Chapter 38 Study Notes

Chapter 38: Communities and Ecosystems (Species Interactions II)

Learning Objectives

  • Distinguish species richness from species evenness.

  • Identify the steps in ecological succession.

  • Describe the energy flow in ecosystems and the trophic levels.

Overview of Communities and Ecosystems

  • Community Definition:

    • A community is comprised of all populations representing multiple species within a specific region.

  • Ecosystem Definition:

    • An ecosystem includes the biotic (living) community along with the abiotic (nonliving) environment.

Describing Diversity and Composition

  • Key Terms: Species Richness and Evenness:

    • Ecologists utilize species richness and species evenness as metrics to describe diversity and composition within a community, exemplified by environments such as the African savannah.

Species Richness

  • Definition:

    • Species richness is defined as the total number of species that occupy a given habitat.

  • Comparison Example:

    • A community represented in yellow displays lower species richness compared to the community depicted in blue.

Species Evenness

  • Definition:

    • Species evenness refers to the proportion of individuals of each species in a community.

  • Characteristics:

    • A community exhibiting high species evenness contains approximately equal numbers of individuals from each species, whereas the yellow community shows lower species evenness than the blue community.

Ecosystem Dynamics

  • Ecosystems are dynamic and exhibit shifts in:

    • Population sizes.

    • Species composition.

    • Physical conditions.

    • Interconnectedness among all components of the ecosystem.

Ecological Succession

  • Definition:

    • Succession is characterized by gradual changes in a community’s species composition over time.

  • Measurement of Changes:

    • Ecologists assess species evenness and richness over time to identify changes within a community.

Types of Succession
Primary Succession

  • Definition:

    • Primary succession occurs in environments where no previous community has existed, such as areas following a volcanic eruption.

  • Predictability:

    • The process of primary succession unfolds in a predictable pathway where each species modifies the habitat, thereby influencing which subsequent species can inhabit the area.

Pioneer Species

  • Definition:

    • Pioneer species, such as lichens, are the initial colonizers of newly formed or disturbed habitats.

Next Stages in Primary Succession

  • Initial Vegetation:

    • Decomposing pioneer species foster soil formation, allowing for the establishment of the first herbs and weeds.

  • Further Plant Growth:

    • As soil continues to develop, larger plants begin to root in the area.

Secondary Succession

  • Definition:

    • Secondary succession takes place following disturbances that do not obliterate the community (e.g., fire, clearing).

  • Adaptation of Species:

    • Species adapt to fill new ecological niches created by partial disturbances.

Case Studies of Ecological Succession

  • Examples include:

    • Secondary succession at an abandoned mine site.

    • Primary succession post-volcanic eruption.

    • Secondary succession following the Chernobyl disaster.

Energy Flow in Ecosystems

  • Many organisms rely on others for energy; an organism’s trophic level defines its place in the food chain.

  • Understanding trophic levels is critical to comprehending community structure, as energy transfers within an ecosystem are rarely simple.

Producers vs. Consumers

  • Producers (Autotrophs):

    • Primary producers convert energy from inorganic sources (e.g., sunlight) into organic molecules.

  • Consumers (Heterotrophs):

    • Consumers acquire energy by consuming producers or other consumers:

    • Herbivores: Primary consumers.

    • Carnivores: Secondary, tertiary consumers, etc.

Decomposers

  • Function of Decomposers:

    • Decomposers, like fungi and bacteria, break down detritus—consisting of dead tissue and organic waste—to recycle inorganic nutrients back into the environment.

Food Webs

  • Definition:

    • A food web represents a more intricate depiction of the interrelated feeding relationships within a community.

Keystone Species and Community Dynamics

  • Definition of Keystone Species:

    • Keystone species are essential for maintaining diversity in a community as many species depend on them for survival.

  • Example:

    • The American bison is crucial for maintaining species diversity in the Great Plains ecosystem.

Prairie Dogs as Ecological Engineers

  • Contributions Include:

    • Soil aeration and nutrient cycling via burrowing that mixes and aerates soil and redistributes organic matter.

    • Enhancing water infiltration through tunneling, which aids drainage and groundwater recharge.

    • Creating habitats through abandoned burrows that provide shelter for various species.

    • Promoting plant diversity by establishing patches that encourage early-successional species.

Yellowstone Case Study

  • Historical Context (1800-1900s):

    • Agricultural exploitation and intensive hunting of deer and livestock took place, along with a national predator control program leading to local extinction of wolves in 1926.

  • Consequences:

    • Resulted in unchecked deer population growth and behavioral changes, causing excessive plant consumption and significant effects on various interdependent species.

Reintroduction of Gray Wolves

  • Year of Impact (1995):

    • The reintroduction led to natural control of herbivore populations, prompting a resurgence of secondary growth and positively impacting ecosystem diversity.

Giant Sea Otters Case Study

  • Overview of Decline (1700-1800):

    • Overhunting significantly diminished sea otter populations, resulting in ecological imbalance.

  • Ecosystem Role:

    • Sea otters are crucial predators of herbivorous sea urchins, which consume kelp. The loss of otters led to uncontrolled sea urchin populations, resulting in the destruction of kelp forests and subsequent species die-off.

  • Reintroduction Consequences:

    • Sea otter reintroduction revived the ecosystem by restoring balance.

Trophic Cascades and Their Implications

  • Definition:

    • Trophic cascades are significant ecological consequences that stem from alterations at one trophic level, affecting others.

  • Examples of Keystone Species Involved in Cascades:

    • Wolves, otters, reptiles, amphibians, panthers, and beavers.

Energy Flow in Ecosystems

  • Energy Input Requirement:

    • Ecosystems require constant energy input, typically sourced from sunlight.

  • Energy Transfer Efficiency:

    • Energy transfer is inefficient, with only about 10% available to the following trophic level after loss at each level.

Primary Production

  • Definition:

    • Primary production signifies the energy harnessed by producers and converted into food molecules for consumers.

Sustainability and Diet Choices

  • Energy Efficiency Comparison:

    • A plant-based diet is argued to be more energy efficient than one comprising meat due to the energy retention from producers to herbivores compared to higher trophic levels.

Biomagnification

  • Definition:

    • Biomagnification refers to the increasing concentration of toxins at higher trophic levels.

  • Characteristics of Biomagnifying Chemicals:

    • Chemicals that dissolve in fat and are not readily degraded are prone to biomagnification.

Mercury Biomagnification Example

  • Process Overview:

    • Mercury released from coal-fired plants converts into methylmercury via bacteria, accumulating in aquatic food chains.

    • At the apex of the food chain, organisms can exhibit up to 100,000 times higher mercury concentrations than those at the food web's base.

Impacts on Human Health

  • Consumer Role in Biomagnification:

    • Although humans occupy lower trophic levels, consumption patterns resemble those of apex predators, leading to exposure to high levels of toxins such as methylmercury and PCBs.

  • Feedback Loop:

    • Humans introduce persistent toxins into ecosystems and subsequently reabsorb them through the consumption of affected organisms, establishing a closed feedback loop in biomass accumulation.

Conclusion

  • Human Accountability:

    • The unique human role as both the source and endpoint of global biomagnified contaminants underscores the need for increased awareness and mitigation in ecosystem management strategies.