Plants do not suffer greater losses to seed predation towards the tropics

Abstract

Aim Biotic interactions have traditionally been predicted to be stronger towards the tropics. However, all previous studies about the latitudinal gradient in seed predation were either based on single species or compiled data from studies that used different methods in different ecosystems. Our goal was to provide the first broad‐scale quantification of the latitudinal gradient in seed predation at both cross‐species and within‐species levels. Location Twenty‐five sites spanning 28° of latitude on the east coast of Australia. Methods Using consistent protocols, we measured pre‐dispersal seed predation for 256 species × site combinations (including a total of 170 species) and post‐dispersal seed removal for 126 species × site combinations (including a total of 91 species), as well as removal of standard rice grains at each site. The relationships between seed predation and latitude were quantified using generalized mixed‐effects models and meta‐analyses. Results Contrary to expectations, the proportion of seeds lost to pre‐dispersal seed predation increased with latitude at the cross‐species level and showed no latitudinal trend at the within‐species level. There was no significant relationship between latitude and the proportion of seeds lost to post‐dispersal seed removal, either within species or across species. We found significantly higher removal of standard rice grains at lower latitudes, but this trend reversed if we excluded the four island sites from analysis. Main conclusions Seed predation on natural seeds was not higher at lower latitudes. Thus, our study joins a growing body of evidence contrary to the idea that tropical species suffer greater losses to herbivores than do temperate species. The latitudinal gradient in removal rates for standard rice grains suggests that natural seeds from low latitudes may have achieved a similar predation rate to high‐latitudinal species through increased investment in defenses. However, additional data will be necessary to test this hypothesis.

Factors shaping large‐scale gradients in seed physical defence: Seeds are not better defended towards the tropics

“…Our findings join a growing body of studies that cast doubt on the generality of the latitudinal gradient in biotic interactions and consequences (HilleRisLambers, Clark, & Beckage, ; Kozlov et al, ; Moles & Ollerton, ; Moles & Westoby, ; Moles, Bonser, Poore, Wallis, & Foley, ; Ollerton & Cranmer, ; Onoda et al, ; Poore et al, ; Schleuning et al, ; Zhang et al, ). Together with the finding that seed predation is not stronger at low latitudes (Chen, Hemmings et al, ), our large‐scale empirical studies cast serious doubt on the traditional idea of a latitudinal gradient in seed predation and resistance to seed predation and our previous understanding of the factors shaping the high diversity of tropical plant species.…”

“…(b) Post‐dispersal seed physical defence versus seed predation. Data for seed predation are from Chen, Hemmings et al (), collected from the same individual plants at the same study sites. Each data point represents the mean value for a species–site combination.…”

“…However, there is a high level of controversy surrounding latitudinal gradients in herbivory and defence (Kozlov, Lanta, Zverev, & Zvereva, ; Moles & Ollerton, ; Onoda et al, ; Poore et al, ; Zhang, Zhang, & Ma, ). Intraspecific studies (Anstett, Naujokaitis‐Lewis, & Johnson, ; García, Zamora, Gómez, Jordano, & Hodar, ; Sanz & Pulido, ; Toju & Sota, ), large‐scale data syntheses (Hulme & Kollmann, ; Moles & Westoby, ; Peco et al, ) and a large‐scale cross‐species study (Chen, Hemmings et al, ) have all failed to support the hypothesis that seed predation of natural seeds in their natural habitats is more intense towards the equator. One possibility is that the latitudinal gradient in seed predation is being obscured by a latitudinal gradient in seed defence (Chen, Hemmings et al, ).…”

“…Intraspecific studies (Anstett, Naujokaitis‐Lewis, & Johnson, ; García, Zamora, Gómez, Jordano, & Hodar, ; Sanz & Pulido, ; Toju & Sota, ), large‐scale data syntheses (Hulme & Kollmann, ; Moles & Westoby, ; Peco et al, ) and a large‐scale cross‐species study (Chen, Hemmings et al, ) have all failed to support the hypothesis that seed predation of natural seeds in their natural habitats is more intense towards the equator. One possibility is that the latitudinal gradient in seed predation is being obscured by a latitudinal gradient in seed defence (Chen, Hemmings et al, ). However, there has been only one previous study on the latitudinal gradient in seed physical defence, which found that the pericarp of Japanese camellia ( Camellia japonica ) was thicker towards lower latitudes across 6° of temperate latitude (Toju et al, ).…”

“…Larger seeds are more likely to be encountered by predators (Feeny, ). However, they do not seem to suffer greater levels of seed predation (Chen, Hemmings et al, ; Moles et al, ), indicating that larger seeds might be armed with greater defences. The relationship between seed mass and seed physical defence has received little attention at the pre‐dispersal stage, and there are only three cross‐species studies at the post‐dispersal stage.…”