L. E. Hurd

Food Production and the Energy Crisis

The principal raw material of modern U.S. agriculture is fossil fuel, whereas the labor input is relatively small (about 9 hours per crop acre). As agriculture is dependent upon fossil energy, crop production costs will also soar when fuel costs increase two- to fivefold. A return of 2.8 kcal of corn per 1 kcal of fuel input may then be uneconomical. Green revolution agriculture also uses high energy crop production technology, especially with respect to fertilizers and pesticides. While one may not doubt the sincerity of the U.S. effort to share its agricultural technology so that the rest of the world can live and eat as it does, one must be realistic about the resources available to accomplish this mission. In the United States we are currently using an equivalent of 80 gallons of gasoline to produce an acre of corn. With fuel shortages and high prices to come, we wonder if many developing nations will be able to afford the technology of U.S. agriculture. Problems have already occurred with green revolution crops, particularly problems related to pests (57). More critical problems are expected when there is a world energy crisis. A careful assessment should be made of the benefits, costs, and risks of high energy-demand green revolution agriculture in order to be certain that this program will not aggravate the already serious world food situation (58). To reduce energy inputs, green revolution and U.S. agriculture might employ such alternatives as rotations and green manures to reduce the high energy demand of chemical fertilizers and pesticides. U.S. agriculture might also reduce energy expenditures by substituting some manpower currently displaced by mechanization. While no one knows for certain what changes will have to be made, we can be sure that when conventional energy resources become scarce and expensive, the impact on agriculture as an industry and a way of life will be significant. This analysis is but a preliminary investigation of a significant agricultural problem that deserves careful attention and greater study before the energy situation becomes more critical.

Top-Down Cascade from a Bitrophic Predator in an Old-Field Community

We tested the hypothesis that a bitrophic (third and fourth level) arthropod predator can exert a cascading, top—down influence on other arthropods and plants in an early successional old field. First—stadium mantids, Tenodera sinensis, were added to replicated open—field plots in numbers corresponding to naturally occurring egg hatch density and allowed to remain for

Disturbance, coral reef communities, and changing ecological paradigms

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Stability and Diversity at Three Trophic Levels in Terrestrial Successional Ecosystems

mo. Sticky—trap dispersal barriers around both control and mantid—addition plots allowed us to monitor emigration of arthropods continuously during the experiment. Biomass of herbivores, carnivores, and plants, and abundances of arthropod taxa within plots were determined at the beginning, middle, and end of the experiment. The impact of mantids on the community was a top—down trophic cascade, beginning at the fourth trophic level and evident at each of the lower three levels. Mantids induced marked behavioral responses in other predators, but inteference among predators did not prevent the trophic cascade. The most common predators, cursorial spiders, emigrated from mantid addition plots in significantly greater numbers than from controls. This behavioral response may have resulted from avoidance of predation or competition. Mantids decreased biomass of herbivorous arthropods through predation, and this decrease in turn increased biomass of plants. Therefore, these generalist predators were able to decrease herbivory enough to affect plant growth. This and other recent studies indicate that top—down effects can be important in structuring terrestrial communities. Ours is the first example of a top—down cascade by a generalist arthropod predator in a nonagricultural ecosystem and illustrates the importance of detecting behavioral responses in studies of trophic interactions.

Hatch Density Variation of a Generalist Arthropod Predator: Population Consequenes and Community Impact

We examine changing ecological theory regarding the role of disturbance in natural communities and relate past and emerging paradigms to coral reefs. We explore the elements of this theory, including patterns (diversity, distribution, and abundance) and processes (competition, succession, and disturbance), using currently evolving notions concerning matters of scale (temporal and spatial), local versus regional species richness, and the equilibrium versus nonequilibrium controversy. We conclude that any attempt to categorize coral reef communities with respect to disturbance regimes will depend on the question being asked and the desired level of resolution: local assemblage versus regional species pool, successional versus geological time, and on the taxonomic and tropic affinities of species included in the study. As with many communities in nature, coral reefs will prove to be mosaics of species assemblages with equilibrial and nonequilibrial dynamics.

Stability in Relation to Nutrient Enrichment in Arthropod Consumers of Old‐Field Successional Ecosystems

An ecosystem perturbation experiment in which inorganic fertilizer was added to two successional old fields of different ages indicated that stability, expressed as resistance to modification by enrichment, increased during succession only at the primary trophic level. The perturbation was propagated throughout the arthropod herbivores and carnivores. Both herbivores and carnivores were less stable in the older field. In contrast to current ecological theory, greater diversity at a trophic level was accompanied by lower stability at the next higher level.

