Getting over the Hump

For decades, ecologists have toiled to nail down principles explaining why some habitats have many more plant and animal species than others. Much of this debate is focused on the idea that the number of species is determined by the productivity of the habitat. Shouldn't a patch of prairie contain a different number of species than an arid steppe or an alpine tundra?

Exactly how biodiversity relates to productivity, however, has been a highly controversial issue in ecology over the past few decades. Initially, for example, many researchers believed that the relationship between species richness and net primary productivity (often expressed as the number of grams of carbon produced within a square meter of an ecosystem over a year) could be visualized as a 'hump-shaped' curve, with richness first rising (as stress is alleviated) and then declining with increasing productivity (due to increasing effects of competition).

I've just been involved in a global study that tries to shed light on the hump-shaped relationship between species richness and productivity in grasslands across the globe. Our paper in Science has just been published and the results were, to put it mildly, rather surprising.

 

Locations of the 48 sites that provided data for this study



In a multiscale assessment of 48 herbaceous plant communities on five continents (including four Australian sites), we demonstrate that the modal productivity-diversity pattern is quite rare in nature, rather than the dominant relationship. This is really interesting given that so much attention had been devoted to this theory.



Our study shows no clear relationship between productivity and the number of plant species in small study plots when you look at data collected from across a range of grassy vegetations from across the globe. These range in temperature, rainfall and geological and evolutionary history. Indeed, the evidence for the predicted 'humped-back' reponse is rather weak within most of the sites studied regardless of these big differences in history.



Within-site relationships between productivity, measured as peak live biomass (dry weight)

and species richness. The inset shows the frequencies of relationships that were

nonsignificant (NS, thin dashed lines), positive or negative linear (thick dashed lines),

and concave-up (+) or -down (–) (solid curves). The marginal histograms show the frequency

 of species richness and peak live biomass across all sites.



The findings suggest that ecological understanding may advance more rapidly if ecologists focus on exploring a range of topics that are germane to the productivity-diversity relationship in a changing world (e.g. how will loss of species affect productivity, stability and resilience of ecosystems to change), rather than continuing the search for a dominant pattern ("the Holy Grail"). Our work not only sheds light on this classic question, it also demonstrates the power of a network approach. Working in a network with ecologists who share your passion and interest in theoretical ecology is a great way to facilitate insights into the functioning of nature, insights that aren't possible in a focus on individual ecosystems.

Here is a link to the paper:

And an accompanying Perspectives piece

And an NSF press release:

Offspring for the next generation

One of the interesting questions our Lab asks is: how is it that many plant species can coexist together at high density? In the diverse woodlands of western Victoria, for instance, Brandon Schamp, Jodi Price and I are trying to understand just how diversity is partioned in space (due to microsite variation - or 'niche availability') and time (how fluctauting resources such as rainfall affect coexistence). These are two really valid approaches to studying patterns of diversity at small scales, but I've recently started to think about this problem in another way.

Diversity in woodlands in southern Australia can be very high
at small scales: up to 45 species per square metre (Photo: John Morgan)


  Within the crowded natural plant populations of species that we work with, the traditional prediction is that most of the offspring from which future generations are drawn will be contributed by the relatively few individuals belonging to the larger size classes. Yet, the extent to which this is true should depend on whether the inevitably more numerous, but relatively small suppressed plants within the population manage not only to survive suppression, but also to reproduce before death. If they can do this, then species coexistence should be ensured because R (the intrinsic rate of growth) >0 for most of the coexisting species.

Hence, resident species are successful not because they are relatively large, but because they produce numerous descendants from numerous (often small) offspring. Lonnie Aarssen has coined the term 'reproductive economy' to describe this phenomenon. So, if species can survive to reproduction despite the clear disadvantages of growing at high density, there is an opportunity for that species to maintain itself in that space - and if many species can do this, then there is a simple mechanism to explain how species can coexist.

We don't have much data on reproductive success and size of individuals in a population. Last year, we harvested all flowering individuals of four herbaceous species in grassy woodlands in plots that were about 100 m2 in area. We then dried and weighed these individuals and plotted the frequency of plants by weight. The data for one species (Arthropodium strictum) is shown below (and yes, the sample size is only 87 - but we are collecting more data this year).



The number of flowering plants by their size in one population.
Note: most reproductive plants in this population are small.



What appears to be clear from this data is that the reproductive size profile for this species is conspicously right-skewed. Most flowering plants are small, but these numerous plants are likely to contribute lots of seed to this population. This is better than leaving no progeny for the next generation. Hence, if small plants of many species can successfully reproduce at high density, this hints that species might be able to persist. Rather than being the weaklings, their collective reproductive output vastly exceeds that of their larger (but fewer) conspecific neighbours.

We intend to collect data for a large range of species this spring and see if most individuals are (a) small but (b) successful at flowering and if so, to quantify just how many seeds enter a population and how many are contributed by the smallest of the individuals. Stay tuned.