Population responses of small mammals to food supply and predators: a global meta-analysis

1. The relative importance of food supply and predation as determinants of animal population density is a topic of enduring debate among ecologists. To address it, many studies have tested the potential effects of food on population density by experimentally supplementing natural populations, with much focus on terrestrial vertebrates, especially small mammals. 2. Here we perform a meta-analysis of such experiments, testing two complementary hypotheses: (i) small mammal populations are bottom-up limited and (ii) population increases in response to food supplementation are constrained by predation, a top-down limitation. 3. In the 148 experiments recorded, food supplementation had an overall positive and significant effect, increasing population densities by 1.5-fold. Larger population increases occurred when predation was reduced and populations were open to immigration. Predation appeared to be unimportant when populations were closed to immigration. Immigration was the major mechanism underlying increases in abundance by increasing local population density and crowding. Contributions of increased reproductive rate could be detected, but were minor compared to immigration, and no effects were detected from survival. 4. Our analyses support the view that animal population density is determined by both bottom-up and top-down forces. They also suggest the possibility that food supplementation experiments might unintentionally create ecological traps by aggregating both prey and predators in small areas of the landscape. We suggest an alternative experimental design to increase the contribution that food supplementation experiments can make in future.

Population Characteristics and Dosage Trajectory Analysis for Mesocriconema xenoplax in California Prunus Orchards

The Mesocriconema xenoplax population increased exponentially in a newly planted peach orchard. The rate of increase was greater on Nemaguard than on Lovell rootstock and was reduced by postplant nematicides. Population levels were more stable in an established almond orchard on Nemaguard rootstock. All life stages of the nematode were present year round; lower ratios of juveniles to adults in summer suggested adverse effects of temperature and dry soil. Also in summer, there was a smaller proportion of the population in the upper 30 cm of soil than at greater depths. Nematode dosage, average nematode density multiplied by accumulated degree-days (physiological time) of the sampling interval, was useful in quantifying nematode stress on trees and as an indicator of the nematode management effectiveness. The annual trajectory of the nematode dosage could be determined by two samplings, one in spring and one in fall. A nematode predator, the parasitic fungus Hirsutella rhossiliensis, did not regulate ring nematode populations in the newly planted orchard; a recovery period was necessary for increase in the prevalence of parasitism.

New frontiers in nematode ecology

Future areas of emphasis for research and scholarship in nematode ecology are indicated by pressing agricultural and environmental issues, by new directions in applied nematology, and by current technological advances. Studies in nematode ecology must extend beyond observation, counting, and simple statistical analysis. Experimentation and the testing of hypotheses are needed for understanding the biological mechanisms of ecological systems. Opportunities for fruitful experimentation in nematode ecology are emerging at the ecosystem, community, population, and individual levels. Nematode ecologists will best promote their field of study by closely monitoring and participating in the advances, initiatives, developments, and directions in the larger field of ecology.