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Habitat destruction is the major cause of species extinctions1?3. Dominant species often are considered to be free of this threat because they are abundant in the undisturbed fragments that remain after destruction. Here we describe a model that explains multispecies coexistence in patchy habitats4 and which predicts that their abundance may be fleeting. Even moderate habitat destruction is predicted to cause time-delayed but deterministic extinction of the dominant competitor in remnant patches. Further species are predicted to become extinct, in order from the best to the poorest competitors, as habitat destruction increases. More-over, the more fragmented a habitat already is, the greater is the number of extinctions caused by added destruction. Because such extinctions occur generations after fragmentation, they represent a debt?a future ecological cost of current habitat destruction.

References

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Tilman, D., and D. Wedin. 1991a. Plant traits and resource

reduction for five grasses growing on a nitrogen gradient.

Ecology 72:685–700.

Tilman, D., and D. Wedin. 1991b. Dynamics of nitrogen

competition between successional grasses. Ecology 72:

1038–1049.

Wedin, D. A. and D. Tilman. 1996. Influence of nitrogen loading and species composition on the carbon balance of grasslands. Science 274: 1720-1723.

Tilman, D., D. Wedin, and J. Knops. 1996. Productivity and

sustainability influenced by biodiversity in grassland ecosystems.

Nature 379:718–720.

Abstract

The functioning and sustainability of ecosystems may depend on their biological diversity1?8. Elton's9 hypothesis that more diverse ecosystems are more stable has received much attention1,3,6,7,10?14, but Darwin's proposal6,15 that more diverse plant communities are more productive, and the related conjectures4,5,16,17 that they have lower nutrient losses and more sustainable soils, are less well studied4?6,8,17,18. Here we use a well-replicated field experiment, in which species diversity was directly controlled, to show that ecosystem productivity in 147 grassland plots increased significantly with plant biodiversity. Moreover, the main limiting nutrient, soil mineral nitrogen, was utilized more completely when there was a greater diversity of species, leading to lower leaching loss of nitrogen from these ecosystems. Similarly, in nearby native grassland, plant productivity and soil nitrogen utilization increased with increasing plant species richness. This supports the diversity?productivity and diversity?sustainability hypotheses. Our results demonstrate that the loss of species threatens ecosystem functioning and sustainability.

References

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Turelli, M. 1981. Niche overlap and invasion of competitors

in random environments. 1. Models without demographic

stochasticity. Theoretical Population Biology 20:1–56.

Abstract

The relationship between persistent, small to moderate levels of random environmental fluctuations and limits to the similarity of competing species is studied. The analytical theory hinges on deriving conditions under which a rare invading species will tend to increase when faced with an array of resident competitors in a fluctuating environment. A general approximation scheme predicts that the effects of low levels of stochasticity will typically be small. The technique is applied explicitly to a class of symmetric, discrete-time stochastic analogs of the Lotka-Volterra equations that incorporate cross-correlation but no autocorrelation. The random environment limits to similarity are always very close to the corresponding constant environment limits. However, stochasticity can either facilitate or hinder invasion. The exact limits to similarity are extremely model-dependent. In addition to the symmetric models, an analytically tractable class of models is presented that incorporates both auto- and cross-correlation and no symmetry assumptions. For all of the models investigated, the analytical theory predicts that small-scale stochasticity does little, if anything, to limit similarity. Extensive Monte Carlo results are presented that confirm the analytical results whenever the dynamics of the discrete time models are biologically reasonable in the sense that trajectories do not exhibit unrealistic crashes. Interestingly, the class of stochastic models that is well behaved in this sense includes models whose deterministic analogs are chaotic. The qualitative conclusion, supported by both the analytical and simulation results, is that for competitive guilds adequately modeled by Lotka-Volterra equations including small to moderate levels of random fluctuations, practical limits to similarity can be obtained by ignoring the stochastic terms and performing a deterministic analysis. The mathematical and biological robustness of this conclusion is discussed.

Usher, M. B. 1988. Biological invasions of nature reserves: A search for generalizations.

Biological Conservation. 44:(1-2) 119-135

Abstract

Each one of the 24 nature reserves in the preceding case studies has received introduced species of plants and vertebrates (and invertebrates where the data exist). Some of these have become invasive, although the probability that an island nature reserve is invaded is greater than a savanna or dry woodland. Arid lands and Mediterranean-type reserves showed a negative relationship between the proportion of species that are introduced and the reserve's area. Examples demonstrate that after a period of about 1000 years it is difficult to distinguish between native and introduced species.

Invasive species affect both the structure and function of an ecosystem. Management priority has to be given both to invasive species that threaten endemic species with extinction and to species that have a strong landscape effect. The cost of controlling invasive species can utilise a large proportion of a reserve manager's recurrent budget. Tourism poses dangers for reserves since there is a positive correlation between visitation rate and the number of introduced species.

The most important generalisation is that all nature reserves, except those in Antarctica, appear to have invasive species. Managers face the problem of how best to conserve ecological variety.

Badgery, W.B., Kemp, D.R., Michalk, D.L., King, W.M.C.G. 2005.

Competition for Nitrogen between Australian Native Grasses and the Introduced Weed Nassella trichotoma (summary not full paper)

Annals of Botany 2005 96(5):799-809;

• Background and Aims Nassella trichotoma is an unpalatable perennial grass weed that invades disturbed native grasslands in temperate regions of south-eastern Australia. This experiment investigated whether elevated N levels, often associated with disturbance, increases the competitiveness of N. trichotoma relative to C3 and C4 native Australian grasses.

• Methods A pot experiment investigated competitive interactions between four native grasses, two C3 species (Microlaena stipoides and Austrodanthonia racemosa) and two C4 species (Themeda australis and Bothriochloa macra), and N. trichotoma at three different N levels (equivalent to 0, 60 and 120 kg ha–1) and three competing densities (zero, one and eight neighbouring plants), using an additive design.

• Key Results All native grasses were competitive with N. trichotoma at low N levels, but only M. stipoides was competitive at high N. High densities of native grasses (8 : 1) had a major competitive effect on N. trichotoma at all N levels. The competitive ranking of native grasses, across all N levels, on N. trichotoma was: M. stipoides > A. racemosa > B. macra > T. australis. The C3 species were generally more competitive than the C4 species and C4 grasses were not inherently more productive at low N levels, in contrast to the results of other studies.

• Conclusion To resist invasion from N. trichotoma, these native grasses need to be maintained at a high density and/or biomass. The results do not support the theory that species such as N. trichotoma, with high tissues density, are always less competitive than those of low tissue density; in this case competitiveness depended on N levels. The ability of N. trichotoma to accumulate biomass at a higher rate than these native grasses, helps to explain why it is a major weed in disturbed Australian native grasslands.