Upward movement of buried seeds: another ecological role of dung beetles promoting seedling establishment

Abstract

Abstract:Dung beetles are known to perform important ecological functions, such as secondary seed dispersal of vertebrate-defecated seeds. We found that dung beetles also move buried seeds upwards, with positive consequences for seedling establishment. In the Lacandon rain forest of southern Mexico we conducted field experiments to address three questions: (1) What proportions of different-sized seeds buried by dung beetles are exhumed by them? (2) Does upward relocation of seeds caused by dung beetle activity promote seedling establishment? (3) Does recurrent beetle activity increase seedling establishment? Using 4-mm, 8-mm and 12-mm beads as seed mimics, embedded in howler-monkey dung, we found that 2–6% of buried beads were later exhumed by beetles, with smaller beads exhumed more often. In small plots (N = 100) where beetles were allowed to bury dung and seed rain was excluded, seedling establishment was over three times higher compared with plots without beetle activity. In plots (N = 8) where we placed dung on four occasions in 1 mo, seedling establishment was more than twice as high compared with plots with single-time dung placement. We believe that our findings open up interesting research opportunities to help further elucidate this newly discovered ecological function of dung beetles.

Dung beetle activity affects rain forest seed bank dynamics and seedling establishment

“…Is re‐distribution of the seed bank through dung beetle activity likely to promote seedling establishment? An increase in seedling establishment from the seed bank in sites where dung beetles had been active has been observed in two studies (Ocampo‐Castillo & Andresen, ; Santos‐Heredia & Andresen, ). While the upward movement of seeds and a consequent increase in germination seems like a plausible mechanism explaining the observed pattern, it could also be that dung presence stimulates other processes that could promote germination and/or seedling establishment (e.g., increased activity of other soil organisms, increased soil fertility or water content; Warr, Thompson, & Kent, ; Freymann, Buitenwerf, Desouza, & Olff, ).…”

“…Dung burial is achieved through the construction of underground tunnels, and the consequent excavation of soil from deep layers to the surface (Pouvelle, Feer, & Ponge, ). Many important ecological functions are derived from this behavior (Nichols et al., ; Santos‐Heredia & Andresen, ; Santos‐Heredia et al., ), including the accidental movement of seeds, either when seeds in feces are buried (i.e., vertical secondary seed dispersal; Andresen & Feer, ), or when seeds already buried in the soil are moved along their tunnels (Pouvelle, Jouard, Feer, Tully, & Ponge, ). Secondary seed dispersal by tropical dung beetles has been the focus of numerous studies (e.g., Andresen & Feer, ; Culot, Huynen, & Heymann, ; Culot, Mann, Muñoz‐ Lazo, Huynen, & Heymann, ; Griffiths, Bardgett, Louzada, & Barlow, ; Lawson, Mann, & Lewis, ; Shepherd & Chapman, ).…”

“…Secondary seed dispersal by tropical dung beetles has been the focus of numerous studies (e.g., Andresen & Feer, ; Culot, Huynen, & Heymann, ; Culot, Mann, Muñoz‐ Lazo, Huynen, & Heymann, ; Griffiths, Bardgett, Louzada, & Barlow, ; Lawson, Mann, & Lewis, ; Shepherd & Chapman, ). On the other hand, the vertical re‐distribution (i.e., upward and downward movement along the vertical axis) of seeds already buried in the soil has been proposed (Pouvelle et al., ), and quantified in one study, but only for seed exhumation, that is, when buried seeds are brought back to the surface (Santos‐Heredia & Andresen, ). The general re‐distribution patterns of the buried soil seed bank due to dung beetle activity remain unassessed.…”

“…One of the main factors affecting secondary dispersal of defecated seeds by beetles is seed size; seeds are “dung contaminants” from the beetles' perspective and larger seeds tend to be excluded (Andresen & Feer, ). It could be that larger seeds in the soil also represent obstacles to be avoided during soil excavation and tunnel building, and one study found that the probability of seed exhumation by beetles was lower for larger seeds (Santos‐Heredia & Andresen, ). Also, the depth at which seeds are buried may affect their movement probability.…”

“…Seedling establishment has often been found to be higher in primate latrines, an effect attributed to the increase in the seed rain (e.g., Anzures‐Dadda, Andresen, Martínez, & Manson, ; Bravo, ). However, it is possible that increased dung beetle activity in latrines also contributes to the increased seedling establishment in these sites (Santos‐Heredia & Andresen, ).…”