Conclusions the need for a landscape perspective

The lottery hypothesis and the notion of the founder-controlled community were important steps in the development of our understanding of the range of community dynamics that can occur.

However, these should be viewed not as hard and fast rules to which some communities are subject, but rather as extremes on a continuum from dominance to founder control. Real communities may be closer to one or other end of this continuum, but in reality component species or component patches may be dominance controlled or founder controlled within the same community. Syms and Jones (2000), for example, acknowledge that more than half of within-reef variation in fish species composition in their study of patch reefs in the Great Barrier Reef was attributable to unexplained, and thus possibly stochastic, factors such as those emphasized in the lottery hypothesis. But a significant proportion of variation could be explained by specific habitat requirements of the constituent species.

More generally, no community is truly the homogeneous, temporally invariant system described by simple Lotka-Volterra mathematics and exemplified by laboratory microcosms, although some are less variable than others. In most real communities, population dynamics will be spatially distributed and temporal variation will be present. In a closed system, composed of a single patch, species extinctions can occur for two very different reasons: (i) as a result of biotic instability caused by competitive exclusion, overexploitation and other strongly destabilizing species interactions; or (ii) as a result of environmental instability caused by unpredictable disturbances and changes in conditions. By integrating unstable patches of either of these types into the open system of a larger landscape (consisting of many patches out of phase with each other), persistent species-rich communities can result (DeAngelis & Waterhouse, 1987). This is the principal message to emerge from the patch dynamics perspective, and its larger scale counterpart, 'landscape ecology' (Wiens et al., 1993), stressing the importance of the spatial scale at which we view communities and the open nature of most of them. Note the strong link between the patch dynamics view of community organization and metapopulation theory, which deals with the effects on the dynamics of populations of dividing them into fragments (see Section 6.9). In a model that combines extinction-colonization dynamics (the metapopulation approach) with the dynamics of patch succession, Amarasekare and Possingham (2001) show that persistence of a species in the landscape depends: (i) on the net rate at which suitable patches arise relative to the species' colonization ability; as well as (ii) the longevity of the dormant stages (e.g. seed bank) relative to disturbance frequency.

Future development of ideas about patch dynamics is likely to concern the consequences of multiple classes of disturbance. Steinauer and Collins (2001) have made a start by showing that disturbances caused by urine deposition and grazing by bison (Bos bison) interact with each other. The abundance of four common grass species, and of all of them combined, increased plants in grassland or forest founder and dominance control as a continuum of possibilities importance of a 'landscape ecology' perspective multiple classes of disturbance...

on urine patches in ungrazed prairie grassland. However, the abundance of the grass Andropogon gerardii, and all grasses combined, decreased on urine patches in grazed prairie. The changed dynamics reflect the fact that bison preferentially graze on urine patches. In addition, grazed areas initiated on urine patches tend to expand well beyond the area of urine deposition, increasing the size and severity of disturbance by grazing.

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Finally, just as we can readily see

... may interact how community dynamics may vary to determine according to the order in which colon-

community patterns izing species happen to appear after a disturbance, it is equally the case that the order in which different kinds of disturbance occur may affect the outcome. Fukami (2001) addressed this issue by introducing two classes of disturbance (drought or the addition of predatory mosquito larvae) in various sequences to laboratory micrososms of protists and small metazoans (analogous to natural communities in water-filled bamboo stumps). Different disturbance sequences drove the microcosms into different successional trajectories, sometimes leading to divergence in final community composition (both in terms of species richness and relative abundance of the component species). This is illustrated graphically by ordination diagrams (see Section 16.3.2) that show the sequence of positions of communities in the same ordination space through experiments where disturbances were imposed in different sequences (Figure 16.22). It may often be the case that a knowledge of the disturbance history will be needed to predict the responses of communities to disturbances imposed in the future (such as global climate change).

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