Posted by: chrismaser | May 24, 2012

REPAIRING ECOSYSTEMS—CUMULATIVE EFFECTS, LAG PERIODS, AND THRESHOLDS

Nature, which has only intrinsic value, allows each component of a prairie to develop its prescribed structure, carry out its ecological function, and interact with other components through their evolved, interdependent processes and self-reinforcing feedback loops. No component is more or less important than another; each may differ in form, but all are complementary in function.

Our intellectual challenge is recognizing that no given factor can be singled out as the sole cause of anything. All things operate synergistically as cumulative effects that exhibit a lag period before fully manifesting themselves. Cumulative effects, which encompass many little, inherent novelties, cannot be understood statistically because ecological relationships are far more complex and far less predictable than our statistical models lead us to believe. Our inability to recognize cumulative effects arises, in part, because we live in the invisible present.

THE INVISIBLE PRESENT

The invisible present is our inability to stand at a given point in time and see the small, seemingly innocuous effects of our actions as they accumulate over weeks, months, and years. Consider that all of us can sense change—day becoming night, night turning into day, a hot summer changing into a cold winter, and so on. Some people who live for a long time in one place can see longer-term events and remember the year of the exceptionally deep snow when pronghorn antelope starved to death on the prairie or the year of the intense grass fires.

In spite of such a gift, only unusual people can sense, with any degree of precision, the changes that occur over the decades of their lives. At this scale of time we tend to think of the world as being in some sort of steady state (with the exception of technology), and we typically underestimate the degree to which change has occurred—like the amount of recovery exhibited by perennial grasses following the removal of domestic livestock. We are unable to directly sense slow changes, and we are even more limited in our abilities to interpret the relationships of cause and effect in these changes. Hence, the subtle processes that act quietly and unobtrusively over decades reside cloaked in the invisible present, such as gradual declines in habitat quality.

CHANGES IN HABITAT

Changes in habitat often have a delayed, negative effect on the species occupying the habitat; an effect that may not be recognized for years, as demonstrated by a study of the cooperatively breeding acorn woodpecker. The study was conducted in Water Canyon, which is located in the Magdalena Mountains of south-central New Mexico.

Acorn woodpeckers rely on self-constructed sites in “granary trees” to store acorns for use during winter and spring. Most granaries, which consist of precisely made holes, are in dead trunks and limbs of narrow-leafed cottonwood trees. Groups of woodpeckers with large storage facilities, which equate to high-quality territories, have greater reproductive success and better survival than do pairs or groups with poorly developed storage sites.

In the study, most territories, which had contained birds in 1985, were unoccupied by 1995. This drastic decline was associated with the gradual collapse of individual granary trees in the invisible present, a collapse that ultimately resulted in the loss of nearly all the large storage facilities. The lack of new, high-quality granaries from 1975 to 1995 was probably due in part to the distinctly bimodal age structure of the cottonwood trees—nearly all of which were either very young or old. The scarcity of middle-aged trees reflected a period of intensive grazing by cattle, during which production of young cottonwoods was suppressed because cattle tend to eat them.1

THE DYNAMICS OF CAUSE AND EFFECT

Hidden within this scale of time (1975 to 1995) are the interwoven threads of cumulative effects, lag periods, and thresholds. To understand the dynamics of cause and effect, we will visit the Serengeti-Mara Ecosystem, which is a component of the Serengeti Plain in Africa. Here, long-term ecosystem monitoring has highlighted the following complex of interactions.

The limitation of food has clear effects on regulating the populations of wildlife, particularly the migratory wildebeest and the non-migratory Cape buffalo. In turn, predation limits populations of smaller resident ungulates and small carnivores. Thus, systems can be self-regulating through the dynamics of food availability and predator-prey interactions. Interactions between African elephants and their food, for example, both allow and maintain the coexistence of savanna and grassland communities. However, with increased woodland vegetation, predators’ success in capturing their prey increases. Under these circumstances, artificially regulating a population’s size may not be required. In addition, periodic physical events, such as droughts and floods, create disturbances that affect the survivorship of ungulates and birds through feedbacks among the three spheres—atmosphere, hydro-lithosphere, and biosphere.

