Peter S. Petraitis

The Maintenance of Species Diversity by Disturbance

Hypotheses that relate disturbance to the production and maintenance of species diversity are reviewed. The hypotheses have been classified traditionally by the effect of the disturbance on the community: Those that involve selective mortality, which maintains the species diversity of a community in equilibrium, and those that invoke causing random, localized, mass mortality, which prevent the community from reaching and equilibrium. Regardless of this difference, most hypotheses predict that the greatest number of species will occur at intermediate leves of disturbance. We develop graphical models for equilibrium and nonequilibrium situations, which we show to differ only in respect to temporal and spatial scale. We maintain that the usual outcome-the greatest number of species at intermediate levels of disturbance-can be explained by assuming that there are trade-offs in species-specific abilities that place constraints on immigration to, and extinction in, patches. Changes in immigration and extinction, the process that govern patch diversity, depend on species-specific abilities to fend off competitors or to endure disturbances. Members of a species can resist competitors or resist disturbance, but they cannot excel at both. If this trade-off does not exist, then the highest level of species diversity will not occur at intermediate levels of disturbance. Furthermore, we show that if this assumption is relaxed or if interactions between the frequency and magnitude of disturbances are considered, other unexpected outcomes are posible.

THE IMPORTANCE OF SCALE IN TESTING THE ORIGINS OF ALTERNATIVE COMMUNITY STATES

The possibility that different species assemblages may represent persistent alternative community states remains largely unexplored by experimental ecologists because of a variety of conceptual and experimental problems. We discuss some of the conceptual roadblocks to experimentation and propose several avenues for attacking the problem experimentally. We address the conceptual issues involved in (1) the blurring of the distinction between the processes that initiate the switch among alternative states and the positive-feedback processes that maintain those states, and (2) the role of spatial scale in initiating the switch. We suggest that the switch between alternative states requires, first, a disturbance that removes species involved in the positive feedbacks needed for maintenance and, second, the arrival of other individuals that initiate the switch to the alternative assemblage. The removal of the species that maintain the system must be large enough and over a long enough time to allow the arrival ...

Direct and indirect effects of predation, herbivory and surface rugosity on mussel recruitment

SummaryThe predatory gastropod Nucella lapillus, commonly preys upon the mussel, Mytilus edulis, and is thought to control the distribution and abundance of mussels on the rocky shores of New England, USA. In this study, done in Maine, USA, not only the presence of Nucella lapillus but also the roughness of the experimental surface and the presence of the herbivorous gastropod, Littorina littorea, were manipulated. Four types of surfaces were used as recruitment substrata for mussels: smooth bare granite, aggregations of the barnacle, Semibalanus balanoides, fiberglass resin castings of smooth bare granite and resin castings of aggregations of S. balanoides. To ensure that caged N. lapillus were not starving, barnacles were provided as alternative prey. Experiments showed no detectable effect of N. lapillus on the recruitment of M. edulis. Mussel recruitment was enhanced by surface rugosity and depressed by the activities of L. littorea. Analysis of covariance, using the number of algal species as the covariate, suggested that L. littorea reduced the number of newlyrecruited mussels by removing algae that provided recruitment sites, but no manipulations were done to test this conjecture. It is likely that previous reports of N. lapillus controlling mussel abundance are attributable to N. lapillus preying upon barnacles, which increase surface rugosity and enhance mussel recruitment. Review of literature on feeding preferences of N. lapillus supports this view. When handling times and prey availability are taken into account, Nucella shows a clear preference for barnacles over mussels.

The Role of Growth in Maintaining Spatial Dominance by Mussels (Mytilus Edulis)

Spatial dominance is maintained by the balance of three processes: mortality, recruitment, and growth. Patches of mussels facing high rates of mortality may show no change in percent cover (i.e., dominance) as long as growth by the remaining survivors can fill in the vacant space. A graphical model is developed to examine the possibility that growth may ameliorate the effects of predation and physical disturbance on spatial dom- inance. In the model, growth, mortality, and recruitment are depicted as vectors on a set of isoclines for log percent cover. The 100% isocline is analogous to the self-thinning line. Self-thinning data for the mussel Mytilus edulis are used to establish the log-percent iso- clines. A review of published growth and mortality data suggests mussels can use growth to offset the effects of mortality. Small mussels can easily double in length in 1 yr, and this is sufficient to withstand 60% mortality per year without a loss in percent cover. Since both growth and mortality tend to be size specific, the point at which growth can no longer offset mortality depends on the environmental conditions that affect rates of growth and mortality. The model and analysis can be easily extended to other species and suggest that species with rapid growth and large variation in body size need not rely on recruitment to maintain spatial dominance, even when faced with high rates of predation.

