David J. Gibson

Experimental Analysis of Intermediate Disturbance and Initial Floristic Composition: Decoupling Cause and Effect

The intermediate disturbance hypothesis predicts that richness will be highest in communities with moderate levels of disturbance and at intermediate time spans following disturbance. This model was proposed as a nonequilibrium explanation of species richness in tropical forests and coral reefs. A second model of succession, initial floristic composition, states that nearly all species, including late seral species, are present at the start of suc- cession. This leads to the prediction that richness should be highest immediately following disturbance. We tested these predictions using plant species composition data from two long-term field experiments in North American tallgrass prairie vegetation. In contrast to one prediction of the intermediate disturbance hypothesis, there was a significant monotonic decline in species richness with increasing disturbance frequency, with no evidence of an optimum, in both field experiments. Species composition on an annually burned site was a subset of that of infrequently burned sites. The average number of species per quadrat and the number of grass, forb, and annual species were lowest on annually burned sites compared to unburned sites and sites burned once every 4 yr. The second prediction of the intermediate disturbance hypothesis, however, was supported. Richness reached a maximum at an intermediate time interval since the last disturbance. This contradicts the prediction from the initial floristic composition model of succession. These results also suggest that the two predictions of the intermediate disturbance hypothesis are independent and unre- lated. We propose that this may be explained by uncoupling the effects of disturbance as a single, relatively discrete event from system response to disturbance. From this perspec- tive, disturbance becomes an extinction-causing event in these grasslands, whereas recovery following disturbance is a balance between immigration and extinction.

Effects of fire, topography and year-to-year climatic variation on species composition in tallgrass prairie

Native unploughed tallgrass prairie from Konza Prairie, Kansas, USA is described with respect to plant species compositional changes over a five year period in response to fire and topography. The principal gradient of variation in the vegetation is related to time since burning. Species show an individualistic response in terms of relative abundance to this gradient. Both the percentage of and cover of C4 species and all grasses decrease as the prairie remains unburnt. Forb and woody plant species numbers and abundance increase along this gradient. A secondary gradient of variation reflects topography (i.e. upland versus lowland soils). Upland soils support a higher species richness and diversity. Upland and lowland plant assemblages are distinct except on annually burnt prairie. The interaction between burning regime, topography and year-to-year climatic variation affects the relative abundance of the plant species differentially. The most dominant species overall, Andropogon gerardii, was affected only by year-to-year variation (i.e. climate). Its position at the top of the species abundance hierarchy was unaffected by burning regime or soil type. The other dominant species showed a suite of varying responses to these factors.

Controls of nitrogen limitation in tallgrass prairie

SummaryThe relationship between fire frequency and N limitation to foliage production in tallgrass prairie was studied with a series of fire and N addition experiments. Results indicated that fire history affected the magnitude of the vegetation response to fire and to N additions. Sites not burned for over 15 years averaged only a 9% increase in foliage biomass in response to N enrichment. In contrast, foliage production increased an average of 68% in response to N additions on annually burned sites, while infrequently burned sites, burned in the year of the study, averaged a 45% increase. These findings are consistent with reports indicating that reduced plant growth on unburned prairie is due to shading and lower soil temperatures, while foliage production on frequently burned areas is constrained by N availability. Infrequent burning of unfertilized prairie therefore results in a maximum production response in the year of burning relative to either annually burned or long-term unburned sites.Foliage biomass of tallgrass prairie is dominated by C4 grasses; however, forb species exhibited stronger production responses to nitrogen additions than did the grasses. After four years of annual N additions, forb biomass exceeded that of grass biomass on unburned plots, and grasses exhibited a negative response to fertilizer, probably due to competition from the forbs. The dominant C4 grasses may out-compete forbs under frequent fire conditions not only because they are better adapted to direct effects of burning, but because they can grow better under low available N regimes created by frequent fire.

Effects of small mammal and invertebrate herbivory on plant species richness and abundance in tallgrass prairie

SummaryA factorial field experiment was designed to test the effects of small mammals and above- and below-ground invertebrates on plant species richness and composition in native tallgrass prairie at Konza Prairie Research Natural Area, northeast Kansas. Over a 4-year period, Microtus ochrogaster densities were maintained by live-trapping in fenced plots, and invertebrate levels were reduced using the pesticides carbaryl for above-ground invertebrates and an organophosphate (isofenphos) for belowground invertebrates. ANOVA according to a split-plot design of plant species biomass data harvested in 1984 and 1986 revealed few significant effects of either small mammal densities or pesticide application. Of 54 species harvested from both sample dates, only 10 were significantly affected by either treatment. Analysis of species richness according to 8 life-form classes provided a clearer pattern of response than did biomass either by species or life-form class. For example, numbers of C4 grasses were reduced by increasing small mammal densities, whereas numbers of C4 annual forbs were lowest when above-ground herbivory was reduced. While consumers have been shown to have strong effects on successional communities, the few significant results observed in this study suggests that the manipulated levels of small mammals and insects had few effects on a mature tallgrass prairie.

