Multiple metrics of latitudinal patterns in insect pollination and herbivory for a tropical‐temperate congener pair

Abstract

The biotic interactions hypothesis posits that biotic interactions are more important drivers of adaptation closer to the equator, evidenced by "stronger" contemporary interactions (e.g., greater interaction rates) and/or patterns of trait evolution consistent with a history of stronger interactions. Support for the hypothesis is mixed, but few studies span tropical and temperate regions while experimentally controlling for evolutionary history. Here, we integrate field observations and common garden experiments to quantify the relative importance of pollination and herbivory in a pair of tropical-temperate congeneric perennial herbs. Phytolacca rivinoides and P. americana are pioneer species native to the Neotropics and the eastern United States, respectively. We compared plant-pollinator and plant-herbivore interactions between three tropical populations of P. rivinoides from Costa Rica and three temperate populations of P. americana from its northern range edge in Michigan and Ohio. For some metrics of interaction importance, we also included three subtropical populations of P. americana from its southern range edge in Florida. This approach confounds species and region but allows us, uniquely, to measure complementary proxies of interaction importance across a tropicaltemperate range in one system. To test the prediction that lower-latitude plants are more reliant on insect pollinators, we quantified floral display and reward, insect visitation rates, and self-pollination ability (autogamy). To test the prediction that lower-latitude plants experience more herbivore pressure, we quantified herbivory rates, herbivore abundance, and leaf palatability. We found evidence supporting the biotic interactions hypothesis for most comparisons between P. rivinoides and north-temperate P. americana (floral display, insect visitation, autogamy, herbivory, herbivore abundance, and young-leaf palatability). Results for subtropical P. americana populations, however, were typically not intermediate between P. rivinoides and north-temperate P. americana, as would be predicted by a linear latitudinal gradient in interaction importance. Subtropical young-leaf palatability was intermediate, but subtropical mature leaves were the least palatable, and pollination-related traits did not differ between temperate and subtropical regions. These nonlinear patterns of interaction importance suggest future work to link interaction importance to climatic or biotic thresholds. In sum, we found that the biotic interactions hypothesis was more consistently supported at the larger spatial scale of our study.

Balancing the risks of mating: biogeographic evidence of cleistogamy as a bet hedging strategy

“…We first contrast members of Poaceae, or wind pollinated grasses, with all other species as a proxy for pollination mode. We find that non-Poacea species are more prevalent in colder environments, where biotic pollinator abundance is the most variable (Ollerton [2017], Baskett et al [2020]). We also find significant differences in the distribution of complete vs. dimorphic cleistogamy, regardless of pollinator mode.…”

“…In colder biomes (i.e. boreal forests, temperate forests, and tundra), it is unsurprising that cleistogamy will be more prevalent in biotic pollinated species, as colder mean temperatures are correlated with decreased biotic pollinator abundance (Ollerton [2017], Baskett et al [2020]). However, it is interesting that cleistogamy is more prevalent in non-Poaceae species in tropical and subtropical moist broadleaf forests, given this biome is characterized by a consistently warm and wet climate (Loidi et al [2022], Atangana et al [2014]).…”

Is there a latitudinal diversity gradient for symbiotic microbes? A case study with sensitive partridge peas

“…Mutualism may be more prevalent in tropical regions than in temperate zones because the warmer and more constant climate promotes strong year‐round biotic interactions (Schemske et al., 2009). Mutualism could therefore play an important role in generating the high species richness observed at low latitudes through facilitating allopatric speciation (Gómez & Verdú, 2012) or more intense coevolution with, or adaptation to, mutualistic partners (Baskett et al., 2020; Hembry et al., 2014; Weber & Agrawal, 2014). Alternatively, high species richness in the tropics could create more opportunity for species interactions to occur, and thus influence the generation and persistence of mutualism in those regions (Chomicki & Renner, 2017).…”

Tempo and mode of morphological evolution are decoupled from latitude in birds

“…One hypothesis posits that species interactions are stronger in the tropics, and, therefore, play a more important role in many processes (e.g., diversification) in tropical lineages [2][3][4][5][6] (but see [7]). Previous tests of this "biotic interactions hypothesis" have generally focused on latitudinal gradients in the strength of ecological interactions between predator and prey, herbivore and plant, or pathogen and host [8][9][10][11]. Latitudinal gradients in the strength of competition between members of the same trophic level have been less explored, although they have been highlighted as one of the most important research directions for testing the biotic interaction hypothesis [5].…”