Functional Traits and Community Assemblage. What / Which Main Factors Drive Community Assembly (FTH)? Damien Bonal

Functional traits and community assemblage. What / Which main factors drive community assembly (FTH)? Damien Bonal

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Damien Bonal. Functional traits and community assemblage. What / Which main factors drive community assembly (FTH)?. Species Interactions in Mixed Forest Ecosystems (MIXFOR). 2014 NFZ.forestnet Summer School, Aug 2014, Nancy, France. 53 p. hal-01268789

HAL Id: hal-01268789 https://hal.archives-ouvertes.fr/hal-01268789 Submitted on 5 Jun 2020

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. 1 3.4 Functional traits and community assemblage

What / Which main factors drive community assembly (FTH)?

D. BONAL

2 Plant ecologists seek to understand where plants grow and why they grow there

3 Which species, present in a larger geographical region, will be found at a local site?

4 What / Which main factors drive community assembly? How is it possible for a number of species to coexist in a relatively unstructured environments all competing for the same resources?

How does high species diversity can be maintained within communities?

How is it that so many species can co-exist? Why are some species common and others rare?

5 Questions at the interface of Functional Ecology and Community Ecology 1987

 Functional Ecology

… the relationships between the functions of biological systems and the environmental conditions

To find the general rules that allow to explain …  Community Ecology

… the coexistence of species living together at a given place and at a given time

6 Questions at the interface of Functional Ecology and Community Ecology

Why are these questions at this interface?

Because: Mechanistic explanations of community assembly state that “community membership is constrained to those species with the appropriate functional traits to reach a site and establish under the circumstances set by the environment and other organisms”

7 Questions at the interface of Functional Ecology and Community Ecology

Why are these questions at this interface?

Because: In the framework of the Biodiversity-Ecosystem Function debate, the functional significance of biodiversity arises primarily from diversity of functional traits among the species in a community.

8 Functional Plant Diversity

9 Many ecologists believe that the key to understand these questions depends on our understanding of intra- and interspecific variation in plant functional traits

Huge amount of research studies to characterize the functional diversity of species

10 Functional diversity of leaf traits

14 tree species in French Guiana 11 NOTE: Functional diversity measures at community level

12 Correlation and trade-off among traits

Study on seedlings of 32 species in French Guiana Leaf functional and performance traits

1.0 20

0.8 ) -1 d

-1 15

0.6

(mg g (mg

0.4 25% PAR 25%

0.2 RGR Understory Survival Rate Survival Understory

0.0 0 2 2 3 3 A 4 A 4 sat (umol 6m sat (umol 6m 2 8 2 8 -2 -2 10 s -1 10 s -1 ) 12 1 ) 12 1 14 N fol (%) 14 N fol (%) 0 0

Baraloto et al. in prep. Oecologia 13 Correlation and trade-off among traits

There are fundamental differences among species in functional traits and in the trade-offs of these functions

Functional traits provide an important tool in community ecology as they reflect trade-offs that determine species performance in a given environment and consequently species abundances along environmental gradients

The quantification of these trade-offs as multivariate trait spectrums is consistent with the idea of Plant Strategies

Plant strategies is central to developing a trait-based theory of community assembly and a general understanding of the dynamics and functioning of ecosystems

14 Plant strategies

First edition in 1974

15 Plant strategies Grime introduced the CSR theory : “Each plant species has a blend of three strategies” : C (competitive), S (stress tolerant) and R (ruderal, or rapid propagation).

Species differ in the “proportion” of these strategies This theory states that environmental stress and disturbance agents are providing the principal constraints on plant evolution and community structure. According to habitat, each species must adopt a predictable compromise with regard to how they use the resources in order to survive.

The compromise involves the investment of resources in either the effort to acquire more resources, the tolerance of factors that reduce metabolic performance, or reproduction. 16 Ecological niche The ecological niche describes how an organism or population responds to the distribution of resources and competitors and how it in turn alters those same factors Many definitions: A term used to describe a species’ habitat and distribution or a place in the environment that can be empty or filled?

