In this post, Functional Ecology provides an introduction to each article that can be found in our Animal Functional Traits Special Focus. This collection of studies shows how precise measurements of morphological or physiological traits can increase mechanistic understanding of community assembly across trophic levels, particularly of the mechanisms underpinning large-scale biodiversity patterns. Further, a clearer picture is emerging of systematic animal responses to environmental change that shape the trait composition of ecological communities and affect ecosystem functioning.
Future efforts should narrow the gap in the availability of trait data and concepts between plants and animals. By bridging this gap, functional ecology can take a crucial step forward and fundamentally improve analyses of the structure and function of whole ecosystems.
1) Animal functional traits: towards a trait-based ecology for whole ecosystems
With this Special Focus, we showcase a collection of studies on animals ranging in size from springtails to crocodiles and from primary consumers to apex predators. In the context of these studies, we use this editorial to consider three prominent questions guiding current and future research on animal functional traits. (1) How can we use animal functional traits to expand insights to whole food webs and to the critical interactions between plants and animals, including herbivory, pollination and seed dispersal? (2) Can we use these insights to better understand global patterns of biodiversity? And (3) is it possible to leverage functional traits to improve predictions of how biodiversity will respond to environmental change and what this means for ecosystem functioning?
2) Local-scale temperature gradients driven by human disturbance shape the physiological and morphological traits of dung beetle communities in a Bornean oil palm–forest mosaic
The authors collected data on traits linked to thermal sensitivity (upper thermal limits, body size, cuticle lightness and hairiness) for 46 dung beetle species across a mixture of forest and oil palm plantation in Malaysian Borneo. They found that physiological thermal limits predict how species respond to maximum temperatures. Increased body size and decreased hairiness are also associated with higher temperatures, while cuticle lightness shows a complex relationship with temperature across the disturbance gradient. Their findings highlight the potential causes of the decline of sensitive species in human-modified landscapes that result in changes to community patterns.
3) Ecological strategies of (pl)ants: Towards a world-wide worker economic spectrum for ants
It has long been recognised that ants and plants have many similarities. This is because ants are sedentary, i.e., they are based around a central location, the nest. Further, ants divide the colony into individuals with different tasks, including some that reproduce (e.g., the queen) and some that (largely) cannot reproduce (workers). In this article, the authors explore how these similarities may lead to similar ecological strategies. They suggest how many of these strategies may be interlinked and highlight directions for future research into the ecological strategies of ants that may help to bridge the gaps in knowledge between plants and animals.
4) Towards a modular theory of trophic interactions
The authors created a framework to model predator-prey interactions across trophic levels. The reason this framework is needed is because, at the moment, whenever someone divides up an interaction to build a model like this, they divide it based on the particular interaction and traits they’re studying. While that makes a lot of sense for an individual study, it makes it very difficult to compare across, and build from, different studies to gain an overarching understanding of how traits affect interactions. The use of this framework with eight explicit steps will ensure that we consider clearly which steps are most important for a given system, that we can easily compare across systems, and that we can borrow modules from one system to apply to another system.
5) Trait-mediated responses to aridity and experimental drought by springtail communities across Europe
In this paper the authors investigated the combined effects of regional aridity gradients and local drought episodes harnessing data from standardized experiments of climate manipulation distributed across six European shrublands. They crossed community data (near 20K specimens classified into 102 species) with 22 ecological and morphological traits and reconstructed their evolutionary relationships by means of short DNA sequences to track the evolution of adaptations to live at different soil depths, which is key to cope with desiccation.
They found a strong convergent evolution, so that the species from all three major collembolan lineages developed the same adaptations to live at different soil layers. The regional aridity gradient promoted the traits of the species living on the surface or in the topsoil, while the experimental drought selected those species living deeper in the soil profile, thus confirming our expectation of a disparity between the effects of long and short-term environmental gradients. This suggests that the effects of our short-term climate change experiments over springtails communities may be transient community responses.
