Issue 104:4 is now online and the Editor’s Choice paper from this issue is Latitudinal variation in herbivory: hemispheric asymmetries and the role of climatic drivers by Zhang et al.
Associate Editor Richard Shefferson has written a post about the paper below.
The Big Bang and thereafter, across latitude
The Earth is a celestial object, and its position in the universe in many ways creates the conditions that have allowed intelligent life to develop here. Astronomy is currently enjoying a golden age of discovery, with astronomers now having found thousands of planets in other parts of the universe. The connection between biology and astrophysics is therefore of great importance to our understanding of life’s origins. However, our place in the universe also impacts the world’s ecological patterns. Although to many people, ecology and astronomy seem incompatible fields, nonetheless some of ecology’s longest standing questions have concerned patterns that are clearly due to some aspects of the Earth’s unique characteristics within the Solar System, and even the Milky Way galaxy.
In the 1770s, the German botanist Johann Forster joined the legendary Captain James Cook on a voyage around the world. This voyage led to many achievements, among them one of the earliest known observations of a latitudinal gradient in ecology: in this case, the observation that there were more plant species in the Tropics than in mid- to high-latitudes (Forster 1778). Thus began approximately 250 years of observation and conjecture into what is ultimately an astrobiological question – why does latitude, with its gradients of light, UV radiation, temperature, and moisture all dependent on Earth’s position and stable rotation around the Sun, affect biological organisms so strongly as to create ecological patterns in diversity? Indeed, life on Earth is only possible because of Earth’s astrophysical characteristics, such as its general shape, the stability of its axis, the strong tidal influence of the Moon, and, of course, the Earth’s distance from the Sun, and these same characteristics must somehow result in these gradients. It is only natural, then, that one of the oldest preoccupations of ecologists is in the impact of geography, particularly latitude, on ecological processes and phenomena.
In this month’s issue of Journal of Ecology, Zhang, Zhang & Ma (2016) present an ambitious attempt to understand a particularly important hypothetical latitudinal gradient in a major species interaction: herbivory. The authors note that although a hypothesized decrease in herbivory with increasing distance from the Tropics has been commonly accepted in the literature, only about 1/3 of empirical studies have supported it. In part, this acceptance might be due to the use of small-scale studies that are perhaps more likely to support these patterns occasionally simply due to sample size; it may also be due, at least in part, to the natural inclination many scientists have to view negative results as less important than positive results. The authors deal with these problems by collecting data on herbivory levels in plant populations and communities from around the Earth, yielding a huge, worldwide dataset (1890 data points from 291 studies and >1000 plant species). Using this dataset, they find support for a gradient of decreasing herbivory with latitude in the Northern Hemisphere, but not in the Southern. Further, herbivory increases with increasing mean temperature in the Northern Hemisphere, but decreases within increasing mean temperature in the Southern Hemisphere.
The inconsistency between the hemispheres is of particular significance to ecologists. Hypotheses to explain latitudinal gradients are many and varied, but most if not all involve ultimate mechanisms that rely on large-scale features of the Earth itself, and so should not vary with hemisphere (Cox, Moore & Ladle 2016). For example, the hypothesis that temperature, or length of growing season, drives latitudinal gradients should yield reasonably similar and consistent patterns moving northward or southward from the Tropics. In this study, however, temperature had effects separate from latitude, and these effects were in opposing directions in the two hemispheres.
Why do we such inconsistent patterns across the hemispheres? The authors suggest a number of potential hypotheses for exploration. First, leaf lifespans and other leaf traits may vary between hemispheres. This hypothesis suggests a potentially strong role for deep evolutionary branching in the terrestrial plants, since such ecological traits would vary in ways suggestive of differing evolutionary histories resulting from vicariance. Second, different patterns in biodiversity decline between the hemispheres might have impacts on herbivory both through herbivore communities and the plant communities that they feed on. This is also a credible hypothesis, although one would still need to explain why biodiversity declines differently in the two hemispheres. And, of course, it is entirely possible that the climates of sampled Southern Hemisphere sites do not strongly overlap with those of Northern Hemisphere sites, a situation that might impact statistical analyses. This is particularly possible given the strong difference in sampling effort between the hemispheres, with over 3 times more sites sampled in the North than in the South. It is clear that the large-scale sampling effort of this study should lead to further work to explain these fascinating patterns (it is my own hope that we will publish some of these follow-ups in the Journal of Ecology).
I think that Journal of Ecology readers will enjoy this strong foray into a classic question in ecology. Analyses involving large, worldwide datasets such as this may finally settle some long-standing controversies about global diversity patterns. Studies like this can also lead us further into the development of comprehensive tests of mechanisms resulting in latitudinal gradients – studies that are still sorely lacking in the literature. I would argue that ultimately, understanding the mechanisms resulting in these patterns will give us a far clearer understanding of our own place in the universe, and how likely complex life really is on other worlds.
Associate Editor, Journal of Ecology