Agents of change can be abiotic or biotic

Communities, and the species contained within them, change in response to a number of abiotic and biotic factors (TABLE 17.1). We have considered many of these factors in previous chapters.

the form of climate, soils, nutrients, and water, vary over daily, seasonal, decadal, and even 100,000-year time scales. This variation has important implications for community change. For example, in Indian Ocean coral reef communities (see Figure 17.3), unusually high water temperatures driven by large-scale climate change have been implicated in recent losses of symbiotic algae from corals, resulting in coral bleaching. If the symbiotic algae do not return, the corals will eventually die, thus creating the conditions for species replacement. Likewise, increases in sea level can decrease the amount of light that reaches the corals. If light availability falls below the physiological limits of some coral species, they could slowly be replaced by more tolerant species, or even by macroalgae (seaweeds). Finally, increasing ocean acidification can dissolve the skeletons of corals, hindering their growth (see Chapter 25 and Chapter 16's Climate Change Connection for more information on climate change and ocean acidification). Because these abiotic conditions are constantly changing, communities are doing the same, at a pace consistent with their environment.

TABLE 17.1 Examples of Abiotic and Biotic Agents of Stress, Disturbance, and Change in Communities

or climate

Source: Adapted, with additions, from W.

Abiotic agents of change can be placed into two categories, both of which can have either natural or human origins, but which differ in the effects they have on species: disturbances and stresses. A disturbance is an abiotic event that physically injures or kills some individuals and creates opportunities for other individuals to grow or reproduce. Some ecologists also consider biotic events such as digging by animals to be disturbances. In our coral reef example, the 2004 tsunami can be viewed as a disturbance because the force of water passing over the reef injured and killed many coral individuals. Likewise, the outlawed practice of blast fishing, which involves using dynamite to stun or kill fish for easy collection, can cause massive injury and death in coral reefs. Even biotic events such as coral boring by snails or predation by parrot fishes can be considered disturbances because they remove coral tissue and weaken coral skeletons. Stress, on the other hand, occurs when some abiotic factor reduces the growth, reproduction, or survival of individuals and creates opportunities for other individuals. A stress in our coral reef might be the effect of warmer water temperatures or sea level rise on the growth, reproduction, or survival of corals. Examples of other stresses and disturbances are included in Table 17.1. Both disturbance and stress are believed to play critical roles in driving succession.

How do biotic factors influence community change? In Unit 4, we saw that species interactions, both negative and positive, can result in the replacement of one species with another through stress and disturbance. In our coral reef (see Figure 17.3), change might be driven by competition between, for example, platelike corals and branched corals, with the platelike forms eventually dominating over time. Coral diseases are another example of a species interaction that can initiate change in communities by causing particular coral species to grow more slowly or eventually die.

Finally, it is important to realize that abiotic and biotic factors often interact to produce change in communities. We can see this interaction in the case of ecosystem engineers such as beavers, which cause changes in abiotic conditions that in turn cause species replacement (see Figure 16.18). Similarly, abiotic factors such as wind, waves, or temperature can act by modifying species interactions, either positively or negatively, thus creating opportunities for other species. We have seen examples of this kind of effect on sea palms in the rocky intertidal zone (see Figure 14.18), plants in alpine regions (see Figure 15.9), and stream insects in Northern California (see Figure 16.19).