Community Ecology



Community Ecology

Applied community ecology and future directions

 

Outline:

1. Threats to biodiversity

A. Habitat fragmentation

B. Habitat degradation

i. Biological magnification/bioaccumulation

ii. Restoration ecology

a. restoration vs. rehabilitation vs. replacement

C. Exotic/invasive species

i. Ecological release

D. Climate change

 2. Some emerging ideas in CE / hot topics:

a. Neutral theory vs. niche theory

b. Metacommunities

c. Community phylogenetics

d. Implications of the pattern that biodiversity isn’t being lost at local scales

e. Invasion paradox

f. Identification of what drives regional biodiversity

g. Determining how many species are needed to ensure successful ecosystem functioning

h. Examining effects of climate change

i. Examining effects of urbanization

3. Examples of problems that might be solved through the imaginative application of insights obtained from community ecology

4. Course recap & final thoughts

Community ecology has provided insights to ecology and to knowledge as a whole about species interactions, environmental pattern-process relationships, and the causes and effects of biodiversity. The future of community ecology and of all of us lies in applying what we have learned to solve mounting problems of biodiversity conservation, environmental degradation and reparation, and wise and efficient resource use.

Threats to Biodiversity:

1. Habitat fragmentation

Habitat loss

Isolation of remnant habitat patches

2. Degradation of habitat: individuals are stressed, habitat supports fewer numbers of each species

Mechanisms:

- Pollution (air, water, soil, etc.)

-as we go up the food chain, the relative concentration of toxins increase in concentration (biological magnification/bioaccumulation): If we were cannibals, we couldn't eat each other because we wouldn't pass the inspections by the USDA!

- Edge effects: changes in heat, light, wind, aridity, soil moisture, exposure, sound pollution from humans, etc.

Restoration ecology: "The acid test of our understanding is not whether we can take ecosystems to bits on pieces of paper, however scientifically, but whether we can put them together in practice and make them work." (Bradshaw 1983)

restoration vs. rehabilitation vs. replacement

Regardless of which of these is being done, there are three things to keep in mind:

1)

2)

3)

Restoration ecology is often seen as “vague, varied, and difficult to quantify” (Standish et al. 2014). And shifting baselines pose challenges to conservation (how can we tell what is pristine, or natural?):



The aims in restoration ecology are often romantic rather than pragmatic in that they try to establish a community that would represent a situation before human influence (or, in North America, before European human influence). But this ignores the fact that humans have long been part of the biotic community and have been shaping the landscape for tens of thousands of years (e.g. Native Americans used fire, probably hunted some large mammals to extinction, altered watercourses). So to get to a "pre-human" state, you’d be dealing with a community from so long ago that the very climate would be different for a given area! The 1492 landing of Columbus or the 1804 Lewis & Clark expedition are often used as bellwether dates for North American biodiversity, yet by then (esp. the latter date) the continent was already biotically impoverished. For a fascinating interpretation of how even the pre-Columbian landscape was human dominated, I recommend C.C. Mann’s 2006 book 1491.

We ignore history at our peril (lest we be “doomed to repeat it”) yet because of variation, the past may not provide an adequate baseline for comparison with the present or the future. (There are no precedents for climate change, for example.) But some federal agencies (e.g. US Forest Service) are mandated to manage within the “Historical Range of Variation,” where past conditions are seen as a reference, better than current conditions and therefore a goal to work towards in ecological restoration.

But do we have any other choice but to try? Or will we be like Nero and fiddle while Rome burns?

3. Exotic/Invasive Species

Why do exotic/introduced species incur so many effects on communities?

See Pimentel et al. 2005

Ecological release

Hardest hit location: _______________

4. Climate Change

See Hoegh-Guldberg et al. 2007

Global climate change: a.k.a. global warming (although warming is a misnomer since some places will experience cooler average temperatures) caused by greenhouse effect, with implications for temperature, precipitation patterns, sea level, watershed structure, species’ geographic ranges/distributions

-gas emissions are primarily CO2 (most abundant), NO, CFCs, and methane

G.E. Hutchinson (1949) first suggested that industrial emissions (esp. after Industrial Revolution) and deforestation could disrupt atmospheric CO2 levels/global carbon cycle

There has been a significant increase in CO2 production since the early 1800s (corresponding with increased industrialization).

-What will it do?

