Résumé
L'abondance relative de différentes espèces de plantes peut varier selon la densité de cervidés par l'action du broutement sélectif qui tend à diminuer l'abondance des plantes préférées tout en favorisant les plantes évitées. Ceci peut avoir des impacts sur les arthropodes qui sont associés à ces plantes, mais très peu d'études se sont penchées sur le sujet. Nous avons étudié les communautés d'arthropodes herbivores, pollinisateurs et prédateurs associées à quatre espèces de plantes (épilobe, chardon, cornouiller du Canada et framboisier) dont l'abondance variait différemment selon la densité de cerfs de Virginie sur l'île d'Anticosti dans un dispositif répliqué comprenant trois densités contrôlées de cerfs et une densité élevée non contrôlée. Les herbivores sont les plus affectés par la densité de cerfs et les communautés associées au chardon, une plante abondante à densité non contrôlée, mais dont l'abondance diminue avec la densité de cerfs, sont les plus modifiées.
Introduction
Cervids overabundance can strongly modify plant communities (Rooney and Waller 2003). Selective browsing tends to reduce population size and density of favoured plant species (Augustine et al. 1998), thus leaving space and resources for unbrowsed or less exploited plant species (Horsley et al. 2003). High deer density can thus increase plant richness (Schütz et al. 2003), but if an unbrowsed plant benefits to the point of becoming dominant, the overall effect can also be negative (Horsley et al. 2003). These characteristics of plant communities can influence abundance and diversity of insects associated with them, and thus, deer can indirectly influence insect-plant relationships in different ways (Den Herder et al. 2004, Vázquez and Simberloff 2004).
Few studies have examined deer impact on insect-plant interactions. Vázquez and Simberloff (2003, 2004) did not find any effect of deer and cattle browsing on the number
of visits by pollinators to flowers of four plant species of which three were more abundant and one was less abundant in absence of browsing, although pollinator communities were affected. The main difference was a tendency for dominant relations to be less important when plant density was reduced. Browsing by reindeer (Rangifer tarandus (L.)) on tea- leaved willow (Salix phylicifolia L.) in Finland reduced the abundance of associated herbivorous leaf beetles (Chrysomelidae) and galling sawflies (Tenthredinidae) (Den Herder et al. 2004), whereas a similar study on winter browsing by moose on silver birch (Betula pendula Roth) revealed its positive impact on aphid abundance, but not on Curculionidae, Lepidoptera and Acari (Den Herder et al. 2009). These results are concordant with the observation that sap-feeding insects tend to react differently than folivorous and galling insects to plant stress (Larsson 1989).
Insect-plant relationships are diverse, including antagonism by herbivorous species, mutualism by pollinators, and more or less neutral commensalisms with predators that use plants as foraging habitat where they prey on either herbivores or pollinators (Gullan and Cranston 2005). Each of these groups could react differently to density and isolation of their host plant, as well as to plant diversity. The main factor influencing herbivorous insects seems to be plant diversity, insects showing consistently higher abundance and diversity in sites with higher plant diversity (Root 1973, Siemann 1998, Symstad et al. 2000, Borges and Brown 2001). Furthermore, positive relations were found between the abundance of a host plant and the abundance and diversity of its herbivorous insects. For example, stem borers of Canada thistle (Cirsium arvense (L.) Scopoli) were found to be more diverse and abundant in Lower Saxony, Germany when the plant was more abundant (Kruess 2003); the abundance of the thephritid fly Urophora cardui (L.) in Bavaria, Germany was also greater when it host plant C. arvense was more abundant (Eber and Brandl 2003). Similar relationships were found for pollinators.
The population size of sticky catchfly (Lychnis viscaria L.) and Wild mustard (Sinapis arvensis L.) has been shown respectively to have a positive impact on the abundance of Bumblebees (Bombus spp.) (Mustajarvi et al. 2001), and pollinators in general (Kunin 1997). Also, plant isolation (i.e., the distance between patches of the same
plant species) has been reported to have a negative impact on the abundance and diversity of Apoidea, whereas Syrphidae and other pollinators did not seem to be affected (Steffan- Dewenter and Tscharntke 1999). Finally, few studies addressed the impact of plant community structure on plant dwelling predators. However, it seems that web-spiders, foraging on plants are more abundant in complex plant communities (Borges and Brown 2001, Miyashita et al. 2004). Thus, the main factors affecting herbivorous arthropods and pollinators seem to be the population size and the isolation of their host plant, while predators are more influenced by the complexity of plant architecture. By inducing changes in the relative abundance of different plant species (Frelich and Lorimer 1985, Virtanen et al. 2002) and by simplifying plant architecture (Miyashita et al. 2004), deer overbrowsing can influence insect communities associated with them in different ways.
Here, we examine the impact of a reduction in white-tailed deer (Odocoileus
virginianus (Zimmermann)) density, in a forest ecosystem strongly disturbed by deer
overbrowsing, on the arthropod herbivores, pollinators and predators that are associated with three different plant species responding differently to deer browsing. Herbivorous and predatory arthropods were sampled on Canada thistle, fireweed (Epilobium angustifolium L.) and raspberry (Rubus idaeus L.), whereas pollinators were sampled on Canada thistle, dwarf cornel (Cornus canadensis L.) and fireweed. To assess the effects of a reduction in deer density on arthropods, replicated exclosures at three controlled deer densities (i.e.: 0, 7.5 and 15 deer/km2) were compared to nearby sites at uncontrolled deer density (>20 deer/km2) for arthropod abundance and community changes.
Our goal was to assess how reducing deer density may affect insect-plant relationships. More precisely, our study aimed to answer the following questions: 1) Do herbivores, pollinators and predators respond similarly to reduced deer density? 2) Do insect communities associated to plant species that are recovering versus those that are declining at reduced deer density react similarly to deer density reduction? First, we hypothesized that arthropod communities most directly associated with plants should be the most affected by changes in abundance and isolation of their host plant induced by reduction in deer density (chapter 1). Secondly, arthropod communities associated with
invasive plants at uncontrolled deer densities should be affected more rapidly than those associated with recovering plants.