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Joseph T. Springer
Department of Biology
University of Nebraska at Kearney
Kearney, Nebraska 68849
To cite this paper: Springer, J. T. 1998. Mammalian effects on Loess Hills prairie. Platte Valley Review 26(2):104-116.
The General Ecology textbook that was required reading when I was an undergraduate had an entire chapter devoted to trophic levels within ecosystems (Odum 1959, pp. 43-87). The basic concept is that any ecosystem can be divided into groups (trophic levels) based on how the species obtain their energy. If energy comes directly from the sun, the species is a Producer (e. g. grass). If energy comes from consuming producers, the species is a Primary Consumer, or Herbivore (e. g. mice that eat grass). If energy comes from consuming herbivores, the species is a Secondary Consumer, or Primary Carnivore (e. g. snakes that eat mice). And, if energy comes from consuming primary carnivores, the species is a Tertiary Consumer, or Secondary Carnivore (e. g. hawks that eat snakes).
Throughout my professional career as a mammalian ecologist, I have preached this basic ecological premise. In terrestrial ecosystems, members of one trophic level do not control the level below them; rather they are controlled by the level below them. This is a simplistic generalization, but one which makes intuitive sense.
Only a small fraction of one trophic level can be consumed by members the next level at best. If the Producer level is diminished by a certain percentage (due to the construction of a shopping mall for instance), all higher levels will be diminished by at least that same percentage to the point where there is just not enough energy available for any members of the highest level. This explains why some of the world’s most endangered species are carnivores, such as the bald eagle (Haliaeetus leucocephalus), black-footed ferret (Mustela nigripes), jaguar (Felis onca), peregrine falcon (Falco peregrinus), and tiger (Panthera tigris).
An ecosystem includes all the components found within an area, both living (plants, fungi, bacteria, protozoans and animals) and non-living (soil, water, light, wind, temperature, etc.) Ecology is the science of studying how various components of an ecosystem interact. But if each trophic level controls the one above it, how can any animal species affect the grassland portion of a prairie ecosystem?
“Controlling” and “having an effect” are two different things. As will be noted elsewhere in this issue, the loess hills prairie is comprised of many kinds of plants which can be generally put into three groups: graminoids (grasses and plants that look like grass), forbs (broad-leaf vegetative plants), and woody plants (trees and shrubs). From our understanding of trophic levels, these three groups would probably all still be represented even if there were no mammals in the prairie. However, which exact species will comprise these three groups and in what proportion will depend to a large extent on what herbivores are present and in what concentration.
Many kinds of animals are herbivores in the prairie: nematodes (e. g. round worms), annelids (e. g. earthworms), mollusks (e. g. snails and slugs), crustaceans (e. g. pill bugs), insects (e. g. caterpillars of butterflies, ants), reptiles (e. g. turtles), birds, and mammals. The animal herbivore classes that consume the most volume of plant material are insects, birds and mammals. The first two classes will be discussed elsewhere in this issue.
There are seven ways mammals can significantly affect loess hills prairie. I will discuss each of these starting with impacts we know the least about and ending with impacts we understand better.
But what about prairie mammals? No research has been done to my knowledge as the effects of large herbivores such as cattle (Bos taurus), elk (Cervus elephas), and bison (Bison bison) on pollination of plants that are normally wind-pollinated, the typical mode for grasses. In this part of the Great Plains, the prevailing wind direction is from the southwest. Certainly the wind during the pollination season will sometimes come from any other direction, but most days and most times of day it will be from the southwest. Thus, one would expect any given grass to receive pollen predominantly from individuals that grow southwest of it resulting in a virtual one-way gene flow throughout the grasses of the Great Plains. Is this the case? Or do the large herbivores accidentally carry pollen upwind: pollen that has temporarily attached to the fur of their bellies? From my personal observation, wild herbivores will commonly walk into the wind, presumably to better smell potential predators. Using radioactive tracer techniques, these questions could be answered relatively easily.
Medium-sized mammals such as badgers (Taxidea taxus), coyotes (Canis latrans), raccoons (Procyon lotor), red foxes (Vulpes vulpes), actually tend to have latrine sites (Springer 1979) for defecation. This concentrates fecal material resulting in less overall impact on the total environment. Urine for these medium-sized is often used to mark the territory and is therefore sprayed high up onto vegetation or rocks, therefore having little impact on soil fertility.