Raise the ambient global temperature

Raise the oceans' temperatures and ocean acidification (due to rise in atmospheric carbon dioxide ( increase in carbonic acid formation in oceans) ( interferes with ability of corals, crustaceans, etc. to make calcium carbonate exoskeletons; also causes coral bleaching from death of algal symbionts (especially worrisome given coral’s role as ecosystem engineers in supporting marine biodiversity)

Melt the glaciers ( sea level rise (by 0.8-2 m by 2100) ( inundation of coastal land, forcing most of the world's population to flee inland and degrade other land ( destroying imperiled coastal habitats, human displacement, famine, degradation and imperiling terrestrial ecosystems, etc.

Changes in distributions (ranges) of organisms

Changes in phenology

For more info:

In these and other areas, community ecologists can have make important and lasting contributions. Community ecologists work in academic, governmental, non-profit, and private business settings on natural resource use, conservation, ecotoxicology, captive breeding programs (for zoos, aquaria, and botanical gardens), integrated pest management, wildlife management, ecotourism, and environmental impact assessment. So what is the future for community ecology and for students interested in community ecology? The very origin of ecology was rooted in community-level studies. Ecology as a discipline is incomplete if community interactions are ignored. Therefore, for as long as ecology exists, community ecology will continue to be important.

 

Some emerging ideas in CE / hot topics

By definition, community ecology is concerned with interspecific interactions. Such biotic interactions have been considered to be crucial in explaining organism-environment relationships and patterns in the abundance and distribution of organisms. Adaptive trade-offs preclude the evolution of a “super-species” that is superior at everything, so there is co-occurrence of interacting species. Of the biotic interactions, competition and predation have been considered the most ubiquitous and influential (with parasitism and mutualisms less so, and other types of interactions only rarely important and only under certain, limited circumstances). Interspecific competition has been particularly invoked (and attacked) by community ecologists since it involves multiple species simultaneously (and thus is in the purview of CE).

-competition is implicit in MANY aspects of CE, including niche theory (limiting similarity, character displacement, fundamental vs. realized niche), succession, community assembly and priority effects, etc.

-however, the ubiquity of competition has been questioned: for example, it has been said that competition may be important primarily during “ecological crunches” (Wiens); more recently, an even greater downplaying of the importance of competition has emerged in neutral theory

• Neutral theory - Hubbell (2001), Rosindell et al. (2011)

-“unified theory of biodiversity and biogeography”: IB doesn’t predict species abundances, and species-abundance models don’t predict species richness; neutral theory attempts to combine these two

-Hubbell’s definition of community - "a group of trophically similar, sympatric species that actually or potentially compete in a local area for the same or similar resources" ( how might this definition “stack the deck” in favor of neutral theory?

“ecological equivalence

Importance of dispersal rather than competition

-“ecological drift”

community structure is donor-controlled from input of new species (via dispersal) and speciation

Example:

Imagine a tropical forest saturated with K trees (up to S species in the species pool). Each tree individual occupies an area and resists invasion by other trees until it dies. Suppose a random disturbance like a storm kills D trees, with mortality randomly distributed across species, with the losses suffered by each species in proportion to that species’ abundance. Now there are D vacancies, so D new trees can fill the vacancies, with the proportion of replacement trees contributed by each species in the pool given by the proportional abundance of a species in the community after the disturbance. In other words, no species is inherently better than any other in occupying a vacant site. Over a long period of time, what would the tree community look like?

[pic]

Over the short term, the process leads to a log-normal species-abundance distribution, and over the long term, all but one species will be lost by local extinction. Which one will win? (Answer: green.) So over the longer term, the distribution becomes more of a geometric series. How quickly will a species in this community be lost to local extinction? Depends on D and K: for D = 8 and K = 512, it will take 90,000 disturbance events to lead to an extinction of a species with a starting population of 256 individuals. This is a loooooooong time for even a small community with small abundances, so Hubbell argues that species can be viewed as essentially immune to random extinctions over geologically significant time spans (long enough for speciation to act as well).

