Category Archives: Ecology

Learning humility

I’m not a forester, though I sometimes forget that. I’m not an expert on sustainability, or environmental policy, or environmental history. While I know a lot about these things, my knowledge is riven with holes – and I’m often unaware of those holes.

When I teach environmental science, I know that I’m a dabbler when I talk about the atmosphere, or weather, or even ocean currents. I know my geology is self-taught. As a consequence, I know enough to make sure I know what I know and avoid stepping off the cliff into ignorance. But when I talk about environmental history or policy, when I talk about sustainability or land-use or economics, I don’t know what I don’t know. I built a decent edifice of knowledge on a foundation honeycombed with ignorance. He who knoweth not, and knoweth not he knoweth not.

There are things you know because you have a solid foundation, but not a whole lot of updated, specialised knowledge. But what you know serves you well. I can teach chemistry or physics to environmental science students and not feel like they are ill-served. My physics ends at A Levels and my chemistry with my first year undergrad, but what I’m teaching is barely O Level.  It’s stuff I know inside-out, backward and forward – well enough to take apart and explain to the science-phobic. When I hit an unknown, it doesn’t come as a surprise – it’s either higher-level knowledge that I never got to (but may be aware exists) or it’s something I once knew, but have forgotten. Forgetting can be annoying, but it’s just part of what happens to specific bits of information you haven’t though about in 25 years.

Hitting a gap in ecology, forestry, or environmental history can be vertigo-inducing.

I remember not knowing who Aldo Leopold was, early in my career as a grad student, when everyone else seemed conversant with his work. It was very different from discovering how much I didn’t know about the importance of Howard Odum or Robert MacArthur because at least I had heard their names.

So what sparked the current crisis of faith? Aldo Leopold, again.

Aldo Leopold (1887 – 1948) developed from his long professional experience in Pinchot’s Forest Service, his discussions with the British ecologist Charles Elton and his encounter with the German ‘Dauerwald’ experiments.

Dauerwald. The term doesn’t even have an article in the English Wikipedia, though it does exist in the German one (and, apparently, its Russian and Estonian counterparts). Not knowing about something that I don’t know exists shouldn’t really bother me, but once I started looking I felt like I shouldn’t have known about this.

Dauerwald is a forestry system that eschewed clear-cutting and defined timber quotas for a system that more closely mimicked nature. I’m immediately reminded of the Periodic Block System. It was required by the Nazi government. Really? And it was, of course, influential on Leopold.

There’s just too much there I should know, but don’t. Discovering this for the first time makes me feel profoundly ignorant.

Dauerwaldrevier2 Baerenthoren

What’s new in tropical ecology I: Mapping tropical dry forests

Blogs began as virtual notebooks, places people kept track of the things that caught their interest. And the flood of information makes it hard to keep track of anything on a regular basis. So why not challenge yourself?
Pick up an atlas and you’ll find vegetation maps of some sort which delineate different vegetation types (biomes, life zones, or something else of the sort). At broad scales you’ll see things like savannas and rainforests, at finer scales you’ll see things like oak-hickory forests. Everywhere you look, you’ll see vegetation mapped with what appears to be a high degree of confidence. But ever stop to wonder what those colourful areas on the map come from?

Traditionally there were two ways to map vegetation – ground surveys and aerial surveys. In the 19th century people like Charles Flauhert at Montpellier began mapping and classifying vegetation. People like Robert Smith brought this work back to Britain; after his death, his brother William Gardner Smith continued his work. W.G. Smith was one of the organising members of the British Vegetation Committee (together with Arthur Tansley and others), which later gave rise to the British Ecological Society. They used field surveys to map different vegetation types, and standardised sampling procedures to characterise the different forms of vegetation. In America, E. Lucy Braun, Edgar Transeau and William S. Cooper played a similar pioneering role in vegetation mapping.

The availability of aerial surveys, starting in the 1920s and 30s, added another dimension to vegetation surveys. Now, it was possible to collect data over a large area, but the quality of the data was low – you could probably distinguish forest from grassland, farmland from urban. As aerial photography developed it was possible to extract more information about vegetation – coniferous forest from deciduous forest, open forest from closed forest, even distinguish younger successional forest by the roughness of the canopy (younger forests tend to be dominated by even-aged stands of the same species, so the canopy is made up of relatively similar trees. Older forests are more uneven in age and species composition, leading to a canopy that varied more in height and colour.) Things began to take off as additional sensors, many of them space-based, became available. By measuring additional wavelengths, outside the visible spectrum, and by actually collecting quantitative data about the absolute reflectivity in each band, it was possible to develop much more structured data from remotely sensed data. Some of these sensors can estimate the amount of chlorophyll in vegetation, while others can measure vegetation structure.

