Scott L. Collins

The Keystone Role of Bison in North American Tallgrass Prairie

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Species diversity enhances ecosystem functioning through interspecific facilitation

Facilitation between species is thought to be a key mechanism by which biodiversity affects the rates of resource use that govern the efficiency and productivity of ecosystems; however, there is no direct empirical evidence to support this hypothesis. Here we show that increasing the species diversity of a functional group of aquatic organisms induces facilitative interactions, leading to non-additive changes in resource consumption. We increased the richness and evenness of suspension-feeding caddisfly larvae (Insecta, Trichoptera) in stream mesocosms and found that the increased topographical complexity of the benthic habitat alters patterns of near-bed flow such that the feeding success of individuals is enhanced. Species diversity reduces ‘current shading’ (that is, the deceleration of flow from upstream to downstream neighbours), allowing diverse assemblages to capture a greater fraction of suspended resources than is caught by any species monoculture. The fundamental nature of this form of hydrodynamic facilitation suggests that it is broadly applicable to freshwater and marine habitats; in addition, it has several analogues in terrestrial ecosystems where fluxes of energy and matter can be influenced by biophysical complexity. Thus, changes in species diversity may alter the probability of positive species interactions, resulting in disproportionately large changes in the functioning of ecosystems.

INTERACTION OF DISTURBANCES IN TALLGRASS PRAIRIE: A FIELD EXPERIMENT'

A field study was conducted to address the roles of fire, cattle grazing, and the interaction of these two disturbances on plant species abundance and community structure in a tallgrass prairie. In 1985 and 1986, species composition was sampled in three replicates of the following grassland treatments: ungrazed + unburned (undisturbed), grazed + unburned, ungrazed + burned, and grazed + burned. Cover of the matrix-forming species Andropogon gerardii was significantly greater, while cover of the nonmatrix species Bromus tectorum was significantly lower, on the burned treatments than on the unburned treatments. The number and cover of annuals were significantly higher on the grazed treatments than on the ungrazed treatments. Species richness increased with increasing disturbance intensity. Diversity, however, was lowest on the ungrazed + burned treatment and highest on the grazed + burned treatment. Burning significantly reduced species diversity on ungrazed treatments, and grazing significantly increased diversity on the burned treatment. This study documented the dissimilar effects of different natural disturbances on grass- land species, growth form characteristics, and community structure. In addition, the results emphasize the important role of interaction among disturbances on plant community structure in grasslands.

An integrated conceptual framework for long-term social-ecological research

The global reach of human activities affects all natural ecosystems, so that the environment is best viewed as a social–ecological system. Consequently, a more integrative approach to environmental science, one that bridges the biophysical and social domains, is sorely needed. Although models and frameworks for social–ecological systems exist, few are explicitly designed to guide a long-term interdisciplinary research program. Here, we present an iterative framework, “Press–Pulse Dynamics” (PPD), that integrates the biophysical and social sciences through an understanding of how human behaviors affect “press” and “pulse” dynamics and ecosystem processes. Such dynamics and processes, in turn, influence ecosystem services –thereby altering human behaviors and initiating feedbacks that impact the original dynamics and processes. We believe that research guided by the PPD framework will lead to a more thorough understanding of social–ecological systems and generate the knowledge needed to address pervasive environmental problems.

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.

Fire Frequency and Community Heterogeneity in Tallgrass Prairie Vegetation

Pattern in plant communities is a function of many biotic and abiotic factors and these factors operate on different species at different spatial scales (Pielou 1960, Greig-Smith 1979, Legendre and Fortin 1989). One product of the nonrandom distribution of species at different spatial scales is community heterogeneity. In this paper, spatial heterogeneity is the mean degree of dissimilarity in species composition from one point to another in a community (Inouye et al. 1987, Collins 1990). Heterogeneity differs from pattern analysis in that the latter is a measure of the degree of spatial autocorrelation, or distance decay in the case of gradients, in a community (Palmer 1988, Legendre and Fortin 1989).