-"Dispersal-assembly perspective” vs. "niche-assembly perspective" of Chesson and Huntly 1997)

zero-sum game

-criticisms: Enquist et al. (2002), Whitfield (2002), Clark (2009), Purves and Turnbull (2010), and others

-claim of a “unified theory of biodiversity and biogeography” is pretentious because it is unlikely that a single theory will explain all patterns and processes at all scales at all times

-perhaps ecological equivalence is the result of past competition? (Chave 2004)

-very difficult to study because of all the species involved, issues of inclusion/exclusion, interactions and dispersal difficult to observe, timescales

-but see empirical tests: reviewed by Holyoak and Loreau (2006), McGill et al. (2006)

-one of its key tenets (demographic equivalence of individuals of different species) is clearly false in many (most?) cases (Holyoak and Loreau 2006, Leibold and McPeek 2006)

-often used as a null model, although doing so promotes duality thinking even though both neutral and nonneutral factors may be present within a community (Tilman 2004)

-useful because one doesn’t need to supply species-specific details about all members of a community to capture overall patterns (that is, neutral theory is simpler)

Because so much of CE invokes interspecific competition (from niche theory, food web dynamics, community assembly, succession, etc.), neutral theory has been both lauded and ridiculed ( a paradigm shift in the making?

“One of the main goals in producing the neutral theory was to stir the scientific pot vigorously, which in my opinion has been overdue in community ecology for a long time.” (Hubbell 2003)

Dispersal is also important in another recent development in community ecology: metacommunity dynamics:

• A metacommunity (Leibold et al. 2004, Leibold and Chase 2018) is a “community of communities” linked by dispersal of multiple, potentially interacting species. A metacommunity represents the integration of local and regional processes, partitioned along axes of species attributes (especially dispersal) and environmental heterogeneity.

The development of the metacommunity concept illustrates the growing complexity in our thinking about biodiversity in determining why certain species co-occur. But some (e.g. Cavender-Bares et al. 2009) have argued that these processes are not relevant on evolutionary time scales, and argue for a more phylogenetic approach to examining how communities are structured and maintained:

[pic]

• In the above graph, on the one hand, environmental filtering will select for species with similar traits in the same environment, but then fierce competition among those similar species will limit the similarity that is possible. Community phylogenetics examines the relative importance of competitive exclusion and character displacement in community assembly. This approach is closely associated with trait-based community assembly. The premise is that if environmental filtering dictates community structure, then species in a community should be more similar in traits than expected (trait clustering). But if competition is the driver, then trait overdispersion would be expected in the community.

But even this isn’t straightforward and is scale-dependent: at a regional scale, environmental filtering appears to lead to trait clustering, but at a local scale, competition/limiting similarity leads to trait overdispersion (see e.g. Cavender-Bares et al. 2004 on FL oaks, Graham et al. 2009 on tropical hummingbirds).

• Mark Vellend (2016) argues that there is a mismatch of spatial scales when using how biodiversity loss affects ecosystem functioning as a justification for conservation, because biodiversity is not declining at local scales (species losses are being offset by global biotic homogenization, invasive species, etc.) ( what are the implications of this? And what should we use as a justification for conservation instead?

• In nature, there is a positive correlation between native and exotic species richness, yet experiments have repeatedly shown that the most species-rich communities show the lowest rate of invasion by exotics (e.g. Fargione and Tilman 2005) – this is the “invasion paradox” (Fridley et al. 2007):

[pic]

Most broad-scale observations in nature find positive correlations between native and exotic species richness (the tropics may be an exception, represented as the circle with the question mark). But local-scale studies (most of which are experimental) tend to find negative correlations. Why?

• What drives (supports) regional biodiversity?

• The question of how important biodiversity is in nature is still unresolved, meaning that we still do not yet know how biodiversity losses will affect us. The simplest of questions—How many species (and which ones?) are needed to ensure the successful functioning on an ecosystem?—is still unanswered.

Vellend (2010) argues that there are 4 types of community diversity regulatory mechanisms: selection (which affects interspecific interactions), drift (with random changes in diversity), dispersal (species move in/out of communities), and speciation. But is this testable? Is it useful?

• How will climate change affect species distributions and species interactions?

• Do urban ecosystems impose novel evolutionary pressures on species?