In any case, slow and rapid changes of different spatial scales that initiate and maintain multiple conditions within an ecosystem become apparent only over several decades; hence there may be no a priori need to maintain one particular state. Beyond that, anthropogenic disturbances have direct (hunting) and indirect (transfer of disease to wildlife) effects on wildlife. Therefore, conservation must accommodate both infrequent and unpredictable events and long-term trends by planning for the time scale of those events—without aiming to maintain the status quo.2

At length, cumulative effects, gathering themselves below our level of conscious awareness, suddenly become visible. By then, it is too late to retract our decisions and actions even if the outcome they cause is decidedly negative with respect to our intentions. So it is that the cumulative effects of our activities multiply unnoticed until something in the environment shifts dramatically enough for us to see the effects through casual observation. That shift is defined by a threshold of tolerance in the ecosystem, beyond which the system as we knew it, suddenly, visibly, becomes something else.

In Africa, for example, the decline in the geographical distribution and numbers of elephants, as a result of expanding human activity, is recognized as one of the continent’s serious conservation problems. Elephants and people coexist at various levels of human density, but when a threshold in human numbers and activity is reached, elephant populations disappear. This threshold is apparently related to a particular stage in the process of land being converted to agriculture, a situation in which farms become spatially dominant over the woodland that constitutes elephant habitat.3

However, if the African people had wanted to protect the elephants from extirpation in a particular area, repair of the elephant’s habitat would need to have been initiated long before the threshold of vulnerability was reached. If, at this stage, habitat loss has seriously eroded the demographic potential of the species, halting the decline in population is limited more by demographic factors than by the amount of available habitat. Under this circumstance, it is not enough to conclude that mending the habitat will be sufficient to rescue a declining population.4 Why? One reason has to do with our limited understanding of the effects of spatial scale on habitat quality.

THE INFLUENCE OF HABITAT EDGES

Consider, for example, that although habitat edges are a ubiquitous feature of modern, fragmented landscapes (such as prairie remnants), researchers have an abiding tendency to focus their sampling designs on relatively small spatial scales. Their findings, therefore, can elucidate the influence of edge effects on animal communities over distances of only 66 to 820 feet, whereas edge effects can penetrate as far as 6/10 of a mile into a habitat patch. Such large-scale edge effects can lead to an 80% reduction in the population size of interior species, even in very large habitat fragments.

In fact, edge effects of this magnitude can drive the interior-dwelling species to local extinction, whether in a remnant of prairie or forest. With respect to forests, a global analysis of protected areas suggests that edge effects of 6/10 of a mile may compromise the ability of more than 3/4 of the world’s forested reserves to maintain the uniqueness of their interior community structures.5

INVASIVE EXOTIC SPECIES

Returning to our example of the North American prairie, native grasslands fall into three categories: short grass, tall grass, and mixed grass. They also vary in quality, as determined by the abundance of indigenous plants versus exotic species. With respect to the exotic species, two kinds of common organisms (invasive plants and invasive insects) illustrate the synergistic dynamics of prairie-grassland composition, structure, and function coupled with cumulative effects, lag periods, and thresholds.