In the March 2013 issue of Biotropica, Sebastián Martinuzzi, William Gould and others used LIDAR data to classify dry forest1 in Guánica, Puerto Rico, into “forest type” (semi-deciduous forest, semi-evergreen forest, scrub forest, dwarf forest and mesquite forest, a “relatively homogeneous stand of Prosopis pallida with a dense herbaceous understory”) and beyond that, into successional stage: mid-secondary forest with a logging2 past, late secondary forest with a logging past, late-secondary forest with an agricultural past, and ‘primary’ forest (undisturbed for more than 90 years). This part of their analysis they restricted to semi-deciduous forest areas. Since LIDAR penetrates the forest canopy, it can identity the underlying land surface (and thus, elevation and topography), and the height not only of the forest canopy, but also of various layers in the canopy.

It’s hard to explain how appealing the idea of being able to map and age these forests using remote sensing can be. I’ve mapped forest cover using aerial photographs – first you map current forest cover, then you map historical forest cover, and use that to estimate areas of “older growth” forest. Not only is it tedious, there’s also a risk of ending up with subjective results. If an area was cut-over and regrew in the interval between two images, you may never noticed it. And if an area lost all its understorey without losing its canopy, you’d probably be unaware.

While avoiding subjectivity in that regard, this approach added a different sort of subjectivity.

We used GPS surveys and visual interpretation of 1-m spatial resolution color aerial photos supported by expert knowledge, for a total of 83 sample locations. All samples represented an area of at least 30 m by 30 m of the same forest type (to coincide with the geospatial grain size used by this study), and were separated by >60 m (consistent with Agosto Diaz 2008).

As far as I know, the definitions of the different forest types in Guánica Forest are subjective. They are somewhat self-evident, as long as you stay in the park (and avoid certain grey areas), but I’m not aware of any formal delineations of these vegetation types. Granted, there’s nothing unique about that, but having poked around in Guánica Forest, uncertain (at times) whether I was in one forest type or another…I’m a tad bothered.

Perhaps the thing that interested me most was this

The most important predictors in the LiDAR canopy model included the median absolute deviation of vegetation heights (HMAD), the 90th percentile of vegetation heights (H90th), and the percent of returns >1.0 m (CDENSITY2; Table 3).

Canopy closure is a useful, but incomplete predictor of forest type. The most closed canopies tend to be in ravines and arroyos (semi-evergreen forest), but certain young secondary forests could also have a closed canopy (like the Prosopis forest they mentioned). But when you combine that with with the variation in vegetation height, you can probably separate out the species-poor secondary forests dominated by Prosopis or Leucaena leucocephala, since these forests lack structural diversity – most of the trees are the same age and belong to just a single species. This has me wondering…

  1. Martinuzzi, Sebastián, William A. Gould, Lee A. Vierling, Andrew T. Hudak, Ross F. Nelson, and Jeffrey S. Evans. 2013. Quantifying Tropical Dry Forest Type and Succession: Substantial Improvement with LiDARBiotropica 45(2): 135-146
  2.  Logging in this case means harvest for fence posts and charcoal production; this process left the rootstocks intact and allowed rapid coppice regeneration.
  3. Lugo, Ariel E., Jose A. Gonzalez-Liboy, Barbara Cintron, and Ken Dugger . 1978. Structure, productivity, and transpiration of a subtropical dry forest in Puerto Rico. Biotropica10: 278–291.

Fundamental questions for ecology I.a

In my last post, I began discussing William J. Sutherland et al.’s article Identification of 100 fundamental ecological questions – specifically, the first question What are the evolutionary consequences of species becoming less connected through fragmentation or more connected through globalization? 

This isn’t the first time I’ve given thought to this issue. In the opening chapter of my dissertation I wrote:

The viability of small populations can also be affected by the loss of genetic diversity. Most tropical trees are outbreeders with complex incompatibility systems (Bawa 1974, Zapata and Arroyo 1978, Bawa et al. 1985, Bawa 1990). Since they are less prone to inbreeding in natural conditions, outbreeders are likely to carry a moderately high genetic load of slightly deleterious alleles (Lande 1995). Minimum viable population sizes for tropical forest trees needed to ensure long-term survival have been estimated at effective populations sizes (Ne) of about 5000 (Alvarez-Buylla et al. 1996).

