Abstract
Density-dependent interactions between plants and their natural enemies, including fungal pathogens and insect herbivores, help maintain plant species coexistence and diversity at local scales (α-diversity). However, turnover in plant species composition across space also contributes to biodiversity at larger spatial scales (β-diversity). Despite mounting evidence that enemies can maintain α-diversity, we know little about their contributions to β-diversity. Additionally, in the light of widespread habitat fragmentation and potentially modified insect and pathogen communities in forest fragments, the effects of fragment area on their diversity-maintaining roles are largely unknown. We carried out a field experiment to investigate how natural enemies in impact tree α and β-diversity in a fragmented rainforest landscape in the Western Ghats, India. In twenty-one rainforest fragments, we suppressed insects and fungi/oomycetes with pesticides, and examined changes in the diversity of tree seedlings. We found that fungicide had no effect on α-diversity, but significantly decreased β-diversity (species turnover among plots). The facilitative effects of fungi and oomycetes on β-diversity, however, weakened as fragments decreased in area, indicating that certain specialized plant-pathogen interactions may be lost when fragments become smaller. Insecticide, in contrast, increased α-diversity. However, insects tended to increase β-diversity between distant plots. In summary, we found that interactions between plants and their natural enemies help maintain β-diversity in large forest fragments but not in small fragments. Small fragments are often viewed as future reservoirs of biodiversity in human-dominated landscapes, but our findings suggest that modified interactions with natural enemies may result in the erosion of this diversity over time.
Highlights
Throughout the tropics, contiguous rainforest is being replaced by fragments of forest isolated within agricultural and urban landscapes (Laurance et al, 2011; Haddad et al, 2015)
The effects of the insecticide and fungicide treatments are contrasted with effects in the control treatments
Despite the lack of evidence that insects and pathogens increased plant α-diversity, our results suggest that primarily insects enhanced dissimilarity between distant plots, and increased diversity at large spatial scales (β-diversity)
Summary
Throughout the tropics, contiguous rainforest is being replaced by fragments of forest isolated within agricultural and urban landscapes (Laurance et al, 2011; Haddad et al, 2015). One such ecological process is described by the JanzenConnell hypothesis (Connell, 1971; Janzen, 1971) Specialized natural enemies such as insect herbivores and pathogens (which include fungi, oomycetes, bacteria, nematodes, viruses, and others) cause elevated mortality on seedlings growing near conspecific adults or at high conspecific density (Augspurger, 1984; Bell et al, 2006; Freckleton and Lewis, 2006), facilitating the long-term persistence of locally-rare species. Attack by both fungal (Mangan, 2010; Bagchi et al, 2014) and oomycete (Augspurger, 1984; Augspurger and Wilkinson, 2007) pathogens has been linked to density dependent mortality of tropical plant seedlings. The role of natural enemies in maintaining α-diversity is well-studied, little is known about their role in maintaining β-diversity
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