction of the exotic, Melaleuca, (and draining perpetuated) by Melaleuca. In some cases a complete community shift to monoculture.

Identifying and maintaining that delicate balance of tolerable perturbation and its role in ecological equilibria has long been a goal of resource conservation. Mega bucks and man hours have been spent attempting to rid systems of invaders instead of "arming" the natives with conditions that favor their competitive abilities. Holling's paper argues that it is the evolutionary ability of functional systems to effectively deal with variation that maintains its diversity. In this example then, could the system resist invasional changes due to its attributes of evolutionary maturity in multiple stable equilibria if natural variation is reestablished? If this variation results in multiple equilibria, is the system more susceptible to invasions when between the various stable states? How does the state of stability relate to exotic species invasions? These questions refocus the NIS issue from individual species control to the causes of stability and instability that reduce resistance of native to invasions.

As demonstrated by the above example, NIS invasions are usually not simple additions of species but are often accompanied by human activities. This important association may set them apart from the usual arguments of immigration of nearby colonizing species which "complex, well-developed diverse, mature, tightly cycling, well connected, genetically rich" communities are said to resist in terms of being invaded. All these community attributes leading to maximum niche efficiencies may pale when NIS are introduced along with chainsaws and backhoes. Resilience to change that depends on memory, learning and genetic adaptability may not be possible if continuous human intervention results in fast paced, technologically innovative disturbances occurring on a continuous basis.


John Craig - An Amazonian El Dorado?

I am curious about multiple equilibria on a fairly large spatial and temporal scale. It has to do with the legend of El Dorado. When the Spanish arrived in South America looking for gold, they heard legends of a huge lake where a king, a sort of South American Midas who had hair of gold, would go to bathe. The sediments of that lake reportedly were full of gold dust, or dandruff, that flaked off. But the Spanish searched all over the continent looking for the lake and couldn't find it. In recent years, large deposits of gold were found in the Northern Amazon region of Brazil, near Venezuela. Instead of the Amazonian rain forest, the center of this region has large expanses of grassland. In the last decade, geologic studies indicate that the area used to be a huge lake, over 200 km across! The lake drained about 500 years ago, just as the Spanish arrived. Apparently, the region used to drain northward, to the Orinoco basin, then in a gradual geologic uplift process, it leveled off, forming the lake, and suddenly, it started to drain southward to the Amazon, eroding a channel, and the whole lake was gone in less than 20 years. This was corroborated by the local Indians, who have passed down stories of their ancestors who used to paddle canoes across the lake, fish in it, and live on islands. These Indians were not very interested in gold, that is why the Spanish hadn't slaughtered them all.

Anyway, I was curious why the former lake had not developed into Amazon rainforest. Apparently the rainfall is less in that local area than elsewhere. Estimates indicate that at least 50% of the rain that falls in the Amazon basin comes from transpiration from the forest itself. It may be that lacking forest cover in such a large area, it has created a drier climate where it is more difficult for the forest to be established. This notion feeds the fears that some people have of catastrophic changes occurring if too much of the forest is cut down. In other words, another stable state for the Amazon basin may be grassland, or perhaps even Amazon desert!

Just a thought.

Marianne Donohue - Fire Supression in Yellowstone Natural Park

BACKGROUND Equilibrium-centered Nature Constant: This viewpoint emphasizes the stability of an ecosystem near an equilibrium steady state; it focuses on constancy, spatial homogeneity, linear causation, and predictability. Management under this traditional view involves maintaining the functional efficiency an ecosystem.

Multiple equilibrium Nature Resilient: This viewpoint emphasizes the existence of more than one stable state; it focuses on variability, spatial heterogeneity, non-linear causation, and uncertainty. Management under this view involves the understanding of ecosystem dynamics and behavior as a means for maintaining the resilience and functional diversity of the system.

An example of ecosystem management that will focus on and defend the concept of multiple equilibrium states is forest fire suppression in the national parks of the United States. In many national parks, ecosystem resilience and functional diversity were lost because of the suppression of fires in the system. The cause was due to the need to increase the constancy of "production"; there was a desire to increase the recreational activities in the parks, ultimately at the expense of natural variability and ecosystem function. Such management policy was based on the equilibrium-centered view of constant nature.

Initially, fire suppression was deemed successful because it met the immediate needs of the management institution and of society. However, as the success continued, the objective of the management agenices shifted from the original socioeconomic objective to one of increased efficiency of forest fire detection and control. As a consequence of reducing the variability of forest fires, features of the system that were originially viewed as constants began to change and evolve into a qualitatively different state. According to Holling (1986), reduction of forest fires led to the closing of the forest canopy and to the accumulation of fuel so that "what were once modest ground fires affecting limited areas and causing minor tree mortality became catastrophic fires covering large areas and causing massive tree mortality".

