Fragmentation and matrix contrast favor understory plants through negative cascading effects on a strong competitor palm

Linking changes in landscape structure to insect herbivory in forest edges and interiors of Atlantic Forest remnants

Human activities have triggered profound changes in natural landscapes, resulting in species loss and disruption of pivotal ecological interactions such as insect herbivory. This antagonistic interaction is affected by complex pathways (e.g., abundance of herbivores and predators, plant chemical defenses, and resource availability), but the knowledge regarding how forest loss and fragmentation affect insect herbivory in human-modified tropical landscapes still remains poorly understood. In this context, we assessed multi-pathways by which changes in landscape structure likely influence insect herbivory in 20 Atlantic forest fragments in Brazil. Using path analysis, we estimated the direct effects of forest cover and forest edge density, and the indirect effect via canopy openness, number of understory plants and phenolic compounds, on leaf damage in understory plants located in the edge and interior of forest fragments. In particular, plants located in forest edges experienced greater leaf damage than interior ones. We observed that landscape edge density exerted a positive and direct effect on leaf damage in plants sampled at the edge of forest fragments. Our findings also indicated that forest loss and increase of edge density led to an increase in the canopy opening in the forest interior, which causes a reduction in the number of understory plants and, consequently, an increase in leaf damage. In addition, we detected that phenolic compounds negatively influence leaf damage in forest interior plants. Given the increasing forest loss in tropical regions, in which forest fragments become stranded in highly deforested, edge-dominated and degraded landscapes, our study highlights the pervasive enhancement in insect herbivory in remaining forest fragments-especially along forest edges and canopy gaps in the forest interior. As a result, increased herbivory is likely to affect forest regeneration and accelerate the ecological meltdown processes in these highly deforested and disturbed anthropogenic landscapes.

Maintenance of different life stages of old-growth forest trees in deforested tropical landscapes

Tropical tree species are increasingly being pushed to inhabit deforested landscapes. The habitat amount hypothesis posits that, in remaining forest patches, species diversity in equal-sized samples decreases with decreasing forest cover in the surrounding landscape. We tested this prediction by taking into account three important factors that can affect species responses to forest loss. First, forest loss effects can be linear (proportional) or nonlinear, as there can be threshold values of forest loss beyond which species extirpation may be accelerated. Second, such effects are usually scale dependent and may go unnoticed if assessed at suboptimal scales. Finally, species extirpation may take decades to become evident, so the effects of forest loss can be undetected when assessing long-lived organisms, like adult old-growth forest trees. Here, we evaluated the linear and nonlinear effects of landscape forest loss across different spatial scales on site-scale abundance and diversity of old-growth forest trees, separately for four plant-life stages (seeds, saplings, juveniles, and adults) in two rainforest regions with different levels of deforestation. We expected stronger (and negative) forest loss effects on early plant-life stages, especially in the region with the highest deforestation. Surprisingly, in 13 of 16 study cases (2 responses × 4 life stages × 2 regions), null models showed higher empirical support than linear and nonlinear models at any scale. Therefore, the species richness and abundance of local tree assemblages seem to be weakly affected by landscape-scale forest loss independently of the spatial scale, life stage, and region. Yet, as expected, the predictive power of forest cover was relatively lower in the least deforested region. Our findings suggest that landscape-scale forest loss is poorly related to site-scale processes, such as seed dispersal and seedling recruitment, or, at least, such effects are too small to shape the abundance and diversity of tree assemblages within forest patches. Therefore, our findings do not support the most important prediction of the habitat amount hypothesis but imply that, on a per-area basis, a unit of habitat (forest) in a highly deforested landscape has a conservation value similar to that of a more forested one, particularly in moderately deforested rainforests.

