Rare species contribute disproportionately to the functional structure of species assemblages

Abstract

There is broad consensus that the diversity of functional traits within species assemblages drives several ecological processes. It is also widely recognized that rare species are the first to become extinct following human-induced disturbances. Surprisingly, however, the functional importance of rare species is still poorly understood, particularly in tropical species-rich assemblages where the majority of species are rare, and the rate of species extinction can be high. Here, we investigated the consequences of local and regional extinctions on the functional structure of species assemblages. We used three extensive datasets (stream fish from the Brazilian Amazon, rainforest trees from French Guiana, and birds from the Australian Wet Tropics) and built an integrative measure of species rarity versus commonness, combining local abundance, geographical range, and habitat breadth. Using different scenarios of species loss, we found a disproportionate impact of rare species extinction for the three groups, with significant reductions in levels of functional richness, specialization, and originality of assemblages, which may severely undermine the integrity of ecological processes. The whole breadth of functional abilities within species assemblages, which is disproportionately supported by rare species, is certainly critical in maintaining ecosystems particularly under the ongoing rapid environmental transitions.

An island view of endemic rarity—Environmental drivers and consequences for nature conservation

“…At this scale (~700 km 2 ), species range sizes likely have very complex shapes and might be disjunct (c.f. Gaston, 1994), which seemingly cannot be grasped by less precise measures such as the occurrence frequency method and convex hulls; especially, convex hulls, which are widely applied in macroecology (e.g., Leitão et al, 2016;Romeiras et al, 2016), might strongly overestimate range sizes by connecting and covering areas and environmental conditions that the species actually cannot inhabit (Blonder, 2016).…”

“…Indeed, areas where rare species aggregate are often preferentially chosen as conservation areas (Crain, White, & Steinberg, 2011)-also because rare species contribute disproportionally to the functional structure of species assemblages (Leitão et al, 2016). Identifying hotspots of endemic rarity is of high importance in determining areas with high conservation value and priority, not only for local conservation efforts but also for preserving global biodiversity (Kier et al, 2009).…”

Functional Rarity: The Ecology of Outliers

“…Complementing this, quantifying functional rarity must include the extent to which species traits, used as proxies to represent functions, trophic links and niche axes [42][43][44][45][46][47], are more or less distinct or redundant within local communities or larger-scale species assemblages [40,48,49] (Box 1).…”

Manipulation of Non-random Species Loss in Natural Phytoplankton: Qualitative and Quantitative Evaluation of Different Approaches

“…These characteristics in combination with stochastic processes make rare species more likely to go extinct than common species (Lande, 1993;Frankham, 2005;O'Grady et al, 2006;Leitão et al, 2016). Due to numerical disadvantages, rarity is often accompanied by competitive inferiority which can lead to extinction by competitive exclusion.…”

“…Despite their increased risk of extinction, rare species are important in communities because they can maintain ecosystem functions under changing environmental conditions when they substitute for dominant species that are lost or decline in numbers (Walker et al, 1999;Norberg et al, 2001;Elmqvist et al, 2003;Lyons et al, 2005;Mouillot et al, 2013;Jain et al, 2014;Leitão et al, 2016;Jousset et al, 2017). Hence, manipulating the loss of rare species in bloom building phytoplankton can provide important information about possible future scenarios, in which these species might be lost at disproportionally high rates.…”