Examples of problems that might be solved through the imaginative application of insights obtained from community ecology:

• Epidemiology of animal-borne diseases

• Biological control of invasive species

• Biomanipulation of water quality

• Bioremediation of environmental disasters

• Optimal design of nature preserves

• Management of multispecies fisheries

• Predicting and managing responses to global environmental change

• Maximization of yield in mixed-species agroecosystems

• Assembly of viable communities in novel environments

Course recap:

- a community is a multispecies assemblage living within a circumscribed area

- members of a community interact with each other, both directly and indirectly; these interactions are characterized in terms of costs and benefits and include competition, predation, parasitism, and mutualism

- communities have a structure based on diversity properties

- communities are dynamic over time

- community structure is often repeated over space (due to similar environmental conditions and/or convergent evolution), resulting in consistent community patterns

- these patterns inspire hypotheses about processes, which must be tested against null models about community structure

- these patterns must be recognized as being scale-dependent

- anthropogenic changes to the environment impact community structure

Perhaps most importantly, you have been exercising your critical thinking skills throughout this course. Critical thinking is a vital skill in evaluating scientific claims in community ecology and claims in life in general. There is a risk in exercising this skill, however: you soon realize just how little is known with certainty! Or as the German writer/philosopher Johann Wolfgang von Goethe put it, "Doubt grows with knowledge."

There are numerous “reflections” on community ecology, ranging from historical (Roughgarden 2009) to personal and pointedly critical (Lawton 2000) to unneeded borrowing from population genetics (Vellend 2010). I urge you to read these (and others) to gain greater perspective about the field, for there is far more than I have had time to cover in this course.

Final thoughts:

If the 4.5-billion-year history of Earth is represented as a single 24-hour day, then Homo sapiens have been present for just under 3 seconds. In that span, look what we have accomplished: we have journeyed to the stars and trod on the moon. But look also at what we have wrought: our activities—land-cover change, fire and fire suppression, habitat fragmentation, anthropogenic climate change, overhunting/overfishing, and many others—are directly and indirectly causing extinctions at a rate over 1000x the background rate, resulting in a biologically impoverished world. Each of the past 10 generations has experienced fewer songbirds, large predators, legendary fish, and exquisite wildflowers. These extinctions are even more painful when we consider that we don’t know exactly what we have lost.

We have a better understanding of how many stars there are in the Milky Way than of how many species there are on Earth, so these calculations will be approximations, but consider this:

There are an estimated 200-2000 extinctions that occur per year. That wide variation is due to the uncertainty we have in documenting true absences (vs. still present but just not detected) and also in extrapolating rates from named species to estimated numbers of as-yet unnamed species. Let’s be conservative and assume there are about 300 extinctions per year. That’s 0.01% of the 3 million or so named species that we lose every year. That’s 5.77 species lost each week. There are 15 weeks in a semester at TTU, so 87 *named* species have been lost forever since the start of this semester. Nearly every large animal is predicted to go extinct (at least in the wild if not altogether) by the year 2100 (your great-great-grandchildren’s generation) if current population trends are not altered.

These problems seem insurmountable. You may be asking yourself, “What can I, an individual who is not rich, not famous, not powerful, do about all this?” Individual actions like recycling are OK, but we must work together and collectively take on problems at their source: corporations use collective resources (land, air, water, and biodiversity) in privatizing profits while socializing losses (i.e., treating profits as property of shareholders, whereas losses, spills, disasters are treated as a responsibility that society must shoulder). This is clearly unsustainable (and unfair). A symptom of this problem is that fossil fuel subsidies are 10 times greater than what we spend on education in the US.

Worldwide, a mere 100 companies are responsible for 71% of carbon emissions since 1988; your choice of using a reusable straw is an infinitesimally small effect in comparison to what effects these corporations have on the environment. A long-term and systematic pressure upon governments to deregulate and incorporate wealth via enormous lobbying efforts have obstructed conservation efforts. If we want things to change at the speed warranted by the gravity of the situation, then we need to enact laws that are for the collective good. To do so, we must overcome the current paradigm of the role of individuals as consumers. We simply cannot address the Earth’s needs without doing so as a group and collectively taking on corporate power to pollute and destroy. Corporate power can be shaped by large-scaled individual spending patterns, such as boycotts. But it is more effectively shaped via legislation...and that requires you to vote for candidates who are willing to write and support such legislation. Or perhaps run for office!

Community ecology is the science of biodiversity. Global biodiversity is now less than it was at the start of this semester. What are we going to do about it? Watch it diminish, or put our knowledge and skills to use? The ideas you have been exposed to and the skills you have learned in this class are ones that are needed to educate others and to convince individuals to act collectively for all our sakes.

 

References:

 

See also Chapter 22 in Begon, M., C.R. Townsend, and J.L. Harper. 2006. Ecology: From Individuals to Ecosystems. Blackwell Publ., Malden, MA. [Discusses many other applications for community ecology.]