Large mammals, the grazers and browsers, of the loess hills prairie can have significant impacts on soil fertility. We have no way of knowing exactly what effect herds of giant ground sloths (Nothrotherium spp.), mammoths (Mammuthus spp.), mastodons (Mammut spp.), and wild horses (Equus spp.) might have had in this area up to 10,000 years ago before they went extinct. Common sense would suggest that their collective fecal and urine output would have been substantial.
The best we can do is look at the large herbivore that is still with us: the American bison and its immigrant cousins, domesticated cattle. Hobbs (1996) stated that large herbivores return N to the soil in a heterogeneous way via their urine and feces, creating patches with more N, leaving areas with depleted N.
Steinauer & Collins (1995) conducted a research project where they applied “simulated” bovine urine to prairie vegetation in Kansas and in north central Nebraska. They showed that plant abundance increased in general on urine patches, but the effect seemed to greater when the amount of litter (last year’s dead grass) was low—as in an area that had been recently burned. During the growing season, Bison tend to graze on areas that were recently burned (Vinton et al. 1993), and that is where their urine would have the greatest fertilizing effect.
Cattle compact the soil to considerable depths, particularly when the soil is moist (Stoddard et al. 1955). Compact soils absorb precipitation poorly, which is a double-edged sword to the prairie: plants get less water to the roots than they should, and water runoff can lead to serious erosion of the soil (Jeltsch et al. 1997, Mwendera & Saleem 1997, Trimble & Mendel 1995). Furthermore, plant seeds cannot penetrate compacted soil, adding another problem to the plant populations.
Damage done to rangelands in North America by overgrazing cattle has become of serious concern (Gillis 1991). In terms of water eroding soil, it has been virtually impossible to determine how much has been due to soil compaction, removal of vegetation that would have held the water back, or the development of pavement like paths (done by the cattle) which become runoff ditches when it rains. No doubt, all three factors are involved.
Also, it is unlikely that unconfined grazers would ever have done much soil compaction except in rare settings such as natural salt licks or display grounds, or during an occasional harsh winter when herds were confined by deep snow.
In fact, some range managers believe that this hoof action is essential for the health of the prairie (Brown 1994). First of all, this ecosystem has existed for thousands of years with large, grazing, hoofed mammals feeding daily on its vegetation. Hooves essentially poke holes through the crust of the soil, allowing better water infiltration. This action will also bury seeds, which gives them a better chance to avoid being eaten and to germinate (Bai & Romo 1996, 1997). Vander Wall (1993) showed that if seeds had been air-dried, rodents could not detect buried seed caches, but caches were readily found if seeds were hydrated. If that is true, then buried individual seeds that had air-dried would not be found either.
Hoof action is not the only way soil disturbance by mammals can benefit grasslands. Activities by small mammals can also be beneficial. Peart (1989b) reported that pocket gopher (Thomomys spp.) mounds resulted in huge increases in dominant perennial grass species recruitment even in annual-dominated vegetation. Activity relating plains pocket gophers (Geomys bursarius) to plant communities has also been reported by Steuter et al. (1995). Prairie dogs (Cynomys spp.) also makes soil mounds in the prairie (Carlson & White 1988). While a particular burrow is active, it seems unlikely that new vegetation would ever get established in the soil of a prairie dog mound. But once a burrow is abandoned, buried seeds can germinate and take hold.
Kangaroo rats (Dipodomys spp.) and pocket mice (Perognathus spp.) disturb soil when they sandbathe as a form of territorial communication (Randall 1991, 1993), and this could also result in covering seeds.
Fredrickson et al. (1997) examined the potential of domestic sheep (Ovis ares) as dispersal agents for grass seed. The seeds they studied had a normal germination rate of 98%. When these seeds were subjected to digestion by sheep, 37% of the viable seeds germinated within 21 days of being recovered from sheep feces. These seeds had been added to the sheep digestive tracts through a hole (fistula) in the body wall, so that any destruction to the seeds was due to chemical action, not being crushed by the sheep’s teeth. A separate experiment was then conducted to determine the effect of mastication by the sheep. It was shown that 35% of the seeds eaten entered the stomach without damage. This would indicate that of all grass seeds consumed by sheep, about 13% are defecated intact.