Anyone who endeavors to repair a prairie remnant must determine whether invasive plants already exist in the area or have access to it. Either way, transportation to and from the area is of critical importance because roadsides are preferential migration corridors for exotic plants and can act as starting points for invasion into adjacent habitats. Because vehicles transport large amounts of seeds, it would be wise to examine the existing roadsides carefully for exotic plants and map their distribution; the rapid spread and interrupted patterns of a plant’s distribution can indicate long-distance dispersal along roads. Dispersal of plants by vehicles can greatly accelerate invasion by exotic plants into a remnant, where they can induce rapid changes in the patterns of biodiversity by altering the species composition of the native plants and thus the remnant’s overall physical structure and biophysical function.6

These changes occur within the realm of cumulative effects, lag periods, and thresholds, so by the time an invasive species is noticed, it may already be well established. In addition, many exotic plants have an advantage over indigenous prairie species because they do not require the obligate, symbiotic, mycorrhizal fungus in order to survive, whereas close to 100% of the native species in a prairie remnant do. (“Mycorrhiza” is derived from the Greek mykes, “fungus” and rhiza, “a root.”) In fact, the vast majority of indigenous plants in all terrestrial ecosystems require this mutualistic relationship, in which the host plant provides simple sugars from photosynthesis to the mycorrhizal fungus, which lacks chlorophyll and generally is not a competent saprophyte (a living plant that derives its nutrients from dead or decaying organic material). Fungal hyphae penetrate the tiny rootlets of the host plant to form a balanced, beneficial relationship with the roots. The fungus absorbs minerals, other metabolites, and water from the soil and translocates them into its host. Furthermore, nitrogen-fixing bacteria, which occur inside the mycorrhizae, use a fungal extract as food and in turn fix atmospheric nitrogen, which becomes available in usable form to both the fungus and its host plant.

In essence, mycorrhizal fungi serve as a highly effective extension of the host root system. Many of these fungi also produce growth regulators that induce production of new root tips and increase their useful life span. At the same time, a host plant prevents its mycorrhizal fungus from damaging its roots. Mycorrhizal colonization enhances resistance to attack by pathogens. Indeed, some mycorrhizal fungi also produce compounds that prevent pathogens from contacting the root system. Moreover, these fungi are dispersed throughout prairie and other ecosystems by such organisms as earthworms and small mammals, which eat the belowground fruiting-bodies and defecate the viable spore onto and within the soil as they move about.7

In addition to the likelihood of an exotic plant’s being mycorrhizal-free, how closely related it is to the native species will largely determine not only how invasive it is but also how likely it is to undermine the repair of a relic piece of prairie. The more closely related an alien is to the indigenous plants, the less invasive it is likely to be, whereas the less related it is, the more invasive it will be. This relationship between the invader and the existing native community may explain why foreign species are not uniformly toxic in all novel habitats. Therefore, the degree of an invader’s relatedness to the native biota may be a useful criterion for prioritizing whether to use it in a preventative mode or, if necessary, in an after-the-fact corrective one.8

A further advantage that some invasive exotic species have over native ones is their capability for independent seed production through self-fertilization or autonomous seed production. These plants usually have small, shallow flowers. Such plants are much more easily distributed than are species that are self-compatible, which means that the male gamete can fertilize the female gamete of the same plant, but the process requires an unrelated organism, such as a bee, to do the pollinating. Autonomous seed production increases an invasive plant’s ability to extend its geographical range farther than can a plant that requires a bee or some other organism to accomplish pollination. Moreover, polycarpic plants (from the Greek polys, “many” and karpos, “fruit”), those that flower and set seeds many times before dying, have a vast competitive advantage over monocarpic species (from the Greek monos, “single” and karpos, “fruit”), those that flower and set seeds only once before dying.9

Understanding the behavior of invasive plants is critical. For example, in a study of fragments of tall-grass prairie, the ten most frequently occurring and abundant species of exotic plant were cool-season species, in contrast to those of the native-plant community, which was dominated by warm-season species. Timing is thus important for exotic species to succeed in the tall-grass prairie. Because it is biologically possible for an invasive species to out-compete a native one, it would be wise in any repair initiative to protect existing small, but relatively intact, fragments of tall-grass prairie as long-term refugia for indigenous species. Such refugia could be a source of genetically viable material for mending the prairie ecosystem should it be damaged by invasive exotics. (A refugium (plural: refugia), sometimes termed simply a refuge, is a location of an existing isolated or relict population of a species that was once more widespread.) In addition, an intact fragment could be the source of genetic variability to augment a prairie remnant in which ecological repair is ongoing.10

