Hunting alters seedling functional trait composition in a Neotropical forest

Abstract

Abstract. Defaunation alters trophic interactions between plants and vertebrates, which may disrupt trophic cascades, thereby favoring a subset of plant species and reducing diversity. If particular functional traits characterize the favored plant species, then defaunation may alter community-wide patterns of functional trait composition. Changes in plant functional traits occurring with defaunation may help identify the species interactions affected by defaunation and the potential for other cascading effects of defaunation. We tested the hypotheses that defaunation would (1) disrupt seed dispersal, thereby favoring species whose dispersal agents are not affected (e.g., small birds, bats, and abiotic agents), (2) reduce seed predation, thereby favoring larger-seeded species, and (3) reduce herbivory, thereby favoring species with lower leaf mass per area (LMA), leaf toughness, and wood density. We examined how these six traits responded to vertebrate defaunation caused by hunters or by experimental exclosures among more than 30 000 woody seedlings in a lowland tropical moist forest. Exclosures reduced terrestrial frugivores, granivores, and herbivores, while hunters also reduced volant and arboreal frugivores and granivores. The comparison of exclosures and hunting allowed us to parse the impacts of arboreal and volant species (reduced by hunters only) and terrestrial species (reduced by both hunters and exclosures). The loss of terrestrial vertebrates alone had limited effects on plant trait composition. The additional loss of volant and arboreal vertebrates caused significant shifts in plant species composition towards communities with more species dispersed abiotically, including lianas and low wood-density tree species, and fewer species dispersed by large vertebrates. In contrast to previous studies, community seed mass did not decline significantly in hunted sites. Our exclosure results suggest this is because reducing seed predators disproportionately benefits large-seeded species, partially compensating for the reduction of seed dispersers at hunted sites. Our results demonstrate that decreased seed dispersal and seed predation are important determinants of seedling community compositional change as a consequence of defaunation. Defaunation may also negatively impact carbon storage in tropical forests, by favoring lianas and low wood density tree species.

Dispersal limitation induces long-term biomass collapse in overhunted Amazonian forests

“…S4) (20, 23), suggesting that overhunting-induced reductions in seedling recruitment are disproportionately stronger in heavy-wooded tree species. These patterns are corroborated by studies in Mesoamerica (11,15), Africa (13,14,43,44), and southern Asia (12), showing that severe depletion of large frugivores negatively affects the recruitment, relative abundance, and population growth rate of large-seeded trees. However, our simulations may conservatively exclude some very large-seeded species (typically with seed lengths >45 mm) from our list of candidate taxa projected to undergo severe dispersal limitation, even though these species are often associated with high wood density (Fig.…”

Seed dispersal increases local species richness and reduces spatial turnover of tropical tree seedlings

“…Our findings highlight that global declines in vertebrate populations (14-17) could significantly impact forest regeneration through reducing seed dispersal. While Guam represents an extreme case of complete disperser loss, leading to gap seedling richness being approximately halved and seedling turnover among gaps being doubled, other studies have demonstrated that partial loss of dispersers can reduce seedling diversity and disproportionally alter species abundances (20,23,45), particularly of light-demanding pioneer species (22,46). While we have to wait to see how gap dynamics play out in the presence and absence of dispersers on these islands, our findings show that global vertebrate losses have the potential to profoundly alter tropical forest composition.…”

“…Key issues here are that even high hunting pressure has not led to the complete loss of vertebrate dispersers (e.g., ref. 14); hunting also removes vertebrate seed predators and seedling browsers (20,22,23), making it difficult to isolate an effect of seed disperser loss alone; and the treatment of interest (removal of vertebrate dispersers) can be confounded with other site differences. Hunting pressure, for example, is often higher in areas without formal conservation protection, meaning these areas experience additional human pressures that affect tree populations, such as forest fragmentation and tree harvesting (14,20,24,25).…”

Out on a Limb: Arboreal Camera Traps as an Emerging Methodology for Inventorying Elusive Rainforest Mammals

“…Part of this complexity is evident in the vertical stratification of different faunal communities between terrestrial and canopy layers [3][4][5][6][7]. Research suggests that arboreal rainforest mammal species should be high conservation priorities because habitat alteration due to anthropogenic activities causes a greater disruption to arboreal than to terrestrial biodiversity [4,[7][8][9][10][11] and, as with rainforest mammals in general, they are often prey to human disturbance in the form of hunting.…”