Effects of livestock on arthropod biodiversity in Iberian Holm oak savannas revealed by metabarcoding
Tara Caneloa,b*, Daniel Marquinac,d*, Sergio Chozase, Johannes Bergstenf, Álvaro Gaytáng,, Carlos Pérez-Izquierdoa, Raúl Bonala, h
a Dpto. Ingeniería del Medio Agronómico y Forestal. Grupo de Investigación Forestal (INDEHESA), Centro Universitario de Plasencia, Universidad de Extremadura, Avda. Virgen del Puerto 2, 10600 Plasencia, Cáceres, Spain
b Centro de Ecologia Aplicada “Prof. Baeta Neves” (CEABN), Instituto Superior de Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal
c Department of Bioinformatics and Genetics, Swedish Museum of Natural History, Stockholm, Sweden
d Department of Zoology, Stockholm University, Stockholm, Sweden
e cE3c - Centre for Ecology, Evolution and Environmental Changes & CHANGE - Global Change and Sustainability Institute. FCUL: Sciences Faculty of the University of Lisbon Campo Grande, Portugal
f Department of Zoology, Swedish Museum of Natural History, Stockholm, Sweden
g Instituto de Recursos Naturales y Agrobiología de Sevilla, Consejo Superior de Investigaciones Científicas (IRNAS-CSIC), Sevilla, Spain
h Department of Biodiversity, Ecology and Evolution. Complutense University of Madrid, Spain
* These authors contributed equally to the study.
Corresponding author: Tara Canelo, taracanelo@unex.es
Running head: Effects of livestock on arthropod biodiversity
ABSTRACT
Increasing food production while avoiding negative impacts on biodiversity constitutes one of the main challenges of our time. Traditional silvopastoral systems like Iberian oak savannas (“dehesas”) set an example, where free-range livestock has been reared for centuries while preserving a high natural value. Nevertheless, factors decreasing productivity need to be addressed, one being acorn losses provoked by pest insects. An increased and focalized grazing by livestock on infested acorns would kill the larvae inside and decrease pest numbers, but increased livestock densities could have undesired side effects on ground arthropod communities as a whole. We designed an experimental setup including areas under trees with livestock exclosures of different ages along with controls, using DNA metabarcoding (mitochondrial markers COI and 16S) to rapidly assess arthropod communities’ composition. Livestock removal quickly increased grass cover and arthropod taxonomic richness and diversity, which was already higher in short-term (1-year exclosures) than beneath the canopies of control trees. Interestingly, arthropod diversity was not highest at long-term exclosures (≥10 years), although their community composition was the most distinct. The diversity peak at short term exclosures would support the intermediate disturbance hypothesis, which relates it with the higher microhabitat heterogeneity at moderately disturbed areas. Thus, we propose a rotatory livestock management in dehesas: plots with increased grazing should co-exist with temporal short-term exclosures. Ideally, a few long-term excluded areas should be also kept for the singularity of their arthropod communities. This strategy would make possible the combination of biological pest control and arthropod conservation in Iberian dehesas.
Key-words : wood-pastures, grazing, livestock exclusion, metabarcoding, arthropods, intermediate disturbance hypothesis
INTRODUCTION
One of the great human challenges for the 21st century is to produce enough food for an increasing population, but to do so in an environmentally-friendly way (FAO 2022; IPBES 2019). In this context, traditional agroecosystems play a very important role combining the preservation of natural values with economic activities. Iberian silvopastoral oak savannas (so called “dehesas” in Spain and “montados” in Portugal) are a good example (Sucena-Paiva et al., 2022). Century-long shrub and tree clearing of Mediterranean forests has produced landscapes with oaks interspersed within a grassland matrix, in which livestock rearing is the main use (Bugalho et al., 2011; Moreno & Pulido, 2009; Sá-Sousa, 2014). Dehesas are also included in the Habitats Directive of the European Union, as they host a wide range of rare, threatened, or endemic animal and plant species (EC 2013).
As in any other agricultural system, pest insects reduce the productivity in oak dehesas (Canelo et al., 2021a), but the preservation of their high natural value preclude the use of any chemical treatment. Current approaches follow the logic of ecological intensification, based on managing service-providing organisms that make a quantifiable direct or indirect contribution to agricultural and forest productivity (Bommarco et al., 2013; Montesinos, 2019). Parasitoids (frequently species of the order Hymenoptera), or vertebrates (e.g. birds), are among them (Diaz-Siefer et al., 2021; García et al., 2021; Redlich et al., 2018; Zhu et al., 2019). However, for oak dehesas, we previously proposed a novel way of biological control based on livestock management (Canelo et al., 2021b).
In dehesas, acorns are a key food resource for livestock, but their availability is limited by insect pre-dispersal seed predators, mainlyCurculio weevils (Coleoptera: Curculionidae), which can infest half of the year’s acorn production (Bogdziewicz et al., 2019; Bonal et al., 2007). The weevil larvae, however, are vulnerable to predation by large vertebrates for a short time, namely the period of 20 days that they spend inside the acorns on the ground after the premature abscission of the seed (Bonal & Muñoz, 2007). Thus, we proposed an increased livestock grazing on infested acorns during that period that proved successful to reduce pest numbers (Canelo et al., 2021b). But it is unclear whether this increased grazing could have undesired negative side effects on arthropod communities coexisting with the pest and the livestock.
