Beyond predator satiation: Masting but also the effects of rainfall stochasticity on weevils drive acorn predation

Abstract

Escaping seed predation is a classic “economy of scale” hypotheses (predator satiation hypothesis, Psh) to explain the selection for the synchronous production of massive and nil seed crops (masting) in plants. The Psh postulates that predator satiation occurs through a combination of (1) “functional satiation,” as not all seeds can be consumed during a massive crop, and (2) “numerical satiation,” as predator populations collapse during poor crop years. Many studies advocate for the Psh, but few have investigated the importance of masting compared to other factors for the control of predation extent. Namely, environmental cues prompting masting could also determine predator's success and, ultimately, influence directly and independently seed predation intensity. We explored this question in Mediterranean oaks, as they exhibit strong masting behavior; acorns are heavily predated upon by weevils; and rainfall stochasticity drives masting and the emergence of adult weevils from the soil. Results of two mid‐term studies (4 and 11 yr) showed that acorn production and predation were highly variable across years, while the abundance of adult weevils was positively related to autumn rainfall and to the number of infested acorns the previous years. Ultimately, acorn predation was negatively influenced by inter‐annual fluctuation of seed production (masting) yet, mainly and positively, prompted by autumn rainfall and acorn crop size (only in one site). Our results highlight the relevance of masting to reduce seed predation. Yet evidences that rainfall stochasticity directly determines the success of weevils, and it independently influences seed predation extent, indicate that environmental cues prompting masting may also fine‐tune the output of this reproductive behavior. Additionally, local differences suggest that the relevance of masting may change with tree characteristics (low vs. high seed production) and landscape structure (isolated vs. dense forests). We also discuss what can be the effects of increasing drought in Mediterranean areas for this antagonistic interaction, triggered by rainfall.

Rapid aggregative and reproductive responses of weevils to masting of North American oaks counteract predator satiation

“…, Espelta et al. ). Thus, past acorn production should positively correlate with current predation rates (prediction 2).…”

“…, Espelta et al. ). In that case, seed damage rate should be negatively correlated with the change in reproductive effort of plants between subsequent years (prediction 3).…”

“…Thus, we first tested for the satiation of seed predators, i.e., a decreasing proportion of predated seeds with increasing seed abundance (prediction 1). The second component of the PSH involves invertebrate starvation, i.e., lean seed years followed by mast years should favor increased seed survival (Kelly and Sullivan 1997, _ Zywiec et al 2013, Espelta et al 2017. Thus, past acorn production should positively correlate with current predation rates (prediction 2).…”

“…Thus, past acorn production should positively correlate with current predation rates (prediction 2). Eventually, if the number of predators is reduced by low seed production in a previous year, the predator population as a whole cannot cope with increased seed production (Kelly and Sullivan 1997, Espelta et al 2017. In that case, seed damage rate should be negatively correlated with the change in reproductive effort of plants between subsequent years (prediction 3).…”

Nature beyond Linearity: Meteorological Variability and Jensen's Inequality Can Explain Mast Seeding Behavior

“…Hypotheses accounting for ultimate causes (i.e., masting would be a selective trait that increases fitness) are based on the benefits of economies of scale, i.e., massive but random reproductive events would be more beneficial than producing regular crops of similar but smaller size (e.g., the predator-satiation hypothesis, see Curran and Webb, 2000;Espelta et al, 2008Espelta et al, , 2017. Hypotheses accounting for proximate causes, though, are focussed on the mechanisms by which plants produce fruits (Norton and Kelly, 1988;Kelly and Sork, 2002).…”