6) Towards an animal economics spectrum for ecosystem research
In analogy to the plant economics spectrum, the authors propose the animal economics spectrum (AES) based on broadly available traits related to organismal size, biological times and rates. These traits include adult and offspring body mass, life span, age at first reproduction, and reproductive and metabolic rates. They compiled these traits for 98 terrestrial taxa from seven selected animal classes that were mapped into an exemplary quantitative trait space.
They suggest future steps towards an application of the AES should aim at the understanding of ecological processes and ecosystem functions, which facilitates predictions of how the functional composition and diversity of animal communities can be affected by global change. Finally, they urge the compilation of databases of comparable functional traits across the animal kingdom and beyond, with the ultimate goal to map the economic spectrum of life.
7) Geographical variation in the trait-based assembly patterns of multitrophic invertebrate communities
The authors surveyed aquatic invertebrates (largely insects, but also worms, leeches, snails, crustaceans) in the rainwater-filled leaf axils of bromeliads. These miniature animal communities occur naturally in bromeliads throughout the Neotropics, and we sampled bromeliads at 26 sites from Argentina to Mexico. They tested if trait characteristics of invertebrate communities at each site changed in a similar fashion along three common environmental gradients.
Their results suggest that, while the traits of animal communities may differ by geographic location, general patterns in traits do exist once we take into account biogeography and climate.
8) Variable relationships between trait diversity and avian ecological functions in agroecosystems
For two years, the authors focused on birds of woodland pastures and cider apple orchards, two common agroecosystems in Asturias (northern Spain). They evaluated the direct and indirect (i.e., biodiversity-mediated) effects of environmental gradients (forest cover and resource availability) and biodiversity components (trait diversity, phylogenetic diversity, and abundance) on two relevant avian ecological functions, i.e. seed dispersal and insect predation, across agroecosystems.
Their findings reveal that trait diversity showed positive effects on insectivory but almost neutral effects on seed dispersal, which was mostly affected by species abundance. They found a strong effect of environmental gradients of forest cover and resource availability as filters of all biodiversity components. Finally, environmental factors also showed a direct effect on seed dispersal, but not on insect predation.
9) Functional diversity and trait filtering of insectivorous bats on forest islands created by an Amazonian mega dam
In this study, the authors used bats that consume insects during flight as a model group and compared the morphology of the bats that inhabit the islands with those of the adjacent continuous forest in the Balbina dam, located in the Brazilian Amazon. They measured the wing shape, weight, characteristics of echolocation calls and the height of the forest that bats fly. They also related bat morphology to vegetation characteristics.
Their results indicate that only bats with specific characteristics are able to use isolated small islands, while a greater variety of bat morphologies can be found on large islands and continuous forest. Therefore, they recommend that, when designing and licensing new dams, extensive protected areas around the dam need to be created to minimize the harmful impacts of small isolated islands and to safeguard the full complement of key ecological functions provided by insectivorous bats.
10) Using functional traits to identify conservation priorities for the world’s crocodylians
To understand better the functional diversity of crocodylians, the authors collected a database of functional traits of all species of crocodylian. These traits are measurable qualities – such as skull shape or saltwater tolerance – that allow us to understand the different ecological role of species, and how similar and different species are from one another. They found certain traits reduced species risk of extinction, in particular species that invest more in reproduction, are habitat generalists, and can tolerate climate extremes.
The authors predict 32-38% of crocodylian functional diversity is likely to be lost within the next hundred years, with Asia being a hotspot of predicted losses. They calculated values that allow us to prioritise conservation of functionally unique species at high risk of extinction (EcoDGE species). They found conserving species such as the gharial and Chinese alligator would have a positive impact on conserving global crocodylian functional diversity.
11) Is species richness mediated by functional and genetic divergence? A global analysis in birds
Here the authors looked at the two principal dimensions of species’ ecology: functional traits (aspects of species external morphology that determine the way in which they move, where they live, and how they eat), and their genetic diversity (how much variability there is in their DNA). The authors used a collection of morphological measurements for every living species of bird, and over 100,000 genetic sequences collected by researchers over multiple decades.
