Birds help plants: a meta-analysis of top-down trophic cascades caused by avian predators

Abstract

The tritrophic interactions between plants, herbivores and avian predators are complex and prone to trophic cascades. We conducted a meta-analysis of original articles that have studied birds as predators of invertebrate herbivores, to compare top-down trophic cascades with different plant responses from different environments and climatic areas. Our search found 29 suitable articles, with a total of 81 separate experimental study set-ups. The meta-analysis revealed that plants benefited from the presence of birds. A significant reduction was observed in the level of leaf damage and plant mortality. The presence of birds also positively affected the amount of plant biomass, whereas effects on plant growth were negligible. There were no differences in the effects between agricultural and natural environments. Similarly, plants performed better in all climatic areas (tropical, temperate and boreal) when birds were present. Moreover, both mature plants and saplings gained benefits from the presence of birds. Our results show that birds cause top-down trophic cascades and thus they play an integral role in ecosystems.

Herbivore diet breadth mediates the cascading effects of carnivores in food webs

“…In support of the EFScascade hypothesis, we found that the effect of herbivore diet breadth cascaded down to the first trophic level, implicating herbivore diet breadth as a potentially widespread source of variation in the indirect effects of generalist predators on plant damage due to herbivory. Recent metaanalyses of the indirect beneficial effect of vertebrate insectivores on plants were unable to explain significant variation seen across tritrophic studies based on ecosystem properties [climate zone, agricultural vs. natural (28,41)], plant growth stage (41), or intraguild predation (26). Based on the evidence presented here, we suggest that diet breadth of the herbivore assemblage and the defensive traits associated with dietary specialization may provide some missing explanatory power.…”

“…21), thus leaving open the possibility that inferences are biased by nonindependent comparisons (25). In addition, all tests of the EFS hypothesis have focused on predatory insects (predatory wasps, ants, and hemipterans), and it is unknown whether dietary specialization provides EFS from generalist vertebrate predators such as insectivorous birds, bats, and lizards; these vertebrates are important consumers of plant-feeding insects and are known to indirectly benefit plants (26)(27)(28). Finally, past empirical work has been limited to predation trials in which herbivores are presented to predators divorced from the full context of their host plants or habitat, thus preventing herbivores from fully using their host plant for EFS.…”

Natural enemy interactions constrain pest control in complex agricultural landscapes

“…1. that this difference can lead to a dampening of biological control. Previous work has shown that vertebrate predators can have net positive effects on plant productivity by reducing herbivorous arthropods, even as they also reduce intermediate arthropod predators (21,22). However, most of these studies were performed in forests, agroforests, or plantations where spatial structures are presumably much less distinct than in mosaic agricultural landscapes dominated by annual crops.…”

“…The present data confirm these results in simple, structurally homogeneous landscapes, but not in complex, structurally heterogeneous landscapes, where contiguity of very distinct habitats may create entirely different conditions for intraguild predation. However, despite its importance (29), no previous study of the effects of vertebrate predators on pest control has taken into account the landscape context in a replicated design (21,22).…”

“…The direction of these responses depends on the type of interaction occurring between enemy species or functional groups, which may be antagonistic, neutral, additive, or synergistic and involve intraguild predation, functional redundancy, niche partitioning, or facilitation, respectively (11,12,20). In contrast, little is known about the role of landscape context in determining trophic interactions and their consequences for ecosystem service provision (15,18,21,22), despite the documented importance of landscapes in regulating species functional diversity (5,6,(23)(24)(25). Given the implications for landscape-wide effectiveness of biological control and other biodiversity-dependent services, scaling up the interaction mechanisms of functional species groups to a landscape perspective is critical for the development of sustainable management strategies (6,13,15,26) as well as for increased predictability of ecosystem functioning (9,13,16,18,19).…”