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Lecture 16

Lecture 16 - Change in Communities

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Western University
Biology 2483A
Hugh Henry

LECTURE 16 – CHANGE IN COMMUNITIES  Mount St. Helens (1980): new habitats devoid of living organisms  Communities change over time, and one of the ways we see this, is when we see recovery from natural disasters such as the eruption of Mount St. Helen  Effects of the eruption varied depending on distance from the volcano and habitat type  A surprising number of species survived. Some were still dormant under winter snows. Others were in burrows, or under ice-covered lakes, or were plants with underground parts  Gophers survived in their tunnels. Grassy meadows, their preferred habitat, expanded after eruption  Their burrowing activities facilitated plant succession by bringing organic soil, seeds, and fungal spores to the surface – thus plant growth increased where there was increased gopher activity  Newly-formed and isolated ponds were colonized by amphibians much faster than was thought possible  Frogs and salamanders were using tunnels created by northern pocket gophers to make their way from one pond to another  Multiple mechanisms were responsible for primary succession: o Facilitation by dwarf lupines on the Pumice Plain. They trap seeds and detritus, and have nitrogen- fixing bacteria o Lupines were inhibited by insect herbivores, which controlled the pace of succession o Tolerance: Douglas fir and herbaceous species lived together in some habitats  Succession occurs become some species grow quicker than others, so succession can occur even when there is no interaction between species but simply because one specie can outgrow the other Introduction  Communities are always changing, some more than others  Human actions are becoming one of the strongest forces behind community change, and we have an imperfect understanding of the consequences of those actions  Agents of change act on communities across all temporal and spatial scales  Consider a coral reef community in the Indian Ocean. If you could view it over the last few decades, you would observe slow and subtle changes, as well as catastrophic ones Succession  Succession is the directional change in species composition over time as a result of abiotic and biotic agents of change o Different from seasonal change as it is not a cycling change  Studies of succession often focus on vegetative change, but the roles of animals, fungi, bacteria, and other microbes are equally important  Agents of change vary in  Two types of succession: frequency and intensity 1. Primary succession – involves the colonization of habitats devoid of life  In general, low frequency and (e.g., volcanic rock) high intensity events are frequent 2. Secondary succession – involves re-establishment of a community in which some, but not all, organisms have been destroyed  Primary succession can be very slow. Initial conditions are very inhospitable.  The first colonizers (pioneer or early successional species) tend to be stress- tolerant, and transform the habitat in ways that benefit their growth and that of other species.  Secondary succession occurs after fires, storms, logging, etc. the legacy of the pre-existing species and their interactions with colonizing species play larger roles than in primary succession  One of the pioneers of modern ecology was Henry Cowles (1899), who studied succession on sand dunes along Lake Michigan  He assumed that plant assemblages farthest from the lake’s edge were the oldest; the ones nearest the lake were the youngest, representing a time series of successional stages  The first stages were dominated by a hardy ecosystem engineer, American beach grass, which traps sand and creates hills, providing refuge for plants less tolerant of burial and scouring  Cowles could predict how communities would change over time without actually waiting for the pattern to unfold, which would have taken decades to centuries (space for time substitution) o Pattern that was being seen was a substitution for time – instead of waiting for how change occurs, moving inland would allow for the study of successional change  Frederick Clements believed plant communities are like “superorganisms,” groups of species working together toward some deterministic end. Thus, succession is similar to the development of an organism  Each community reaches a stable end point called the “climax community,” which is composed of dominant species that persist over many years and provide stability that can be maintained indefinitely  Henry Gleason thought communities are the random product of fluctuating environmental conditions acting on individual species  Communities are not the predictable and repeatable result of coordinated interactions among species; each community is unique  Connell and Slatyer (1977) reviewed the literature on succession and proposed three models: 1. Facilitation model – inspired by Clements. Early species modify the environment in ways that benefit later species. The sequence of species facilitations leads to a climax community 2. Tolerance model – also assumes the earliest species modify the environment, but in neutral ways that neither benefit nor inhibit later species 3. Inhibition model – assumes early species modify conditions in negative ways that hinder later successional species Mechanisms of Succession  Glacier Bay, Alaska is one of the best studied examples of primary succession  Melting glaciers have led to a sequence of communities that reflect succession over many centuries  Not only does the number of species change, but we see a shift in types of species – early successional species are eventually pushed out  Soil organic matter, moisture, and nitrogen concentration increase as succession progresses  In field experiments, spruce seeds were added to each successional stage. Germination, growth, and survival were monitored over time  Neighboring plants had both facilitative and inhibitory effects on the spruce seedlings, but the direction and strength of those effects varied with successional stage  Glacier Bay illustrated all three mechanisms in Connell and Slatyer’s models: o
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