Relative ranges of mating and dispersal modulate Allee thresholds in sessile species

Abstract

Sessile organisms, including plants and benthic macrofauna, are often restricted in the ranges over which they are able to reproduce and disperse. This leads to spatial patterning within populations, causing the effective population density around each individual to depart from the average across its range. This has important implications for population dynamics, in particular the minimum density at which populations are able to maintain positive growth (the Allee threshold). Here we compare the population dynamics of species with three syndromes -spatially restricted mating, dispersal or both -against a null model of a species with no spatial limitations. First we demonstrate mathematically that the population density at which Allee effects occur systematically shifts in each case. Next we use individual-based models representing three exemplar species to simulate the implications for the Allee threshold of each within a fixed area. In the case where mating occurs over long ranges but dispersal is restricted (e.g. the wind-pollinated silver fir, Abies alba Mill.), there is a negligible impact on the Allee threshold. When mating is also spatially restricted (e.g. the dipterocarp tree Shorea curtisii Dyer ex King), the Allee threshold reduces, unless high death rates prevent the stabilisation of aggregations. This occurs because offspring remain within the range of potential mating partners. * Corresponding author Email address: markus.eichhorn@nottingham.ac.uk (Markus P. Eichhorn) URL: treesinspace.com (Markus P. Eichhorn) Preprint submitted to Ecological Modelling May 24, 2017Finally we consider a case in which mating is short-ranged, and dispersal effectively unrestricted, but in which individuals choose to locate themselves in the vicinity of conspecifics (e.g. acorn barnacles, Semibalanus balanoides). This has the effect of maintaining clustering in the face of high dispersal, reducing the Allee threshold, and compensating for the apparent cost of short-range mating.Incorporating information on ranges of mating and dispersal can lead to more effective models for the management of populations at low density, in particular the identification of species with syndromes which make them vulnerable to Allee effects. Most notably, mechanisms which increase the degree of clustering in populations increase both their resilience and persistence when finding a mate is the greatest challenge faced by a sessile organism.

Ecological correlates of reproductive status in a guild of Afrotropical understory trees

“…The spatial and temporal patterns of reproduction of sedentary organisms can have important implications for offspring fitness, population diversity, persistence, and species interactions in a community (Levin, 1992; Velazquez-Castro & Eichhorn, 2017). From the perspective of both population and community dynamics, flowering remains a complex and understudied aspect of the reproduction of tropical trees, yet it is a critical life history event (Abe et al, 2016.).…”

“…Species rarity or abundance in a community may alter the costs and benefits of temporal and spatial dispersion, particularly during events requiring phenological synchrony between individuals, such as reproduction (Elzinga et al, 2007; Velazquez-Castro & Eichhorn, 2017). In trees, flowering patterns in space and time are determined not only by the distribution and abundance of adults but by variation in the reproductive status among individuals.…”