Resolving the SLOSS dilemma for biodiversity conservation: a research agenda

The legacy of the 'SL > SS principle', that a single or a few large habitat patches (SL) conserve more species than several small patches (SS), is evident in decisions to protect large patches while down-weighting small ones. However, empirical support for this principle is lacking, and most studies find either no difference or the opposite pattern (SS > SL). To resolve this dilemma, we propose a research agenda by asking, 'are there consistent, empirically demonstrated conditions leading to SL > SS?' We first review and summarize 'single large or several small' (SLOSS) theory and predictions. We found that most predictions of SL > SS assume that between-patch variation in extinction rate dominates the outcome of the extinction-colonization dynamic. This is predicted to occur when populations in separate patches are largely independent of each other due to low between-patch movements, and when species differ in minimum patch size requirements, leading to strong nestedness in species composition along the patch size gradient. However, even when between-patch variation in extinction rate dominates the outcome of the extinction-colonization dynamic, theory can predict SS > SL. This occurs if extinctions are caused by antagonistic species interactions or disturbances, leading to spreading-of-risk of landscape-scale extinction across SS. SS > SL is also predicted when variation in colonization dominates the outcome of the extinction-colonization dynamic, due to higher immigration rates for SS than SL, and larger species pools in proximity to SS than SL. Theory that considers change in species composition among patches also predicts SS > SL because of higher beta diversity across SS than SL. This results mainly from greater environmental heterogeneity in SS due to greater variation in micro-habitats within and across SS habitat patches ('across-habitat heterogeneity'), and/or more heterogeneous successional trajectories across SS than SL. Based on our review of the relevant theory, we develop the 'SLOSS cube hypothesis', where the combination of three variables - between-patch movement, the role of spreading-of-risk in landscape-scale population persistence, and across-habitat heterogeneity - predict the SLOSS outcome. We use the SLOSS cube hypothesis and existing SLOSS empirical evidence, to predict SL > SS only when all of the following are true: low between-patch movement, low importance of spreading-of-risk for landscape-scale population persistence, and low across-habitat heterogeneity. Testing this prediction will be challenging, as it will require many studies of species groups and regions where these conditions hold. Each such study would compare gamma diversity across multiple landscapes varying in number and sizes of patches. If the prediction is not generally supported across such tests, then the mechanisms leading to SL > SS are extremely rare in nature and the SL > SS principle should be abandoned.

Landscape structure shapes the diversity of tree seedlings at multiple spatial scales in a fragmented tropical rainforest

The maintenance of seedling diversity of animal-dispersed tree species is fundamental for the structure and function of forest patches in fragmented tropical rainforests. Nonetheless, the effects of landscape structure at different spatial scales on α- and β-diversity of tree seedling communities are recently explored. Using a multi-scale approach, we assessed the relative effect of landscape composition and configuration on α- and β-diversity of animal-dispersed seedlings within 16 forest patches in the Lacandona rainforest, Mexico. We assessed these effects at 13 spatial scales (from 300 to 1500 m radius, at 100 m intervals) for three metrics of effective number of species considering α- and β-diversity. We found that α-diversity was largely affected by landscape composition and β-diversity by landscape configuration. On the one hand, the amount of secondary forest influenced α-diversity. Additionally, species richness increased in landscapes with highly aggregated forest patches. On the other hand, β-diversity was affected positively by forest fragmentation and negatively by the edge contrast of forest patches with the surrounding matrix. Our findings indicate that landscape configuration is a strong driver of seedling diversity in highly deforested rainforests. Promoting forest patches and secondary forests through payment for ecosystem services' programs, favoring matrix quality within land-sharing schemes of smallholder agriculture and secondary forest management, and identifying restoration opportunities for assisted or unassisted natural regeneration are urgently needed for conservation of seedling diversity in human-modified tropical landscapes.

Designing optimal human-modified landscapes for forest biodiversity conservation

Agriculture and development transform forest ecosystems to human-modified landscapes. Decades of research in ecology have generated myriad concepts for the appropriate management of these landscapes. Yet, these concepts are often contradictory and apply at different spatial scales, making the design of biodiversity-friendly landscapes challenging. Here, we combine concepts with empirical support to design optimal landscape scenarios for forest-dwelling species. The supported concepts indicate that appropriately sized landscapes should contain ≥ 40% forest cover, although higher percentages are likely needed in the tropics. Forest cover should be configured with c. 10% in a very large forest patch, and the remaining 30% in many evenly dispersed smaller patches and semi-natural treed elements (e.g. vegetation corridors). Importantly, the patches should be embedded in a high-quality matrix. The proposed landscape scenarios represent an optimal compromise between delivery of goods and services to humans and preserving most forest wildlife, and can therefore guide forest preservation and restoration strategies.