Livestock plays a major role in ecosystem functioning (Eldridge et al., 2016, 2017; Eskelinen et al., 2022), but grazing consequences on biodiversity remain not fully understood (Filazzola et al., 2020; Yuan et al., 2016). Some effects of large herbivores are direct, like unintentional predation on insects while feeding (Bonal & Muñoz, 2007; Gómez & González-Megías, 2002, 2007; Retamosa et al., 2004). By contrast, others are indirect and mediated by biotic and abiotic variables (Maestre et al., 2022). For example, grazing can alter the richness, abundance, and species composition of different animal communities through changes in plant cover and/or diversity (Adler et al., 2001; Brambila et al., 2020; Kirk et al., 2019; Magnano et al., 2019; Sims et al., 2019; et al., Song et al., 2020). Livestock also increases soil hardness and changes its physicochemical properties (Armas-Herrera et al., 2019; Zhang et al., 2020), as well as water composition and quality (Kilgarriff et al., 2020; O’Callaghan et al., 2018; O’Sullivan et al., 2019). Grazing effects are not always easy to predict because they result from complex cascade effects (Abdala-Roberts et al., 2019; Eldridge et al., 2016, 2017; Evans et al., 2015; Sankaran & Augustine, 2004; Vandegehuchte et al., 2017), which transcend ecosystem boundaries (Knight et al., 2005). Moreover, the variability of direct and indirect effects, along with their interactions with environmental factors (Vojta et al., 2020), may make the consequences of grazing seem counterintuitive, as different organisms may respond differently to the same disturbance events (Didham et al., 2009; Gossner et al., 2016; Jackson et al., 2015). For oak dehesas, we proposed a temporary localized increased grazing to reduce acorn pests, rotated among different farm areas every 2 -3 years (Canelo et al. 2021b). In principle, grazing would be expected to reduce arthropod numbers and diversity compared to areas with no livestock. Nonetheless, it could depend on grazing intensity, as generally predicted by the intermediate disturbance hypothesis (IDH).
The intermediate disturbance hypothesis (IDH) states that species diversity peaks at intermediate levels of environmental stress (Connell, 1978; Gao & Carmel, 2020; Roxburgh et al., 2004; Svensson et al., 2007; Yan et al., 2015). On the one hand, high disturbances result in constant selection pressure allowing only a few species to survive; on the other hand, the lack of disturbance benefits just a few strong competitors. However, in between, the competition between existing species decreases and new species can rapidly colonize. Consequently, species richness rises under intermediate disturbance regimes. This hypothesis has been proposed to explain the high arthropod diversity recorded in areas with moderate levels of grazing (Joubert et al., 2016; Kaltsas et al., 2013; Kati et al., 2012; Lazaro et al., 2016a, 2016b; Qin et al., 2017; Winck et al., 2019).
Livestock exclusion is seen as a good strategy to restore degraded ecosystems (Kröpfl et al. 2011; Prober et al. 2011; Su et al. 2015). At the same time, it is a very efficient way to evaluate the effects of grazing on other organisms (Adams, 1975; Trigo et al., 2020; Wassie et al., 2009). Such effects usually appear over time, and not always in a linear fashion (Filazzola et al., 2020), hence, the use of exclosures of different age is recommendable to study them. We did so in the present study, building exclosures and, after one year, comparing them with adjacent grazed areas and places in which livestock had been absent for a longer period of time. We performed extensive and detailed analyses on the effects on arthropod communities considering all trophic and taxonomic levels. To achieve this ambitious goal, we used DNA metabarcoding approach.
In arthropod biodiversity studies, species identification is always the main challenge (Moretti et al., 2004). Gaps in taxonomic identification are usually related to the nature of taxonomic knowledge: due to the outstanding arthropod diversity, researchers need to focus on specific taxonomic groups (Didham et al., 2009; Jackson et al., 2015), and taxonomic expertise is not always available for every study. Metabarcoding overcomes this taxonomic impediment, allowing species identification at a community level by delimiting molecular operational taxonomic units (MOTUs), which correspond to putative species (Gaytán et al., 2020). These MOTUs can be determined to the Order, Family, Genus or Species level depending on the availability of identified reference barcodes. In this sense, using more than one marker for species identification enhances detection likelihood (Kaunisto et al., 2017; Wangensteen et al., 2018). For terrestrial arthropods, the two mitochondrial genes cytochrome oxidase I (COI) and the 16S rRNA gene (16S) have shown to complement each other very well (Marquina et al., 2019a; Roger et al., 2022).
The general objective of this study is to assess, for the first time, the effects of grazing on arthropod communities of Iberian oak dehesas using DNA metabarcoding, along with the environmental factors that may drive them. Our specific goals were: i) to compare the success of two genetic markers (mitochondrial genes COI and 16S) used for arthropod species detection by DNA metabarcoding; ii) evaluate changes in vegetation structure after livestock exclusion and its effects on arthropod diversity; and iii) assess arthropod species richness and community composition across trophic levels throughout a gradient of grazing exclusion.
MATERIAL AND METHODS