Umesh Srinivasan

A slippery slope: logging alters mass–abundance scaling in ecological communities

Summary 1. Natural ecosystems face ever-increasing anthropogenic threats from activities such as logging. It is therefore important to: (i) understand anthropogenic impacts on key community characteristics with implications for community structure and function and (ii) identify metrics with mechanistic underpinnings, providing a more functional understanding of anthropogenic impacts on biodiversity. Mass–abundance scaling is the most fundamental property of ecological communities, with implications on energy and resource partitioning and on how species and population traits translate to community structure and function. 2. In habitat patches representing a continuum of logging intensity, I examined the impact of logging on mass–abundance scaling in understorey bird species using mist netting and bird ringing. I used regression quantiles to estimate the slope and intercept of upper bound of the polygonal local-scale mass–abundance relationship. 3. I show that this slope becomes more negative (and the intercept higher) as logging intensity increases. Logging might therefore significantly alter resource and energy partitioning among species in natural communities. 4. The mechanism underlying this pattern is likely to be an interaction between resource depletion, body size and density compensation. Larger species face more severe resource shortages from depleted resource availability and are expected to decline in abundance following logging, leading to more negative mass–abundance scaling. Density compensation by smaller species following large species decline can ‘push up’ the intercept of the mass–abundance relationship. This might result in both steeper slopes and higher intercepts in logged habitats. 5. Synthesis and applications. This study shows for the first time that anthropogenic habitat change can alter fundamental community properties such as mass–abundance scaling and that this property of ecological communities was better at detecting logging impacts than other standard community measures. Identifying key metrics that provide a functional understanding of anthropogenic impacts on biodiversity is a crucial need, for both the assessment of these impacts and the continued monitoring of habitat change. This study highlights the need to supplement commonly used community descriptions with more mechanistic measures of human impacts on biodiversity.

To Eat and Not Be Eaten: Modelling Resources and Safety in Multi-Species Animal Groups

Using mixed-species bird flocks as an example, we model the payoffs for two types of species from participating in multi-species animal groups. Salliers feed on mobile prey, are good sentinels and do not affect prey capture rates of gleaners; gleaners feed on prey on substrates and can enhance the prey capture rate of salliers by flushing prey, but are poor sentinels. These functional types are known from various animal taxa that form multi-species associations. We model costs and benefits of joining groups for a wide range of group compositions under varying abundances of two types of prey–prey on substrates and mobile prey. Our model predicts that gleaners and salliers show a conflict of interest in multi-species groups, because gleaners benefit from increasing numbers of salliers in the group, whereas salliers benefit from increasing gleaner numbers. The model also predicts that the limits to size and variability in composition of multi-species groups are driven by the relative abundance of different types of prey, independent of predation pressure. Our model emphasises resources as a primary driver of temporal and spatial group dynamics, rather than reproductive activity or predation per se, which have hitherto been thought to explain patterns of multi-species group formation and cohesion. The qualitative predictions of the model are supported by empirical patterns from both terrestrial and marine multi-species groups, suggesting that similar mechanisms might underlie group dynamics in a range of taxa. The model also makes novel predictions about group dynamics that can be tested using variation across space and time.

Patterns of species participation across multiple mixed-species flock types in a tropical forest in northeastern India

We studied mixed-species bird flocks in northeastern India to (a) develop a framework for quantifying species participation in mixed-species flocks, (b) characterize the ecology and behaviour of participant species, and (c) explore mechanisms influencing the coexistence of different mixed-species flock types. To characterize participation in mixed-species flocks, we implement a new method incorporating species abundances, minimizing potential biases in measuring participation arising from differences in the availability of flocking species. There are at least three distinct flock types in the lowland forests of northeastern India; these flock types differ in the body mass and vertical stratum use of participant species. The “core” of mixed-species flocks was composed of a species group that differed much more in their foraging method in comparison with “attendant” species. The exchange of benefits and minimization of interspecific competition might lead to, and maintain, heterogeneity in foraging methods among core species of mixed-species flocks.