Empirical and theoretical evidence for herd size as a risk factor for swine diseases

abstract: Herd size is frequently studied as a risk factor for swine diseases, yet the biological rationale for a reported association with herd size (whether positive or negative) is rarely adequately discussed in published epidemiological studies. Biologically plausible reasons for a positive association between herd size and disease include a greater risk of introduction of pathogens from outside the herd, greater risk of transmission of pathogens within and among herds when the herd is large, and effects of management and environmental factors that are related to herd size. However, compared with owners of small herds, owners of large herds might more frequently adopt management and housing practices that mitigate this theoretically increased risk. Studies on pleuritis, pneumonia and pseudorabies were used to describe the epidemiological issues involved in evaluations of the relationship between management factors, herd size and disease. In future studies, it is recommended that: (1) herd size be measured in a way that best characterizes the true population at risk; (2) studies that evaluate management-related risk factors should account for herd size wherever possible; (3) population-based studies of the interrelationships among management factors and between management factors, herd size, herd density and pig density must be conducted; (4) likely biological reasons for any herd-size effect should be postulated; and (5) the distribution of herd sizes in the source population and the study sample must be described

Uneven global distribution of food web studies under climate change

Abstract Trophic interactions within food webs affect species distributions, coexistence, and provision of ecosystem services but can be strongly impacted by climatic changes. Understanding these impacts is therefore essential for managing ecosystems and sustaining human well-being. Here, we conducted a global synthesis of terrestrial, marine, and freshwater studies to identify key gaps in our knowledge of climate change impacts on food webs and determine whether the areas currently studied are those most likely to be impacted by climate change. We found research suffers from a strong geographic bias, with only 3.5% of studies occurring in the tropics. Importantly, the distribution of sites sampled under projected climate changes was biased?areas with decreases or large increases in precipitation and areas with low magnitudes of temperature change were under-represented. Our results suggest that understanding of climate change impacts on food webs could be broadened by considering more than two trophic levels, responses in addition to species abundance and biomass, impacts of a wider suite of climatic variables, and tropical ecosystems. Most importantly, to enable better forecasts of biodiversity responses to climate change, we identify critically under-represented geographic regions and climatic conditions which should be prioritized in future research.Peer reviewe