Does succession consistently improve ecosystem function?

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In a guest post, Timothy Crews quotes two representative statements from my book in which I argue that (in contrast to natural selection's improvement of individual adaptations) neither natural selection nor other natural processes have improved the overall organization of natural ecosystems over millennia. My doubts about "other natural processes" were based on actual data comparing the productivity and stability of natural and managed ecosystems. Dr. Crews argues that shorter-term ecological processes, specifically succession, may improve "ecosystem function with respect to agricultural goals." He doesn't seem to be claiming longer-term improvements, such as succession improving ecosystems more today than it did millions of years ago, which would be analogous to the longer-term improvement of individual adaptations by natural selection.

I don't doubt that some successional changes in some ecosystems qualify as improvements by criteria relevant to agriculture. So I agree that agriculture might "benefit from studies of niche complementarity" etc. As I wrote on page 1:

"Once we drop the assumption of perfection, however, we can learn much from studying natural communities."
"The assumption of perfection" may be an exaggeration of the viewpoint I intended to criticize. I could rephrase as "Once we drop the assumption that there are ecological processes that consistently improve the overall organization of natural ecosystems in ways that would make it safe for agriculture to copy their organization without first testing all the effects of that organization", but that seemed a bit lengthy for page 1.

But how consistently do successional processes improve productivity, stability, nutrient retention, etc.? If there's a good recent review of this question, I'd appreciate hearing about it. The papers I've found do not support the hypothesis that these measures of ecosystem performance consistently improve with succession.

Gower et al.(Gower et al. 1996) analyzed 13 datasets for forests around the world and reported that "Aboveground net primary production (ANPP) commonly reaches a maximum in young forest stands and decreases by O-76% as stands mature." In agriculture, productivity is arguably our most-important criterion, even for environmentalists, as it reduces the amount of land needed to grow a given amount of food. But other measures of performance are also important.

Crews' guest post emphasizes nutrient retention. Vitousek and Reiners (Vitousek and Reiners 1975) presented data compared nitrate loss in streams from younger versus older forests. The older forests lost more nitrate. They also discussed similar data from other forests. Similarly, Lamb (Lamb 1980) found higher nitrate levels in soils of older rather than younger tropical rainforests.

Comparisons among ecosystems are complicated by the possibility of differences unrelated to succession. The closest thing I know to a controlled comparison is that of Wardle et al.(Wardle et al. 1997) They compared islands whose differences in successional stage depended on how recently they had been struck by lightning and burned. This depended mostly on their size. Species diversity was greatest on islands in later successional stages, but "ecosystem process rates were lowest on those islands," perhaps because microbial biomass was greatest at early successional stages. The effects of succession on overall productivity and stability were apparently not measured, unfortunately.

Until I see some data to the contrary -- more than an isolated example or two -- I conclude that succession does not improve ecosystem function consistently enough that we can safely copy the overall organization of even late-successional ecosystems without extensive testing.

References

Gower, S. T., R. E. McMurtrie, and D. Murty. 1996. Aboveground net primary production decline with stand age: potential causes. Trends in Ecology & Evolution 11:378-382.

Lamb, D. 1980. Soil nitrogen mineralisation in a secondary rainforest succession. Oecologia 47:257-263.

Vitousek, P. M., and W. A. Reiners. 1975. Ecosystem succession and nutrient retention: a hypothesis. Bioscience 25:376-381.

Wardle, D. A., O. Zackrisson, G. Hornberg, and C. Gallet. 1997. The influence of island area on ecosystem properties. Science 277:1296-1299.


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"-- those projects are worthy of funding." With those words Timothy Crews finishes his guest post here. And Ford is also fond of making the case that future research funding would do well to invest in research which explores how evolution might be imitated or at least considered for its wealth of "wisdom" from such long term testing of various and sundry variation.

Without belaboring the setup, I want to complain here a bit about the use of 'natural' to indicate a system without us. A natural ecosystem being one without Homo sapiens, and natural selection being selection outside the hand of man. But we do seem to be accepting of leaf cutter ants "farming" their fungal counterparts as somehow being a natural ecosystem. [if they could blog, would it be cease to be a natural system?]

So Homo sapiens is an invasive species. So Homo sapiens have now seemingly ascended to a point where through their (our) collective action the whole of the Earth should tremble. Good grief. We evolved, right? Seems pretty natural.

So now let's return to the beginning and have a look at funding. Where does "funding" come from? Private property, commerce, capitalism, socialism, communism, public property, the commons... all manifestations of our human world view. We've created the concepts. And one can even make a pretty salient argument that some of these concepts have undergone a sort of selection (natural or otherwise) over human history.

Perhaps I'm too naive, but to me much of the 'value' in our arguments comes back to who is doing the valuing. A farmer in Iowa, USA will have a different value proposition than an ecologist in Australia. But both inhabit this same little blue marble. What to do?

The farmer in Iowa will fund his activities (or through market forces find extramural funding) - and if successful will continue to do so until he retires or fails to be successful. If the ecologist is independently wealthy, she could fund her work. But one imagines most modern research efforts are funded by outside sources of wealth. Grant writers compete for funding - and the effort takes on a particularly keen resemblance to selection. The fittest survive.

We are not at the end of the evolutionary road. The farms we Homo sapiens create take up significant stretches of the landscape. On a unit space measure these same farms are more productive today than they were in former times [though even here one can argue what this productivity means when all externalities are measured and different value propositions are considered]. But my point is with these farms we currently feed [most of] ourselves, thus allowing us 'funding' to continue to improve our systems with the hope of feeding ourselves into the future. And if we get good enough at it we might even spare some of the landscape for a couple other species.