Although the literature was far from cutting edge even when I wrote it (and more than a little long-of-tooth today), I think the basic point is applicable to the fragmentation question – inbreeders should be expected to purge genetic load, outbreeders more likely to tolerate moderate genetic load. So inbreeders and outbreeders are likely to experience fragmentation differently. (There’s are important caveats here, of course: plants may not respond to fragmentation the same way that animals do; tropical plants may differ from extra-tropical plants; trees will probably not react in the same way that shorter-lived species would.)

Thinking about this point reminded me of something other than Preston Aldrich’s work – Gemma White worked on gene flow in fragmented populations of Swietenia humilis, a species of mahogany restricted to the Pacific coast of Mexico. White and colleagues found that increased gene flow between populations in a fragmented landscape counteracted the effects of fragmentation (in this one species in this one context). The problem with this is that Aldrich and Hamrick’s paper was published in 1998, while White et al. is a 2001 vintage. So what has happened since?

This is where Google Scholar becomes both a boon and a trap. Not only does it make it much easier to find literature, it also makes it easy to find who has cited the papers that interest you. Granted, Web of Knowledge does the same thing, but Google Scholar is quicker and has a more intelligent search engine (as you’d expect from Google). White’s PNAS paper, it turns out, has been cited about 226 times, far too often for casual curiosity. And searching those 226 papers can itself be difficult – the top hits may not be the ones that cite White et al. for precisely the reasons you’re interested. At the same time, it does make an exercise in casual curiosity, like this one, feasible.

So what does the literature say? Not surprisingly (given the question posed), it says “we’re not sure”. Caesalpinia echinata in fragmented Brazilian Atlantic Rainforest fragments showed substantial genetic structure in forest fragments – populations in different fragments have become different, presumably due to reduced gene flow. On the other hand, the Australian rainforest tree Elaeocarpus grandis, showed reduced diversity and increased inbreeding in fragmented landscapes, but no increase in genetic structure. But Andrea Kramer and colleagues found that although “theory predicts widespread loss of genetic diversity from drift and inbreeding in trees subjected to habitat fragmentation, yet empirical support of this theory is scarce”, and suggested that ecological degradation, rather than genetic degradation, might be a far more important cause for concern.

These are just a few random excerpt from the literature, but it makes me think that I might been on the right track when I wrote:

However, the studies that led to these conclusions were not done on insular populations. Species native to the Greater and Lesser Antilles should be adapted to much smaller populations than are mainland populations; historically small populations are likely to be more inbred and, as a consequence of this, to carry lower genetic loads (Alvarez-Buylla et al. 1996). This makes them less susceptible to inbreeding depression (reduced viability, seed production and growth rates caused by the segregation of partially recessive lethal alleles).

There’s still a long way to go, but I suspect that a lot of insight from naturally fragmented systems, especially areas like the Caribbean (and obviously, some people have done just that).

Fundamental questions for ecology I

Why not start a series of potentially 100 blog posts? Aim big, right…?
In January, the Journal of Ecology published a massively multi-authored article Identification of 100 fundamental ecological questions. The article, the full text of which is freely available, was an interesting read – interesting, but not terribly surprising…I rather doubt I would have been able to come up with a list like that, but I my response to most of them was “yeah, that makes sense”. But even if you don’t learn a whole lot from them, they strike me as a good way to focus your thoughts. So, I though, by not?

The authors grouped the questions into six group which were, more or less, what you’d expect them to be: evolution and ecology, populations, diseases and microorganisms, communities and diversity, ecosystems and functioning, and humans impacts and global change. One thing I found notable was how things I would consider to be within my area of interest, were scattered up and down the list. It probably says something both about ecology itself (its subfields are notoriously poorly demarcated), but it probably also explains why I sometimes find the term “plant community ecologist” to be an ill-fitting label. Communities are, after all, made up of populations nested within species which are nested within landscapes and ecosystems, shaped by evolution and biogeography, and structured by human disturbance (both at the local and global level). So…Puerto Rican dry forests, you say…

1. What are the evolutionary consequences of species becoming less connected through fragmentation or more connected through globalization?

