When anthropogenic‐related disturbances overwhelm demographic persistence mechanisms

Abstract

Summary1. Population decline is associated with increased vulnerability to extinction, but also with possible density-, frequency-or distance-related 'rarity advantages' that increase recruitment success as individuals become isolated from their congeners. Distinguishing between these alternatives (risk vs. recovery of rare populations via demographic processes) has become critical, given how anthropogenic disturbances are causing population declines globally. 2. Here, we demonstrate how distance-related rarity advantages are evident in spatially isolated recruits of a canopy-dominant but regionally rare species of oak that appears to be suffering recruitment collapse. As distance from parent trees increased, seedlings had significantly more leaves and experienced reduced insect browsing and intraspecific competition. Long-term field-based experimental treatments revealed these advantages to be associated with rapid rates of juvenile maturation and survival that are unobserved in natural settings. 3. The discrepancy between the experimental and natural settings was explained by trophic collapse and habitat loss -two changes ubiquitous to many terrestrial ecosystems -that combine to concentrate vertebrate herbivores in habitat remnants and cause 100% juvenile mortality via the browsing of taller juveniles. Exotic grass cover, long associated with oak recruitment failure, significantly suppressed seedling height and leaf production, but appeared to delay mortality by hiding shorter seedlings from vertebrate herbivores. 4. Synthesis. Our work demonstrates how rarity advantages have the potential to positively influence the population performance of a declining species, but are short-circuited by intense herbivory associated with human-based environmental change. Regionally, there appear to be few existing conditions on the contemporary landscape that favour juvenile survival, suggesting ongoing recruitment difficulties without intervention. Our work clarifies how extinction risk can in some cases be best defined by how anthropogenic disturbances affect, and are offset by, demographic-based persistence mechanisms, than simply by present-day abundance or distribution.

Interacting livestock and fire may both threaten and increase viability of a fire‐adapted Mediterranean carnivorous plant

“…In Mediterranean and other fire-prone ecosystems, which occupy~40% of terrestrial habitats world-wide (Bond, Woodward & Midgley 2005), human disturbances have been increasingly replacing or interacting with natural fire disturbances in the last century (Turner 2010;Duwyn & MacDougall 2015;Tye et al 2016). Our study suggests a more effective conservation management of fire-adapted seeder species by combining different disturbance regimes, rather than substituting human disturbances for fire (Fuhlendorf et al 2009;Fernandes et al 2013).…”

“…In Mediterranean and other fire‐prone ecosystems, which occupy ~40% of terrestrial habitats world‐wide (Bond, Woodward & Midgley ), human disturbances have been increasingly replacing or interacting with natural fire disturbances in the last century (Turner ; Duwyn & MacDougall ; Tye et al . ).…”

Temporal rarity is a better predictor of local extinction risk than spatial rarity

“…Rare species are generally assumed to be more sensitive to human perturbations than abundant, common, or broadly distributed species (Gaston 1994, Manne and Pimm 2001, Dawson et al. 2012, Duwyn and MacDougall 2015). The sensitivity of rare species is often attributed to having a narrower niche, more conservative resource acquisition, or a slower growth rate relative to their neighbors (Grime 1977, Rabinowitz 1981, Rabinowitz et al.…”

“…However, determining which species will be adversely affected by global change is challenging because shifts in abundance and extinctions arise not only from altered environmental conditions, but also from stochastic events (Shoemaker et al 2020). Rare species are generally assumed to be more sensitive to human perturbations than abundant, common, or broadly distributed species (Gaston 1994, Manne and Pimm 2001, Dawson et al 2012, Duwyn and MacDougall 2015. The sensitivity of rare species is often attributed to having a narrower niche, more conservative resource acquisition, or a slower growth rate relative to their neighbors (Grime 1977, Rabinowitz 1981, Rabinowitz et al 1984, Reich et al 1999.…”

Long‐term and interactive effects of different mammalian consumers on growth, survival, and recruitment of dominant tree species

“…Throughout the world, an increasing number of trees species are reported to be declining in abundance (Allen et al, 2010;Cohen et al, 2016;van Mantgem et al, 2009;Millar & Stephenson, 2015;Trumbore, Brando, & Hartmann, 2015). In many cases, this decline is due to increased mortality of established trees, whereas in other cases it involves reduced recruitment, with younger trees being either less abundant or failing to reach the adult stage (Duwyn & MacDougall, 2015;Fei, Kong, Steiner, Moser, & Steiner, 2011;Gibbons et al, 2008;Kueppers, Snyder, Sloan, Zavaleta, & Fulfrost, 2005;Manning, Fischer, & Lindenmayer, 2006). Understanding the patterns of tree decline and their drivers is of great importance given that forests, woodlands, and savannas cover 30% of the world's terrestrial landscapes (FAO, 2006) and have major influences on ecological systems, including carbon sequestration, water purification, and habitat provisioning (Trumbore et al, 2015).…”