Since sheep (like bison, cattle, deer and elk) are ruminants with 4-chambered stomachs, food passage rate is slow—70 to 100 hours (Pough et al. 1996, p. 661). Ocumpaugh et al. (1994) even suggested that livestock could be managed to distribute viable seeds of desirable plants as a mechanism of revegetating degraded grasslands.
Malo & Suarez (1995) studied herbivorous-mammal seed dispersal in a dehesa habitat (open woodland that is used for hunting and ranching). They looked at rabbits, fallow deer (Dama dama), red deer (Cervus elephas), and cattle; North American equivalents would be cottontail rabbits (Sylvilagus floridanus), white-tailed deer (Odocoileus virginianus), elk, and bison. Malo & Suarez (1995) collected fecal samples from each species throughout the seeding season, then cultivated whatever seedling emerged within a greenhouse. These mammals disperse many seeds, averaging 6 to 15 seeds per gram of dry fecal material, and representing an average of 52 to 78 plant species. The four herbivore species dispersed different species and densities of seeds. It was concluded that many plant species have adapted to the dispersal of their seeds via herbivore consumption. This is in line with the theory that plants provide fruit to certain animals in the evolutionary hope that the animal will disperse the seeds found in that fruit. In this case, the foliage of the grasses serve as the “fruit” (Quinn et al. 1994).
In range management, we refer to certain plants as decreasers, increasers, and invaders based on their responses to livestock grazing (Stoddard et al. 1975). Decreasers are those plants most preferred by livestock, and would be the first to show signs of grazing stress. Clearly, native bison prefer to graze on those plants generally considered to be decreasers (Steuter et al. 1995). If grazing pressure continues without a break, these plants would lose vigor, produce minimal annual growth, slow reproduction, and eventually die out. However, in the equilibrium that has been established between native herbivores and the prairie plant communities, even when the herbivore benefits at the expense of a plants species it feeds on, it would be unlikely that the herbivore would actually drive the plants to extinction. Bison, elk, and deer probably would have moved on to greener pastures before destroying their preferred plant species. Although we cannot know exactly what these native herbivores would have done before fencing and cattle changed the equilibrium, we know that this is what happens in in East Africa (Talbot 1962). Parasites rarely kill the host. To do so would mean their own death (Hickman & Roberts 1994, p. 154). So it should be with native herbivores and the plants they feed on.
Anderson & Briske (1995) discussed the mechanisms by which herbivore-induced shifts in species composition occur. They recognized that decreasers, increasers and invaders are the result of food preferences by the herbivores and not due to an inability of the plant to tolerate herbivory.
Small mammals can consume large quantities of vegetation, particularly members of the vole subfamily, which also includes lemmings and muskrats (Batzli & Pitelka 1971). Brown (1980) reported that meadow vole (Microtus pennsylvanicus) diets were 100% herbaceous. Hahus & Smith (1990) reported that prairie vole diets were over 95% herbaceous. Hulme (1996) showed that rodents (versus mollusks and arthropods) exerted the greatest influence on plant performance, reducing plant biomass by as much as 50%.
Fossorial herbivorous mammals—pocket gophers and black-tailed prairie dogs (Cynomys ludovicianus) can effect vegetation in the mixed-grass prairie. The pocket gophers primarily consume roots and tubers (Huntly & Reichman 1994), whereas the prairie dogs consume above-ground herbage (Uresk 1984). Pocket gopher tunnel systems can have strong local effects on overlying vegetation, according to Reichman & Smith (1985) in Minnesota. They found that plant biomass over gopher burrows was 30 to 50% less than in control areas.
Reichman et al. (1993) found biomass reductions of 86% in tallgrass prairie over gopher burrows. Gopher activity need not always be detrimental. Zinnel (1992) found that gopher nest sites were recognizable above ground because the vegetation was particularly green and productive. She concluded that nest sites and below ground food caches improved the soil-nitrogen profile.
Uresk & Bjugstad (1983) conducted four experiments to examine cattle-prairie dog forage relationships: no grazing by either cattle or prairie dogs, grazing by prairie dogs only, grazing by cattle only, and grazing by both cattle and prairie dogs. They were surprised to find after four years of these treatments that plant production of above-ground herbage was higher than in the other three treatments. Furthermore, the cattle plus prairie dogs treatment produced more herbage that the treatment with cattle alone. The completely ungrazed treatment produced the least herbage.