Listed under “evolution and ecology”, this encapsulated the nature of ecological questions nicely. If a formerly continuous population is fragmented, you’d generally expect a reduction in gene flow between populations. The consequences of reduced gene flow cover a wide spectrum from extinction due to inbreeding depression to speciation. If you had just read a conservation biology textbook you’d probably come away with the former answer. If you had read an evolution text, you’d be more likely to come up with the latter. Still, given the simplified way in which we teach ecology and evolutionary biology, you’d probably think that this wouldn’t be the kind of problem that would called a “fundamental question”.

Reality is always more complicated than we like to imagine. To begin with, the population genetic impact of fragmentation differs between species. Is the species unisexual or bisexual, and if it is the latter, is it an obligate (or even predominant) outcrosser? I always remember Preston Aldrich’s work on Symphonia globulifera in Costa Rica. Dan Janzen characterised individual trees in pasture as “the living dead” – although adults could survive in pasture, there was almost no recruitment. Although the tree might still have many decades of life ahead of it, the odds of it passing its genes on to future generations was almost nil. Aldrich and Hamrick showed that, in fact, a single individual growing in the middle of a pasture was, in fact, supplying most of the pollen to trees in surrounding forest fragments. Despite growing in a habitat that was inhospitable for seedlings, it had a disproportionate impact on the next generation. This was probably tied to the fact that the tree in the pasture experienced less competition and had greater access to light. This gave it more resources to dedicate to flower production and allowed it to dominate pollen flow locally. This doesn’t mean that living in a pasture amidst forest fragments is good for a species – rather, it means that these things can be difficult to predict. The harder it is to predict something, the longer it’s likely to linger on a list of fundamental (and unanswered) questions in ecology.

Difficult, of course, does not mean intractable. Nor must a rule be perfect. If “in general” is known, and we understand something of the nature of the deviations from it, we’d still have made great substantial progress.

History of ecology: Frank Edwin Egler

When it comes to mainstream ecology, I rarely encounter something that’s entirely new to me. There’s plenty that I don’t know a whole lot about, but usually when I come across something that I’ve never heard of, it’s merely a new name for a concept I’m already familiar with (macroecology and metacommunity ecology being two terms that I still remember encountering for the first time). Even when a concept is new to me, it’s generally built from pieces with which I’m already familiar.

The same cannot be said about the history of ecology. It seems like everywhere I turn I come across a new and important figure that I’ve never heard of before. Today on Wikipedia, for example, I encountered Frank Edwin Egler. A student of William S. Cooper, he went on to play a role in Rachel Carson’s Silent Spring. And his property in Connecticut now forms a protected area, Aton Forest.

There’s also this bit in the article that intrigues me. Like too much in Wikipedia, it’s written by someone who failed to include much back-story or context:

A consequence was that a passage in Silent Spring having some of Egler’s sarcasm received the most criticism from Ian Baldwin in his famously negative review in Science ([13]). Egler rose to defend Carson’s (and his) views in a series of publications that led to his censure by the Entomological Society of America—and censure of a journal that published his views. That incident helped both to focus and to polarize the issues of professionalism and environmentalism in the science of ecology ([14][15]).

Like much of the article, this passage refers back to the Aton Forest website, which contains a long biography of Egler. But without reading it, much of the article is obscure. (Ian Baldwin’s famously negative review? I’m familiar with neither Baldwin – the source calls him “an agricultural scientist at the University of Wisconsin” – nor his review. And Wikipedia contains neither a biography of Baldwin nor mention of his review in its Silent Spring article.)

Natural Selection as a Community Process

Continuing a few pages in Shipley’s book brings another new idea…that natural selection can function as a process driving community assembly. Um, yeah…that’s actually pretty obvious if you think about it (as most good ideas are, once someone points then out to you). Starting with a given assemblage of species (rather than genotypes) the environment will select the ‘fittest’ individuals…those that are favoured by the specific environmental conditions will leave the most offspring – or, will occupy  the largest proportion of the site. Seems pretty obvious once you think about it.

Playing with loaded dice

I happened to come across Bill Shipley’s From Plant Traits to Vegetation Structure. In it he uses a fascinating analogy for plant community assembly: “a never-ending game of crooked dice”. The craps tables are the different physical environments in which plants can grow, and each one has different characteristics (foam, steel, maple syrup stains) which interact different with the millions of dice (each face representing a species) that are thrown across their surface.

