Organization of a Plant‐Arthropod Association in Simple and Diverse Habitats: The Fauna of Collards (Brassica Oleracea)

Abstract

Collards were grown at Ithaca, New York, in two experimental habitats: pure stands and single rows that were bounded on each side by diverse, meadow vegetation. The arthropods associated with these plants were sampled on 20 dates over a 3—year period. The status of the herbivore species was measured by their rank in biomass in each sample. The two most prominent species, Phyllotreta cruciferae and Pieris rapae, maintained high status throughout the investigation, but another important species, Brevicoryne brassicae, was absent for an entire season. Pit feeders usually formed the most important herbivore guild. Nevertheless, the guild spectrum, which describes the functional structure of the fauna, varied widely in time and space. The size distributions of species and of individuals were both highly skewed toward the smaller sizes. Herbivore loads, the mean biomass of herbivores per 100 g of consumable foliage, were consistently higher in the pure stands. Moreover, herbivore loads varied significantly with season in each experimental habitat. Both the number of herbivore species and the diversity of the herbivore load were greater in the diverse habitat. Biomass was more heavily concentrated among the prominent herbivores in the pure stands; increased dominance, rather than differences in species richness, appeared to be the major cause for the lower herbivore diversity in this habitat. The diversity of predators and parasitoids was higher in the pure stands. Most of the abundant species found on collards shared a similar narrow range of hosts. As a result the species in this core group of herbivores and parasitoids were regularly associated with each other. Predators and the less abundant herbivores tended to be less specialized and served to link the collard association with the surrounding community. Plant—arthropod associations are representative of component communities, well—integrated systems that form portions of larger compound communities. This distinction facilitates the analysis of community structure. Microclimates and the effectiveness of "enemies" did not appear to differ sufficiently in the two experimental habitats to account for the observed differences in the herbivore load. The results suggest a new proposition, the resource concentration hypothesis, which states that herbivores are more likely to find and remain on hosts that are growing in dense or nearly pure stands; that the most specialized species frequently attain higher relative densities in simple environments; and that, as a result, biomass tends to become concentrated in a few species, causing a decrease in the diversity of herbivores in pure stands.

Spatial diversification of agroecosystems to enhance biological control and other regulating services: An agroecological perspective

“…Chaplin-Kramer et al (2011) and Veres et al (2013) proposed explanations for the variable effects of landscape complexity on insect pests across studies. Despite the resource concentration hypothesis (Root, 1973), large fields may favour pest dilution, resulting in a reduced abundance in regard to the field size. Moreover, although non-crop habitats can enhance natural enemies and in turn pest control, landscape complexity may also complicate their search for prey or hosts.…”

“…Because development of specialised herbivores is facilitated in homogeneous fields (i.e. resource concentration hypothesis of Root, 1973), diversifying cropping areas by mixing crops (i.e. intercropping), crop with non-crop plants (i.e.…”

Plant diversity effects on insect herbivores and their natural enemies: current thinking, recent findings, and future directions

“…'Resource Concentration Hypothesis' poses that herbivores frequently forage in a density-dependent manner, and therefore increasing plant species or genotype number while keeping plant density constant reduces the probability of finding a preferred host plant species (or genotype), ultimately leading to lower herbivore abundance and damage on individual plants [21].…”

“…A rich body of theory has been developed to predict the effects of plant diversity on communities at higher trophic levels ( [16][17][18][19][20][21][22], see Box 1). Despite this vast collection of theory behind plant diversity effects on associated faunas and the large number of empirical studies conducted thus far, formal evaluations of the mechanisms behind the observed patterns have been developed in natural communities (but see [20] for e.g.…”

“…Based on these findings, we argue that a push towards studies that explicitly manipulate diversity in ecologically relevant traits, particularly in the case of intra-specific diversity, is needed in order to gain a predictive understanding of plant diversity effects on plant-centred arthropod communities. Moreover, we recommend manipulating plant diversity on the basis of functional traits with particularly strong effects on consumers and for which we know the 2 Ecology/parasites/parasitoids/biological control Box 1 Ecological theories for the effects of plant diversity on communities at higher trophic levels and the mechanisms underpinning such effects.'Resource Specialization Hypothesis' argues that increased plant diversity provides a greater diversity of resources which favours an increase in consumer species richness due to an accumulation of consumers specializing on different resources [16,17].'More Individuals Hypothesis' postulates that high plant diversity increases plant productivity, such that an increase in the resource base positively influences consumer abundance, and that this in turn leads to an abundance-driven accumulation of consumer species [18].'Resource Concentration Hypothesis' poses that herbivores frequently forage in a density-dependent manner, and therefore increasing plant species or genotype number while keeping plant density constant reduces the probability of finding a preferred host plant species (or genotype), ultimately leading to lower herbivore abundance and damage on individual plants [21].'Resource Dilution Hypothesis' posits exactly the opposite: as the density of plant items (species or genotypes) decreases (i.e. dilution), herbivores concentrate on the few available items and thus cause more damage in species-rich than in species-poor communities [22].…”