As a result of fire suppresion and spatial homogenization of the forest canopy structure, the park ecosystems became more fragile and more dependent on fail-safe/error-free management. According to Holling (1986), the idea of Nature Constant encountered the reality of Nature Resilient. If the control of the management institution failed, the intensitiy and extent of the forest fires could be great enough to overwhelm the system and flip it into an irreversible state.

In general, the response to ecosystem change and socioeconomic crises is alarm, denial, or adaptation. According to Holling (1986), forest fire policy in the national parks of the western US has recently changed to reinstate fire as the natural manager of the forests. Such adaptation was not made easily and rested upon the existence of alternative policies and increased understanding of ecosystem behavior, structure, and function.

Maynard L. Hiss - Longleaf Pine

Longleaf pine is a highly competitive species that facilitates fire in order to maintain its' dominance. Fire helps to reduce competition from other woody species and can alter plant communities in ways that favor longleaf. Under the right fire regime longleaf can become established in many different plant community types, and along a wide range of topographic gradients. Longleaf has a symbiotic relationship with prairie and savannah plant species, which in the past have helped to foster the spread of fires and longleaf communities.

Longleaf's relationship with fire makes its' equilibrium erratic, since sources of ignition and conditions under which fire occur are highly variable, and cannot be controlled by longleaf. Under natural conditions longleaf was able to adapt ways to reduce fluctuations in ignition sources by developing traits that take advantage of fire sources. Longleaf facilitates frequent fires by providing a fine fuels that foster the spread fire under a variety of environmental conditions. Dead and down resinous old-growth longleaf can maintain fire for months, allowing for re-ignition of the landscape when fuel becomes available again.

Longleaf have many adaptations to frequent fires of moderate intensity. Young longleaf protect their terminal bud with a dense cluster of needles and by growing low to the ground for the first few years. After developing reserves in the grass stage they bolt, making rapid height growth. Fire prunes their lower branches and encourages height which reduces the risks of crown fire in older trees. Yearly needle drop helps increase the frequency and intensity of fires, which give longleaf a competitive advantage.

Many other species are adapted to fire, however they often loose their dominance and remain in the groundcover or understory in the more frequent fire regimes that are facilitated by longleaf. Once established hardwoods can compete with pines in areas where fire is excluded or where there are infrequent low intensity fires. They do this be creating a micro climate and a fuel source where fuel sources are not easily ignited. This strategy works especially well when there is a mixture environmental microclimates, gradients, and weather conditions that increase the heterogeneity of the fire. Once hardwoods are established, they can rebound quickly even if top killed, because of the large reserves within their roots.

Currently the longleaf equilibrium is tenuous at best. At the present time most of the longleaf has been eliminated throughout it's range. The original forest has been reduced from 70-90 million acres to 3 million, with 75% of the stands being on 100 acres of less. Less than 10,000 acres of old growth remains. Much of the remaining acreage of longleaf is suitable for harvest and could be eliminated rapidly, since it is not protected. Longleaf forestry practices have been slow to develop because of lack of understanding; e.g. ecological relationship with fire, unique reproductive strategy, and growth characteristics.

Natural conditions under which longleaf evolved have been eliminated or are controlled by humans. While longleaf pine was never at a state of equilibrium, it has in the past few thousand years played a dominant role in the southeastern landscape. Ironically, what threatens the longleaf most is its' desirability. Unless longleaf can find a niche in the noosphere it fate is not assured.

Jackie Wilson - Columbia River Basin

I chose to look at the Columbia River Basin (CRB) to determine if there are multiple equilibria in the system. First, I think the equilibrium of the CRB can be dependent on the different elements of the system (water quality, water flow, fish, etc.) The state of these elements determines the condition of the river and whether or not it is in a state of equilibrium. What has been focused on the most in class has been the salmon element. However, I want to discuss the other elements, as well as the salmon, and look at the states of equilibria over time.

I do think that the CRB system has multiple states of equilibria. The CRB was in an original equilibrium before European settlement. Elements like water quality, water flow and fish productivity were minimally impacted by humans (Native Americans), but this disturbance did perturb the elements enough in order to displace them into to other states of equilibria. After European settlement, the system became heavily impacted by humans. The elements like water quality and water flow (I am sure there are several other elements, but I do not know enough about the system in order to name them) were changed. Water quality was impacted by agricultural, industrial and urban uses. However, I think that these elements have not been essentially destroyed. They have changed (there is more pollution in the river today than before development, and water flow has been drastically changed by dams and diversion for urban uses), but the river itself has not gone through eutrophication and still flows despite the damming. In a sense, it has reached another equilibrium level-it is not in the state that it was before, but it is still a productive river (for the most part). Additionally, these elements have reached new equilibria in a shorter period of time (within the last 60 years) than the next element I want to discuss-salmon productivity.

