A sense of community: how plants determine which insects live together

Anthropocentric influences of the last century has left our natural world in a declining state, with decreased natural habitats and a resulting decrease in biodiversity. Generally, the scientific community more heavily focuses on the diversity of vertebrates and plants, with these species abundance and diversity declines more publicized and accessible by the general public. However, insect biodiversity may be declining at an even quicker pace than that of vertebrates and plants, and the lack of public awareness of this requires a more effective method to quantify the risk this organismal group is under. Because arthropods can be categorized into a wide range of trophic levels, or roles within in the food chain, numerous factors determine arthropod community structure, and defining which variables influence species composition the most is essential for conservational purposes.

           

Figure 1. Before this study, the species making up an insect community were though to be determined mainly by environmental conditions (such as soil pH, moisture, etc.) and by vegetation structure (map A.) However, this study brought to light that really only plant species composition has a direct impact on the insect community. Map B demonstrates that this is because the environment influences plant species presence/absence, which in turn, determines vegetation structure. Picture from Shaffers 2008.

            Andrè P.Shaffers’ (et al.) ‘Arthropod Assemblages are Best Predicted by Plant Species Composition’ sought to compare local factors to define what best predicted arthropod species presence, also known as ‘assemblage.’ Specifically, the factors he studies were plant species composition (what species form the plant community,) landscape composition, vegetation structure (the way plants grow vertically,) and environmental conditions. Two years of sampling focused on 47 sites ranging from open grasslands, to hay meadows, to dense tall-herb grasslands in the Netherlands, concentrating on seven arthropod groups (spiders, grasshoppers, ground beetles, weevils, plant-hoppers, hoverflies, and bees.)

Surprisingly, environmental conditions and vegetation structure played a weak role in predicting arthropod species occurrence, while plant species composition was a highly influential factor. Before this study, species presence was thought to be attributable to these weaker factors, but Shaffers shows instead that it is the species of the plant community that end up having a direct impact on arthropod assemblage. Intuitively, this makes sense as specific plant species, products of environmental conditions, are relied upon by herbivorous arthropod species for food, and thus determine the herbivorous insect community. As higher trophic level species, such as predator and parasitic arthropods, prey upon the lower trophic levels that are the herbivorous species, the herbivorous species composition will define the rest of the arthropod community. In sum, arthropod community is determined by which plants can successfully grow in the environment to support herbivorous insect species, which in turn, influence which predator species will be present in the plant community as well. By focusing first on plant species composition, certain arthropod communities can be better targeted for conservation, management, or research.

Figure 2. Variations in ‘Vegetation Structure’ can be seen in these pictures by the differing heights and densities of plant growth. (Picture from Shaffers 2008)

            Though Shaffers’ study did not look into the effects of urbanization upon arthropod communities, his findings end up having implications for our own study concerning the differences in ground beetle species between restored prairie sites and flowerbeds across the Grinnell College campus. Plant species composition will undoubtedly be very different between these two types of sample sites, with the resorted prairie being a ‘naturally developing ecosystem’ while the flowerbeds, aesthetically designed by landscapers, will not result in the same complex plant community structure seen in prairie systems. Theoretically, ground beetle assemblages will differ between the sites as a result of the differing plant community compositions, which may have future implications concerning the importance of flowerbed arrangements towards maintaining arthropod diversity and abundance. If we are concerned about the decline in arthropod biodiversity, even small-planted plots in urbanized areas can affect insect communities and their conservation.

Reference:   Schaffers AP, et al. 2008. Arthropod assemblages are best predicted by plant species composition. Ecology 89:782-794