Seed predation increases from the Arctic to the Equator and from high to low elevations

Abstract

Species interactions have long been predicted to increase in intensity towards the tropics and low elevations, due to gradients in climate, productivity, or biodiversity. Despite their importance for understanding global ecological and evolutionary processes, plant-animal interaction gradients are particularly difficult to test systematically across large geographic gradients, and evidence from smaller, disparate studies is inconclusive. By systematically measuring post-dispersal seed predation using 6980 standardized seed depots along 18 mountains in the Pacific cordillera, we found that seed predation increases 18% from the Arctic to Equator and 16% from 4000 masl to sea level. Clines in total predation, likely driven by invertebrates, were consistent across tree-line ecotones and in continuous forest, and were better explained by climate seasonality than by productivity, biodiversity, or latitude. These results suggest that species interactions play predictably greater ecological and evolutionary roles in tropical, lowland, and other less seasonal ecosystems.One Sentence Summary: Post-dispersal seed predation increases from the Arctic to the Equator and from high elevations to sea level. Main Text:Few biological patterns are as striking as latitudinal and elevational changes in biotic communities. Biodiversity and ecosystem productivity increase dramatically toward low latitudes (1, 2) and elevations (3,4). Biologists have long speculated that greater diversity and productivity should generate corresponding increases in the intensity of species interactions (5-7). However, tests for gradients in interaction intensity (8)(9)(10)(11)(12) or their expected ecological and evolutionary signatures (e.g. density dependence 13, 14, defenses 15, 16) find contradictory results. While latitude and elevation are often considered analogues, their effects on interaction strength are rarely tested together. This likely contributes to the variability of experimental results, and limits our understanding of their joint effects on global patterns in species interactions.More intense interactions toward low latitudes and elevations underpin several iconic biogeographic hypotheses. Antagonistic species interactions are thought to maintain high tropical diversity by limiting species dominance (the Janzen-Connell hypothesis; 17, 18), amplify tropical diversity by accelerating speciation (7,19), and play a predictably greater role in determining species' warm (low-latitude and elevation) vs. cool range limits (5,6). For example, stronger tropical seed predation-an interaction that shapes plant communities and distributions (20, 21)-is proposed to explain the greater tropical diversity of trees (14,17,18) and adaptations for seed defense (22). The strength and predictability of interaction gradients is therefore pivotal to understanding their role as macroevolutionary and biogeographic agents.Despite an outsized role in theory, assessing the generality of interaction gradients is hampered by constraints of existing evidence (23). Mos...

Environmental drivers of disease depend on host community context

“…Warmer temperatures can also increase parasite overwintering success (Burdon & Elmqvist 1996; Pfender & Vollmer 1999) or allow parasites to produce more generations during a longer growing season (Garrett et al 2006). These results corroborate past studies suggesting that environmental gradients can directly alter the strength of biotic interactions (Schemske et al 2009; Pellissier et al 2014; Descombes et al 2017; Roslin et al 2017; Hargreaves et al 2019), including host-parasite interactions (Nunn et al 2005; Abbate & Antonovics 2014; LaManna et al 2017; Allen et al 2020).…”

Predation and parasitism on herbivorous insects change in opposite directions in a latitudinal gradient crossing a boreal forest zone

“…The LBIH, which was originally developed for among‐biome comparisons (Dobzhansky, 1950; Schemske et al., 2009), has been supported by several macroecological studies of predation (Hargreaves et al., 2019; Jeanne, 1979; Roslin et al., 2017). A few studies found a poleward decrease in predation rates within the temperate biome (DeGregorio et al., 2016; Thompson & Ribic, 2012).…”

“…(DeGregorio et al., 2016; Roslin et al., 2017; Thompson & Ribic, 2012; Zvereva et al., 2019). These latitudinal changes in predation are frequently associated with changes in the abundance of invertebrate predators (Hargreaves et al., 2019; Roslin et al., 2017). We suggest that the decline in ant predation in our latitudinal gradient was also driven mainly by a decline in ant abundance: analysis of the soil macrofauna in the same gradient showed a substantial poleward decrease in the density of predatory ants (M. Kozlov, unpubl.…”

“…Therefore, natural enemies have been suggested as a factor driving latitudinal patterns in insect herbivory (Björkman et al., 2011). However, latitudinal studies of predator–prey interactions focused mostly on seed predation (Hargreaves et al., 2019; Moles & Westoby, 2003; Peco et al., 2014) and bird nest predation (Kubelka et al., 2018; McKinnon et al., 2010). Only a few studies tested the LBIH with data on the natural enemy impact on herbivores, and these studies demonstrated specific responses of different groups of enemies.…”

The latitudinal gradient in rates of evolution for bird beaks, a species interaction trait

“…2016; Moles & Ollerton 2016; Roslin et al . 2017; Baskett & Schemske 2018; Hargreaves et al . 2019; Freeman et al .…”