Seasonality, Optimal Foraging, and Prey Coexistence

Abstract

Several empirical studies suggest that herbivores may promote coexistence between plants by relaxing the strength of resource competition. In contrast, recent mathematical models predict that food-limited herbivory instead cause exclusion through apparent competition, regardless of whether herbivore selectivity is constant or density dependent. This study extends existing theory to consider a strongly seasonal system. Herbivores with fixed diet preferences have the same effect regardless of seasonality, but there is a marked difference when the diet selectivity of herbivores conforms to a simple optimal-foraging model. An optimally foraging herbivore in a seasonal environment is able to promote plant coexistence among many species. The mechanism involves diet switching, occurring over narrow density intervals. For this to have an effect in a nonseasonal model, equilibrium resource densities must be in this interval, which requires close parameter fitting. In seasonal environments, resource densities change through the year and may frequently move across narrow regions in which diet changes occur. The potential of gray-sided voles to promote coexistence between two arctic dwarf shrubs is evaluated in terms of the model. For this system, it is shown that vole herbivory has the potential to reverse competitive dominance.

Seasonally varying marine influences on the coastal ecosystem detected through molecular gut analysis

“…Temporal variation in consumer diets is a common cause of indirect effects between prey species through shared consumers (Hambäck, ; Holt & Kotler, ; Holt & Lawton, ). This temporal variation in indirect effects may be caused both by differences in diet among consumer life history stages and by variation in resource availability within and between years (Hambäck, ).…”

Vole–vegetation interactions in an experimental, enemy free taiga floor system

“…Our results indicate, that seasonality can contribute to the lack of herbivore impact on the peak biomass of herbaceous vegetation in communities, where herbivory is intense enough to have a strong impact on the species composition (Hambäck 1998, Chase et al 2000). By mammalian standards, field voles have a spectacular reproductive capacity and an introduced vole in an enclosure of 33 m 2 corresponds to a density of 300 voles ha −1 .…”

“…This supports Leibold's (1989)Leibold's (1996) conjecture of dynamics in food webs with heterogeneous trophic levels. Hambäck (1998) and Norrdahl et al (2002) proposed that the ultimate reason for the contrasting responses of woody and herbaceous plants is seasonality. Woody plants are accessible in the limiting season, when herbivorous mammals are compelled to use even the marginal forage, whereas herbaceous plants are only accessible in the growing season, when grazers are literally wading in forage and can thus be expected to be maximally selective.…”

Seasonality, Parasite Diversity, and Local Extinctions in Plasmodium falciparum Malaria

“…Phenology may be most familiar as an aspect of plant ecology (Fenner 1998), but it is also important in insect ecology (Grist and Gurney 1997), including the biological control of pest species (DeBach 1974). Seasonal variation can be a significant force in population regulation (Hamback 1998, Tkadlec and Zejda 1998, Hansen et al 1999, sustainable harvesting (Kokko and Lindstrom 1998), predator-prey dynamics (Scheffer et al 1997), and life history evolution (Hairston 1988). Increased risks of extinction with small population sizes are familiar, e.g., from studies of island biogeography (Pimm et al 1988, Rosenzweig andClark 1994), and are often evaluated in the context of species conservation (Soule 1987).…”