Hutchinson, 1957: Fundamental niche - hypervolume (>3 axes) within which a species can survive or reproduce

The niche defines the total ecological space in which a species could survive

http://bblonder.wordpress.com/2014/02/26/the-ecological-niche-and-the-n-dimensional-hypervolume/

17 Community Assembly

18 Reminder : Mixture effect on ecosystem functioning

19 Community assembly

20 Community assembly Assembly rules: • Predators cannot invade a habitat where there are no prey • Abiotic conditions limit species ranges (e.g. temperature, presence of soil, etc)

A lynx being a predator! A lynx just exercising!

21 Community assembly Assembly rules: • Predators cannot invade a habitat where there are no prey • Abiotic conditions limit species ranges (e.g. temperature, presence of soil, etc) • Priority rules: species that arrive first can potentially exclude later-arriving species

22 Community assembly Assembly rules:

• Only some combinations of species exist, some never do

• Stable combination types vary within a given environment

23 Community assembly

Temporal variation in assembly Community assembly Keep in mind that …

… a great uncertainty persists regarding the importance of different ecological processes in shaping species distribution in complex communities

25 Community assembly

Theory of Community: Speciation, Drift, Extinction, Birth and Dispersal interact to determine community composition and dynamics across spatial scales Extinction Speciation

Drift Metacommunity Birth

Local community Dispersal Drift

Death

26 Ecological processes in shaping species distribution

Numerous studies show how plant functional traits affect the performance of individual species, but our ability to predict community assembly from these relationships is nevertheless limited.

Community-level studies analyzing functional traits and species abundances have shown that niche-related processes clearly play a role in community assembly, while unpredictable chance events or the influence of local environmental conditions are also proposed to play a leading role …

27 Neutrality

28 Neutral theory

The distribution of species in a given environment is related to stochastic processes operating at the different levels:

1967 2001 Extinction S.P. Hubbell Speciation

Metacommunity Birth

Local community All individuals in a community are strictly Dispersal equivalent regarding their prospects of reproduction and death

This theory implies that biodiversity arises Death at random, as each species follows a “random walk” 29 Neutral theory The probabilities of immigration from the metacommunity, and the probabilities of survival and reproduction of all species, are equal (demographic neutrality).

Subsequent population dynamics in the local community are determined purely by random drift.

“Surprisingly”, this theory is able to quite-well summarize a large array of empirical patterns of species abundance!

Particularly in highly-diverse tropical rainforests So, even if one cannot rule out other potential mechanisms to explain patterns, why are these null models so well- describing some of the community assemblage? 30 Neutral theory - Example

Example with tropical rainforest plots

Gaussian model = Stochastic description of species distribution31 Environmental filtering Niche based

32 Environmental filtering Grime: A plant strategy is systematically associated with particular environmental conditions

The most common trait values in a local community will be possessed by those individuals having the greatest probabilites of survival, reproduction and immigration.

∑piti = twm Community mean trait value

i=1 offilters passing

This average trait value will probability relative abundance~ relative reflect the selective A B C D E species advantage/disadvantage of this tA tB tC td tE traits of species trait in passing through the various abiotic and biotic filters 33 Environmental filtering Consequences of Grime’s ideas for community assembly:

• Understanding how community functional composition varies across environmental gradients is necessary to understand community assembly

• If environmental conditions strongly filter species among habitats, you can expect directional shifts in mean trait values among habitats and reductions in the range and variance of these traits at the community level

34 Environmental filtering The influence of environmental filtering on tree species throughout the ontogeny

Enviro1 = white Enviro2 = gray

Species colours indicate species environmental preference. Newly emerged seedling communities are almost the same in the two sites. The neutral theory would have predicted that the community of adult tree is a random sample

of the seedling community 35 Environmental filtering - Example

74 plots in Amazonia (French Guiana and Peru) Gradients of soil fertility, of length of dry season, and of forest basal area

Yellow = White sand Blue = Seasonally flooded Red = Terra firme

36 Environmental filtering - Example

Bivariate differences in Community Trait Clouds between the 3 habitats

Trait clouds were built in wood (WSG, Stem Wood Stem Gravity) and leaf (SLA, specific leaf area) dimensions, integrating species abundances Yellow = White sand Blue = Seasonally flooded Red = Terra firme

• Environmental filtering shapes the functional composition of these highly diverse tropical forests at large scales across the three habitats

• Environmental factors drive and allow the prediction of variation in community functional composition among habitat types in these Amazonian forests. 37 Competitive exclusion Niche based

38 Competitive exclusion

With competition, the niche space of a species may be reduced Fundamental niche of sp1

Realized niche with 1 competitor

Competition prevents occupation of all of the fundamental niche.

