Functional responses of contrasting seed predator guilds to masting in two Mediterranean oak species

Abstract

The predator satiation hypothesis poses that synchronous and variable seed production during masting events increases seed escape through seed predator satiation. The success of this strategy depends upon the type of consumer functional response, in this case defined as the change in seed consumption rate by a predator as a function of change in seed density. Type II (where the proportion of seed consumed is highest at low levels of seed availability) and type III (where the proportion of seed consumed is highest at some intermediate level of seed availability and then declines towards zero) functional responses describe negative density-dependence and indicate predator satiation. The type of function response should be contingent upon herbivore traits: type II responses are predicted for dietary specialist predators with low mobility, and type III responses are predicted for highly mobile, dietary generalist predators. Surprisingly, most studies have not evaluated whether functional responses vary among seed predator guilds. Here we describe the functional responses at population and individual tree level of highly mobile generalist (birds and rodents) and less mobile specialist (insects) pre-dispersal seed predators attacking acorns of two sympatric oaks (Quercus suber and Q. canariensis) over a 10-year period. Our results showed that in most cases specialist seed predators exhibited the predicted type II functional response at both the individual tree and population level for both oak species. However, generalist seed predators did not exhibit the predicted type III response; instead, they also exhibited a type II response at the individual tree and population level for both oak species. By independently assessing the effects of multiple seed predators associated with the same host tree species, our work highlights the influence of herbivore traits on the outcome of plant-seed predator interactions in masting species, and thus furthers our understanding of the ecological and evolutionary mechanisms underlying masting behaviour.

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

“…In contrast to some previous studies, we did not observe a higher proportion of seeds consumed at low levels of seed availability, the so‐called type II functional response of seed consumers (sensu Holling , see also Moreira et al. ), indicating that the effects of masting may be somewhat idiosyncratic. In fact, although the effects of masting and early autumn rain on acorn predation were similar in the two localities, they differed in the intensity of these effects and the importance of other variables: greater importance of masting in Collserola and of rainfall in Huecas and significant effect of the current acorn crop sizes in the former site (see Tables and ).…”

“…, Tachiki and Iwasa , Moreira et al. ). This variable is the ratio of the size of the current acorn crop available to previous year crops, as the proxy of the potential number of adult weevils present in year t .…”

“…, Moreira et al. ), as the use of the crude number of seeds depredated can be tightly linked to the number of seeds available. The rationale behind the inclusion of the different independent variables in the model and their expected impact (positive or negative) on acorn predation is listed below:…”

“…, Moreira et al. ). However, few studies have tested whether environmental factors (proximate causes) influencing masting might also be directly involved in controlling seed predator's populations and thus also contribute to escape seed predation (but see Poncet et al.…”

“…Evolutionary hypotheses for the advantages of masting as a reproductive strategy are based on an "economy of scale" principle: A single but extraordinary reproductive episode should be more beneficial than continuous but moderate reproductive events (Kelly and Sork 2002). In this sense, the advantage of masting as a mechanism to reduce seed predation (the seed predation satiation hypothesis sensu Janzen 1971, see also Silverstown 1980) has probably been one of the hypotheses receiving greatest attention (Kelly et al 2000, Yu et al 2003, Espelta et al 2008, Fukumoto and Kajimura 2011, Archibald et al 2012, Moreira et al 2017. As proposed by , the synchronous, irregular production of large and nil seed crops would lead to predator satiation by combining two different mechanisms: (1) "functional satiation" (large seed crops surpass the feeding capacity of predators, see also Ims 1990) and (2) "numerical satiation" (the current number of predators may be too low to consume many seeds if the previous year's crop was small, Bonal et al 2012).…”