Some exotics, such as tall fescue, can introduce unwanted, virulent pathogens into a prairie remnant. Tall fescue, an indigenous grass of Eurasia and North Africa, is now widely spread in the United States. The challenge for prairie repair with this particular grass is its endophyte, which is a fungus that lives within the grass. This fungal endophyte is particularly troublesome because it causes fescue toxicosis—-a fungal poison that can affect livestock. (“Endophyte” is from the Greek endon, “growing inward” and phyton, “plant.”)

Such hereditary fungal endophytes can increase the host plant’s ability to compete, tolerate drought, and resist consumption by herbivores. Two endophyte-related mechanisms in tall fescue benefit the endophyte-infected plants growing in phosphorus-deficient soils. These mechanisms appear in the morphology of a grass’s roots in the form of reduced diameters and longer root hairs. A chemical modification of the rhizosphere also results from the exudation of phenolic-like compounds. (The rhizosphere is the area surrounding the roots of plants wherein complex relations exist among the plant, soil microorganisms, and the soil itself.) Although it’s unclear whether these ecological benefits alter the dynamics of the endophyte-host relationship over time, the presence of herbivores—both mammals and insects—temporarily increases the frequency of the fungal infection in tall fescue.11

In one study, a normal, background level of mammalian grazing increased the endophyte frequency and thereby shifted the plant-species composition toward greater relative biomass of infected tall fescue. These results demonstrate that herbivores can drive plant-microbe dynamics and, in doing so, modify plant-community structure directly and indirectly. Part of the dynamics is low concentrations of copper in the tall fescue because of its fungal symbiont, which may result in lower copper in livestock—a partial cause of fescue toxicity.12

In addition to tall fescue, cool-season grasses infected with endophytes have an extraordinary impact on the ecology of grasslands. As in tall fescue, these endophytes induce adaptive mechanisms (morphological, physiological, and biochemical) that help the infected grasses avoid, tolerate, and recover from drought.13

Mineral nutrition (nitrogen, phosphorus, calcium), as mediated by the grass’s endophyte, affects the production of ergot alkaloids. These alkaloids are produced by the ergot fungus, which looks like black smut on various grasses. The ergot alkaloids are at once potent neurotoxins and vasoconstrictors, which cause a condition called ergotism, or ergot poisoning, to which both livestock and people are susceptible.

The ergot fungus is common throughout North America, where it can infect such domestic crops as wheat and oats, a process I witnessed as a young man working on cattle ranches. I therefore include a brief description of ergot poisoning, or St. Anthony’s fire, as the human type of the disease was known in medieval Europe. The outbreaks of “dancing mania” between the thirteenth and sixteenth centuries have sometimes been attributed to ergot poisoning, which was finally identified as the cause in the seventeenth century. It produces a common set of symptoms: gangrene with burning pain in the extremities or convulsions, hallucinations, severe psychosis, and death.

A ninth-century author described an outbreak of ergot poisoning: “A great plague of swollen blisters consumed the people by a loathsome rot so that their limbs were loosened and fell off before death.” The cause of this great plague was the ingestion of toxic amounts of the alkaloids produced by the ergot fungus that infested rye, the growth of which was promoted by the cold, damp conditions common in France and Germany. Repeated epidemics occurred throughout the Middle Ages, when whole populations became infected from eating bread made from contaminated rye.14

THE IMPORTANCE OF LOCATION

As with invasive plants, it makes a difference where in the landscape one chooses a prairie remnant to repair. This is a critical consideration because many of the remaining patches of prairie are surrounded by intensive agriculture, with its insect pests. In southeast Minnesota, for instance, large numbers of corn-rootworm beetles invade prairie remnants from surrounding cornfields in late summer and attack the resident sunflowers.

