Horizontal seed dispersal by dung beetles reduced seed and seedling clumping, but did not increase short-term seedling establishment

Abstract

Dung beetles are secondary seed dispersers, incidentally moving many of the seeds defecated by mammals vertically (seed burial) and/or horizontally as they process and relocate dung. Although several studies have quantified this ecological function of dung beetles, very few have followed seed fate until seedling establishment, and most of these have focused on the effects of seed burial. We know very little about the effects of horizontal seed movement by dung beetles, though it is generally assumed that it will affect plant recruitment positively through diminishing seed clumping. The objective of our study was to assess the effects of dung beetle activity on the spatial distribution of seeds and seedlings, and on the probability of seedling establishment. In a tropical rainforest in Mexico we carried out two complementary field experiments for each of two tree species (Bursera simaruba and Poulsenia armata), using seeds experimentally imbedded in pig dung and recording their fate and spatial location over time. For both species, dung beetle activity reduced the spatial clumping of seeds and seedlings; however, it did not increase the probability of seedling establishment. We discuss the context- and species-specificity of the combined effects of horizontal and vertical dispersal of seeds by dung beetles, and the need to quantify long-term seedling fates to more accurately determine the effects of seed movement by dung beetles on plant recruitment.

Linking howler monkey ranging and defecation patterns to primary and secondary seed dispersal

“…Endozoochory by howler monkeys is largely recognized to result in a higher abundance and richness of seeds in the soil seed bank, and may influence spatial dynamics (Feer et al, 2013;Juliot,;Pouvelle 2009;Santos-Heredia & Andresen, 2014; Urrea-Galeano, Andresen, Coates, Mora-Ardila, & Ibarra-Manríquez, 2019; Urrea-Galeano, Andresen, Coates, Mora-Ardila, Rojas, et al, 2019), and also the higher abundance of seedlings and saplings recruiting under defecation areas compared with control sites (Bravo & Cueto, 2020;Juliot,). It is important to consider, however, that recruits in latrines are likely to be influenced by positive and negative effects: at the same time that latrines represent areas with high income of nutrients (Feeley, 2005;Santos Neves et al, 2010), the higher density of saplings may reduce chances to survive due to negative density-dependent processes (Russo, 2005;Russo and Augspurger, 2004).…”

“…Both horizontal and vertical distances were dependent on bead size, with the large seed mimics reaching shorter horizontal distances and buried shallower. Previous beetle-mediated seed dispersal studies in tropical forests have demonstrated that the influence of burial depth on the chances of seed germination and establishment are both species and trait-specific (Culot et al, 2011;Griffiths et al, 2016;Leishman et al, 2000;Shepherd & Chapman, 1998;Urrea-Galeano, Andresen, Coates, Mora-Ardila, Rojas, et al, 2019). Yet, burial between 1 and 4 cm, where most of the seed mimics in both latrines and fruit-feeding trees were deposited, are recognized to result in increased germination success and survival when compared with seeds on the soil surface or buried deeper (e.g., Cavalcante, 1991;Shepherd & Chapman, 1998).…”

“…Seeds surrounded by fecal matter that are not buried by beetles may suffer higher predation rates than seeds left on the surface without dung (Andresen, 2001;Andresen & Levey, 2004). Furthermore, the additional horizontal movement provided by dung beetles may reduce the spatial aggregation of seeds in fecal clumps, but it is not clear if this has positive effects on the probability of seedling establishment (Lawson et al, 2012;Urrea-Galeano, Andresen, Coates, Mora-Ardila, Rojas, et al, 2019).…”

Dung beetle activity had no positive effect on nutrient concentration or performance of established rainforest seedlings

“…3.6.1; R Core Team, 2019) using packages 'lme4' (Bates et al, 2015), 'stats' (R Core Team, 2019), 'coxme' (Therneau, 2019), 'car' (J. Fox & Weisberg, 2011), and 'emmeans' (Lenth, 2019).…”

Seed size and pubescence facilitate secondary dispersal by dung beetles

“…There is evidence that seedling survival is higher when both mammals and dung beetles act together as primary and secondary dispersers, respectively (Culot et al, 2018). Understanding secondary dispersal by dung beetles could be critical to our understanding of forest dynamics and seedling establishment because dung beetles reduce seed clumping (Lawson et al, 2012;Urrea-Galeano et al, 2019), which should decrease density-dependent seed mortality (e.g., pathogen and seed predation) (Andresen & Levey, 2004;Beaune et al, 2012;Bell et al, 2006;Vulinec, 2002). When dung beetles bury seeds, they drastically reduce seed discovery by seed predators.…”