Rare species advantage? Richness of damage types due to natural enemies increases with species abundance in a wet tropical forest

Abstract

Summary1. The Janzen-Connell hypothesis (JC) is one potential mechanism to explain the maintenance of high alpha diversity of tree species in tropical forests, operating through differential pressure by natural enemies. 2. We proposed that this differing pressure could arise from the richness of damage types due to natural enemies (RDNE). Following a community compensatory trend (CCT), we hypothesized greater RDNE on common species than on rare species. 3. We evaluated this novel interpretation of the JC by assessing damage patterns on leaves, as a proxy for natural enemy species in 44 tree species. We first evaluated which abiotic and biotic factors affect RDNE. Then, we tested whether increasing RDNE leads to an increasing amount of foliar damage. 4. We found that RDNE a. Was affected by biotic environments: RDNE increased with mean seedling species abundance. RDNE was higher on species occurring near more closely related neighbours. b. Was not impacted by abiotic factors. Yet, seedlings of shade-tolerant species hosted a higher RDNE than seedlings of shade-intolerant species. c. Was positively correlated with amount of foliar damage at the species level.5. Finally, we tested whether RDNE increased seedling mortality risk. We found that a. Foliar damage, species abundance and RDNE 2 increased mortality risk.b. Richness of damage types due to natural enemies linearly decreased mortality risk more strongly than RDNE 2 increased it.c. Seedling age decreased mortality risk. 6. Synthesis. The richness of damage types due to natural enemies increased with abundance of the host species, suggesting an important role of enemy diversity in the maintenance of tree diversity. Supporting a novel interpretation of the Janzen-Connell hypothesis, we found a greater mortality risk with increasing RDNE 2 , but not with increasing RDNE. There was a stronger negative linear effect of RDNE on mortality risk. Rare species with low RDNE as well as species with very high RDNE suffered greater mortality than species hosting intermediate RDNE, reinforcing the complexity of the effect of multiple enemies on prey.

The advantage of the extremes: tree seedlings at intermediate abundance in a tropical forest have the highest richness of above‐ground enemies and suffer the most damage

“…Previous studies (Strong, Lawton & Southwood ; Moran et al . ; Bachelot & Kobe ) suggested that the richness of enemies should increase with conspecific seedling density. Our results were partially consistent with these theories as we found that the relationship between species abundance and richness of above‐ground enemies hosted by an individual seedling exhibited a hump‐shaped pattern with richness, peaking at intermediate densities of conspecific seedlings.…”

“…At the local and community scales, enemy richness is expected to increase linearly with conspecific tree density (Moran et al . ; Bachelot & Kobe ) and foliar damage (Ness, Rollinson & Whitney ; Schuldt et al . ; Cárdenas et al .…”

Nonrandom, diversifying processes are disproportionately strong in the smallest size classes of a tropical forest

“…Although the relative contributions of nonrandom processes at each life cycle stage to determining patterns of abundance and diversity in the mature canopy are unknown, one long-standing paradigm is that community assembly is mediated primarily by events occurring from seed dispersal through seedling germination and small-sapling establishment (13-17). However, despite suggestive patterns (6,7,18,19), evidence is lacking for the comparative strength of early-stage dynamics in determining canopy abundance and diversity.Numerous studies demonstrate significant interspecific variation in the susceptibility of tropical tree seedlings to postgermination hazards, including natural enemies (20, 21), adverse climatic or edaphic conditions (22), physical damage (23), and the crowding or shared-enemies effects of con-and heterospecific neighbors (24,25). In other words, the per capita probability of seedling mortality is nonrandom because the probability of death is not the same for all individuals in a local community -it is dependent to some degree on species identity.…”

“…Numerous studies demonstrate significant interspecific variation in the susceptibility of tropical tree seedlings to postgermination hazards, including natural enemies (20, 21), adverse climatic or edaphic conditions (22), physical damage (23), and the crowding or shared-enemies effects of con-and heterospecific neighbors (24,25). In other words, the per capita probability of seedling mortality is nonrandom because the probability of death is not the same for all individuals in a local community -it is dependent to some degree on species identity.…”

Calculation of narrower confidence intervals for tree mortality rates when we know nothing but the location of the death/survival events

“…In the case of tropical forest tree species, negative density-dependent processes are assumed to be weak beyond 20 m from the focal individual (Bachelot & Kobe, 2013;Chanthorn, Caughlin, Dechkla, & Brockelman, 2013;Kobe & Vriesendorp, 2011;Ledo & Schnitzer, 2014;Zhu, Comita, Hubbell, & Ma, 2015), and it is difficult to think of trees affecting each other at distances larger than the typical canopy tree height (40-50 m in Lambir). In the case of tropical forest tree species, negative density-dependent processes are assumed to be weak beyond 20 m from the focal individual (Bachelot & Kobe, 2013;Chanthorn, Caughlin, Dechkla, & Brockelman, 2013;Kobe & Vriesendorp, 2011;Ledo & Schnitzer, 2014;Zhu, Comita, Hubbell, & Ma, 2015), and it is difficult to think of trees affecting each other at distances larger than the typical canopy tree height (40-50 m in Lambir).…”