Their results suggest that variation in biodiversity across the globe is not impacted by changes in species ecology to facilitate coexistence as birds can apparently accumulate more species within a region without any readily apparent costs. Instead, alternative factors; for example, the different rates at which new species form may influence global patterns of biodiversity.
12) Community-wide trait adaptation, but not plasticity, explains ant community structure in extreme environments
In this study of ants in Canada, the authors investigated how changes in selection pressures across a latitudinal gradient influence community-wide mean trait values, and how an increase in environmental stress in northern latitudes influences trait variance, whether that is by (1) reducing trait variance by selecting only species with optimal trait values or (2) increase trait variance by selecting for species with high trait plasticity.
Their findings suggest that extreme environments exert a strong selection pressure, causing shifts through adaptation but not plasticity in community-wide optimal trait values, which allow species to persist under these climatic conditions. Combining spatial and community-wide intraspecific functional trait data provides a promising way forward to gain new insights into the processes underpinning community structure along environmental gradients.
13) High-resolution 3D forest structure explains ecomorphological trait variation in assemblages of saproxylic beetles
The authors showed that saproxylic beetle communities are indeed shaped by the environment, with species with certain traits more prevalent in certain forest types. Beetle traits related to their ecological functions (e.g., preference for wood in a certain stage of decay) influenced the presence of individual species at sites more than did the traits related to their body measurements. However, much remains unexplained, and we still know very little about which additional traits could potentially better predict species’ presence and abundance. This work has important implications for future changes in climate and forest structure, because we demonstrate that such changes may influence ecosystem functions by altering the trait composition of beetle communities.
14) Bird extinctions threaten to cause disproportionate reductions of functional diversity and uniqueness
The authors’ analyses show that birds with the most unique traits are the ones under most immediate threat of extinction. They also find that extinction of threatened bird species would result in unexpectedly large losses of morphological diversity. In other words, the loss of morphological diversity under projected extinctions is much larger than if species extinctions were occurring randomly. They also find evidence that this is not simply because extinction risk increases with body size, but because more specialised birds with relatively unique shapes are more highly threatened. Altogether, their results show that we risk losing many distinctive bird species to extinction along with their unique ecological roles, potentially resulting in less resilient ecosystems.
15) Interactions between protea plants and their animal mutualists and antagonists are structured more by energetic than morphological trait matching
Little known whether similar types of functional traits shape mutualistic and antagonistic plant-animal interactions. The authors addressed this question by studying Protea plants and their mutualistic pollinators, as well as their antagonistic seed herbivores in South African heathlands. Protea plants generate copious amounts of nectar, show a morphological diversity and are pollinated by insects, birds and mice. Furthermore, most Proteas retain their seeds until after a bushfire. Hence, the seeds remain in cones on the mother plant for years and are therefore an important food source for many herbivorous animals in these nutrient-poor heathlands.
They found consistent effects of energetic traits on interactions between plants and their animal mutualists and antagonists. In general, large animals with high metabolic demands dominated energy-rich resources, whereas smaller animals were displaced towards less rewarding resources. In contrast, effects of morphological traits on plant-animal interactions were more variable. Inflorescence shape and morphological defense against seed predation were important, but the shape of relationships was variable across animal taxa and interaction types.
16) Genome size does not influence extinction risk in the world’s amphibians
Could scientists predict a species’ risk of extinction from the size of their genomes? Amphibians (frogs, salamanders, caecilians) stand out as unparalleled models to test this theory – these animals not only have the largest range of genome sizes among vertebrates, but also their alarming rates of human-driven extinctions have made them the ‘poster child’ of the modern biodiversity crisis. Using the worldwide diversity of amphibians, this study presents the largest-scale test of the theory that larger genomes increase extinction risk. Contrary to expectations, no evidence to support this prediction was found. A number of factors, including the size of the geographic region occupied by species, their body size and UV-B levels help to explain risk of extinction. Yet, no such role seems to exist for larger genomes.
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