Testing predictions of the Janzen–Connell hypothesis: a meta‐analysis of experimental evidence for distance‐ and density‐dependent seed and seedling survival

Abstract

The Janzen–Connell hypothesis proposes that specialist natural enemies, such as herbivores and pathogens, maintain diversity in plant communities by reducing survival rates of conspecific seeds and seedlings located close to reproductive adults or in areas of high conspecific density. Variation in the strength of distance- and density-dependent effects is hypothesized to explain variation in plant species richness along climatic gradients, with effects predicted to be stronger in the tropics than the temperate zone and in wetter habitats compared to drier habitats.We conducted a comprehensive literature search to identify peer-reviewed experimental studies published in the 40+ years since the hypothesis was first proposed. Using data from these studies, we conducted a meta-analysis to assess the current weight of evidence for the distance and density predictions of the Janzen–Connell hypothesis.Overall, we found significant support for both the distance- and density-dependent predictions. For all studies combined, survival rates were significantly reduced near conspecifics compared to far from conspecifics, and in areas with high densities of conspecifics compared to areas with low conspecific densities. There was no indication that these results were due to publication bias.The strength of distance and density effects varied widely among studies. Contrary to expectations, this variation was unrelated to latitude, and there was no significant effect of study region. However, we did find a trend for stronger distance and density dependence in wetter sites compared to sites with lower annual precipitation. In addition, effects were significantly stronger at the seedling stage compared to the seed stage.Synthesis. Our study provides support for the idea that distance- and density-dependent mortality occurs in plant communities world-wide. Available evidence suggests that natural enemies are frequently the cause of such patterns, consistent with the Janzen–Connell hypothesis, but additional studies are needed to rule out other mechanisms (e.g. intraspecific competition). With the widespread existence of density and distance dependence clearly established, future research should focus on assessing the degree to which these effects permit species coexistence and contribute to the maintenance of diversity in plant communities.

Forest tree neighborhoods are structured more by negative conspecific density dependence than by interactions among closely related species

“…This is further supported by the fact that the proportion of neighbors that were conspecific declined significantly with focal tree size at all sites, particularly for sapling-sized neighbors. Numerous studies have reported strong conspecific negative density-dependent mortality in both tropical and temperate forests (Augspurger 1984, Terborgh et al 2008, Uriarte et al 2004a, Stoll and Newbery 2005, Chen et al 2010, Swamy and Terborgh 2010, Bai et al 2012, Comita et al 2014), particularly at early life stages (e.g., seedling and sapling stages; Zhu et al 2015). Our results are consistent with these other studies and demonstrate that such density-dependent mortality patterns are strong enough to structure forest composition.…”

“…Our results are consistent with these other studies and demonstrate that such density-dependent mortality patterns are strong enough to structure forest composition. Negative interactions with conspecific neighbors can result from strong intraspecific competition for resources, but a large body of literature points to distance/density-responsive, host-specific natural enemies (e.g., pathogens, insect herbivores) as the underlying mechanism, consistent with the Janzen-Connell hypothesis (e.g., Janzen 1970, Connell 1971, Mangan et al 2010, Bagchi et al 2014, Comita et al 2014.…”

Plant density can increase invertebrate postdispersal seed predation in an experimental grassland community

“…However, a recent meta‐analysis by Comita et al. (2014), which addressed distance‐ and density‐driven Janzen–Connell effects based on a larger number of studies, supports the general validity of both distance‐ and density‐driven Janzen–Connell effects across different biomes and habitats. One prominent cause of Janzen–Connell effects is the predation of seeds by granivorous animals.…”

“…2014). Although some studies examined postdispersal seed predation in temperate experimental and semi‐natural grassland systems, they focused on the effects of plant species richness (Pufal and Klein 2013; Preukschas et al.…”

Parent tree distance-dependent recruitment limitation of native and exotic invasive seedlings in urban forests

“…In natural forest ecosystems, native species coexistence is maintained by native seedling recruitment limitation assuming that host specific parasites (e.g., pathogens and herbivores) are more likely to damage conspecific seeds and seedlings the closer they germinate to the parent tree (distance-dependent coexistence mechanism explained by the Janzen and Connell Hypothesis (Connell 1971;Janzen 1970)). Biogeographic studies have supported the parent tree distance-dependent mechanism to explain species coexistence across latitudinal gradients from boreal to temperate systems and across different ecological guilds (Comita et al 2014). However, it is still unknown if distance-dependent mechanisms in regard to recruitment limitation apply to closed-canopy urban forest ecosystems invaded by exotic invasive plant species (hereafter invasive species).…”