Abstract
In the present article we discuss why, in our view, the term ‘generalism’ to define the dietary breadth of a species is a misnomer and should be revised by entomologists/ecologists with the more exact title relating to the animal in question’s level of phagy—mono-, oligo, or polyphagy. We discard generalism as a concept because of the indisputable fact that all living organisms fill a unique ecological niche, and that entry and exit from such niches are the acknowledged routes and mechanisms driving ecological divergence and ultimately speciation. The term specialist is probably still useful and we support its continuing usage simply because all species and lower levels of evolutionary diverge are indeed specialists to a large degree. Using aphids and parasitoid wasps as examples, we provide evidence from the literature that even some apparently highly polyphagous agricultural aphid pest species and their wasp parasitoids are probably not as polyphagous as formerly assumed. We suggest that the shifting of plant hosts by herbivorous insects like aphids, whilst having positive benefits in reducing competition, and reducing antagonists by moving the target organism into ‘enemy free space’, produces trade-offs in survival, involving relaxed selection in the case of the manicured agro-ecosystem.
Highlights
This year marks the centenary of the death of the great German biologist, evolutionist, proto-ecologist and scientific artist, Ernst Haeckel (1834–1919), who, having read Charles Darwin’sOrigin of Species [1] formulated the concept of ‘ecology’ in his 1866 writings [2]
We suggest that the apparent extensive polyphagy seen by many pest aphid species on crops is the direct result of relaxed selection arising from a reduction in secondary plant antifeedant titre in artificially designed and reared crops within the manicured agroecosystem, cossetted plants that would normally be unsuitable as aphid hosts in their wild form
It is not possible to survey the whole of the animal kingdom to support our views on this matter, but in the case of aphids and wasp parasitoids, our own fields of speciality, we provide examples of why so-called highly polyphagous species are in actuality much less so
Summary
This year marks the centenary of the death of the great German biologist, evolutionist, proto-ecologist and scientific artist, Ernst Haeckel (1834–1919), who, having read Charles Darwin’s. If a species were truly generalist, its evolution would stop, as the act of evolving to fill a new niche, perhaps due to intra- or interspecific - or in the case of asexual aphids (Insecta: Hemiptera: Aphididae), phloem sucking plant parasites - intra- or interclonal competition, is the main driving force of evolution itself. With interspecific pressure, this may relate indirectly to predator/parasitoid pressure, and the requirement to evolve into a new ‘enemy free space’, or more exactly, niche [6]. In a truly wild scenario, both chemical defences and predator pressure would exclude such flexible behaviour and would limit the insect to their original, long co-evolved natal host/s, either a single host or taxonomically closely related hosts within the same plant family
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