I’m really interested to see how he develops this model, but my initial reaction is very positive. While it may not possess the simplicity and tractability of Hubbell’s neutral model, it identifies from the very start what’s missing from so much in community ecology: a role for species as something more than interchangeable placeholders.

Disturbance and recovery in tropical dry forests

[Repost from my old blog]
When people think about the destruction and degradation of tropical forests, they tend to focus on rainforests. Tropical dry forests tend to get overlooked. They aren’t as striking – no cathedral-like understorey, no mind-boggling biodiversity. But more importantly, they often just aren’t there. Over much of their potential range they have simply been erased from the landscape. They may have covered as much as 42% of the land area in the tropics1, but have been reduced to less than 27% of their former range in Mexico2, and as little as 2% in Central America3 and New Caledonia4.

Despite the fact this, tropical dry forests are often seen as being quite well-adapted to human disturbance. Being less species-rich than wetter forests, they tend to support fewer rare species, and may be less extinction-prone. In addition, dry forests are dominated by trees that sprout after being cut. This means that if you cut down a patch of dry forest, most of the stumps will re-sprout. This type of recovery is much quicker than you would get if the trees had to germinate from seeds – not only does it take much longer for seedlings to grow large (stump sprouts can draw on resources stored in the roots of the tree), but there’s likely to be a time lag as seeds disperse into the area from surviving trees (tropical forests tend to lack long-lived seedbanks).

Much of our understanding of succession in tropical dry forests comes from Jack Ewel’s dissertation work. Ewel looked at the effect of cutting and herbicide application on succession in a series of plots across the Neotropics. One of his important findings was the dry forests were quicker to recover their stature that wetter forests. Since most of the recovery comes from stump sprouts, the recovering forest is also close to the original forest in terms of species composition.

While lightly used dry forest sites recover rapidly, recovery is slower in more intensively used sites. Seedling survival rates are very low in dry forests – while seedlings establish in the wet season, most (often all) of them die in the subsequence dry season. So while intensively used sites in Guánica Forest recovered well in terms of structure, biomass and leaf fall in 50 years after abandonment, the recovery of species composition was very slow6.

Resilience is the rate of recovery of disturbed sites to their pre-disturbed state. Ewel’s work helped to establish the idea that dry forests are more resilient than wetter forests. But there is no single rate – or pathway – of recovery. Measures of “recovery” depend on the parameter measured – canopy height, biomass, species richness, nutrient cycling… It also depends on the baseline against which recovery is measured: if the same site is measured before and after disturbance, you need to know if the site represented “mature” forest before disturbance. If another site is used, you need to wonder if it is really representative of initial conditions in your experimental plot.

In an article7 published in the journal Biotropica, Edwin Lebrija-Trejos and coauthors looked at what it really means to say that tropical dry forests are more resilient than wetter forests. They looked at a sequence of 15 sites in Oaxaca, Mexico, which had been cultivated and then abandoned for 0-40 years, and compared them with nearby mature forest. All of the sites had been cultivated for a short period (1-2 years) and then abandoned without being converted to pasture8. They considered a variety of different ways to measure resilience – they looked at forest height, plant density, basal area (the area occupied by tree stems), crown cover, species richness, species density (number of species per 100 m2), Shannon evenness and Shannon diversity. Not surprisingly, they found that certain features (canopy height, plant density, crown cover) recovered rapidly (in less than 20 years) while others (including basal area and species richness) had not recovered after 40 years.

When compared their sites with other comparable studies, they found that their sites were among the quickest to recover canopy cover and height. On the other hand, they found that their sites were among the slowest to recover species diversity and average in terms of the recovery of species richness. Overall, in terms of the structural measures that Ewel focussed on, it’s reasonable to conclude that dry forests are more resilient that wetter forests. On the other hand, with regards to things like basal area and species richness, the assertion of resilience for dry forests isn’t well supported.