The beetles feed extensively on sunflower pollen and occasionally on other flower parts, such as petals. Sunflowers located nearer cornfields sustain more floral damage than those farther from the cornfields. The beetles can also reduce the maturation of seeds, thereby interfering with the successful reproduction of sunflowers—and possibly other prairie composites that flower in late summer. The small size of most prairie remnants and the abundance of this flower-feeding beetle in landscapes dominated by corn agriculture may affect the sustainability of prairie-plant populations.15

AN EXOTIC-INDUCED FEEDBACK LOOP

A particularly dramatic example of the effect that an introduced insect can have on an ecosystem is the balsam woolly adelgid’s role in the death of endemic, relict forests of Fraser fir trees at the southern limit of their distribution on the highest ridges of the southern Appalachian Mountains. Here, over a period of 21 years, the avian community populating the montane Fraser fir forests changed in response to the introduction of the balsam woolly adelgid insect.

A combined historical-geographic study was conducted at Mount Collins in the Great Smoky Mountains. Investigators looked at the distribution of birds over time within five southern Appalachian mountain ranges variably affected by the adelgid. Fraser fir was virtually eliminated on Mount Collins, and the canopy cover was reduced to half its previous level, as was the combined density for all breeding birds.

Birds that foraged in the canopy and midstory declined more significantly than those species that foraged on the tree’s trunks and close to the ground. In addition, birds invading from open and disturbed forests diluted the boreal character of the original avifauna. In the other southern Appalachian mountain ranges, where habitat is not as extensive, the adelgid invasion resulted in even greater declines in the abundance of avian species, as well as having more pronounced effects on sensitive species. There also were more prominent avian invasions than in the past by species normally associated with forest succession.16

If we are to have a landscape with a desirable legacy to pass forward, we must protect existing biological, genetic, and functional diversity—including habitats and plant-community types—to foster the long-term ecological wholeness and biological richness of the patterns we create. Thus, to repair a prairie, or anything else for that matter, we have to understand the integration of it components.


Repairing Ecosystems:

• Historical Abuse

• Six Lessons From History

• Restoration, As We Currently Think of It

• Why Restoration Is Not Possible

• Basic Considerations

• Biophysical Dynamics

      1. Composition, Structure, And Function

      3. Habitat Components And Animal Behavior

      4. Habitat Configuration, Size, And Quality

      5. Mending The Prairie Through Fire And Grazing

      6. Special Considerations

• Monitoring Your Efforts

Related Posts:

• Principle 1: Everything is a relationship

• Principle 6: All relationships are self-reinforcing feedback loops

• Principle 7: All relationships have one or more tradeoffs

• Principle 8: Change is a process of eternal becoming

• Principle 9: All relationships are irreversible

• Principle 13: Systemic change is based on self-organized criticality

• Principle 14: Dynamic disequilibrium rules all systems

• Biodiversity—Our Social-Environmental Insurance Policy


ENDNOTES

1. The preceding three paragraphs are based on: J. David Ligon and Peter B. Stacey. Land Use, Lag Times and the Detection of Demographic Change: The Case of the Acorn Woodpecker. Conservation Biology, 10 (1996):840–846.

2. The previous two paragraphs are based on: A.R.E. Sinclair, Simon A. R. Mduma, J. Grant, and others. Long-Term Ecosystem Dynamics in the Serengeti: Lessons for Conservation. Conservation Biology, 21 (2007):580–590.

3. Richard E. Hoare and Johan T. Du Toit. Coexistence between People and Elephants in African Savannas. Conservation Biology, 13 (1999):633–639.

4. Gregory R. Schrott, Kimberly A. With, and Anthony W. King. Demographic Limitations of the Ability of Habitat Restoration to Rescue Declining Populations. Conservation Biology, 19 (2005):1181–1193.

5. The preceding two paragraphs are based on: Robert M. Ewers and Raphael K. Didham. Pervasive Impact of Large-Scale Edge Effects on a Beetle Community. Proceedings of the National Academy of Sciences, 105 (2008):5426–5429.