Effects of disturbance on diversity in mixed-grass prairie

Seven grassland treatments representing different disturbance regimes were sampled within a large area of mixed-grass prairie in southwestern Oklahoma, USA. Species diversity was low on the undisturbed and most severely disturbed grasslands. The results also indicate, however, that grassland diversity was not a simple function of disturbance rate, size or intensity. Instead, species diversity was maximized under a combination of natural disturbances. To some extent, the increase in species diversity was the result of increased habitat diversity associated with a type of disturbance. That is, disturbances such as grazing and wallowing have a cumulative effect one grassland diversity. Overall, the structure one grassland communities can not be accurately determined without considering the structure of the natural disturbance regime.

Disturbance Frequency and Community Stability in Native Tallgrass Prairie

Ecological communities are spatially and temporally variable in response to a variety of biotic and abiotic forces. It is not always clear, however, if spatial and temporal variability leads to instability in communities. Instability may result from strong biotic interactions or from stochastic processes acting on small populations. I used 10–15 yr of annual data from the Konza Prairie Long‐Term Ecological Research site to examine whether plant, breeding bird, grasshopper, and small mammal communities in tallgrass prairie exhibit stability or directional change in response to different experimentally induced fire frequencies. Based on ordination and ANOVA, plant and grasshopper communities on annually burned sites differed significantly from plant and grasshopper communities on less frequently burned sites. Breeding birds and small mammals differed among sites as well, but these differences were not clearly related to disturbance frequency. A modified time series analysis indicated that plant communities were undergoing directional change (unstable) on all watersheds, regardless of fire frequency. Contrary to expectations, directional change was greatest on the annually burned sites and lowest on the infrequently burned sites. Unlike the plant communities, breeding bird, grasshopper, and small mammal communities were temporally stable, despite high‐compositional variability from 1 yr to the next. Stability among the consumer communities within these dynamic plant communities occurs because three‐dimensional vegetation structure does not change over time, despite changes in plant species composition. Evidence suggests that instability in the plant community results from strong biotic interactions among temporally persistent core species and stochastic dynamics among infrequent satellite species. Overall, community stability cannot be assessed if the pattern of temporal dynamics is unknown. Long‐term empirical studies of different taxa under different disturbance regimes are needed to determine over what time frames and spatial scales communities may be stable. Such studies are essential for the development of generalities regarding the relationship between disturbance frequency and community stability in terrestrial and aquatic systems.

Caterpillar Leaf Damage, and the Game of Hide‐and‐Seek with Birds

In an aviary, field—captured Black—capped Chickadees, Parus atricapillus, learned to forage preferentially at trees with either artificially or caterpillar—damaged leaves. The birds also distinguished between species of broad—leaved trees. These individuals showed differences in foraging behavior, possibly related to previous conditioning in the field. When captured, two of the birds were already using leaf morphology (rolls and discoloration) to search for caterpillars. Two others initially disregarded leaf morphology and instead scanned for prey directly. However, when the potential prey was highly cryptic or hidden, the birds all learned to use both tree species and leaf damage simultaneously as cues in their searching effort for specific prey. Caterpillars that were palatable to the chickadees had feeding behaviors that tended to minimize their apparent feeding damage on leaves, while caterpillars that were not eaten did not have these behaviors. See full-text article at JSTOR

Woody encroachment decreases diversity across North American grasslands and savannas

Woody encroachment is a widespread and acute phenomenon affecting grasslands and savannas worldwide. We performed a meta-analysis of 29 studies from 13 different grassland/savanna communities in North America to determine the consequences of woody encroachment on plant species richness. In all 13 communities, species richness declined with woody plant encroachment (average decline = 45%). Species richness declined more in communities with higher precipitation (r2 = 0.81) and where encroachment was associated with a greater change in annual net primary productivity (ANPP; r2 = 0.69). Based on the strong positive correlation between precipitation and ANPP following encroachment (r2 = 0.87), we hypothesize that these relationships occur because water-limited woody plants experience a greater physiological and demographic release as precipitation increases. The observed relationship between species richness and ANPP provides support for the theoretical expectation that a trade-off occurs between richness and productivity in herbaceous communities. We conclude that woody plant encroachment leads to significant declines in species richness in North American grassland/savanna communities.