Impact of pathogenic fungi, herbivores and predators on secondary succession of tropical rainforest vegetation

Abstract

There is growing evidence that top–down biotic factors play an important role in plant community dynamics and are able to maintain the high plant diversity of primary tropical forests. However, the top–down impact by herbivores on the accumulation of plant biomass, richness and the community assembly process remains poorly known for the initial stages of secondary ecological succession. Here we test the top–down effects of natural enemies on the biomass, richness, diversity, community composition, trait dynamics and randomness during the assembly process on an early successional community in a lowland tropical forest in Papua New Guinea. We initiated secondary succession on 36 vegetation plots (5 × 5 m) organized in six blocks. After 1 year we evaluated successional community characteristics in each block containing a control plot and five treatment plots: experimentally excluding (a) insects by insecticides, (b) fungal pathogens by fungicides, (c) vertebrate predators and ants with exclosures and traps and experimentally adding (d) low and (e) high levels of a generalist herbivore species. Insects were responsible for increased plant diversity, shifts in plant community composition and a decrease in community weighted mean of leaf dry matter content. The lack of herbivores resulted in a diversifying effect but did not affect plant biomass and stem density. No other treatment had a significant effect on plant diversity during succession. The highest level of artificially increased herbivore density reduced plant biomass, while the fungicide treatment decreased stem density of woody plants. Using a null model approach, we showed that the removal of insects and their vertebrate predators increased randomness during the community assembly processes for woody plants, whilst the same effect was not found for the removal of fungi or elevated herbivore abundances. Synthesis. Early rainforest succession was not entirely plant driven. During the early succession of lowland tropical forest, insect herbivores enhanced plant diversity and caused shifts in community structure by promoting species with acquisitive leaf traits. As expected, the early successional communities were highly variable, but insects and vertebrate predators reduced randomness during the assembly process. Insect and vertebrate communities can thus impact the regeneration dynamics of tropical forests. In contrast, fungal pathogens were less important in maintaining high diversity of successional communities in our system.

Limited contributions of plant pathogens to density‐dependent seedling mortality of mast fruiting Bornean trees

“…Within fungicide‐treated pots, seedlings were sprayed with a combination of Amotan 22.8SC ® , a selective methalaxyl‐based systemic fungicide (Advansia Ltd, Malaysia, active ingredient: azoxystrobin), and Kencozeb M45 ® , a broad‐spectrum di‐thiocarbamate non‐systemic fungicide (Kenso corporation, Selangor, Malaysia, active ingredient: mancozeb). Active ingredients of both fungicides are frequently used to test fungal contributions to seedling mortality (Krishnadas et al., 2018; Szefer et al., 2020; Xu et al., 2015), providing protection against plant pathogenic fungi and oomycetes, with low toxicity to other soil biota and with minimal inhibitory effects on arbuscular mycorrhizal associations with tropical seedlings (Gripenberg et al., 2014). Fungicides were applied to seedling leaves, or to the surface of germinated seeds prior to leaf development, once every 10 days for 60 days (6 treatments) at the recommended concentration (Amotan22.8SC ® ; 0.05 ml/m 2 , Kencozeb M45 ® ; 0.11 g/m 2 ) using a hand mister, with 50 ml of solution per pot.…”

Weak effects of birds, bats, and ants on their arthropod prey on pioneering tropical forest gap vegetation

“…This should have led to increased recruitment of seedlings from small seeds, increased density, reduced diversity, and elevated plant biomass independently of the indirect effect of the focal taxa of predators (Paine & Beck, 2007). However, observable (but not significant) positive LRR of woody plant biomass were not coupled with any detectable differences in density nor diversity between control and exclosure plots (Szefer et al, 2020). This suggests that negative effects of small rodents were weak or, more probably, compensated by increased pressure of herbivorous insects (Williams et al, 2021).…”

“…This study is a part of a larger experiment focused on evaluating the role of biotic factors in secondary forest succession. The advantages of using abandoned gardens for studying secondary successional have been previously discussed in Szefer et al (2020). Within a radius of 3 km from the village, we selected six experimental gardens (blocks) 3-5 years old, each covering an area of approximately 400 m 2 , separated by a minimal distance of 200 m. At each garden, we initiated regeneration by clearing all aboveground vegetation.…”

“…However, top‐down effects of the higher order predators in tropics can be highly variable (Maas et al, 2016). It has been recently shown that in tropical forest gaps herbivorous insects do shape community composition of pioneering plants, but higher order predators had surprisingly weak cascading effect on vegetation (Szefer et al, 2020). In mature tropical forest canopies, positive correlations between insect herbivore abundance and food quality (e.g., foliar nitrogen) or availability (e.g., leaf biomass) suggest that arthropod communities in late successional forests are shaped mainly by vegetation (Mottl et al, 2020; Whitfeld et al, 2012).…”

Natural Enemies and the Maintenance of Tropical Tree Diversity: Recent Insights and Implications for the Future of Biodiversity in a Changing World

“…In forest ecosystems, studies of succession have largely focused on how abiotic variables, namely light, drive shifts in composition from early to later successional plant species. However, a recent study by Szefer et al (2020) reported that natural enemies (specifically insect herbivores) played a role in shaping initial stages of succession in lowland tropical forest in Papua New Guinea. Thus, a key area of future research is to assess how plant-enemy interactions contribute to the speed and trajectory of tropical forest succession following human disturbance.…”