6. (1) W. Schmidt. Plant Dispersal by Motor Cars. Vegtatio, 80 (1989):147–152, and (2) Moritz Von Der Lippe and Ingo Kowarik. Long-Distance Dispersal of Plants by Vehicles as a Driver of Plant Invasions. Conservation Biology, 21 (2007):986–996.

7. The discussion of mycorrhizae is based on: (1) Zane Maser, Chris Maser, and Randy Molina. Small-Mammal Mycophagy in Rangelands of Central and Southeastern Oregon. Journal of Range Management, 41 (1988):309–312; (2) David Read. The Ties That Bind. Nature, 388 (1997):517–518; (3) David Read. Plants on the Web. Nature, 396 (1998):22–23; (4) Marcel G. A. van der Heijden, John N. Klironomos, Margot Ursic, and others. Mycorrhizal Fungal Diversity Determines Plant Biodiversity, Ecosystem Variability and Productivity. Nature, 396 (1998):69–72; and (5) M. C. Wicklow-Howard. Mycorrhizal Ecology of Shrub-Steppe Habitat. Pp. 207–210. In: Proceedings—Ecology and Management of Annual Rangelands. S. B. Monsen and S. G. Kitchen (eds.) Intermountain Research Station, USDA Forest Service, Fort Collins, CO. 1994.

8. Sharon Y. Strauss, Campbell O. Webb, and Nicolas Salamin. Exotic Taxa Less Related to Native Species Are More Invasive. Proceedings of the National Academy of Sciences, 103 (2006):5841–5845.

9. (1) William E. Kunin and Avi Shmida. Plant Reproductive Traits as a Function of Local, Regional, and Global Abundance,” Conservation Biology, 11 (1997):183–192, and (2) Mark Van Kleunen and Steven D. Johnson. Effects of Self-Compatibility on the Distribution Range of Invasive European Plants in North America. Conservation Biology, 21 (2007):1537–1544.

10. Anne C. Cully, Jack F. Cully Jr., and Ronald D. Hiebert. Invasion of Exotic Plant Species in Tallgrass Prairie Fragments. Conservation Biology, 17 (2003):990–998.

11. Ibid.

12. (1) S. B. Dennis, V. G. Allen, K. E. Saker, and others. Influence of Neotyphodium Coenophialum on Copper Concentration in Tall Fescue. Journal of Animal Science, 76 (1998):2687–2693, and (2) Keith Clay, Jenny Holah, and Jennifer A. Rudgers. Herbivores Cause a Rapid Increase in Hereditary Symbiosis and Alter Plant Community Composition. Proceedings of the National Academy of Sciences, 102 (2005):12465–12470.

13. Dariusz P. Malinowski and David P. Belesky. Adaptations of Endophyte-Infected Cool-Season Grasses to Environmental Stresses: Mechanisms of Drought and Mineral Stress Tolerance. Crop Science, 40 (2000):923–940.

14. Carol Hart. Forged in St. Anthony’s Fire. Modern Drug Discovery, 2 (1999):20–21, 23–24, 28, 31.

15. The preceding two paragraphs are based on: (1) Mark J. McKone, Kendra K. McLauchlan, Edward G. Lebrun, and Andrew C. McCall. An Edge Effect Caused by Adult Corn-Rootworm Beetles on Sunflowers in Tallgrass Prairie Remnants. Conservation Biology, 15 (2001):1315–1324.

16. The preceding three paragraphs are based on: Kerry N. Rabenold, Peter T. Fauth, Bradley W. Goodner, and others. Response of Avian Communities to Disturbance by an Exotic Insect in Spruce-Fir Forests of the Southern Appalachians. Conservation Biology, 12 (1998):177–189.


Text © by Chris Maser 2012. All rights reserved.

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If you want to contact me, you can visit my website. If you wish, you can also read an article about what is important to me and/or you can listen to me give a presentation.



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