Jonsson M 2012 Biodiversity Loss & the Functioning of Ecosystems. ECOLOGY.INFO 30

Biodiversity Loss and the Functioning of Ecosystems

Micael Jonsson
Department of Ecology and Environmental Science
Umeå University
Sweden

Note: This online review is updated and revised continuously, as soon as results of new scientific research become available.  It therefore presents state-of-the-art information on the topic it covers.

Should we worry about species becoming extinct? Until recently, diversity of life had been increasing to the highest known levels in Earth’s history (Chapin et al. 2000). However, human exploitation of nature has had, and continues to have, detrimental consequences for global biodiversity. As many as 150 unique types of organisms are estimated to go extinct every day (Lamont 1995). It is a fact that many species of plants and animals are disappearing, and will continue to do so, due to past and present human activities (Chapin et al. 1996), but does this loss affect how ecosystems function and influence human welfare?

Almost two million species are known to science, but at least 10 (and maybe even up to 30) million species are thought to exist (May 1990). With this great number of species, and the vast diversity it represents, does it really matter if we lose a few, or many for that matter? After all, extinction is a natural process – over 99% of all species that ever existed are today extinct (Leakey 1996). Furthermore, many species are considered to be redundant (Walker 1992), meaning that they perform the same functions within an ecosystem. Thus, losing all but one species performing a certain function should not matter. Or should it?

First, any possible negative effect on ecosystem functioning is due not only to the loss of species per se, but to the rate at which they are currently disappearing. Today, species disappear 100-1,000 times faster than before human existence, and the additional loss of currently threatened species could accelerate this loss significantly (Chapin et al. 1998). Also, for every 10,000 species that go extinct, only one new species evolves (Chapin et al. 1998). Thus, the current rate of biodiversity loss greatly exceeds the rate that nature can compensate for, and adapt to.

Second, redundant species may to some extent buffer against changes of ecosystem function in the event of species loss. However, organisms classified by us as identical in function have many times been shown to differ enough to be of significant importance for ecosystem functioning. Even if some species are redundant in terms of the function they perform, they often have different environmental optima, thus buffering against ecosystem changes in the event of changed environmental conditions (Chapin et al. 1995). Hence, species loss may not only have direct effects on an ecosystem, but also consequences for its buffering capacity against future changes in the environment.

So, we have established that species are disappearing faster than ever before, that nature cannot keep up with this great rate of extinction, and that ecological equivalent species (if there is such a thing) are important for buffering against future changes in the environment. Thus, there are reasons to worry. But, is there any proof that biodiversity loss has negative effects on the functioning of ecosystems? There are at least some indications, and in the following text I will briefly discuss some results from studies where effects of biodiversity loss have been investigated.

Investigations on effects of biodiversity loss

Although several studies, especially in agricultural science, empirically investigated the importance of multi-species assemblages long ago, it was not until the beginning of the 1990’s that the first studies specifically testing for effects of biodiversity loss on ecosystem processes and functioning were published. Since then, research in the so-called Biodiversity-Ecosystem Functioning (BD-EF) field of ecology has increased dramatically (see Loreau et al. 2001, 2002, for reviews). Despite some problems with experimental designs, statistics and extrapolation of results to natural systems, progress has been made. In what follows, I list and discuss what I consider to be the major achievements of these studies.

Biodiversity matters

The earliest empirical contributions to the field of BD-EF were published in the middle of the 1990’s (Tilman and Downing 1994, Naeem et al. 1994, 1995). Both of these studies concluded that biodiversity mattered for ecosystem functioning. The study by Naeem et al. (1994, 1995) was performed in the Ecotron, England, on artificial ecosystems comprised of several trophic levels (i.e. primary producers, consumers and predators) containing low, medium, or high biodiversity. Biodiversity was found to significantly affect several different ecosystem processes, and some processes increased with biodiversity while others decreased. Tilman and Downing (1994) performed their study on grassland ecosystems at Cedar Creek, Minnesota. In their study, they used experimental treatments containing one to 24 species, and found that both productivity and retention of soil nutrients increased with plant diversity. These studies received much attention when they were published and hence were of great importance for boosting BD-EF-research, and increasing awareness of the consequences of biodiversity loss, both in the scientific community and among decision makers. They also provided a good foundation for future research.

Experimental design matters

Following these first empirical studies on the effects of species loss, there was some debate about what caused the results (Aarsen 1997, Huston 1997). One suggestion was that, instead of biodiversity per se, a few species with strong impacts on ecosystem processes, and the increased probability that these species were included in high diversity assemblages, could be responsible for the correlations between biodiversity and ecosystem functioning. In other words, the results could be fabricated by the experimental design (i.e. ‘the sampling effect’). However, other ecologists argued that the importance of particular species and their higher rate of occurrence in more specious assemblages could be an important property of natural systems as well (Tilman et al. 1997). This issue was somewhat resolved when statistical techniques for separating effects of biodiversity and particular species were presented (Jonsson and Malmqvist 2000, Loreau and Hector 2001). Furthermore, the importance of particular species and particular compositions of species should also be of interest in studies on what affects ecosystem functioning. In any case, this debate was important since it led to more solid experimental designs of biodiversity effects.

Species redundancy

Some have argued that it is not biodiversity per se, but functional group diversity that is important for ecosystem functioning. This argument is based on the belief that species belonging to the same functional group are redundant. According to this line of reasoning, species may be lost without any effect on ecosystem functioning, as long as each functional group is represented by at least one species.

