Vers des stratégies de diversification multi-services ?

Les ennemis naturels et les pollinisateurs étant tous deux potentiellement favorisés par l’augmentation de la diversité végétale, de nombreuses études récentes ont cherché à favoriser simultanément ces groupes, et donc les deux services associés, via des stratégies de diversification communes (Albrecht et al., 2020; Bartual et al., 2019; Grass et al., 2016; Martin et al., 2019; Rosas‐Ramos et al., 2019; Shackelford et al., 2013; Sutter et al., 2017). Néanmoins, les résultats restent tout autant variables. L’étude des interactions entre ces services, et notamment entre les groupes d’insectes à l’origine de ces services, est un domaine très récent. Dans la revue bibliographique qui suit, nous faisons un focus particulier sur les interactions entre ennemis naturels et pollinisateurs et les effets de ces interactions sur la production des services écosystémiques.

3. Interactions entre insectes bénéfiques : conjuguer théorie

écologique et gestion agroécologique

Revue bibliographique

Publication en cours de préparation pour Agriculture, Ecosystem & Environment.

Résumé

Dans le contexte actuel de perte de la biodiversité et des services écosystémiques associés, favoriser simultanément plusieurs groupes d’organismes bénéfiques et leurs services a récemment fait l’objet de nombreuses études. Néanmoins, les interactions qui peuvent exister entre ces services écosystémiques sont encore peu regardées. Ici, nous résumons la littérature existante concernant l’effet des interactions entre ennemis naturels, pollinisateurs, ainsi qu’entre ces deux groupes, et l’impact de ces interactions sur les services écosystémiques rendus. Les interactions recensées entre insectes bénéfiques sont les interactions de prédation, compétition, partage de la ressource et facilitation.

Ces recherches ont mis en évidence de nombreux manques dans nos connaissances de l’effet des interactions au sein et entre groupes d’insectes bénéfiques et des services écosystémiques associés. Premièrement, certains groupes sont drastiquement moins étudiés que d’autres, en particulier en champs. C’est le cas des parasitoïdes, pour lesquels l’effet des interactions sur la structure et le fonctionnement des communautés in natura est très peu connu. Cette question sera abordée dans le chapitre 1 de cette thèse. C’est aussi le cas des syrphes, pour lesquels il manque beaucoup d’informations quant à leur utilisation en champs des ressources florales, et en particulier en présence de compétiteurs, ainsi que sur les mécanismes olfactifs de choix de la ressource florale. Ces questions seront abordées dans les chapitres 2 et 5. Les interactions entre groupes fonctionnels différents ont très peu été étudiés, mais l’hypothèse de la présence d’interactions négatives entre ces groupes est non négligeable, et fera l’objet des

chapitres 4 et 5. D’une façon générale, il est difficile d’extrapoler la présence

d’interactions à la production du service, comme nous le verrons aussi dans le chapitre

Interactions among beneficial arthropods: combining ecological theory

with agroecological management

Emma Jeavons1,2, Cécile Le Lann1, Joan van Baaren1

1Université de Rennes 1, UMR 6553 ECOBIO, 35042 Rennes

2Laboratoire de biologie végétale Yves Rocher, La Croix des Archers, 56200 La Gacilly

Abstract

Interspecific interactions are major drivers shaping ecological communities, which explain their surname of “architecture of biodiversity”. Nevertheless, understanding how and to what extent they affect ecosystem functioning remains a key challenge for both fundamental and applied ecology. In the context of agricultural biodiversity loss, sustaining populations of organisms that provide essential ecosystem services can be achieved through diversification strategies. Nevertheless, favouring several beneficial groups and several ecosystem services remains challenging. Interactions among beneficial organisms may impact organisms’ fitness, which could cascade up to the population, modifying community structure and functioning. Interactions could also impact organisms’ foraging behaviour, therefore impacting directly the function they provide.

Here, we review the effect of interactions within and between two groups of ecosystem services providers: natural enemies and pollinators, that respectively provide biological control of pests and crop pollination services. We show that many gaps remain in the understanding of the effect of interactions, and in particular between groups, on the structure of the community and on service provisioning. Moreover, how resource diversification strategies modify those interactions is poorly known. We discuss future research perspectives that should be undertaken in order to better capture the complexity of biodiversity effects on ecosystem functioning, and propose guidelines to the implementation of more efficient multi-service schemes in agroecological landscapes.

Introduction

In a world facing global warming, unprecedented biodiversity loss, water and air pollution and land-use fragmentation, achieving agricultural needs while conserving biodiversity is one of the greatest and most urgent challenges. Maximizing ecological interactions that enhance ecosystem functions that can benefit crop yield (i.e. ecosystem services (ES)) is a major lead to complete this goal. Biological control of herbivores and crop pollination are two major regulating ecosystem services in agriculture and are partly provided by arthropods (hereafter called “beneficial arthropods”). The first service is the resultant of a trophic interaction between a predator or a parasitoid (“natural enemies”) and a crop