NOTE: Ecology vol. 87 iss. 6 from 2006 has a special feature on neutral theory.

Bradshaw, A.D. 1983. The reconstruction of ecosystems. J. Appl. Ecol. 20:1-17.

 

Cavender-Bares, J., D.D. Ackerly, D.A. Baum, and F.A. Bazzaz. 2004. Phylogenetic overdispersion in Floridian oak communities. Am. Nat. 163:823-843.

Cavender-Bares, J., K.H. Kozak, P.V.A. Fine, and S.W. Kembel. 2009. The merging of community ecology and phylogenetic biology. Ecol. Lett. 12:693-715.

Chave, J. 2004. Neutral theory and community ecology. Ecol. Lett. 7:241-253.

Chesson, P.L., and N. Huntly. 1997. The role of harsh and fluctuating conditions in the dynamics of ecological systems. Am. Nat. 150:519-553.

Clark, J.S. 2009. Beyond neutral science. Trends Ecol. Evol. 24:8-15.

Ebenhard, T. 1988. Introduced birds and mammals and their ecological effects. Swed. Wildl. Res. 13:1-107.

Enquist, B.J., J. Sanderson, and M.D. Weiser. 2002. Modeling macroscopic patterns in ecology. Science 295:1835-1836.

Graham, C.H., J.L. Parra, C. Rahbek, and J.A. McGuire. 2009. Phylogenetic structure in tropical hummingbird communities. Proc. Natl. Acad. Sci. 106:19673-19678.

Holyoak, M., and M. Loreau. 2006. Reconciling empirical ecology with neutral community models. Ecology 87:1370-1377.

Hubbell, S.P. 2001. The Unified Neutral Theory of Biodiversity and Biogeography. Princeton University Press, Princeton, NJ.

Hubbell, S.P. 2003. Modes of speciation and the lifespans of species under neutrality: a response to the comment of Robert E. Ricklefs. Oikos 100:193-199.

Kimura, M., and J.R. Crow. 1964. The number of alleles that can be maintained in a finite population. Genetics 49:725-738.

Lawton, J.H. 2000. Community Ecology in a Changing World. Ecology Institute, Oldendorf, Germany.

Leibold, M.A., and J.M. Chase. 2018. Metacommunity Ecology. Princeton University Press, Princeton, NJ.

Leibold, M.A., M. Holyoak, N. Mouquet, P. Amarasekare, J.M. Chase, M.F. Hoopes, R.D. Holt, J.B. Shurin, R. Law, D. Tilman, M. Loreau, and A. Gonzalez. 2004. The metacommunity concept: a framework for multi-scale community ecology. Ecol. Lett. 7:601-613.

Leibold, M.A., and M.A. McPeek. 2006. Coexistence of the niche and neutral perspectives in community ecology. Ecology 87:1399-1410.

McGill, B.J., B.A. Maurer, and M.D. Weiser. 2006. Empirical evaluation of neutral theory. Ecology 87:1411-1423.

Myers, J.H., D. Simberloff, A.M. Kuris, and J.R. Carey. 2000. Eradication revisited: dealing with exotic species. Trends Ecol. Evol. 15:316-320.

Purves, D., and L. Turnbull. 2010. Different but equal: the implausible assumption at the heart of neutral theory. J. Anim. Ecol. 79:1215-1225.

Rosindell, J., S.P. Hubbell, and R.S. Etienne. 2011. The Unified Neutral Theory of Biodiversity and Biogeography at age ten. Trends Ecol. Evol. 26:340-348.

Roughgarden, J. 2009. Is there a general theory of community ecology? Biol. Philos. 24:521-529.

Schiffman, P.M. 1994. Promotion of exotic weed establishment by endangered giant kangaroo rats (Dipodomys ingens) in a California grassland. Biodiv. Conserv. 3:524-537.

 

Standish, R.J., et al. 2014. Resilience in ecology: Abstraction, distraction, or where the action is? Biol. Conserv. 177:43-51.

Tilman, D. 2004. Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly. Proc. Natl. Acad. Sci. 101:10854-10861.

Vellend, M. 2010. Conceptual synthesis in community ecology. Quart. Rev. Biol. 85:183-206.

Vellend, M. 2016. The Theory of Ecological Communities. Monographs in Population Biology 57, Princeton University Press.

Whitfield, J. 2002. Ecology: neutrality versus the niche. Nature 417:480-481.

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