Wildlife disturbances as a source of conspecific negative density-dependent mortality in tropical trees

Abstract

Large vertebrates are rarely considered important drivers of conspecific negative density-dependent mortality (CNDD) in plants because they are generalist consumers. However, disturbances like trampling and nesting also cause plant mortality, and their impact on plant diversity depends on the spatial overlap between wildlife habitat preferences and plant species composition. We studied the impact of native wildlife on a hyperdiverse tree community in Malaysia. Pigs ( Sus scrofa ) are abnormally abundant at the site due to food subsidies in nearby farmland and they construct birthing nests using hundreds of tree saplings. We tagged 34 950 tree saplings in a 25 ha plot during an initial census and assessed the source mortality by recovering tree tags from pig nests ( n = 1672 pig-induced deaths). At the stand scale, pigs nested in flat dry habitats, and at the local neighbourhood scale, they nested within clumps of saplings, both of which are intuitive for safe and efficient nest building. At the stand scale, flat dry habitats contained higher sapling densities and higher proportions of common species, so pig nesting increased the weighted average species evenness across habitats. At the neighbourhood scale, pig-induced sapling mortality was associated with higher heterospecific and especially conspecific sapling densities. Tree species have clumped distributions due to dispersal limitation and habitat filtering, so pig disturbances in sapling clumps indirectly caused CNDD. As a result, Pielou species evenness in 400 m 2 quadrats increased 105% more in areas with pig-induced deaths than areas without disturbances. Wildlife induced CNDD and this supported tree species evenness, but they also drove a 62% decline in sapling densities from 1996 to 2010, which is unsustainable. We suspect pig nesting is an important feature shaping tree composition throughout the region.

Megafauna extinctions produce idiosyncratic Anthropocene assemblages

“…The rise of some megafauna in degraded forests is also a problem. High densities of wild boars in forest edges produce strong shifts in plant diversity and function ( 50 , 51 ), and their collapse due to African swine fever could be equally disruptive ( 52 ). Ecological cascades research will be especially important for this region.…”

Do natural enemies mediate conspecific negative distance‐ and density‐dependence of trees? A meta‐analysis of exclusion experiments

“…Larger mammalian herbivores play a key role in tree seedling survival and forest regeneration. Evidence suggests that large mammals could contribute to CNDDD at seedling stages (Murphy and Comita 2021, Luskin et al 2021). However, their diversity and abundances have been widely reduced by human activities (Corlett 2007, Benítez‐López et al 2017).…”

“…Our study indicated that CNDDD at the seed stage may be higher in disturbed study sites, because of the loss of large mammals (Supporting information). However, CNDDD at the seedling stage may be higher in pristine study sites as large mammals are the main drivers of CNDDD processes (Murphy and Comita 2021, Luskin et al 2021).…”

“…Field studies have shown significant CNDDD across climate zones and vegetation types (Dirzo and Boege 2008, Comita et al 2014), but also large variation among species (Song et al 2021). Some evidence also suggests the existence of a reverse Janzen–Connell effect where the individual's fitness is increased near conspecifics due to the accumulation of host‐specific mutualists (Luskin et al 2021, Zahra et al 2021).…”

“…Plant‐eating animals can also drive CNDDD (Fricke et al 2014, Forrister et al 2019, Luskin et al 2021, Murphy and Comita 2021). If adult plants and higher densities of seeds or seedlings attract more herbivores, this could increase mortality risk for the offspring (Janzen 1970, Connell 1971).…”