herbivore and the second results from a trophic and mutualistic interaction between a nectar/pollen feeder (“pollinator”) and a flowering crop. Different habitat management strategies have been often proposed to support the richness and abundance of natural enemies and pollinators in agricultural landscapes and increase their respective ecosystem services, either by increasing plant diversity within-field (e.g. intercropping, weed conservation etc…), around-field (e.g. flower strips, hedgerows, field border conservation, etc…) or at the landscape scale (e.g. increasing the amount of semi-natural patches of habitats (woodlands, grasslands, hedgerows) and the connectivity among habitats) (Altieri and Letourneau, 1982; Gurr et al., 2017; Martin et al., 2019). Both natural enemies of crop pests and pollinators have been shown to benefit from plant diversification resources, such as food resources (pollen and nectar resources, preys/hosts), overwintering sites, oviposition sites and shelters (Gurr et al., 2017). Nevertheless, diversification strategies seem to have variable results (Albrecht et al., 2020; Letourneau et al., 2011; Tscharntke et al., 2016), either due to a non-enhancement of beneficial insects, or an enhancement of the beneficial insects but not of their associated ecosystem services.

Additionally, emphasis on managing both pest control and pollination conjointly has only been fairly recent (Albrecht et al., 2020; Bartual et al., 2019; Grass et al., 2016; Martin et al., 2019; Rosas‐Ramos et al., 2019; Shackelford et al., 2013; Sutter et al., 2017). Because natural enemy and pollinator diversity are driven by comparable resources, some studies have suggested that increased availability of flowering plants is likely to provide synergistic effects for pollination and biological control. Nevertheless, investigating how ecosystem services interact is an emerging research field and provides mingled results (Bennett et al., 2009; Garibaldi et al., 2018). Garibaldi et al. (2018) identified only 7 studies looking at how pollination and biological control interact, and found that 3 resulted in positive interactions (synergism), 1 in negative (antagonism), and 3 with no interactions (additive results). To manage multiple ecosystem services conjointly, Bennett et al. (2009) have emphasized the need to understand the mechanisms underlying ecosystem services interactions.

Here, we proposed that ecological interactions among beneficial arthropods could explain the success of failure of diversification strategies, due to a modification of the community structure and functioning (Straub et al., 2008; Tylianakis et al., 2008). While trophic

interactions (i.e. among different trophic levels such as predation, nectar consumption, frugivory, detrivory…) can directly provide ecosystem functions (e.g. respectively population regulation, plant pollination, seed dispersal, organic matter decomposition …), non-trophic interactions also drive the functioning of ecosystems through 1) their impact on individuals’ fitness which could cascade up to the population, thus affecting the community and the provision of the ES; 2) change in the foraging behaviour of a species, such as a resource shift, that could impact its effectiveness to provide the ES (Kéfi et al., 2012). Competition is considered to be one of the main drivers shaping communities: it arises in case of resource overlap between two individuals and affects negatively the fitness of the two participants (usually one more than the other) through the reduction of the access to the resource for the inferior competitor. Competition can either result from resource abundance reduction (i.e. exploitative competition), physical prevention of the access (i.e. interference competition) or the enhancement of a common enemy (i.e. apparent competition) (Tilman, 1982). Reduction of interspecific competition can be achieved through resource partitioning (Levine and HilleRisLambers, 2009) which is based on a certain degree of specialization of each species which reduces niche overlap. Such specialization can occur for the type of resource, the temporal use of the resource, the location of the resource or the responses to environmental conditions (Walter, 1991) and can be achieved by the avoidance of competitors and the discrimination of resources exploited. Finally, facilitation is a positive interaction between two organisms that benefits at least one of the protagonist and causes harm to neither (Stachowicz, 2001). This amelioration can be direct, by changing the habitat’s abiotic conditions, providing shelter or food resources consumed by the other species; and indirect by removing predators or competitors.

Diversification strategies aiming at sustaining beneficial arthropods through the provision of diversified and abundant resources could therefore impact guild interactions, as species from a same guild uses similar resources. Taking this reasoning further, it seems genuine to consider that interactions could also occur and impact ecosystem functioning among species that are not from the same guild but benefit from similar types of resources. Nevertheless, such interactions have only scarcely been studied. Studying how interactions among beneficial arthropods impact their fitness, behaviour and the resulting ES should help to answer two practical questions to optimize diversification strategies for agroecological crop production. Do interactions among beneficial

arthropods impact the effectiveness of diversification strategies to (1) develop the populations of several groups of beneficial arthropod populations? (2) provide the ES in the crop (in other words, is it interesting to favour simultaneously several beneficial arthropod groups)? This paper has three main objectives. First, we propose a concise literature review on interspecific interactions (1) among arthropod natural enemies (thus excluding pathogens) and (2) among pollinators and the effects of those interactions on the associated ecosystem services. Our purpose is not to have an extensive review of all interactions possible as there are already more focused and complete reviews on each subject (cited in each part respectively), but to highlight the gaps in the literature on the knowledge of the effect of interaction on ES provisioning. Secondly, we aim et translating this framework to study possible ecological interactions among ecosystem service providers and understand how it could cascade up to the delivery of ecosystem services. Finally, we propose pathways to guide future research and to optimize ES management in agroecosystems for more sustainable crop production.

1. Intra-guild ecological interactions among beneficial insects and its