However, even though species are seemingly redundant in terms of the function they perform, there are many other ways in which they may be different, i.e. activity in time and space, environmental (climatic) preferences, specific choice of prey, vulnerability to predators, and so on. Supporting the notion that seemingly redundant species differ in enough ways to each be important for how ecosystems function, are studies that have investigated the effects of biodiversity loss within functional groups (e.g. Jonsson and Malmqvist 2000, Jonsson et al. 2001, Cardinale et al. 2002, Dangles et al. 2002, Huryn et al. 2002, Jonsson et al. 2002, Jonsson and Malmqvist 2003a, b). These studies found strong effects of changed biodiversity even though the species used performed the same function. Hence, in addition to definite effects on ecosystem functioning when the last species in a functional group is lost, species loss within functional groups is also of significant importance. Although some of these studies have found increased ecosystem functioning with decreasing biodiversity, they still show that species redundancy, in this sense, is a dysfunctional concept.

Furthermore, redundant species may act to some extent as biological insurance, buffering against changes in ecosystem functioning when environmental conditions change. For example, imagine that two seemingly redundant species (A and B) perform a function and that species A dominates in abundance over species B, since the present environmental conditions favour species A. Then, when the environment changes so that the new conditions favour species B and the performance of species A declines, species B then increases in abundance and performance so that the functioning of the system remains unchanged. If species A had been the only species in the system at the time of environmental change, there would have been a loss of ecosystem functioning. Thus, in this sense, species redundancy is an important trait of natural systems.

Mechanistic explanations to biodiversity effects

Exploring the mechanisms behind effects of biodiversity loss is of critical importance if we are to understand the consequences of the current, rapid biodiversity loss. Niche complementarity is often used as the most likely explanation for effects of changed biodiversity, especially if both ‘niche differentiation’ and ‘facilitation’ are included in the definition (e.g. Loreau and Hector 2001). The characteristics of a species determine how, when and where it utilises resources (the niche). While all individuals within a species share these characteristics, they often differ between species (niche differentiation). Thus, niche differentiation allows species to coexist, to avoid strong competition, and thus to perform a process efficiently (e.g. Volterra 1926, Lotka 1932, Jonsson and Malmqvist 2003a). Loss of species may therefore lead to fewer utilised niches, stronger competition, and lower process rates thus affecting ecosystem functioning negatively. Positive interactions between species, such as facilitation, have the potential to be of great importance for ecosystem functioning. Although several studies have found evidence of facilitation between pairs of species (e.g. Soluk and Collins 1988, Kotler et al. 1992, Soluk 1993, Soluk and Richardson 1997, Cardinale et al. 2002, Jonsson and Malmqvist 2003a), it is not well known how common or important such interactions are in natural ecosystems. However, both niche differentiation and facilitation are likely to be important for maintaining process rates and ecosystem functioning. Thus, in the event of species loss, ecosystem functioning could be negatively affected either by increased competition, niche vacancy, or loss of facilitative interactions.

Investigating random or natural biodiversity loss

To actually test for effects of biodiversity, a study has to use species drawn randomly from a large species pool. Most studies, however, have used particular species, or random species compositions from smaller species pools, thus not being able to draw conclusions regarding effects of biodiversity per se. Instead, the results may be relevant only to the species used in the study. Although it may be interesting to investigate if there are any general effects of biodiversity loss on ecosystem functioning by using species at random, species extinction often follows predictable patterns depending on the species in the system and the type of perturbation. Thus, the most relevant way to study effects of biodiversity loss may be to either subject a natural community to a perturbation (Petchey et al. 1999), or to use a predicted order of extinction (Jonsson et al. 2002). This, of course, limits the general applicability of the results, but at the same time it gives more realistic results and specific knowledge of the effects of species loss in the system studied.

Extrapolation of experimental results to natural systems

The persistence of the biodiversity effects observed in controlled, short-term experiments has been questioned (e.g. Symstad et al. 2003). Since most studies to date have been performed over relatively short periods of time, it is not well known if the (initial) effects are transient or persistent, and thus if they are relevant to effects of biodiversity in natural systems. However, in a long-term, grassland study it was found that the initial effect of biodiversity persisted over time although the underlying mechanisms changed (Tilman et al. 2001). Another problem with most studies so far is that, while natural systems often are highly complex, experimental set-ups have used relatively few species and trophic levels. Studies using low complexity often have obtained quite straightforward results, but results from more complex experimental systems have been difficult to interpret. Thus, there is a trade-off between complexity and interpretability of results, and there are still no good solutions to this problem although attempts to perform useful studies on complex systems are being made (see Finke and Denno 2004, for one example).

The future

So far, studies have shown that biodiversity matters for ecosystem process rates and ecosystem functioning – at least on relatively small spatial scales and over short periods of time. Furthermore, evidence for mechanisms behind biodiversity effects have been found. Thus, the challenge for future studies will be to expand in space, time and complexity so that the obtained results are more relevant to natural systems. The question if, and how, biodiversity matters for the functioning of ecosystems is one of the most important questions in ecology today. Because the current loss of biodiversity seriously  threatens the services that well-functioning ecosystems provide to humanity (Luck et al. 2003), preserving biodiversity may also help us to preserve humanity.

Editor's Note:  ECOLOGY.INFO also publishes a poem about biodiversity loss.  To read this poem, click the following link: 
Daffodils No More.

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Jonsson M  2012   Biodiversity loss and the functioning of ecosystems.  ECOLOGY.INFO 30.

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