In his response to my guest post, Denison asks the important and simple question “How consistently do successional processes improve ecosystem functions.” He presents the classic paper by Vitousek and Reiners (1975) as evidence that succession does not consistently improve one important function, which is nutrient retention. The truth is, I could not have selected a better paper to help unpack this topic further and demonstrate the opposite conclusion.

Vitousek and Reiner’s model is powerful in its simplicity. It describes how at the very beginning of primary succession, when there are no plants and only rock or mineral soil deposits, nutrient inputs (through weathering and atmospheric deposition) will equal nutrient losses (through leaching, runoff and denitrification). As plants become established and grow, and detritus begins to accumulate, nutrients losses from the ecosystem drop precipitously, especially the most limiting elements such as N and P, which are taken up, stored and then recycled in the aggrading biomass—in other words, the biomass functions as a nutrient sink. As long as the ecosystem continues to gain in biomass, both living and dead, nutrients will continue to be retained rather than lost. However, once the stature of an ecosystem levels off in succession, and productivity = ecosystem respiration, then the system returns to the original mass balance that existed at the beginning of succession when elemental inputs = outputs.

They go on to describe how when an ecosystem is disturbed, and secondary succession is initiated, there is the potential “for temporary, rapid losses of nutrients stored in detritus and soils, particularly in organic matter which may decompose rapidly under the conditions of devegetation. Rather sensational rates of nutrient loss can be found for instances in which clear cutting or other disturbances can permit rapid nutrient losses from detritus and soils before the rate of biomass uptake increases to where nutrients are utilized at the rate supplied by mineralization of detritus plus normal atmospheric inputs” (p. 377).

How might the Vitousek and Reiners model be relevant to agriculture? Developing highly productive agroecosystems that avoid the most leaky stages of succession would be one logical application. This is easier said than done given that annual agriculture is based on repeatedly resetting ecosystems back to early stages of secondary succession through plowing or herbicide applications. When native ecosystems are first cleared for agriculture, the net mineralization of soil organic matter (SOM) results in an inevitable flush of nutrients (as the quote above describes) and some fraction of these flushes is captured by crops over years or decades. Eventually, the soil organic matter pool approaches an equilibrium between biomass production and respiration, and the flush of free available nutrients comes to an end. Today, since in most cases the flush of nutrients ended long ago, fertilizers are used to support crop growth, but they are applied at a stage of ecosystem development that affords minimal biological control over nutrient retention. The consequences of farming in this leaky stage of ecosystem development include eutrophication of fresh and saltwater ecosystems, greenhouse gas production, aquifer contamination, depletion of fertilizer resources, reliance on energy-expensive inputs, soil acidification, and fertilization of lands downwind from farms and invasions by weedy species.

What would an agroecosystem look like that occupied the high retention and high net biomass increment slopes of the Vitousek and Reiners model? Without question, the establishment and proliferation of perennial growth forms in succession is central to the mechanisms that underlie the high nutrient retention stage of ecosystem development on land. This is, not surprisingly, one of the rationales for why The Land Institute and other researchers around the world are working to develop versions of perennial grain agriculture (see blog discussion between Denison and Van Tassel under category “Perennial Grains”). The reason Denison cited the Vitousek and Reniers model in the first place was to point out that late successional ecosystems might lose nutrients more than earlier successional ecosystems. But given that agriculture involves removal of moderate to large amounts of biomass in the harvest, there is good reason to believe that perennial agroecosystems would never reach the mature (and leaky) equilibrium stage of succession, but instead would be arrested in the aggrading biomass or nutrient sink stage indefinitely (where biomass production exceeds total respiration).

The successional process of resource partitioning that I discussed in my guest post is another mechanism that underlies the high nutrient sink stage of succession. It is likely less significant than perenniality, but appears to be important in many ecosystems.

The Vitousek and Reiners model of nutrient retention has proven to be very useful in framing a great deal of work over the last four decades. Even when the paper was written in 1975, the authors noted that the occurrence of the high nutrient retention stage of succession was “extensively documented” (see citations, p. 377). In general, it is hard to find studies that do not support the hypothesis. The one important exception that has been well-substantiated is that the conceptual model does not address the behavior of dissolved organic nitrogen (DON), which often leaches from forested ecosystems in concentrations that do not correspond to successional stage (Hedin et al. 1995, Vitousek et al. 1998, Goodale et al. 2002). The inability of forests and other mesic ecosystems to arrest leaching losses of DON illustrates a good example of where the functioning of nature is not particularly well-suited to agriculture. But overall, the contrast in how nutrient retention changes with ecological succession is a robust example of a predictable process that is not natural selection that consistently improves upon an ecosystem function and is highly relevant to agriculture.

Goodale, C.L., J.D. Aber & W.H. McDowell. 2000. The long-term effects of disturbance on organic and inorganic nitrogen export in the White Mountains, New Hampshire. Ecosystems 3:433-450
Hedin, L.O., J.J. Armesto & A.H. Johnson. 1995. Patterns of nutrient loss from unpolluted, old-growth temperate forests: evaluation of biogeochemical theory. Ecology 76:493-509
Vitousek, P.M., L.O. Hedin, P.A. Matson, J.H. Fownes & J.C. Neff. 1998. Within-system element cycles, input-output budgets, and nutrient limitation. Pp: 433-451 In: Successes, limitations, and frontiers in ecosystem science. M.L. Pace & P.M. Groffman (eds). Springer, Berlin.
Vitousek, P.M. & W.A. Reiners. 1975. Ecosystem succession and nutrient retention: a hypothesis. Bioscience 25:376-381

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