This leads to the conclusion that grazing is not necessarily harmful to the plants. There is considerable literature suggesting that the composition of plant species considered to represent true climax prairie is most likely to occur under a grazing regimen. As mentioned earlier, these plant populations have survived thousands of years with grazing by the native herbivores. It would make sense that in general grazing would actually help them thrive. This is illustrated by the work done by Uresk & Bjugstad (1983).
Similarly, one would expect native plants to do better when grazed by bison. Hartnett et al. (1996) examined the effects of bison grazing tallgrass prairie and found that plant species diversity and spatial heterogeneity were significantly increased by bison. They concluded that increased heterogeneity and mean species richness in grazed prairie (40 species per sample site) compared to ungrazed prairie (29 species per site) were likely a result of greater microsite diversity generated by bison.
Pfeiffer & Hartnett (1995) showed that bison graze differently on little bluestem (Schizachyrium scoparium) depending on how recently the area was burned. In unburned prairie, little bluestem accumulates a persistent clump of standing dead tillers that appear to serve as a physical deterrent to grazing. Recently burned little bluestem tends to have a greater basal area, produce more tillers, and grow more densely. If bison are then allowed to graze in the area, these three parameters were decreased by more than 50%. Bison clearly preferred burned areas; bison fed on little bluestem three times more frequently than on unburned sites. Grazing shifted the population size distribution toward higher frequencies of smaller individuals (less than 50 cm2 basal area), whereas burning increased the frequency of large (greater than 200 cm2 basal area) individuals.
Within the loess hills prairie, the most common seed predators are the Ord’s kangaroo rats (D. ordii), pocket mice (Perognathus flavescens and P. hispidus), deer mice (Peromyscus maniculatus), harvest mice (Reithrodontomys megalotis and R. montanus), and Franklin’s ground squirrels (Spermophilus franklinii) whose scientific name literally means “seed lover” (Brooks 1995, Brown 1980, Clark & Kaufman 1991, Clark et al. 1991, Hahus & Smith 1990, Heske et al. 1994, Houtcooper 1978, Kantak 1981, Kaufman & Kaufman 1990, Randall 1993, Reese et al. 1997, Stallman & Best 1996, Vander Wall 1993, Vickery et al. 1994, Whitaker 1966).
The numbers of seeds that fall to the ground from the plants that produce them are collectively referred to as “seed rain” and the seeds that manage to get themselves underground are collectively referred to as the “seed bank” (Bai & Romo 1997, Clark 1996, Gutierrez et al. 1997, Hulme 1994, Hutchings & Booth 1990, Johnson & Anderson 1986, Kollmann, & Pirl 1995, Lagroix-McLean 1990, Peart, D. R. 1989a, Price & Joyner 1997, Rabinowitz & Rapp 1980, Santanachote 1992, Smyth et al. 1997).
The most costly loss of seeds to rodents (from a plant’s point of view) occurs during and shortly after the seed rain itself. According to Vander Wall (1993), once dried seeds are below ground, rodents seem unable to detect them. Also, the seed bank does not seem to grow larger year after year, which implies that very little of the seed rain makes it into the bank (Bai & Romo 1997). Santanachote (1992) noted that the seed bank bore little similarity to the seed rain both in Boulder County, Colorado, and at the Konza Prairie in Kansas.
Price & Joyner (1997) monitored the seed band and seed rain over a 19-month period in a desert habitat. They found that the seed bank of 38 g/m2 (106,000 seed/m2 ) was roughly the equivalent of one year’s seed rain, which was admittedly high for such a habitat. (For example, Rabinowitz & Rapp (1980) found 19,700 seeds/m2 in a Missouri tall-grass prairie, and a seed bank of only 5,640/m2 ) They also noted that input from seed rain did not accumulate in the soil; the seed bank actually decreased during their study by 114 seed/m2 each day. It seems unlikely that that many seeds would germinate every day, so one could presume that some kind of granivore consumed them. Since also it seems unlikely that these seed predators would have been small mammals, seed-harvesting ants would be good suspects (Haase et al. 1995).