Experimental evidence for the origin of alternative communities on rocky intertidal shores

Ecological theory of alternative stable communities suggests the switch between different species assemblages occurs when environmental conditions alter species composition so that it crosses a critical threshold and enters a different basin of attraction. The theory implies that once the threshold is crossed, rates of key ecological processes, such as predation and competition, will change. This conjecture was tested on sheltered intertidal shores of New England that are dominated by either mussel (Mytilus edulis) beds or algal (Ascophyllum nodosum) stands. Twelve sets of clearings of different sizes, which mimicked effects of ice scour, were created in algal stands, and mortality of transplanted mussels was monitored. Clearings were 1, 2, 4 and 8 m in diameter. Mussel mortality due to the predatory snail Nucella lapillus declined in a step-like manner in clearings > 4 m in diameter. Initial densities of the predator did not vary among experimental clearings or sites and were not correlated with mussel mortality. Mussel mortality was correlated with predatory snail densities during and at the end of the experiment. These results provide experimental evidence supporting conjectures about the switch between alternative states in ecological communities.

SCALE‐DEPENDENT RECRUITMENT AND DIVERGENCE OF INTERTIDAL COMMUNITIES

Sheltered rocky bays in the Gulf of Maine support communities usually dominated either by the fucoid rockweed Ascophyllum nodosum or the mussel Mytilus edulis. In order to test whether clearing size determines the subsequent pattern of recruitment by intertidal organisms, we examined four common species in clearings in stands of A. nodosum. Circular experimental clearings of 1, 2, 4, and 8 m in diameter, which mimicked severe ice scour, and uncleared controls were established at 12 sites spread across four bays. Recruitment of A. nodosum and of three species not common in A. nodosum stands (mussels, barnacles, and the rockweed Fucus vesiculosus) varied with clearing size and the patterns did not vary among bays. Recruitment of A. nodosum declined exponentially with clearing size. Fucus and barnacles recruited in greater densities to large clearings (≥4 m) compared to small clearings (<4 m). Recruitment of F. vesiculosus was also site specific, reflecting patterns of local abundance, but that of A. nodosum was not. Mussels recruited onto artificial substrates in fewer numbers in large clearings, but in marginally greater numbers to rock underneath these substrates in large clearings. Recruitment densities of mussels and barnacles were strongly site specific. These results suggest that (1) the rate of succession varies site specifically, and (2) the trajectory of succession, either convergence toward or divergence from the surrounding community, is contingent upon the interplay between species interactions and the size of the clearing.

Factors organizing rocky intertidal communities of New England: Herbivory and predation in sheltered bays

In New England, U.S.A., shores exposed to severe wave action are dominated by the common blue mussel Mytilus edulis L. while moderately protected areas are covered with perennial algae. It is thought that algae are limited by mussels which are a superior competitor. Because the effectiveness of predators is inhibited by wave activity, it is assumed that the rate of predation, which varies across this environmental gradient, accounts for the observed distribution of mussels and algae. Shores along sheltered bays appear to be an exception to this pattern and this study addresses some of the possible causes. In New England bays, mussels and barnacles Semibalanus balanoides (L.) are the most common organisms on the solid surfaces in the lower intertidal zone. Perennial macroalgae, such as Chondrus crispas Stackhouse and Fucus vesiculosus L., are rare. The distribution and abundance of species differs from that on moderately protected shores and is similar to very exposed shores which are dominated by mussels and barnacles. Herbivory by the common periwinkle Littorina littorea (L.) limits the abundance of F. vesiculosus and indirectly affects the success of mussels. During 4 years of experimental manipulations, F. vesiculosus rarely recruited in the presence of periwinkles but dominated experimental surfaces if periwinkles were excluded. When experimental surfaces with F. vesiculosus, which had been protected from herbivory for > 1 year, were exposed to natural conditions, herbivores cleared most of the surfaces within several months. Recruitment by barnacles and mussels was higher when periwinkles were excluded. However, the effect of periwinkles on mussels was indirect; the snails reduced barnacle success and thus reduced mussel recruitment which was enhanced by the surface irregularities provided by barnacles. The occurrence of mussels in sheltered bays is not due to a lack of predators. Predators were commonly seen at all sites. Most mussels on experimental surfaces were removed <4 wk when surfaces were exposed to natural levels of predation. Experiments do not provide an explanation for the occurrence of mussels, although the enhancement of mussel recruitment by barnacles suggests that the availability of settlement sites may be important.

