Bradley J. Bergstrom

Editorial: Drivers of small-mammal community structure in tropical savannas

Small-mammal faunas of tropical savannas consist of endemic assemblages of murid rodents, small marsupials, and insectivores on four continents. Small mammals in tropical savannas are understudied compared to other tropical habitats and other taxonomic groups (e.g., Afrotropical megafauna or Neotropical rainforest mammals). Their importance as prey, ecosystem engineers, disease reservoirs, and declining members of endemic biodiversity in tropical savannas compels us to understand the factors that regulate their abundance and diversity. We reviewed field studies published in the last 35 years that examined, mostly experimentally, the effects of varying three primary endogenous disturbances in tropical savanna ecosystems—fire, large mammalian herbivory (LMH), and drought—on abundance and diversity of non-volant small mammals. These disturbances are most likely to affect habitat structure (cover or concealment), food availability, or both, for ground-dwelling small mammalian herbivores, omnivores, and insectivores. Of 63 studies (included in 55 published papers) meeting these criteria from the Afrotropics, Neotropics, and northern Australia (none was found from southern Asia), 29 studies concluded that small mammals responded (mostly negatively) to a loss of cover (mostly from LMH and fire); four found evidence of increased predation on small mammals in lower-cover treatments (e.g., grazed or burned). Eighteen studies concluded a combination of food- and cover-limitation explained small-mammal responses to endogenous disturbances. Only two studies concluded small-mammal declines in response to habitat-altering disturbance were caused by food limitation and not related to cover reduction. Evidence to date indicates that abundance and richness of small savanna mammals, in general (with important exceptions), is enhanced by vegetative cover (especially tall grass, but sometimes shrub cover) as refugia for these prey species amid a “landscape of fear,” particularly for diurnal, non-cursorial, and non-fossorial species. These species have been called “decreasers” in response to cover reduction, whereas a minority of small-mammal species have been shown to be “increasers” or disturbance-tolerant. Complex relationships between endogenous disturbances and small-mammal food resources are important secondary factors, but only six studies manipulated or measured food resources simultaneous to habitat manipulations. While more such studies are needed, designing effective ones for cryptic consumer communities of omnivorous dietary opportunists is a significant challenge.

Wiebers & Feigin (2020) make a strong argument for measures that would limit future zoonoses, such as the COVID-19 pandemic, by closing live-animal markets, changing our habits of food consumption and production, and reducing habitat destruction. These would help human health, animal welfare, and conservation of at-risk wildlife all at the same time. China’s command-and-control government can accomplish some of these things by edict, but slower-to-act western democracies play a surprisingly large role in these global problems by the power of their consumerism, including the illicit wildlife trade. We citizens need to insist that our government use all of its diplomatic soft power to engage (or re-engage) with other nations, and with global institutions such as the World Health Organization, various United Nations programs, and parties to treaties (some of which we still need to ratify, others of which we need to enforce at home) aimed at solving the interlocking global crises of species endangerment, habitat destruction, climate change, and emerging infectious disease.

We related presence and burden of stomach nematodes to body mass and reproductive allocation in hispid cotton rats (Sigmodon hispidus) from two long-running field studies in Virginia (1983–1984, n = 286; and 1988–1990, n = 425) and one from Georgia 1987–1989 (n = 459). Eighty percent of rats from the earlier Virginia sample were infected, with mean nematode mass of 1,311 mg. In the later samples, 23% (Virginia) and 33% (Georgia) were infected with mean nematode mass of 493 and 769 mg, respectively. Presence of nematodes was positively correlated with host body length for each sex in each sample. We used analysis of covariance to examine length-adjusted residuals for presence of nematodes and mass of nematodes for association with somatic and reproductive response variables. Both body and reproductive masses were either positively associated or not related to nematode presence in the two low-prevalence samples, and either negatively associated or not related to nematode presence in the high-prevalence sample. No relationships were detected between host mass and nematode mass per host in either sex in any sample. There was no effect of nematode presence on litter size of pregnant females, but there was a positive effect of nematode mass on litter size in Georgia. Recent theory provides several possible explanations for such neutral-to-positive effects of stomach nematodes on host fitness, including the evolution of host tolerance to the parasites, fecundity compensation by the hosts, and positive effects on host health via immune modulation.

Most small birds wintering in the tropics should show little subcutaneous fat deposition (SFD), except in habitats where food availability may decline in late winter or, for some resident species, to prepare for incubation or brooding fasts. However, these predictions need re‐examination in light of a new, precise, cross‐validated method to compare SFD among habitats and species. We sampled 170 Nearctic‐Neotropical migrant and 279 resident birds during early and late winter in 1993 and 1994 in Jamaica, West Indies. Habitats, from greatest to least expected availability of insect prey, were (1) mangrove forest, (2) montane/foothills forest and cultivation, (3) dry limestone forest, and (4) acacia scrub. Percent lipid, estimated from multiple‐regression models using visual fat scoring (0–8 scale), total‐body electrical conductivity, and a variety of morphometrics, was categorized by percentile ranks to determine if SFD varied by habitat, season, or age for all species, resident species, migrant species, and several individual species. SFD averaged ~ 13% total mass for all birds, ranging from 8–24% for well‐sampled species. The few bird species in acacia scrub, primarily two facultative long‐distance migrants, averaged ~ 26% lipid content, significantly more than birds in other habitats. Most birds did not vary in SFD in the other three habitats, although Common Yellowthroats (Geothlypis trichas) had greater SFD in dry limestone habitat than in montane habitat. Bananaquits (Coereba flaveola) and Jamaican Euphonias (Euphonia jamaica) in montane habitat, especially in early winter, had higher SFD than other resident species. Contrary to our prediction, adults and juveniles had similar SFD, with the exception of juveniles having more SFD than adults in acacia scrub habitat. Winter fat deposition (or, in some cases, muscle‐protein catabolism) in the tropics may be an overlooked strategy, potentially important as a hedge against fasting for floaters, facultative migrants, some territorial migrants in habitats with seasonal declines in food resources, and some resident species prior to breeding.