The change in salmon productivity is one of the most highlighted elements of the CRB. The productivity of these fish has definitely been disturbed, but it is not apparent if it has reached a new equilibrium state. The temporal scale of this element seems to be longer than the temporal scale of the other elements. I see this part of the system to be in the Stage 4: Reorganization phase of the model that was shown during last class. The fish migrations are still declining-they have not become sustainable at lower numbers. The question is what will this new equilibrium be. Will the wild stocks survive with the new management techniques that are being tried, but in lower numbers than what was present before European settlement? Will hatcheries' populations replace wild stocks and reach an equilibrium? Will the salmon migrations completely collapse (i.e., there was one equilibrium and the productivity was so disturbed that the whole thing collapses)? Unfortunately, it seems apparent that the adaptive management in place at the moment is not really working (Volkmann and McConnaha 1993). The fate of the salmon productivity (and its new equilibrium point, if it has one) will only be known over time.


Kate Moran - Global Sea Turtle Populations

I'm taking a bit of a risk this week, in my choice of resource systems, since I'm not sure that what I have in mind is an entirely valid topic. More specifically, I decided to loosely interpret the term "resource", and am going to look at global sea turtle populations (for the sake of time, I won't differentiate between species). Whether or not sea turtles are conventionally thought of as resources, I can see them as having existed and currently existing in 2 distinct equilibria.

Sea turtles are some of the oldest organisms living today, and have survived many of the episodic changes that Holling discusses. Turtles were existing in a self-sustaining state prior to the advent and intensive utilization of modern human technology. It was the sudden change in human population growth (yep, that's the culprit!) and the accompanying effects that flipped turtle populations into a declining state, which is apparently either irreversible or only slowly reversible; who knows if we can ever attain sustainable turtle populations? The present state is not necessarily an equilibrium, in the sense that a decline can hardly be categorized as stable. However, I definitely think that the current level of sea turtle populations is distinct from previous healthy levels. This equilibrium "flip" is not so much dependent upon the turtles themselves as upon variables affecting them. In the interest of time, I'm talking mostly about human factors which caused the switch. There may not be a very blatant difference when and if the state switches back, but at this point, the critical variables have gone beyond "maintaining the (previous) stability landscape" (Holling, p. 3). The story of sea turtle stability is definitely one of maintaining _existence_ , rather than, of function; the question for the turtles' has become one of passing on their genes at all costs.

Critical processes in this system are focused at radically different rates; in terms of ecology, sea turtles are extremely long lived species which take 20-50 years to reach sexual maturity, but their mortality due to incidental catch in shrimp nets, slaughter for their carapaces, and entanglement in/ingestion of anthropogenic debris occurs on a daily basis. Unchanged, the latter set of conditions will never allow for the former life history patterns. As a resource, sea turtles also have spatial considerations, since policies and practices that are carried out on one beach or in one nation can have global impacts, due to the migratory nature of the resource.

While destabilizing forces are important in maintaining diversity and resilience of the system (i.e. storms changing the temperature and compaction of a beach, and different turtles preferring to nest on different types of beaches), stabilizing forces are also important in productivity (i.e. beach renourishment--if sand temp. is too cold, more males will be produced, changing the population structure; if sand moisture or compaction changes, success of digging nests can decrease) and there can be dire consequences if those forces are disturbed too much. In the recent past, management has not been flexible enough in maintaining the stability of the present "equilibrium". Acceptance of sea turtles on the CITES Appendix I is far from universal, the use of TED's in shrimp and other fishing nets have not been enforced in many countries, and beach regulations (i.e. right here in Florida), while having a huge impact on sea turtle nesting, are disturbingly lax.

At least the measurement of (ecological) resilience is high in this case, because the magnitude of disturbance that has been absorbed into the system is enormous. Watching a green turtle going through its stereotypical nesting motions on the black sand shores of Tortuguero, Costa Rica, as it is surrounded by over 20 tourists, makes me realize just how "determined" turtles are to overcome destabilizing influences. With respect to turtles, if we can see multiple equilibria as 2 valleys with a big hill in between, then we're in the position of Sisyphus, forever rolling a stone up a hill, in search of a return to the original equilibrium state of sea turtle populations. I'm still optimistic that anthropogenic factors can come to play a much less detrimental role in the equation of the turtles' ecological stability.

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Copyright Last modified: Feb 14 1996