Cryptic variation, molecular data, and the challenge of conserving plant diversity in oceanic archipelagos: the critical role of plant systematics

Abstract

Plant species on oceanic islands comprise nearly 25% of described vascular plants on only 5% of the Earth's land surface yet are among the most rare and endangered plants. Conservation of plant biodiversity on islands poses particular challenges because many species occur in a few and/or small populations, and their habitats on islands are often disturbed by the activity of humans or by natural processes such as landslides and volcanoes. In addition to described species, evidence is accumulating that there are likely significant numbers of "cryptic" species in oceanic archipelagos. Plant systematists, in collaboration with others in the botanical disciplines, are critical to the discovery of the subtle diversity in oceanic island floras. Molecular data will play an ever increasing role in revealing variation in island lineages. However, the input from plant systematists and other organismal biologists will continue to be important in calling attention to morphological and ecological variation in natural populations and in the discovery of "new" populations that can inform sampling for molecular analyses. Conversely, organismal biologists can provide basic information necessary for understanding the biology of the molecular variants, including diagnostic morphological characters, reproductive biology, habitat, etc. Such basic information is important when describing new species and arguing for their protection. Hybridization presents one of the most challenging problems in the conservation of insular plant diversity, with the process having the potential to decrease diversity in several ways including the merging of species into hybrid swarms or conversely hybridization may generate stable novel recombinants that merit recognition as new species. These processes are often operative in recent radiations in which intrinsic barriers to gene flow have not evolved. The knowledge and continued monitoring of plant populations in the dynamic landscapes on oceanic islands are critical to the preservation of their plant diversity.

Independent homoploid hybrid speciation events in the Macaronesian endemic genusArgyranthemum

“…Interspecific hybridization is frequent in island floras (Carine, Robba, Little, Russell, & Guerra, ; Friar et al., ; Howarth & Baum, ; Jones, Reyes‐Betancort, Hiscock, & Carine, ; Lindhardt, Philipp, Tye, & Nielsen, ). This is likely the result of endemic lineages possessing little intrinsic reproductive isolation and the dynamic ecological landscape of oceanic islands with natural and anthropogenic disturbances bringing previously isolated species into contact (Crawford & Stuessy, ; Francisco‐Ortega, Santos‐Guerra, Kim, & Crawford, ; Kerbs et al., ; Van Hengstum et al., ). In addition, the variety of distinctive habitats in close proximity to oceanic islands also offers the opportunity for the establishment of hybrids free from competition with their parental progenitors.…”

Scandinavian Oncophorus (Bryopsida, Oncophoraceae): species, cryptic species, and intraspecific variation

“…A significant proportion of species diversity is either not yet recognized, such as undescribed species, or hidden within morphologically closely similar or virtually identical species (Hawksworth 2001;Bickford et al 2006;Crawford & Stuessy 2016). During the last years this has been shown repeatedly for different bryophyte genera (Heinrichs et al 2010;Carter 2012aCarter , 2012bBuchbender et al 2014;Hedenäs et al 2014;Draper et al 2015).…”

Island floras as model systems for studies of plant speciation: Prospects and challenges

“…If hybrids have fitness that equals or exceeds one or both of the parents in the parental habitats, the hybrids may reduce the ability of the parents to replace themselves (Levin et al, ). Lastly, repeated backcrossing to the rarer parental species could result in its extinction by genetic assimilation (Levin et al, ; Crawford & Stuessy, ). Suffice it to say that basic biological data about hybrids are important for informed decisions on conservation strategies.…”