We are always puzzled by the widespread belief amongst native plant advocates that native animals are dependent upon native plants and the corollary argument that non-native plants are invasive because they have no predators. We suspect that one of many reasons for this assumption is a lack of understanding about evolution. That is, if you believe that animals are unable to adapt to new plant species, then you probably assume that the new plant species are not useful nor are they prey to native animals.
The Gallup Poll tracks the opinions of Americans regarding evolution. In 2010 a surprisingly small percentage of Americans (16%) believed in the evolution of man unguided by God. Even amongst those who believe in evolution, it is often seen as an historical process that moves too slowly to be perceived. Science has only recently found living examples of on-going evolution:
“A growing appreciation that organic evolution, like mountain building, is an ongoing rather than simply historical process has stimulated an infusion of evolutionary thinking into mainstream ecology.”(1)
The Soapberry Bug
The soapberry bug (Jadera haematoloma) is an example of a native insect that has changed genetically in less than 100 generations over a period of 20 to 50 years in response to a new non-native plant host.
The soapberry bug is named for the plant upon which it depends for both food and reproduction, the Sapindaceae family (‘Soapberry’ family). In southern Florida, the native host plant of the soapberry bug was the balloon vine (Cardiospermum corindum). As its name suggests, its seed is large and round. The soapberry bug that feeds on that seed has a large jaw–up to about 70% of its body length–that enables it to get the seed into its mouth.
In the 1950s a new member of the Sapindaceae family of plants was introduced to southern Florida, the Chinese flametree (Koelrueleria elegans) as an ornamental. Its seed is much smaller than the seed of the balloon vine. The soapberry bug quickly made a transition to its new host and over time it evolved several adaptations to it. The jaw of the soapberry bug that feeds on the flametree is much smaller, as little as 50% of its body length.
The life cycle of the soapberry bug has also changed and is better suited to the brief, simultaneous availability of seeds of its new host, the flametree: “The flametree-specialized race [of soapberry bug] has a briefer development time (and thus an earlier age at first reproduction), greater fecundity, and exhibits greater expenditure of effort towards reproduction than the balloon vine race of J. haematoloma from which they originated.”(2)
In south Florida, the soapberry bug now has two genetically distinct races that are suited to their specific hosts–one native, one not. The original race has not changed where its host is the native balloon vine. The soapberry bug is not very mobile, so these two populations are physically separated. This is an example of increased genetic biodiversity in response to an introduced plant.
There are 400 genera and 1,500 species of plants in the Sapindaceae family all over the world(3), so we should not be surprised to find many other examples in the scientific literature of insect hosts that are adapted to them, whether they are native or introduced plants, as well as differences in those insects that are suited to the specific plants and/or their locations. The soapberry bug isn’t an isolated example of an insect that has rapidly evolved to adapt to new hosts. On the other hand, science cautions us against generalizing to all insects.
We offer our readers three sources of information, depending upon their scientific knowledge. The National Public Radio story about soapberry bug evolution is addressed to the layman. At the opposite extreme, the citation in our footnotes is addressed to scientists with expertise in genetics. The middle ground, from which we drew most heavily, is a website about soapberry bugs.
As we often do, we conclude cheerfully that nature is remarkably resilient. Although nature is less fragile than native plant advocates believe it to be, we don’t take that as an invitation to abuse it. We treat nature with respect, and that includes taking care of what is here, whether it is native or non-native.
(1) Carroll, Scott P., et. al., “Genetic architecture of adaptive differentiation in evolving host races of the soapberry bug, Jadera haematoloma,” Genetica, 112-113: 257-272, 2001