Treves et al. (2019) argue that individual organisms should be protected by the courts. This already happens in many countries for rare and endangered species and for symbolically important species (e.g., raptors, songbirds, and bats in the U.S.), and through the international treaty for whales. But protection of these individuals stems from a focus on preserving their populations, whereas hunting of individuals is still a culturally accepted practice. Killing of individuals should be limited to valid reasons (e.g., eating the meat) and covered by fair-chase and humane-killing laws (which is not always the case). But “futurity” will not inherit present-day individuals. What we owe them are healthy populations and species, and functioning, diverse biotic systems. Conservation biology is already rightly focused on this holistic view. Finally, empowerment of women and reform of economic systems will do more to stem the loss of biodiversity than adjudication beyond that already in service of protecting vulnerable species.

Large mammalian herbivores (LMH) are known to suppress populations of small mammals in African savanna ecosystems; whether this suppression is driven by depletion of nutrients and food resources, or of cover, is poorly understood. Cattle management creates scattered, persistent, nutrient‐enriched areas (glades). Similarly, prescribed fire may enhance forage nutrition of burned patches. Both enriched microhabitats attract wild and domestic LMH and are fertilized by their wastes, but removal of vegetative cover by LMH or fire may negate the benefits of enhanced nutrition to risk‐averse small‐mammal species or individuals. We used replicated LMH exclusion experiments on red sandy loam and black‐cotton clay soils, and a multi‐scale burn experiment on black cotton to examine abundance, diversity, and biomass of small mammals across sites varying in vegetation cover and enrichment. Small‐mammal species composition varied dramatically among habitats. Species diversity and density on red sands were roughly double that of black cotton soils. Small‐mammal densities and diversity were dramatically greater inside LMH exclosures; maximal densities occurred inside fenced, nutrient‐rich, tall‐grass glades. Small‐mammal density was similar between black‐cotton burn sites and unburned matrix habitat but was significantly greater on unburned than burned patches within the burn sites. Burned patches contained less grassy cover up to 50 months post‐burn, although vegetation differences diminished following significant rains. Mice captured on burned patches traveled farther but were not heavier than mice captured on unburned patches. Small mammals were nearly 20‐fold more abundant on totally fenced glades than matrix habitat on both soils and ninefold more abundant on fenced bushland (non‐glades) on red sands. Unfenced glades supported intermediate densities in black cotton but lower densities in red sands because of close cropping by LMH. Total small‐mammal biomass tended to be greater on red sands (though mean body mass was greater on black cotton for two common species), within exclosures, and on glades. Both the grass rat and pouched mouse reached maximal densities within glade LMH exclosures on both soils. This and the positive association of small‐mammal densities with grass height and dead‐stem density suggest loss of cover is a primary driver of large‐herbivore suppression of certain savanna small mammals.

For 90 years, the American Society of Mammalogists (ASM) has made science-based challenges to widespread lethal control of native mammals, particularly by the United States federal government targeting carnivores in the western states. A consensus is emerging among ecologists that extirpated, depleted, and destabilized populations of large predators are negatively affecting the biodiversity and resilience of ecosystems. This Special Feature developed from a thematic session on predator control at ASM’s 2013 annual meeting, and in it we present data and arguments from the perspectives of ecology, wildlife biology and management, social science, ethics, and law and policy showing that nonlethal methods of preventing depredation of livestock by large carnivores may be more effective, more defensible on ecological, legal, and wildlife-policy grounds, and more tolerated by society than lethal methods, and that total mortality rates for a large carnivore may be driven higher than previously assumed by human causes that are often underestimated.

For more than 100 years, the US government has conducted lethal control of native wildlife, to benefit livestock producers and to enhance game populations, especially in the western states. Since 2000, Wildlife Services (WS), an agency of the US Department of Agriculture, has killed 2 million native mammals, predominantly 20 species of carnivores, beavers, and several species of ground‐dwelling squirrels, but also many nontarget species. Many are important species in their native ecosystems (e.g., ecosystem engineers such as prairie dogs and beavers, and apex predators such as gray wolves). Reducing their populations, locally or globally, risks cascading negative consequences including impoverishment of biodiversity, loss of resilience to biotic invasions, destabilization of populations at lower trophic levels, and loss of many ecosystem services that benefit human society directly and indirectly. Lethal predator control is not effective at reducing depredation in the long term. Instead, we recommend that WS and its government partners involved in wildlife conflict management emphasize training livestock producers in methods of nonlethal control, with sparing use of lethal control by methods that are species‐specific, and cease all lethal control in federal wilderness areas and for the purpose of enhancing populations of common game species.

It is well established that massive consumption of plants by the abundant and diverse assemblage of savanna ungulates in East Africa competitively su