The portion actually occupied in

the presence of competition is Now with 2 the realized niche competitors Competitive exclusion - Example

Environmental heterogeneity in woodlands of north-central Florida Gradients of water availability, soil fertility, and fire frequency and severity Hypothesis: Oak species partition these gradients, allowing many species to coexist

40 Competitive exclusion - Example

41 Competitive exclusion - Example

• Species not randomly distributed across the Florida landscape • Clear patterns of ecological sorting and specialization along the environmental gradients (soil moisture, soil fertility, and fire regime) • Niche differentiation can explain species diversity as well as functional diversity42 Niche vs. Competition vs. Neutrality debate

43 Niche vs. Competition vs. Neutrality debate

44 Niche vs. Competition vs. Neutrality debate Theories of neutrality or niche differentiation and environmental filtering still largely debated

Many ecologists remain skeptical of the assumption that species are essentially equivalent in how they function in the community!

Hubbell showed that the neutral model well predicted the species richness and relative abundance in the area.

BUT it must be pointed out that as early as in 1997, Hubbell doesn’t dispute that some species differ in their ability to compete! But he suggested that competition was not really an important factor in determining what plants grew where!

45 Niche vs. Competition vs. Neutrality debate

A matter of community richness scale?

46 Niche vs. Competition vs. Neutrality debate

Objectives: Partition the total variance of species’ means of 17 functional traits measured on trees in each of nine 1-ha plots in the tropical rainforest in French Guiana.

Variance explained into:

1. That due only to immigration 2. That due only to local trait-based selection 3. That due jointly to immigration and traits (correlations with meta-community) 4. Unexplained variation due to demographic stochasticity

47 Niche vs. Competition vs. Neutrality debate

Species’ means of 17 functional traits in each of nine 1-ha plots.

Trait-based filtering was the most important process at the 1-ha scale Tropical forest in French Guiana (34%), whereas demographic at 1-, 0.25- and 0.04-ha scales. stochasticity was the most important at smaller scale (37-53%)

A matter of community richness scale?

48 Niche vs. Competition vs. Neutrality debate Study of the co-occurrence patterns of over 1100 tree species in a 25-ha Amazonian forest plot in relation to field-measured functional traits. Traits : SLA, N, leaf size, seed mass, wood density, maximum dbh

Support for a niche-based view of tropical forest dynamics, in which subtle but omnipresent habitat specialization and strategy differentiation contribute to species coexistence. 49 Niche vs. Competition vs. Neutrality debate Study of the co-occurrence patterns of over 1100 tree species in a 25-ha Amazonian forest plot in relation to field-measured functional traits. Traits : SLA, N, leaf size, seed mass, wood density, maximum dbh Strategy differentiation among species, rather than stochastic processes, contribute to the maintenance of diversity in these highly diverse forests Author conclude: “Forces included in neutral theory (such as demographic stochasticity and dispersal limitation) may not be sufficient to explain species distributions and the maintenance of diversity in these forests, even though they are occurring.” Support for a niche-based view of tropical forest dynamics, in which subtle but omnipresent habitat specialization and strategy differentiation contribute to species coexistence. 50 Conclusions

Taken solely, none of these models (environmental filtering, niche differentiation, neutrality) are able to perfectly simulate observed species distribution in communities

Natural distributions are probably resulting from all three processes, with different degrees according to species richness and local environmental conditions

51 Perspectives

How do these processes act in the context of climate / environmental changes?

How will environmental changes affect community assemblage, and how can we predict them?

How can we combine Community Ecology and Evolutionary biology? How genetic variation and evolution within one species can shape the ecological properties of entire communities? How community context can govern species evolutionary processes and patterns?

52 Thanks for your attention