The generalism-specialism debate: the role of generalists in the life and death of species

Abstract

Specialisms on resources and for niches, leading to specialization, have been construed to be tantamount to speciation and vice versa, while the occurrence of true generalism in nature has also been questioned. We argue that generalism in resource use, biotope occupancy, and niche breadth not only exists, but also forms a crucial part in the evolution of specialists, representing a vital force in speciation and a more effective insurance against extinction. We model the part played by generalism and specialism in speciation and illustrate how a balance may be maintained between the number of specialists and generalists within taxa. The balance occurs as an ongoing cycle arising from turnover in the production of specialists and generalists, speciation, and species extinction. The nature of the balance depends on the type of resources exploited, biotopes, and niche space occupied. These vary between different regions and create taxonomic biases towards generalists or specialists. We envisage that the process may be sympatric/parapatric, although it is more likely initiated by allopatry driven by abiotic forces.

Reconsidering the Specialist-Generalist Paradigm in Niche Breadth Dynamics: Resource Gradient Selection by Canada Lynx and Bobcat

“…It follows that species can either focus their resource choice to exploit a few resources well, or else broaden their choice to use more resources adequately [1]–[3]. The variance in resource use is the metric by which niche breadth is quantified, and although species’ differentiation according to this metric has been questioned [4], the general consensus is that evolution can give rise to individual species with clear differences in niche breadth dimensions [5]. Species with narrow niches, “specialists”, are presumably favoured during periods of environmental stability or homogeneity, whereas those with broad niches, “generalists”, likely are favoured during environmental instability or heterogeneity [5], [6].…”

Ten years of the resource-based habitat paradigm: the biotope-habitat issue and implications for conserving butterfly diversity

“…8) in which generalism enables regional specialism. This process was first suggested by the shifting balance theory (Wright 1932(Wright , 1982 and can represent a basis for emergence of new species or changes in any natural history trait (Mallet 2010;Dennis et al 2011). As a key part of this process, increasingly suitable conditions for colonising new biotopes (different resource contexts), such as climatic changes, could lead to adaptive radiation based on ecological fitting (via phenotypic plasticity and novel phenotypic responses, exaptations, resource use mistakes; Janzen 1985; Agosta et al 2010;Nylin & Janz 2009) and host range use, thus increased variability in resources and conditions (variability in RBH), in effect another round of generalism (oscillation hypothesis: Janz & Nylin 2008;Nylin et al 2014).…”

“…Upturns describe conditions that lead to higher population density, distribution infilling, and range expansion of any butterfly species; downturns, the reverse. See Dennis et al 2011 for details of variables and effects. HTNS, host taxonomic neighbourhood size.…”

The Search Behavior of Omnivorous Thrips Larvae is Influenced by Spider Mite Cues

“…Whether omnivory is adaptive or a transitory stage towards specialization is a topic of debate (Dennis et al 2011;Loxdale et al 2011). Omnivorous insects, such as thrips (Thysanoptera: Thripidae), are sometimes considered 'opportunistic' predators because they do not rely on prey to survive and reproduce, and use plant tissue as their main food source (Trichilo and Leigh 1986;Mound 2005;Zhi et al 2006).…”