GRAZING PATTERNS OF THE PERIWINKLE AND THEIR EFFECT ON SESSILE INTERTIDAL ORGANISMS

The periwinkle, Littorina littorea, a common gastropod herbivore in the New England intertidal, strongly affects two abundant, sessile organisms, the green alga Enteromorpha and the barnacle Balanus balanoides. At very low densities, Littorina can maintain initially bare areas clear of Enteromorpha while higher densities are required to eliminate established patches. At low peri- winkle densities, Enteromorpha interferes with Balanus settlement, while at high densities, Littorina appears to dislodge newly settled barnacle cyprids. Balanus abundance is greatest at intermediate Littorina densities. Experimental manipulation of periwinkle behavior through the use of partial cages shows that Enteromorpha can persist due to an interaction between snail behavior and surface irreg- ularities. Certain areas are consistently missed which would normally be reached by a randomly moving grazer. There is no such refuge for Balanus. These results suggest that behavior-mediated effects may play an important role in maintaining diversity in the rocky intertidal.

Plant response to climate change varies with topography, interactions with neighbors, and ecotype

Predicting the future of any given species represents an unprecedented challenge in light of the many environmental and biological factors that affect organismal performance and that also interact with drivers of global change. In a three-year experiment set in the Mongolian steppe, we examined the response of the common grass Festuca lenensis to manipulated temperature and water while controlling for topographic variation, plant-plant interactions, and ecotypic differentiation. Plant survival and growth responses to a warmer, drier climate varied within the landscape. Response to simulated increased precipitation occurred only in the absence of neighbors, demonstrating that plant-plant interactions can supersede the effects of climate change. F. lenensis also showed evidence of local adaptation in populations that were only 300 m apart. Individuals from the steep and dry upper slope showed a higher stress/drought tolerance, whereas those from the more productive lower slope showed a higher biomass production and a greater ability to cope with competition. Moreover, the response of this species to increased precipitation was ecotype specific, with water addition benefiting only the least stress-tolerant ecotype from the lower slope origin. This multifaceted approach illustrates the importance of placing climate change experiments within a realistic ecological and evolutionary framework. Existing sources of variation impacting plant performance may buffer or obscure climate change effects.

Experimental confirmation of multiple community states in a marine ecosystem

Small changes in environmental conditions can unexpectedly tip an ecosystem from one community type to another, and these often irreversible shifts have been observed in semi-arid grasslands, freshwater lakes and ponds, coral reefs, and kelp forests. A commonly accepted explanation is that these ecosystems contain multiple stable points, but experimental tests confirming multiple stable states have proven elusive. Here we present a novel approach and show that mussel beds and rockweed stands are multiple stable states on intertidal shores in the Gulf of Maine, USA. Using broad-scale observational data and long-term data from experimental clearings, we show that the removal of rockweed by winter ice scour can tip persistent rockweed stands to mussel beds. The observational data were analyzed with Anderson’s discriminant analysis of principal coordinates, which provided an objective function to separate mussel beds from rockweed stands. The function was then applied to 55 experimental plots, which had been established in rockweed stands in 1996. Based on 2005 data, all uncleared controls and all but one of the small clearings were classified as rockweed stands; 37% of the large clearings were classified as mussel beds. Our results address the establishment of mussels versus rockweeds and complement rather than refute the current paradigm that mussel beds and rockweed stands, once established, are maintained by site-specific differences in strong consumer control.