Temple University, Department of Biology, Philadelphia, PA, USA.
Ann Bot. 2018 Mar 5;121(3):513-524. doi: 10.1093/aob/mcx184.
Habitat fragmentation has transformed landscapes globally, leaving remnants embedded within a complex matrix that is rapidly becoming more developed. For many plant populations, the associated factors of decreased size and intensification of land use surrounding them are expected to increase pollen limitation ('PL'), unless autonomous self-pollination provides reproductive assurance ('RA'). Decreased pollinator visitation is often assumed to drive these patterns, but other, less studied mechanisms might include increased heterospecific pollen transfer or decreased conspecific pollen availability via florivory. I investigate how PL and RA and their potential underlying mechanisms vary with population size and land use intensity surrounding populations in the biennial Sabatia angularis (Gentianaceae).
I estimated the capacity for seed production via autonomous self-pollination (i.e. autofertility). Over 2 years in 22 S. angularis populations across a fragmented landscape, I performed emasculation and pollen supplementation experiments measuring RA and PL, and quantified visitation rates of potential pollinators and a pollen consumer, conspecific pollen loads and rates of heterospecific pollen deposition.
Autofertility based on fruit mass was 93 % under PL but only 51.6 % relative to maximal conditions. PL and RA were significant on average across populations in the first year of study. Variation in RA was significantly influenced by the interaction between population size and land use intensity, which in turn rendered PL independent of these factors. Visitation and heterospecific pollen deposition rates were greatest in small populations and declined with population size, while conspecific pollen loads were greatest in intermediate sized populations.
Increased reliance on RA is predicted in small S. angularis populations surrounded by intense development, which can explain elevated selfing rates in fragmented populations of plant species more generally. Results from this study point toward forces such as heterospecific pollen transfer, self-pollen limitation or resource availability influencing the need and ability to rely on RA.
栖息地破碎化已在全球范围内改变了景观，所留下的残余部分嵌入在一个快速发展的复杂基质中。对于许多植物种群而言，其周围的面积缩小和土地利用强度增加相关因素预计会增加花粉限制（“PL”），除非自主自交提供生殖保证（“RA”）。通常认为传粉者访问量的减少会导致这些模式，但其他研究较少的机制可能包括增加异交花粉转移或通过食花减少同种花粉的可用性。我研究了 RA 和 PL 及其潜在的潜在机制如何随种群大小以及种群周围土地利用强度而变化在两年生 Sabatia angularis（龙胆科）中。
我通过自主自交（即自交）来估计种子产生的能力。在一个破碎的景观中，在 22 个 S.angularis 种群中进行了 2 年的实验，我进行了去雄和花粉补充实验，以衡量 RA 和 PL，并量化了潜在传粉者和花粉消费者的访问率、同种花粉负荷和异交花粉沉积率。
在 PL 下，基于果实质量的自交能力为 93%，但相对于最大条件仅为 51.6%。在研究的第一年，RA 和 PL 在平均水平上对种群均具有重要意义。RA 的变化受种群大小和土地利用强度之间的相互作用的显著影响，这反过来又使 PL 不受这些因素的影响。访问率和异交花粉沉积率在小种群中最大，并随种群大小而下降，而同种花粉负荷在中等大小的种群中最大。
在周围发展密集的小 S.angularis 种群中，预计对 RA 的依赖程度会增加，这可以解释更广泛的植物物种在破碎化种群中自我繁殖率升高的原因。本研究的结果表明，异交花粉转移、自花粉限制或资源可用性等因素会影响对 RA 的需求和能力。