If there is already a stand of perennial grass and forbs, as is typical of a prairie, how many seeds need to germinate? The life span of a typical prairie plant can be many years, and sometime within that life span the plant needs to produce one offspring on the average in order for the population of that species to remain stable. Yet each year many of these plants produce great numbers of seeds.
The plant can afford to lose virtually all of these seeds to predators, yet they do have “tricks” to fall back on to improve their success. One is to occasionally produce far more seeds that they do most years. This is called “masting” (Haase et al.1995, Kelly & Sullivan 1997). The potential seed predators are simply overwhelmed with food one out of every four or five years. Seed-eater populations, particularly mammals with relatively low reproductive potentials, have to be adjusted to the average food supply (Gonzalez et al. 1989). The consequence is that in these mast years, many seeds are left alone to germinate. A second “trick” is to release several seeds at the same moment. Hulme (1994) pointed out that seeds in groups of 10 were encountered by rodents a little more than twice as frequently as single seeds. All things being equal, a group of 10 seeds should be encountered 10 times more often in order to suffer the same predation risk. Thus a plant releasing many seeds at once gives them all a better chance of survival than if they were released one at a time.
However, there have been situations where natural mammalian activity clearly has prevented the establishment of what would probably be considered the normal plant community. Two situations will be examined, and although neither involves Nebraska loess hills prairie, both involve mammal species that either live here or are close relatives of species that do.
When ranching first started in earnest in Arizona, areas we now see as desert shrubland were often characterized as grassland. However, intensive cattle grazing soon degraded the range, and the grasslands have nearly disappeared (Humphrey 1953). What Brown & Heske (1990) did in this habitat was to investigate what would happen in this kind of degraded rangeland if certain seed-eating animals were kept out. In an area with no livestock grazing, they set up 24 plots that were fenced with fine wire mesh. Then each plot was given 1 of nine kinds of treatment.
In the end, only two factors proved to be important: were kangaroo rats (Dipodomys spp.) present or absent? Brown & Heske (1990) concluded that treatments in which all rodents or all kangaroo rats had been removed ended up similar to each other in terms of plant variables. Therefore, the eight plots where all kangaroo rats were removed were compared to 14 plots where all kangaroo rats were present.
The second discovery that Brown & Heske (1990) made was that long-term removal of kangaroo rats changed the desert shrubland into desert grassland. They noted that in the absence of kangaroo rats, tall grasses colonized the open spaces between shrubs and increased in numbers: the perennial grass Eragrostris lehmanniana increased more than 20-fold. Two species of annual grasses (Bouteloua aristidoides and B. barbata) became significantly less abundant when kangaroo rats were removed.
Mesquite seeds, pods, seedlings (newly germinated) and saplings (with a stem diameter of 5-6 mm) were presented within the prairie dog colonies at three zones: on colony, off colony, and the transition zone in between. The disappearance of seeds and pods was between 3 and 99 times greater within the prairie dog colony than off colony. Most of the seeds were removed by ants, whose activities were increased by a factor of four within prairie dog colonies. Most of the pods were removed either by prairie dogs or vertebrates associated with them such as rabbits. Prairie dogs were observed eating the fleshy pods and discarding the seeds.
Seedlings that had been kept within exclosures were subjected to prairie dog consumption in mid-May at which point they averaged 84 mm. By mid-August, 51% of them were still alive, but averaged only 36 mm in height while protected seedlings had grown to 162 mm. All mesquite saplings that had been placed within a prairie dog colony were girdled and destroyed within two days of planting.
As part of their study, Weltzin et al. (1997) looked at nearby sites where prairie dogs had been eradicated. They found that honey mesquite had successfully invaded and accounted for about 60% of the canopy—close to the canopy cover of areas in which prairie dogs had not lived. This showed that plenty of viable seeds remained available, and that prairie dogs had not eliminated mesquite but had only suppressed it.
Weltzin et al. (1997) noticed that the majority of stems and leaves of mesquite plants were not consumed, which they found to be consistent with other observations of nearly random clipping and felling of standing vegetation (e. g. King 1955). This is thought to be a predator detection strategy and therefore not done to get rid of mesquite itself but to get rid of any tall obstacle. Weltzin et al. (1997) felt that this behavior would probably extend to other woody plants that might try to invade a prairie dog town.
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