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.