Arthropod Community Responses to Manipulation of a Bitrophic Predator Guild

We examined density dependence in population attributes and community impact of a generalist predator by experimentally mimicking natural variation in initial cohort densities produced by synchronous egg hatch in Mantis religiosa (Mantodea: Man- tidae). Mantid cohorts within the normal range of emergence from a single egg mass were established in a replicated, well-controlled open field experiment. On the scale of the progeny from a single female, density-dependent food limitation caused mortality and ontogenetic asynchrony to increase with increasing density. All cohorts converged to a common level of abundance and biomass because both development rate and population size declined with increasing initial density. Numbers and biomass of other arthropods generally declined with increasing initial density of mantids, although there were both positive and negative effects on different taxa. The abundance of hemipterans (almost exclusively herbivorous mirids) increased in the presence of mantids; this was an indirect effect as large in magnitude as any of the direct reductions in abundance of other taxa. Per capita interaction strengths of mantids on most taxa generally were weak except for the strong positive interaction with hemipterans. In spite of different mantid development rates among treatments, predator load (proportion of arthropod biomass present as predators) for all three treatments, attributable mainly to mantid biomass, converged to approximately five times control level by the end of the experiment. The differences in predator loads between control and treatment plots thus may represent different levels of predator saturation: one for control plots, where predator load was constant over time and in which generalists contributed relatively little to predator biomass, and a higher one for treatment plots, in which generalists comprised the bulk of predator biomass. Predator load may therefore be an indicator of the relative importance of generalist vs. specialist predators in terrestrial arthropod assemblages.

A trophic cascade in a diverse arthropod community caused by a generalist arthropod predator

Two adjacent, abandoned hayfields in central New York State were subjected to nutrient enrichment perturbation by means of a single application of 10-10-10 N, P, K fertilizer early in the growing season of 1970. Aboveground arthropod herbivores and carni- vores were monitored with respect to net productivity (dB/dt) and diversity for one growing season (1970) in the younger field (6 yr old), and for two growing seasons (1970 and 1971) in the older field (17 and 18 yr old). Sampling of arthropods was done with a gasoline- powered suction sampler between 1200 and 1600 h, twice weekly. Stability of the old-field arthropods was defined as resistance to change imposed by external (fertilizer) perturbation and was measured in terms of (1) magnitude (amplitude of deflection from ground state, (2) rate of initial response to perturbation, and (3) rate of damping or return to ground state. The degree of deflection from ground state was determined by com- paring fertilized (treated) plots with unfertilized (control) plots in each field. During the first growing season the older, more species-rich field was less stable with respect to magnitude and rate of productivity deflection than the younger, less species-rich field at both arthropod consumer levels. In addition, the magnitude of productivity deflection from ground state was lower in the carnivore level than in the herbivore trophic level. By the second growing season, the productivity of the arthropod consumers in the treated plots of the older field had not yet returned to control levels. The productivity of arthropod herbivores in the older field exhibited a relatively lower magnitude of response in 1971 than in 1970, while the arthropod carnivores in the older field exhibited a significantly greater difference from ground state in 1971 than in 1970. It was concluded that (1) the magnitude of deflection caused by the enrichment perturba- tion was decreased up the trophic levels from the herbivore to the carnivore level, (2) the rate of response to perturbation decreased up the trophic levels, and (3) although in control plots diversity increased and productivity decreased with increasing successional age for both trophic levels, stability was not positively related to either higher diversity or to increasing age in either consumer trophic level in this old-field successional ecosystem.

Cannibalism reverses male-biased sex ratio in adult mantids: female strategy against food limitation?

We used replicated field enclosures to manipulate population densities of two species of bitrophic predators in a terrestrial old—field community: a mantid (Tenodera sinensis), and a wolf spider (Lycosa rabida). The treatments consisted of adding mantids alone (8 individuals/enclosure), lycosids alone (10 individuals/enclosure), and lycosids and mantids together (8 + 10 individuals). A control consisted of enclosures to which no predators were added. The impact of these predators on numbers and biomass of other arthropods in the community was examined on several levels: overall community, different size (body length) categories, and major taxa. We asked whether the impact of these predators in combination could be predicted from their separate effects. Mantids depressed total numbers (10—15%) and biomass (50%), in the arthropod community over the course of 10 d. This effect was confined to the largest size categories in the community. Lycosids had no measurable effect at this level of resolution. Mantids depressed abundance of acridids in both mantid and mantid/lycosid enclosures, but again lycosids had no impact. Lycosids alone enhanced abundance of gryllids, but not in the presence of mantids. Both mantids and lycosids depressed numbers of small spiders (also members of this guild), but this effect was not additive. Interactions among members of bitrophic generalist predator guilds may contribute to the commonness of nonadditive and higher order effects in manipulative experiments. Depending upon the level of resolution, it may be impractical to predict the impact of the whole guild from summing the individual effects of single predator species on terrestrial arthropod communities.

Cursorial spiders and succession: age or habitat structure?

Abstract We tested the hypothesis that a generalist arthropod predator, Tenodera sinensis Saussure, could trigger a trophic cascade in an old-field ecosystem. These mantids had relatively weak effects on abundance and biomass of other carnivorous arthropods as a group. The effect of mantids on herbivores was stronger than on carnivores, mainly concentrated in Homoptera and Diptera. Herbivore load was reduced by mantids with the consequence that overall plant biomass (mainly grasses) was increased. Percapita interaction strengths between mantids and other arthropod taxa were, for the most part, weakly negative. Our study demonstrates that a significant trophic cascade can be triggered by a generalist predator even within the framework of a diverse community with relatively diffuse interactions.