Pre‐dispersal seed predation and pollen limitation constrain population growth across the geographic distribution of Astragalus utahensis

Abstract

A central focus of ecology is to understand the conditions under which biotic interactions affect species’ abundance and distribution. Classic and recent studies have shown that biotic interactions can strongly impact local or regional patterns of species abundance, but two fundamental questions remain largely unaddressed for non‐competitive biotic interactions. First, do the effects of these interactions on population performance change predictably with environmental context? Second, to what extent do population‐scale effects contribute to limiting species’ geographic distributions? To address these questions, we experimentally assessed the extent to which pollen limitation and insect seed predators affected the fecundity and projected population growth rate (λ) of the native forb Astragalus utahensis. We studied populations at the centre and northern edge of the latitudinal range of A. utahensis that occur across a gradient in abiotic harshness characterized primarily by declining mean annual precipitation. Supplementing pollen and suppressing pre‐dispersal seed predators increased seed production similarly within A. utahensis populations at the centre and northern edge of the range. Integral projection population models revealed that relaxing these checks on seed production tended to increase λ in most populations, regardless of their location within the range. Synthesis. Our results suggest that pollen limitation and insect herbivores limit population growth in A. utahensis similarly across the centre‐to‐north portion of its latitudinal distribution. However, because A. utahensis population growth barely reaches the level of replacement at the northern range edge, the reduction in λ resulting from these interactions may contribute to limiting expansion at the northern edge of A. utahensis’ latitudinal range.

Declining demographic performance and dispersal limitation influence the geographic distribution of the perennial forb Astragalus utahensis (Fabaceae)

“…The discrete seedling stage of each model comprised constants calculated from rates of seedling survival and the size of surviving seedlings for each transition. The results of a seed addition experiment (Baer & Maron, ) informed the constants that described estimated recruitment from seeds produced in the previous season and from the seed bank; these remained identical for each transition year within a population. We calculated these constants by sowing seeds sourced from within the same study population in replicate blocks located throughout each study population.…”

“…Plant size was defined as the area of the basal rosette and calculated using measures of the diameter according to the formula for the area of an ellipse. Basal rosette area is strongly correlated with other measures of plant size such as above‐ground biomass (Baer & Maron, ). We calculated yearly growth rates by comparing the size of a plant from each year to the subsequent year.…”

Biotic Interactions Contribute to the Geographic Range Limit of an Annual Plant: Herbivory and Phenology Mediate Fitness beyond a Range Margin

“…Given the strong effects of herbivory on individual plant fitness, population growth, and local and elevational distributions (Louda 1982;Quinn 1986;Bruelheide and Scheidel 1999;Fine et al 2004), it is surprising that only one recent study has examined herbivory's role in modulating plant species' geographic ranges (Baer and Maron 2018). To our knowledge, ours is the first study to explore the effects of herbivory on a geographic range limit using experimental transplants beyond the range boundary, which is optimal for the testing of range limit hypotheses.…”

“…Given the central role of environmental gradients in structuring species' distributions, identifying important gradients is usually a first goal of range limit studies, with climatic variables being likely candidates. While climatic niche limits often do explain species' distributions (Lee-Yaw et al 2016), it is increasingly recognized that biotic interactions can contribute to large-scale distributional limits (Bruelheide and Scheidel 1999;Hochberg and Ives 1999;Case and Taper 2000;Briers 2003;Case et al 2005;deRivera et al 2005;Araújo and Luoto 2007;Holt and Barfield 2009;Gravel et al 2011;Stanton-Geddes and Anderson 2011;Moeller et al 2012;HilleRisLambers et al 2013;Afkhami et al 2014;Hargreaves et al 2014;Louthan et al 2015;Baer and Maron 2018). However, most evidence for biotic interactions influencing range limits is correlational, and there is a paucity of empirical studies that connect spatial gradients in biotic interactions to population demography and the geographic range limits of native species (Louthan et al 2015).…”

Maladaptation beyond a geographic range limit driven by antagonistic and mutualistic biotic interactions across an abiotic gradient

“…As far back as Darwin (1859), it was proposed that biotic interactions could set geographic range limits, and this idea is well supported by theory (Hochberg and Ives 1999;Case and Taper 2000;Gravel et al 2011). Empirical evidence, especially experimental tests, is relatively scarce, but studies have supported the notion that mutualists (Moeller et al 2012;Afkhami et al 2014), competitors (Bullock et al 2000;Ettinger and HilleRisLambers 2017), and predators (Bruelheide and Scheidel 1999;deRivera et al 2005;Baer and Maron 2018;Benning et al 2019) can influence the location of geographic range limits.…”