Michael L. Rosenzweig

Paradox of Enrichment: Destabilization of Exploitation Ecosystems in Ecological Time

Six reasonable models of trophic exploitation in a two-species ecosystem whose exploiters compete only by depleting each others resource supply are presented. In each case, increasing the supply of limiting nutrients or energy tends to destroy the steady state. Thus man must be very careful in attempting to enrich an ecosystem in order to increase its food yield. There is a real chance that such activity may result in decimation of the food species that are wanted in greater abundance.

Graphical Representation and Stability Conditions of Predator-Prey Interactions

The general nature of the predator-prey interaction has been depicted as a graph of predator versus prey densities from which conditions for stability of the interaction are predicted. An example of a three-species interaction is also presented. Variations of the graph are introduced, and it is shown that an otherwise unstable interaction may be stabilized by the presence of either an inviolable prey hiding place, or extremely low predation pressure at moderate predator and high prey densities, or another predator-limiting resource. Stability is always conferred when the predator is severely limited at its equilibrium density by one of its resources other than its supply of prey. Predators should tend to be limited at their equilibrium densities by more than one of their resources. When either of the two foregoing situations pertains, regular predator-prey oscillations should not be observable. The stability of the interaction close to equilibrium was found to depend exclusively, in the mathematically-continuous model, upon the slopes of two lines in the graph at equilibrium. Stability can be asymptotic rather than oscillatory in type. An equation for the period of oscillatory interactions is also advanced. The effects of Natural Selection on the isoclines, and thus the stability, is not clear-cut. Selection of the prey tends to stabilize the interaction; the opposite is true for selection on the predator.

Net Primary Productivity of Terrestrial Communities: Prediction from Climatological Data

Actual evapotranspiration (AE) is shown to be a highly significant predictor of the net annual above-ground productivity in mature terrestrial plant communities. Communities included ranged from deserts and tundra to tropical forests. It is hypothesized that the relationship of AE to productivity is due to the fact that AE measures the simultaneous availability of water and solar energy, the most important rate-limiting resources in photosynthesis.

Don't judge species on their origins

Conservationists should assess organisms on environmental impact rather than on whether they are natives, argue Mark Davis and 18 other ecologists.

Population Ecology of Desert Rodent Communities: Habitats and Environmental Complexity

We investigated the relative densities of granivorous, nocturnal desert rodents in small plots within two arid regions of Arizona to study how sympatric species avoid competitive extinction. The most common rodents were kangaroo rats, Dipodomys spp., and pocket mice, Perognathus spp. We attempted correlating the density of each species with several environmental measurements, derived from the soils i) depth; ii) texture or iii) resistance to sheer stress; or from the plants i) species diversity; ii) growth forms or iii) foliage density. Successful variables were derived from plant growth form and foliage density. The soils resistance to sheer stress also seemed important for a few species. In general, kangaroo rats were associated with sparseness of vegetation; pocket mice with denseness. One group of mice, which we term bush mice, seemed to require bushes and included two Perognathus spp., three Peromyscus spp., and probably a harvest mouse (Reithrodontomys fulvescens). Two other Perognathus spp. were taken in grassy habitats. Some suitable habitats tended to be complementary to others, suggesting that species associated with them are competitors. Comparisons of the density and distribution of D. merriami, present in both regions but under different biotic circumstances, reinforces the opinion that competition is responsible for the complementariness of habitats. In some cases the evidence suggests that competitive coexistence is accounted for by the fact that different specializations are needed to escape predation in different environments. We use variables which correlate with the relative density of various species to construct a model of habitat complexity. The rodent species diversities obtained in our plots can be approximately accounted for by this model. The model is based on the premises that the rodents collectively discriminate four qualities of soil surface, and three heights and two densities of vegetation. In general, specializations based on biotic variables appear most important.

Reconciliation ecology and the future of species diversity

Species-area relationships (SPARs) dictate a sea change in the strategies of biodiversity conservation. SPARs exist at three ecological scales: Sample-area SPARs (a larger area within a biogeographical province will tend to include more habitat types, and thus more species, than a smaller one), Archipelagic SPARs (the islands of an archipelago show SPARs that combine the habitat-sampling process with the problem of dispersal to an island), and Interprovincial SPARs (other things being equal, the speciation rates of larger biogeographical provinces are higher and their extinction rates are lower, leading to diversities in proportion to provincial area). SPARs are the products of steady-state dynamics in diversity, and such dynamics appears to have characterized the earth for most of the last 500 million years. As people reduce the area available to wild species, they impose a linear reduction of the earths species diversity that will follow the largest of these scales, i.e. each 1% reduction of natural area will cost about 1% of steady-state diversity. Reserving small tracts of wild habitat can only delay these reductions. But we can stop most of them by redesigning anthropogenic habitats so that their use is compatible with use by a broad array of other species. That is reconciliation ecology. Many pilot projects, whether intentionally or inadvertently espousing reconciliation ecology, are demonstrating that it can be done.