  1. Brown, S., and A. E. Lugo. 1982. The storage and production of organic matter in tropical forests and their role in the global carbon cycle. Biotropica 14:161-187.
  2. Trejo, I., and R. Dirzo. 2002. Floristic diversity of Mexican seasonally dry tropical forests. Biodiversity and Conservation 11:2063–2084
  3. Janzen, D. H. 1988. Tropical dry forests: The most endangered major ecosystem. In E. O. Wilson (Ed.). Biodiversity, pp. 130–137. National Academy Press, Washington, DC
  4. Gillespie, T. W., and T. Jaffré. 2003. Tropical dry forests in New Caledonia. Biodiversity and Conservation 12:1687–1697.
  5. Ewel, J. J. 1971. Experiments in arresting succession with cutting and herbicide in five tropical environments. Ph.D. University of North Carolina, Chapel Hill.
  6. Molina Colón, S., and A. E. Lugo. 2006. Recovery of a subtropical dry forest after abandonment of different land uses. Biotropica 38:354–364.
  7. Lebrija-Trejos, E., Bongers, F., Pérez-García, E.A., Meave, J.A. (2008). Successional Change and Resilience of a Very Dry Tropical Deciduous Forest Following Shifting Agriculture. Biotropica 40(4):422-431 DOI: 10.1111/j.1744-7429.2008.00398.x
  8. Conversion to pasture tends to slow recovery significantly; not only does the prolonged period eliminate almost all root stocks, it also establishes a grassy layer that makes it more difficult for tree seedlings to establish.

Speciose or species-rich?

[Repost from my old blog]
As a graduate student I came across the word “speciose”.  It had an alluring sound to it that was lacking in its more pedestrian synonym “species-rich”.  Equally appealing, I suspect, was the fact that it supplied a formal-sounding alternative that was less accessible to the average person.  (If you’re lucky, you outgrow that affectation and learn that clear communication is what matters most.)

In the December 2008 issue of TREEMichael Hart delves into the origin and use of the word speciose.  Although similar to “species”, speciose actually shares a root derives from “specious” in ‘beautiful’ or ‘lovely’.  Hart sees value in speciose – it’s no longer than “species-rich” and solves the hyphenation problem (i.e., the problem of not knowing when to join the words “species” and “rich” with a hyphen).  Both “species-rich” and “speciose” first show up in the Web of Knowledge database in 1957, and use of both terms has grown fairly consistently.  Although he cites Gill’s plea to cease ‘the misuse of ‘‘speciose’’ in the evolutionary biological literature,’ Hart sees value in this “lovely word” and urges “deliberate consideration” as to its future and fate.

I embraced “speciose” in my first or second year as a grad student.  I happily embraced it, using it both in writing and conversation.  And then, to my horror, I discovered Gill or some other pedant who insisted that “speciose” was being misused by ecologists.  With that discovery, I banished the word from my vocabulary.  The only thing worse than using big words is misusing them.  Granted, it had been wearing thin already – my doctoral advisor, for example, had seen no inclination to adopt the word despite my repeated use of it.

And that’s where it’s stood for me, until now.  Al Gentry used to word, and being as amazing a biologist as he was, he had the right to use whatever word he wanted, however he wanted to…and be right.  He was, after all Al Gentry.  (And he had tragically passed away, doing a rapid assessment of biodiversity.) Reading Hart made me re-think my opposition to “speciose”.  We have the right to re-define words from time to time, and this might be a good candidate.  I’m not sure if it’s for me (it’s been four years since I wrote this post and I have not started using it), but I should be willing to consider it an acceptable term.

Hart, Michael W. 2008. Speciose versus species-rich. Trends in Ecology & Evolution,23 (12):660-661 doi:10.1016/j.tree.2008.09.001

Survival and rebound of Antillean dry forests: Role of forest fragments


Antillean dry forests have experienced high levels of human impact for almost five centuries. Economic changes in the second half of the 20th century have facilitated forest recovery in Puerto Rico. We quantified the extent of forest cover and the community composition of representative forest fragments in the subtropical dry forest life zone (sensu Holdridge, 1967) in southwestern Puerto Rico. Forest cover, which was largely eliminated by the 1940s, stood at 48% in the western dry forest life zone in 1993. Fragments varied in land-use history and supported from 1% to 86% of the reference species sampled in Guánica Forest, a 4000-ha protected area. Reference species were well represented in forest fragments, even those smaller than 1 ha, if they had never been completely cleared, but were uncommon in forests regenerating on previously cleared sites. The studied fragments are novel ecosystems which combine native and introduced elements; Leucaena leucocephala (Lam.) De Wit, an introduced legume, was the most common species, regardless of land-use history.

I.A., Murphy, P.G., Burton, T.M., Lugo, A.E.  2012. Survival and rebound of Antillean dry forests: role of forest fragments. Forest Ecology and Management 284:124-132.

Full article: ScienceDirect (subscription) or International Institute of Tropical Forestry.