Endophyte infection levels of native and naturalized fescues in Illinois and England

SPYREAS, G., D.J. GIBSON, AND M.A. BASINGER. (Department of Plant Biology, Southern Illinois University at Carbondale, Carbondale, IL 62901). Endophyte infection levels of native and naturalized fescues in Illinois and England. J. Torrey Bot. Soc. 128:25-34. 2001.-Fungal endophytes are thought to play a major role in the ecology of many cool-season grasses. However, the frequency of endophyte infection in the wild remains poorly known. Populations of wild fescues were tested for systemic fungal endophyte infection by taking samples from four native or naturalized species throughout southern Illinois (Festuca arundinacea, F. rubra, F. obtusa, and F. paradoxa), and one of these species (F. arundinacea) from native populations in England. The two grasses native to southern Illinois (F. obtusa and F. paradoxa) were highly infected (mean infection of 91 % and 100 %, respectively). The introduced grass F. rubra was uninfected. Festuca arundinacea showed variable levels of infection among populations in both southern Illinois and England (71 % and 64 %, respectively). The role of endophytes in natural systems is contingent upon infection levels that appear to vary widely among these four species. Furthermore, the use of agricultural cultivars may differentially affect infection levels of naturalized plants.

Stream insect occupancy-frequency patterns and metapopulation structure

An understanding of the distribution patterns of organisms and the underlying factors is a fundamental goal of ecology. One commonly applied approach to visualize these is the analysis of occupancy-frequency patterns. We used data sets describing stream insect distributions from different regions of North America to analyze occupancy-frequency patterns and assess the effects of spatial scale, sampling intensity, and taxonomic resolution on these patterns. Distributions were dominated by satellite taxa (those occurring in ≤10% of sites), whereas the occurrence of core taxa (occurring in ≥90% of sites) determined the overall modality of occupancy-frequency patterns. The proportions of satellite taxa increased with spatial scale and showed positive relationships with sampling intensity (r2=0.74–0.96). Furthermore, analyses of data sets from New York (USA) showed that generic-level assessments underestimated the satellite class and occasionally shifted occupancy-frequency distributions from unimodal to bimodal. Our results indicate that, regardless of species- or generic-level taxonomy, stream insect communities are characterized by satellite species and that the proportion of satellite species increases with spatial scale and sampling intensity. Thus, niche-based models of occupancy-frequency patterns better characterize stream insect communities than metapopulation models such as the core-satellite species hypothesis.

Learning Ecology by Doing Ecology: Long-Term Field Experiments in Succession.

We believe that ecology field laboratories are not being taught as they should be. In these laboratories, students should be using the scientific method and working in carefully designed long-term field experiments. Instead, many undergraduate ecology laboratories involve mensurative field observations, such as observational guided nature walks, listing species abundances from contrasting areas, or short-term manipulative experiments in the greenhouse. At best, these approaches are natural snapshot experiments (Diamond 1986) in which ecosystems that differ from one another in only one or two characteristics (for example, number of species in polluted versus unpolluted areas) are compared. These experiences are invaluable in the teaching of environmental sciences (Carter 1993) and are well integrated into many environmental science curricula. However, they do not provide a thorough training in the scientific method or reflect the way in which much of the best ecological research is conducted. The typical approach outlined above is in contrast to a large proportion of empirical ecological and environmental research that is based upon field experiments and the scientific method. Further, there is an increasing reliance on long-term field experiments in the development and testing of modern

Additions to the Vascular Flora of Konza Prairie Research Natural Area, Kansas

(KPRNA) over the past several years has yielded a number of vascular plants previously unreported from the research site. The following list is provided as an addendum to the annotated list of vascular plants of KPRNA (Freeman and Hulbert, 1985). These additions bring the number of species, genera, and families known from KPRNA to 467, 289, and 93, respectively. Continued research on Konza may reveal additional new taxa, however, the number of new reports would undoubtedly be few. Most of the taxa reported herein are weedy species or rare natives. Voucher specimens for all reports have been deposited in the Kansas State University Herbarium (KSC) with duplicate specimens in the reference herbarium at KPRNA Headquarters. The format and sequence of presentation in the list follows that outlined in Freeman and Hulbert (1985). Nomenclature is in accord with the Flora of the Great Plains (Great Plains Flora Association, 1986).

College Students' Perceptions on Adequacy of High School Science Curriculum as Preparation for College Level Biology.

W ITH the increasing number of high school graduates going on to college, there is an even greater need to insure the adequacy of the high school curriculum in preparing students for college (Uno 1988). The science education system of the United States has been criticized as providing a poor preparation for college science, lagging behind other countries such as Japan, Europe and the (former) Soviet Union (Barinaga 1990). The high school biology curriculum, in particular, has been criticized. Indeed, U.S. high school students ranked 13 out of 13 countries on knowledge of this subject in a 1988 study (Moore 1990). The strong level of concern by American educators for this issue is reflected in several national committee reports such as Project 2061: Science for All Americans, developed by the American Association for the Advancement of Science (AAAS 1989) and Fulfilling the Promise: Biology Education in the Nations Schools (National Research Council 1990), and in surveys of professional biology educators (Leonard et al. 1991). Much of the discussion on the adequacy of high school curricula as a preparation for college level science concerns the content of high school courses, the development of science process skills (Costenson & Lawson 1986), the relationships to technology and society and science teaching methods (Lawson 1988). While these are important concerns, less discussion has dealt with college student perceptions and attitudes towards biology and, in particular, the perceived adequacy of their own background prior to taking college biology courses. Smith (1979), for example, showed that student attitudes led to the alienation of female and black students from public school science. A negative attitude towards science is one of the main problems facing college freshmen enrolled in lower-division biology courses (Uno 1988). Such attitudes have been shown to have a negative effect on learning (Lawrenz 1976). The goal of our investigation was to survey college students taking introductory biology courses on their attitudes and perceptions on aspects of biology that should have been covered in high school science. Since all high school students in Florida are required to take a high school biology class with a mandatory lab (Florida Department of Education 1990), one of the key indicators of the adequacy of the high school curriculum is its adequacy as a preparation for college-level introductory biology courses.