Soil fertility and anthropogenic disturbances drive mammal species richness and assemblage composition on tropical fluvial islands

Abstract

Floodplain areas comprise some 30% of the area in the Amazon, but are currently under severe anthropogenic threat. Across the Amazon Basin, forest-dwelling non-volant mammals play crucial roles in maintaining the integrity of forest functionality, yet have been poorly studied in fluvial island forests. Mammal assemblages may be affected by edaphic characteristics that operate indirectly via food nutritional quality, by patch attributes, and/or can be modulated by anthropogenic disturbances. Here, we conducted systematic and quantitative mammal surveys across fluvial islands of an Amazonian archipelago, to assess the influence of edaphic factors (soil fertility), island attributes (island area and degree of isolation) and anthropogenic characteristics (distance from human settlement and logging) on the patterns of mammal species composition and richness. On 28 islands, we conducted spoor surveys and deployed 49 camera traps (total effort of 2940 camera trap-days). Subsequently, we performed multiple regression analysis to investigate the influence of environmental and anthropogenic predictors on mammal species richness, while dbRDA (distance-based redundancy analysis) was used for species composition. We found that mammal species richness was positively correlated with soil fertility, and in combination with anthropogenic characteristics, both variables affected the species assemblage composition. In particular, smaller species were found across a variety of levels of soil fertility and anthropogenic disturbances, while larger mammals were mostly recorded at sites with higher soil fertility and low levels of anthropogenic disturbances. Understanding the contribution of environmental and anthropogenic characteristics to the observed mammalian species richness and assemblage composition patterns will help optimise management and conservation efforts on Amazonian fluvial islands. In particular, we suggest enforcing hunting and logging restrictions within fluvial islands through surveillance activities, especially in more fertile islands.

Productivity correlates positively with mammalian diversity independently of the species’ feeding guild, body mass, or the vertical strata explored by the species

“…Lower investment in secondary compounds makes the leaves more easily digestible for fauna of various body sizes (Coley et al 1985), leading to increased herbivory (Pilbeam 2018). These mechanisms may scale up through the trophic web, increasing energy availability and affecting the number of individuals and species richness of arboreal (Peres 2008) or terrestrial mammals (Ferreira Neto et al 2021a). While some of these productivity proxies are direct measures of plant biomass or cover, which could influence mammal diversity due to an influence on resource availability, some of them are coarse proxies, which could influence mammal diversity indirectly (Mittelbach et al 2001, Groner & Novoplansky 2003, Hillebrand & Cardinale 2010).…”

“…Researchers have corroborated the relationship between productivity and both species richness and individual abundance of mammals on several continents. Some remarkable examples are marsupials in Australia (Pavey et al 2014, Swan et al 2020), arboreal mammals in Asia (Wang et al 2013), ungulates in Africa (Kimuyu et al 2017), and terrestrial mammals in South America (Ferreira Neto et al 2021a). However, whether or how much mammalian body mass, feeding guilds, and vertical strata explored by the species influence the diversity–productivity relationship is not fully understood (Brown 1981, 2014).…”

Island size predicts mammal diversity in insular environments, except for land‐bridge islands

“…For instance, there are currently 191 hydroelectric dams operating only across the Amazon basin, with other 246 already planned to be built in the following years (Lees et al, 2016). Different anthropogenic stressors may further threaten forest species diversity in archipelago landscapes, such as cropping, urban expansion, grazing, climate change, logging, hunting, and species introduction (Canning & Waltham, 2021; Ferreira Neto et al, 2021; Rickart et al, 2016). Understanding the magnitude and strength that island area predicts species diversity in different insular environments is central to implementing effective practical management actions and proposing guidelines for reserve design in different island environments (Lomolino & Weiser, 2001).…”

Composition of terrestrial mammal assemblages and their habitat use in unflooded and flooded blackwater forests in the Central Amazon

“…First, the influence of the nutrient load in flood water on the mammal assemblages seemed to be particularly important for the total biomass, which was twice as abundant in terra firme than in the igapó forest. Soil fertility is a key driver of forest dynamics in terms of productivity, tree turnover, and cation exchange capacity, and is strongly associated with the presence of arboreal and terrestrial mammals in the Amazon ( Buendía et al, 2018 ; Ferreira Neto et al, 2021 ). This association involves bottom-up mechanisms by which the limitation of soil nutrients affects the cost-effectiveness of plant investments in reproduction and fruiting bodies ( Chave et al, 2010 ).…”

“…Terrestrial mammals form a key group for forest functionality and provide vital services in flooded and non-flooded ecosystems in the Amazon ( Haugaasen & Peres, 2005a ), mainly in nutrient-poor soil habitats where these animals may significantly contribute to dissipating the large-scale P gradient from rivers and seasonally flooded and terra firme forests ( Buendía et al, 2018 ; Ferreira Neto et al, 2021 ). Improving our understanding of the spatial distribution of mammal assemblages might help us to predict the persistence of species and safeguard them in low resilience ecosystems such as those in flooded areas ( Castello et al, 2013 ; Flores et al, 2017 ).…”