Habitat Selection Experiments with a Pair of Coexisting Heteromyid Rodent Species

The kangaroo rat, Dipodomys merriami, occurs with the pocket mouse, Perognathus penicillatus, in stands of mesquite with little herbaceous cover. The dipo avoids areas that have thick cover between 8 cm and 45 cm. The pocket mouse prefers areas that have considerable cover over 45 cm. Those facts have been established both by previous studies and those reported here. This paper reports the results of altering the vegetation in small patches of land and observing concomitant changes in the land—use patterns of the rodents. The difference between the species appears to be crucial to their coexistence.

Competition and food selection: field tests of a theory

A graphical model describing the optimal choices of two species competing for resources in two types of habitats is tested with hummingbirds in the field. In this model, both species prefer taking resources from one of the habitat types. However, one of the species (the dominant), by virtue of interference competition, can gain access to the better patch more readily than the other (the subordinate). The model begins with the result of single—species optimal foraging models: at low densities of birds, only the better patch type should be selected, but as density increases, both should be used. Interspecific competition should not lead to qualitatively different behaviors for the dominant species because the effects of the subordinate are weak. For the subordinate, however, there is a third class of behaviors; under the pressure of high densities of the dominant, the subordinate may totally avoid the better patch and use only the poorer. We tested the validity of the models predictions using three species of hummingbirds: Blue—throated (Lampornis clemenciae), Rivolis (Eugenes fulgens), and Black—chinned (Archilochus alexandri) coming to feeders containing 1.2 mol/L or 0.35 mol/L sucrose solution. There were no detectable effects of Rivolis on Black—chinned or vice—versa. This allowed us to test the models predictions about the interactions between the other pairs of species. Blue—throateds were dominant in both cases. The amount of time birds spent feeding (or in the case of Blue—throateds, feeding and defending) was analyzed, rather than actual densities. Wide ranges of feeding and defending times were obtained using species removals and natural seasonal changes in densities. These two pairs of species exhibited all the models features. In particular, the dominants aggressive behavior forced the subordinate to restrict itself to a patch type known to be inferior.

GRANIVORY IN NORTH AND SOUTH AMERICAN DESERTS: RODENTS, BIRDS, AND ANTS'

Experiments which measure seed removal rates of ants, birds, and mammals were conducted in the Sonoran Desert of North America and the Monte Desert of South America. Rodents were the primary consumers in North America, but ants also took large amounts of seeds there. Ants were the major granivores in the Monte, where mammal seed consumption was insignificant. Birds were relatively unimportant consumers in both deserts. Total seed removal rates were more than an order of magnitude larger in the Sonoran Desert. These results are much in contrast with the great overall similarity of the 2 systems in climate and flora. The extinction, 1 or 2 million yr ago, of a marsupial family, Argyrolagidae, which appears to have been the ecological equivalent of the rodent granivores, Heteromyidae, may have caused the differences in granivory either because the Argy- rolagidae were mutualistic with the plants or, in an evolutionary sense, with the granivorous ants.

Habitat Selection: An Experimental Field Test with Two Gerbil Species

We provide experimental evidence for the isoleg theory of habitat selection in a pair of psammophilic gerbil species. Gerbillus allenbyi (mean mass: 26 g) and G. pyramidum (mean mass: 40 g) coexist in Israels Negev desert in areas that may contain three distinct sandy habitats: stabilized sand fields, semistabilized dunes, and drifting dunes. When all three habitat types are available, coexistence between the two species has been explained by a centrifugal model of community organization that has been untested until now. To begin testing it, we conducted a field experiment in six 1 ha enclosures, each containing similar proportions of two of the sandy—habitat types (stabilized sand and semi—stabilized dune). This experiment tested the following hypotheses concerning the coexistence of the two species: (1) both species prefer the same primary habitat type; (2) G. allenbyi and G. pyramidum exhibit intraspecific density—dependent habitat selection; (3) habitat preference of both G. allenbyi and G. pyramidum...