Flower‐heads, herbivores, and their parasitoids: food web structure along a fertility gradient

Abstract

1. The ways in which a soil fertility gradient affects three trophic level food webs defined by plants of the family Asteraceae, flower-head herbivores, and their parasitoids was investigated. It was tested how the fertility gradient alters: (i) the abundance and richness of plants, herbivores, and their parasitoids, (ii) the herbivoreplant ratio, and (iii) the connectance of the plant-herbivore community matrix.2. From April to May 2000, plants and insects were sampled in 16 Brazilian Cerrado (sensu stricto) sites along a physiognomic gradient varying from open shrublands (cerrado) to closed woodlands (cerrada˜o). Sites were objectively positioned along the physiognomic gradient by a single index, tree density. Sixty-seven per cent of the variation in tree density among sites was correlated to two principal components of a PCA, representing gradients of soil fertility.3. Asteraceae abundance, richness, and flower-head availability were negatively related to tree density due to their preference for sunny environments, despite the surplus of soil nutrients. The abundance and richness of Diptera and Lepidoptera, the most important flower-head herbivores, were also negatively related to tree density. Parasitoid abundance decreased with tree density; however, the number of parasitoids per hosts was lower in cerrado, suggesting that top-down forces are not getting stronger in more productive sites, as could be expected.4. Community allometry analyses showed that the herbivore to plant ratio was independent of community richness and did not respond to tree density. 5. Connectance of the plant-herbivore matrix was dependent on the community matrix size. Proportionally, species-rich cerrado sites had fewer interactions than their species-poor counterparts. Nevertheless, after removing the effect of the matrix size, connectance was not related to tree density.6. Soil fertility, as the primary cause of the cerrado-cerrada˜o physiognomic gradient, strongly affected the abundance and richness of plants, herbivores and their parasitoids; however, it had little effect on important community attributes, such as the herbivore-plant ratio and the connectance of the plant-herbivore matrix.

Species richness in natural and disturbed habitats: Asteraceae and Flower-head insects (Tephritidae: Diptera)

“…The insect-plant communities were sampled from April to May 2001, during the main reproductive period of Asteraceae in the region. The sampling methods were similar to those used by Fonseca et al (2005) and Almeida et al (2005Almeida et al ( , 2006 in the same areas. Sampling was carried out in 15 random transects (cerrados and the pasture at Águas de Santa Bárbara) and in 30 transects (eucalyptus stands and all the other pastures, due to their low Asteraceae population density), each measuring 30 m x 5 m. In each site, the presence/absence and abundance of Asteraceae were recorded and voucher specimens collected.…”

“…Rare species were sampled by active search outside transects. The insect species richness in each habitat type was estimated through sampling of fl ower heads for each species of host plant, with a standardized effort of four person-hours (see Fonseca et al 2005). Flower-head samples were kept in plastic containers with mesh lids and examined daily for emerging insects over eight weeks, so that any insects that emerged could be removed.…”

História natural e interação flores-besouros em espécies de Cerrado

“…Food resources are patchily distributed for most animals, regulating their feeding behavior, population dynamics, and ultimately their evolution , Schöps 2002, Fonseca et al 2005. Although plant phenological variation exists at every spatial and temporal scale, we rarely know the degree of phenological difference among patches.…”

“…In this context, the Neotropical savanna -Cerrado -can be viewed as an adequate 21 scenario because the high spatial variation in resources and environmental conditions among its phytophysiognomies related mainly to soil fertility and ground water regimes , Fonseca et al 2005. Additionally, in this Biome several plant species provide food and shelter for beetles .…”

“…By supposing that plant richness and abundance can affect the abundance and richness of beetles through the bottom-up control (Fonseca et al 2005), we expected to find great richness and abundance of beetles in areas of great floral resource availability. Additionally, as beetles have an ancient history with wood plants, we also expect to find high occurrence of beetle in the Cerrado areas, because of its high abundance of woody species.…”