Dangerously few liaisons: a review of mate‐finding Allee effects

Abstract

In this paper, we review mate-finding Allee effects from ecological and evolutionary points of view. We define 'mate-finding' as mate searching in mobile animals, and also as the meeting of gametes for sessile animals and plants (pollination). We consider related issues such as mate quality and choice, sperm limitation and physiological stimulation of reproduction by conspecifics, as well as discussing the role of demographic stochasticity in generating mate-finding Allee effects. We consider the role of component Allee effects due to mate-finding in generating demographic Allee effects (at the population level). Compelling evidence for demographic Allee effects due to matefinding (as well as via other mechanisms) is still limited, due to difficulties in censusing rare populations or a failure to identify underlying mechanisms, but also because of fitness trade-offs, population spatial structure and metapopulation dynamics, and because the strength of component Allee effects may vary in time and space. Mate-finding Allee effects act on individual fitness and are thus susceptible to change via natural selection. We believe it is useful to distinguish two routes by which evolution can act to mitigate mate-finding Allee effects. The first is evolution of characteristics such as calls, pheromones, hermaphroditism, etc.which make mate-finding more efficient at low density, thus eliminating the Allee effect. Such adaptations are very abundant in the natural world, and may have arisen to avoid Allee effects, although other hypotheses are also possible. The second route is to avoid low density via adaptations such as permanent or periodic aggregation. In this case, the Allee effect is still present, but its effects are avoided. These two strategies may have different consequences in a world where many populations are being artificially reduced to low density: in the first case, population growth rate can be maintained, while in the second case, the mechanism to avoid Allee effects has been destroyed. It is therefore in these latter populations that we predict the greatest evidence for mate-finding Allee effects and associated demographic consequences. This idea is supported by the existing empirical evidence for demographic Allee effects. Given a strong effect that mate-finding appears to have on individual fitness, we support the continuing quest to find connections between component mate-finding Allee effects (individual reproductive fitness) and the demographic consequences. There are many reasons why such studies are difficult, but it is important, particularly given the increasing number of populations and species of conservation concern, that the ecological community understands more about how widespread demographic Allee effects really are, and why.

Extraordinarily rapid life-history divergence between Cryptasterina sea star species

“…Our population genetic analyses all indicate a significant population bottleneck associated with the speciation event, especially in southern C. pentagona populations and most pronounced in C. hystera. An associated Allee effect-scarcity of mates-may have strongly selected for self-fertilization as a form of reproductive assurance [60,61]. The associated inbreeding depression may be slight if populations are already highly inbred, e.g.…”

Dispersal Evolution in the Presence of Allee Effects Can Speed Up or Slow Down Invasions

“…A handful of theoretical studies have recently provided evidence that evolution can indeed help a population mitigate an Allee effect (Kanarek and Webb 2010;Cushing and Hudson 2012;Shaw and Kokko 2014b). It may be tempting to predict that any adaptation that allows an individual to overcome an Allee effect should have been selected for, since Allee effects negatively influence an individual's fitness (Taylor and Hastings 2005;Gascoigne et al 2009). However, this may not be the case if low densities have been rare during the evolutionary history of a species, and current anthropogenically changed conditions have placed populations in a novel selective environmentsuch as that encountered by any spreading population.…”

Caught between two Allee effects: Trade-off between reproduction and predation risk

“…The mechanisms include male-female encounters in motile animals, encounters between pollen grains and egg cells in plants, and encounters between male and female gametes in many sessile or semi-sessile marine A c c e p t e d m a n u s c r i p t 4 invertebrates (Gascoigne et al, 2009). Evidence for Allee effects affecting reproduction in general and for mate-finding Allee effects in particular is quite widespread; observations of the latter span a wide spectrum of taxa from sheep ticks (Rohlf, 1969) to polar bears (Molnár et al, 2008) among terrestrial animals and from copepods (Kiørboe, 2006) to queen conch (Stoner and Ray-Culp, 2000) among marine species.…”

“…Evidence for Allee effects affecting reproduction in general and for mate-finding Allee effects in particular is quite widespread; observations of the latter span a wide spectrum of taxa from sheep ticks (Rohlf, 1969) to polar bears (Molnár et al, 2008) among terrestrial animals and from copepods (Kiørboe, 2006) to queen conch (Stoner and Ray-Culp, 2000) among marine species. Other mechanisms by which larger population size or density may enhance reproduction success include sperm limitation, physiological stimulation of reproduction and female choice (Gascoigne et al, 2009). …”